GB2189076A

GB2189076A - Phosphorescent material for electroluminescent display

Info

Publication number: GB2189076A
Application number: GB08708379A
Authority: GB
Inventors: David Glaser
Original assignee: Cherry Corp
Current assignee: Cherry Corp
Priority date: 1986-04-09
Filing date: 1987-04-08
Publication date: 1987-10-14
Also published as: DE3712004A1; FR2597111A1; KR870010154A; JPS6310687A; GB8708379D0

Description

SPECIFICATION Phosphorescent material for electroluminescent device This invention relates to a phosphorescent material for electroluminescent display. More particularly, it relates to improvements in the phosphorescent layer of a zinc sulfide powder phosphor electroluminescent display panel, such as a matrix display panel or a segmented display panel, especially such a panel intended for operation in the direct current (DC) mode; but also applicable to display panels intended for operation in the alternating current (AC) mode. Electroluminescence is the emission of light from a crystalline phosphor due to the application of an electric field. A commonly used phosphor material is zinc sulfide activated by the introduction of less than one mole percent of various elements such as magnanese into its lattice structure. When such a material is subjected to the influence of an electric field of a sufficient magnitude, it emits light of a color which is characteristic of the composition of the phosphor. Zinc sulfide activated with manganese referred to as a zinc sulfide: manganese or ZnS:Mn phosphor) produces a pleasant yellowish orange centered at 585 nanometers (nm) wavelength. ZnS:Mn phosphors are characterized by high luminance, luminous efficiency and discrimination ratio, and long useful life. Luminance is brightness or luminous intensity when activated by an electric field, and is commonly measured in lamberts, i.e. candelas per pi square centimeters, or in foot-lamberts, i.e. candelas per pi square feet. Luminous efficiency is light produced compared to power consumed by the device, commonly measured in lumens per watt. Discrimination ratio is the ratio of luminance in response to an "on" voltage to luminance in response to an "off" voltage. A wide range of colors can be obtained by substituting or supplementing the manganese with other materials such as copper or alkaline earth activators, or by substituting or supplementing the zinc sulfide with other similar phosphorescent materials such as zince selenide. Phosphor materials can be formulated into a wide variety of electroluminescent configurations to serve numerous functions. In many electroluminescent devices the electroluminescent display is a panel which is divided into a matrix of individually activated pixels (picture elements). Two major subdivisions of electroluminescent devices are AC and DC intended operating mode. In DC configurations, electrons from an external circuit pass through the pixels in the panel. In AC configurations, the pixels are capacitatively coupled to an external circuit. Electroluminescent devices are also made using either powder or thin-film phosphor configurations. Power phosphors are formed by precipitating powder phosphor crystals of the proper grain size, suspending the powder in a lacquer-like vehicle, and then applying the suspension to a substrate, for example by spraying, screening or doctor-blading techniques. Thin-film phosphors are grown from condensation of evaporants from vacuum vapor depositions, sputtering, or chemical vapor depositions. Two configurations to which the present invention has high applicability are the powder phosphor electroluminescent matrix and segmented display panels, intended for operation in the direct current (DC) mode. Matrix display panels can be used for a variety of applications, and in general, can find utility as substitutes for cathode ray tubes (CRTs), wherever CRTs are used. For example, matrix display panels can be used for such applications as oscilloscopes, television sets and monitors for computers. A particularly advantageous application for the matrix display panel is as the monitor for a microcomputer, or personal computer. By avoiding the need for a CRT, an electroluminescent matrix display panel can make a personal computer more compact and thus more easily portable. Segmented display panels find utility for example as alphanumeric displays in such apparatus as digital clocks; pocket calculators; and gasoline pump indicators for price, volume delivered and cost of amount delivered. The use of electroluminescent matrix display panels as monitors for personal computers, and for various other applications, is known. Electroluminescent display panels, however, are subject to various modes of degradation after a period of use, and in due course the panels must be replaced. In U.S. Patent Application Serial No. 752,317, filed July 3, 1985, by David Glaser, there is disclosed a PHOSPHORESCENT MATERIAL FOR ELECTROLUMINESCENT DISPLAY having increased useful life as compared to materials known before that invention. Even the material of that invention, however, has proved to be somewhat subject to "further forming", i.e., progression of the forming process beyond that desirable to cause luminescence, resulting in a lowering of the capacitance of phosphor elements made with the phosphorescent material. Additional reduction in the tendency for "further forming" was found to be desirable.

