Note: Descriptions are shown in the official language in which they were submitted.
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Calcium Fluoride
The present' invention relates to calcium fluoride, and in
particular to ion doped calcium fluoride and methods of
manufacture thereof.
"Ion-beam induced white luminescence of calcium fluoride
implanted with both Eu and Tb ions", Aono et al, Jpn. J. Appl.
Phys. Vo1.32 (1993) pp 3851-3853, Part 1, No. 9A, September
1993 (Aono) discloses a technique for manufacturing Eu and Tb
ion implanted calcium fluoride (CaF2). A single crystal
substrate of CaF2 is implanted using fluences of 1x1019-1x101s
ions/cm2 at 100 keV at room temperature with a relatively low
beam current density of 0 . C2 r.tl1/ cm2. P;hen the implanted single
crystal substrate was subjected to Ar bombardment light was
output at between 400-460 nm and 600-700 nm due to the
presence of Eu+z and Eu+3 ions . The ion implantation technique
used in Aono does not represent a very practical method for
the manufacture of commercially viable quantities of ion doped
CaF2 or an ion doped CaFz phosphor. Furthermore, the Aono ion
implantation technique allows neither the manufacture of CaFz
nor a CaF2 powder within a relatively short period of time or
which has a relatively uniform grain size. It will be
appreciated that any such ion implantation technique is both
expensive and slow. Furthermore, the ion bombardment will
also produce lattice damage in the substrate or crystal being
implanted.
If the single crystal Eu ion implanted substrate of Aono is
crushed to produce a CaFz powder, the light output efficiency
and intensity of the resulting powder are relatively poor.
Producing a CaFz powder by such crushing also has the
disadvantage that the powder is not uniformly doped with Eu
ions.
It will be appreciated that there is a physical lower limit to
the grain size that can be produced by crushing bulk grown
CaF2. Attempting to produce grain sizes below this lower limit
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leads '~o contamination of the resulting powder with material
constituting the crushing or grinding surfaces.
It is an object of the present invention to at least mitigate
some of the problems of the prior art.
Accordingly, a first aspect of the present invention provides
a calcium fluoride precipitate doped with ions.
Preferably, an embodiment provides a calcium fluoride
precipitate which is doped with at least one of the lanthanide
ions, preferably Eu+Z, Eu+3, ions of Tb or Dy, or at least one
group IIIb ion, preferably, ions of T1.
AonU does not allow the manufacture of a CaF2 powder having a
controllable or relatively uniform grain size.
Advantageously, an embodiment of the present provides a
calcium fluoride precipitate and phosphor wherein the grain
size of the calcium fluoride is between 5 nm and 10,000 nm and
is relatively uniform. It will be appreciated that the actual
grain size intended to be manufactured will depend upon the
application of the CaF2 powder or phosphor.
Given the macroscopic size of the substrates used for
implantation, it is very likely that the substrates contain
stresses which, in turn, lead to relatively inferior
luminescence properties. Furthermore, the crushing process
used to produce crushed bulk grown CaFz also impairs or deforms
crystal structure, which, again, results in poor luminescence
properties. In contrast, the present invention produces good
quality crystals having little or no defects. Still further,
it has been found that the smaller grain size of the present
invention results in better performance of a CASPAR detector
by affecting the nuclear/electron recoil discrimination.
Preferably, the grain size is substantially 800 nm for CAS PAR
applications.
An embodiment provides a calcium fluoride precipitate capable
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of being used to generate and a phosphor capable of generating
visible light, preferably having a wavelength of between 360
nm and 780 nm.
It has been found that the CaFz(Eu) or calcium fluoride
phosphor derived therefrom has a photoluminescence light
output efficiency of greater than or of the order of five
times that of crushed bulk grown CaFz(Eu) crystal.
Accordingly, a further aspect of the present invention
provides a calcium fluoride phosphor powder comprising a
calcium fluoride phosphor derived from a CaFz(Eu) precipitate.
The stresses within crystals of the embodiments of the present
inver~~ion are substantially reduced or eliminated.
