US3346757A - Electroluminescent lamp having an aluminum electrode, a layer of di-electric material and an aluminum oxide layer disposed between the aluminum electrode and the dielectric layer - Google Patents

Description

Oct. 10, 1967 5. H. A. E. RSSEN I INESCENT LAMP HAVING CTROL AL NUM ELECTRODE, A LAYE LE IC MATERIAL AND AN ALUMIN OXIDE LAYER DISPOSED B EEN THE ALUMINUM ELECTRODE AND THE DIELECTRIC LAYER Filed Oct. 24, 1962 m Quin mm mm VA Nan vm\m New mm mm mm m an an m WM Amy 49w JAN 49 am: A?

lnvervtor'i GUTWlThT H. AE; Diew'ss en mi sg United States Patent 3,346,757 ELECTRQLUMLNESCENT LAMP HAVING AN ALUMINUM ELECTRODE, A LAYER OF DI- ELECTRIC MATERIAL AND AN ALUMINUM OXIDE LAYER DISPOSED BETWEEN THE ALUMINUM ELECTRODE AND THE DIELEC- TRIC LAYER Gunther H. A. E. Dierssen, Mentor-on-the-Lake, Ohio, assignor to General Electric Company, a corporation of New York Filed Oct. 24, 1962, Ser. No. 232,699 4 Claims. (Cl. 313-408) The present invention relates, in general, to electroluminescent devices such as electroluminescent cells or lamps, and more particularly to an insulating antibreakdown or barrier layer therefor.

As known at present, electroluminescent cells or lamps, sometimes referred to as luminous capacitors, comprise in general a thin layer of a field-responsive phosphor sandwiched between a pair of electrically conductive layers or electrodes at least one of which is transparent or is, at least, light-transmitting. When an alternating voltage is applied across the electrodes, the phosphor emits visible light which then escapes through the transparent electrode.

In order to obtain superior operating characteristics, in particular, less susceptibility to arcing and greater efliciency, such electroluminescent cells or lamps have been customarily provided with a thin anti-breakdown or barrier layer of a dielectric or insulating material between the phosphor-bearing layer and one of the electrically conductive electrode layers, usually the non-light-transmissive back electrode layer. By employing for such insulating or barrier layer a material of improved permittivity and electrical breakdown strength as compared with the phosphor layer, the electroluminescence of the cell or lamp is increased since a greater voltage stress can be applied to the electroluminescent phosphor without breakdown. In addition, where the insulating layer employed is of light-reflecting character, good reflection of the light produced by the phosphor is also effected by the insulating layer, thereby producing greater lamp brightness at the light-transmitting conducting surface of the cell or lamp.

The compositions heretofore generally employed for such insulating or barrier layers in electroluminescent cells or lamps have been composed at least in part of a solventpermeable organic or resinous material. Thus, the preferred insulating or barrier layer compositions most commonly employed have consisted of a dispersion of a powdered ceramic such as polycrystalline barium titanate or titanium dioxide in a suitable organic plastic or resin such as, for example, cyanoethyl cellulose plasticized with cyanoethyl phthalate. The insulating layer is customarily formed by coating the non-light-transmitting back electrode of the lamp or cell with a layer of a suspension of the powdered barium titanate or titanium dioxide material in a solution of the organic plastic matrix material, and then drying the wet coating to form the finished barrier layer. I

It is an object of the invention to provide an improved electroluminescent device having a simple, inexpensive and effective insulating barrier layer for permitting the application to the device of electric fields of greater intensity, without likelihood of electrical breakdown, to thereby increase the light-emitting efliciency thereof.

Another object of the invention is to provide an electroluminescent device having an insulating barrier layer of a solvent-impermeable and mechanically stable type which affords greater stability of electrical characteristics with time.

