Note: Descriptions are shown in the official language in which they were submitted.
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DISPLAY PANEL BACKPLATE
This application claims the benefit of U. S. Provisional Application;
Serial Number 60/085;122, filed May 12, 1998 entitled EMISSIVE DISPLAY
PANEL, by Ronald L. Stewart and Donald M. Trotter.
FIELD OF THE INVENTION
Backplates for display panels, display panels embodying such
backplates, and methods for producing the backplates.
BACKGROUND OF THE INVENTION
Fiat panel, information displays maybe of an emissive or non-emissive
nature. Emissive displays, such as electroluminescent and plasma displays
characteristically respond to stimulation from an external source. This
stimulation makes or modifies the light that they emit fo present an image for
viewing. Non-emissive displays, such as liquid crystal displays, modulate
light
from an external source.
Both types of displays essentially consist of front and backplates.
Active, structured layers of material between the plates generate, or
modulate,
light.
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2 ..
Transmissive, non-emissive displays require high optical transparency in
both plates. This permits passage of light from an external source. Emissive
displays, as well as reflective, non-emissive displays, also require a front
plate
of high optical transparency to permit passage of light for viewing. Glass
sheets are typically used for front plates in each type of display panel.
The backpiate for an emissive, or a reflecting, non-emissive display,
however, does not need to transmit light. Therefore, it need not be
transparent. Indeed, to enhance the light emitted through the front plate for
viewing, it can be advantageous to have a reflective backplate.
In any display, the front and backplates are sealed together, often at a
relatively high temperature. Further, in many applications, material patterns
on
the two plates must remain in registration over a range of temperatures.
Therefore, it has become common practice to form both plates of the same
material, for example, the same glass. This ensures a good match of thermal
expansions when a seal is made.
The present invention arose in connection with electroluminescent (EL)
displays. Accordingly, particular attention is given to such displays, and to
the
solution of problems in their production. However, the broader applicability
of
the invention to other displays will become apparent.
An electroluminescent display consists of an electroluminescent
phosphor layer sandwiched between two conducting electrodes. At high
voltages, a form of breakdown occurs which causes currents to pass through
the phosphor. As a consequence; the phosphor emits light.
Voltages tend to be quite high, that is, greater than 100 volts. Since the
phosphor layers are quite thin, the electric frelds are very high. To limit
current,
the displays are typically operated on alternating currents by inserting a
dielectric, insulating Payer. Current passes on each half cycle until the
capacitance of the device is charged.
The capacitance is proportional to the dielectric constant of the material
divided by the thickness of the layer. Therefore, with a material having a
high
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dielectric constant, the thickness of the layer can be greater. This is
beneficial
since the thicker layer is less prone to manufacturing defects, such as
pinholes.
Present EL display panels have row and column electrodes arranged
orthogonaliy with respect to each other. These electrodes are connec#ed to
drivers through contact at the periphery of an insulating substrate. Each
pixel,
then, is defined by a row and column intersection.
Traditionally, EL displays have been fabricated on ceramic or glass
substrates. Glass substrates provide the required electrically insulating
characteristics, but the transparency provided by glass is unnecessary in the
backplate of an EL display panel. Also, glasses are generally not sufficiently
refractory to withstand the temperatures involved in material processing.
Consequently, the requirements of an EL display panel are somewhat
different from those of a non-emissive display panel. The active materials are
formed on the backplate, for example by silk-screening, and are not
environmentally sensitive. The front plate essentially acts as a shield
against
damage to the active material, and no accurate registration needs to be
maintained between the plates. With the need for a thermal expansion match
relaxed, the front and backplates may be bonded together with a simple,
compliant, polymer material:
The manufacture of an inorganic, EL display panel typically involves one
of two processes, depending on the thickness of the active material layer. In
one process; a thin film is vacuum deposited on the plate surface, and this is
followed by an annealing step. The other process involves silk-screening a
thick film on the plate and firing to produce an adherent layer. Either
process,
requires that the back plate, upon which the material is applied, withstand a
high temperature, albeit for a.short time. Typically the cycle is about
850° C.
for about fifteen minutes.
