US3673450A

US3673450A - Electroluminescent techniques and devices

Info

Publication number: US3673450A
Application number: US7157A
Authority: US
Inventors: Sam L Leach
Original assignee: Spectra Tech Corp
Current assignee: Spectra Tech Corp
Priority date: 1970-01-30
Filing date: 1970-01-30
Publication date: 1972-06-27
Anticipated expiration: 1989-06-27
Also published as: CA925219A

Abstract

New electroluminescent techniques and devices are disclosed which render a product that is more commercially saleable, reliable, and economically feasible than heretofore was the case in the art. A stacked, multi-layered electroluminescent device is generally provided, comprising a first supporting layer or sheet of Mylar, for example, a front electrode that is transparent or at least translucent to visible light, a phosphor-filled layer of dielectric material, an additional dielectric layer if desired, and a rear electrode plate. Disposed in intimate contact with the front electrode and preferably between the electrode and the phosphor-filled layer is a web of electrically conductive filaments to obviate deleterious effects of electrode separations. The provision of a photo-sensitive electrically conductive material as the rear electrode is contemplated, which material can be photographically exposed and then developed so as to produce any desired pattern or configuration of electroluminescence. The phosphor-filled layer itself is contemplated to comprise a chemically inert layer of polypropolyene, for example, this layer being made electrically conductive by the insertion of a plurality of electrically conductive particulates dispersed throughout so as to thereby effectively control the dielectric constant thereof. Desirably, the electrically conductive particulates comprise phosphor particles novelly processed to include an adsorbed layer in the form of a glass coating thereon, the glass coating itself being provided with a discontinuous layer of stannous oxide.

Description

United States Patent Leach [451 June 27, 1972 [54] ELECTROLUMINESCENT 21 Appl. No.: 7,157

Primary Examiner-Herman Karl Saalbach Assistant Examiner-Wm. H. Punter Attorney-Jacobi, Lilling & Siegel [57] ABSTRACT New electroluminescent techniques and devices are disclosed which render a product that is more commercially saleable, reliable, and economically feasible than heretofore was the case in the art. A stacked, multi-layered electroluminescent device is generally provided, comprising a first supporting layer or sheet of Mylar, for example, a front electrodethat is [5 2] [1.5. CI. ..3l3/108 A, 96/36.1, 117/215 transparent or at least translucent to visible light, a phosphor- [51] Int. Cl. ..H0l j 1/62 filled layer of dielectric material, an additional dielectric layer [58] Field of Search ..3l3/108, 108 R, 108 A; 96/49, if desired, and a rear electrode plate. Disposed in intimate 96/36.1; 29/572; 117/21 1, 215 contact with the front electrode and preferably between the electrode and the phosphorfilled layer is a web of electrically References Cited conductive filaments to obviate deleterious effects of electrode separations. The provision of a photo-sensitive electri- UNITED STATES PATENTS cally conductive material as the rear electrode is contem- 3,475,640 lO/1969 Litank ..313/108 p which material can be Photographically exposed and 3,368,099 2/1968 Arnold... ...313/108 then developed so as to produce any desired pattern or con- 3,310,703 3/1967 Brooks 313/108 figuration of electroluminescence. The phosphor-filled layer 2,885,560 5/1959 Destriau 313/108 itself is contemplated to comprise a chemically inert layer of 3,376,453 4/1968 Leach 313/108 polypropolyene, for example, this layer being made electri- 3,371,243 2/ 1968 Braml'ey. 315/169 cally conductive by the insertion of a plurality of electrically 3,379,927 4/1968 Yando ..315/55 conductive particulates dispersed throughout so as to thereby 3, 9 7/ R mm 117/215 effectively control the dielectric constant thereof. Desirably, 2,972,303 1961 Ko y..-- the electrically conductive particulates comprise phosphor 2,772,160 1 1/1956 p 49 particles novelly processed to include an adsorbed layer in the 3,005,708 10/1961 l-lesse ..96/36.l f f a glass coating thereon, the glass coating it lf being 2,873,189 2/1959 Evans ..96/36.1 provided with a discontinuous layer f Stannous i 2,756,167 7/1956 Barnett ..96/36.l

11 Claims, 4 Drawing Figures f IO vise l [2 q k 7 22 1 ELECTROLUMINESCENT TECHNIQUES AND DEVICES BACKGROUND OF THE-INVENTION This invention generally relates to electrolurninescence and particularly concerns novel electroluminescent techniques and structure contemplated to provide a more commercially saleable, reliable, and economically feasible electroluminescent device than heretofore was possible in the art.

