Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
PHOSPHOR POWDERS,
METHODS FOR MAKING PHOSPHOR POWDERS
AND DEVICES INCORPORATING SAME
BACKGROUND OF THE INVENTION
1. Fietd of the invention
The present invention relates to phosphor powders, methods for
producing phosphor powders and devices, such as display devices,
incorporating the powders., In particular, the present invention is directed
to
phosphor powders having well-controlled chemical and physical properties. The
present invention also relates to a method for producing such powders by spray-
conversion.
2. Description of Related Art
Phosphors are compounds that are capable of emitting useful quantities
of radiation in the visible and/or ultraviolet spectrums upon excitation of
the
~5 phosphor by an external energy source. Due to this property; phosphor
compounds have long been utilized in cathode ray tube (CRT) screens for
televisions, in lighting elements and in other devices. Typically, inorganic
phosphor compounds include a host material doped with a small amount of an
activator ion.
2o There are a number of requirements for phosphor powders, which can
vary dependent upon the specific application of the powder. Generally,
phosphor powders should have one or more of the following properties: high
purity; high crystallinity; small particle size; narrow particle size
distribution;
spherical morphology; controlled surface chemistry; homogenous distribution of
25 the activator ion; good dispersibility; and controlled porosity. The proper
combination of the foregoing properties will result in a phosphor powder with
high luminescent intensity and long lifetime that can be used in many
applications. It is also advantageous for many applications to provide
phosphor
powders that are surface passivated or coated, such as with a thin, uniform
3o dielectric or semiconducting coating.
1
CA 02345300 2001-03-23
WO 00/12649 PCT/US99119582
Phosphor powders can be categorized by the excitation mechanism,
which causes luminescence in the phosphor. In this regard, phosphor powders
can be categorized as cathodoluminescent, photofuminescent,
electroluminescent or x-ray phosphors.
Cathodoluminescent phosphor powders luminesce when the phosphor
powder is excited by an electron source, usually a high-energy electron
source.
For example, cathodoluminescent phosphors are utilized in CRT devices. More
recently, cathodoluminescent phosphors have been utilized in advanced display
devices that provide illuminated text, graphics or video output. In
particular,
o there has been significant growth in the field of flat panel displays such
as field
emission displays (FEDs). FEDs are similar in principal to CRTs, wherein
electrons emitted from a tip stimulate the phosphors, which then emit light of
a
preselected color.
Photaluminescent phosphor powders are phosphor powders, which emit
~5 tight when excited by light (photons) of a different wavelength. For
example,
photoluminescent phosphor powders are used in fluorescent lighting elements
including common industrial lights. Photoluminescent phosphors are also used
as the back lights for display screens.
More recently, photoluminescent phosphor powders have been used in
2o advance display devices such as plasma displays. Plasma display panels
utilize
a gas trapped between transparent layers wherein the gas emits ultraviolet
light
when excited by an electric field. The ultraviolet fight stimulates the
photoluminescent phosphors on the screen to emit visible light. Plasma
displays
are particularly useful for larger displays, such as greater than about 20
diagonal
25 inches. In addition, photoluminescent phosphor powders can also be used as
a means of identifying an article of manufacture, such as a confidential
document, currency, postage and the like.
Electroluminescent phosphor powders emit light when excited by an
electric freld. Electroluminescent phosphor powders can also be used in fiat
3o panel display devices such as thick film and thin film electroluminescent
displays.
2
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
Thin film and thick film efectroiuminescent displays (TFEL) utilize a film of
electroluminescent material trapped between glass plates and electrodes that
emit light in an electric field. Electroluminescent phosphor powders are also
used in electroluminescent lamps {EL), which include an electroluminescent
phosphor powder deposited on a polymer substrate which emits fight when an
electric field is applied.
X-ray phosphor powders are used in image intensifers, particularly for
medical devices.
Numerous methods have been proposed for producing phosphor
~o particles. One such method is referred to as the solid-state method. In
this
process, the phosphor precursor materials are mixed in the solid state and are
heated so that the precursors react and form a powder of the phosphor
material.
For example, U.S. Patent No. 4,925,703 by Kasenga et al. discloses a method
for the production of a manganese activated zinc silicate phosphor
(ZnSi04:Mn).
The method includes a step of dry blending a mixture of starting components
such as zinc oxide, silicic acid and manganese carbonate and firing the
blended
mixture at about 1250°C. The resulting phosphor is broken up or crushed
into
smaller particles. Solid-state routes, and many other production methods,
utilize
a grinding step to reduce the particle size of the powders. The mechanical
2o grinding damages the surface of the phosphor, forming dead layers, which
inhibit
the brightness of the phosphor, powders.
Phosphor powders have also been made by liquid precipitation. In these
methods, a solution, which includes phosphor particle precursors, is
chemically
treated to precipitate phosphor particles or phosphor particle precursors.
These
25 particles are typically calcined at an elevated temperature to produce the
phosphor compound. The particles must often be further crushed, as is the case
with solid-state methods.
In yet another method, phosphor particle precursors or phosphor particles
are dispersed in a solution, which is then spray dried to evaporate the
liquid.
3o The phosphor particles are thereafter sintered in the solid state at an
elevated
3
CA 02345300 2001-03-23
WO 00/I2b49 PCT/US99/19582
temperature to crystallize the powder and form a phosphor. For example, U.S.
Patent No. 4,948,527 by Ritsko et al. discloses a process for producing
Y203:Eu
phosphors by dispersing yttrium oxide in a europium citrate solution to form a
slurry which is then spray dried. The spray-dried powder was then converted to
s an oxide by firing at about 1000°C for two hours and then at
1600°C for about
four hours. The fired powder was then lightly crushed and cleaned to recover
useful phosphor particles.
U.S. Patent No. 5,644,193 by Matsuda et al. discloses phosphor powders
having an average particle size of up to 20 gym. The phosphors can include
rare
earth oxides, rare earth oxysulfides and tungstates. The particles are
produced
by fusing phosphor particles in a thermal plasma and rapidly cooling the
particles.
Despite the foregoing, there remains a need for phosphor powders having
well-controlled properties for various applications. Desirable properties
typically
~s include a high luminescent intensity, particles having a substantially
spherical
morphology, narrow particle size distribution, a high degree of crystallinity
and
good homogeneity. The powder should have good dispersibility and the ability
to be fabricated into thin layers having uniform thickness. Phosphor powders
having these properties will be particularly useful in display devices and
other
2o advanced applications.
SUMMARY OF THE INVENTION
The present invention provides improved phosphor powder batches
including phosphors having a small particle size, spherical morphology and
good
crystallinity. The present invention also provides methods for forming
phosphor
25 powder batches and devices, such as advanced display devices and fighting
elements, incorporating the powder batches.
4
CA 02345300 2001-03-23
WO 40/12649 PCTIUS99/19582
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a process block diagram showing one embodiment of the
process of the present invention.
Fig. 2 is a side view in cross section of one embodiment of aerosol
s generator of the present invention.
Fig. 3 is a top view of a transducer mounting plate showing a 49
transducer array for use in an aerosol generator of the present invention.
Fig. 4 is a top view of a transducer mounting plate for a 400 transducer
array for use in an ultrasonic generator of the present invention.
~o Fig. 5 is a side view of the transducer mounting plate shown in Fig. 4.
Fig. fi is a partial side view showing the profile of a single transducer
mounting receptacle of the transducer mounting plate shown in Fig. 4.
Fig. 7 is a partial side view in cross-section showing an alternative
embodiment for mounting an ultrasonic transducer.
s Fig. 8 is a top view of a bottom retaining plate for retaining a separator
for
use in an aerosol generator of the present invention.
Fig. 9 is a top view of a liquid feed box having a bottom retaining plate to
assist in retaining a separator for use in an aerosol generator of the present
invention.
2o Fig. 10 is a side view of the liquid feed box shown in Fig. 9.
Fig. 11 is a side view of a gas tube for delivering gas within an aerosol
generator of the present invention.
Fig. 12 shows a partial top view of gas tubes positioned in a liquid feed
box for distributing gas relative to ultrasonic transducer positions for use
in an
z5 aerosol generator of the present invention.
Fig. 13 shows one embodiment for a gas distribution configuration for the
aerosol generator of the present invention.
Fig. 14 shows another embodiment for a gas distribution configuration for
the aerosol generator of the present invention.
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/I9582
Fig. 15 is a top view of one embodiment of a gas distribution platelgas
tube assembly of the aerosol generator of the present invention.
Fig. 16 is a side view of one embodiment of the gas distribution plate/gas
tube assembly shown in Fig. 15.
Fig. 17 shows one embodiment for orienting a transducer in the aerosol
generator of the present invention.
Fig. 18 is a top view of a gas manifold for distributing gas within an
aerosol generator of the present invention.
Fig. 19 is a side view of the gas manifold shown in Fig. 18.
~o Fig. 20 is a top view of a generator lid of a hood design for use in an
aerosol generator of the present invention.
Fig. 21 is a side view of the generator lid shown in Fig. 20.
Fig. 22 is a process block diagram of one embodiment of the process of
the present invention including a droplet classifier.
~5 Fig. 23 is a top view in cross section of an impactor of the present
invention for use in classifying an aerosol.
Fig. 24 is a front view of a flow control plate of the impactor shown in Fig.
23.
Fig. 25 is a front view of a mounting plate of the impactor shown in Fig.
20 23.
Fig. 26 is a front view of an impactor plate assembly of the impactor
shown in Fig. 23.
Fig. 27 is a side view of the impactor plate assembly shown in Fig. 26.
Fig. 28 is a process block diagram of one embodiment of the present
25 invention including a particle cooler.
Fig. 29 is a top view of a gas quench cooler of the present invention.
Fig. 30 is an end view of the gas quench cooler shown in Fig. 29.
Fig. 31 is a side view of a perforated conduit of the quench cooler shown
in Fig. 29.
6
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
Fig. 32 is a side view showing one embodiment of a gas quench cooler
of the present invention connected with a cyclone.
Fig. 33 is a process block diagram of one embodiment of the present
invention including a particle coater.
s Fig. 34 is a block diagram of one embodiment of the present invention
including a particle modifier.
Fig. 35 shows cross sections of various particle morphologies of some
composite particles manufacturabie according to the present invention.
Fig. 36 is a block diagram of one embodiment of the process of the
~o present invention including the addition of a dry gas between the aerosol
generator and the furnace.
Fig. 37 illustrates pixel regions on a display device according to the prior
art.
Fig. 38 illustrates pixel regions on a display device according to an
~ 5 embodiment of the present invention.
Fig. 39 illustrates a schematic view of a CRT device according to an
embodiment of the present invention.
Fig. 40 illustrates a schematic representation of pixels on a viewing
screen of a CRT device according to an embodiment of the present invention.
20 Fig. 41 schematically illustrates a field emission display according to an
embodiment of the present invention.
Fig. 42 schematically illustrates a cross-section of an electroluminescent
display device according to an embodiment of the present invention.
Fig. 43 schematically illustrates an exploded view of an
25 electroluminescent display device according to an embodiment of the present
invention.
Fig. 44 illustrates an electroluminescent lamp according to an
embodiment of the present invention.
Fig. 45 schematically illustrates a plasma display panel according to an
3o embodiment of the present invention.
7
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
Fig. 46 schematically illustrates a fluorescent lighting element according
to an embodiment of the present invention.
Fig. 47 schematically illustrates a photoluminescent phosphor powder
layer dispersed on a lighting element tube according to an embodiment of the
present invention.
Fig. 48 illustrates the use of an x-ray image intensifier according to an
embodiment of the present invention.
Fig. 49 illustrates a cross-section of an x-ray image intensifier according
to an embodiment of the present invention.
Fig. 50 illustrates an SEM photomicrograph of a phosphor powder
according to an embodiment of the present invention.
Fig. 51 illustrates the particle size distribution of a phosphor powder
according to an embodiment of the present invention.
Fig. 52 illustrates an x-ray diffraction pattern of a phosphor powder
~5 according to an embodiment of the present invention.
Fig. 53 illustrates an SEM photomicrograph of a phosphor powder
according to an embodiment of the present invention.
Fig. 54 illustrates the particle size distribution of a phosphor powder
according to an embodiment of the present invention.
2o Fig. 55 illustrates an x-ray diffraction pattern of a phosphor powder
according to an embodiment of the present invention.
Fig. 56 illustrates an SEM photomicrograph of a phosphor powder
according to an embodiment of the present invention.
Fig. 57 illustrates an SEM photomicrograph of a phosphor powder
25 according to an embodiment of the present invention.
Fig. 58 illustrates the particle size distribution of a phosphor powder
according to an embodiment of the present invention.
Fig. 59 illustrates an SEM photomicrograph of a phosphor powder
according to an embodiment of the present invention.
8
CA 02345300 2001-03-23
WO 00112649 PCT/US99/19582
Fig. 60 illustrates the particle size distribution of a phosphor powder
according to an embodiment of the present invention.
Fig. 61 illustrates an x-ray diffraction pattern of a phosphor powder
according to an embodiment of the present invention.
DESCRIPTION OF THE INVENTION
The present invention is generally directed to phosphor powders and
methods for producing the powders, as well as devices which incorporate the
powders. Phosphors emit light, typically visible light, upon stimulation by an
o energy source such as electrons (cathodoluminescent phosphors}, photons
(photoluminescent phosphors), an electric field (eiectroluminescent
phosphors),
or x-ray energy (x-ray phosphors). Examples of particular phosphor compounds
are listed in detail below. It will be appreciated that some phosphor
compounds
will fall into two or more of these categories.
15 In one aspect, the present invention provides a method for preparing a
particulate product. A feed of liquid-containing, flowable medium, including
at
least one precursor for the desired particulate product, is converted to
aerosol
form, with droplets of the medium being dispersed in and suspended by a
carrier
gas. Liquid from the droplets in the aerosol is then removed to permit
formation
2o in a dispersed state of the desired particles. In one embodiment, the
particles
are subjected, while still in a dispersed state, to compositional or
structural
modification, if desired. Compositional modification may include, for example,
coating the particles. Structural modification may include, for example,
crystallization, recrystallization or morphological alteration of the
particles. The
z5 term powder is often used herein to refer to the particulate product of the
present
invention. The use of the term powder does not indicate, however, that the
particulate product must be dry or in any particular environment. Although the
particulate product is typically manufactured in a dry state, the particulate
product may, after manufacture, be placed in a wet environment, such as in a
3o paste or slurry.
9
CA 02345300 2001-03-23
WO 00/12b49 PCT/US99/19582
The process of the present invention is particularly well suited for the
production of particulate products of finely divided particles having a small
weight
average size. In addition to making particles within a desired range of weight
average particle size, with the present invention the particles may be
produced
with a desirably narrow size distribution, thereby providing size uniformity
that
is desired for many applications.
In addition to control over particle size and size distribution, the method
of the present invention provides signifiicant flexibility for producing
phosphor
particles of varying composition, crystallinity and morphology. For example,
the
~o present invention may be used to produce homogeneous particles involving
only
a single phase or multi-phase particles including multiple phases. In the case
of multi-phase particles, the phases may be present in a variety of
morphologies.
For example, one phase may be uniformly dispersed thraughout a matrix of
another phase. Alternatively, one phase may form an inferior core while
another
phase forms a coating that surrounds the core. Other morphologies are also
possible, as discussed more fully below.
Referring now to Fig. 1, one embodiment of the process of the present
invention is described. A liquid feed 102, including at least one precursor
for the
desired particles, and a carrier gas 104 are fed to an aerosol generator 106
2o where an aerosol 108 is produced. The aerosol 108 is then fed to a furnace
110
where liquid in the aerosol 108 is removed to produce particles 112 that are
dispersed in and suspended by gas exiting the furnace 110. The particles 112
are then collected in a particle collector 114 to produce a particulate
product 116.
As used herein, the liquid feed 102 is a feed that includes one or more
flowable liquids as the major constituent(s), such that the feed is a flowabie
medium. The liquid feed 102 need not comprise only liquid constituents. The
liquid feed 102 may comprise only constituents in one or more liquid phase, or
it may also include particulate material suspended in a liquid phase. The
liquid
feed 102 must, however, be capable of being atomized to form droplets of
3o sufficiently small size for preparation of the aerosol 108. Therefore, if
the liquid
CA 02345300 2001-03-23
WO 00/12649 PCT/US99119582
feed 102 includes suspended particles, those particles should be relatively
small
in relation to the size of droplets in the aerosol 108. Such suspended
particles
should typically be smaller than about 1 ,um in size, preferably smaller than
about 0.5 ,um in size, and more preferably smaller than about 0.3 ,um in size
and
s most preferably smaller than about 0.1 ,um in size. Most preferably, the
suspended particles should be colloidal. The suspended particles could be
finely
divided particles, or could be agglomerate masses comprised of agglomerated
smaller primary particles. For example, 0.5 ,um particles could be
agglomerates
of nanometer-sized primary particles. When the liquid feed 102 includes
1o suspended particles, the particles typically comprise no greater than about
10
weight percent of the liquid feed.
As noted, the liquid feed 102 includes at least one precursor for
preparation of the particles 112. The precursor may be a substance in either a
liquid or solid phase of the liquid feed 102. Frequently, the precursor will
be a
~5 material, such as a salt, dissolved in a liquid solvent of the liquid feed
102. The
precursor may undergo one or more chemical reactions in the furnace 110 to
assist in production of the particles 112. Alternatively, the precursor
material
may contribute to formation of the particles 112 without undergoing chemical
reaction. This could be the case, for example, when the liquid feed 102
includes,
2o as a precursor material, suspended particles that are not chemically
modified in
the furnace 110. In any event, the particles 112 comprise at least one
component originally contributed by the precursor.
The liquid feed 102 may include multiple precursor materials, which may
be present together in a single phase or separately in multiple phases. For
25 example, the liquid feed 102 may include multiple precursors in solution in
a
single liquid vehicle. Alternatively, one precursor material could be in a
solid
particulate phase and a second precursor material could be in a liquid phase.
Also, one precursor material could be in one liquid phase and a second
precursor material could be in a second liquid phase, such as could be the
case
3o when the liquid feed 102 comprises an emulsion. Different components
11
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
contributed by different precursors may be present in the particles together
in a
single material phase, or the different components may be present in different
material phases when the particles 112 are composites of multiple phases.
Specific examples of preferred precursors for the phosphor particles of the
present invention are discussed more fully below.
The carrier gas 104 may comprise any gaseous medium in which droplets
produced from the liquid feed 102 may be dispersed in aerosol form. Also, the
carrier gas 104 may be inert, in that the carrier gas 104 does not participate
in
formation of the particles 112. Alternatively, the carrier gas may have one or
more active component{s) that contribute to formation of the particles 112. In
that regard, the carrier gas may include one or more reactive components that
react in the furnace 110 to contribute to formation of the particles 112.
Preferred
carrier gas compositions for the phosphor particles of the present invention
are
discussed more fully below.
1s The aerosol generator 106 atomizes the liquid feed 102 to farm droplets
in a manner to permit the carrier gas 104 to sweep the droplets away to form
the
aerosol 108. The droplets comprise liquid from the liquid feed 102. The
droplets
may, however, also include nonliquid material, such as one or more small
particles held in the droplet by the liquid. For example, when the particles
112
2o are composite particles, one phase of the composite may be provided in the
liquid feed 102 in the form of suspended precursor particles and a second
phase
of the composite may be produced in the furnace 110 from one or more
precursors in the liquid phase of the liquid feed 102. Furthermore the
precursor
particles could be included in the liquid feed 102, and therefore also in
droplets
zs of the aerosol 108, for the purpose only of dispersing the particles for
subsequent compositional or structural modification during or after processing
in the furnace 110.
An important aspect of the present invention is generation of the aerosol
108 with droplets of a small average size and narrow size distribution. In
this
12
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
manner, the particles 112 may be produced at a desired small size with a
narrow
size distribution, which are advantageous for many applications.
The aerosol generator 106 is capable of producing the aerosol 108 such
that it includes droplets having a weight average size in a range having a
lower
s limit of about 1 ~cm and preferably about 2 ,um; and an upper limit of about
10
,um; preferably about 7 ~cm, more preferably about 5 ,um and most preferably
about 4 gym. A weight average droplet size in a range of from about 2 ,um to
about 4 ,um is more preferred for most applications, with a weight average
droplet size of about 3 ,um being particularly preferred for same
applications.
o The aerosol generator is also capable of producing the aerosol '108 such
that it
includes droplets in a narrow size distribution. Preferably, the droplets in
the
aerosol are such that at least about 70 percent (more preferably at least
about
80 weight percent and most preferably at least about 85 weight percent) of the
droplets are smaller than about 10 ,um and more preferably at least about 70
s weight percent {more preferably at least about 80 weight percent and most
preferably at least about 85 weight percent) are smaller than about 5 ~cm.
Furthermore, preferably no greater than about 30 weight percent, more
preferably no greater than about 25 weight percent and most preferably no
greater than about 20 weight percent, of the droplets in the aerosol 108 are
20 larger than about twice the weight average droplet size.
Another important aspect of the present invention is that the aerosol 108
may be generated without consuming excessive amounts of the carrier gas 104.
The aerosol generator 106 is capable of producing the aerosol 108 such that it
has a high loading, or high concentration, of the liquid feed 102 in droplet
form.