prior art, for use in an electroluminescent dis-play panel such as an electroluminescent ma- It is, therefore, a purpose of this invention to provide an electroluminescent material for use in electroluminescent display panels, having increased useful life, and in particular, a decreased tendency for "further forming." A phosphorescent material according to the trix display panel, comprises: (a) phosphor particles of a size from about 0.1 to about 2.5 microns, comprising zinc suIfide containing from about 0.1 to about 1.0% by weight manganese; (b) a coating of copper sulfide on the phosphor particles; and (c) a dielectric binder. The present invention provides, as an improvement in such a phosphorescent material, dehydrating the phosphorescent material to less than 6 micrograms of water per gram of total phosphor and binder. Preferably, the amount of water is reduced to below 3, or even 2, micrograms of water per gram of total phosphor and binder. A preferred method of reducing the amount of water to the desired level is freeze drying. Another method, which may be preferable in some cases, is to remove water by the simultaneous application of heat and partial vacuum. Freeze drying, however, has the advantage that water is removed without heating. Heating may degrade phosphorescent elements, for example by causing undesirable chemical reactions to occur. Freeze drying has been found capable of removing water which heat alone cannot remove, without degradation of the phosphorescent elements. It has been found that dehydration to the level indicated results in a phosporescent material which is less subject to "further forming". An electroluminescent display panel according to the prior art comprises: (1) a transparent, flat, electrically non-conductive substrate; (2) at least one anode, applied to one side of the transparent electrically nonconductive substrate; (3) a phosphorescent layer from about 15 to about 40 microns thick, comprising at least one phosphor element, applied to one side of the transparent electrically nonconductive substrate, in electrical contact with the anode, each phosphor element comprising: (a) phosphor particles of a size from about 0.1 to about 2.5 microns, comprising zinc sulfide containing from about 0.1 to about 1.0%, preferably 0.4%, by wieght manganese; (b) a coating of copper sulfide on the phosphor particles; and (c) a dielectric binder; and (4) at least one conductive cathode, each cathode being in electrical contact with a phosphor element. In the case of an electroluminescent matrix display panel, the device comprises: (1) a transparent, flat, electrically non-conductive substrate; (2) a plurality of mutually parallel transparent electrically conductive anodes, applied to one side of the transparent electrically nonconductive substrate; (3) a phosphorescent layer from about 15 to about 40 microns thick, comprising a plurality of mutually parallel phosphor elements, applied to one side of the transparent electrically nonconductive substrate, over the transparent electrically conductive anodes, in a direction oblique to the transparent, electrically conductive anodes, each phosphor element comprising (a) phosphor particles of a size from about 0.1 to about 2.5 microns, comprising zinc sulfide containing from about 0.1 to about 1.0%, preferably 0.4%, by weight manganese;(b) a coating of copper sulfide on the phosphor particles; and (c) a dielectric binder; and (4) a plurality of mutually parallel electrically conductive cathodes, each cathode being applied to a phosphor element. The present invention provides, as an improvement in such electroluminescent display panels or electroluminescent matrix display panels, removing (preferably by freeze drying) any water in the phosporescent material used in the panel, in excess of 6 (preferably 3, or even 2) micrograms of water per gram of total phosphor and binder. An electroluminescent matrix display panel according to the prior art can be made by a method which comprises: (1) applying a plurality of mutually parallel transparent electrically conductive anodes, to one side of a transparent electrically nonconductive substrate; (2) preparing a homogeneous powder of zinc sulfide crystals containing from about 0.1 to about 1.0% by weight manganese, to obtain crystal grains of a size between 0.1 and 2.5 microns; (3) immersing the crystal grains in an aqueous salt solution containing a salt selected from the group consisting of copper chloride and copper nitrate, whereby to effect a surface replacement of zinc with copper and to yield zinc sulfide: manganese particles coated with copper sulfide;(4) mixing dielectric binder with sufficient amount of a thinner to provide the dielectric binder with a viscosity enabling a mixture of the dielectric binder, thinner and the zinc sulfide: manganese particles coated with copper sulfide to be applied to the transparent electrically nonconductive substrate; (5) mixing the mixture of dielectric binder and thinner with the zinc sulfide: manganese particles coated with copper sulfide; (6) applying the mixture of dielectric binder, thinner and zinc sulfide: manganese particles coated with copper sulfide on the transparent electrically nonconductive substrate, over the parallel transparent electrically conductive anodes, and in stripes which are parallel to each other but oblique to the angle of the parallel transparent electrically conductive anodes;(7) evaporating the thinner from the mixture of dielectric binder, thinner and zinc sulfide:- manganese particles coated with copper suIfide, to leave a series of stripes of dielectric binder and zinc sulfide : manganese particles coated with copper sulfide; (8) applying cathodes 17, one cathode over each stripe 15 of dielectric binder and zinc sulfide: manganese particles coated with copper sulfide; (9) passing a sufficient forming current through the cathodes, dielectric binder and zinc sulfide: manganese particles coated with copper sulfide and anodes to render sections of the stripes of dielectric binder and zinc suIfide:manganese particles coated with copper sulfide into a matrix of electroluminescent pixels. The present invention