A particularly promising area of environmental utilization for electroluminescent devices is to be found in the advertising industry specifically as concerns the employment of electroluminescent devices in advertising displays, signs and the like, and as further concerns the replacement of commonly utilized neon-tube illuminating devices with electroluminescent apparatus. To date, however, the state of the art, particularly when viewed from a commercial standpoint, has not been able to provide the technology by which the use of electroluminescent devices in such environments would be feasible.

This failure is not at all surprising when one considers the peculiar requirements in this contemplated environment. For example, an electroluminescent device that is desired to be utilized in an out-of-doors display would be subjected to thermal shocks," and bending or ilexure stresses as well as to excessive amounts of moisture and other efficiency degrading substances. Specifically, the electroluminescent devices now generally available typically comprise a plurality of stacked adjacent layers formed of difierent materials having different coefficients of expansion, and when a device of this type is subjected to thermal shock, the various layers expand or contract in differing amounts and tend to pull apart. More im portantly, the different degrees of expansion oftentimes creates cracks or area separations of the very thin transparent front electrode which is employed in these devices, thus rendering the devices inoperative.

Further, the typical layered-type of electroluminescent device is highly succeptable to moisture and vapor degradations and, as such, such devices cannot feasibly be utilized in high moisture environments such as the out-of-doors. As is known, in devices of this type, free zinc tends to be liberated from phosphor particles and/or the phosphor, and this zinc is available for organic reaction with the dielectric layer. This reaction results in a darkening of the dielectric in the area of the reaction thereby decreasing the effective light output of the unit. Additional problems concerning the light output arises in connection with the destructive effect of reactable oxygen with the phosphor particles themselves. If moisture is permitted within the area of the phosphor particles, then, due to the ionization which takes place within the device during normal operation thereof, oxygen from the moisture or water particles tends to become free for reaction with the phosphor. The oxygen reacts photochemically with the phosphor grain, thereby darkening the same.

Electroluminescent devices desired to be utilized in advertising displays, for example, generally would have to generate some predetermined pattern or display configuration to meet the requirements of the user. For example, if electroluminescent devices were desired to be utilized as replacements for neon lettering, the electroluminescent device in and of it self must serve to generate the required letter or letters. Furthermore, esoteric advertising effects which can presently be effected with neon tube techniques must also be reproducable by the electroluminescent replacement, yet, the current state of the art has not produced any such electroluminescent device.

SUMMARY OF THE INVENTION Thus, as should be apparent, a need exists for electroluminescent technology which will obviate the aforementioned disadvantages of the currently available electroluminescent devices and specifically which will enable the utilization of electroluminescent devices as advertising displays and the like. It is the primary object of the instant invention to provide the technology to satisfy this need. However, it should be understood at the outset that, while this invention is primarily directed to the adverfising industry, each of the techniques disclosed herein have general widespread applicability.

It is an additional, though more specific, object of the instant invention to provide an electroluminescent device which is substantially free from the degrading efiects of moisture and the like.

It is a further object of the instant invention to provide an electroluminescent device which can be readily constructed in an economical manner and which will generate predetermined patterns and displays of illumination.

It is another object of the instant invention to provide a new manufacturing technique as well as materials for the phosphor-filled layer of the stacked, multi-layer electroluminescent device, which materials serve to increase the light output of the device, increase the lifetime of the device, and render the device suitable for construction by mass-production techniques.

A further advantageous object of the instant invention concerns the provision of an electroluminescent device of the type described which is capable of withstanding thermal stresses and shocks as well as bending and flexure stresses that normally would tend to render a device inoperative by creating cracks or area separations of the thin light-transmitting front electrode provided.