Sheets of ordinary glass are not sufficiently refractory to withstand such
processing temperatures. As used herein, "refractory" means that a material is
capable of withstanding a temperature on the order of 850° C. without
undergoing destructive chemical or physical change, or distortion
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The problem just noted with glass has ied to use of high temperature
ceramics, since transparency is not required. For example, tape cast, alumina
sheets have been employed as backplates. Also, vitreous silica has been
proposed. Except for the latter, glasses generally lack the required
refractoriness.
It is difficult and expensive to manufacture either sintered alumina or
vitreous silica sheets. When the sheet size has a diagonal measurement
greater than about 20 cm. (8 inches), the process becomes prohibitively
expensive. Also, such large alumina sheets tend to be insuffrciently flat for
silk-
screening, or other patterning processes. Vitreous silica has a very low CTE.
This makes it difficult to fire a thick film pattern on the sheet without
cracking.
A CTE greater than 40x10-'1°C., and preferably in the range of 40-
70x10''/°C. is
considered necessary.
The desire for larger EL display panels makes it imperative that an
alternative, substrate material be provided. It is a basic purpose of the
present
invention to meet this need: Another purpose is to provide a novel backplate
for an emissive display panel. A further purpose is to provide a backplate for
an EL display panel that is readily produced in relatively large sizes. A
still
further purpose is to provide a backplate for an EL display panel that is
mechanically rugged, and that is sufficiently refractory to withstand a
processing temperature of at least 850° C.
SUMMARY OF THE INVENTION
The invention resides, in part, in a backplate for a display panel
comprising a thin layer of a glass-ceramic that receives the active display
material on its surface, the glass-ceramic being sufficiently refractory to
withstand a processing temperature of at least 850° C., that has a
coefficient of
thermal expansion greater than 40, but not over about 70x10''/°C. and
that has
a predominant crystal phase selected from spines, enstatite, a mixture of
spinet
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and enstatite, alpha-quartz, sapphirine, mixtures of alpha-quartz and
sapphirine, cristobalite, sanbomite and mixtures.
The invention also resides in a display device comprising a display
panel having a backplate as just described.
5 The invention further resides in a method of producing a backplate for a
display panel which comprises melting the precursor glass for a glass-ceramic
that has a crystal phase selected from spine!, enstatite, a mixture of spine!
and
enstatite, alpha-quartz, sapphirine, a mixture of alpha-quartz and sapphirine,
cristobalite, sanbornite and mixtures forming a rigid layer of said glass,
heat
treating the glass layer to produce uniform crystallization throughout the
layer.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawing, FIGURE 1 is a side view of a typical EL
display panel illustrating the present invention, and
FIGURE 2 is a perspective view of an alternative form of EL display
panel in accordance with the invention.
DESCRIPTION OF THE INVENTION
The present invention arose from a need for a refractory material to
replace alumina in the backplate of a display panel. The need was critical for
a
material that could be produced in large sheet form for use in large size
panels.
In particular, panels having a diagonal measurement greater than 20 cm. (8
inches).
It has been found that the need can be met with selected glass-ceramic
materials. These materials may be employed either as a thin sheet of glass-
ceramic as such, or as a coating on a metal base. The invention will be
described, initially, with respect to the free-standing, glass-ceramic sheet
aspect.
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Alumina, the material previously used in producing a backplate, has a
CTE of about 70x10'/°C. Accordingly, display device processing was
geared
to materials having a CTE compatible with alumina. Thus, it was very
desirable that a replacement material for alumina have a CTE of at least about
40x10''1°C. in order to minimize processing changes.
While a backplate need not transmit light, it must be relatively smooth.
This is necessary to permit uniform silk-screening, or other applications, of
the
active components for a display device. Of great importance is refractoriness,
that is, the ability to withstand processing temperature on the order of
850° C.
This refractory requirement essentially eliminates glasses other than fused
silica.
it has been found that certain glass-ceramic composition families are
capable of meeting the requirements for a display panel backplate. Lack of a
need for light transmission permits use of non-transmissive materials for this
purpose. These glass-ceramics can be initially drawn in large, glass sheets.
