US3154712A - Electroluminescent lamp - Google Patents

Description

Oct. 27, 1964 E. c. PAYNE 3,154,712

ELECTROLUMINESCENT LAMP Filed May 19, 1952 United States Patent O arsenic ELESTRGLUNT nSCENT LABT? Elmer C. Payne, Binghamton, NX., assigner to Sylvania Electric iroduets inc., Salem, Mass., a corporation of Massachusetts Filed May 19, i952, No. 233,62@ 7 Claims. (Qi. 3io-NS) This invention relates to lamps in which light is produced by the application of an electric iield to a region including a phosphor. The invention relates also to phosphors particularly suited for such use, and to methods of preparing them.

Electric lamps utilizing phosphors have been previously known. ln one commercial type, the electric iield is applied to a gas and the resultant ultraviolet radiation used to excite the phosphor. ln another, the iield is applied to an evacuated space in which electrons are accelerated and the resultant cathode ray beam used to excite the phosphor. But in neither of these devices is the light produced by the direct application of a suiliciently intense field to the immediate region of the phosphor.

Moreover, both of these devices, like the well-known incandescent lamp, require an enclosing bulb or tube hermetically sealed, with consequent diiliculties in manufacture and limitations in geometry. The devices of my invention are far less limited in such respects, requiring no sealed bulb or tube for their operation, and can be made in practically any convenient size and shape, thus making possible for the first time such new lighting applications as the illumination of a room by a complete ceiling of electrically-luminous plaques.

As in certain specic embodiments of the invention described below, such plaoues `advantageously have a plate of conductive glass in contact with one side of a thin layer of phosphor embedded in a light-transmitting dielectric material, and a conducting layer in Contact with Ithe other side of said phosphor layer.

The device has a positive volt-ampere characteristic and hence requires no ballast. lt can be operated directly from the usual 110 volt, 60-cycle per second power line, although in some cases a transformer may be desirable for higher voltage operation.

The device is in effect a luminous capacitor, and the light produced appears to be due to the action of the electric field on the phosphor, or on the phosphor and the embedding material.

The appearance of a faint glow upon the application of an electric held to a phosphor film appears to have been observed heretofore, but only as an obscure scientiiic phenomenon. Attempts by earlier scientists to produce light of practical illuminating intensities by such means appear to have been unsuccessful, as pointed out by Leverenz in his recently-published book, An Introduction to the Luminescence of Solids (McMillan, New York, 1959, page 290). Leverenz found that excitation of allegedly held-excited phosphor lrns produced only a feeble blue glow, barely perceptible to a partially darkadapted eye, and he attributed the glow to excitation of aerospheres of atmospheric nitrogen trapped in the embedding medium, rather than to excitation of the phosphor itself by the field.

In contrast to this, the light produced by the present invention is not at all the blue glow of atmospheric nitrogen, and can be made blue, green, yellow, red or other color depending on the choice of phosphor. And instead of the faintly perceptible glow of the films examined by Leverenz, my lamps can have brightness as high as foot-lamberts or more. This brightness can be maintained over large areas to yield a very large total light output.

rThese unexpected results appear to be due to direct excitation of the phosphor by the field. I have discovered that useful electroluminescent phosphors generally comprise a host crystal containing not only an activator impurity, but also a donator impurity, the latter providing electrons wmch can be excited into the conduction band of the crystal, with the expenditure of a small energy; and from there can be accelerated by the iield until they have enough energy to excite the activator atoms. For example, one very effective electroluminescent phosphor comprises zinc sulfide with a copper activator impurity and a lead-donator impurity.

Copper-activated Zinc sulde containing lead has been known as an infra-red responsive phosphor, but the amount of lead used for such infra-red phosphors is considerably more than can be used in an eiicient electroluminescent phosphor. The amount of retained lead in infra-red phosphors is greater than the highest amount which gives appreciable electroluminescence. An infrared phosphor does not ordinarily electroluminesce, and an electroluminescent phosphor does not ordinarily give infra-red response.