Now, these objects as well as other objects which will become apparent as the description proceeds are implemented by the instant invention which, from a general configurational format, will be seen to comprise a multi-layer unit including a first supporting layer or sheet of Mylar, for example, upon which the remaining layers of the stacked, multilayer device is constructed. A light-transmitting, such as at least a translucent though preferably transparent, front electrode is then provided, this electrode being constructed of materials such as vaporized gold, silver or aluminum having typical thicknesses of 300 400 angstroms. Of course, other electrode materials could be utilized if desired, one such material being micronsized glass spheres, the glass spheres being rendered electrically conductive by the provision of a metalized coating thereon.

Beneath the light-transmitting front electrode, a phosphorfilled dielectric layer is provided, the dielectric layer being formed of a translucent dielectric material which, in the prefered inventive embodiment, will be seen to comprise a chemically inert material such as polypropolyene, for example, this layer being electrically conductive by means of the random dispersion therein of micron-size conductive particulates such as copper, silver, aluminum and the like. Underneath the phosphor-filled layer, as additional dielectric layer may be provided, if desired, and, subsequently, a rear electrode completes the stacked package.

As briefly alluded to above, one important feature of the instant invention concerns the particular materials utilized in the phosphor-filled dielectric layer and specifically, the construction of this layer in and of itself. As mentioned above, a specific deficiency of prior art devices concern their suseptability to the degrading etfects of moisture and particularly the resultant chemical reaction which takes place between the free zinc liberated from the phosphor particles and the surrounding dielectric layer. The instant invention proposes to utilize a chemically inert material as the dielectric layer, this chemically inert material preferably comprising polypropolyene although Teflon and polyethylene have also been found to be suitable.

Yet, a chemically inert dielectric material such as the materials under consideration, although eliminating the deleterious chemical reactions, is not adequately electrically conductive material, and, accordingly, does not inherently serve to distribute the ionic current throughout the phosphor dispersed therein. The instant invention, however, contemplates the elimination of this disadvantagous feature while still retaining the advanatages associated with chemical inertness by providing a random dispersion of micron-size conductive particulates of such material as copper, silver, or aluminum, for example, throughout the chemically inert dielectric. Accordingly, a psuedo dielectric is formed in which the phosphor particulates are dispersed, this psuedo dielectric having outstanding capabilities and physical characteristics and having a controllable dielectric constant.

In an alternative form of the dielectric layer; the need for separate electrically conductive particulates randomly dispersed throughout is obviated, as the phosphor particles themselves are contemplated to provide the necessary electrical conductivity for proper electroluminescent operation. With this aspect or form of the instant invention, the phosphor particles are contemplated to be treated in a particular manner such that they can be coated with a glass layer resulting in phosphor particles having a substantially monomolecular glass coating rendering the particles insensitive to the effect of environmental moisture or vapor. In this respect, the instant invention contemplates an improved processing treatment for the phosphor particles which comprises extruding a blend of the same with a particular glass frit which melts at a temperature of between 800 900 F. and is free of lead or cadmium which would react with the sulfide in the phosphor grains to form an undesirable chemical reaction, a polyethelene resin or a like being utilized as a binder during the extrusion procedure. The rod or wire thus produced is fed through a plasma or flame gun to spray the same into a collector thereby melting the glass and coating the phosphor particles with the desired substantially monomolecular layer.

The glass covered phosphor particles are further contemplated to be sprayed with a stannous chloride material which converts to form a partially conductive coating of stannous oxide. Alternatively, since the particles eminating from the flame or plasma gun are at the desired temperature, they may be sprayed directly into an atmosphere of stannous chloride whereby the partial coating of conductive stannous oxide will be simultaneously effected.

The glass coated, partially conductive, stannous oxide coated phosphor particles so formed in this manner may then be mixed with a suitable dielectric material, such as the chemically inert polypropolyene discussed above, to form the phosphor-filled dielectric, the conductive coating on the glass particles accordingly eliminating the need for-the insertion of additional conductive particulates into the chemically inert dielectric material. As should be apparent, the dielectric constant of the material utilized can be varied as necessary either by the insertion of the additional conductive particulates or by the utilization of coated phosphor particulates as above described.