Briefly stated, in accordance with one aspect of the invention, that electrode layer of an electroluminescent device which is separated from the phosphor layer thereof by an insulating or voltage barrier layer of the solventpermeable type such as referred to above, is comprised of a thin sheet or foil of aluminum at least that surface of which, at the interface with the said insulating layer of the device, is anodized to transform it into a thin solventimpermeable insulating layer of aluminum oxide. In accordance with a further aspect of the invention, the anodized surface layer of the aluminum sheet or foil electrode is constituted of a coarsely crystalline and comparatively porous primary layer portion, and a dense secondary layer portion of relatively fine crystallinity filling the pores of the primary layer portion.

Further objects and advantages of the invention will appear from the following detailed description of a species thereof and from the accompanying drawing.

The single figure of the drawing is a sectional view, on a greatly enlarged scale, of an electroluminescent device incorporating the invention.

Referring to the drawing, the electroluminescent cell or lamp there shown is comprised, in general, of the usual layer 1 of phosphor-impregnated dielectric material sandwiched between a pair of electrically conductive layers 2 and 3 which form the front and back electrodes, respectively, of the device. At least one of the electrode layers, e.g., the front electrode layer 2, is transparent or, at least, capable of transmitting light so as to permit the light generated by the phosphor-bearing layer 1 to pass out through the said front electrode layer. The assembly of the phosphor-bearing layer 1 and the two electrode layers 2 and 3 together constitute the electrically active elements of the electroluminescent cell or lamp.

To prevent the ingress of moisture from the atmosphere into the interior of the cell or lamp such as would cause the rapid deterioration thereof, the electrically active assembly of the cell is preferably encased in a thin outer encapsulating envelope 4 of thermoplastic material of high moisture impermeability such as, for example, polychlorotrifluoroethylene fihn, known as Kel F. The encapsulating envelope 4 may be formed of two sheets 5 and 6 of such highly water-impermeable thermoplastic material between which sheets the electrically active assembly of the cell is laminated under heat and pressure, while the space between the sheets is evacuated, to thereby efiect the softening and heat-sealing together of the plastic sheets around their marginal edges so as to encapsulate the electrically active cell assembly. A thin, flexible film 7 of a suitable thermoplastic material such as low density polyethylene, or nylon 6 such as that known as Caplene, is preferably laminated into the cell between the front electrode layer 2 and the plastic top sheet 5 of the encapsulating envelope 4 for the purpose, among other things, of firmly holding the front electrode layer 2 in the place and cementing it to both the underlying phosphor layer 1 and the overlying layer 5 of the encapsulatin-g envelope 4. The electroluminescent cell or lamp is energized by applying a suitable potential such as an alternating voltage, for example, 120 volts cycles AG,

to electrical conductors 8 and 9 connected to the respective electrodes 2 and 3 and sealed through and projecting laterally from the edges of the outer envelope 4. The conductors 8 and 9 are preferably formed of relatively fine mesh wire cloth, for example, 200' to 300 mesh, of suitable electrically conductive material such as copper or Phosphor bronze, for instance, and they may be sealed between the two plastic sheets 5 and 6 at the time they are heat-sealed together to form the outer encapsulating envelope 4. As shown, the conductor 8 is electrically insulated from the back electrode 3 by a small strip 8' of suitable insulating material.

The back electrode layer 3, which customarily is of non-light-transmissive or opaque character, is composed of metal sheet or foil as described more fully hereinafter. The other or light-transmitting front electrode layer 2 may consist of any of the well-known types of light-transmitting electrode layers conventionally employed for such purpose in electroluminescent cells or lamps. For example, it may consist of a thin, light-transmitting coating of tin oxide or other metallic oxide such as indium oxide on a suitable substrate such as glass or light-transmitting plastic sheet, or it way be in the form of electrically conductive paper comprised of glass fibers which are coated with a thin light-transmitting film of electrically conductive material such as, for example, a metal or a metallic oxide, or an indium compound as described more fully in U.S. Patent 2,849,339, Jaffe, dated Aug. 26, 1958, and assigned to the assignee of this invention. As shown, the front electrode layer 2 is of smaller surface area size than the back electrode 3 and the phosphor layer 1 thereon so as to leave a narrow margin of the phosphor layer 1 exposed all around its marginal edges. The phosphor and electrode layers 1, 2 and 3 as well as the plastic layer 7, have a configuration generally corresponding to that of the outer encapsulating envelope 4 which may be of any desired configuration such as rectangular, square, circular, elliptical, or any other shape.