2~ In that regard, the aerosol 108 preferably includes greater than about 1 x
106
droplets per cubic centimeter of the aerosol 108, more preferably greater than
about 5 x 106 droplets per cubic centimeter, still more preferably greater
than
about 1 x 10' droplets per cubic centimeter, and most preferably greater than
about 5 x 10' droplets per cubic centimeter. That the aerosol generator 106
can
so produce such a heavily loaded aerosol 108 is particularly surprising
considering
13
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
the high quality of the aerosol 108 with respect to small average droplet size
and
narrow droplet size distribution. Typically, droplet loading in the aerosol is
such
that the volumetric ratio of liquid feed 102 to carrier gas 104 in the aerosol
108
is larger than about 0.04 milliliters of liquid feed 102 per liter of carrier
gas 104
in the aerosol 108, preferably larger than about 0.083 milliliters of liquid
feed 102
per liter of carrier gas 104 in the aerosol 1 O8, more preferably larger than
about
0.167 milliliters of liquid feed 102 per liter of carrier gas 104, still more
preferably
larger than about 0.25 milliliters of liquid feed 102 per liter of carrier gas
104, and
most preferably larger than about 0.333 milliliters of liquid feed 102 per
liter of
o carrier gas 104.
This capability of the aerosol generator 106 to produce a heavily loaded
aerosol 108 is even more surprising given the high droplet output rate of
which
the aerosol generator 106 is capable, as discussed more fully below. It will
be
appreciated that the concentration of liquid feed 102 in the aerosol 108 will
~5 depend upon the specific components and attributes of the liquid feed 102
and,
particularly, the size of the droplets in the aerosol 108. For example, when
the
average droplet size is from about 2 ~m to about 4 ,um, the droplet loading is
preferably larger than about 0.15 milliliters of aerosol feed 102 per liter of
carrier
gas 104, more preferably larger than about 0.2 milliliters of liquid feed 102
per
20 liter of carrier gas 104, even more preferably larger than about 0.2
milliliters of
liquid feed 102 per liter of carrier gas 104, and most preferably larger than
about
0.3 milliliters of liquid feed 102 per liter of carrier gas 104. When
reference is
made herein to liters of carrier gas 104, it refers to the volume that the
carrier
gas 104 would occupy under conditions of standard temperature and pressure.
2s The furnace 110 may be any suitable device for heating the aerosol 108
to evaporate liquid from the droplets of the aerosol 108 and thereby permit
formation of the particles 112. The maximum average stream temperature, or
conversion temperature, refers to the maximum average temperature that an
aerosol stream attains while flowing through the furnace. This is typically
3o determined by a temperature probe inserted into the furnace. Preferred
14
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
conversion temperatures far the phosphor particles according to the present
invention are discussed more fully below.
Although longer residence times are possible, for many applications,
residence time in the heating zone of the furnace 110 of shorter than about 4
seconds is typical, with shorter than about 2 seconds being preferred, such as
from about 1 to 2 seconds. The residence time should be long enough,
however, to assure that the particles 112 attain the desired maximum stream
temperature for a given heat transfer rate. In that regard, with extremely
short
residence times, higher furnace temperatures could be used to increase the
rate
~o of heat transfer so long as the particles 112 attain a maximum temperature
within the desired stream temperature range. That mode of operation, however,
is not preferred. Also, it is preferred that, in most cases, the maximum
stream
temperature not be attained in the furnace 110 until substantially at the end
of
the heating zone in the furnace 110. For example, the heating zone will often
include a plurality of heating sections that are each independently
controllable.
The maximum stream temperature should typically not be attained anti! the
final
heating section, and more preferably until substantially at the end of the
last
heating section. This is important to reduce the potential for thermophoretic
losses of material. Also, it is noted that as used herein, residence time
refers to
2o the actual time for a material to pass through the relevant process
equipment.
In the case of the furnace, this includes the effect of increasing velocity
with gas
expansion due to heating.
Typically, the furnace 110 will be a tube-shaped furnace, so that the
aerosol 108 moving into and through the furnace does not encounter sharp
zs edges on which droplets could collect. Loss of droplets to collection at
sharp
surfaces results in a lower yield of particles 112. More important, however,
the
accumulation of liquid at sharp edges can result in re-release of undesirably
large droplets back into the aerosol 108, which can cause contamination of the
particulate product 116 with undesirably large particles. Also, over time,
such
CA 02345300 2001-03-23
WO 00112649 PCT/US99/19582
liquid collection at sharp surfaces can cause fouling of process equipment,
impairing process performance.
The furnace 110 may include a heating tube made of any suitable
material. The tube material may be a ceramic material, for example, mullite,
silica (quartz) or alumina. Alternatively, the tube may be metallic.
Advantages
of using a metallic tube are low cost, ability to withstand steep temperature
gradients and large thermal shocks, machinability and weldability, and ease of
providing a seal between the tube and other process equipment. Disadvantages
of using a metallic tube include limited operating temperature and increased
o reactivity in some reaction systems. One type of tube that is particularly
useful
according to the present invention is a fined metallic tube, such as a metal
tube
whose interior surface is lined with alumina.
When a metallic tube is used in the furnace 110, it is preferably a high
nickel content stainless steel alloy, such as a 330 stainless steel, or a
nickel-
based super alloy. As noted, one of the major advantages of using a metallic
tube is that the tube is relatively easy to seal with other process equipment.
In
that regard, flange fittings may be welded directly to the tube for connecting
with
other process equipment. Metallic tubes are generally preferred for spray-
converting phosphor particles that do not require a maximum tube wall
2o temperature of higher than about 1100°C during particle manufacture,
which is
the case for the phosphor particles according to the present invention.
Also, although the present invention is described with primary reference
to a furnace reactor, which is preferred, it should be recognized that, except
as
noted, any other thermal reactor, including a flame reactor or a plasma
reactor,
could be used instead. A furnace reactor is, however, preferred, because of
the
generally even heating characteristic of a furnace for attaining a uniform
stream
temperature.
The particle collector 114, may be any suitable apparatus for collecting
particles 112 to produce the particulate product 116. One preferred embodiment
of the particle collector 114 uses one or more filter to separate the
particles 112
16
CA 02345300 2001-03-23
W4 00/12649 PCT/US99/19582
from gas. Such a filter may be of any type, including a bag filter. Another
preferred embodiment of the particle collector uses one or more cyclone to
separate the particles 112. Other apparatus that may be used in the particle
collector 114 includes an electrostatic precipitator. Also, collection should
normally occur at a temperature above the condensation temperature of the gas
stream in which the particles 112 are suspended. Also, collection should
normally be at a temperature that is low enough to prevent significant
agglomeration of the particles 112.
Of significant importance to the operation of the process of the present
o invention is the aerosol generator 106, which must be capable of producing a
high quality aerosol with high droplet loading, as previously noted. With
reference to Fig. 2, one embodiment of an aerosol generator 106 of the present
invention is described. The aerosol generator 106 includes a plurality of
ultrasonic transducer discs 120 that are each mounted in a transducer housing
122. The transducer housings 122 are mounted to a transducer mounting plate
124, creating an array of the ultrasonic transducer discs 120. Any convenient
spacing may be used for the ultrasonic transducer discs 120. Center-to-center
spacing of the ultrasonic transducer discs 120 of about 4 centimeters is often
adequate. The aerosol generator 106, as shown in Fig. 2, includes forty-nine
2o transducers in a 7 x 7 array. The array configuration is as shown in Fig.
3, which
depicts the locations of the transducer housings 122 mounted to the transducer
mounting plate 124.
With continued reference to Fig. 2, a separator 126, in spaced relation to
the transducer discs 120, is retained between a bottom retaining plate 128 and
2s a top retaining plate 130. Gas delivery tubes 132 are connected to gas
distribution manifofds 134, which have gas delivery ports 136. The gas
distribution manifolds 134 are housed within a generator body 138 that is
covered by generator lid 140. A transducer driver 144, having circuitry for
driving
the transducer discs 120, is electronically connected with the transducer
discs
30 120 via electrical cables 146.
17
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
During operation of the aerosol generator 106, as shown in Fig. 2, the
transducer discs 120 are activated by the transducer driver 144 via the
electrical
cables 146. The transducers preferably vibrate at a frequency of from about 1
MHz to about 5 MHz, more preferably from about 1.5 MHz to about 3 MHz.
s Commonly used frequencies are at about 1.6 MHz and about 2.4 MHz.
Furthermore, all of the transducer discs 110 should be operating at
substantially
the same frequency when an aerosol with a narrow droplet size distribution is
desired. This is important because commercially available transducers can vary
significantly in thickness, sometimes by as much as 10%. It is preferred,
~o however, that the transducer discs 120 operate at frequencies within a
range of
5% above and below the median transducer frequency, more preferably within
a range of 2.5%, and most preferably within a range of 1 %. This can be
accomplished by careful selection of the transducer discs 120 so that they all
preferably have thicknesses within 5% of the median transducer thickness, more
15 preferably within 2.5%, and most preferably within 1 %.
Liquid feed 102 enters through a feed inlet 148 and flows through flow
channels 150 to exit through feed outlet 152. An ultrasonically transmissive
fluid,
typically water, enters through a water inlet 154 to fill a water bath volume
156
and flow through flow channels 158 to exit through a water outlet 160. A
proper
2o flow rate of the ultrasonically transmissive fluid is necessary to cool the
transducer discs 120 and to prevent overheating of the ultrasonically
transmissive fluid. Ultrasonic signals from the transducer discs 120 are
transmitted, via the ultrasonically transmissive fluid, across the water bath
volume 156, and ultimately across the separator 126, to the liquid feed 102 in
25 flow channels 150.
The ultrasonic signals from the ultrasonic transducer discs 120 cause
atomization cones 162 to develop in the liquid feed 102 at locations
corresponding with the transducer discs 120. Carrier gas 104 is introduced
into
the gas delivery tubes 132 and delivered to the vicinity of the atomization
cones
30 162 via gas delivery ports 136. Jets of carrier gas exit the gas delivery
ports 136
18
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
in a direction so as to impinge on the atomization cones 162, thereby sweeping
away atomized droplets of the liquid feed 102 that are being generated from
the
atomization cones 162 and creating the aerosol 108, which exits the aerosol
generator 106 through an aerosol exit opening 164.
s Efficient use of the carrier gas 104 is an important aspect of the aerosol
generator 106. The embodiment of the aerosol generator 106 shown in Fig. 2
includes two gas exit ports per atomization cone 162, with the gas ports being
positioned above the liquid medium 102 over troughs that develop between the
atomization cones 162, such that the exiting carrier gas 104 is horizontally
3o directed at the surface of the atomization cones 162, thereby efficiently
distributing the carrier gas 104 to critical portions of the liquid feed 102
for
effective and efficient sweeping away of droplets as they form about the
ultrasonically energized atomization cones 162. Furthermore, it is preferred
that
at least a portion of the opening of each of the gas delivery ports 136,
through
s which the carrier gas exits the gas delivery tubes, should be located below
the
top of the atomization cones 162 at which the carrier gas 104 is directed.
This
relative placement of the gas delivery ports 136 is very important to
efficient use
of carrier gas 104. Orientation of the gas delivery ports 136 is also
important.
Preferably, the gas delivery ports 136 are positioned to horizontally direct
jets of
2o the carrier gas 104 at the atomization cones 162. The aerosol generator 106
permits generation of the aerosol 108 with heavy loading with droplets of the
carrier liquid 102, unlike aerosol generator designs that do not efficiently
focus
gas delivery to the locations of droplet formation.
Another important feature of the aerosol generator 106, as shown in Fig.
25 2, is the use of the separator 126, which protects the transducer discs 120
from
direct contact with the liquid feed 102, which is often highly corrosive. The
height of the separator 126 above the top of the transducer discs 120 should
normally be kept as small as possible, and is often in the range of from about
1
centimeter to about 2 centimeters. The top of the liquid feed 102 in the flow
3o channels above the tops of the ultrasonic transducer discs 120 is typically
in a
19
CA 02345300 2001-03-23
WO OO/I2649 PCT/US99/19582
range of from about 2 centimeters to about 5 centimeters, whether or not the
aerosol generator includes the separator 126, with a distance of about 3 to 4
centimeters being preferred. Although the aerosol generator 106 could be made
without the separator 126, in which case the liquid feed 102 would be in
direct
contact with the transducer discs 120, the highly corrosive nature of the
liquid
feed 102 can often cause premature failure of the transducer discs 120. The
use of the separator 126, in combination with use of the ultrasonically
transmissive fluid in the water bath volume 156 to provide ultrasonic
coupling,
significantly extending the life of the ultrasonic transducers 120. One
disadvantage of using the separator 126, however, is that the rate of droplet
production from the atomization cones 162 is reduced, often by a factor of two
or more, relative to designs in which the liquid feed 102 is in direct contact
with
the ultrasonic transducer discs 102. Even with the separator 126, however, the
aerosol generator 106 used with the present invention is capable of producing
a high quality aerosol with heavy droplet loading, as previously discussed.
Suitable materials for the separator 126 include, for example, polyamides
(such
as Kapton? membranes from DuPont) and other polymer materials, glass, and
piexiglass. The main requirements for the separator 126 are that it be
ultrasonically transmissive, corrosion resistant and impermeable.
2o One alternative to using the separator 126 is to bind a corrosion-resistant
protective coating onto the surface of the ultrasonic transducer discs 120,
thereby preventing the liquid feed 102 from contacting the surface of the
ultrasonic transducer discs 120. When the ultrasonic transducer discs 120 have
a protective coating, the aerosol generator 106 will typically be constructed
25 without the water bath volume 156 and the liquid feed 102 will flow
directly over
the ultrasonic transducer discs 120. Examples of such protective coating
materials include platinum, gold, TEFLON?, epoxies and various plastics. Such
a coating typically significantly extends transducer fife. Also, when
operating
without the separator 126, the aerosol generator 106 will typically produce
the
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
aerosol 108 with a much higher droplet loading than when the separator 126 is
used.
One surprising finding with operation of the aerosol generator 106 of the
present invention is that the droplet loading in the aerosol may be affected
by the
temperature of the liquid feed 102 as well as the temperature of the water
bath
volume 156. It has been found that when the liquid feed 102 andlor the water
bath volume includes an aqueous liquid at an elevated temperature, the droplet
loading increases significantly. The temperature of the liquid feed 102 and/or
the
water bath volume 156 is preferably higher than about 30°C, and more
o preferably higher than about 35°C. If the temperature becomes too
high,
however, it can have a detrimental effect on droplet loading in the aerosol
108.
Therefore, the temperature of the liquid feed 102 and/or the water bath volume
should generally be lower than about 50°C, and preferably lower than
about
45°C. Either the liquid feed 102 or the water bath volume 156 may be
~5 maintained at the desired temperature in any suitable fashion. For example,
the
portion of the aerosol generator 106 where the liquid feed 102 is converted to
the aerosol 108 could be maintained at a constant elevated temperature.
Alternatively, the liquid feed 102 could be delivered to the aerosol generator
106
from a constant temperature bath maintained separate from the aerosol
2o generator 106.
The design for the aerosol generator 106 based on an array of ultrasonic
transducers is versatile and is easily modifed to accommodate different
generator sizes for different specialty applications. The aerosol generator
106
may be designed to include a plurality of ultrasonic transducers in any
25 convenient number. Even for smaller scale production, however, the aerosol
generator 106 preferably has at least nine ultrasonic transducers, more
preferably at least 16 ultrasonic transducers, and even more preferably at
least
25 ultrasonic transducers. For larger scale production, however, the aerosol
generator 106 includes at least 40 ultrasonic transducers, more preferably at
30 least 100 ultrasonic transducers, and even more preferably at least 400
21
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
ultrasonic transducers. In some large volume applications, the aerosol
generator
may have at least 1000 ultrasonic transducers.
Figs. 4-21 show component designs for an aerosol generator 106
including an array of 400 ultrasonic transducers. Referring first to Figs: 4
and 5,
the transducer mounting plate 124 is shown with a design to accommodate an
array of 400 ultrasonic transducers, arranged in four subarrays of 100
ultrasonic
transducers each. The transducer mounting plate 124 has integral vertical
walls
172 for containing the ultrasonically transmissive fluid, typically water, in
a water
bath similar to the water bath volume '! 56 described previously with
reference
o to Fig. 2.
As shown in Figs. 4 and 5, four hundred transducer mounting receptacles
174 are provided in the transducer mounting plate 124 for mounting ultrasonic
transducers for the desired array. With reference to Fig. 6, the profile of an
individual transducer mounting receptacle 174 is shown. A mounting seat 176
~5 accepts an ultrasonic transducer for mounting, with a mounted ultrasonic
transducer being held in place via screw holes 178. Opposite the mounting
receptacle 176 is a flared opening 180 through which an ultrasonic signal may
be transmitted for the purpose of generating the aerosol 108, as previously
described with reference to Fig. 2.
20 A preferred transducer mounting configuration, however, is shown in Fig.
7 for another configuration for the transducer mounting plate 124. As seen in
Fig. 7, an ultrasonic transducer disc 120 is mounted to the transducer
mounting
plate 124 by use of a compression screw 177 threaded into a threaded
receptacle 179. The compression screw 177 bears against the ultrasonic
25 transducer disc 120, causing an o-ring 181, situated in an o-ring seat 182
on the
transducer mounting plate, to be compressed to form a seal between the
transducer mounting plate 124 and the ultrasonic transducer disc 120. This
type
of transducer mounting is particularly preferred when the ultrasonic
transducer
disc 120 includes a protective surface coating, as discussed previously,
because
3o the seal of the o-ring to the ultrasonic transducer disc 120 will be inside
of the
22
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
outer edge of the protective seal, thereby preventing liquid from penetrating
under the protective surface coating from the edges of the ultrasonic
transducer
d isc 120.
Referring now to Fig. 8, the bottom retaining plate 128 for a 400
transducer array is shown having a design for mating with the transducer
mounting plate 124 (shown in Figs. 4-5). The bottom retaining plate 128 has
eighty openings 184, arranged in four subgroups 186 of twenty openings 184
each. Each of the openings 184 corresponds with five of the transducer
mounting receptacles 174 (shown in Figs. 4 and 5) when the bottom retaining
plate 128 is mated with the transducer mounting plate 124 to create a volume
for a water bath between the transducer mounting plate 124 and the bottom
retaining plate 128. The openings 184, therefore, provide a pathway for
ultrasonic signals generated by ultrasonic transducers to be transmitted
through
the bottom retaining plate.
Referring now to Figs. 9 and 10, a liquid feed box 190 for a 400
transducer array is shown having the top retaining plate 130 designed to fit
over
the bottom retaining plate 128 (shown in Fig. 8), with a separator 126 (not
shown) being retained between the bottom retaining plate 128 and the top
retaining plate 130 when the aerosol generator 106 is assembled. The liquid
2o feed box 190 also includes vertically extending walls 192 for containing
the liquid
feed 102 when the aerosol generator is in operation. Also shown in Figs. 9 and
is the feed inlet 148 and the feed outlet 152. An adjustable weir 198
determines the level of liquid feed 102 in the liquid feed box 190 during
operation
of the aeroso) generator 106.
25 The top retaining plate 130 of the liquid feed box 190 has eighty openings
194 therethrough, which are arranged in four subgroups 196 of twenty openings
194 each. The openings 194 of the top retaining plate 130 correspond in size
with the openings 184 of the bottom retaining plate 128 (shown in Fig. 8).
When
the aerosol generator 106 is assembled, the openings 194 through the top
3o retaining plate 130 and the openings 184 through the bottom retaining plate
128
23
CA 02345300 2001-03-23
WO 00112649 PCT/US99/19582
are aligned, with the separator 126 positioned therebetween, to permit
transmission of ultrasonic signals when the aerosol generator 106 is in
operation.
Referring now to Figs. 9-11, a plurality of gas tube feed-through holes 202
extend through the vertically extending wails 192 to either side of the
assembly
including the feed inlet 148 and feed outlet 152 of the liquid feed box 190.
The
gas tube feed-through holes 202 are designed to permit insertion therethrough
of gas tubes 208 of a design as shown in Fig. 11. When the aerosol generator
106 is assembled, a gas tube 208 is inserted through each of the gas tube feed-
o through holes 202 so that gas delivery ports 136 in the gas tube 208 will be
properly positioned and aligned adjacent the openings 194 in the top retaining
plate 130 for delivery of gas to atomization cones that develop in the liquid
feed
box 190 during operation of the aerosol generator 106. The gas delivery ports
136 are typically holes having a diameter of from about 1.5 millimeters to
about
3.5 millimeters.
Referring now to Fig. 12, a partial view of the liquid feed box 190 is shown
with gas tubes 208A, 208B and 208C positioned adjacent to the openings 194
through the top retaining plate 130. Also shown in Fig. 12 are the relative
locations that ultrasonic transducer discs 120 would occupy when the aerosol
2o generator 106 is assembled. As seen in Fig. 12, the gas tube 208A, which is
at
the edge of the array, has five gas delivery ports 136. Each of the gas
delivery
ports 136 is positioned to divert carrier gas 104 to a different one of
atomization
cones that develop over the array of ultrasonic transducer discs 120 when the
aerosol generator 106 is operating. The gas tube 2088, which is one row in
from
the edge of the array, is a shorter tube that has ten gas delivery ports 136,
five
each an opposing sides of the gas tube 2088. The gas tube 2088, therefore,
has gas delivery ports 136 for delivering gas to atomization cones
corresponding
with each of ten ultrasonic transducer discs 120. The third gas tube, 208C, is
a
longer tube that also has ten gas delivery ports 136 for delivering gas to
3o atomization cones corresponding with ten ultrasonic transducer discs 120.
