US2932879A - Single crystal electroluminescence - Google Patents

Description

April 19, 1960 w. w. PIPER 2 ,932,879

SINGLE CRYSTAL ELECTROLUMINESCENCE Original Filed Dec. 5. 1955 Fig.2.

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United States Patent 2,932,879 SINGLE CRYSTAL ELECTROLUMINESCENCE William W. Piper, Scotia, N.Y., assignor to General Electric Company, a corporation of New York Original application December 5, 1955, Serial No. 550,992, new Patent N 2,841,730, dated July 1, 1958. Divided and this application April 21, 1958, Serial No. 729,693

7 Claims. (Cl. 2925.14)

The present invention relates to electroluminescent cells,

and, more particularly, to electroluminescent cells utilizing a single crystal of an activated electroluminescent phosphor as the light emitting member. This application is a division of application Serial No. 550,992, filed December 5, 1955, now Patent No. 2,841,730 issued July 1, 1958, which is a continuation-in-part of my copending application Serial No. 274,237, filed February 29, 1952, now abandoned, both of which are assigned to the same assignee as the present invention.

Electroluminescent cells, or luminous capacitors, as they are sometimes called, resemble a flat plate capacitor wherein the dielectric has incorporated within it a quantity of electroluminescent phosphor and one of the plates is a transparent conducting layer. When an alternating current potential is impressed between the two capacitor plates luminousity effects are observed Within the dielectric layer. A device of this character is disclosed and claimed in the application of Jerome S. Prener, Serial No. 245,696, filed September 8, 1951, and assigned to the same assignee as the present invention.

The electroluminescent cells of the prior art operate only on alternating current, and the intensity of light given off thereby increases with voltage and frequency up to a certain point. Beyond a certain point, higher frequency is not accompanied by increasing brightness. The limiting factor on raising the voltage is dielectric breakdown.

' It is an object of this invention to provide an electroluminescent cell which will operate upon unidirectional as well as alternating current potentials,

Another object of the invention is to provide a light producing unit utilizing a single crystal of an electroluminescent phosphor.

A further object of the invention is toprovide an electroluminescent cell of small dimensions utilizing a single crystal of phosphor as the light emitter.

A further object of the invention is to provide a method for producing electroluminescent cells composed of the single crystal of an electroluminescent phosphor.

Briefly stated, in accordance with one aspect of my invention, I provide an electroluminescent cell by producing a single crystal containing a metal activator and one or more substances selected from the group consisting of zinc sulfide, zinc selenide, cadmium sulfide, cadmium selenide, and positioning a pair of oppositely dis posed electrodes on the crystal.

The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, together with further objects and advantages thereof may best be understood by referring to the following description taken in conjunction with the drawing in which Figs. 1 and 2 show, in greatly enlarged form, electroluminescent cells constructed in accord with the invention.