Typically, the Aono process must use single crystal substrates
that are contaminated with at least 10 ppb U and Th. This, in
low level background scintillator applications, leads to an
undesirable level of background radiation. Therefore, an
embodiment provides a calcium fluoride phosphor containing
impurities of less than 10 ppb of at least one of either U or
Th.
As discussed above, the ion implantation process is expensive,
slow and produces inefficient CaF2 having a relatively poor
light output.
Accordingly, a second aspect of the present invention provides
a method for manufacturing a calcium fluoride precipitate
doped with ions and phosphor derived therefrom, the method
comprising the steps of
producing a dopant solution using a salt of at least a first
dopant and a solvent for that salt:
producing a solution of CaCl2;
mixing the dopant solution and CaCl2 solution with Hydrogen
Fluoride to produce a precipitate.
The precipitate comprises grains of CaFZ doped with ions of the
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first dopant. It will be appreciated that the doped calcium
fluoride is made and doped in solution in a single step.
An embodiment further comprises the step of treating the
precipitate to activate the dopant ions by incorporating them
into the CaFz crystal lattice thereby producing a phosphor
doped with ions of the first dopant.
An embodiment provides a method wherein the salt of the first
dopant is in the form of a powder.
A further embodiment provides a method wherein the salt of the
dopant is selected such that a required ionisation state is
not readily oxidisable in the so?vent at above a
predeterminable rate of oxidation. A dopant ion is considered
to be not readily oxidisable if not more than 10~ oxidisation
of the dopant ion occurs, preferably, during the immersion in
the solvent used from the time of first dissolving the dopant
salt in the solvent to the time the HF is added to it (and the
CaCl2 solution).
Still, further, there is provided an embodiment wherein the
salt of the dopant is a salt of a lanthanide or of a group
IIIb element. Preferably, the lanthanide is Dy, Tb, Eu'2 or
Eu'3 and/or the group IIIb element is T1.
A still further embodiment provides a method wherein is used a
number of moles of the first dopant equal to between 0.05 and
10~ of the number of moles of CaCl2 used. A preferred
embodiment provides a method wherein the number of moles of
the first dopant is equal to between 0.5$ and 5$ of the number
of moles of CaCl2. A still more preferred embodiment provides a
method wherein the number of moles of the first dopant
represents about 1$ of the number of moles of CaCl2.
Yet another embodiment provides a method wherein the step of
producing the dopant solution comprises the step of adding an
acetic acid solution, preferably a glacial acetic acid
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solution. Advantageously, the acetic acid is a solvent for
EuCl2 that does not oxidise the Eu+2. Preferably, an embodiment
is provided wherein 1 cm3 of acetic acid solution is provided
per gram of salt of the first dopant.
5
An embodiment provides a method wherein the step of producing
a solution of CaCl2 comprises the steps of dissolving CaClz.6H20
crystals in a solvent; and adding hydrogen fluoride.
Preferably, the step of adding HF comprises adding a
stoichiometric quantity of HF at a predeterminable
concentration. Preferably, the concentration is 48~.
A further embodiment provides a method wherein the step of
treating comprises the steps of annealing the prccipii:ate at a
predeterminable temperature. Preferably, an embodiment
provides a method wherein the predetermined temperature is
between 700°C and 1200°C, preferably 800°C to
1000°C. Still
more preferably, the predetermined temperature is 900°C. The
temperature or temperature range are selected to balance the
activation of the dopant in the shortest period of time while
reducing sintering of grains that occurs at higher
temperatures.
An embodiment provides a method wherein the step of heating
spans a predeterminable period of time. Preferably, the
duration of the predeterminable period of time is set
according to the required ionisation states of at least the
first dopant and/or the required light output characteristics.
A further embodiment provides a method wherein the step of
annealing is performed in a predetermined atmosphere or under
a vacuum. An embodiment is provided wherein the atmosphere is
an inert gas, preferably He.
A still further embodiment provides a method further
comprising the step of subjecting the mixture of dopant
solution, CaCl2 solution and Hydrogen Fluoride to an ultrasonic
field to produce a precipitate having a predeterminable range
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of grain sizes. The characteristics of the ultrasonic field
are such that grains of a required size are produced.