The
24
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
design shown in Fig. 12, therefore, includes one gas delivery port per
ultrasonic
transducer disc 120. Although this is a lower density of gas delivery ports
136
than for the embodiment of the aerosol generator 106 shown in Fig. 2, which
includes two gas delivery ports per ultrasonic transducer disc 120, the design
s shown in Fig. 12 is, nevertheless, capable of producing a dense, high-
quality
aerosol without unnecessary waste of gas.
Referring now to Fig. 13, the flow of carrier gas 104 relative to atomization
cones 162 during operation of the aerosol generator 106 having a gas
distribution configuration to deliver carrier gas 104 from gas delivery ports
on
o both sides of the gas tubes 208, as was shown for the gas tubes 208A, 2088
and 208C in the gas distribution configuration shown in Fig. 11. The carrier
gas
104 sweeps both directions from each of the gas tubes 208.
An alternative, and preferred, flow for carrier gas 104 is shown in Fig. 14.
As shown in Fig. 14, carrier gas 104 is delivered from only one side of each
of
the gas tubes 208. This results in a sweep of carrier gas from all of the gas
tubes 208 toward a central area 212. This results in a more uniform flow
pattern
for aerosol generation that may significantly enhance the efficiency with
which
the carrier gas 104 is used to produce an aerosol. The aerosol that is
generated, therefore, tends to be more heavily loaded with liquid droplets.
2o Another configuration for distributing carrier gas in the aerosol generator
106 is shown in Figs. 15 and 16. In this configuration, the gas tubes 208 are
hung from a gas distribution plate 216 adjacent gas flow holes 218 through the
gas distribution plate 216. In the aerosol generator 106, the gas distribution
plate 216 would be mounted above the liquid feed, with the gas flow holes
25 positioned to correspond with an underlying ultrasonic transducer.
Referring
specifically to Fig. 16, when the ultrasonic generator 106 is in operation,
atomization cones 162 develop through the gas flow holes 218, and the gas
tubes 208 are located such that carrier gas 104 exiting from ports in the gas
tubes 208 impinge on the atomization cones and flow upward through the gas
so flow holes. The gas flow holes 218, therefore, act to assist in efficiently
CA 02345300 2001-03-23
WO 00/12549 PCT/US99/19582
distributing the carrier gas 104 about the atomization cones 162 for aerosol
formation. It should be appreciated that the gas distribution plates 218 can
be
made to accommodate any number of the gas tubes 208 and gas flow holes
218. Far convenience of illustration, the embodiment shown in Figs. 15 and 16
s shows a design having only two of the gas tubes 208 and only 16 of the gas
flow
holes 218. Also, it should be appreciated that the gas distribution plate 216
could be used atone, without the gas tubes 208. In that case, a slight
positive
pressure of carrier gas 104 would be maintained under the gas distribution
plate
216 and the gas flow holes 218 would be sized to maintain the proper velocity
of carrier gas 104 through the gas flow holes 218 for efficient aerosol
generation.
Because of the relative complexity of operating in that mode, however, it is
not
preferred.
Aerosol generation may also be enhanced through mounting of ultrasonic
transducers at a slight angle and directing the carrier gas at resulting
atomization
~s cones such that the atomization cones are tilting in the same direction as
the
direction of flow of carrier gas. Referring to Fig. 17, an ultrasonic
transducer disc
120 is shown. The ultrasonic transducer disc 120 is tilted at a tilt angle 114
{typically less than 10 degrees), so that the atomization cone 162 will also
have
a tilt. It is preferred that the direction of flow of the carrier gas 104
directed at the
2o atomization cone 162 is in the same direction as the tilt of the
atomization cone
162.
Referring now to Figs. 18 and 19, a gas manifold 220 is shown for
distributing gas to the gas tubes 208 in a 400 transducer array design. The
gas
manifold 220 includes a gas distribution box 222 and piping stubs 224 for
2s connection with gas tubes 208 (shown in Fig. 11 ). Inside the gas
distribution box
222 are two gas distribution plates 226 that form a flow path to assist in
distributing the gas equally throughout the gas distribution box 222, to
promote
substantially equal delivery of gas through the piping stubs 224. The gas
manifold 220, as shown in Figs. 18 and 19, is designed to feed eleven gas
tubes
zs
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
208. For the 400 transducer design, a total of four gas manifolds 220 are
required.
Referring now to Figs 23 and 24, the generator lid 140 is shown for a 400
transducer array design. The generator lid 140 mates with and covers the
liquid
feed box 190 (shown in Figs. 9 and 10). The generator lid 140, as shown in
Figs. 20 and 21, has a hood design to permit easy collection of the aerosol
108
without subjecting droplets in the aerosol 108 to sharp edges on which
droplets
may coalesce and be lost, and possibly interfere with the proper operation of
the
aerosol generator 106. When the aerosol generator 106 is in operation, the
o aerosol 108 would be withdrawn via the aerosol exit opening 164 through the
generator cover 140.
It is important that the aerosol stream that is fed to the furnace 110 have
a high droplet flow rate and high droplet loading as would be required for
most
industrial applications. With the present invention, the aerosol stream fed to
the
~5 furnace preferably includes a droplet flow of greater than about 0.5 liters
per
hour, more preferably greater than about 2 liters per hour, still more
preferably
greater than about 5 liters per hour, even more preferably greater than about
liters per hour, particularly greater than about 50 liters per hour and most
preferably greater than about 100 liters per hour; and with the droplet
loading
2o being typically greater than about 0.04 milliliters of droplets per liter
of carrier
gas, preferably greater than about 0.083 milliliters of droplets per liter of
carrier
gas 104, more preferably greater than about 0.167 milliliters of droplets per
liter
of carrier gas 104, still more preferably greater than about 0.25 milliliters
of
droplets per liter of carrier gas 104, particularly greater than about 0.33
milliliters
25 of droplets per liter of carrier gas 104 and most preferably greater than
about
0.83 milliliters of droplets per liter of carrier gas 104.
In addition to the foregoing, it has been found to be advantageous
according to the present invention to provide means for adjusting the
concentration of precursor in the liquid feed. More specifically, it has been
found
3o that during aerosol production, the precursor solution can concentrate due
to the
27
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
preferential evaporation of water from the liquid. As a result, it is
desirable to
provide water to the liquid either on a substantially continuous basis or
periodically to maintain the concentration of the precursors within an
acceptable
range. In some instances, it may also be necessary to add small amounts of
precursors if there is any preferential evaporation of precursor materials
from the
liquid.
The aerosol generator 106 of the present invention produces a
concentrated, high quality aerosol of micro-sized droplets having a relatively
narrow size distribution. It has been found, however, that for many
applications
the process of the present invention is significantly enhanced by further
classifying by size the droplets in the aerosol 108 prior to introduction of
the
droplets into the furnace 110. In this manner, the size and size distribution
of
particles in the particulate product 116 are further controlled.
Referring now to Fig. 22, a process flow diagram is shown for one
~5 embodiment of the process of the present invention including such droplet
classification: As shown in Fig. 22, the aerosol 108 from the aerosol
generator
106 goes to a droplet classifier 280 where oversized droplets are removed from
the aerosol 108 to prepare a classified aerosol 282. Liquid 284 from the
oversized droplets that are being removed is drained from the droplet
classifier
2o 280. This drained liquid 284 may advantageously be recycled for use in
preparing additional liquid feed 102.
Any suitable droplet classifier may be used for removing droplets above
a predetermined size. For example, a cyclone could be used to remove over-
size droplets. A preferred droplet classifier for many applications, however,
is
25 an impactor. One embodiment of an impactor for use with the present
invention
will now be described with reference to Figs. 23-27.
As seen in Fig. 23, an impactor 288 has disposed in a flow conduit 286
a flow control plate 290 and an impactor plate assembly 292. The flow control
plate 290 is conveniently mounted on a mounting plate 294.
28
CA 02345300 2001-03-23
WO 00112649 PCTIUS99/19582
The flow control plate 290 is used to channel the flow of the aerosol
stream toward the impactor plate assembly 292 in a manner with controlled flow
characteristics that are desirable for proper impaction of oversize droplets
on the
impactor plate assembly 292 for removal through the drains 296 and 314. One
embodiment of the flow control plate 290 is shown in Fig. 24. The flow control
plate 290 has an array of circular flow parts 296 for channeling flow of the
aerosol 108 towards the impactor plate assembly 292 with the desired flow
characteristics.
Details of the mounting plate.294 are shown in Fig. 25. The mounting
o plate 294 has a mounting flange 298 with a large diameter flow opening 300
passing therethrough to permit access of the aerosol 108 to the flow ports 296
of the flow control plate 290 (shown in Fig. 24).
Referring now to Figs. 26 and 27, one embodiment of an impactor plate
assembly 292 is shown. The impactor plate assembly 292 includes an impactor
plate 302 and mounting brackets 304 and 306 used to mount the impactor plate
302 inside of the flow conduit 286. The impactor plate 302 and the flow
channel
plate 290 are designed so that droplets larger than a predetermined size will
have momentum that is too large for those particles to change flow direction
to
navigate around the impactor plate 302.
2o During operation of the impactor 288, the aerosol 108 from the aerosol
generator 106 passes through the upstream flow control plate 290. Most of the
droplets in the aerosol navigate around the impactor plate 302 and exit the
impactor 288 through the downstream flow control plate 290 in the classified
aerosol 282. Droplets in the aerosol 108 that are too large to navigate around
25 the impactor plate 302 will impact on the impactor plate 302 and drain
through
the drain 296 to be collected with the drained liquid 284 (as shown in Fig.
23).
The configuration of the impactor plate 302 shown in Fig. 22 represents
only one of many possible configurations for the impactor plate 302. For
example, the impactor 288 could include an upstream flow control plate 290
so having vertically extending flow slits therethrough that are offset from
vertically
29
CA 02345300 2001-03-23
WO 40/12649 PCTIUS99/19582
extending flow slits through the impactor plate 302, such that droplets too
large
to navigate the change in flow due to the offset of the flow slits between the
flow
control plate 290 and the impactor plate 302 would impact on the impactor
plate
302 to be drained away. Other designs are also possible.
In a preferred embodiment of the present invention, the droplet classifier
280 is typically designed to remove droplets from the aerosol 108 that are
larger
than about 15 ,um in size, more preferably to remove droplets larger than
about
~m in size, even more preferably to remove droplets of a size larger than
about 8 ~m in size and most preferably to remove droplets larger than about 5
,um in size. The droplet classification size in the droplet classifier is
preferably
smaller than about 15 ,um, more preferably smaller than about 10 ,um, even
more
preferably smaller than about 8 ,um and most preferably smaller than about 5
~cm. The classification size, also called the classification cut point, is
that size
at which half of the droplets of that size are removed and half of the
droplets of
1s that size are retained. Depending upon the specific application, however,
the
droplet classification size may be varied, such as by changing the spacing
between the impactor plate 302 and the flow control plate 290 or increasing or
decreasing aerosol velocity through the jets in the flow control plate 290.
Because the aerosol generator 106 of the present invention initially
2o produces a high quality aerosol 108, having a relatively narrow size
distribution
of droplets, typically less than about 30 weight percent of liquid feed 102 in
the
aerosol 108 is removed as the drain liquid 284 in the droplet classifier 288,
with
preferably less than about 25 weight percent being removed, even more
preferably less than about 20 weight percent being removed and most preferably
25 less than about 15 weight percent being removed. Minimizing the removal of
liquid teed 102 from the aerosol 108 is particularly important for commercial
applications to increase the yield of high quality particulate product 116. It
should be noted, however, that because of the superior performance of the
aerosol generator 106, it is frequently not required to use an impactor or
other
3o droplet classifier to obtain a desired absence of oversize droplets to the
furnace.
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
This is a major advantage, because the added complexity and liquid losses
accompanying use of an impactor may often be avoided with the process of the
present invention.
With some applications of the process of the present invention, it may be
possible to collect the particles 112 directly from the output of the furnace
110.
More often, however, it will be desirable to cool the particles 112 exiting
the
furnace 110 prior to collection of the particles 112 in the particle collector
114.
Referring now to Fig. 28, one embodiment of the process of the present
invention is shown in which the particles 112 exiting the furnace 110 are sent
to
~o a particle cooler 320 to produce a cooled particle stream 322, which is
then feed
to the particle collector 114. Although the particle cooler 320 may be any
cooling
apparatus capable of cooling the particles 112 to the desired temperature for
introduction into the particle collector 114, traditional heat exchanger
designs are
not preferred. This is because a traditional heat exchanger design ordinarily
5 directly subjects the aerosol stream, in which the hot particles 112 are
suspended, to cool surfaces. In that situation, signifcant losses of the
particles
112 occur due to thermophoretic deposition of the hot particles 112 on the
cool
surfaces of the heat exchanger. According to the present invention, a gas
quench apparatus is provided for use as the particle cooler 320 that
significantly
2o reduces thermophoretic tosses compared to a traditional heat exchanger.
Referring now to Figs. 29-31, one embodiment of a gas quench cooler
330 is shown. The gas quench cooler includes a perforated conduit 332 housed
inside of a cooler housing 334 with an annular space 33fi located between the
cooler housing 334 and the perforated conduit 332. In fluid communication with
25 the annular space 336 is a quench gas inlet box 338, inside of which is
disposed
a portion of an aerosol outlet conduit 340. The perforated conduit 332 extends
between the aerosol outlet conduit 340 and an aerosol inlet conduit 342.
Attached to an opening into the quench gas inlet box 338 are two quench gas
feed tubes 344. Referring specifically to Fig. 31, the perforated tube 332 is
3o shown. The perforated tube 332 has a plurality of openings 345. The
openings
31
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
345, when the perforated conduit 332 is assembled into the gas quench cooler
330, permit the flow of quench gas 346 from the annular space 336 into the
interior space 348 of the perforated conduit 332. Although the openings 345
are
shown as being round holes, any shape of opening could be used, such as slits.
Also, the perforated conduit 332 could be a porous screen. Two heat radiation
shields 347 prevent downstream radiant heating from the furnace. in most
instances, however, it will not be necessary to include the heat radiation
shields
347, because downstream radiant heating from the furnace is normally not a
significant problem. Use of the heat radiation shields 347 is not preferred
due
o to particulate losses that accompany their use.
With continued reference to Figs. 29-31, operation of the gas quench
cooler 330 will now be described. During operation, the particles 112, carried
by
and dispersed in a gas stream, enter the gas quench cooler 330 through the
aerosol inlet conduit 342 and flow into the interior space 348 of perforated
conduit 332. Quench gas 346 is introduced through the quench gas feed tubes
344 into the quench gas inlet box 338. G~uench gas 346 entering the quench
gas inlet box 338 encounters the outer surface of the aerosol outlet conduit
340,
forcing the quench gas 346 to flow, in a spiraling, swirling manner, into the
annular space 338, where the quench gas 346 flows through the openings 345
2o through the walls of the perforated conduit 332. Preferably, the gas 346
retains
some swirling motion even after passing into the interior space 348. In this
way,
the particles 112 are quickly cooled with low losses of particles to the walls
of the
gas quench cooler 330. In this manner, the quench gas 346 enters in a radial
direction into the interior space 348 of the perforated conduit 332 around the
entire periphery, or circumference, of the perforated conduit 332 and over the
entire length of the perforated conduit 332. The cool quench gas 346 mixes
with
and cools the hot particles 112, which then exit through the aerosol outlet
conduit 340 as the cooled particle stream 322. The cooled particle stream 322
can then be sent to the particle collector 114 for particle collection. The
3o temperature of the cooled particle stream 322 is controlled by introducing
more
32
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
or less quench gas. Also, as shown in Fig. 29, the quench gas 346 is fed into
the quench cooler 330 in counter flow to flow of the particles. Alternatively,
the
quench cooler could be designed so that the quench gas 346 is fed into the
quench cooler in concurrent flow with the flow of the particles 112. The
amount
of quench gas 346 fed to the gas quench cooler 330 will depend upon the
specific material being made and the specific operating conditions. The
quantity
of quench gas 346 used, however, must be ufficient to reduce the temperature
of the aerosol steam including the particles 112 to the desired temperature.
Typically, the particles 112 are cooled to a temperature at least below about
0 200°C, and often lower. The only limitation on how much the particles
112 are
cooled is that the cooled particle stream 322 must be at a temperature that is
above the condensation temperature for water as another condensable vapor
in the stream. The temperature of the cooled particle stream 322 is often at a
temperature of from about 50°C to about 120°C.
Because of the entry of quench gas 346 into the interior space 348 of the
perforated conduit 322 in a radial direction about the entire circumference
and
length of the perforated conduit 322, a buffer of the cool quench gas 346 is
formed about the inner wall of the perforated conduit 332, thereby
significantly
inhibiting the loss of hot particles 112 due to thermophoretic deposition on
the
2o cool wall of the perforated conduit 332. In operation, the quench gas 346
exiting
the openings 345 and entering into the interior space 348 should have a radial
velocity (velocity inward toward the center of the circular cross-section of
the
perforated conduit 332) of larger than the thermophoretic velocity of the
particles
112 inside the perforated conduit 332 in a direction radially outward toward
the
perforated wall of the perforated conduit 332.
As seen in Figs. 29-31, the gas quench cooler 330 includes a flow path
for the particles 112 through the gas quench cooler of a substantially
constant
cross-sectional shape and area. Preferably, the flow path through the gas
quench cooler 330 will have the same cross-sectional shape and area as the
3o flow path through the furnace 110 and through the conduit delivering the
aerosol
33
CA 02345300 2001-03-23
WO OO/I2649 PCT/US99/19582
108 from the aerosol generator 106 to the furnace 110. In one embodiment,
however, it may be necessary to reduce the cross-sectional area available for
flow prior to the particle collector 114. This is the case, for example, when
the
particle collector includes a cyclone for separating particles in the cooled
particle
stream 322 from gas in the cooled particle stream 322. This is because of the
high inlet velocity requirements into cyclone separators.
Referring now to Fig. 32, one embodiment of the gas quench cooler 330
is shown in combination with a cyclone separator 392. The perforated conduit
332 has a continuously decreasing cross-sectional area for flow to increase
the
velocity of flow to the proper value for the feed to cyclone separator 392.
Attached to the cyclone separator 392 is a bag filter 394 for final clean-up
of
overflow from the cyclone separator 392. Separated particles exit with
underfiow
from the cyclone separator 392 and may be collected in any convenient
container. The use of cyclone separation is particularly preferred for powder
having a weight average size of larger than about 1 ~cm, although a series of
cyclones may sometimes be needed to get the desired degree of separation.
Cyclone separation is particularly preferred for powders having a weight
average
size of larger than about 1.5 ,um. Also, cyclone separation is best suited for
high
density materials. Preferably, when particles are separated using a cyclone,
the
2o particles are of a composition with specific gravity of greater than about
5.
In an additional embodiment, the process of the present invention can
also incorporate compositional modification of the particles 112 exiting the
furnace. Most commonly, the compositional modifcation will involve forming on
the particles 112 a material phase that is different than that of the
particles 112,
such as by coating the particles 112 with a coating material. One embodiment
of the process of the present invention incorporating particle coating is
shown in
Fig. 33. As shown in Fig. 33, the particles 112 exiting from the furnace 110
go
to a particle coater 350 where a coating is placed over the outer surface of
the
particles 112 to form coated particles 352, which are then sent to the
particle
3o collector 114 for preparation of the particulate product 116. Coating
34
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
methodologies employed in the particle coater 350 are discussed in more detail
below.
With continued reference primarily to Fig. 33, in a preferred embodiment,
when the particles 112 are coated according to the process of the present
s invention, the particles 112 are also manufactured via the aerosol process
of the
present invention, as previously described. The process of the present
invention
can, however, be used to coat particles that have been premanufactured by a
different process, such as by a liquid precipitation route. When coating
particles
that have been premanufactured by a different route, such as by liquid
precipitation, it is preferred that the particles remain in a dispersed state
from the
time of manufacture to the time that the particles are introduced in slurry
form
into the aerosol generator 106 for preparation of the aerosol 108 to form the
dry
particles 112 in the furnace 110, which particles 112 can then be coated in
the
particle coater 350. Maintaining particles in a dispersed state from
manufacture
~5 through coating avoids problems associated with agglomeration and
redispersion of particles if particles must be redispersed in the liquid feed
102 for
feed to the aerosol generator 106. For example, for particles originally
precipitated from a liquid medium, the liquid medium containing the suspended
precipitated particles could be used to form the liquid feed 102 to the
aerosol
2o generator 106. It should be noted that the particle coater 350 could be an
integral extension of the furnace 110 or could be a separate piece of
equipment.
In a further embodiment of the present invention, following preparation of
the particles 112 in the furnace 110, the particles 112 may then be
structurally
modified to impart desired physical and chemical properties. Referring now to
25 Fig. 34, one embodiment of the process of the present invention is shown
including such structural particle modification. The particles 112 exiting the
furnace 110 go to a particle modifier 360 where the particles are structurally
modified to form modified particles 362, which are then sent to the particle
collector 114 for preparation of the particulate product 116. The particle
modifier
30 360 is typically a furnace, such as an annealing furnace, which may be
integral
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
with the furnace 110 or may be a separate heating device. Regardless, it is
important that the particle modifier 360 have temperature control that is
independent of the furnace 110, so that the proper conditions for particle
modification may be provided separate from conditions required of the furnace
s 110 to prepare the particles 112. The particle modifier 360, therefore,
typically
provides a temperature controlled environment and necessary residence time
to effect the desired structural andlor chemical modifcation of the particles
112.