Once an electron is freed from a donator atom in the crystal, it must be accelerated through a considerable number of atom spacings by the iield. For example, with 600 volts across three-thousandths of an inch layer of phosphor, the two-volt drop necessary for visible emission may require a distance of about 2.5 X10-5 centimeters, or about 2000 atom-spacings. The mean free path in looselypacked crystals such as zinc sullide, under the influence of a iield, may be itself as great as 10-5 or more, so that the mean free path and the required distance of travel may not be greatly different. The electron can travel for a considerable number of mean free paths, if the energy losses at the end of each path are small. Loosely packed crystals such as the sultides, selenides, and silicates of cadmium, zinc, and the like, have large mean free paths and are thus suitable for electroluminescence, when -a suitable activator and donator are present.

When the iield is suciently high, it may produce free electrons from atoms, by shifting electrons directly from a lower band to the conductive band, and the presence of some famt electroluminescence may be obtained under such circumstances without a donator impurity, but the addition of a donator impurity will greatly increase the light emission even in such cases. For example, in one copper-activated, lead donator, zinc sulfide phosphor, the light emission was increased from a barely visible illumination of 3 units to a bright illumination of 460 units by the addition of only 0.001% lead by Weight.

ln the speciiic embodiments of the invention described below, the phosphor is placed between two conductors across which a voltage can be applied. In a preferred embodiment, a thin layer of a dielectric material having me crystalline phosphor particles embedded therein iills the space between two electrically conducting layers, at least one of which is light-transmitting such as a conducting glass or plastic. ln the absence of the embedding materi l, the crystals appear to glow only at their points of contact with a conductor. The presence of the embedding medium greatly enhances the glow, which can be seen under the microscope to spread out over the entire crystal. The phosphor in its embedding dielectric appears to be excited directly by the field.

The response of a particular phosphor to this method of excitation cannot be -redicted from its response to other forms of excitation. Some of the best iluorescent lamp phosphors do not resnond to the electric field, but the zinc sulphide type phosphor can be made very eiective for such purposes. The crystals of one effective zinc sulphide type phosphor appear to have small bumps on at surfaces, and those bumps may be smaller crystals, of zinc oxide aihxed to the main zinc sulphide crystals, or they may be due to some other distor'tl'ng inliuence. If the bumps or projections are zinc oxide crystals, then there may be a very high iield at the interface between these and the main zinc sulphide crystal. And if they are not Zinc oxide, the bumps may still increase the iield because of their distortion of the crystal shape. In any event, the oxide, the activators and other impurities added to the crystal would seem to insure suliicient irregularity inside the crystal to alford the possibility of localized build-up of the iield at some point in the crystal.

' A certain amount of zinc oxide is helpful in the zinc sulphide mixture while the latter is i'lred, presumably because it aids in formation of the proper type and consti-tution of crystals. Nonetheless washing the phosphor with a suitable solvent for zinc oxide, such as a solution ofacetic acid or `of ammonium acetate, improves and multiplies the brightness of such phosphors. Such solvents may also have the eifect of removing surface impurities from the surfaces oi the active crystals, and the latter may actually be the determining factor, but in any case the solvents do remove the excess zinc oxide which is of low resistivity. This has the efiect of increasing the resistivity of the phosphor about 100 times in some cases and the resultant high resistivity improves the phosphor.

The emission from the sulphide phosphors may be a surfaceV eifect, or due to a surface layer between the dielectric medium and the main portion of the crystal, but the great brightness of these high resistivity phosphors may indicate that the iield penetrates well into them and excites a larger portion of the crystal material than would be -the case with phosphors of lower resistivity.

Other features and advantages of the invention will be apparent Vfrom the following detailed description of specific embodiments thereof.

In the accompanying drawings, three devices embodying aspects of the invention areshown, FIG. 1 being a perspective -view partly in section of one such device; FG. 2 being a perspective view of a second device; FIG. 3 being 2in-enlarged cross-sectional View of the device in FIG. 2; FIG. 4 being a plan cross-sectional view of a third device along the line 4 4 in FlG. 5; FIG. 5 being an enlarged cross-sectional view of the device in FIG. 4, along the line 5 5, and FIG. 6 being an energy diagram of a phosphor according to the invention.

VThe device shown in FIG. 1 has a glass plate 1,

having -a transparent conductive surface 2, over which is a thin layer 3 of phosphor-impregnated dielectric material, with a metal backing layer 4 over that and in intimate contact therewith. This completes an illumination source, suitable for use as a luminous plaque for walls and ceilings, for example. One terminal of a prope source of varying or alternating voltage can be connected to the metal backing layer d, the other to a metal tab 5 which is connected to the conducting surface 2.