In accordance with yet another important aspect or embodiment of the instant invention, a web or mat of conductive filaments is contemplated to be placed in intimate contact with the light-transmitting front electrode. The web or mat, in the preferred inventive embodiment, comprises minute micronsize filaments constructed of conductive material such as copper, aluminum, silver, stainless steel, or glass or quartz having a coating of stannous oxide, these filaments being randomly dispersed preferably between the front electrode and the underlying phosphorfilled dielectric layer. As discussed above, thermal shocks and bending stresses oftentimes create cracks or area separations within the thin transparent front electrode, these cracks or separations rendering the device unusable. However, the random dispersion of conductive filaments contemplated herein eliminates this problem for, if a crack or area separation in the thin front electrode should occur, the conductive filaments themselves will bridge the gap and insure the continued distribution of elec trical voltage potential throughout the desired lighting area. This aspect of the invention in and of itself represents a significant advance in the art and can provide an electroluminescent device suitable for use in aggravated environments.

7 In accordance ,with yet another particularly advantageous aspect of the instant invention, the rear or back electrode of the device is contemplated to be constructed of a photo-sensitive yet electrically conductive coating. This coating is contemplated to be exposed to electromagnetic energy such as light, for example, which may be transmitted through a screen or a negative so as to impinge upon the photo-sensitive coating in any desired pattern or configuration. The photo-sensitive coating is then developed by the application of suitable developing chemicals which results in the removal of all except the desired portions of the rear electrode. Accordingly, complex area designs can readily be created on the rear electrode of the electroluminescent device causing the device to generate light only in desired areas and in any configuration or pattern. This feature of the instant invention widely expands the field of use of electroluminescent devices perse as, for the first time, any pattern or light generating configuration canbe created at will, economically, and without requiring prefabrication or physical cutting of rear electrode patterns.

BRIEF DESCRIPTION OF THE DRAWINGS The invention itself will be better understood and further advantageous features thereof will become apparent from the following detailed description of a preferred inventive embodiment, such description making reference to the appended sheets of drawings, wherein:

FIG. 1 is a perspective view, partially in section and partially broken away for a illustrative clarity, depicting the overall constructional features of the instant invention;

FIG. 2 is a perspective illustration, also partially in section, depicting the novel process disclosed herein whereby, through photographic techniques, the rear electrode of the device can be etched such that the electroluminescent device will radiate or generate light in any desired display, configuration or pattern;

FIG. 3 is a schematic, block diagram illustration of a novel process disclosed herein whereby the phosphor particles utilized in the phosphor-filled dielectric layer are coated with glass and are subsequently made conductive by means of a coating of stannous chloride, these particles being contemplated for utilization in the preferred inventive embodiment in a dielectric layer such as polypropolyene which is chemically inert and which normally is electrically non-conducting; and

FIG. 4 is a sectional view of a phosphor particle with various coatings thereon such as produced by the process of FIG. 3.

DETAILED DESCRIPTION OF PREFERRED INVENTIVE EMBODIMENTS Referring now to the drawing, and particularly to FIG. 1 thereof, a preferred construction of the novel electroluminescent device is illustrated, this device being of the stacked, multi-layer variety. Specifically, the novel electroluminescent device will be seen to comprise a first supporting layer or sheet 10 formed of a substantially moisture-proof, at least translucent or transparent insulating material such as Mylar (polyethylene terephthalate) or Saran (vinylidene chloride), although other obvious insulating layers can be readily substituted therefore. The supporting layer or sheet comprises a major portion of the thickness of the overall device and, in accordance with the preferred constructional technique, is utilized as the starting material in the fabrication of the device.

A front electrode 12 is then provided and can either be sprayed or roller-coated to the supporting sheet 10. The front electrode must be light-transmitting, i.e. at least translucent, though preferably transparent, such that radiant energy generated in the layers immediately below can be transmitted there through. The front electrode 12 is therefore constructed of materials such as vaporized gold, silver, or aluminum, having thicknesses in the range of 300 400 angstroms as is typical in the art. Other electrode materials could also be utilized, if so desired, in accordance with the inventive concepts herein and, in this respect, one such substitutional material could comprise micron-size glass sphere which are rendered electrically conductive by the provision of a metalized coating thereon.