The structural modification that occurs in the particle modifier 360 may be
any modification to the crystalline structure or morphology of the particles
112.
o Preferably, the particles 112 are heat treated in the particle modifier 360
to
further convert and densify the particles 112 or to recrystallize the
particles 112
into a polycrystalline or single crystalline phosphor form. Also, especially
in the
case of composite particles 112, the particles may be annealed for a
sufficient
time to permit redistribution within the particles 112 of different material
phases.
~s Particularly preferred parameters for such processes are discussed in more
detail below.
The initial morphology of composite particles made in the furnace 110,
according to the present invention, could take a variety of forms, depending
upon the specified materials involved and the specific processing conditions.
2o Examples of some possible composite particle morphologies, manufacturable
according to the present invention are shown in Fig. 35. These marphoiogies
could be of the particles as initially produced in the furnace 110 or that
result
from structural modification in the particle modifier 360. Furthermore, the
composite particles could include a mixture of the morphological attributes
25 shown in Fig. 35.
Aerosol generation with the process of the present invention has thus far
been described with respect to the ultrasonic aerosol generator. Use of the
ultrasonic generator is preferred for the process of the present invention
because
of the extremely high quality and dense aerosol generated. In some instances,
3o however, the aerosol generation for the process of the present invention
may
36
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
have a different design depending upon the specific application. For example,
when larger particles are desired, such as those having a weight average size
of larger than about 3 ,um, a spray nozzle atomizer may be preferred. For
smaller-particle applications, however, and particularly for those
applications
utilizing particles smaller than about 3 ,um, and preferably smaller than
about 2
,um in size, as is often desired with the phosphor particles of the present
invention, the ultrasonic generator, as described herein, is particularly
preferred.
In that regard, the ultrasonic generator of the present invention is
particularly
preferred for when making particles with an average size of from about 0.2 ,um
o to about 3 ,um.
Although ultrasonic aerosol generators have been used for medical
applications and home humidifiers, the use of ultrasonic generators for spray
pyrolysis particle manufacture has largely been confned to small-scale,
experimental situations. The ultrasonic aerosol generator of the present
invention described with reference to Figures 5-24, however, is well suited
for
commercial production of high quality powders with a small average size and a
narrow size distribution. In that regard, the aerosol generator produces a
high
quality aerosol, with heavy droplet loading and at a high rate of production.
Such a combination of small droplet size, narrow size distribution, heavy
droplet
loading, and high production rate provide significant advantages over existing
aerosol generators that usually suffer from at least one of inadequately
narrow
size distribution, undesirably low droplet loading, or unacceptably low
production
rate.
Through the careful and controlled design of the ultrasonic generator of
the present invention, an aerosol may be produced typically having greater
than
about 70 weight percent (and preferably greater than about 80 weight percent)
of droplets in the size range of from about 1 ,um to about 10 ~cm, preferably
in a
size range of from about 1 ,um to about 5 ~m and more preferably from about 2
~cm to about 4 ,um.
37
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
Also, the ultrasonic. generator of the present invention is capable of
delivering high output rates of liquid feed in the aerosol. The rate of liquid
feed,
at the high liquid loadings previously described, is preferably greater than
about
25 milliliters per hour per transducer, more preferably greater than about
37.5
milliliters per hour per transducer, even more preferably greater than about
50
milliliters per hour per transducer and most preferably greater than about 100
millimeters per hour per transducer. This high level of performance is
desirable
for commercial operations and is accomplished with the present invention with
a relatively simple design including a single precursor bath over an array of
ultrasonic transducers. The ultrasonic generator is made for high aerosol
production rates at a high droplet loading, and with a narrow size
distribution of
droplets. The generator preferably produces an aerosol at a rate of greater
than
about 0.5 liter per hour of droplets, more preferably greater than about 2
liters
per hour of droplets, still more preferably greater than about 5 liters per
hour of
~5 droplets, even more preferably greater than about 10 liters per hour of
droplets
and most preferably greater than about 40 liters per hour of droplets. For
example, when the aerosol generator has a 400 transducer design, as described
with reference to Figures 7-24, the aerosol generator is capable of producing
a
high quality aerosol having high droplet loading as previously described, at a
2o total production rate of preferably greater than about 10 liters per hour
of liquid
feed, more preferably greater than about 15 liters per hour of liquid teed,
even
more preferably greater than about 20 liters per hour of liquid feed and most
preferably greater than about 40 liters per hour of liquid feed.
Under most operating conditions, when using such an aerosol generator,
2s total particulate product produced is preferably greater than about 0.5
gram per
hour per transducer, more preferably greater than about 0.75 gram per hour per
transducer, even more preferably greater than about 1.0 gram per hour per
transducer and most preferably greater than about 2.0 grams per hour per
transducer. The mass of powder produced per unit time will be influenced by
the
so molecular weight of the compound.
3$
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/195$2
One significant aspect of the process of the present invention for
manufacturing particulate materials is the unique flow characteristics
encountered in the furnace relative to laboratory scale systems. The maximum
Reynolds number attained for flow in the furnace 110 with the present
invention
is very high, typically in excess of 500, preferably in excess of 1,000 and
more
preferably in excess of 2,000. In most instances, however, the maximum
Reynolds number for flow in the furnace will not exceed 10,000, and preferably
will not exceed 5,000. This is significantly different from lab-scale systems
where the Reynolds number for flow i.n a reactor is typically lower than 50
and
rarely exceeds 100.
The Reynolds number is a dimensionless quantity characterizing flow of
a fluid, which, for flow through a circular cross sectional conduit is defined
as:
Re =
where: ? = fluid density;
v = fluid mean velocity;
d = conduit inside diameter; and
a = fluid viscosity.
It should be noted that the values for density, velocity and viscosity will
vary
along the length of the furnace 110. The maximum Reynolds number in the
furnace 110 is typically attained when the average stream temperature is at a
maximum, because the gas velocity is at a very high value due to gas expansion
when heated.
One problem with operating under flow conditions at a high Reynolds
number is that undesirable volatilization of components is much more likely to
occur than in systems having flow characteristics as found in laboratory-scale
systems. The volatilization problem occurs with the present invention, because
the furnace is typically operated over a substantial section of the heating
zone
in a constant wall heat flux mode, due to limitations in heat transfer
capability.
This is significantly different than operation of a furnace at a laboratory
scale,
39
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
which typically involves operation of most of the heating zone of the furnace
in
a uniform wall temperature mode, because the heating load is sufficiently
small
that the system is not heat transfer limited.
With the present invention, it is typically preferred to heat the aerosol
stream in the heating zone of the furnace as quickly as possible to the
desired
temperature range for particle manufacture. Because of flow characteristics in
the furnace and heat transfer limitations, during rapid heating of the aerosol
the
wall temperature of the furnace can significantly exceed the maximum average
target temperature for the stream. This is a problem because, even though the
~o average stream temperature may be within the range desired, the wall
temperature may become so hot that components in the vicinity of the wall are
subjected to temperatures high enough to undesirably volatilize the
components.
This volatilization near the wall of the furnace can cause formation of
significant
quantities of ultrafine particles that are outside of the size range desired.
Therefore, with the present invention, it is preferred that when the flow
characteristics in the furnace are such that the Reynolds number through any
part of the furnace exceeds 500, more preferably exceeds 1,000, and most
preferably exceeds 2,000, the maximum wall temperature in the furnace should
be kept at a temperature that is below the temperature at which a desired
2o component of the final particles would exert a vapor pressure not exceeding
about 200 millitorr, more preferably not exceeding about 100 millitorr, and
most
preferably not exceeding about 50 millitorr. Furthermore, the maximum wall
temperature in the furnace should also be kept below a temperature at which an
intermediate component, from which a final component is to be at least
partially
derived, should also have a vapor pressure not exceeding the magnitudes noted
for components of the final product.
In addition to maintaining the furnace wall temperature below a level that
could create volatilization problems, it is also important that this not be
accomplished at the expense of the desired average stream temperature. The
so maximum average stream temperature must be maintained at a high enough
CA 02345300 2001-03-23
WO 00112649 PCT/US99/19582
level so that the particles will have a desired high density. The maximum
average stream temperature should, however, generally be a temperature at
which a component in the final particles, or an intermediate component from
which a component in the final particles is at least partially derived, would
exert
a vapor pressure not exceeding about 100 millitorr, preferably not exceeding
about 50 millitorr, and most preferably not exceeding about 25 millitorr.
So long as the maximum wall temperature and the average stream
temperature are kept below the point at which detrimental volatilization
occurs,
it is generally desirable to heat the stream as fast as possible and to remove
~a resulting particles from the furnace immediately after the maximum stream
temperature is reached in the furnace. With the present invention, the average
residence time in the heating zone of the furnace may typically be maintained
at
shorter than about 4 seconds, such as from about 1 to 2 seconds.
Another significant issue with respect to operating the process of the
~5 present invention, which includes high aerosol flow rates, is loss within
the
system of materials intended for incorporation into the final particulate
product.
Material losses in the system can be quite high if the system is not properly
operated. If system losses are too high, the process would not be practical
for
use in the manufacture of particulate products of many materials. This has
2o typically not been a major consideration with laboratory-scale systems.
One significant potential for loss with the process of the present invention
is thermophoretic losses that occur when a hot aerosol stream is in the
presence
of a cooler surface. In that regard, the use of the quench cooler, as
previously
described, with the process of the present invention provides an efficient way
to
25 cool the particles without unreasonably high thermophoretic losses. There
is
also, however, significant potential for losses occurring near the end of the
furnace and between the furnace and the cooling unit.
It has been found that thermophoretic losses in the back end of the
furnace can be significantly controlled if the heating zone of the furnace is
30 operated such that the maximum stream temperature is not attained until
near
41
CA 02345300 2001-03-23
WO OOI12649 PCT/US99/19582
the end of the heating zone in the furnace, and at least not until the last
third of
the heating zone. When the heating zone includes a plurality of heating
sections, the maximum average stream temperature should ordinarily not occur
until at least the last heating section. Furthermore, the heating zone should
typically extend to as close to the exit of the furnace as possible. This is
counter
to conventional thought, which is to typically maintain the exit portion of
the
furnace at a low temperature to avoid having to seal the furnace outlet at a
high
temperature. Such cooling of the exit portion of the furnace, however,
significantly promotes thermophoretic losses. Furthermore, the potential for
operating problems that could result in thermophoretic losses at the back end
of
the furnace are reduced with the very short residence times in the furnace for
the
present invention, as discussed previously.
Typically, it would be desirable to instantaneously cool the aerosol upon
exiting the furnace. This is not possible. It is possible, however, to make
the
residence time between the furnace outlet and the cooling unit as short as
possible. Furthermore, it is desirable to insulate the aerosol conduit
occurring
befween the furnace exit and the cooling unit entrance. Even more preferred is
to insulate that conduit and, even more preferably, to also heat that conduit
so
that the wall temperature of that conduit is at least as high as the average
stream
2o temperature of the aerosol stream. Furthermore, it is desirable that the
cooling
unit operate in a manner such that the aerosol is quickly cooled in a manner
to
prevent thermophoretic losses during cooling. The quench cooler, described
previously, is very effective for cooling with low losses. Furthermore, to
keep the
potential far thermophoretic losses very tow, it is preferred that the
residence
time of the aerosol stream between attaining the maximum stream temperature
in the furnace and a point at which the aerosol has been cooled to an average
stream temperature below about 200°C is shorter than about 2 seconds,
more
preferably shorter than about 1 second, and even more preferably shorter than
about 0.5 second and most preferably shorter than about 0.1 second. In most
3o instances, the maximum average stream temperature attained in the furnace
will
42
CA 02345300 2001-03-23
WO 00/12649 PCTIUS99/19582
be greater than about 700°C. Furthermore, the total residence time from
the
beginning of the heating zone in the furnace to a point at which the average
stream temperature is at a temperature below about 200°C should
typically be
shorter than about 5 seconds, preferably shorter than about 3 seconds, more
preferably shorter than about 2 seconds, and most preferably shorter than
about
1 second.
Another part of the process with significant potential for thermophoretic
losses is after particle cooling until the particles are fnally collected.
Proper
particle collection is very important to reducing losses within the system.
The
potential for thermophoretic losses is significant following particle cooling
because the aerosol stream is still at an elevated temperature to prevent
detrimental condensation of water in the aerosol stream. Therefore, cooler
surfaces of particle collection equipment can result in significant
thermophoretic
losses.
To reduce the potential for thermophoretic losses before the particles are
finally collected, it is important that the transition between the cooling
unit and
particle collection be as short as possible. Preferably, the output from the
quench cooler is immediately sent to a particle separator, such as a filter
unit or
a cyclone. In that regard, the total residence time of the aerosol between
2o attaining the maximum average stream temperature in the furnace and the
final
collection of the particles is preferably shorter than about 2 seconds, more
preferably shorter than about 1 second, still more preferably shorter than
about
0.5 second and most preferably shorter than about 0.1 second. Furthermore,
the residence time between the beginning of the heating zone in the furnace
and
final collection of the particles is preferably shorter than about 6 seconds,
more
preferably shorter than about 3 seconds, even more preferably shorter than
about 2 seconds, and most preferably shorter than about 1 second.
Furthermore, the potential for thermophoretic Posses may further be reduced by
insulating the conduit section between the cooling unit and the particle
collector
3o and, even more preferably, by also insulating around the filter, when a
filter is
43
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
used for particle collection. The potential for losses may be reduced even
further by heating of the conduit section between the cooling unit and the
particle
collection equipment, so that the internal equipment surfaces are at least
slightly
warmer than the aerosol stream average stream temperature. Furthermore,
s when a filter is used for particle collection, the filter could be heated.
For
example, insulation could be wrapped around a filter unit, with electric
heating
inside of the insulating layer to maintain the walls of the filter unit at a
desired
elevated temperature higher than the temperature of filter elements in the
filter
unit, thereby reducing thermophoretic particle losses to walls of the filter
unit.
o Even with careful operation to reduce thermophoretic losses, some losses
will still occur. For example, some particles will inevitably be lost to walls
of
particle collection equipment, such as the walls of a cyclone or filter
housing.
One way to reduce these losses, and correspondingly increase product yield, is
to periodically wash the interior of the particle collection equipment to
remove
particles adhering to the sides. In most cases, the wash fluid will be water,
unless water would have a detrimental effect on one of the components of the
particles. For example, the particle collection equipment could include
parallel
collection paths. One path could be used for active particle collection while
the
other is being washed. The wash could include an automatic or manual flush
2o without disconnecting the equipment. Alternatively, the equipment to be
washed
could be disconnected to permit access to the interior of the equipment for a
thorough wash. As an alternative to having parallel collection paths, the
process
could simply be shut down occasionally to permit disconnection of the
equipment
for washing. The removed equipment could be replaced with a clean piece of
2~ equipment and the process could then be resumed while the disconnected
equipment is being washed.
For example, a cyclone or filter unit could periodically be disconnected
and particles adhering to interior walls could be removed by a water wash. The
particles could then be dried in a low temperature dryer, typically at a
so temperature of lower than about 50°C.
44
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
Another area for potential losses in the system, and far the occurrence
of potential operating problems, is between the outlet of the aerosol
generator
and the inlet of the furnace. Losses here are not due to thermophoresis, but
rather to liquid coming out of the aerosol and impinging and collecting on
conduit
and equipment surfaces. Although this loss is undesirable from a material
yield
standpoint, the loss may be even more detrimental to other aspects of the
process. For example, water collecting on surfaces may release large droplets
that can lead to large particles that detrimentally contaminate the
particulate
product. Furthermore, if accumulated liquid reaches the furnace, the liquid
can
cause excessive temperature gradients within the furnace tube, which can cause
furnace tube failure, especially for ceramic tubes.
One way to reduce the potential for undesirable liquid buildup in the
system is to provide adequate drains, as previously described. In that regard,
it is preferred that a drain be placed as close as possible to the furnace
inlet to
15 prevent liquid accumulations from reaching the furnace. The drain should be
placed, however, far enough in advance of the furnace inlet such that the
stream
temperature is lower than about 80°C at the drain location:
Another way to reduce the potential for undesirable liquid buildup is for
the conduit between the aerosol generator outlet and the furnace inlet to be
of
2o a substantially constant cross sectional area and configuration.
Preferably, the
conduit beginning with the aerosol generator outlet, passing through the
furnace
and continuing to at least the cooling unit inlet is of a substantially
constant cross
sectional area and geometry.
Another way to reduce the potential for undesirable buildup is to heat at
25 least a portion, and preferably the entire length, of the conduit between
the
aerosol generator and the inlet to the furnace. For example, the conduit could
be wrapped with a heating tape to maintain the inside walls of the conduit at
a
temperature higher than the temperature of the aerosol. The aerosol would then
tend to concentrate toward the center of the conduit due to thermophoresis.
CA 02345300 2001-03-23
WO 00/12649 PCTIUS99119582
Fewer aerosol droplets would, therefore, be likely to impinge on conduit walls
or
other surfaces making the transition to the furnace.
Another way to reduce the potential for undesirable liquid buildup is to
introduce a dry gas into the aerosol between the aerosol generator and the
furnace. Referring now to Fig. 36, one embodiment of the process is shown for
adding a dry gas 118 to the aerosol 108 before the furnace 110. Addition of
the
dry gas 118 causes vaporization of at least a part of the moisture in the
aerosol
108, and preferably substantially all of the moisture in the aerosol 108, to
form
a dried aerosol 119, which is then introduced into the furnace 110.
The dry gas 118 will most often be dry air, although in some instances it
may be desirable to use dry nitrogen gas or some other dry gas. If a
sufficient
quantity of the dry gas 118 is used, the droplets of the aerosol 108 are
substantially completely dried to beneficially form dried precursor particles
in
aerosol form for introduction into the furnace 110, where the precursor
particles
are then pyroiyzed to make a desired particulate product. Also, the use of the
dry gas 118 typically will reduce the potential for contact between droplets
of the
aerosol and the conduit wall, especially in the critical area in the vicinity
of the
inlet to the furnace 110. !n that regard, a preferred method for introducing
the
dry gas 118 into the aerosol 108 is from a radial direction into the aerosol
108.
2o For example, equipment of substantially the same design as the quench
cooler,
described previously with reference to Figs. 29-31, could be used, with the
aerosol 108 flowing through the interior flow path of the apparatus and the
dry
gas 118 being introduced through perforated wall of the perforated conduit. An
alternative to using the dry gas 118 to dry the aerosol 108 would be to use a
low
25 temperature thermal preheater/dryer prior to the furnace 110 to dry the
aerosol
108 prior to introduction into the furnace 110. This alternative is not,
however,
preferred.
Still another way to reduce the potential for losses due to liquid
accumulation is to operate the process with equipment configurations such that
3o the aerosol stream flaws in a vertical direction from the aerosol generator
to and
46
CA 02345300 2001-03-23
WO 00112649 PCT/US99/19582
through the furnace. For smaller-size particles, those smaller than about 1.5
,um,
this vertical flow should, preferably, be vertically upward. For larger-size
particles, such as those larger than about 1.5 ,um, the vertical flow is
preferably
vertically downward.
Furthermore, with the process of the present invention, the potential for
system losses is signifcantiy reduced because the total system retention time
from the outlet of the generator until collection of the particles is
typically shorter
than about 10 seconds, preferably shorter than about 7 seconds, more
preferably shorter than about 5 seconds and most preferably shorter than about
0 3 seconds.
Many phosphors can be difficult to produce using conventional methods
such that the powders have the desirable physical, chemical and luminescent
characteristics. Many phosphor compounds can be difficult to produce even
using a standard spray pyrofysis technique.
s These compounds can advantageously be produced according to the
present invention using a process referred to as spray-conversion. Spray-
conversion is a process wherein a spray pyrolysis technique, as is described
above, is used to produce an intermediate particulate product that is capable
of
being subsequently converted to a particulate phosphor having the desirable
2o properties. The intermediate product advantageously has many of the
desirable
morphological properties discussed hereinbelow, such as a srnali particle size
and a narrow particle size distribution. Further, the intermediate particles
individually include the chemical components needed to form the phosphor
compound. This is distinguished from individual particles of different
2s composition that are co-mixed. The intermediate precursor particles of the
present invention can advantageously be converted to the corresponding
phosphor at a reduced temperature with reduced agglomeration.
As is discussed above, precursor materials including water-soluble
precursors, such as nitrate salts and insoluble precursors such as colloidal
silica,
3o are placed into a liquid solution, atomized and are converted at a
relatively low
47
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
temperature, such as less than about 1000°C, to intermediate precursor
particles
that typically include low crystallinity oxide phase(s). The intermediate
precursor
particles have a small size and, preferably, a narrow particle size
distribution, as
is described in more detail below. The intermediate precursor particles are
then
s converted by further treatment, such as by heat treating at an elevated
temperature, to form a phosphor compound having high crystallinity and good
luminescence characteristics. The resulting powder advantageously does not
require any further milling to reduce particle size or reduce hard
agglomerates
since the intermediate particles have the desired size and hard agglomeration
is avoided during the subsequent heat treatment. The resulting end product is
a highly crystalline phosphor powder having the desirable morphological and
luminescent properties. The average particle size and morphological
characteristics are determined by the characteristics of the intermediate
product.
Thus, the precursors can preferably be spray-converted at a temperature
15 Of at least about 700°C, such as from about 750°C to
950°C, to form a
homogeneous admixture, typically having low crystallinity. The intermediate
particles can then be heat treated at a temperature of, for example,
1100°C to
1600°C, to form phosphor particles having high crystallinity and good
luminescent properties.