In a modification of this device the layer 4 can also be of conductive glass, instead of being of metal, thus providing a plaoue which emits light from both sides, and when not energized is translucent. Such a device can be used in various ways, for example in table lamps and otherlighting iixtures or even as a window pane which transmits sunlight by day and emits its own light at night.

A conducting surface 2 of good transparency or translucency is difficult to obtain, because good electric conductors are generally good reflectors of light, rather than Vso transmitters of it. However, although other coatings may be used, I iind that a particularly effective conductive surface lmay be provided by heating the glass and exposing it while hot to vapors of the chlorides of silicon, tin, or titanium, and afterward placing the treated glass in a slightly reducing atmosphere. Where the application in the vapor state is not convenient, good results i may be obtained by mixing stannie chloride with absolute alcohol and glacial acetic acid and dipping the glass plate into the mixture.

However applied, the resultant conductive surface appears to contain stannic (or silicic or titanic) oxide, probably to some extent at least reduced to a form lower than the dioxide, although the exact composition is diliicult to determine.

The conductive surface 2 so applied will have a resistance of about ohms per square, that is a resistance of 100 ohms taken between the entire opposite sides of any square on the surface 2.

TheV phosphor-impregnated layer 3 placed over the transparent conductive layer 2 is a phosphor of copperactivated zinc sulphide as described below, in the form of line particles embedded in plasticized nitro-cellulose.

For example, about 20 grams of nitrocellulose with a suitable plasticizer such as chlorinated diphenyl can be dissolved in about 8O cc. of a suitable solvent such as butyl acetate, and about l0 grams of finely-divided phosphor suspended therein. The plasticizer in the above example is included in the weight of the nitrocellulose, which can be of quarter-second viscosity, although any convenient viscosity can be used, the proportions and constituents of the above mixture being capable of considerable variation. A large number of plasticizers are well known, but for best results those with high resistivity and high dielectric constant should be chosen. The plastieizer may be used in considerable quantity, if desired, and may even comprise the major portion of the combined weight of nitrocellulose and plasticizer, as shown in the application of Eric L. Mager tiled concurrently herewith, and a non-acid dielectric medium, or one of 10W acidity or acid number may be used as shown in that application.

The backing layer 4 is of metal, preferably a good retiecting metal such as aluminum or chromium, which will not react appreciably with the phosphor or embedding material used. The metal layer or conductive surface d is preferably of low resistance and can be applied in any convenient manner, taking care not to damage the cellulose-phosphor layer. However, best results have been obtained by vacuum-deposition of the metal. The glass plate'll, with its conductive surface 2, is coated with the embedded phosphor layer 3, placed in a bell jar and the latter evacuated. The coating 3 is then heated for a moment, for example by passing a current through the conductive surface 2. The heating is preferably ofrthe order of that used for drying, and should not, of course, be sutiicient to char the embedding material 'inphosphor layer 3. The heating is not essential to producing a plaque of good initial brightness, but aids in maintaining the brightness throughout the life of the lamp.V

The aluminum or other metal'is then deposited on the phosphor layer in a vacuum, for example by being placed on a tungsten filament and the latter heated by the passage of an electric current therethrough, as shown for example in U.S. Patent 2,123,706, issued July 12,

1938, to O. H. Biggs.

Various other plastics can be used instead of nitrocellulose. Glass and various enamels may be used, particularlyV glass of low enough melting point to insure that the phosphor crystals remain unmelted.

The thickness of the various layers can be altered to suit` various voltage conditions and the like. The voltage necessarily will depend on the phosphor used, the thickness of the phosphor layer 3, and the brightness desired, but voltagesbetween'ZS'volts and 2500 volts and even higher have been used. A lamp operable from a 11G-volt alternating current power line can be made with the conducting surface 2 of a thickness of about a wavelength of light, producing an iridescent effect when viewed at an angle, the phosphor layer 3 of about 2 onethousandthsY of an` inch, and the metal layer 4 of a fraction of a thousandth of an inch. The plate 1 can have any convenient thickness and should be transparent or translucent.

A highly eective phosphor can be prepared by intimately mixing as tine powders about 75 parts by weight of zinc sulphide and parts zinc oxide, with about 1.0 part zinc chloride, about .075 part copper added as copper sulphate, and about 1 part lead sulphate.