Disposed generally below the front electrode 12 is a phosphor-filled dielectric layer 14 formed as a continuous layer containing the electroluminescent phosphor particles 16. Immediately below the phosphor-filled dielectric layer 14, an additional dielectric layer 18 is provided, the dielectric layer 18 preferably being reflective to light. Finally, a rear electrode structure 20 is provided, formed of an electrically conductive material and, if desired, although not illustrated, an electrically insulating, moisture-proof material or lacquer can be subsequently disposed beneath, and in intimate contact with the rear electrode 20.

With the arrangement described immediately above, when an alternating electrical field (AC) is applied across leads 22 and 24 respectively connected to the transparent front electrode l2 and to the rear electrode 20, a light output is produced through the supporting sheet or layer 10. The electrical field is established between the electrically-conducting

layers

12 and 20, and such field serves to excite the electroluminescent phosphor particles 16 in the phosphor-filled dielectric layer 14. Accordingly, these particles emit light and the light either passes directly outwardly through the

layers

14, 12 and 10, or first rearwardly to the reflective layer 18, and then outwardly through the

layers

14, 12 and 10.

As briefly discussed above, certain aspects of the novel invention are concerned with the particular materials and the construction utilized in and about the various layers of the multi-layered device of FIG. 1. One such inventive aspect contemplates the provision of a web or mat of conductive filaments 26 disposed in intimate contact with the thin, trans parent front electrode 12 and, preferably, between the front electrode 12 and the phosphor-filled dielectric layer 14. With the preferred inventive construction, the web or mat of conductive filaments comprises minute, micron-size filaments constructed of a conductive material such as copper, aluminum, silver, stainless steel, or glass or quartz with a coating such as stannous oxide. These filaments 26 are preferably randomly dispersed throughout the expanse of the electroluminescent device beneath the front, transparent electrode 12.

As discussed above, thermal stresses as well as bending or flexure stresses to which the electroluminescent device is subjected oftentimes will create cracks or area separations in the thin, transparent electrode 12, which cracks or area separations can render the electroluminescent device inoperative or, at the very least, substantially reduce the light generating areas thereof. The conductive filaments 26, however, effectively serves to bridge the gap" between any crack or electrode area separation and thus renders the device capable of withstanding such abusive stresses. The light output from the device is not at all reduced by the provision of the random dispersion of conductive filaments 26 due to the minute size of such filaments and, in the fabrication of the device, these filaments can literally be sprinkled" over the conducting electrode 12 prior to the application of the phosphor-filled dielectric layer 14. While the disposition of the mat or web of conductive filaments 26 has been indicated to preferably lie between the front transparent electrode 12 and the phosphorfilled dielectric layer 14, it should be understood that, from a conceptual standpoint, the prime requirement as to disposition of the conductive filaments is that the conductive filaments be in intimate contact with such front electrodes so as to insure the even disposition of electrical potential throughout the electrode area. The provision of the conductive web or mat 26 in and of itself constitutes a major advance in the art, widely expanding the possible environments of utility of an electroluminescent device.

Consistent with the prime objectives of the instant invention to provide a device suitable for utilization in the advertising industry, the novel inventive device incorporates constructional features by which the device can generate radiant energy in any desired display, configuration or pattern. For example, and again referring to FIG. 1, let it be assumed that one would desire the electroluminescent devices depicted therein to radiate light only the pattern of a letter such as the letter A, for example, as shown in dotted lines, through the supporting sheet or layer 10. To achieve this effect, the instant invention contemplates the provision of a rear electrode 20 constructed not only to be electrically conductive, but to also be photosensitive. In this respect, the rear electrode 20 comprises a photosensitive coating which contains a mixture of electrically-conductive particulates which, in the preferred inventive embodiment, comprise approximately 20 percent by weight of the coating and has a grain size of approximately 3 to 20 microns. The photo-sensitive coating itself is contemplated in the preferred inventive embodiment to comprise cellulose acetate or vinyl methyl-ether/maleic-anhydride copolymer sensitized with a diazo which itself may comprise and ester-amide of 2- diazol-naphthol-S-sulfonic acid.