provides, as an improvement in such a process, performing the final indicated step (passing a forming current through the cathodes, zinc sulfide: manganese particles and anodes to form pixels) in a dry air atmosphere, in a dry box; then (10) placing the panel in a chamber to which vacuum can be applied, and into which inert gas can be introduced; (11) lowering the temperature of the panel in the chamber to less than -10[deg]C, preferably less than -30[deg]C, to freeze the water in the panel into ice; (12) applying a partial vacuum to the chamber, through a vacuum conduit provided with a closure valve, to reduce the pressure to less than 25 microns of mercury absolute pressure, preferably less than 12 microns of mercury, to sublime the ice in the panel, and to remove the sublimed ice from the chamber, whereby to remove water from the panel;(13) maintaining the partial vacuum until water no longer is being removed from the panel, typically for approximately 20 to 60 minutes; (14) closing the closure valve on the vacuum conduit; (15) introducing an inert gas, preferably dry helium or argon, into the chamber; (16) sealing a back cap over the anodes, phosphor elements and cathodes, to the substrate, using a low permeation cement, to permanently encase the anodes, phosphor elements and cathodes in the inert gas; (17) testing the seal between back cap and substrate for leaks; and (18) aging the panel by passing current through the cathodes, pixels and anodes to cause phosphorescence under normal operating conditions until pixel response (production of posphorescence in the various pixels in response to current passage) is sufficiently uniform, typically from 1 to 2 hours. The dielectric binder can be organic, such as nitrocellulose; or it can be inorganic, such as tin sulfide or a ceramic material. In the drawings: Figure 1 is a schematic representation, in perspective, of an electroluminescent matrix display panel according to the invention, prior to application of a back cap. Figure 2 is an expanded end view of the electroluminescent matrix display panel of Fig. 1, illustrating detail of its construction, and taken along line 2-2 of Fig. 1; and Figure 3 is similar to Fig. 1, showing application of the back cap. The electroluminescent matrix display panel shown in Fig. 1 is in a position opposite that in which it would be seen by a viewer in actual use. A portion of the panel is shown in Fig. 2 in a position perpendicular to that in which it would be seen by a viewer in actual use. Panel 10 consists of a substrate 11 upon which are deposited, on one side, various components hereinafter described. Those components produce electroluminescence at the interface between those components and substrate 11. The electroluminescent matrix display panel is intended for viewing by observer 12 through substrate 11, along line of sight 13. The general structure and operation of electroluminescent matrix display panels are known in the prior art; see for example E. L. Tannas, Electroluminescent Displays, chapter 8 in E. L. Tannas, Ed., Flat-Panel Displays and CRTs (1984); Vecht, U.S. Patent 3,731,353; Kirton et al., U.S. Patent 3,869,646; and Vecht et al., U.S. Patent 4,140,937. Additional description is given in U.S. Patent Application Serial No. 752,317, filed July 3, 1985, by David Glaser. The following explanation, however, will allow an understanding of the invention without reference to the prior art. Substrate 11 is transparent, flat and electrically nonconductive. The preferred materials for substrate 11 are glasses such as sodalime glass and borosilicate glass. A plurality of mutually parallel transparent electrically conductive anodes 14 are applied to one side of the transparent electrically nonconductive substrate 11. Anodes 14 can be tin oxide or indium-tin oxide. A phosphorescent layer from about 15 to about 40 microns thick, preferably about 25 microns thick, comprising a plurality of mutually parallel phosphor elements 15, is applied to one side of the transparent electrically nonconductive substrate 11, over the transparent electrically conductive anodes 14. The direction of application of the mutually parallel phosphor elements 15 is oblique, and preferably perpendicular, to the transparent, electrically conductive anodes 14. Phosphor elements 15 comprise phosphor particles 16 (see Fig. 2) of a size from about 0.1 to about 2.5 microns; and a dielectric binder. The phosphor particles 16 comprise zinc sulfide containing from about 0.1 to about 1.0%, preferably about 0.4%, by weight manganese; preferably also about 0.05% by weight copper ; and a coating of copper su- Ifide on the phosphor particles. The dielectric binder is, according to one preference, organic, such as nitrocellulose. As noted above, an inorganic binder such as tin sulfide or a ceramic material could also be used. A plurality of mutually parallel electrically conductive cathodes 17, preferably of aluminum, is applied over the phosphor elements 16, each cathode 17 being applied to a phosphor element 16. By indicating that the phosphor elements 16 are applied in stripes, and that cathodes 17 are applied over phosphor elements 16, it is intended to specify the configuration ultimately provided for the phosphor elements 16 and the positioning of the cathodes 17, not the order in which the device is constructed. It is convenient to apply phosphor particles and binder as a sheet and aluminum for the cathodes 17 in another sheet, and then to scribe both simultaneously to form phosphor elements 16 and cathodes 17. As is known in the art, there are also other methods of simultaneously forming individual phosphor elements and electrodes, which can also be used. As part of applying the binder, phosphor and cathodes, steps performed may include trimming the applied binder, phosphor and cathodes to size, if they were applied to a greater area than on which they should remain in the finished panel; and applying bridging links between the cathodes and the terminals to which they are to be connected. Current will later flow between cathodes 17 and anodes 14, first to render sections of the stripes of organic dielectric binder and zinc sulfide: manganese particles coated with copper sulfide into a matrix of electroluminescent pixels 18, and later to cause those pixels 18 to luminesce. The current will flow in the most direct path between the cathodes 17 and anodes 14, i.e. in the square columnar parts of the phosphor elements 15 which are within the squares which are defined at one end by the width of the anodes 14, and on the other end by the width of cathodes 17. Each such square columnar part of phosphor elements 15 is a pixel 18. Each pixel 18 may be caused to luminesce independently, by circuitry known in the art to sequentially address each combination of cathodes 17 and anodes 14 on a time division multiplexing basis. The anodes 14 and cathodes 17 are preferably each spaced about 0.25 millimeter apart, center-to-center, resulting in a density of about 16 pixels per square millimeter, or 1600 pixels per square centimeter. Electroluminescent matrix display panels 10 can be made by: (1) applying a plurality of mutually parallel transparent electrically conductive anodes 14, preferably tin oxide or indium-tin oxide, to one side of a transparent electrically nonconductive substrate 11, preferably soda-lime or borosilicate glass; (2) preparing a homogeneous powder of zinc sulfide crystals containing from about 0.1 to about 1.0%, preferably about 0.4%, by weight manganese, and preferably also about 0.05% copper, to obtain crystal grains of a size between 0.1 and 2.5 microns; (3) immersing the crystal grains in an aqueous salt solution containing a salt selected from the group consisting of copper chloride and copper nitrate, whereby to effect a surface replacement of zinc with copper and to yield zinc sulfide: manganese particles coated with copper sulfide;(4) mixing dielectric binder with sufficient amount of a thinner to provide the dielectric binder with a viscosity enabling a mixture of the dielectric binder, thinner and the zinc sulfide: manganese particles coated with copper sulfide to be applied to the transparent electrically nonconductive substrate; (5) mixing the mixture of dielectric binder and thinner with the zinc sulfide: manganese particles coated with copper sulfide; (6) applying the mixture of dielectric binder, thinner and zinc sulfide: manganese particles coated with copper sulfide on the transparent electrically nonconductive substrate 11, over the parallel transparent electrically conductive anodes 14, and in stripes 15 which are parallel to each other but oblique, preferably perpendicular, to the angle of the parallel transparent electrically conductive anodes 14;(7) evaporating the thinner from the mixture of dielectric binder, thinner and zinc sulfide:- manganese particles coated with copper sulfide, to leave a series of stripes 15 of dielectric binder and zinc sulfide: manganese particles coated with copper sulfide; (8) applying cathodes 17, one cathode over each stripe 15 of dielectric binder and zinc sulfide: manganese particles coated with copper sulfide; (9) in a dry air atmosphere, in a dry box, passing a sufficient forming current through the cathodes 17, dielectric binder and zinc suIfide : manganese particles coated with copper sulfide and anodes 14 to render sections of the stripes of dielectric binder and zinc sulfide: manganese particles coated with copper sulfide into a matrix of electro-luminescent pixels 18;(10) placing the panel 10 in a chamber (not shown) to which vacuum can be applied, and into which inert gas can be introduced; (11) lowering the temperature of the panel 10 in the chamber to less than -10[deg]C, preferably less than -30[deg]C, to freeze the water in the panel 10 into ice; (12) applying a partial vacuum to the chamber, through a vacuum conduit (not shown) provided with a closure valve, to reduce the pressure to less than 25 microns of mercury absolute pressure, preferably less than 12 microns of mercury, to sublime the ice in the panel, and to remove the sublimed ice from the chamber, whereby to remove water from the panel; (13) maintaining the partial vacuum until water no longer is being removed from the panel, typically for approximately 20 to 60 minutes; (14) closing the closure valve on the vacuum conduit;(15) introducing an inert gas, preferably dry helium or argon, into the chamber; (16) sealing a back cap 20 (see Fig. 3) over the anodes 14, phosphor elements 15 and cathodes 17, to substrate 11, using a low permeation cement, to permanently encase the anodes 14, phosphor elements 15 and cathodes 17 in the inert gas; (17) testing the seal between back cap 20 and substrate 11 for leaks; and (18) aging the panel 10 by passing current through the cathodes 17, pixels 18 and anodes 14 to cause phosphorescence under normal operating conditions until pixel response (production of phosphorescence in the various pixels 18 in response to current passage) is sufficiently uniform, typically from 1 to 2 hours. Additional details concerning preparation of the display panel are given in U.S. Patent Application Serial No. 752,317, filed July 3, 1985, by David Glaser, the disclosure of which is incorporated by reference. Application of back caps and testing for leaks are, in themselves, known in the art. Back cap 20 is preferably made of aluminum, applied so as to not cause electrical contact between any of the cathodes 17 and anodes 14. Back cap 20 can also be made of glass. Back cap 20 is sealed over the anodes 14, phosphor elements 15 and cathodes 17, to substrate 11, using a low permeation cement, such as a low outgassing epoxy resin, i.e., a resin which does not generate significant amounts of gas during its curing. A suitable cement is Bacon FA-1 epoxy resin adhesive, an unfilled gyro-grade adhesive sold by Bacon Industries Inc., of Watertown, Mass. and Irvine, California. Testing for large leaks can be accomplished for example by submerging the panel with back cap attached in warm water and watching for bubbles. Small leaks can be detected by placing the sealed panel in a vacuum chamber, applying a partial vacuum within the chamber, and checking the chamber for the presence of the inert gas which was used to permanently encase the anodes 14, phosphor elements 15 and cathodes 17.