These are the preferred values for best results, but the copper, calculated as metallic copper, can be varied over a range of about 0.03% to 0.3%, and the amount of chloride, calculated as zinc chloride, should be between 0.4% to 2.0%, although if a fluoride is used the amount added should not ordinarily be greater than about 0.1%. The amount of lead, calculated as the sulphate, should be between and 5%, the higher amounts being used only when there is a considerable flow of itrogen or other inert gas during the firing, to carry away the excess lead. The amount of lead retained in the nal phosphor should be only between about 0.01% and 0.000l% by weight for best brightness.

The components should be thoroughly mixed in the form of line powders, and heated to between 900 C. to 1250 C. in an inert atmosphere, for example in a gastight electric furnace lled with nitrogen, and having a chamber of nitrogen connected thereto to pass a sloW stream of the gas therethrough. A batch of 200 grams has been tired in an electric furnace at about 1000" C. in a small silica boat in a silica tube 3 inches in diameter and inches long sealed at both ends, with a quarter inch silica tube feeding nitrogen into one end of the tube and a quarter inch tube for exhausting the nitrogen at the other end. The rate of flow or" nitrogen was about 0.1 liter per minute.

In tiring the phosphor, there are three distinct stages. In the lirst stage, the mixture emits fumes of the halide and the lead compound used, and turns a yellow color, which deepens with heating. lf removed from the furnace at this stage, the material will not have any appreciable electroluminescence. In the second stage, the evolution of fumes decreases greatly and the color of the phosphor darkens somewhat to a greenish-gray. If removed from the furnace during this stage the phosphor will luminesce. Finally there is a third stage, where the phosphor darkens further and becomes gritty, gradually losing its electroluminescent capability. The phosphor should be removed from the tiring furnace toward the end of the second stage or the beginning of the third. The soft, tlutly mass can then be crumbled or shaken to separate the powder particles.

After tiring of the phosphor, a treatment with acetic acid or ammonium acetate improves the luminescence of the phosphor. ri`he brightness is usually increased several thnes, and in many cases the treatment makes the difence between a good brightness and no brightness at in treating the tired phosphor powder with acetic acid, a solution of about 5% of the acid in Water is heated to between 60 C. and 100 C., preferably nearer to 60 C., and poured over the phosphor while the latter is subjected to a gentle grinding action until thoroughly treated, for example, about 2 minutes, and the suspension is then filtered, washed with Water, and dried. The temperature of the solution is kept at about 60 C. to 100 C. during the entire treatment, even during the iiltering.

lWhile the foregoing treatment improves the phosphor, an ammonium acetate treatment may be preferred because it is less critical in use and is more effective in increasing the brightness. In some cases, the acetate has given a 50% brighter phosphor than the acetic acid. ln using this treatment, enough of a saturated solution of ammonium acetate in water is added to the phosphor to rgive a slurry, which is stirred in a motar and thoroughly ground until all the large aggregates are broken up into their component particles. Then a quantity of half-saturated acetate solution is added, in proportions of say 200 cc. for every grams of phosphor, enough to give a thinner slurry and the supernatant suspension poured oir". A similar amount of half-saturated acetate solution is then added to the phosphor arid poured off or iiltered oli. The process is repeated with successive dilutions, two ltreatments with one-eihth saturated solution, then two with one-twelfth saturation, two with onesixteenth and several with pure water. The treatment could, if desired, be continuous with two streams of liquid, one being of water and one orF acetate solution, pouring onto :the phosphor, the stream of acetate solution being gradually reduced in ilow. if the acetate conentration on the phosphor is not diluted gradually, the dissolve zinc oxide will precipitate out again over the phosphor.

The eiectiveness of the treatment is apparently in removing the excess zinc oxide, leaving the sulphide and presumably leaving also any small particles of zinc oxide Whh may have attached themselves to the sulphide, or any zinc oxide distributed throughout the sulphide crystals.

The treatment is also found to remove a considerable fraction of the lead.

if the acetic acid solution used, it should be Weak enough to remove the oxide without also removing the sulphide. With the :acetate solution, the sulphide is unaffected, and the pl-i of the solution may be varied from 4 to 9, by varying the proportions o the ammonium acetate radicals, and still be edective. This is a helpful circumstance, for commercial ammonium acetates generally vary in composition, diering considerably trom stoichiometric. The acetate solution may possibly remove also any surface lm on the sulphide crystals and perhaps some of the copper, although the latter seems less likely.