A still further embodiment provides a method further
comprising the step of processing the CaCl2 solution to remove
impurities, for example, actinide impurities, preferably, by
passing the solution through a DIPHONIX column.
The calcium fluoride precipitate or phosphor derived therefrom
can be used for many purposes. Therefore, an embodiment of
the present invention provides a method for making a
fluorescent transparent polycrystalline solid comprising the
steps of pressing or sintering it, with or without a binding
agent (LOr example, potassium bromide) using a technique such
as Hot lsostatic Pressing (HIP).
Further, an embodiment of the present invention provides a
method for making a fluorescent paint comprising the steps of
mixing a calcium fluoride phosphor with a binding agent.
Still further, an embodiment provides a method for making a
fluorescent polymer comprising the steps of mixing a calcium
fluoride phosphor with a polymer, for example, polytri-
fluorochloroethylene (PTFCE).
A further embodiment provides a method for making a
fluorescent transparent liquid or a fluorescent gel comprising
a liquid or gel having a refractive index matched to the
calcium fluoride phosphor. Preferably, the tolerance of the
matching is such that the difference in the refractive indices
is less than 0.05 and still more preferably less than 0.01.
Dioxan and naphthalene could be used for the above purposes.
Once an efficient, high-light output calcium fluoride phosphor
powder has been produced, it can be applied in numerous
applications. Accordingly, embodiments of the present
invention provide, for example, a VDU or television comprising
a tube with a coating of a calcium fluoride phosphor, a flat-
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panel or field-emission display containing a coating of a
calcium fluoride phosphor, a paint comprising a calcium
fluoride phosphor, a light source comprising a calcium
fluoride phosphor, preferably a white light source. It is not
always desirable to have a white light source. Accordingly,
an embodiment provides a visible light phosphor comprising a
calcium fluoride phosphor.
Embodiments of the present invention will now be described, by
way of example only, with reference to the accompanying
drawings in which:
figure 1 shows the photoluminescence spectra of raw
CaF2 (Eu+z) powder and annealed CaFz (0.5gM Eu'2) according to an
embodiment of the present invention;
figure 2 shows the photoluminescence spectra for annealed
CaF2(1$M Eu+Z) according to embodiments of the present invention
as compared to crushed bulk CaF2(Eu) and undoped CaF2:
figure 3 shows the photoluminescence spectra for annealed
CaF2(Eu+3) according to an embodiment of the present invention
showing a strong 610 nm emission; and
figure 4 shows the photoluminescence spectra for sintered
CaF2(Eu+3) according to an embodiment of the present invention
having reduced peaks at 610 nm and increased peaks at 435 nm.
Sufficient EuCl2 powder for doping the CaFZ is obtained. In
the specific embodiment, 1~ of the total number of moles of
initial CaC12.6HZ0 was used. Thus to prepare lg (1.28x10-2
moles) of CaFz product, 0.0285g (1.28x10-" moles) of EuClz was
used along with a stoichiometric quantity of CaC12.6Hz0 (2.8g
or 1.28x10-Z moles). Other embodiments can use equivalent molar
fractions between 0.05$ and 10$. However molar fractions
between 0.5~ and 5~ are preferred. The percentage doping
represents a balance between too little doping, which will
result in poor light output, and too much doping, which will
cause self absorption of the phosphor's own fluorescence, that
is, quenching of the light output.
It will be appreciated that the molar quantity of CaClz.6H20
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used will be identical to that of the final CaF2 product
assuming stoichiometric quantities of CaClz.6H20 and HF and
assuming that all Ca+2 ions form CaF2. Thus a 1$ molar fraction
of EuClz relative to the number of moles of CaC12.6Hz0 is, under
these assumptions, equivalent to a 1$ molar fraction relative
to the number of moles of CaF2 product. Then, if it is assumed
that all Eu dopant ions are taken up into the CaF2, the molar
concentration of dopant ions in the CaF2 is also about l~s.