2o The present invention is applicable to a wide variety of phosphors,
including cathodoluminescent (CL), electroluminescent (EL), phototuminescent
(PL) and x-ray (XR) phosphors. Phosphors typically include a matrix compound,
referred to as a host material, and the phosphor also includes one or more
dopants, referred to as activator ions, to emit a specific color or to enhance
the
25 luminescence characteristics. Examples of host materials to which the
present
invention is applicable include yttrium oxides, yttrium oxysutfides,
gadolinium
oxysulfides, sulfides such as zinc sulfide, calcium sulfide and strontium
sulfide,
silicates such as zinc silicate and yttrium silicate, thiogailates such as
strontium
thiogallate and calcium thiogallate, gallates such as zinc gallate, calcium
gallate
30 and strontium gallate, aluminates such as barium aluminate or barium
48
CA 02345300 2001-03-23
WO 00/12649 PCT/US99l19582
magnesium aluminate (BAM) and borates such as yttrium-gadolinium borate.
Specific examples of phosphor compounds according to the present invention
are listed in Tabie I.
49
CA 02345300 2001-03-23
WO 00/12649 PCT/US99119582
Table I Examples of Phosphor Compounds
HOST MATERIAL ACTIVATOR ION (S) EXCITATION MECHANISM
Y2O3 Eu, rare earths, CL, PL
Tb
Y202S Eu, Tb CL
ZnS Au, AI, Cu, Ag, CI, CL, EL
Mn
SrGa2S4 Eu, Ce CL, EL
Y3AI50,2 Tb, Cr CL
Y3(Ga,AI)50,2 Tb, Cr CL
Zn2Si04 Mn CL, PL
Y2Si05 Tb, Ce CL
CaS Eu, Ce EL
SrS Eu, Ce EL
CaGa2S4 Eu, Ce EL
ZnGa20a Mn, Cr EL
~5 CaGa204 Eu, Ce EL
SrGa204 Eu, Ce EL
Ga203 Dy, Eu EL
Ca3Ga206 Eu EL
Zn2Ge04 Mn EL
2o Zn2(Ge,Si)04 Mn EL
(Y,Gd)B03 Eu PL
BaMgAI,o0,7 Mn, Eu PL
BaAIXOy Mn PL
Gd202S Tb XR
25 (Y,Gd)2Si05 Tb XR
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
According to the present invention, the liquid feed preferably includes the
chemical components that will form the phosphor particles. For example, the
liquid feed can include a solution of metal salts such as nitrates, chlorides,
sulfates, hydroxides or oxalates of the phosphor components. In addition, the
liquid feed can include particulate precursors such as Si02 or AI2O3.
Particulate
Si02 is a preferred precursor for the silicate compounds, and it may be
advantageous to provide an excess of silica to obtain phosphor powders having
a highly crystalline structure.
A preferred precursor are . the nitrates, such as yttrium nitrate,
~o Y(N03)3A6H20, for the production of yttrium oxide phosphor particles.
Nitrates
are typically highly soluble in water and the solutions maintain a low
viscosity,
even at high concentrations. A typical reaction mechanism for the formation of
yttrium oxide would be:
2Y{N03)3 + H20 + heat ________> Y203 + NOX + H20
It has been found that the precursor solution for the production of yttrium
oxide
should preferably include from about 4 to about 6 weight percent of the
precursors.
Similarly, yttrium oxysulfides can advantageously be produced from a
similar precursor system wherein the intermediate precursor particles form low
2o crystaliinity oxides which are subsequently heated in a sulfide-containing
atmosphere, such as in H2S.
Preferred precursors for the production of silicates, such as Zn2Si04:Mn,
include metals salts, particularly nitrate salts, for the Zn and the Mn. For
the
silica component; it is preferred to use dispersed particulate silica. It has
also
been found that for the production of zinc silicate that the precursor
solution
should include an excess of silica. It has been found that, for example, a
preferred precursor solution includes an excess of about 10 atomic percent
silica. Yttrium silicate can be produced in a similar fashion wherein yttrium
nitrate is substituted for zinc nitrate.
51
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
Zinc sulfide can be produced from a precursor solution including thiourea
and zinc nitrate. Y5{Ga,AI)50,2 can advantageously be produced from a solution
comprising all metal salts or can include particulate alumina.
Thiogallates such as SrGa2S4 can be produced from salts such as nitrate
salts and pyrolyzed in air to produce intermediate precursor particles that
are
composed mainly of low crystallinity oxides such as SrGa204. The intermediate
precursor particles are then treated in a sulfur-containing atmosphere to
convert
the oxides to the sulfide phase.
Oxide based phosphors can~typicaliy be produced from simple metal
salts. For example, SrGaXOy can be produced from strontium carbonate and
gallium nitrate and CaGaXOy can be produced from calcium carbonate and
gallium nitrate.
For the production of europium doped barium magnesium aluminate
(BAM:Eu) phosphors according to the present invention, the liquid feed can
include a solution of metal salts such as nitrates, chlorides, sulfates,
hydroxides
or oxalates of the phosphor components. According to a preferred embodiment,
BAM:Eu is formed from a precursor solution comprising barium nitrate,
magnesium nitrate and fumed (particulate) alumina for the BAM host material,
as well as europium nitrate to provide the Eu dopant ion or manganese nitrate
2o to provide the Mn dopant ion. It has been found that aluminum nitrate as a
precursor to aluminum oxide is not preferred, but alumina, or a similar
aluminum
compound such as boehmite is preferred. It has also been unexpectedly found
that the precursor solution should include an amount of aluminum that is in
excess of the stoichiometric amount required to form BAM. In one preferred
embodiment, at least about 20 atomic percent excess alumina is added to the
precursor solution. Barium aluminate can be formed in a similar manner. About
5 to 10 weight percent of the precursor in the solution is preferred.
(Y,Gd)B03 can be formed from a precursor solution comprising yttrium
and gadolinium salts, such as the nitrate salts, as well as boric acid.
Preferably
so the precursor solution includes on excess of boric acid, which
advantageously
52
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
produces highly crystalline (Y,Gd}B43 powders. It is preferred to use a
precursor
concentration of from about 5 to about 10 weight percent, more preferably from
about 7 to 8 weight percent, based on the equivalent weight of the phosphor
compound.
The foregoing precursorsolutionsldispersions are preferably not saturated
with the precursor to avoid precipitate formation in the liquid. The solution
preferably includes, for example, sufficient precursor to yield from about 1
to 50
weight percent , such as from about 1 to 15 weight percent, of the phosphor
compound. That is, the solution concentrations are measured based on the
o equivalent weight percent of the phosphor product. The final particle size
of the
phosphor particles is also influenced by the precursor concentration.
Generally,
lower precursor concentrations in the liquid feed will produce particles
having a
smaller average size.
As is discussed above, the liquid feed preferably includes the dopant
~5 (activator ion} precursor, typically a soluble metal salt such as a
nitrate: The
relative concentrations of the precursors can be adjusted to vary the
concentration of the activator ion in the host material.
Preferably, the solvent is aqueous-based for ease of operation, although
other solvents, such as toluene, may be desirable. However, the use of organic
20 solvents can lead to undesirable carbon contamination in the phosphor
particles.
Further, the pH of the aqueous-based solutions can advantageously be adjusted
to alter the solubility characteristics of the precursors in the solution.
In addition to the foregoing, the liquid feed may also include other
additives that contribute to the formation of the particles. For example, a
fluxing
25 agent can be added to the solution to increase the crystallinity and/or
density of
the particles. The addition of urea to metal salt solutions, such as a metal
nitrate, can increase the density of particles produced from the solution. In
one
embodiment, up to about 1 mole equivalent urea is added to the precursor
solution, as measured against the moles of phosphor compound in the metal salt
3o solution. Small amounts, e.g. less than 1 weight percent, of boric acid
added to
53
CA 02345300 2001-03-23
WO 00/12649 PCT/US99I19582
a precursor solution can also enhance crystallinity without substantially
altering
the composition of the powder. Further, if the particles are to be coated
phosphor particles, as is discussed in more detail below, a soluble precursor
to
both the phosphor compound and the coating can be used in the precursor
solution wherein the coating precursor is an involatile or volatile species.
For producing the phosphor particles of the present invention, the carrier
gas may comprise any gaseous medium in which droplets produced from the
liquid feed may be dispersed in aerosol form. Also, the carrier gas may be
inert,
in that the carrier gas does not participate in formation of the phosphor
particles.
Alternatively, the carrier gas may have one or more active components) that
contribute to formation of the phosphor particles. In that regard, the carrier
gas
may include one or mare reactive components that react in the furnace to
contribute to formation of the phosphor particles. In many applications for
the
spray-conversion of phosphor particles according to the present invention, air
will
be a satisfactory carrier gas for providing oxygen. In other instances, a
relatively
inert gas such as nitrogen may be required.
When the phosphors of the present invention are coated phosphors, such
as phosphors coated with a metal oxide, precursors to metal oxide coatings can
be selected from volatile metal acetates, chlorides, alkoxides or halides.
Such
2o precursors are known to react at high temperatures to form the
corresponding
metal oxides and eliminate supporting ligands or ions. For example, SiCl4 can
be used as a precursor to Si02 coatings when water vapor is present:
SiCl4 c9> + 2HZO~s? -______> Si02cs? + 4 HCI~s~
SiCl4 also is highly volatile and is a liquid at room temperature, which makes
transport into the reactor more controllable. Aluminum trichloride (AIC13) can
be
used in a similar manner to form an alumina coating.
Metal alkoxides can be used to produce metal oxide films by hydrolysis.
The water molecules react with the alkoxide M-O bond resulting in clean
elimination of the corresponding alcohol with the formation of M-O-M bonds:
Si(OEt)4 + 2H20 --------> Si02 + 4EtOH
54
CA 02345300 2001-03-23
WO OO/I2649 PCT/US99/I9582
Most metal alkoxides have a reasonably high vapor pressure and are therefore
well suited as coating precursors.
Metal acetates are also useful as coating precursors since they readily
decompose upon thermal activation by acetic anhydride elimination:
Mg(02CCH3)z ----------> Mg0 + CH3C(O,}OC(O)CH3
Metal acetates are advantageous as coating precursors since they are water
stable and are reasonably inexpensive.
Coatings can be generated on the particle surface by a number of
difFerent mechanisms. One or more precursors can vaporize and fuse to the hot
~ o phosphor particle surface and thermally react resulting in the formation
of a thin-
film coating by chemical vapor deposition (CVD). Preferred coatings deposited
by CVD include metal oxides and elemental metals. Further, the coating can be
formed by physical vapor deposition (PVD) wherein a coating material
physically
deposits an the surface of the particles. Preferred coatings deposited by PVD
5 include organic materials and elemental metals. Alternatively, the gaseous
precursor can react in the gas phase forming small particles, for example less
than about 5 nanometers in size, which then diffuse to the larger particle
surface
and sinter onto the surface, thus forming a coating. This method is referred
to
as gas-to-particle conversion (GPC). Whether such coating reactions occur by
2o CVD, PVD or GPC is dependent on the reactor conditions such as precursor
partial pressure, water partial pressure and the concentration of particles in
the
gas stream. Another possible surface coating method is surface conversion of
the surface of the particle by reaction with a vapor phase reactant to convert
the
surface of the particles to a different material than that originally
contained in the
2s particles.
In addition, a volatile coating material such as PbO, Mo03 or V205 can be
introduced into the reactor such that the coating deposits on the particle by
condensation. Highly volatile metals, such as silver, can also be deposited by
condensation. Further, the phosphor powders can be coated using other
3o techniques. For example, a soluble precursor to both the phosphor powder
and
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
the coating can be used in the precursor solution wherein the coating
precursor
is involatile (e.g. AI(N03)3) or volatile (e.g. Sn(OAc)a where OAc is
acetate). In
another method, a colloidal precursor and a soluble phosphor precursor can be
used to form a particulate colloidal coating on the phosphor.
s The phosphor powders produced by the foregoing method may be fully
converted to the crystalline phosphor compound during the pyrolization step.
However, it is preferred that the powders are spray-converted to an
intermediate
form. It is then necessary to heat the spray-converted intermediate precursor
particles to convert the intermediate precursor powder to a luminescent
phosphor compound and to increase the crystaliinity (average crystallite size)
of
the powder. Thus, the powders can be heat-treated for an amount of time and
in a preselected environment, as is discussed above. Increased crystallinity
can
advantageously yield an increased brightness and efficiency of the phosphor
particles. If such heat treating steps are performed, the heat treating
~s temperature and time should be selected to minimize the amount of
interparticle
sintering. Table II illustrates examples of preferred phosphor powders
according
to the present invention with the preferred conversion and heat treatment
conditions.
56
CA 02345300 2001-03-23
WO 00/12649 PCT/US99119582
TABLE II Examples of Spray-Converted Phosphor Compounds
Host MaterialConversion Carrier Heat Heat
Temperature Gas Treatment Treatment
(pyroiization) Temperature Gas
Y2p3 850-1000 Air 1350-1500 Air
C C
Zn2Si04 850-1000 Air 1100-1300 Air
C C
ZnS 300-900 N2 500-900 C H2SIN2
C
SrGa2S4 700-900 Air 800-1100 C H2S/N2
C
SrGaxOy 700-900 Air 1100-1450 Air
C C
CaGaXOy 700-900 Air 1100-1450 Air
C C
BaMgAl,oO" 750-950 Air 1200-1700 HZIN2
C C
(Y,Gd)B03 900-950 Air 1100-1400 Air
C C
BaAIXOy 750-950 Air 1200-1700 Air
C C
(Y,Gd)2Si05 900-950 Air 1100-1200 Air
C C
The heat treatment time is preferably not more than about 2 hours and
can be as little as about 1 minute. To reduce agglomeration, the intermediate
particles are preferably heat treated under sufficient agitation to minimize
the
agglomeration of the particles. One preferred method for agitating during heat
treatment is to heat treat the powders in a rotary kiln, wherein the powders
are
constantly moving through a tubular furnace that is rotating on its major
axis.
Further, the crystallinity of the phosphors can be increased by using a
2o fluxing agent, either in the precursar solution or in a post-formation
annealing
step. A fluxing agent is a reagent, which improves the crystallinity of the
material
when the reagent and the material are heated together, as compared to heating
the material to the same temperature and for the same amount of time in the
absence of the fluxing agent. The f#uxing agents typically cause a eutectic to
25 form which leads to a liquid phase at the grain boundaries, increasing the
diffusion coefficient. The fluxing agent, for example alkali metal halides
such as
NaCI or KCI or an organic compound such as urea (CO(NH2)2), can be added
57
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
to the precursor solution where it improves the crystallinity andlor density
of the
particles during their subsequent formation. Alternatively, the fluxing agent
can
be contacted with the phosphor powder batches after they have been collected.
Upon heat treatment, the fluxing agent improves the crystallinity of the
phosphor
powder, and therefore improves other properties such as the brightness of the
phosphor powder. Also, in the case of composite particles, the particles may
be
annealed for a sufficient time to permit redistribution within the particles
of
different material phases.
The phosphor host material can be doped with an activator ion, typically
o in an amount of up to about 30 atomic percent, such as from about 0.02 to
about
20 atomic percent. The preferred concentration of the activator ian will vary
depending on the composition and the application of the phosphor, as is
discussed in more detail below. The activator ion should also be in the proper
oxidation state.
One advantage of the present invention is that the activator ion is
homogeneously distributed throughout the host material. Phosphor powders
prepared by solid-state methods do not give uniform concentration of the
activator ion, particularly in small particles, and solution routes also do
not give
homogenous distribution of the activator ian due to different rates of
2o precipitation.
The powder characteristics that are preferred will depend upon the
particular application of the phosphor powders. Nonetheless, it can be
generally
stated for most applications that the powders should have one or more of:
small
particle size; narrow size distribution; spherical morphology; high
crystalfinity;
2s controlled porosity and homogenous dopant distribution of activator ion
throughout the host material. The efficiency of the phosphor, defined as the
overall conversion of excitation energy to visible photons, should be high.
According to the present invention, the phosphor powder includes
particles having a small average particle size. Although the preferred average
so size of the phosphor particles will vary according to the application of
the
58
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
phosphor powder, the average particle size of the phosphor particles is at
least
about 0.1 ~m and more preferably is at least about 0.3 um. Further, the
average
particle size is not greater than about 20 gym. For most applications, the
average
particle size is preferably not greater than about 10 gym, such as not greater
than
about 5 ,um and more preferably is not greater than about 3 ,um, such as from
about 0.3 ,um to about 3 ,um. As used herein, the average particle size is the
volume average particle size.
According to the present invention, the powder batch of phosphor
particles can also have a narrow particle size distribution, such that the
majority
14 of particles are substantially the same size. Preferably, at least about 70
volume
percent of the particles and more preferably at (east about 80 volume percent
of
the particles are not larger than twice the average particle size. Thus, when
the
average particle size is about 2 ~cm, it is preferred that at least about 70
volume
percent of the particles are not larger than 4 um and it is more preferred
that at
least about 80 volume percent of the particles are not larger than 4 gym.
Further,
it is preferred that at least about 70 volume percent of the particles, and
more
preferably at least about 80 volume percent of the particles, are not larger
than
about 1.5 times the average particle size. Thus, when the average particle
size
is about 2 ~cm, it is preferred that at least about 70 volume percent of the
2o particles are not larger than about 3 ~cm and it is more preferred that at
least
about 80 volume percent of the particles are not larger than about 3 gym.
Powders produced by the processes described herein, particularly those
that have experienced a post treatment step, generally exit as soft
agglomerates
of primary spherical particles. It is well known to those in the art that
micrometer-
sized particles often form soft agglomerates as a result of their relatively
high
surface energy, as compared to larger particles. It is also known to those
skilled
in the art that such soft agglomerates may be dispersed easily by treatments
such as exposure to ultrasound in a liquid medium or sieving. The average
particle size and particle size distributions described herein are measured by
3o mixing samples of the powders in a medium such as water with a surfactant
and
59
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
a short exposure to ultrasound through either an ultrasonic bath ar horn. The
ultrasonic treatment supplies sufficient energy to disperse the soft
agglomerates
into primary spherical particles. The primary particle size and size
distribution
is then measured by light scattering in a Microtrac instrument. This provides
a
good measure of the useful dispersion characteristics of the powder because
this simulates the dispersion of the particles in a liquid medium such as a
paste
or slurry that is used to deposit the particles in a device, such as an
cathodoluminescent display devices. Thus, the references to particle size
herein refer to the primary particle size, such as after lightly dispersing
the soft
o agglomerates of particles. .
Further, it is advantageous according to the present invention that the
foregoing description of the average size and size distribution of the
phosphor
particles also applies to the intermediate precursor particles that are
produced
during the pyrolization step. That is, the size and size distribution of the
particles
15 changes very little, if at all, during the heat treatment step after
pyrolization. The
morphological properties of the final phosphor powder are substantially
controlled by the properties of the intermediate precursor particles.
The phosphor particles of the present invention are comprised of a
number of crystallites. According to the present invention, the phosphor
particles
2o are highly crystalline and it is preferred that the average crystallite
size
approaches the average particle size such that the particles are composed of
only a few large crystals. The average crystallite size of the particles is
preferably at least about 25 manometers, more preferably is at least about 40
manometers, even more preferably is at least about fi0 manometers and most
25 preferably is at least about 80 manometers. In one embodiment, the average
crystallite size is at least about 100 manometers. As it relates to particle
size, the
average crystallite size is preferably at least about 10 percent, more
preferably
at feast about 20 percent and most preferably is at least about 30 percent of
the
average particle size. Such highly crystalline phosphors are believed to have
CA 02345300 2001-03-23
WO 00/12649 PCT/US99119582
increased luminescent efficiency and brightness as compared to phosphor
particles having smaller crystallites.
The phosphor particles of the present invention advantageously have a
high degree of purity, that is, a low level of impurities. Impurities are
those
materials that are not intended in the final product and that negatively
affect the
properties of the phosphor. Thus, an activator ion is not considered an
impurity.
The level of impurities in the phosphor powders of the present invention is
preferably not greater than about 1 atomic percent, more preferably is not
greater than about 0.1 atomic percent and even more preferably is not greater
than about 0.01 atomic percent. It will be appreciated that some of the
phosphor
particles according to the present invention will include a second-phase that
does not hinder the properties of the phosphor. Far example, the Zn2Si04
phosphor particles of the present invention are advantageously produced using
a slight excess of silica, and therefore the particle wilt include silica
crystallites
dispersed throughout a Zn2Si04 matrix. Such a second phase is not considered
an impurity.
The formation of hollow particles is common in spray pyrolysis and can
occur in spray conversion. Hollow phosphor particles may be detrimental in a
number of applications of phosphor powders. In the present invention, it has
2o been found that the formation of hollow or dense particles can be
controlled
through a combination of the control over pyroiysis temperature, residence
time,
precursor selection and solution concentration. For example, with constant
pyrolysis temperature and residence time, the morphology of some powders
show the presence of progressively more hollow particles as the solution
concentration is raised above 5 weight percent.
The phosphor particles of the present invention are also substantially
spherical in shape. That is, the particles are not jagged or irregular in
shape.
Spherical particles are particularly advantageous because they are able to
disperse and coat a device more uniformly with a reduced average thickness.