Other ammonium sts, such as the chloride, are also effective, but ammonia itseli is not satisfactory. The reason appears to be that in a reaction between zinc oxide and ammonium acetate a complex zinc diammine acetate is formed, plus water. Ammonia itself lacks a negative radical to form such a complex for removing both the oxide and zinc portion 'of the Zinc oxide. But ammomurn chloride and many other ammonium salts would worl: like the acetate.

The effects of the 4treatments described on a ZnS-ZnO phosphor, suitably activated, are indicated in the following table:

Relative Brightness Percent ZnS in Starting Mixture Untreetod Treated The treatment in the above tests was with acetic acid. the ammonium acetate treatment generally gives about 50% more brightness near the maximum point.

it is seen from the table that the treatment had no effect on the 100% zinc sulphide sample, presumably because there was in that case no Zinc oxide to be removed, but in the other cases the treatment had a remarkable effect in increasing the brightness. The untreated samples -appeared to have no appreciable luminosity before treatment and this may have been due to the screening effect of the zinc oxide, which has an electrical conductivity high with respect to that of the sulphide.

The test device actually passed 100 times as much current with untreated phosphors as with treated ones. With a sinusoidal 100 volts at 60 cycles per second on a test device using `a cell 0.01 inch thick and 5 sq. in. in area, between a metal plate at the bottom and a piece of conductive glass at its top, using 1.5 grams of phosphor in 1.2 cc. of castor oil, the current passed was milliamperes before treatment and 0.05 milliampere after treatment. Thus a high-resistivity phosphor is demed for the purposes of this specification as one which passes of the order of 0.05 milliampere when tested in the above apparatus under the conditions specified, that is one which does not pass more than 0.5 milliampere.

The resistivity of the treated phosphor is even greater than the current passed throughtne cell might at rst seem :to indicate, because a large part of the current through the cell is due to capacitance. The current of 0.05 milliampere when the cell is iilled with the castor oil and treated phosphor in the above example is about double that passe when the cell is lilled with castor oil alone. Since the phase angle between voltage and current is only about tive or ten degrees, only a small part of the increase in current is due to the conductivity of the treated phosphor. The greater increase appears to be due to the increased dielectric constant of the mixture with the treated phosphor added. Since the dielectric constant of the phosphor-oil mixture appears to be double that of the oil alone, the constant of the treated phosphor appears to be quite high, say greater than ten, because the phosphor makes up only about a third of the volume of the mixture in the cell.

But when the phosphor is untreated, there is a considerableV amount of zinc oxide present, and since the dielectric constant for the oxide appears to be only about 2.5, the great increase in current with the untreated phosphor is due to its high conductivity.

A further example of a phosphor useful in electrolurninescent devices has been prepared by intimately mixing the following ingredients as iinely-divided powders,

in the proportions indicated:

Grams Zinc sulphide (ZnS) 75.60 Zinc oxide (ZnO) 24.42 Lead carbonate (PbCO3) 1.37

Cuprous oxide (CuO) 0.0637

The sulphide used contained 5 grams of water and 1% zinc chloride. rl`he water was removed by drying the mixture at'160 C. The batch was then placed in a quart Vmill and milled with acetone for half an hour, after which it was again dried and then tired at 1000 C. in an open silica tra for half an` hour in a furnace. Pre-puried nitrogen was iiowed over the mixture in the furnace at a rate of 0.06 liter per minute, during the tiring. After this heating, the phosphor was ground lightly in a mortar to break up the resultant fluly cake into its component particles or into its smaller aggregates of particles.

The powder was then treated with a boiling solution of 5% acetic acid in water, Ithen with a 1/2 solution of the 'same and then washed with water. In each case the phosphor Was placed in the acetic acid solution and the whole raised to boiling temperature in about ten minutes, continuing the boiling for five minutes.

In phosphors using about oxide in the starting mixture, the percentage has been found to be still substantially 25% after iiring. But after treatment with the solutions as above described, the Aamount of oxide present has been reduced to 5% or less, so that the iinal phosphor is about 95% sulphide or more.