Claims

1. A phosphorescent material for electroluminescent display, comprising phosphor particles of a size from about 0.1 to about 2.5 microns, comprising zinc sulfide containing from about 0.1 to about 1.0% by weight manganese, a coating of copper sulfide on the phosphor particles, and a dielectric binder, said material containing less than 6 micrograms of water per gram of total phosphor and binder.

2. A material according to claim 1, comprising less than 3 micrograms of water per gram of total phosphor and binder.

3. A material according to either claim 1 or claim 2, comprising less than 2 micrograms of water per gram of total phosphor and binder.

4. A material according to any one of claims 1-3, wherein the phosphorescent material is dehydrated by freeze drying.

5. A material according to any one of claims 1-3, wherein the phosphorescent material is dehydrated by simultaneous application of heat and partial vacuum.

6. An electroluminescent matrix display panel comprising: (1) a transparent, flat, electrically non-conductive substrate; (2) a plurality of mutually parallel transparent electrically conductive anodes, applied to one side of the transparent electrically nonconductive substrate; (3) a phosphorescent layer from about 15 to about 40 microns thick, comprising a plurality of mutually parallel phosphor elements, applied to one side of the transparent electrically nonconductive substrate, over the transparent electrically conductive anodes, in a direction oblique to the transparent, electrically conductive anodes, each phosphor element comprising phosphor particles of a size from about 0.1 to about 2.5 microns, comprising zinc sulfide containing from about 0.1 to about 1.0% by weight manganese, a coating of copper sulfide on the phosphor particles, and a dielectric binder, in which there is less than 6 micrograms of water per gram of total phosphor and binder; and (4) a plurality of mutually parallel electrically conductive cathodes, each cathode being applied to a phosphor element.

7. A display panel according to claim 6, wherein each phosphor element comprises less than 3 micrograms of water per gram of total phosphor and binder.

8. A display panel according to claim 6, wherein each phosphor element comprises less than 2 micrograms of water per gram of total phosphor and binder.