The phosphor-bearing layer 1 may be any of the usual types employed for such purpose in electroluminescent cells or lamps, such phosphor-bearing layers comprising, in general, an electroluminescent phosphor such as, for example, zinc sulfide-zinc oxide with suitable activators such as copper, manganese, lead or silver, dispersed in a suitable dielectric matrix material. As is well known in the art, the dielectric matrix material of the phosphor layer 1 may consist either of a glass frit, or of a suitable organic polymeric material, preferably one having a high dielectric constant such as cyanoethyl cellulose plasticized with cyanoethyl phthalate as disclosed in U.S. Patent 2,951,865, Jaffe et al., issued Sept. 6, 1960, and assigned to the same assignee as the present invention.

In accordance with well-known practice, an insulating barrier layer of dielectric material is interposed between the back electrode layer 3 and the phosphorirnpregnated layer 1 for the purpose, among other things, of imparting higher breakdown strength to the electroluminescent device in order to permit the application thereto of electric fields of greater strength, without electrical breakdown of the device, to thereby produce increased light emission therefrom. The insulating barrier layer 10 is of the well-known type comprising a dispersion of a powdered ceramic such as polycrystalline barium titanate or titanium dioxide in a dielectric matrix material consisting of a suitable organic polymeric material, preferably one having a high dielectric constant such as that referred to above as the dielectric matrix material of the phosphor layer 1 and comprising cyanoethyl cellulose plasticized with cyanoethyl phthalate. The insulating barrier layer 10 is formed by coating the metal sheet or foil back electrode 3 of the lamp or cell with a layer of a suspension of the powdered barium titanate or titanium dioxide material in a solution of the organic plastic matrix material, and then drying the wet coating to form the finished barrier layer.

To prevent the formation of any local electrical short circuits in the cell or lamp during the fabrication thereof, the cell or lamp in accordance with the invention is provided with a solvent-im ermeable supplemental insulating barrier layer 11 interposed between the metal sheet or foil back electrode layer 3 and the phosphor layer 1. For such purpose, the metal sheet or foil electrode 3 is formed of aluminum, preferably full-soft aluminum foil of a thickness of around 2 mils for example, and the supplemental insulating barrier layer 11 is constituted by an anodized surface layer portion of the aluminum sheet or foil electrode 3. As is well known in the art, an anodized aluminum surface layer consists of aluminum oxide which is inherently electrically insulating in character and .4 possesses comparatively good dielectric properties. Moreover, because the anodized layer 11 is formed by a chemical transformation of the surface layer of the aluminum sheet or foil 3 itself, it therefore constitutes an integral part of the aluminum sheet or foil electrode 3 in the form of a tightly adhering film or layer thereon. As a result, it is not subject to separation from the aluminum back electrode layer 3 such as may occur with separate barrier layers 10 coated onto an unanodized aluminum sheet or foil back electrode 3, with resulting deterioration and ultimate failure of the electroluminescent cell or lamp. In addition, because of the improved adherence of such separate barrier layer coatings 10 to an anodized surface 11 of an aluminum sheet or foil electrode 3 as compared to a bare or unanodized aluminum electrode, there is little likelihood of the barrier layer coating 1% separating from the anodized surface layer 11 of the aluminum back electrode 3 such as to cause deterioration of the cell or lamp.

Because of the high temperature stability of anodized aluminum and its non-susceptibility to being softened by heat, and further because of its solvent-impermeable character such as renders it non-susceptible to being softened by the common solvents customarily employed in the coating suspensions which are commonly used to form the phosphor-bearing layers 1 of electroluminescent devices, the use in accordance with the invention of an anodized aluminum surface layer 11 as a component part of a composite dielectric barrier layer in an electroluminescent device entirely eliminates the possibility of any electrical short circuits being formed in the body of the device during its fabrication. As a result, a wider latitude in the choice of manufacturing procedures for the fabrication of electroluminescent devices is afforded through the use of a composite dielectric barrier layer 14 11 according to the invention and comprised at least in part of an anodized aluminum surface layer 11. In addition, the provision in accordance with the invention of an anodized aluminum surface layer 11 on the back aluminum sheet or foil electrode 3 affords the added advantage of increasing the dielectric strength of the barrier layer beyond that of the barium titanate or titanium dioxide barrier coating 10 alone. As a consequence, electric fields of greater strength may be applied to the electroluminescent device without electrical breakdown thereof, to thereby produce increased light emission from the device.