Phosphors made Without any oxide in the starting materials have luminesced but were only about 20% as bright las those made with oxide. The copper content for even the 20% brightness should be much greater than the optimum values for the phosphors using oxides. A small amount of oxide may possibly be formed in this phosphor during firing, since it is diicult to insure that every trace of 'oxygen is absent from the nitrogen `atmosphere used during tiring.

The sulphide phosphor prepared as in the preceding examples iluoresces a greenish-yellow under excitations b at 60 cycles per second and fluoresces blue under excitation of about 2000 cycles.

A phosphor uorescing orange-yellow may be prepared by using manganese in activating amounts with the 'sulphide. In an example, such a phosphor is produced by mixing 87 parts by weight of zinc sulphide and 13 parts zinc oxide as finely divided powders, together with about 2.1% manganous sulfate and 0.8% zinc chloride and 1% lead sulfate. The constituents are blended as dry powders, preferably finely-divided, or they may be blended wet, for example, in `a water slurry and dried. Some of the compounds will remain yas powders and some will dissolve in the Water.

In any case, after the components 4are thoroughly mixed together, the resultant mixture should be fired, preferably in an inert atmosphere, as with the previously-described phosphor, .at a temperature of about 900 C. to 1200 C., preferably about 1000 C. After firing, the resultant mass is crumbled or milled to a desired particle size, although tne less the milling, the better will be the phosphor.

The phosphor should then be given a 'treatment such as the acetic acid or ammonium acetate treatment previously described, to improve its brightness. Such a treatment appears to be less marked in its effect on the manganese phosphor, probably on account of the smaller quantity'of oxide in the starting mixture.

Among the other manganese-activated phosphors which exhibit electroluminescence are the cadmium silicate and zinc fluoride phosphors. Electroluminescent sulphide phosphors can also be made in which the zinc is replaced partly or entirely by calcium or strontium.

FIGS. 2 to 5 show forms of the invention in which paired long spaced narrow conductors 6 and 7, 8 and 9, are placed side by side, the conductors and the space between them being occupied by -a coating or layer 10, 11 consisting of an electroluminescent phosphor embedded in a dielectric material. The conductors and the layer `are carried by insulating supports 12 and 13. In FIG. 2 the conductors 6, 7 are wires, having an enamel insulating layer 14, Wound side by side `and close together but spaced apart a distance of a few thousandths of au inch or less.

A lamp is defined for the purposes of this specification as a device which produces light of practical illuminating intensities. YIntensities below =a foot-lambert are practical for some application, although the lamps herein described have given several foot-lamberts on 60 cycles per second alternating voltage supply, and 15 to 20 foot-lamberts on a supply of several thousand cycles per second.

Such lamps are therefore useful for general illumination purposes including use as luminous panels for ceilings, as lighting sources for table lamps, as luminous signs and clock faces, as luminous face plates for household electrical switches, for street lighting `and for many other applications.

A typical energy level diagram for an electroluminescent phosphor is shown in FIG. 6. Between the highest lilled band 15 of the host crystal and the lowest unlled band 16, there is an energy gap 17. It the phosphor is to emit visible light, the width of 'this gap must be greater than 1.5 electron volts, which corresponds to the energy equivalent to the extreme red end of the Visible spectrum, that is to the lowest energy which can produce visible light. In zinc sullide the gap is about 3.5 volts.

The highest occupied band 18 of the activator material will be spaced from the lowest unfilled band 16 by an yamount less than for the gap 17, Iand in the case of copper in zinc suliide will be spaced about 2.7 volts. The donator impurity will have its highest occupied level 19 just below the bottom of the lowest unfilled band 16, .and the gap between the two Will generally be only a few tenths of a volt, for example between -about 0.1 volt and 0.4 volt for a lead impurity in copper-activated zinc sulfide. Other activating materials can be used, for example silver atea-,712

or manganese, and the most suitable material will be different for dilerent host crystals. A host material ot fairly open crystal structure and consequent long mean free path facilitates the acceleration of the electrons to a value suitable for exciting the activator material. Metailic sulphides, selenides fand silicates are among the comA pounds which can be used as host crystals. Zinc, cadmium, and calcium are among the substances which can be used as the metallic component or the compounds.