With careful processing, these glass sheets can then be cerammed, that is,
caused to undergo separation of the glass to form crystalline phases
throughout the glass.
A suitable glass-ceramic composition will be chemically durable and will
be sufficiently viscous at 850° C. to undergo minimal distortion in a
period of
fifteen minutes. The latter is a condition imposed by processing of other
materials in a display device. The precursor glass must be one that lends
itself
to a cost-effective, sheet manufacturing process, such as rolling or float
forming.
Glass-ceramic families of particular interest are non-alkali metal
compositions. They provide CTEs over about 40x10''l°C., have the
required
refractory character as indicated by high strain points, and have high elastic
moduli. These families are characterized by crystal phases of spinet,
enstatite,
a mixture of spinet and enstatite, alpha-quartz, sapphirine, cordierite,
silicates
of the alkaline earth metal and solid solutions of such silicates.
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Glass-ceramic sheets are produced employing conventional ceramming
practice as is welt known in the art. A precursor glass is first hatched,
melted
and formed as sheets. The forming may be by rolling, or by the float process.
The glass sheet is then converted to the glass-ceramic state. This is
accomplished by subjecting the glass to a thermal treatment having a tinie-
temperature, crystallizing cycle appropriate to the particular glass:
The invention is further described with reference to the accompanying
drawing wherein FIGURE 1 is a side view of a typical EL display panel. The
panel illustrates the present invention and is generally designated by the
numera110.
Display panel 10 comprises a backplate 12 and a front plate 14. Front
plate 14 has a transparent electrode 16 applied over its interior surface 18.
Backplate 12 has an electrode 20 applied on its interior surface 22. The
respective electrodes may be electrically conductive films applied as parallel
strips on the plates in an orthogonal pattern with respect to each other.
A layer of dielectric material 24 is applied over the electrode 20 on
backplate 12. A layer of electroluminescent material 26 is applied over
dielectric layer 24 followed by transparent electrode 16 on front plate 14. An
AC current from an external source is supplied across panel 10 between
electrodes 16 and 20 to activate the EL material.
The present invention is concerned only with backplate 12. It is not
concerned with the electroluminescent materials, or with the electrode films,
or
with the means of applying the materials and films. Accordingly, no effort is
made to further describe these matters since adequate information is readily
available in the literature.
TABLE I, below, sets forth composition ranges for two glass-ceramic
composition families that have been successfully tested to produce EL display
panel backplates. Glass-ceramics in family A are characterized by a
predominant, spinet-type, crystal phase. Glass-ceramics in family B are
characterized by a mixture of spinet and enstatite crystal phases. Both
families
are typically free of alkali metal oxides.
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TABLE I
(A) (B)
weight Spinet Spinet-Enstatite
%
Si02 40-?0 35-60
AI2O3 10-35 10-35
Zn0 4-25 0-25
Mg0 0-12 4-30
Ti42 0-10 5-20
Zr02 0-10 0-10
FIGURE 2 illustrates the alternative form that backplate 14 may take. In
this form, backplate 14 comprises a thin strip, or sheet, 30 of a refractory
metal
or metal alloy. A glass-ceramic layer 32 covers both faces of metal strip 30.
The size of strip 30 will depend on the size panel produced. Heretofore,
substrates have been limited to a diagonal measurement of less than 20 cm. (8
inches). It is a feature of the present invention that larger sizes can be
readily
produced employing either a glass-ceramic sheet or a glass-ceramic layer on a
metal base.
In the interest of conserving space and weight, refractory metal strip 30
will be as thin as is consistent with other requirements. Primarily, this will
depend on whether or not the strip must be sufficiently inflexible to resist
bending, or other distortion during subsequent processing. Where distortion is
not a problem, a thin roll of foil up to about 0.01 cm (a few mils) thick
might be
used in a continuous process. The foil would be coated, dried, fired and re-
rolled for storage or shipping.