For the purposes of the invention, the anodized aluminum surface layer 11 should be of substantially greater thickness than thevery thin natural skin of aluminum oxide which forms on bare aluminum when exposed to the atmosphere inasmuch as the manufacture of anodized aluminum layers as thin as such natural skins of aluminum oxide and having the requisite uniformity of thickness throughout to assure unifOrrn surface brightness for the finished electroluminescent device, is too critical an operation to control to be practical from a manfacturing standpoint. For this reason, therefore, the anodized aluminum surface layer 11 preferably should have a thickness of at least around /2 micron or so, which is considered to be the lower thickness limit practical for the production manufacture of anodized aluminum surface layers 11 of the requisite uniformity in thickness. On the other hand, the anodized aluminum surface layer 11, like the barrier layer coating 10, should be kept as thin as possible since the thinner it is the greater will be the capacitance of the electroluminescent device and the lower will be the voltage loss across the anodized aluminum layer 11. As a result, a greater proportion of the operating voltage applied to the electroluminescent device will be impressed across the phosphor-bearing layer 1, thus producing maximum light output therefrom. To this end, therefore, the anodized aluminum surface layer 11 should have a thickness preferably no greater than approximately one micron or so, for electroluminescent lamps of maximum brightness operated at volts and 60 cycles. However,

for higher voltage lamps, it may be desirable to employ anodized aluminum layers 11 of much greater thicknesses even ranging up to as high as 25 microns or so.

Because of the added dielectric strength imparted by the anodized aluminum surface layer 11, the dielectric barrier layer coating therefore need not be as thick as that which otherwise would be necessary, in the absence of such an anodized layer 11, in order to impart the desired dielectric strength to the device. Thus, for a barium titanate barrier layer coating 10 such as described hereinbefore, a final dried coating thickness of around 10 to 25 microns or so has been found entirely satisfactory.

While for the purposes of the invention the anodization of the aluminum sheet or foil back electrode layer 3 to form the aluminum oxide surface layer 11 thereon may be carried out by the use of most any of the well-known processes for anodizing aluminum, for best results it is preferred to form the anodized surface layer 11 by a process which results in the formation on the aluminum sheet or foil electrode 3 of a composite aluminum oxide layer comprised of a comparatively porous primary layer portion of relatively coarse crystallinity and comparatively dense aluminum oxide secondary layer portion of relatively fine crystallinity which fills the pores of the porous layer portion to thereby produce a composite aluminum oxide film 11 of good dielectric strength and substantially free of pinholes such as might result in greater electrical losses in the operation of the electroluminescent device or in a non-uniform or grainy lighted appearance thereof. A composite anodized aluminum film or layer 11 of the character referred to above may be produced by a twostage electrolytic anodizing process wherein the bare aluminum sheet or foil 3 the surface of which may, if not sufficiently clean, be suitably cleaned as by immersion in a chromic acid bath for example, is first electrolytically anodized in an electrolyte consisting of an approximately 3% aqueous solution of oxalic acid to thereby produce the porous primary layer of aluminum oxide of relatively coarse crystallinity which constitutes the major portion of the total thickness of the final composite aluminum oxide film 11. The so-anodized aluminum sheet or foil 3 is then subjected to a secondary electrolytic anodization process in an electrolyte consisting of an approximately 10% aqueous solution of boric acid, to thereby form the dense secondary layer of aluminum oxide of relatively fine crystallinity which fills and seals off the pores of the porous primary layer of aluminum oxide. The secondary anodization step is continued until the desired overall thickness of the composite aluminum oxide film 11 is attained on the aluminum sheet or foil electrode 3.