The manner or technique by which the electroluminescent device of FIG. 1 is caused to generate only selective patterns of light can best be seen by reference to FIG. 2 wherein the multi-layered device is again illustrated in perspective view and wherein like layers and components have identical reference numerals as those utilized with respect to the description of the device in FIG. 1.

Initially, a pattern or negative such as a mask 28 having a cutout portion 30 in the shape of the desired letter A, for example, is disposed between an optical system comprising a source of light 32 and an associated lens 34, and the rear electrode 20 of the electroluminescent device itself. Radiant energy as schematically illustrated by lines 36 impinge from the light source 22 onto the rear electrode 20 in the pattern or configuration defined by negative or cut-out portion 30 of the mask 28. In this manner, the photo-sensitive rear electrode 20 is exposed in the desired pattern or configuration of subsequent light generation. The non-exposed portions of the rear electrode 20 are then chemically-developed and removed so as to actually form the depicted pattern of the letter A. In accordance with the preferred inventive techniques, the chemical developer utilized is contemplated to comprise, by volume, approximately 50 ozs. of gum arabic, 30 ozs. distilled water, and 20 ozs. of methanol. During the developing procedure, the undesired electrode area is removed and the remaining portions of the electrode covered with a lacquer, which is filled with conductive particulates. Alternatively, the chemical developer could itself incorporate the lacquer and, in the instance, the chemical developer is contemplated to comprise by volume, approximately 60 ozs. of ethylene dichloride, 20 ozs. of epoxy resin, and 20 ozs. of electrically-conductive particulates, which particulates assist in the distribution of current and potential throughout the remaining portion of the rear electrode 20. The epoxy resin of the developer itself could comprise an unmodified hydroxl containing resinous glycidyl polyether of a dihydric phenol. As a further alterna tive, and from a more general standpoint, the chemical developer incorporating the lacquer could comprise, by volume, 60 ozs. of ethylene dichloride, 20 ozs. of electricallyconductive particulates, as well as 20 05. of polyvinyl alcohol, vinyl acetate or emulsions of both vinyl and acrylic. Of course, other suitable chemical developers and the like will be apparent to those of ordinary skill in the art.

The phosphor-filled dielectric layer 14 itself constitutes an important aspect of the instant invention significantly contributing to the overall efi'rciency and reliability of the novel device in its field of intended use. The phosphor-filled dielectric layer, as mentioned above, includes the phosphor particles 16 carried within and randomly dispersed throughout a dielectric matrix 38 such as is shown in FIG. 1. The dielectric material 38 must be translucent, or transparent for optimum efficiency and, this layer should preferably be chemically inert so as to make the overall device less susceptible to the degrading effects of moisture, and particularly to eliminate the possibilities of chemical reactions which take place in ordinary devices between free zinc liberated from the phosphor articles and the surrounding dielectric layer. In this respect, the instant invention contemplates to utilize as the dielectric layer a chemically inert material preferably comprising polypropylene although Teflon and polyethylene have also been found to be suitable.

Yet, a chemically inert dielectric such as the polypropylene contemplated for use is not a good enough conductor of electricity to insure the proper ionic current flow throughout the layer enabling operation of the device. Accordingly, the instant invention contemplates the provision of a random dispersion of micron-size conductive particulates 40 throughout the dielectric layer, these conductive particulates rendering the chemically inert dielectric material sufficiently electrically-conductive. Suitable materials for utilization as the conductive particulates have been found to comprise copper, silver, aluminum and the like although other metallic ingredients could be substituted therefore.

As an alternative to the utilization of separate electricallyconductive particulates 40 throughout the chemically inert dielectric layer 14, the phosphor particles 16 can themselves be provided with a partially-conductive discontinuous coating of metallic material. However, before an explanation is set forth as to the manner of producing such an electrically conductive discontinuous coating 42 on the phosphor particles 16 as shown in FIG. 4, a further feature of theinvention will be discussed, this feature being concerned with the provision of a coating on the phosphor particles intermediate the phosphor particles and the electrically-conductive material, which coating is moisture-proof and vapor-proof thereby rendering the particles insensitive to deterioration in this respect and acting in cooperation with the chemically inert dielectric material 38 to reduce the overall degrading effects of moisture.