3o Although the particles are substantially spherical, the particles may
become
61
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
faceted as the crystallite size increases while maintaining a substantially
spherical morphology.
In addition, the phosphor particles according to the present invention
advantageously have a low surface area. The particles are substantially
spherical, which reduces the total surface area for a given mass of powder.
Further, the elimination of larger particles from the powder batches
eliminates
the porosity that is associated with open pores on the surface of such larger
particles. Due to the elimination of the large particles, the powder
advantageously has a lower surface area. Surface area is typically measured
using a BET nitrogen adsorption method, which is indicative of the surface
area
of the powder, including the surface area of accessible surface pores on the
surface of the powder. For a given particle size distribution, a lower value
of a
surface area per unit mass of powder indicates solid or non-porous particles.
Decreased surface area reduces the susceptibility of the phosphor powders to
~5 adverse surface reactions, such as degradation from moisture. This
characteristic can advantageously extend the useful life of the phosphor
powders.
The surfaces of the phosphor particles according to the present invention
are typically smooth and clean with a minimal deposition of contaminants on
the
2o particle surface. For example, the outer surfaces are not contaminated with
surfactants, as is often the case with particles produced by liquid
precipitation
routes. Since the particles do not require milling, the particle surfaces do
not
include major surface defects that typically result from milling and can
decrease
the brightness of the powders.
25 In addition, the powder batches of phosphor particles according to the
present invention are substantially unagglomerated, that is, they include
substantially no hard agglomerates of particles. Hard agglomerates are
physically coalesced lumps of two or more particles that behave as one large
particle. Hard agglomerates are disadvantageous in most applications of
3o phosphor powders. !t is preferred that no more than about 1 weight percent
of
62
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
the phosphor particles in the powder batch of the present invention are in the
farm of hard agglomerates. More preferably, no more than about 0.5 weight
percent of the particles are in the form of hard agglomerates and even more
preferably no more than about 0.'f weight percent of the particles are in the
form
s of hard agglomerates. In the event that hard agglomerates of the powder do
form, they can optionally be broken up, such as by jet-milling the powder.
According to one embodiment of the present invention, the phosphor
particles are composite phosphor particles, wherein the individual particles
include at least a first phosphor phase and at least a second phase associated
~o with the phosphor phase. Preferably, the second phase is dispersed
throughout
the matrix of the first phosphor phase. The second phase can be a different
phosphor compound or can be a non-phosphor compound. Such composites
can advantageously permit the use of phosphor compounds in devices that
would otherwise be unusable. Further, combinations of different phosphor
~5 compounds within one particle can produce emission of a selected color. The
emission of the two phosphor compounds could combine to approximate white
light. Other advantages can also be realized. For example, in
cathodoluminescent applications, the matrix material can accelerate the
impingent electrons to enhance the luminescence of the particles.
2o According to another embodiment of the present invention, the phosphor
particles are surface modified or coated phosphor particles that include a
particulate coating {Fig. 35d) or non-particulate (film) coating {Fig. 35a)
that
substantially encapsulates an outer surface of the particles. The coating can
be
a conductive metallic material, including a metal, a non-metallic compound
2~ including a semiconductor or an organic compound.
Coatings are often desirable to reduce degradation of the phosphor
powder due to moisture or other influences. The thin, uniform coatings
according to the present invention will advantageously permit use of the
phosphor powders under corrosive conditions. Coatings also create a diffusion
3o barrier such that activator ions cannot transfer from one particle to
another,
63
CA 02345300 2001-03-23
WO 00/12649 PCTIUS99/19582
thereby altering the luminescence characteristics. Coatings can also control
the
surface energy levels of the particles.
The coating can be a metal, metal oxide or other inorganic compound
such as a metal sulfide, or can be an organic compound. For example, a metal
oxide coating can advantageously be used, such as a metal oxide selected from
the group consisting of Si02, MgO, AI ~J 3 ZnO, Sn0 2 or In ~J 3 Particularly
preferred are Si02 and AI203 coatings. Semiconductive oxide coatings such as
Sn02 or In203 can be advantageous in some applications. In addition,
phosphate coatings, such as zirconium phosphate or aluminum phosphate, can
also be advantageous for use in some applications.
The coatings should be relatively thin and uniform. The coating should
encapsulate the entire particle, but be sufficiently thin such that the
coating does
not substantially interfere with light transmission. Preferably, the coating
has an
average thickness of not greater than about 200 manometers, more preferably
~5 not greater than about 100 manometers, and even more preferably not greater
than about 50 manometers. The coating preferably completely encapsulates the
phosphor particle and therefore should have an average thickness of at least
about 2 manometers, more preferably at feast about 5 manometers. In one
embodiment, the coating has an average thickness of from about 2 to 50
2o manometers, such as from about 2 to 10 manometers. Further, the particles
can
include more than one coating substantially encapsulating the particles to
achieve the desired properties.
The coating, either particulate or non-particulate, can also include a
pigment or other material that alters the fight characteristics of the
phosphor.
2s Red pigments can include compounds such as the iron oxides (Fe203), cadmium
sulfide compounds (CdS) or mercury sulfide compounds (HgS). Green or blue
pigments include cobalt oxide (Co0), cobalt aluminate (CoAl2C?4) or zinc oxide
(Zn0). Pigment coatings are capable of absorbing selected wavelengths of light
exiting the phosphor, thereby acting as a filter to improve the color contrast
and
so purity. Further, a dielectric coating, either organic or inorganic, can be
used to
64
CA 02345300 2001-03-23
WO 00112649 PCT/US99/19582
achieve the appropriate surface charge characteristics to carry out deposition
processes such as electrostatic deposition.
In addition, the phosphor particles can be coated with an organic
compound such as PMMA (polymethylmethacrylate), polystyrene or similar
s organic compounds, including surfactants that aid in the dispersion and/or
suspension of the particles in a flowable medium. The organic coating is
preferably not greater than about 100 nanometers thick and is substantially
dense and continuous about particle. The organic coatings can advantageously
prevent corrosion of the phosphor particles and also can improve the
dispersion
o characteristics of the particles in a paste or other flowable medium.
The coating can also be comprised of one or more monolayer coatings,
such as from about 1 to 3 monolayer coatings. A monoiayer coating is formed
by the reaction of an organic or an inorganic molecule with the surface of the
phosphor particles to form a coating layer that is essentially one molecular
layer
s thick. En particular, the formation of a monolayer coating by reaction of
the
surface of the phosphor powder with a functionalized organo silane such as
halo- or amino-silanes, for example hexamethyldisilazane or trimethyl-
silylchloride, can be used to modify and control the hydrophobicity and
hydrophiiicity of the phosphor powders. Monolayer coatings of metal oxides
20 (e.g. Zn0 or Si02) or metal sulfides (e.g. Cu2S) can be formed as monolayer
coatings. Monolayer coatings can allow for greater control over the dispersion
characteristics of the phosphor powder in a wide variety of paste compositions
and other flowable mediums.
The monolayer coatings may also be applied to phosphor powders that
25 have already been coated with an organic or inorganic coating, thus
providing
better control over the corrosion characteristics {through the use of a
thicker
coating) as well as dispersibility (through the use of a monolayer coating) of
the
phosphor powder.
CA 02345300 2001-03-23
WO OOlt2b49 PCT/US99/19582
As a direct result of the foregoing powder characteristics, the phosphor
powders of the present invention have many unique and advantageous
properties that are not found in phosphor powders known heretofore.
The phosphor powders of the present invention have a high efficiency,
sometimes referred to as quantum efficiency. Efficiency is the overall
conversion
of excitation energy to visible photons emitted. The high efficiency of the
phosphor powders according to the present invention is believed to be due to
the
high crystallinity and homogenous distribution of activator ion in the host
material
as well as a substantially defect-free particle surface.
o The phosphor powders also have well-controlled color characteristics,
sometimes referred to as emission spectrum characteristics or chromaticity.
This
important property is due to the ability to precisely control the composition
of the
host material, the homogenous distribution of the activator ion and the high
purity
of the powders.
s The phosphor powders also have improved decay time, also referred to
as persistence. Persistence is the amount of time for the light emission to
decay
to 70% of its peak brightness. In display applications, phosphors with long
decay times can result in blurred images when the image moves across the
display. The improved (reduced) decay time of the phosphor powders of the
2o present invention is believed to be primarily due to the homogenous
distribution
of activator ion in the host material.
The phosphor powders also can have an improved brightness over prior
art phosphor powders. That is, under a given application of energy (electrons,
photons, electric field or x-rays), the phosphor powders of the present
invention
25 produce more light.
Thus, the phosphor powders of the present invention have a unique
combination of properties that are not found in conventional phosphor powders.
The powders can advantageously be used to form a number of intermediate
products, for example liquid mediums such as pastes or slurries, and can be
3o incorporated into a number of devices, wherein the devices will have
significantly
66
CA 02345300 2001-03-23
WO 00/12b49 PCT/US99/19582
improved performance resulting directly from the characteristics of the
phosphor
powders of the present invention.
Phosphor powders are typically deposited onto device surfaces or
substrates by a number of different deposition methods which involve the
direct
deposition of the dry powder such as dusting, electrophotographic or
electrostatic precipitation, while other deposition methods involve liquid
vehicles
such as ink jet printing, liquid delivery from a syringe, micro-pens, toner,
slurry
deposition, paste-based methods and electrophoresis. In all these deposition
methods, the powders described in .the present invention show a number of
o distinct advantages over the phosphor powders produced by other methods. For
example, small, spherical, narrow size distribution phosphor particles are
more
easily dispersed in liquid vehicles, they remain dispersed for a longer period
and
allow printing of smoother and finer features compared to powder made by
alternative methods.
~5 For many applications, phosphor powders are often dispersed within a
paste, which is then applied, to a surface to obtain a phosphorescent layer.
The
powders of the present invention offer many advantages when dispersed in such
a paste. For example, the powders will disperse better than non-spherical
powders of wide size distribution and can therefore produce thinner and more
2o uniform layers with a reduced lump count. Such a thick flm paste will
produce
a brighter display due to the increased powder density in the phosphor layer.
The number of processing steps can also be advantageously reduced.
One preferred class of intermediate products according to the present
invention are thick film paste compositions, also referred to as thick film
inks.
25 These pastes are particularly useful for applying the phosphor particles
onto a
substrate, such as for use in display devices.
In the thick film process, a viscous paste that includes a functional
particulate phase, such as phosphor powder, is screen printed onto a
substrate.
A porous screen fabricated from stainless steel, polyester, nylon or similar
inert
3o material is stretched and attached to a rigid frame. A predetermined
pattern is
67
CA 02345300 2001-03-23
WO OOI12649 PCT/US99/19582
formed on the screen corresponding to the pattern to be printed. For example,
a UV sensitive emulsion can be applied to the screen and exposed through a
positive or negative image of the design pattern. The screen is then developed
to remove portions of the emulsion in the pattern regions.
The screen is then affrxed to a printing device and the thick film paste is
deposited on top of the screen. The substrate to be printed is then positioned
beneath the screen and the paste is forced through the screen and onto the
substrate by a squeegee that traverses the screen. Thus, a pattern of traces
andlor pads of the paste material is transferred to the substrate. The
substrate
with the paste applied in a predetermined pattern is then subjected to a
drying
and heating treatment to adhere the functional phase to the substrate. For
increased line definition, the applied paste can be further treated, such as
through a photolithographic process, to develop and remove unwanted material
from the substrate.
~5 Thick film pastes have a complex chemistry and generally include a
functional phase, a binder phase and an organic vehicle phase. The functional
phase can include the phosphor powders of the present invention which provide
a luminescent layer on a substrate. The particle size, size distribution,
surface
chemistry and particle shape of the particles all influence the theology of
the
20 paste.
The binder phase is typically a mixture of inorganic binders such as metal
oxide or glass frit powders. For example, Pb0 based glasses are commonly
used as binders. The function of the binder phase is to control the sintering
of
the film and assist the adhesion of the functional phase to the substrate
andlor
25 assist in the sintering of the functional phase. Reactive compounds can
also be
included in the paste to promote adherence of the functional phase to the
substrate.
Thick film pastes also include an organic vehicle phase that is a mixture
of solvents, polymers, resins or other organics whose primary function is to
3o provide the appropriate theology (flaw properties) to the paste. The liquid
68
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
solvent assists in mixing of the components into a homogenous paste and
substantially evaporates upon application of the paste to the substrate.
Usually
the solvent is a volatile liquid such as methanol, ethanol, terpineol, butyl
carbitol,
butyl carbitol acetate, aliphatic alcohols, esters, acetone and the like. The
other
organic vehicle components can include thickeners (sometimes referred to as
organic binders), stabilizing agents, surfactants, wetting agents and the
like.
Thickeners provide sufficient viscosity to the paste and also acts as a
binding
agent in the unfired state. Examples of thickeners include ethyl cellulose,
polyvinyl acetate, resins such as ~ acrylic resin, cellulose resin, polyester,
o poiyamide and the like. The stabilizing agents reduce oxidation and
degradation, stabilize the viscosity or buffer the pH of the paste. For
example,
triethanolamine is a common stabilizer. Wetting agents and surfactants are
well
known in the thick film paste art and can include triethanolamine and
phosphate
esters.
~5 The different components of the thick film paste are mixed in the desired
proportions in order to produce a substantially homogenous blend wherein the
functional phase is well dispersed throughout the paste. The powder is often
dispersed in the paste and then repeatedly passed through a roll-mill to mix
the
paste. The roil mill can advantageously break -up soft agglomerates of powders
2o in the paste. Typically, the thick film paste will include from about 5 to
about 95
weight percent, such as from about 60 to 80 weight percent, of the functional
phase, including the phosphor powders of the present invention.
Phosphor paste compositions are disclosed in: U.S. Patent No.
4,724,761; U.S. Patent No. 4,806,389; and U.S. Patent No. 4,902,567, each of
25 which are incorporated herein by reference in their entirety. Generally,
phosphors are deaggregated and are combined with organic additives to form
the paste.
Some applications of thick film pastes require higher tolerances than can
be achieved using standard thick-film technology, as is described above. As a
3o result, some thick film pastes have photo-imaging capability to enable the
69
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/I9582
formation of lines and traces with decreased width and pitch. In this type of
process, a photoactive thick film paste is applied to a substrate
substantially as
is described above. The paste can include, for example, a liquid vehicle such
as polyvinyl alcohol that is not cross-linked. The paste is then dried and
exposed to ultraviolet light through a photomask to polymerize the exposed
portions of paste and the paste is developed to remove unwanted portions of
the
paste. This technology permits higher density lines and pixels to be formed.
The combination of the foregoing technology with the phosphor powders of the
present invention permits the fabrication of devices with resolution and
tolerances as compared to conventional technologies using conventional
phosphor powders.
In addition, a laser can be used instead of ultraviolet fight through a mask.
The laser can be scanned over the surface in a pattern thereby replacing the
need for a mask. The laser light is of sufficiently low intensity that it does
not
~5 heat the glass or polymer above its softening point. The unirradiated
regions of
the paste can be removed leaving a pattern.
Likewise, conventional paste technology utilizes heating of a substrate to
remove the vehicle from a paste and to fuse particles together or modify them
in some other way. A laser can be used to locally heat the paste layer and can
zo be scanned over the paste layer thereby forming a pattern. The laser
heating
is confrned to the paste layer and drives out the paste vehicle and heats the
powder in the paste without appreciably heating the substrate. This allows
heating of particles, delivered using pastes, without damaging a glass or even
polymeric substrate.
25 Other deposition methods far the phosphor powders can also be used.
For example, a slurry method can be used to deposit the powder. The powder
is typically dispersed in an aqueous slurry including reagents such as
potassium
silicate and polyvinyl alcohol, which aids in the adhesion of the powder to
the
surface. For example, the slurry can be poured onto the substrate and left to
3o settle to the surface. After the phosphor powder has sedimented onto the
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
substrate, the supernatant liquid is decanted off and the phosphor powder
layer
is left to dry.
Phosphor particles can also be deposited electrophoretically or
electrostaticaily. The particles are charged and are brought into contact with
the
substrate surface having localized portions of opposite charge. The layer is
typically lacquered to adhere the particles to the substrate. Shadow masks can
be used to produce the desired pattern on the substrate surface.
Ink jet printing is another method for depositing the phosphor pawders in
a predetermined pattern. The phosphor powder is dispersed in a liquid medium
o and dispensed onto a substrate using an ink jet printing head that is
computer
controlled to produce a pattern. The phosphor powders of the present invention
having a small size, narrow size distribution and spherical morphology can be
printed into a pattern having a high density and high resolution. Other
deposition
methods utilizing a phosphor powder dispersed in a liquid medium include micro-
1 s pen or syringe deposition, wherein the powders are dispersed and applied
to a
substrate using a pen or syringe and are then allowed to dry.
Patterns of phosphors can also be formed by using an ink jet or micropen
(small syringe) to dispense sticky material onto a surface in a pattern.
Powder
is then transferred to the sticky regions. This transfer can be done is
several
20 ways. A sheet covered with powder can be applied to the surface with the
sticky
pattern. The powder sticks to the sticky pattern and does not stick to the
rest of
the surface. A nozzle can also be used to transfer powder directly to the
sticky
regions.
Many methods for directly depositing materials onto surfaces require
25 heating of the particles once deposited to sinter them together and densify
the
layer. The densification can be assisted by including a molecular precursor to
a material in the liquid containing the particles. The particielmolecular
precursor
mixture can be directly written onto the surface using ink jet, micropen, and
other
liquid dispensing methods. This can be followed by heating in a furnace or
3o heating using a localized energy source such as a laser. The heating
converts
71
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
the molecular precursor into the functional material contained in the
particles
thereby filling in the space between the particles with functional material.
Thus, the phosphor powders produced according to the present invention
result in smoother phosphor powder layers when deposited by such liquid or dry
powder based deposition methods. Smoother phosphor powder layers are the
result of the smaller average particle size, spherical particle morphology and
narrower particle size distribution compared to phosphor powders produced by
other methods. Smoother phosphor powder layers are valuable in various
applications, especially those where the phosphor powders are used in a
display
~o device where a high resolution is critical. For example, a smoother
phosphor
powder layer in a display application where the phosphor layer produces light
that is photographed results in improved definition and distinction of the
photographed image.
A variety of deposition techniques often degrade the properties of the
powders, especially the powder brightness. An example is the three-roll mill
used to form pastes that are photoprinted, screen printed, directly written
with a
microsyringe and others. A method for increasing the brightness of the
phosphor particles once deposited on the surface is to irradiate them with a
laser
(Argon ion, krypton ion, YAG, excimer, etc...). The laser light increases the
2o temperature of the particles thereby annealing them and increasing the
brightness. The laser heating of the particles can be carried out for
particles on
glass or even polymeric substrates since the laser causes local heating of the
particles without heating the glass above its softening point. This approach
is
useful for the phosphor powders of the present invention.
25 The phosphor particle layer deposited onto a surface often needs to be
coated to protect the layer from plasmas, moisture, electrons, photons, etc.
Coatings can be formed by sputtering, but this method requires a mask to avoid
deposition onto undesired areas of the substrate. Laser-induced chemical vapor
deposition (LCVD) of metal oxides and other materials onto particles can allow
so localized deposition of material to coat phosphor particles without coating
other
72
CA 02345300 2001-03-23
WO 00/12649 PCT/US991I9582
areas. The laser heating of the particles that drives the CVD can be carried
out
for particles on glass or even polymeric substrates because the laser causes
local heating of the particles without heating the glass or polymer above its
softening point.
Thus, the phosphor powders of the present invention have a unique
combination of properties that are not found in conventional phosphor powders
. The powders can advantageously be incorporated into a number of devices,
wherein the devices will have significantly improved performance resulting
directly from the characteristics of the phosphor powders of the present
o invention. The devices can include light-emitting lamps and display devices
for
visually conveying information and graphics. Such display devices include
traditional CRT-based display devices, such as televisions, and also include
flat
panel displays. Flat pane! displays are relatively thin devices that present
graphics and images without the use of a traditional picture tube and operate
with modest power requirements. Generally, flat pane! displays include a
phosphor powder selectively dispersed on a viewing panel, wherein the
excitation source lies behind and in close proximity to the panel.
For display devices, it is important for the phosphor layer to be as thin and
uniform as possible with a minimal number of voids. Fig. 37 schematically
2o illustrates a lay down of large agglomerated particles in a pixel utilizing
conventional phosphor powders. The device 3700 includes a transparent
viewing screen 3702 and, in the case of an FED, a transparent electrode layer
3704. The phosphor particles 3706 are dispersed in pixels 3708. The phosphor
particles are large and agglomerated and result in a number of voids and
2s unevenness in the surface. This results in decreased brightness and
decreased
image quality.
Fig. 38 illustrates the same device fabricated utilizing powders according
to the present invention. The device 3810 includes transparent viewing screen
3812 and a transparent electrode 3814. The phosphor powders 3816 are
3o dispersed in pixels in 3818. The pixels are thinner and more uniform than
the
73
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
conventional pixel. in a preferred embodiment, the phosphor layer constituting
the pixel has an average thickness of not greater than about 3 times the
average
particle size of the powder, preferably not greater than about 2 times the
average
particle size and even more preferably not greater than about 7.5 times the
average particle size. This unique characteristic is possible due to the
unique
combination of small particle size, narrow size distribution and spherical
morphology of the phosphor particles. The device will therefore produce an
image having much higher resolution due to the ability to form smaller, more
uniform pixels and much higher brightness since light scattering is
significantly
~o reduced and the amount of light lost due to non-luminescent particles is
reduced.