9. A display panel according to any one of claims 6-8, wherein the phosphor elements are dehydrated by freeze drying.

10. A display panel according to any one of claims 6-8, wherein the phosphor elements are dehydrated by simultaneous application of heat and partial vacuum.

11. A display panel according to any one of claims 6-10, comprising in addition, a back cap sealed over the anodes, phosphor elements and cathodes, to the substrate, with a low permeation cement, to permanently encase the anodes, phosphor elements and cathodes in inert gas.

12. A display panel according to claim 11, wherein the cement is a low outgassing epoxy resin.

13. A display panel according to either claim 11 or claim 12, wherein the inert gas is dry helium.

14. A display panel according to either claim 11 or claim 12, wherein the inert gas is dry argon.

15. A display panel according to any one of claims 11 to 14, wherein the back cap is made of aluminum, applied so as to not cause electrical contact between any of the cathodes and anodes.

16. A display panel according to any one of claims 11 to 14, wherein the back cap is made of glass.

17. A method of making an electroluminescent matrix display panel, comprising the steps of (1) applying a plurality of mutually parallel transparent electrically conductive anodes, to one side of a transparent electrically nonconductive substrate; (2) preparing a homogeneous powder of zinc sulfide crystals containing from about 0.1 to about 1.0% by weight manganese, to obtain crystal grains of a size between 0.1 and

2.5 microns; (3) immersing the crystal grains in an aqueous solution of copper nitrate, whereby to effect a surface replacement of zinc with copper and to yield zinc sulfide: manganese particles coated with copper sulfide; (4) mixing dielectric binder with sufficient amount of a thinner to provide the dielectric binder with a viscosity enabling a mixture of the dielectric binder, thinner and the zinc sulfide: manganese particles coated with copper sulfide to be applied to the transparent electrically nonconductive substrate; (5) mixing the mixture of dielectric binder and thinner with the zinc sulfide: manganese particles coated with copper sulfide; (6) applying the mixture of dielectric binder, thinner and zinc sulfide: manganese particles coated with copper sulfide on the transparent electrically nonconductive substrate, over the parallel transparent electrically conductive anodes, and in stripes which are parallel to each other but oblique to the angle of the parallel transparent electrically conductive anodes; (7) evaporating the thinner from the mixture of dielectric binder, thinner and zinc sulfide:- manganese particles coated with copper sulfide, to leave a series of stripes of dielectric binder and zinc sulfide: manganese particles coated with copper sulfide; (8) applying cathodes, one cathode over each stripe of dielectric binder and zinc sulfide: manganese particles coated with copper sulfide; , (9) in a dry air atmosphere, in a dry box, passing a sufficient forming current through the cathodes, dielectric binder and zinc sulfide: manganese particles coated with copper sulfide and anodes to render sections of the stripes of organic dielectric binder and zinc sulfide: manganese particles coated with copper sulfide into a matrix of electroluminescent pixels; (10) placing the panel in a chamber to which vacuum can be applied, and into which inert gas can be introduced; (11) lowering the temperature of the panel in the chamber to less than 10[deg]C, to freeze the water in the panel into ice; (12) applying a partial vacuum to the chamber, through a vacuum conduit provided with a closure valve, to reduce the pressure to less than 25 microns of mercury absolute pressure, to sublime the ice in the panel, and to remove the sublimed ice from the chamber, whereby to remove water from the panel; (13) maintaining the partial vacuum until water no longer is being removed from the panel; (14) closing the closure valve on the vacuum conduit; (15) introducing an inert gas into the chamber; (16) sealing a back cap over the anodes, phosphor elements and cathodes, to the substrate, with a low permeation cement, to permanently encase the anodes, phosphor elements and cathodes in the inert gas; (17) testing the seal between the back cap and the substrate for leaks; and (18) ageing the panel by passing current through the cathodes, pixels and anodes to cause phosphorescence under normal operating conditions until pixel response is sufficiently uniform.

18. A method according to claim 17, wherein the temperature of the panel in the chamber is lowered to less than -30[deg]C.

19. A method according to either claim 17 or claim 18, wherein the pressure is reduced to less than 12 microns of mercury.

20. A method according to any one of claims 17 to 19, wherein the partial vacuum is maintained for approximately 20 to 60 minutes.

21. A method according to any one of claims 17 to 20, wherein the inert gas is dry helium.

22. A method according to any one of claims 17 to 20, wherein the inert gas is dry argon.

23. A phosphorescent material substantially as described herein.

24. A method of making an electroluminescent matrix display panel substantially as described herein.