The coated roll could be cut to size before, or after, further processing.
In such further processing, it is contemplated that the necessary electrodes
and other materials would be applied over the glass-ceramic. This would be in
accordance with conventional procedures now employed on other, commercial
substrates. It is also contemplated that portions of a panel might be
perforated, or otherwise left uncoated, for such purposes as mounting.
Glass-ceramic coating 32 is produced by initially applying a glass
coating over both faces of metal strip 30. The glass is one that is thermally
convertible to a glass-ceramic coating. This means that the glass can be
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uniformly crystallized in situ to a certain degree by thermal treatment at the
glass crystallization temperature.
There are several features that ace key to success of this aspect of the
present invention. First, the glass must form a continuous coating on the
metal
that is essentially defect-free and relatively uniform in thickness. This is
particularly true where the panel is used for image display. There, light
spots,
caused by pinholes or thin spots in the coating, would be especially
detrimental. Another feature is the glass-ceramic character of the ultimate
coating. This is necessary to permit use of temperatures of 850° C. and
higher
in processing.
Finally, both faces of metal strip 30 are coated. It is, of course,
desirable that coefficients of thermal expansion (CTEs) be relatively closely
matched. However, even with a close match, there may be a tendency for the
panel to warp or curl during processing. This, of course, is unacceptable
where image display is involved. With both faces of metal strip 30 evenly and
equally coated, the tendency is for any effect of expansion difference to
occur
equally on both faces and thus cancel out.
Basically, coating 32 is produced by applying a coating of glass
particles, and thermally softening the glass to wet the metal and form a
continuous, adherent, glass coating thereon. The glass is one capable of
being converted to a glass-ceramic state by uniform crystallization in situ
with
thermal treatment. The glass-coated metal strip is then heated at the
crystallization temperature of the glass for a time sufficient to effect the
desired
conversion to the glass-ceramic state.
Suitable precursor glasses include alkaline earth metal oxide silicates,
borosilicates and aluminosilicates. The modifying alkaline earth metals may be
barium, strontium, magnesium, zinc, and/or calcium, either alone or in
combination. Alkali metal oxides are preferably avoided, except as impurities
in glass batch material. These oxides tend to reduce refractoriness, and also
tend to introduce undesired electrical conductivity.
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Certain glass families containing alkaline earth metal oxides have
proven particularly useful for present purposes. One family is the barium
aluminosilicate family; another is the strontium-nickel aluminosilicate
family. A
barium aluminosilicate glass, when converted to the glass-ceramic state, will
5 have primary crystal phases of sanbornite and cristobalite and a minor phase
of BaA12Si208. The composition family will consist essentially in weight
percent
of 20-65% BaO, 25-65% SI02, and up to 15% AIZO3. A strontium-nickel
aiuminosilicate glass-ceramic will contain primary crystal phases of SrSi03
and
Ni~SiOa, and a minor phase of cristobalite. The glass family will consist
10 essentially, by weight! of 20-60% SrO, 30-70% SI02, up to 15% AI203 and up
to
25% NiO.
Another glass famiiy of interest is based on mixed alkaline earth,
borosilicate glasses containing zinc oxide. These glasses will, when
crystallized, have a primary phase of Ba0-2Mg0-2SiO2, or, if a substantial
amount of AI203 is present, hexacelsian. This family consists essentially of,
in
weight percent on an oxide basis,
Si02 25-4.0 Ba0 10-GO
BZC~3 5-30 Mg0 10-35
AI2O3 0-15 Ca0 0-15
AI2p3+Ca0+Zn0 5-20 Zn0 5-20
TABLE I1 sets forth, in weight percent on an oxide basis, as calculated
from the precursor glass batch; the compositions for several different glass-
ceramics having properties that adapt them to use for present purposes.
Examples 1-6 illustrate alkaline earth metal aluminoborates or borosilicates.
Examples 7-10 illustrate alkaline earth metal silicates which may contain
minor
amounts of alumina or zirconia.
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TABLE I!