A donator impurity is one from which electrons can be treed by thermal agitation or by the field at relatively low energy values, generally not over a few-tenths of a volt. A donator impurity is a substance having a higher valence than one of the atoms or ions of the host crystal so that it can substitute for such an atom or ion, and lose an electron in the process. The lost electron will take up a low energy orbit of large radius around the atom or ion from which it has thus been partly freed, and may then be more nearly completely freed by a small additional energy, the amount supplied by thermal agitation often being suicient. For example, lead, thallium, indium, and tin are among the metals which can replace par-t of the zinc in zinc sulphide and act as donator impurities.

For best brightness, the amount of donator material present should be less than about 0.00005 gram-atom per mole of host crystal material, although with some phosphors electroluminescence can be obtained with two to three times that proportion of donator material. The amount of donator material should, of course, be greater than zero and for best brightness should generally be above 0.0000005 gram-atom per mole of host crystal material.

When chlorine is used in the starting mixture from which the phosphor is made, the amount retained in the tinal Washed phosphor will not ordinarily be greater than 0.1% by weight of the final washed phosphor.

Activators or the types held in part at least interstitially in the phosphor are particularly eiective, and the amount of such activators used is much greater than the amount ordinarily used in cathode ray or ultaviolet excited phosphors, or in infra red-responsive phosphors, as shown by the comparatively high copper content of the electroluminescent zinc sulphide phosphor described above.

The highest occupied band 19 is generally known as the Valence band and the lowest uniilled band 16 as the conduction band.

This application is in part a continuation of my abandoned applications Serial Nos. 105,803, 119,021, and 119,022 and Patent 2,838,715, iiled respectively July 20, 1949, September 30, 1949, September 30, 1949, and August 22, 1950.

What 1 claim is:

1. A high resistivity electroluminescent phosphor consitsing essentially of a host crystal of Zinc suliide, an activating copper impurity therein, said copper being present in an amount between 0.03% and 0.3% of the weight of zinc sulfide, and a donator impurity of lead therein to furnish electrons for excitation of the activating impurity, said lead being present in an amount between about 0.0001% and 0.01% by weight of the zinc suliide.

2. An electroluminescent phosphor consisting essentially of a host crystal of zinc sulfide, an activating impurity therein, and a donator impurity therein to furnish electrons for excitation of the activating impurity, said donator being present in an amount between 0.0000005 and 0.00005 gram atoms per mol of zinc sulfide, and being selected from .the group consisting of lead, thallium, indium and tin.

3. An electroluminescent lamp comprising a first electrode, a second electrode in proximity thereto, and a solid layer therebetween including an electroluminescent phosphor consisting essentially of activated crystals of combined zinc oxide and sulphide and substantially free from any uncombined zinc oxide, and in which the phosphor contains a small amount of lead.

4. The lamp of claim 3, in which the phosphor comprises a fired mixture of zinc sulphide, zinc oxide, a copper compound and a lead compound.

5. The lamp of claim 4 in which a halide is one of the ingredients of the mixture tired.

6. Electroluminescent capacitor phosphor crystals, each said phosphor crystal having a plurality of separated conductive coats each in a separate part thereof and arranged in capacitor structure.

7. The capacitor-phosphor crystal of claim 6 and a dielectric covering overall.

References Cited in the tile of this patent UNITED STATES PATENTS 2,136,871 Wakenhut Nov. 15, 1938 2,447,322 Fonda Aug. 17, 1948 2,474,506 V/Ood .lune 28, 1949 2,559,279 Charles July 3, 1951 2,566,349 Mager Sept. 4, 1951 2,624,857 Mager June 6, 1953 2,745,811 Butler et al May 15, 1956 FOREIGN PATENTS 873,860 France July 22, 1942 OTHER REFERENCES Article by Destriau in the Philosophical Magazine, vol. 38, October 1947, pages 700 to 723.

Claims (1)

1. 3. AN ELECTROLUMINESCENT LAMP COMPRISING A FIRST ELECTRODE, A SECOND ELECTRODE IN PROXIMITY THERETO, AND A SOLID LAYER THEREBETWEEN INCLUDING AN ELECTROLUMINESCENT PHOSPHOR CONSISTING ESSENTIALLY OF ACTIVATED CRYSTALS OF COMBINED ZINC OXIDE AND SULPHIDE AND SUBSTANTIALLY FREE FROM ANY UNCOMBINED ZINC OXIDE, AND IN WHICH THE PHOSPHOR CONTAINS A SMALL AMOUNT OF LEAD.

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