Inasmuch as the anodization of the aluminum sheet or foil electrode 3 by most any of the known aluminum anodizing processes normally results in the formation of an aluminum oxide film (which is inherently electrically insulating) on both sides of the aluminum sheet or foil 3, the electrical connection of the inlead conductor 9 to the anodized aluminum sheet or foil electrode 3 may be made by suitably removing, as by abrading, a portion of the insulating film of aluminum oxide from the outwardly facing side of the aluminum electrode 3 so as to expose the bare aluminum and then positioning the conductor 9 opposite the so abraded surface area of the aluminum electrode so as to be pressed and held thereagainst by the thermoplastic encapsulating sheet 6 to thus effect the electrical connection therewith.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electroluminescent device comprising a layer of aluminum forming a first electrode, a thin transparent layer of conductive material forming a second electrode, a layer of electroluminescent phosphor-impregnated dielectric material between said first and second electrode layers forming an electroluminescent layer, a layer of aluminum oxide tightly adhered to said aluminum layer and disposed between said aluminum layer and said electroluminescent layer, said layer of aluminum oxide being of a thickness greater than the thickness of the natural skin of aluminum oxide which forms on aluminum when exposed to the atmosphere, and an additional layer of dielectric material disposed between said layer of aluminum oxide and said electroluminescent layer.

2. An electroluminescent device according to claim 1 in which said layer of aluminum oxide is of a thickness within the range of approximately /2 to 1 micron.

3. An electroluminescent device comprising a back electrode layer of sheet aluminum, a light-transmitting electrically-conductive front electrode layer, a phosphorbearing layer disposed between the said electrode layers, and an insulating coating of high dielectric constant material on the side of said aluminum sheet electrode contiguous the said phosphor-bearing layer, the said aluminum sheet electrode having an anodized insulating surface layer of aluminum oxide, at its interface with said insulating coating, affording improved adherence of the said insulating coating to the said aluminum back electrode layer, said layer of aluminum oxide being of a thickness greater than the thickness of the natural skin of aluminum oxide which forms on aluminum when exposed to the atmosphere and ranging up to approximately 25 microns.

4. An electroluminescent device as specified in claim 3 wherein the said insulating coating of high dielectric constant material is constituted of barium titanate dispersed in an organic polymeric matrix material.

References Cited UNITED STATES PATENTS 2,866,117 12/1958 Walker et al. 313108 2,929,005 3/ 1960 Lilienfeld 317-230 3,007,070 10/1961 Cargill 313-1081 JAMES W. LAWRENCE, Primary Examiner.

DAVID J. GALVIN, GEORGE N. WESTBY, ROBERT SEGAL, C. R. CAMPBELL, R. JUDD, Examiners.

Claims (1)

1. AN ELECTROLUMINESCENT DEVICE COMPRISING A LAYER OF ALUMINUM FORMING A FIRST ELECTRODE, A THIN TRANSPARENT LAYER OF CONDUCTIVE MATERIAL FORMING A SECOND ELECTRODE, A LAYER OF ELECTROLUMINESCENT PHOSPHOR-IMPREGNATED DIELECTRIC MATERIAL BETWEEN SAID FIRST AND SECOND ELECTRODE LAYERS FORMING AN ELECTROLUMINESCENT LAYER, A LAYER OF ALUMINUM OXIDE TIGHTLY ADHERED TO SAID ALUMINUM LAYER AND DISPOSED BETWEEN SAID ALUMINUM LAYER AND SAID ELECTROLUMINESCENT LAYER, SAID LAYER OF ALUMINUM OXIDE BEING OF A THICKNESS GREATER THAN THE THICKNESS OF THE NATURAL SKIN OF ALUMINUM OXIDE WHICH FORMS ON ALUMINUM WHEN EXPOSED TO THE ATMOSPHERE, AND AN ADDITIONAL LAYER OF DIELECTRIC MATERIAL DISPOSED BETWEEN SAID LAYER OF ALUMINUM OXIDE AND SAID ELECTROLUMINESCENT LAYER.

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