Ex. SiOz BZO, AIZO$ Bata Mg0 CaU Zn0 Zr02 Sr0 Ni0 F
1 15.0 23.5 0.2 17.9 39.1 0.3 - 2.9 0.3 - 0.8
2 - 25.4 18.6 56.0 - - - - - - 6
3 ~ 31.8 13.2 - 16.5 24.5 - 14.0 - - - -
4 9.6 22.2 32.5 - - 35.8 - - - - -
30.6 12.7 3.8 15.9 23.5 - 13.5 - - - -
6 - 27.0 19.8 29.7 7.8 - 15.8 - - _ -
7 65.0 - 6.9 - - - - - 28.1 - -
8 47.2 ~ - - - - - - 12.1 40.7 - -
9 54.1 - 5.7 - - - - - 23.3 16.8 -
62.7 - 5.3 32.0 - - - - - - -
Example 1
A glass having a composition within~family B of TABLE I was melted and
5 a sheet formed therefrom. The composition, in weight percent, consisted of:
47.1 % Si02, 22.1 % Ai2O3, 16.9% MgO, 1.7% Zn0 and 12.3% Ti02. The glass
sheet was heated to a temperature of 800° C., and held at that
temperature for
one hour to nucleate the glass. The nucleated glass was then heated to a
temperature of 1000° ~C., and held at that temperature for two hours,
to grow
10 crystals on the nuclei. This converted the glass to a glass-ceramic having
a
mixture of Mg-spinet and enstatite crystals. The glass-ceramic had a CTE of
65x10''I°C. and a strain point of 997° C.
This glass-ceramic sheet was substituted for sintered alumina as a
backplate in an EL display panel. Performance was reported to be very
satisfactory.
Example 2
A glass having a composition within family A in TABLE I was melted and
a sheet formed therefrom. The composition, in weight percent, consisted of:
59.3% S102, 19.1 % AI203, 2.5% MgO, 9.0% ZrtO, 2.1 % BaO, 5.0% Ti02 arid
3.0% Zr02. The glass sheet was heated at a temperature of 800° C. for a
period of two hours to nucleate the glass. The temperature was then raised to
1000° C., and held at that temperature for four hours to convert the
glass to a
glass-ceramic. The glass-ceramic had a predominant crystal phase of a Zn-
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rich spinet. The glass-ceramic had a CTE of nearly 40x10-'/°C. and a
strain
point of 910° C. With small changes in the processing conditions, this
glass-
ceramic sheet was successfully substituted for sintered alumina as a backplate
in an EL display panel.
Example 3
This is a comparative example to show the ineffectiveness of a non-
crystallizable glass available from Corning as Code 1737. This glass is alkali-
free, has a strain point of 66fi° C.; and is used as: a pane! glass for
LCD panels.
The glass has a composition, in weight percent, consisting of: 57.7 % Si02,
16.4% AI203, 8.9% 8203, 0.7% MgO, 4.1 % CaO, 1.9% SrO, 9.5% BaO, 0.8%
AS203.
When a sheet of this glass was substituted for sintered alumina in an EL
display panel, the glass warped to such an extent during processing of the
panel that it was considered unacceptable.
Example 4
Type 430 stainless steel panels having a thickness of about 0.09 cm
(0.03fi inches) were obtained. Each panel was grit blasted with 100 mesh
alumina at 80 psi to roughen the surface. The panels were then rinsed with
distilled water and isopropanol, and air dried preparatory to coating.
A glass batch, based on composition 5 in TABLE ll, was mixed in
conventional manner using conventional materials. The batch was melted in a
platinum container for six hours at a temperature in the range of 1400 to
1500°
C. The molten glass was poured onto a steel slab and rolled to form thin,
glass
sheets. The sheets were broken into small pieces, ball milled and sized to
provide a glass powder having a mean particle size of about 8 microns.