A concentrated solution of EuCl2 is produced by dissolving the
EuCl2 in a small amount of distilled water. Typically,
approximately 3 cm3 of water is used per gram of EuCl2 powder.
Preferably, a quantity, preferak~ly i cm', of glacial acetic
acid solution is also added to the solution. The EuCl2
concentration selected represents a balance between the need
to avoid oxidation (which occurs at too low a concentration)
and the ability to dissolve the powder (which will not occur
at too high a concentration). The solution is vigorously
shaken until the powder has been completely dissolved. It is
thought, without wishing to be bound by any particular theory,
that maintaining the Eu ions in solution until the HF is added
significantly reduces the resulting grain size.
A solution of CaCl2 is added to the shaken solution.
Preferably, a small amount of acetic acid is also added. The
CaCl2 solution is prepared using CaClz.6H20 crystal. The CaCl2
solution is highly concentrated. Preferably, the solution is
a 5 mole/litre solution. Preferably, concentrated hydrogen
fluoride is added to the solution whilst stirring vigorously,
using a mechanical stirring rod set at 5 revolutions sec-1,
until precipitation of the CaFz results. The concentration of
the hydrogen fluoride governs the grain size; the higher the
concentration, the quicker the reaction and the smaller the
grain size.
The CaF2 precipitate is then washed and dried very thoroughly.
Preferably, the washing process utilises water. Preferably,
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the drying process uses a centrifuge/vacuum oven for drying.
In an embodiment, a MSE Centaur 2 centrifuge and a Gallenkamp
vacuum oven were used for washing and drying. Care should be
taken to ensure that the drying process does not cause caking
of the doped calcium fluoride.
Finally, the powder is annealed at a high temperature in an
inert atmosphere. Alternatively, the powder may be annealed
in a vacuum. The preferred annealing temperature is between
800°C and 1000 °C, preferably, 900°C. The annealing
period is
for between 10 and 15 minutes. The annealing period is
arranged to activate the dopant in the shortest period of time
to at least reduce and preferably avoid the sintering of
grains that occurs at higher temperatures. After annealing
the dopant ions are incorporated into the crystal lattice
thereby activating the characteristic light output of the
doped CaFz. A preferred annealing atmosphere is Helium.
Although the embodiments above have been described with
reference to specific values of concentration or temperature
etc, the present invention is not limited thereto.
Embodiments can equally well be realised using the variants
described below.
Other dopants can be selected provided they satisfy certain
ionisation requirements. The ionisation requirements are such
that the dopant must not be readily oxidisable in or by water.
A salt of the required dopant should preferably be made in
water to produce the required starting ion. For example, use
of a salt of Eu+3 may be used to produce Eu+3 in a red phosphor,
using heat if necessary, see, for example, figure 3. It will
be appreciated that the use of heat or otherwise represents a
balance between the speed of dissolving of the dopant salt and
the oxidation of ions prone to oxidation. If oxidised ions
are required for doping or if the required ions are stable
against oxidation, then heat may be applied until the dopant
salt has been dissolved in the solvent. Other possibilities
include using Tb ions for a green phosphor and other
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Lanthanides such as Dy. A Dy doped powder may be useful in
Thermoluminescent Dosimetry applications, for example,
radiation detection badges.
5 Other phosphors can be made by adding several different
desired dopants to the starting CaClz solution; each dopant
being selected according to a desired emission wavelength.
Alternatively, once phosphor powders having respective dopants
have been produced, they can be mixed to produce a phosphor
10 having various light emission wavelengths.
A prolonged heating step at a high temperature of, for
example, 1000 °C, for several days can reduce any given ions to
ano~her desired state. The initial dopants, annealing came
and temperature may be used to vary the different dopant and
ionisation states that may be present simultaneously after
annealing thereby enabling a mixed light output to be
realised. For example, a phosphor having blue 435 nm + red
600 nm output characteristics can result from Eu+3 initial
dopant ions partially reduced to Eu+2. Eu" can be converted to
Eu+2 by annealing for approximately 3 days at a temperature of
1000 °C, as illustrated in figure 4.