Specifically, the brightness of the phosphor powder layer of the present
invention is enhanced due to the thin, dense nature of the Payer. The powder
layer is comprised of spherical particles of the appropriate particle size
distribution, preferably a bimodal distribution, resulting in a thinner,
brighter layer.
~5 CRT devices, utilizing a cathode ray tube, include traditional display
devices such as televisions and computer monitors. CRTs operate by selectively
firing electrons from one or more cathode ray tubes at cathodoluminescent
phosphor particles, which are located in predetermined regions (pixels) of a
display screen. The cathode ray tube is located at a distance from the display
2o screen, which increases as screen size increases. By selectively directing
the
electron beam at certain pixels, a full color display with high resolution can
be
achieved.
A CRT display device is illustrated schematically in Fig. 39. The device
3902 includes 3 cathode ray tubes 3904, 3906 and 3908 located in the rear
2s portion of the device. The cathode ray tubes generate electrons, such as
electron 3910. An applied voltage of 20 to 30 kV accelerates the electrons
toward the display screen 3912. In a color CRT, the display screen is
patterned
with red (R), green (G) and blue (B) phosphors, as is illustrated in Fig. 40.
Three
colored phosphor pixels are grouped in close proximity, such as group 3914, to
so produce multicolor images. Graphic output is created be selectively
directing the
74
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
electrons at the pixels on the display screen 3912 using, for example,
electromagnets 3916. The electron beams are rastered in a left to right, top
to
bottom fashion to create a moving image. The electrons can also be filtered
through an apertured metal mask to block electrons that are directed at the
wrong phosphor.
The phosphor powder is typically applied to the CRT display screen using
a slurry. The slurry is formed by suspending the phosphor particles in an
aqueous solution which can also include additives such as PVA (polyvinyl
alcohol) and other organic compounds to aid in the dispersion of the particles
in
~o the solution as well as other compounds such as metal chromates. The
display
screen is placed in a coating machine, such as a spin coater, and the slurry
is
deposited onto the inner surface of the display screen and spread over the
entire
surface. The display screen is spun to thoroughly coat the surface and spin
away any excess slurry. The slurry on the screen is then dried and exposed
~s through a shadow mask having a predetermined dot-like or stripe-like
pattern.
The exposed film is developed and excess phosphor particles are washed away
to form a phosphor screen having a predetermined pixel pattern. The process
can be performed in sequence for different color phosphors to enable a full
color
display to be produced.
2o It is generally desired that the pixels are formed with a highly uniform
phosphor powder layer thickness. The phosphors should not peel from the
display screen and no cross contamination of the colored phosphors should
occur. These characteristics are significantly influenced by the morphology,
size
and surface condition of the phosphor particles.
25 CRT devices typically employ cathodoluminescent phosphor particles
rather than thin-film phosphors due to the high luminescence requirements. The
resolution of images on powdered phosphor screens can be improved if the
screen is made with particles having a small size and uniform size
distribution,
such as the phosphor particles according to the present invention. Image
quality
so on the CRT device is also influenced by the packing voids of the particles
and
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
the number of layers of phosphor particles, which are not involved in the
generation of cathodaluminescence. That is, particles, which are not excited
by
the electron beam, will only inhibit the transmission of luminescence through
the
device. Large particles and aggregated particles bath form voids and further
contribute to loss of light transmission. Significant amounts of light can be
scattered by reflection in voids. Further, for a high quality image, the
phosphor
layer should have a thin and highly uniform thickness. Ideally, the average
thickness of the phosphor Payer should be about 1.5 times the average particle
size of the phosphor particles.
1o CRTs typically operate at high voltages such as from about 20 kV to 30
kV. Cathodoluminescent phosphors used for CRTs should have high brightness
and good chromaticity. Cathodoluminescent phosphors, which are particularly
useful in CRT devices include ZnS:Cu or AI far green, ZnS:Ag, Au or CI for
blue
and Y202S:Eu for red. The phosphor particles can advantageously be coated
in accordance with the present invention to prevent degradation of the host
material or diffusion of activator ions. Silica or silicate coatings can also
improve
the rheologicai properties of the phosphor slurry. The particles can also
include
a pigment coating, such as particulate Fe203, to modify and enhance the
properties of the emitted light.
Other CRT-based devices operating on a similar principle are heads-up
and heads-down displays. A heads-up display is a small, high resolution
display
that is placed in close proximity to the eyes of a user, for example a pilot,
so that
the display can provide information to the user without requiring the user to
be
distracted. Such displays should have high brightness and good resolution.
Similarly, heads-down displays are utilized, for example, in airplane cockpits
to
provide data to the pilots. Such phosphors should also be bright and have a
long lifetime. The small, spherical phosphor powders of the present invention
are ideally suited for such applications.
Another device utilizing cathodoluminescent phosphors is a vacuum
3o fluorescent display (VFD). VFD's operate at an excitation voltage of less
than
76
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
about 500 volts, such as about 40 volts. The most common VFD phosphor is
presently ZnO:Zn (blue). ZnS, CdS and Zn, Cd:S are also useful for green and
red. As is discussed above, the phosphor powders of the present invention are
particularly advantageous at such low applied voltages.
s The introduction of high-definition televisions (HDTV) has increased the
interest in projection television (PTV). In this concept, the light produced
by
three independent cathode ray tubes is projected onto a facepiate on the tube
that includes particulate phosphors, to form 3 colored projection images. The
three images are projected onto a display screen by reflection to produce a
full
~o color image. Because of the large magnification used in imaging, the
phosphors
on the faceplate of the cathode ray tube must be excited with an intense and
small electron spot. Maximum excitation density may be two orders of
magnitude larger than with conventional cathode ray tubes. Typically, the
efficiency of the phosphor decreases with increasing excitation density. For
the
s foregoing reasons, the cathodoluminescent phosphor powders of the present
invention are particularly useful in HDTV applications.
One of the problems with CRT-based devices is that they are large and
bulky and have significant depth as compared to the screen size. Therefore,
there is significant interest in developing flat panel displays to replace CRT
2o based devices in many applications.
Flat panel displays (FPDs) offer many advantages over GRTs including
lighter weight, portability and decreased power requirements. Flat panel
displays can be either monochrome or color displays. It is believed that flat
panel displays will eventually replace the bulky CRT devices, such as
televisions,
25 with a thin product that can be hung on a wall, like a picture. Currently,
flat panel
displays can be made thinner, fighter and with lower power consumption than
CRT devices, but not with the visual quality and cost performance of a CRT
device.
The high electron voltages and small currents traditionally required to
3o activate cathodoluminescent phosphors efficiently in a CRT device have
77
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
hindered the development of flat panel displays. Phosphors for flat panel
displays such as field emission displays must typically operate at a tower
voltage, higher current density and higher efficiency than phosphors used in
existing CRT devices. The low voltages used in such displays, such as less
than
about 12 kV, result in an electron penetration depth in the range of several
micrometers down to tens of nanometers, depending on the applied voltage.
Thus, the control of the size and crystallinity of the phosphor particles is
critical
to device performance. If large or agglomerated powders are used, only a small
fraction of the electrons will interact with the phosphor. Use of phosphor
powders having a wide size distribution can also lead to non-uniform pixels
and
sub-pixels, which will produce a blurred image.
Further, the phosphor powders of the present invention are particularly
applicable to excitation voltages of not greater than 12 kV, such as not
greater
than 8 kV and preferably not greater than 5 kV. This is due to the clean
surface
of the particles with a reduced number of surface defects, compared to a
powder
that has been milled.
One type of flat panel display is a field emission display (FED). These
devices advantageously eliminate the size, weight and power consumption
problems of CRTs while maintaining comparable image quality, and therefore
2o are particularly useful for portable electronics, such as for laptop
computers.
FEDs generate electrons from millions of cold rnicrotip emitters with low
power
emission that are arranged in a matrix addressed array with several thousand
tip emitters allocated to each pixel in the display. The microtip emitters are
typically located approximately 0.2 millimeter from a cathodoluminescent
phosphor screen, which generates the display image. This allows for a thin,
light-weight display.
Fig. 41 illustrates a high-magnification, schematic cross-section of an FED
device according to an embodiment of the present invention. The FED device
4180 includes a plurality of microtip emitters 4182 mounted on a cathode 4184,
3o which is attached to a backing plate 4186. The cathode is separated from a
gate
78
CA 02345300 2001-03-23
WO OOII2649 PCTlUS99/19582
or emitter grid 4188 by an insulating spacer 4190. Opposed to the cathode 4184
and separated by a vacuum is a faceplate assembly 4191 including phosphor
pixel 4192 and a transparent anode 4194. The phosphor pixel layers can be
deposited using a paste or electrophoretically. The FED can also include a
transparent glass substrate 4196 onto which the anode 4194 is printed. During
operation, a positive voltage is applied to the emitter grid 4188 creating a
strong
electric field at the emitter tip 4182. The electrons 4198 migrate to the
faceplate
4191, which is maintained at a higher positive voltage. The faceplate
collector
bias is typically about 1000 volts. Several thousand microtip emitters 4182
can
be utilized for each pixel in the display.
Cathodoluminescent phosphors, which are particularly useful for FED
devices, include thiogallates such as SrGa2S4:Eu for green, SrGa2S4:Ce for
blue
and ZnS:Ag or C1 for blue. Y203:Eu can be used for red. ZnS:Ag or Cu can also
be used for higher voltage FED devices. Y2Si05:Tb or Eu can also be useful.
~5 For use in FED devices, these phosphors are preferably coated, such as with
a
very thin metal oxide coating, since the high electron beam current densities
can
cause breakdown and dissociation of the sulfur-containing phosphor host
material. Dielectric coatings such as Si02 and AI~'73 can be used. Further,
semiconducting coatings such as Sn0 or In2O3 can be particularly advantageous
2o to absorb secondary electrons.
Coatings for the sulfur-containing FED phosphors preferably have an
average thickness of from about 1 to 10 manometers, more preferably from about
1 to 5 manometers. Coatings having a thickness in excess of about 10
manometers will decrease the brightness of the device since the electron
25 penetration depth of 1-2 kV electrons is only about 10 manometers. Such
thin
coatings can advantageously be monolayer coatings, as is discussed above.
The primary obstacle to further development of FEDs is the lack of
adequate phosphor powders. FEDs require low-voltage phosphor materials, that
is, phosphors which emit sufficient light under low applied voltages, such as
less
so than about 500 volts, and high current densities. The cathodoluminescent
79
CA 02345300 2001-03-23
WO 00/12649 PCTIUS99119582
phosphor powders of the present invention advantageously have improved
brightness under such low applied voltages and the coated phosphor particles
of the present invention resist degradation under high current densities. The
improved brightness can be attributed to the high crystallinity and high
purity of
the particles. Phosphor particles with low crystallinity and high impurities
due to
processes such as milling do not have the desirable high brightness. The
phosphor particles of the present invention also have the ability to maintain
the
brightness and chromaticity over long periods of time, such as in excess of
10,000 hours. Further, the spherical morphology of the phosphor powder
~o decreases light scattering and therefore improves the visual properties of
the
display. The small average size of the particles is advantageous since the
electron penetration depth is only several manometers, due to the low applied
voltage.
The present invention is also applicable to other displays, including
electroluminescent displays. EL displays are very thin structures, Which can
have very small screen sizes, such as few inches diagonally, while producing a
very high resolution image. These displays, due to the very small size, are
utilized in many military applications where size is a strict requirement such
as
in aircraft cockpits, small hand-held displays and heads-up displays. These
2o displays function by applying a high electric potential between two
addressing
electrodes. EL displays are most commonly driven by an A.C. electrical signal.
The electrodes are in contact with a semiconducting phosphor thin-film and the
large potential difference creates hot electrons, which move through the
phosphor, allowing for excitation followed by light emission.
2s An EL display is schematically illustrated in Figs. 42 and 43. The EL
display device 4220 includes a phosphor layer 4222 sandwiched between two
dielectric insulating layers 4224 and 4226. On the back side of the insulating
layers is a backplate 4228, which includes row electrodes 4230. On the front
of
the device is a glass facepiate 4232, which includes transparent column
3o electrodes 4234, such as electrodes made from transparent indium tin oxide.
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
While current electroiuminescent display configurations utilize a thin film
phosphor layer 4222 and do not typically utilize phosphor powders, the use of
very small monodispersed phosphor particles according to the present invention
is advantageous for use in such devices. For example, small monodispersed
s particles could be deposited on a glass substrate using a thick film paste
and
sintered to produce a well connected film and therefore could replace the
expensive and material-limited CVD technology currently used to deposit such
films. Such a well-connected flm could not be formed from large, agglomerated
phosphor particles. Similarly, composite phosphor particles are a viable
alternative to the relatively expensive multilayer stack currently employed in
electroluminescent displays. Thus, a composite phosphor particle comprising
the phosphor and a dielectric material could be used.
Particularly preferred phosphors for use in electroluminescent display
applications include the metal sulfides such as ZnS:Cu, CaS:Ce, SrS:RE (RE =
rare earth), and ZnS:Mn. Further, mixed metal sulfides such as SrxCayBa,_X_
ySN:Ce can be used. Further, the thiogallate phosphors according to the
present
invention can also have advantages for use in electroluminescent displays.
Oxide-based phosphors can also be useful, such as SrGaxOYEu, preferably
including 5 to 7 atomic percent Eu, and CaGaXOy:Ce, preferably including from
2o about 0.5 to 3 atomic percent Ce.
Another use for electroluminescent phosphor powders according to the
present invention is in the area of electroluminescent lamps.
Electroluminescent
lamps are formed on a rigid or flexible substrate, such as a polymer
substrate,
and are commonly used as back lights for membrane switches, cellular phones,
25 watches, personal digital assistants and the like. A simple
electroiuminescent
lamp is schematically illustrated in Fig. 44. The device 4440 includes a
phosphor powderlpolymer composite 4442 is sandwiched between two
electrodes 4444 and 4446, the front electrode 4444 being transparent. The
composite layer 4442 includes phosphor particles 4448 dispersed in a polymer
3o matrix 4450.
81
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
Electroluminescent lamps can also be formed on rigid substrates, such
as stainless steel, for use in highway signage and similar devices. The rigid
device includes a phosphor particle layer, a ceramic dielectric layer and a
transparent conducting electrode layer. Such devices are sometimes referred
to as solid state ceramic electroluminescent lamps (SSCEL). To form such rigid
devices, a phosphor powder is typically sprayed onto a rigid substrate.
Electroluminescent lamp manufacturers currently have only simple metal
sulfides such as ZnS phosphor powder host material at their disposal. ZnS:Cu
produces a blue color, while ZnS:Mn, Cu produces an orange color. These
materials have poor reliability and brightness, especially when filtered to
generate other colors. Additional colors, higher reliability and higher
brightness
powders are critical needs for the electroluminescent lamp industry to supply
designers with the ability to penetrate new market segments. The phosphor
layers should also be thinner and denser, without sacrificing brightness, to
s minimize water intrusion and eliminate light scattering. Higher brightness
electroluminescent lamps require thinner phosphor layers, which requires
smaller particle size phosphor powders that cannot be produced by conventional
methods. Such thinner layers will also use less phosphor powder. Presently
available EL lamps utilize powders having an average size of about 5 ,um or
2o higher. The phosphor powders of the present invention having a small
particle
size and narrow size distribution, will enable the production of brighter and
more
reliable EL lamps that have an increased life-expectancy. Further, the
phosphor
powders of the present invention will enable the production of EL lamps
wherein
the phosphor layer has a significantly reduced thickness, without sacrificing
25 brightness or other desirable properties. Conventional EL lamps have
phosphor
layers on the order of 100 ~m thick. The powders of the present invention
advantageously enable the production of an EL lamp having a phosphor layer
that is not greater than about 15 ~cm thick, such as not greater than about 10
,um
thick. The phosphor layer is preferably not thicker than about 3 times the
weight
82
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
average particle size, more preferably not greater than about 2 times the
weight
average particle size.
As discussed above, preferred electroluminescent phosphors for use in
electroluminescent lamps include ZnS:Cu far blue or blue-green and ZnS:Mn,
Cu for orange. Other materials that are desirable for EL lamp applications
include BaS:RE, Cu or Mn, CaS:RE or Mn, SrS:RE or Mn, and SrXCaYBa,_x_yS:RE
(where RE is a rare earth a#ement) for other colors. CaS:Ga or Cu and SrS:Ga
or Cu are also useful. The thiogallate phosphors of the present invention,
such
as SrGa2S4 and CaGa S , 4 can be particularly advantageous for use in
electroluminescent lamps. As is discussed above, many of these phosphors
cannot be produced using conventional techniques and therefore have not been
utilized in EL lamp applications. When used in an EL lamp, these phosphors
should be coated to prevent degradation of the phosphor due to hydrolysis or
other adverse reactions. Preferably, such a coaxing has an average thickness
~s of from about 2 to 50 nanometers.
Electroluminescent powders used in AC powder EL (ACEL) are generally
required to have a moderately conductive phase either on their surface or
contained within them. As an example, ZnS:Cu is well known to exist as a ZnS
host with copper sulfide crystallites contained within each particle. The
copper
2o sulfide crystallites are the source of the electric feld that excites the
dopant in
the ZnS lattice the causes light emission. In the case of other
electraluminescent powders, such as the metal oxide-based doped metal
gallates (MXGaYOZ), these powders should also contain either a conductive
additional phase in each particle or a conductive coating on the particle
surface.
25 For example, according to this embodiment of the invention, ZnGaz04
exhibits
better perFormance when it is either coated or contains an additional
conductive
phase such as ZnO.
As stated above, electroluminescent lamps are becoming increasingly
important for back lighting alphanumeric displays in small electronic devices
3o such as cellular phones, pagers, personal digital assistants, wrist
watches,
$3
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
calculators and the like. They are also useful in applications such as
instrument
panels, portable advertising displays, safety lighting, emergency lighting for
rescue and safety devices, photographic backlighting, membrane switches and
other similar applications. One of the problems associated with
s electroluminescent devices is that they generally require the application of
alternating current (AC) voltage to produce light. A significant obstacle to
the
development of the useful direct current (DC) electroluminescent (DCEL)
devices
is a need for a phosphor powder that will function adequately under a DC
electric
field. The phosphor powder for functioning under a DC electric field should
meet
at least three requirements: 1 ) the particles should have a small average
particle
size; 2) the particles should have a uniform size, that is, the particles
should
have a narrow size distribution with no large particles or agglomerates; and
3)
the particles should have good luminescence properties, particularly a high
brightness. The phosphor powders of the present invention advantageously
meet these requirements. Therefore, the electroluminescent phosphor powders
of the present invention will advantageously permit the use of
eiectroluminescent
devices without requiring an inverter to convert a DC voltage to an AC
voltage.
Such devices are not believed to be commercially available at this time. When
utilized in a device applying DC voltage, it is preferred to coat the phosphor
20 particles with a thin layer of a conductive compound, such as a metal, for
example copper metal, or a conductive compound such as copper sulfide.
The present invention is also applicable to photoluminescent phosphors.
Photoluminescent phosphors are useful in plasma display devices. Plasma
displays have an image quality that is comparable to current CRT devices and
25 can be easily scaled to large sizes such as 20 to 60 diagonal inches. The
displays are bright and lightweight and have a thickness of from about 1.5 to
3
inches. A plasma display functions in a similar manner as fluorescent
lighting.
In a plasma display, plasma source, typically a gas mixture, is placed between
an opposed array of addressable electrodes and a high energy electric field is
3o generated between the electrodes. Upon reaching a critical voltage, a
plasma
84
CA 02345300 2001-03-23
WO 00112649 PCT/US99/19582
is formed from the gas and UV photons are emitted by the plasma. Color
plasma displays contain three-color photoluminescent phosphor particles
deposited on the inside of the glass faceplate. The phosphors selectively emit
light when illuminated by the photons. Plasma displays operate at relatively
low
currents and can be driven either by an AC or DC signal. AC plasma systems
use a dielectric layer over the electrode, which forms a capacitor. This
impedance limits current and provides a necessary charge in the gas mixture.
A cross-section of a plasma display device is illustrated in Fig. 45. The
plasma display 4540 comprises two.opposed panels 4542 and 4544 in parallel
~o opposed relation. A working gas, typically including xenon, is disposed and
sealed between the two opposing panels 4542 and 4544. The rear panel 4544
includes a backing plate 4546 on which are printed a plurality of electrodes
4548
(cathodes) which are in parallel spaced relation. An insulator 4550 covers the
electrodes and spacers 4552 are utilized to separate the rear panel 4544 from
the front panel 4542.
The front panel 4542 includes a glass face plate 4554, which is
transparent when observed by the viewer {V). Printed onto the rear surface of
the glass face plate 4554 are a plurality of electrodes 4556 (anodes) in
parallel
spaced relation. An insulator 4558 separates the electrode from the pixels of
2o photoluminescent phosphor powder 4560. The phosphor powder 4560 is
typically applied using a thick film paste. When the display 4540 is
assembled,
the electrodes 4548 and 4556 are perpendicular to each other, forming an XY
grid. Thus, each pixel of phosphor powder can be activated by the addressing
an XY coordinate defined by the intersecting electrodes 4548 and 4556.
One of the problems currently encountered in plasma display devices is
the long decay time of the phosphor particles, which creates a ?tail? on a
moving image. Through control of the phosphor chemistry, such decay-related
problems can be reduced. Further, the spherical, non-agglomerated nature of
the phosphor particles improves the resofutian of the plasma display panel.