In Fig. 1 of the drawing, an electroluminescent cell constructed in accord with the invention comprises a single crystal 10 of an electroluminescent phosphor'having a pair of oppositely disposed transparent electrodes 11 and 12 in good electrical contact with opposite surfaces thereof. For the purposes of this application and the appended claims, a single crystal of an electrolumines cent phosphor material may be defined as one having the following characteristics. A single crystal of a phosphor material generally has a high degree of crystal perfection and is in the form of a regular elongated column having a cross-sectional area which is polygonal in shape. One major dimension of the single crystal cross-sectional area is at least one millimeter in length. The length of the column comprising the single crystal is generally much greater than the cross-sectional dimension thereof, being at least one millimeter, and in many cases being as larg'e' as one centimeter. The minimum weight of single crys-, tals of phosphor materials utilized in this invention is approximately one-half milligram, although some large single crystals may weigh as much as one gram. Such regular geometry and dimensions, and high degree of crystal perfection are to be contrasted with the microcrystals comprising conventional powdered phosphors utilized in electroluminescent cells heretofore. Such microcrystals may have any irregular geometry, may be fused masses which include a plurality of unoriented crystals and may have dimensions from one to forty microns. The single crystals of this invention are also to be distinguished from large crystalline masses of phosphor materials grown by conventional rapid evaporation and condensation techniques in which case the crystalline phosphor body rather than being a single crystal comprises a plurality of Small microcrystals with no particular orientation and a lesser degree of crystal perfection. The single crystal phosphor materials of this invention may also be defined as crystals which are grown by a slow sublimation crystal growth process upona heated base plate which is maintained at a temperature slightly below the condensation point of the material compris ing the crystal. Such a method of crystal formation is described in greater detail hereinafter. Referring again to Fig. l of the drawing, electrodes 11 and 12 may be composed of an opaque conducting substance such as silver paste or a smooth surfaced metal plate or metallic probes. It may be desirable in certain cases to have one or both of electrodes 11 and 12 composed of a transparent conducting layer, such as a glass' backed layer of conducting tin oxide or titanium dioxide. If both electrodes are opaque, luminosity may be' readily detected emanating from the exposed portions of the cry's tal 10 when electrodes 11 and 12 are connected to an energizing potential source. A source of electrical energy 13 is connected to the electrodes 11 and 12 by means of conductors 14 and 15 respectively. Source 13 may be either a source of 'uni-. directional or alternating potential, since the single crystal electroluminescent cells of this invention, as opposed to the polycrystalline electroluminescent cells of the prior art which luminesce only in response to alternating current potentials, become luminescent when excited by either alternating or unidirectional potentials. A suitable exciting potential may be any source sutficien't to impress an electric field of approximately 10 volts per centimeter across single crystal 10.

Crystal 10 may be prepared from a charge comprising one or more materials selected from the group consisting of zinc sulfide, zinc selenide, cadmium sulfide and. cadmium selenide. These materials are mixed with a small quantity of a metallic activator generally in the' form of a chloride since it has been found that the chlo-f ride ion enhances the activation attained thereby. The mixture is then placed in finely divided form in a carefully cleaned quartz tube. The tube is then evacuated and heated to drive off absorbed gases, and'sealed oflEJ The lower half of the tube containing the finely divided vapor sublimed from the charge.

3 mixture is then heated to a temperature atwhich appreciable sublimation of the chargeoccurs hereinafter referred to as the sublimation temperature of the charge. If the charge comprises zinc sulfide, this temperature is approximately 1175 C. For zinc selenide or cadmium sulfide the temperature is approximately 800 C. For cadmium selenide the temperature is approximately 600 C. The top half of the tube is maintained at a temperature approximately 100 C. cooler so that this portion of the tube is below the condensation temperature of the The powder then gradually sublimes from the bottom of the tube and condenses onto the cooler walls of the upper portion of the tube in the form of large single crystals.

Preferred activating agents for my phosphors are copper and silver, but other activators such as manganese and phosphorus and other well known electroluminescent activators are also satisfactory. The activating agents used in the practice of my invention are added to the host phosphor material placed in the evaporated quartz tube in proportions which may rangefrom approximately 0.01% to 5% by weight of the host phosphor. The addition of quantities of activator less than approximately 0.01% by weight does not produce high brightness electroluminescent crystals. The addition of amounts of activaton'particularly highly conductive activators such as copper and silver in excess of 5% by weight tends to decrease the electrical resistivity of the resultant single tube was evacuated and heated to a temperature approximately 250 C. to drive on absorbed gases, and sealed off. The lower half of the tube containing the charge was then heated to a temperature of approximately 1100 C. The upper half of the tube was maintained at a temperature of approximately 1000 C. These temperatures were maintained for approximately 60 hours. At the end of the 60 hour period, the tube was opened and a number of long needlelike single crystals of Zinc sulfide activated with copper were found to have grown on the Walls of the low temperature portion of the tube.

