US 3007070 A
Description (OCR text may contain errors)
Oct. 31, 1961 W. W. CARGILL, JR
ELECTROLUMINESCENT DEVICE Filed Feb. 1, 1960 INVENTOR. \Nmuam W. CA e\L| .,JR.
ATTORNEY United States Patent 3,007,070 ELECTROLUMINESCENT DEVICE William W. Cargill, Jr., Los Angeles, Calif., assignor to Controls Company of America, Schiller Park, 111., a corporation of Delaware Filed Feb. 1, 1960, Ser. No. 5,969 3 Claims. (Cl. 313-108) This invention relates to devices for producing an electromagnetic energy output and particularly to improvements in electroluminescent devices. While this invention is not restricted to devices having electromagnetic energy outputs at frequencies in the light region of the electromagnetic spectrum, my invention will be illustrated and described with reference to such electroluminescent devices.
Basically, devices of this type include a first layer of conductive material forming a first electrode, a second layer of phosphor-impregnated dielectric material forming an electroluminescent layer and a third layer of conductive material forming a second electrode. Light is generated by the application of an electric field to the phosphor particles by applying an alternating voltage to the electrode layers. Phosphors are semi-conductive materials which may form a path of relatively low electric resistance between the conductive layers. This condition tends to dissipate the electric field required to excite the phosphors thus causing reduced light emission.
It is the object of this invention, therefore, to provide an improved device having a simple, inexpensive and effective means for producing electric fields of greater intensity to thereby increase the light emitting efficiency of the device.
In the proposed embodiment, the non-transparent electrode is made from aluminum. To produce electric fields of greater intensity between the electrodes, I provide a tightly adhering film of aluminum oxide to the aluminum layer between such layer and the electroluminescent layer. An aluminum material with the required film of aluminum oxide is produced commercially by anodizing and can be purchased at relatively low cost. Anodized aluminum is easy to handle Without damaging the dielectric layer and produces the desired results when incorporated into an electroluminescent device without the application of a separate dielectric layer as would otherwise be the case. The presence of this aluminum oxide layer produces a non-conductive barrier making it possible to build up a field of greater intensity than could otherwise be obtained. As a further refinement to my improved design I provide an additional optional layer of dielectric material between the aluminum oxide layer and the electroluminescent layer. It has been found that such additional layer of dielectric material permits the use of higher voltages before breakdown than would be possible without it.
Other objects and advantages will be pointed out in, or be apparent from the specification and claims, as will obvious modifications of the single embodiment shown in the drawing, in which:
The figure shown in the drawing is an enlarged sectional view of an electroluminescent device embodying my invention.
Referring to the drawing, my improved device includes the usual layers and 12 of conductive material forming the first and second electrodes and a third layer 14 of phosphor-impregnated dielectric material forming the electroluminescent layer of the device. An optional light intensifier layer 16 of barium titanate, for example, may be provided between the electroluminescent layer 14 and first electrode layer 10 to intensify the electroluminescent phenomena.
Electrode layer 10 may be made from any suitable conductive material and in the preferred embodiment such layer is made from aluminum. Electrode layer 12 is made from any suitable conductive material such as gold, tin oxide, etc. and must be thin enough to be optically transparent. The thin transparent layer 12 is generally applied by vacuum deposition.
Up to this point the construction of the device described above has been of conventional design. However, as previously stated, the phosphors impregnated in the electroluminescent layer 14 are semi-conducting materials which can easily form a path of relatively low resistance between the electrode layers 10 and 12 provided such semi-conducting materials are in direct contact with the electrode layers as in conventional devices. This condition tends to dissipate the electric field required to excite the phosphors causing reduced light emission.
To provide electric fields of greater intensity, I provide a tightly adhering film 18 having high dielectric properties to one face of electrode layer 10 as shown. In the preferred embodiment, layer 10 is made from anodized aluminum having a film or layer 18 of aluminum oxide which has high dielectric properties and which is tightly adhered to the aluminum electrode layer 10. While it is recognized that when bare aluminum is exposed to the atmosphere a very thin natural skin of aluminum oxide forms thereon, such natural skin of aluminum oxide is not of sufficient thickness to produce the desired results. Anodized layer 18, therefore, is considerably thicker than such natural skin. The natural skin of aluminum oxide formed by exposure of the raw metallic aluminum to air varies in thickness from nothing on a freshly exposed surface to a thickness of several molecules depending on the surface state, impurities in the air, etc. While it is difiicult to give an exact thickness because of these variables, the layer is transparent so its thickness must be less than A Wavelength of light. There is a direct relationship between the thickness of anodized layer 18 and its insulating ability. It has been found that a thickness of .001 inch provides good results. From the foregoing it is seen that by providing a high dielectric layer 18 aluminum oxide as described above, a non-conductive barrier is formed making it possible to build up a field of greater intensity for exiting the phosphors than could be obtained Without such barrier.
Anodized aluminum is available commercially at a relatively low cost when compared to the expense of applying an additional layer of dielectric material in the manufacture of an electroluminescent device. Furthermore, anodized aluminum is easy to handle requiring no special care to protect the layer of aluminum oxide.
As a further refinement of my improved design resulting in a device which permits the use of higher voltages before breakdown, I provide an additional optional layer 20 of dielectric material between the aluminum oxide layer 18 and the layer 16 of barium titanate. Layer 20 may be of any suitable dielectric material such as urea formaldehyde and may be applied by spraying such material onto the aluminum oxide layer 18.
The device is completed by the connection of leads 22 to electrode layers 10 and 12 and the application of a final coating 24 of a suitable plastic such as Plexiglas, epoxy, styrene, etc. to protect the finished device from adverse external effects. Protective coating 24 may be applied by spraying, dipping, casting or laminating the plastic material around the device.
Although but one embodiment of the present invention has been illustrated and described, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention or from the scope of the appended claims.
1. An electroluminescent device comprising; alayerofaluminum forming a first electrode, a thin transparent layer of conductive material forming a second electrode,
a layer of electroluminescent phosphor-"impregnated di'-' electric material between said first and 'second electrode layers" forming an electroluminescentlayer,.and a 'layer' of aluminum oxide tightly adhered'to said aluminum lay=- er and disposed between said" aluminum" layer and said electroluminescent layer, said-'layer'of"aluminummxide" b'eirig 70f a" thickness greater than the thickness of the natural skinof aluminum'oxid'e which formson alumi-' num when exposed 'to the atmosphere.
3. An electroluminescent device according to claim 1 in which said-layer of aluminum oxide is at about .001 inch thick.
References Cited in the file-of this patent UNITED STATES PATENTS Walker Dec. 23, 1958 Mash Dec. 29, 1959