To this end, the phosphor particles 16 may be provided with a coating of methylchlorosilanes or colloidal silicate, but according to a preferred embodiment of this invention, the phosphor grains are encapsulated in a substantially monomolecular glass coating. To provide the methyclorosilane coating, the phosphor particles are bathed in a methylchlorosilane solution and then separated and dried, the coating, when adsorbed by the phosphor particles, assumes a polycarbon nature so as to effectively seal any moisture out of the phosphor particles. Moreover, the coating material itself replaces any moisture content in thephosphor particles initially since the phosphor particles have 'a greater affinity for methylchlorosilane than for water, i.e. there is a bi-functional condensation. Additionally, the adsorbed methylchlorosilane coating traps any free zinc within the peripheral limits of the particle so as to again act in conjunction with the chemically inert dielectric to prevent any adverse che micalreaction. The thickness of the coating rovided over the phosphor particles is contemplated to be a minor fractional part of the mean max-- imum dimension of the phosphor particles and, in fact, tends to be of the order of one molecule. I 1

As mentioned above, however, the. preferred intermediate coating between the phosphor particles and discontinuous electrically conductive coating comprises glass as indicated by reference numberal 44 in FIG. 4. The preferred and novel technique for forming such glass-coated phosphor particles is illustrated in FIG. 3 and attention is therein directed. The phosphor particles are initially intimately intermixed in any conventional manner with a frit which must have a melting point of between 800 and 900 F. and be free of lead and cadmium toavoid any color reaction, and a binder which may be in the form of polyethylene or the like, at the station in the block diagram of FIG. 3 indicated by reference 46. This mixtional extruding mechanism as schematically shown at 48 to form an elongated filament or rod which may be stored on reels at station 50 or otherwise prepared in any desired manner for feeding to a conventional flame'or plasma gun designated generally by reference No. 52. A detailed explanation of this apparatus is not necessary since the significant factors to be considered comprise merely the subjection of the elongated rod of phosphor particles, frit and binder to an elevated temperature within the plasma or flame gun'52 prior to the expulsion of the heated mixture from the flame gun in the form of a spray indicated schematically by line 54 which is ing from the plasma or flame gun 52 is approximately in the range of about 900 to l,000 F.

Now, the glass-coated phosphor grains are contemplated to be further processed so as to provide the partial or discontinuous electrically conductive layer 42 referred to hereinabove with reference to FIG. 4. This may be achieved in accordance with a variety of novel techniques as illustrated in FIG. 3. For example, the individually glass-coated phosphor particles may be separately subjected to a stannous chloride atmosphere in a further processing station 58 by passing the same through a spray of stannous chloride, which stannous chloride spray impacts on the glass-coated phosphor at a point in time when the glass coating is still heated in the range of 900 to 1,000 F. Accordingly, the carefully selected glass formulation would react with the stannous chloride to form stannous oxide which is partially conductive as is required to insure electrical conductivity of the chemicall inert dielectric material 38 in which the phosphor particulates are dispersed. The amount of conductivity imparted to the conductive coating 42 on the phosphor particle 16 can be controlled by manipulation of the temperature of the spray, solutionstrength, formulation of the glass, and actual point of impact of the two spray patterns. As

a result, the partial or discontinuous layer 42 can be formed such that its conductivity is sufficient to insure that only an ionic current flow can exist and that a continuous conductive path is not present.

Alternatively, since the spray 54 eminating from the gun is at a desired temperature, the stannous chloride atmosphere may be provided in the collector 56 from a source 60 whereby the discontinuous stannous oxide coating will be concomittantly provided in addition to the glass coating on the phosphor particles.