A single crystal produced by the above-described method is then removed from the sealed tube and two oppositely disposed surfaces are carefully coated or otherwise contacted with conducting electrodes. This may be accomplished by vapor depositing a thin transparent metallic film upon oppositely disposed sides to form electrodes 11 and V 12. Alternatively, electrodes 11. and 12 may comprise transparent conducting coatings of tin oxide or titanium dioxide deposited upon glass backing plates and maintained in pressure contact with oppositely disposed surfaces of the single crystal. The unit thus produced makes a satisfactory electroluminescent cell which operates either on alternatcrystals and lowers the dielectric breakdown voltage 7 thereof. A preferred range of activator percentages which result in the formation of crystals having excellent light producing characteristics is from approximately 0.05% to 1% by weight of the host material. The activator does not need to be initially present in elemental form, and is preferably added as the chloride. If copper is to be the activator incorporated in the host phosphor material, it may be added in the form of copper chloride to the powdered phosphor material placed within the quartz tube. v p

It is to be noted that the above quoted percentages are not necessarily the amount of activator present in the resultant single crystal, but represent the amount of activator added to the host phosphor material before the sublimation crystal growth of single crystals. I have found experimentally that as the percentage of the metallic activator added to the host phosphor in the initial charge is increased the percentage of the activator to be found in the final crystal decreases. Thus, for example, when 0.01% by weight of copper is added to a zinc sulfide charge substantially the same percentage of copper is found in zinc sulfide single crystals grown in accord with this invention. On the other hand, when 0.2% by weight of copper is added to a zinc sulfide charge and single crystals grown therefrom in accord with this invention, the resultant crystals are found to contain only approximately 0.067% by weight of copper. Additionally, single crystals of zinc sulfide grown from a charge initially containing 1% by weight of copper are believed to contain at most 0.2% by weight of copper.

The method by which I produce single crystals of phosphor materials may be utilized to co-crystallize two or more substances. For example, an excellent phosphor is produced from an initial mix of eighty parts zinc sulfide to 20 parts zinc selenide and 0.5% by weight of copper as the chloride.

It is desirable to allow crystal formation to take place over a protracted period of time under conditions of slow crystal growth. Forty eight hours is about the minimum growth time to obtain satisfactory crystals. Obviously, this time may be shortened, but the crystals produced will not be large enough to give satisfactory results.

In one specific example of the formation of single crystals, ten grams of zinc sulfide and 22 milligrams of copper chloride were deposited in the bottom of a closed 'quartz'tube 6" in length and 1" in diameter. The

ing current or unidirectional current potential. A number of these small crystals may be combined into a single electroluminescent lamp interposed in parallel between the same electrodes if desired. This may be accomplished by orienting a plurality of single crystals into a mosaic screen and contacting parallel sides of the properly oriented crystals with oppositely disposed electrodes, at least one of which should be transparent.

The operating characteristics of the electroluminescent cells of the invention are dependent upon the orientation of the electric field with respect to the unique or C-axis of the crystal. The brightness for a given field is much higher when the field is oriented perpendicular to the uniqueaxis ofthe crystal as is shown in Fig. 2. Properly oriented crystals have been observed to emit six times the brightness of non-oriented crystals.

While the present invention has been described with reference to particular embodiments thereof it will be understood that numerous modifications may be made by those skilled in the art without departing fromthe invention. It is my intention therefore by the appended claims to cover all such equivalent variations as come Within the true spirit and scope of the foregoing disclosure; What I claim as new and desire to secure by Letters Patent of the United States is:

1. The method of preparing a phosphor screen which comprises; placing a mixture consisting essentially of at least of a finely divided phosphor material selected from the group consisting of zinc sulfide, zinc selenide, cadmium sulfide, cadmium selenide, and mixtures thereof and approximately 0.01% to 5% by Weight of a metal activator in a tube; evacuating and sealing said tube; heating the portion of said tube with which the mixture is in contact approximately to the sublimation temperature of the phosphor mixture; maintaining at least a portion of the remaining surface of said tube at a slightly lower temperature to provide a surface for the slow condensation of volatile components; removing the condensed particles from the tube; and orienting in parallel relationship a plurality of individual crystals as elements of a phosphor screen 2. The method of claim 1 wherein the phosphor is zinc v sulfide.