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
Plasma display panels typically operate using a xenon gas composition.
The Y203:Eu phosphors of the present invention, doped with from 4 to about 6
atomic percent Eu, are useful for plasma displays for providing red color.
(Y,Gd)B03 phosphors including about 14 to 20 atomic percent Eu are also useful
for red color. Further, the Zn2 Si04:Mn phosphors of the present invention,
preferably with 0.05 to 2 atomic percent Mn, are useful for producing a green
color. BAM phosphors of the present invention are useful far producing a blue
color, particularly when doped with from about 8 to 12 atomic percent Eu. The
phosphors can advantageously be coated, such as with MgO, to reduce
~o degradation from the plasma.
The photoluminescent phosphors of the present invention are also useful
as taggents for security purposes. In this application the phosphors, which
are
undetectable under normal lighting, become visible upon illumination by a
particular energy.
~5 For security purposes, the phosphor particles are dispersed into a liquid
vehicle which can be applied onto a surface by standard ink deposition
methods,
such as by using an ink jet or a syringe, or by screen printing. The phosphor
particles of the present invention, having a small size and narrow size
distribution, advantageously permit better control over the printed feature
size
2o and complexity. The methodology of the present invention also permits
unique
combinations of phosphor compounds that are not available using conventional
methods. Such phosphors can be applied to currency, confidential
documentation, explosives and munitions, or any other item that rnay require
positive identification. The phosphor powders can advantageously be dispersed
25 in an ink, which is then used to form indicia on a document or other item,
such
as a postal envelope.
Useful phosphor compounds for security applications include Y203:Eu,
preferably including 6 to 9 atomic percent Eu. (Y,Gd)B03 including 14 to 20
atomic percent Eu can also be useful. Such phosphors emit visible light upon
so excitation by an infrared source. The phosphor powders of the present
invention
86
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
provide many advantages in such applications. For example, the small,
monodispersed nature of the particles makes the particles easy to supply in
smaller quantities.
In addition to the foregoing, the phosphors of the present invention can
s also be used as target materials for the deposition of phosphor thin-films
by
electron beam deposition, sputtering and the like. The particles can be
consolidated to form the target for the process. The homogenous concentration
of activator ions in the particles will lead to more uniform and brighter
film.
Further, the photoluminescent phosphor particles according to the present
o invention can be used for fluorescent lighting elements, including common
overhead lighting tubes as well as lamps used to backlight LCD displays, which
are commonly used on laptop computers. Typically, the excitation source for
such displays includes mercury vapor.
A fluorescent tubular lighting element is schematically illustrated in Fig.
~5 46. The lighting element 4600 includes a glass tube 4602 that is sealed to
contain a gas composition, typically including mercury. The ends of the glass
tube 4602 include electrodes 4604 and 4606 that ionize the gas composition
thereby stimulating photoluminescent phosphors 4608 that are disposed on the
inner surface of the glass tube 4602. The mixture of different color phosphors
2o produces a white light, which is typically desired in fluorescent lighting
applications.
Fig. 47 schematically illustrates a close-up of the phosphor powders 4608
of the present invention on a glass tube 4602. The small and spherical
phosphors form a uniform thin layer having a reduced average thickness, such
25 as less than about 3 times the average particle size of the phosphor
particles
4608. This uniform and thin surface can advantageously produce bright, uniform
light while consuming less power than conventional phosphor powders. It will
be appreciated that the fighting element can have a variety of shapes and
forms,
and often are in the form of very thin tubing for use as an LCD backlight.
Such
3o LCD backlights are particularly applicable to the present invention since
they
87
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
require phosphor powders that are small, spherical and have a narrow size
distribution so that the powders will readily disperse in the lighting
element.
Preferred photoluminescent phosphor powders for such fluorescent
lighting applications including Y203:Eu, particularly those including from
about
6 to about 9 atomic percent Eu. (Y,Gd) BO3:Eu, including from about 14 to
about
20 atomic percent Eu, are also useful. Other useful phosphors include Zn2
Si04:Mn, particularly from about 0.05 to about 2 atomic percent Mn as well as
BAM:Eu, including from about fi to about 12 atomic percent Eu. Combinations
of the foregoing phosphors can be used to produce a white light element. To
~o form these lighting elements, the phosphor particles are typically
dispersed in a
liquid vehicle such as a slurry which are then applied to the interior of a
glass
tube and dried to form the phosphor layer.
The present invention is also applicable to LEDs. LEDs (light emitting
diodes) are solid-state devices that emit light of a particular wavelength
when
powered by an external electric circuit. The advantages of LEDs are their high
brightness and low power consumption. LED efficiencies are typically around
45 lumens/watt, compared to about 12 lumens/watt for fluorescent lamps. LEDs
are planned to be used for household and commercial lighting -- replacing
incandescent fight bulbs and possibly replacing fluorescent lamps -- traffic
lights,
2o large area visual displays and security devices. For use as a security
device, the
phosphor would be placed in an article such as a document, currency, passport,
stamp, bankcard, or the like and an LED would be used to verify the
authenticity
of the article by making the phosphor iuminesce.
LEDs are generally based on nitrides such as GaN or (In,Ga)N or other
315 or 2/6 compositions such as GaP, GaAs or ZnSe. Polymer LEDs are also
known. The nature of the materials and their band-gap determines the
wavelength of the light that is emitted and the emitted light has a wavelength
that
varies between the UV and red.
Currently, the most common LEDs emit blue light having a wavelength of
3o around 450 nm. It is desirable to have an LED-based white light source, but
this
88
CA 02345300 2001-03-23
WO 00/12649 PCT/US99I19582
is not possible with an LED alone. However, the color of the light emitted by
an
LED can be modified by placing a layer of luminescent material, such as a
photoluminescent phosphor powder, on top of the LED such that the light
emitted by the LED excites the phosphor powder which re-emits the light at a
s different wavelength. By selecting the proper phosphor powder, white LEDs
can
be produced. An example of a blue-to-white LED phosphor is YAG:Ce, where
YAG {yttrium aluminum garnet) is Y3AI50,z. There are many variations on this
composition which result in subtle variations of color temperature, such as
(Y,Gd)3AI50,2:Ce and (Y,Gd)3{Ga,AI~)~0,2:Ce.
Other materials that are useful for LED applications are the existing
plasma TV phosphors, such as BAM:Eu. Future generations of LEDs will be
optimized to emit light in the UV range and UV-excited phosphors such as those
currently used in the lighting and plasma TV industry will be useful to
achieve
RGB colors for displays. There are also LEDs that emit in the IR range, and
~5 upconverter phosphors would be useful to convert the IR to visible light.
The powders of the present invention will be particularly useful in the
foregoing
LED applications. In particular, the phosphor powders will advantageously have
higher efficiencies since milling of the powder is not required, leading to
clean,
defect-free surfaces. The spherical particle morphology and bimadal size
2o distribution will enable higher brightness with less material through
better
phosphor layer characteristics. Further, the versatility of spray processing
will
enable the formation of many existing and new compositions, including complex
compositions.
The present invention is also applicable to x-ray phosphors. One
25 preferred device utilizing the x-ray phosphors of the present invention is
an x-ray
image intensifier. As is illustrated in Fig. 48, a subject 4802 is placed
between
an x-ray source 4804 and an image intensifying screen 4806. Behind the image
intensifying screen is photographic film 4808 that captures the image. As x-
rays
are passed through the subject 4802, some are absorbed or deflected and the
3o resulting pattern of x-ray energy impinges on the intensifying screen 4806.
The
89
CA 02345300 2001-03-23
WO 00112649 PCT/US99/19582
screen 4806 includes x-ray phosphors, which convert the x-ray energy to
visible
light, and the light strikes the photographic film 4808, thus producing an
image.
A cross-section of an x-ray image intensifying screen is illustrated in Fig.
49. The screen 4900 utilizes two intensifying screens 4902 and 4904. The
screens each include a base 4906 and 4908 and a layer of x-ray phosphors
4910 and 4912. The x-rays impinge on the x-ray phosphors, which then emit
visible light and produces an image on the photographic film 4914.
Phosphors for x-ray imaging should have a high x-ray absorption
capability, a high density and a blue or green luminescence to match the
o sensitivity of the film. Particular x-ray phosphors which meet these
requirements
include: Gd202S:Tb, preferably including from about 5 to about 20 atomic
percent Tb; Gd2Si05 or (Y,Gd)2Si05:Tb, particularly including from about 5 to
about 20 mole percent Tb; and (Y,Gd)B03:Tb, preferably including from about
to about 20 atomic percent Tb. Other x-ray phosphors include those based
~5 on Lu2Si05, GdTa04, PbHf03, Hf02 and Gd3Ga50,2. Tungstates such as
CaW04, SrW04 and PbW04 are also useful.
The x-ray phosphor particles of the present invention are also
advantageous for the manufacture of intensifying screens since the phosphors
can form thin, uniform layers having high brightness. The high relative
density
of the phosphor layer can improve the efficiency and resolution of the image
intensifier.
EXAMPLES
A yttrium oxide phosphor powder batch was produced according to the
present invention. An aqueous precursor solution was formed comprising
yttrium nitrate and europium nitrate in a ratio to yield a phosphor comprising
Y203 and 8.fi atomic percent Eu. The total precursor concentration was 7.5
weight percent based on the final product.
The liquid solution was atomized using ultrasonic transducers at a
frequency of 1.6 MHz. Air was used as a carrier gas and the aerosol was
carried
CA 02345300 2001-03-23
WO OU/I2649 PCT/US99/19582
through a tubular furnace having a temperature of 800°C. The total
residence
time in the furnace was about 1-2 seconds. The pyrolyzation at 800°C
resulted
in intermediate precursor particles of a low crystallinity yttrium compound.
The intermediate precursor particles were then heated in batch mode at
a temperature of 1400°C for 60 minutes in air. The heating ramp rate
was
10EC/minute.
The resulting powder is illustrated in the SEM photomicrograph of Fig. 50.
The particle size distribution is illustrated in Fig. 51. The average particle
size
was 2.476 ,um and 90 percent of the particles had a size of less than 4.150
,um.
o The x-ray diffraction pattern illustrated in Fig. 52 shows that the
particles are
substantially phase pure Yz03.
A zinc silicate powder batch was produced according to the present
invention. A precursor solution was formed comprising zinc nitrate and
manganese nitrate along with colloidal silica (Cabot L-90, Cabot Corporation,
Massachusetts). An excess of 50 molar percent silica was used in the precursor
liquid and the concentration of manganese was 5 atomic percent. The total
precursor concentration was about 7.5 weight percent based on the final
product. The liquid solution was atomized using ultrasonic transducers at a
frequency of 1.6 MHz. Air was used as a carrier gas and the aerosol was
carried
2o through a tubular furnace having a temperature of 900°C. The total
residence
time in the furnace was about 1-2 seconds. The pyrofyzation at 900°C
resulted
in intermediate precursor particles having a low crystallinity.
The intermediate precursor particles were then heated in batch mode at
a temperature of 1175°C for 60 minutes in air. The heating ramp rate
was
10EC/minute.
The resulting powder is illustrated in the SEM photomicrograph of Fig. 53.
The particle size distribution is illustrated in Fig. 54. The average particle
size
was 2.533 ~m and 90 percent of the particles had a size of less than 4.467
~cm.
The x-ray diffraction pattern illustrated in Fig. 55 shows that the particles
are
so substantially phase pure Zn2Si04.
91
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
For the production of a thiogallate, an aqueous solution was formed
including 2 mole equivalents gallium nitrate (Ga(N03)3) and 1 mole equivalent
strontium nitrate (Sr(N03)~. About 0.05 mole equivalents of europium nitrate
(Eu(N03}3) was also added.
The aqueous solution was formed into an aerosol using ultrasonic
transducers of a frequency of about 1.6 MHz. The aerosol was carried in air
through a furnace heated to a temperature of about 800°C to spray-
convert the
solution. The intermediate product was a SrGa204 oxide having a small average
particle size and low impurities. .
The oxide intermediate precursor product was then heated at 900°C
under a flowing gas that included H2S and nitrogen in a 1:1 ratio, for about
20
minutes. The resulting powder was substantially phase pure SrGazS4:Eu (3
atomic percent Eu) having good crystallinity and good luminescent
characteristics.
~5 In a further example, SrGa204:Eu was prepared by mixing the appropriate
ratios of strontium carbonate, gallium nitrate and europium nitrate. The total
precursor concentration was 5 weight percent.
The liquid solution was atomized using ultrasonic transducers at a
frequency of 1.6 MHz. Air was used as the carrier gas and the aerosol was
2o carried through a furnace heated to 800°C. This process formed an
SrGa204:Eu
phosphor having about 4 atomic percent Eu. This powder is illustrated in Fig.
56.
The following examples demonstrate the advantages of the BAM
phosphor powders according to the present invention and the method for making
25 the powders.
1. BAM Precursor Selection
Using aluminum nitrate hydrate (AI{N03)3 9H20), a 5 weight percent
solution was made based on Bao.95Euo.oSMgAI,oO" (i.e. 5 atomic percent
europium as a dopant). Stirring the solution resulted in a crystalline
deposition
ao of barium nitrate. Four equivalents of ethylene glycol were added as a
chelating
92
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
agent to increase the solubility of the barium nitrate, however the deposit
still
remained.
Similar solutions containing aluminum nitrate and having a total precursor
concentration of 5, 7.5 and 10 weight percent were prepared. Polyether was
s added as a second chelating agent to increase the solubility of the barium
nitrate. The results are listed in Table III.
Table III
Example Weight Percent SolutionResult
BAM-1 5.0 Mostly in solution
1o BAM-2 7.5 Slight suspension of
solid
BAM-3 10.0 Difficult to put into
solution
The solutions from Examples BAM-1 & BAM-2 were nebulized using an
ultrasonic generator at a frequency of about 1.6 MHz and carried through a
furnace at 750°C, substantially as described herein. Both atomized
poorly (low
~s production rate), although some powder was collected from Example BAM-1.
The collected powder had a brownlblack appearance. Heating the powder to
1350°C under flowing forming gas (7% H~I93%N2} produced a grey powder
that
x-ray diffraction indicated was phase pure. Further heat treatment in air at
1350°C for 2 hours produced a white, phase pure powder.
2o Nonetheless, it was concluded that aluminum nitrate as a precursor was
not a viable alternative due to solubility problems associated with barium
nitrate.
2. BAM Process Temperature
Based on the foregoing, fumed particulate alumina was selected as a
precursor component. Europium nitrate, barium nitrate and magnesium nitrate
25 were used as the remaining precursors, for a total precursor concentration
of 7.5
weight percent based on the final BAM composition. The solution was nebulized
using ultrasonic transducers at a frequency of about 1.6 MHz to form an
aerosol.
93
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
The aerosol was carried through a tubular furnace at varying temperatures, as
indicated in Table IV.
Table iV
Example Crystalline
Conversion Crystalline Phases after
Temperature Phases Heat Treatment
BAM-4. 600C Ba(N03)2 BaA1204, BAM
BAM-5 700 C Ba(N03)2 BaA1204, BAM
BAM-6 800C ~ Amorphous BaA1204, BAM
Mainly amorphous
BAM-7 900 C Some Ba0 BaA1204, BAM
Below 700°C, the barium nitrate did not fully decompose. It was
concluded that
a preferred range for producing a spray-converted intermediate product was at
least about 700°C and preferably 800°C to 900°C.
A sample of each of the spray-converted intermediate powders was heat
treated at 1500°C for 2 hours under a forming gas of 10% H2I 90% N2 and
a
sample of each powder was heat treated at 1400°C for 1 hour in air.
Each post
treatment resulted in a powder that was not phase pure (BaA1204 impurity) and
there was virtually no difference in the x-ray diffraction patterns for the
powders
treated in forming gas versus the powders treated in air.
3. BAM Precursor Concentration
In an effort to eliminate the aluminate impurity identified above, the metal
2o stoichiometry in the precursor solution was varied. The solutions and
results of
processing are summarized in Table V. All solutions were formed into an
aerosol using ultrasonic atomizers at a frequency of about 1.6 MHz and were
spray converted at a temperature of 750°C in a tubular furnace.
94
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
Table V
Heat Weight crystalline
ExamplePrecursor TreatmentLoss Phases Relative
- StoichiometryAtmosphereduring of PL
Heat Final PowderIntensity
Treatment
BAM-8 Ba.ssEu.sM9,.sAl,o~xHz/Nz 10.4% BaAlz04 13
BAM-9 Ba.ssEu.osM9,.~sAllo~.Hz/Nz 8.5% BaAlz04 18
BAM-10 Ba.ssEu.sM9z.oAl,o~xI"Iz/Nz 8.2% BaAl20, 7
BAM-11 Bao,95Euo.osM9Ahz0XHz/Nz 7.8% BaAlz04 42
+
BAM
BAM-12 Bao.aSEu.osM9Al,n~xHz/Nz 7.2% BAM 37
BAM-13 Ba.95Eu.SMgAI,sOxHz/Nz 5.9% BAM 29
BAM-14 Ba,95Eu,SMgAIzOxHZ/Nz 4.5% BAM 14
BAM-15 Ba.ssEuo.osM9o.ssAl,o~xAir 12.0% BaAIz04
+
BAM
BAM-16 Ba,ssEu.aM9i."A~,s~xAir 13.6% BaAlz04
+
BAM
BAM-17 Ba.asEu.asM9Ahs~xAir 7.9% BaAlzO, --
+BAM
BAM-18 Ba.95Eu,SMgAl,zOxAir 7.0% BaA1204 --
+
BAM
BAM-19 Bao.95Eu.SMgAl,40xAir 7.0% BAM --
increasing the magnesium content in Examples BAM-8, 9 and 10 did not
lead to a decrease of the barium aluminate phase. However, when the
aluminum content was increased keeping the barium and magnesium ratio fixed,
phase pure BAM was produced. Therefore, it was concluded that an excess of
aluminum in the precursor, such as 40% or more excess, is desirable for
2o producing phase pure BAM.
The photoluminescence (PL) data indicates that the maximum intensity
occurs for powders treated in a reducing atmosphere, likely due to the
reduction
of europium to the Eu2+ state.
Based on the foregoing examples, BAM powder was produced as follows.
Barium nitrate, europium nitrate, magnesium nitrate and colloidal alumina were
formed into a precursor solution having a concentration of about 8 weight
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
percent based on the equivalent of BAM. Europium was incorporated at 5
atomic percent. The solution was atomized using ultrasonic transducers at a
frequency of about 1.6 MHz. The aerosol was pyrolyzed in a tubular furnace at
a reaction temperature of 950°C to form a spray-converted intermediate
precursor powder. The powder was then heat-treated at 1450°C for 2
hours
under an atmosphere of HZIN2. This powder is illustrated the photomicrograph
of Fig. 57. The particle size distribution of the powder is illustrated in
Fig. 58.
The average particle size was 1.88 ~cm and 90% of the particles by weight were
less than 3.51 ,um in size. The powder exhibited strong photoluminescence
characteristics.
A yttrium gadolinium borate powder batch was also produced according
to the present invention. An aqueous precursor solution was formed comprising
yttrium nitrate, gadolinium nitrate, europium nitrate and boric acid in a
ratio to
yield a (Y,Gd)B03 phosphor having a Y:Gd ratio of 3 and an Eu concentration
of 16 atomic percent. The total precursor concentration was 8.0 weight percent
based on the final product.
The liquid solution was atomized using ultrasonic transducers at a
frequency of 1.6 MHz. Air was used as a carrier gas and the aerosol was
carried
through a tubular furnace having a temperature of 950°C. The total
residence
2o time in the furnace was about 1-2 seconds. The pyrolyzation at 950°C
resulted
in intermediate precursor particles having low crystallinity.
The intermediate precursor particles were then heated in batch mode at
a temperature of 1150°C for 60 minutes in air. The heating ramp rate
was
10EC/minute.
25 The resulting powder is illustrated in the SEM photomicrograph of Fig. 59.
The particle size distribution is illustrated in Fig. 60. The average particle
size
was 2.139 ~m and 90 percent of the particles had a size of less than 3.608
,um.
The x-ray diffraction pattern illustrated in Fig. 61 shows that the particles
are
substantially phase pure yttrium borate and gadolinium borate having a high
3o crystallinity.
96
CA 02345300 2001-03-23
WO 00/12649 PCT/US99/19582
An x-ray phosphor compound, Y,.88Gdo,,2Si~5:Ce, that is useful as an x-
ray phosphor was produced according to the present invention. A liquid
precursor solution was formed including yttrium nitrate, gadolinium nitrate,
cerium nitrate and colloidal silica (Cabot HS-5, Cabot Corporation, Boyertown,
s PA) to yield the foregoing compound including 0.5 atomic percent Ce. The
total
precursor concentration was 5 weight percent based on the equivalent weight
of the final compound. The liquid was atomized into an aerosol using
ultrasonic
transducers operating at a frequency of 1.6 MHz and the aerosol was carried in
air through furnace having a temperature of about 650°C to produce
o intermediate precursor particles.
The intermediate precursor particles were then batch annealed in air at
a temperature of 1350°C for 1 hour. The resulting powder had a small
particle
size and a narrow particle size distribution.
While various embodiments of the present invention have been described
~5 in detail, it is apparent that modifications and adaptations of those
embodiments
will occur to those skilled in the art. However, it is to be expressly
understood
that such modifications and adaptations are within the spirit and scope of the
present invention.
97