In any event, the insertion of additional conductive particulates into the chemically inert dielectric layer or the alternative provision of a discontinuous conductive coating on the phosphor'particulates themselves provides the possibility of controlling the dielectric constant of the chemically inert material '38 in any desired fashion to insure the desired amount of ionic current flow and distribution. Thus, the advantages of a chemically inert material with respect to eliminating both the degrading effects of moisture and vapor as well as the darkening effects of undesired zinc reactions can be retained while the normally poor electrical conductivity of chemically inert materials is obviated.

As should now be apparent, the objects initially set forth at the outset to the specification have been successfully achieved. Accordingly,

What is claimed is:

1. In an electroluminescent device of the type including a plurality of stacked layers defining at least a light-transmitting supporting sheet, a light-transmitting front electrode, a phosphor-filled layer, and a rear electrode, the improvement comprising: a separate layer defining a web of electrically conductive filaments dispersed in intimate contact with said front I electrode.

'ture of phosphor, frit and binder is fed through any conven- It has been found that the temperature of the spray 54 eminat- 2. The improvement defined in claim 1, wherein said electrically conductive filaments are randomly dispersed.

3. The improvement defined in claim 1, wherein each filament has a thickness of the order of one micron.

4. The improvement defined in claim 3, wherein said web is disposed between said front electrode and said phosphor-filled layer. I

5. The improvement defined in claim 4, wherein said filaments are constructed of materials selected from the group consisting of copper, aluminum, silver, stainless steel, and glass or quartz with a coating of stannous oxide.

6. In an electroluminescent device of the type including a plurality of stacked layers defining at least a light-transmitting suppor'dng sheet, a light-transmitting front electrode, a phosphor-filled layer, and a rear electrode, the improvement wherein said rear electrode is constructed of a photo-sensitive, electrically conductive coating.

coating are chemically removed thus defining an electrode pattern, and wherein said conductive particulates comprise 20 percent by weight of said photo-sensitive coating and have a grain size of approximately 3 to 20 microns.

10. The improvement defined in claim 9, wherein said photo-sensitive coating comprises materials selected from the group consisting of cellulose acetate or vinyl methylether/maleic-anhydride copolymer sensitized with a diazo.

11. The improvement defined in claim 10, wherein the diazo comprises an ester amide of 2-diazo-l-naphthol-5-sulfonic acid.

Claims

1. In an electroluminescent device of the type including a plurality of stacked layers defining at least a lighttransmitting supporting sheet, a light-transmitting front electrode, a phosphor-filled layer, and a rear electrode, the improvement comprising: a separate layer defining a web of electrically conductive filaments dispersed in intimate contact with said front electrode.

2. The improvement defined in claim 1, wherein said electrically conductive filaments are randomly dispersed.

4. The improvement defined in claim 3, wherein said web is disposed between said front electrode and said phosphor-filled layer.

6. In an electroluminescent device of the type including a plurality of stacked layers defining at least a light-transmitting supporting sheet, a light-transmitting front electrode, a phosphor-filled layer, and a rear electrode, the improvement wherein said rear electrode is constructed of a photo-sensitive, electrically conductive coating.

7. The improvement in claim 6, wherein selected portions of said photo-sensitive coating are chemically removed thus defining an electrode pattern.

8. The improvement defined in claim 7, wherein said photo-senstiive coating contains a mixture of electrically conductive particulates.

9. In an electroluminescent device of the type including a plurality of stacked layers defining at least a light-transmitting supporting sheet, a light-transmitting front electrode, a phosphor-filled layer, and a rear electrode, the improvement wherein said rear electrode is constructed of a photo-sensitive coating containing a mixture of electrically conductive particulates, wherein selected portions of said photo-sensitive coating are chemically removed thus defining an electrode pattern, and wherein said conductive particulates comprise 20 percent by weight of said photo-sensitive coating and have a grain size of approximately 3 to 20 microns.

10. The improvement defined in claim 9, wherein said photo-sensitive coating comprises materials selected from the group consisting of cellulose acetate or vinyl methyl-ether/maleic-anhydride copolymer sensitized with a diazo.

11. The improvement defined in claim 10, wherein the diazo comprises an ester amide of 2-diazo-1-naphthol-5-sulfonic acid.