of the phosphor mixture; maintaining at least a portion of the remaining surface of said tube at a slightly lower temperature to provide a surface for the slow condensation of volatile components; removing the condensed particles from the tube; orienting in parallel relationship a plurality of individual crystals as elements of a phosphor screen; and contacting each of the opposed surfaces of each crystal with a conducting electrode.

4. The method of claim 3 wherein the phosphor is zinc sulfide.

5. The method of preparing a phosphor screen which comprises placing a mixture consisting essentially of at least 80% of a finely divided phosphor material selected from the group consisting of zinc sulfide, zinc selenide, cadmium sulfide, cadmium selenide, and mixtures thereof and from approximately 0.01% to 5% by weight of a metal activator in a tube; evacuating and sealing said tube; heating the portion of said tube with which the mixture is in contact approximately to the sublimation temperature of the phosphor mixture; maintaining at least a portion of the remaining surface of said tube at a slightly lower temperature to provide a surface for the slow condensation of volatile components; removing the condensed particles from the tube; and orienting a plurality of individual crystals in parallel relationship with the unique axis of the crystals parallel to one another to form a planar phosphor screen, said unique axis being substantially parallel to the plane of said screen.

6. The method of claim 5 in which the phosphor is zinc sulfide.

7. The method of preparing a phosphor screen which comprises; placing a mixture consisting essentially of at least of a finely divided phosphor material selected from the group consisting of zinc sulfide, zinc selenide, cadmium sulfide, cadmium selenide, and mixtures thereof and from approximately 0.01% to 5% by weight of a metal activator in a tube; evacuating and sealing said tube; heating the portion of said tube with which the mixture is in contact approximately to the sublimation temperature of the phosphor mixture; maintaining at least a portion of the remaining surface of said tube at a slightly lower temperature to provide a surface for the slow condensation of volatile components; removing the condensed particles from the tube; and orienting a plurality of individual crystals in parallel relationship with the unique axis of the crystals parallel to one another to form a planar phosphor screen; said unique axes being substantially parallel with the plane of said screen; and contacting opposite surfaces of each of said crystals with a conducting electrode.

No references cited.

Claims (1)

1. THE METHOD OF PREPARING A PHOSPHOR SCREEN WHICH COMPRISES, PLACING A MIXTURE CONSISTING ESSENTIALLY OF AT LEAST 80% OF A FINELY DIVIDED PHOSPHOR MATERIAL SELECTED FROM THE GROUP CONSISTING OF ZINC SULFIDE, ZINC SELENIDE, CADMIUM SULFIDE, CADMIUM SELENIDE, AND MIXTURES THEREOF AND APPROXIMATELY 0.01% TO 5% BY WEIGHT OF A METAL ACTIVATOR IN A TUBE, EVACUATING AND SEALING SAID TUBE, HEATING THE PORTION OF SAID TUBE WITH WHICH THE MIXTURE IS IN CONTACT APPROXIMATELY TO THE SUBLIMATION TEMPERATURE OF THE PHOSPHOR MIXTURE, MAINTAINING AT LEAST A PORTION OF THE REMAINING SURFACE OF SAID TUBE AT A SLIGHTLY LOWER TEMPERATURE TO PROVIDE A SURFACE FOR THE SLOW CONDENSATION OF VOLATILE COMPONENTS, REMOVING THE CONDENSED PARTICLES FROM THE TUBE, AND ORIENTING IN PARALLEL RELATIONSHIP A PLURALITY OF INDIVIDUAL CRYSTALS AS ELEMENTS OF A PHOSPHOR SCREEN.

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