A slip was prepared, containing the glass powder, for application to the
stainless steel panels. The vehicle for the slip was prepared by adding 100
grams of a high molecular weight polybutyl methacrylate to 500 ml of liquid
solvent composed of equal parts of amyl and ethyl acetates. The mixture was
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heated at a low heat on a hot plate to provide a solution. The solution was
poured into a Nalgene roller bottle with zirconia grinding cylinders, and 250
grams of glass powder was added. The bottle was closed and rolled at
medium speed on a roller mill for about three hours. The zirconia cylinders
were removed and the slip was de-aired by rolling overnight at slow speed.
The slip was applied by dipping the metal panel in the slip until fully
immersed. 'the slip-coated panel was removed with a continuous, medium
speed pull and air dried for a half hour. This dipping and drying was repeated
several times to provide a weight gain, after drying, of about 32 mglcm2 (200
mg/inchz).
The coated and dried panel was then heated in a muffle furnace at 1
° C.
rate to 500° C. to remove the binder. The coated metal panels were
removed
while the furnace was heated to 925° C. The panels were reinserted in
the
furnace at 925° C. and held at that temperature for one half hour. This
converted the glass to an adherent, glass-ceramic coating on the metal panel.
The coated panel was cooled in the furnace at furnace rate.
Example 5
A substrate for an electroluminescent display panel was prepared in the
manner described above in Example 4. However, the glass employed had the
composition set forth in composition 3 of TABLE Il. This glass is similar to
that
used in Example 4, but omits alumina. The glass-ceramic produced therefrom
had only a single observable crystal phase, Ba0-2Mg0-2Si02, whereas the
glass-ceramic in Example 5 also showed some Mgt-SI02.
Example 6
In this example, a panel substrate was prepared using a sheet of low
carbon, enameling iron having a typical composition, in weight %, of: 0.003 C,
0.20 Mn, 0.008 P, 0.020 S and the balance, 99.7, iron. The metal sheet was
carefully cleaned, and then nitric acid etched to provide a rough surface. The
surface was then given a nickel flash plating to permit ultimate development
of
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nickel oxide. This promotes adherence.of the glass-ceramic coating to the
metal.
A glass batch, based an composition 1 in TABLE II, was mixed in usual
manner, but in relatively large amount. The batch was melted in small units
and homogenized before being introduced into a larger melting unit operating
at about 1400° C. This melting unit had a discharge orifice that
permitted
running a thin stream of molten glass onto water cooled rollers to produce
thin,
glass flakes. The flakes were gathered in plastic lined jars and milled in dry
propanol using zirconia cylinders in a 20:1 glass to zirconia weight ratio.
This
reduced the glass to a powder having a mean particle size in the range of 3-6
~,m.
The glass powder was applied to the metal by electrophoretic
deposition. A DC voltage was employed with an isopropanol bath having water
added to boost conductivity. The positively charged, glass particles in the
bath
are attracted to the negatively charged, metal sheet.
The metal plate, coated with glass powder, is then dried and heated to a
temperature of about 900° C. At this temperature, the glass powder
sinters
and, after a few minutes, forms an impervious glaze layer. An ascharite
crystal
phase (2Mg0-8203) separates out in a very viscous, residual, glassy matrix.
Example 7
A coating slip was prepared as described in Example 4, but using a
glass having the composition set forth as number 10 in TABLE II. The slip was
applied to a stainless steel strip by dipping and drying several times. The
coated metal strip was then heated to a temperature of 1150° C. and
held at
that temperature to thoroughly wet and adhere to the metal surface. The
temperature was then reduced to about 1050° C., the crystallization
temperature for the glass. It was held at that temperature to effect
conversion
to the glass-ceramic state.
The glass-ceramic coating contained a sanbornite crystal phase and a
minor phase of cristobaiite. The panel just described was deemed satisfactory.
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However, the borosilicate glasses described in Examples 4 and 5 appeared to
better wet the steel prior to any crystal formation. This would better insure.
against defects, such as pinholes, in a larger scale process.
Electroluminescent panels have been prepared by applying electrodes
5 and other electroluminescing accessories on glass-ceramic coated substrates
prepared as just described. The panels performed satisfactorily when
operated, thus indicating an adherent glass-ceramic that insulates the metal
substrate and has no adverse effect on the electroluminescent material.