The grain size of Eu doped CaFz can be selected by performing
the reaction with the hydrofluoric acid in an ultrasonic field
and by varying the concentrations of the reactants. This
permits the formation of very small, 5 nm to 10,000 nm,
crystals of excellent cubic crystal structure. The preferred
grain size varies according to the application of the
phosphor. For example, for neutron/gamma discrimination in
CASPAR the grain size should be preferably approximately 800
nm.
An embodiment of the present invention used 48~ hydrogen
fluoride, 5M CaCl2 solution and an ultrasonic field of 25 kHz
of sufficient power to produce cavitation. A preferred
embodiment used 500 watts to produce a grain size of 100 nm.
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The radio purity of the resulting Eu doped CaF2 phosphor can be
improved by extracting actinide impurities using chelating
agents . The levels of U, TH and K are reduced to improve the
radio purity of the resulting phosphor.
Further modifications to the present invention to improve the
purity, grain size and light output of the resulting Eu doped
CaF2 may result by varying the following parameters:
i) the proportion of doping materials used;
ii) the proportion of water/acetic acid solution or other
solvents needed to dissolve the starting materials;
iii) the concentration of CaCl2 and its pH;
iv) the relative quantity of concentrated hydrogen fluoride
used;
v) the speed and type of mechanical stirrer used in the
reaction;
vi) the use of ultrasound;
vii) the washing and drying techniques employed including the
centrifuge techniques, the use of a vacuum oven, the use of
distilled water as the wash and
viii) the annealing technique, including the temperature, time
and atmosphere used.
In summary, the material concentrations, speeds of
reaction/stirring and the use of ultrasound may be used to
control the CaF2 grain size. The proportion of doping
materials, water and acetic acid solution and annealing can be
varied to influence the colour and magnitude of light output
through efficiency and valency of dopant ions. It will be
appreciated that the annealing affects the quality of the
powder as it determines the amount of sintering of grains.
The washing and drying techniques affect the quality of the
powder and also its optical properties through the
introduction of impurities into the powder.
Advantageously, the light-output power of calcium fluoride
phosphor doped with Eu is substantially five times the light-
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output power of crushed bulk CaFz. The increase in
luminescence arises from the rapid production of small grain
sizes in the solution.
A still further advantage of the present invention resides in
the production of uniform mono-crystals produced during
precipitation on the addition of hydrogen fluoride. Still
further, since the reactants are in solution, they can be
purified in advance.
Without wishing to be bound by any theory it is thought that
doping with other ions should be possible using the present
invention providing they are soluble and stable in water or
some other sol~Tent for short periods and their ionic radii and
charge are similar to Ca+z.
Referring to figure 1 there is shown the
photoluminescence spectra 100 of raw CaFz(0.5~M Eu*z)
precipitate and annealed CaFz(0.5gM Eu+z) precipitate according
to embodiments of the present invention. It can be seen from
the plot 110 for the raw CaFz(0.5$M Eu+z) as compared to the
plot 120 for annealed CaFz(0.5~aM Eu+z) that the latter has a
significantly greater photoluminescence.
With reference to figure 2, there is shown
photoluminescence spectra 200 for annealed CaFz(1$M Eu+z)
according to an embodiment of the present invention as
compared to bulk crushed CaFz (Eu) and undoped CaFz. It can be
seen from the photoluminescence plot 210 for the CaFz (1~SM Eu+z)
that the light output intensity is significantly greater that
the light output intensities 220 and 230 of bulk crushed
CaFz(Eu) and undoped CaFz respectively.
Figure 3 illustrates photoluminescence spectra 300 for
annealed CaFz(Eu+3) according to an embodiment of the present
invention. It can be from the photoluminescence plot 310 that
there is a strong 610 nm emission.
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Referring to figure 4, there is shown the
photoluminescence spectra 400 for sintered CaF2(Eu+3) according
to an embodiment of the present invention. It can be seen
from the plot 410 that the intensities of the emissions 420 at
610 nm have been reduced and the intensities of the emissions
430 at 435 nm have been increased as compared to the
photoluminescence spectra shown in figure 2.
