US3046154A - Method for forming luminescent screens - Google Patents

Description

y 1962 c. FELDMAN 3,046,154

METHOD FOR FORMING LUMINESCENT SCREENS Original Filed April 19. 1957 IlEll TRANSPARENT REFRACTORY GLASS BASE l TRANSPARENT NON-LUMINESCENT REFRACTORY LAYER 31 TRANSPARENT LUMINES'CENT ZINC SILICATE LAYER (GREEN LUMINESCENCE) m TRANSPARENT NON-LUMINESCENT REFRACTORY LAYER ILE E l l l I l LASS BASE 2 ON-LUMINESCENT LAYER 3 MINESCENT zmc SILICATE LAYER 4 (GREEN) 5 LUM|NESGENT LAYER LUMINESCENT MAGNESIUM SILICATE LAYEF (RED) INVENTOR CHARLES FELDMAN ATTORNEY v opaque powder.

greatly improved. The transparent films reflect no light layer exciting it to luminescence.

United States Patent Ofitice 3,046,154 Patented July 24, 1962 3,046,154 METHOD FOR FORMING LUMINESCENT SCREENS Charles 'Feldman, Hollin Hills, Va., assignor to Davohn Corporation, a corporation of Delaware Original application Apr. 19, 1957, Ser. No. 653,867. -Divided and this application Dec. 22, 1959, Ser.; No. 861,731

2 Claims. (Cl. 117-33.5) (Granted under Title 35, US. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government, of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to amethod for forming luminescent screens which are useful in cathode ray tubes for the production of color images;

This application vis a division of my copcnding application Serial No. 653,867, filed April 19, 1957. 7

According to the prior art, opaque white powder have been used for screens of cathoderay tubes or television picture tubes to produce'images. These tubes have the disadvantage of not being sharp because. of the light: dispersion caused by the opaque powder. They have the further disadvantage of reflecting incoming light, thus giving poor contrast between the background and the light producing image.

The tubes therefore fail to provide good daylight viewing. To produce colored images as V in color television, the only practical process before the applicant's invention required placing tiny specs of color producing phosphors in regulated order all over the view ing screen. -'There have been some attempts at using layers of the opaque phosphorescent powder with each layer 1 being a separate color. It has been realized for some time that chromatic variations in a cathode ray tube could be produced by varying the penetration of electrons into a screen composed of layers of different phosphors. In practice, the use of powdered phosphors informing these multi-layer screens is completely out of the question. Each grain of the powdered phosphor is about 1 to 3 microns in diameter and layers several grains thick must phosphor which luminescences a different color than phosphor in the first layer. The color seen by the observer will be a combination of these two. colors. This process can be extended for three or more layers.

-L ittle information exists on electron penetration into and through solids. The existing theoretical treatments fit only the simplest systems and few measured values for solids are available. The situation in phosphors is complicated because the exact mechanism of excitation is not certain. That is, it is diflicult to determine what part of the electron energy that is lost in collision actually goes to excite the luminescent center in. the solid. However, one may draw the following general conclusions from the existing published work:

The depth of electron penetration or the range (R) increases as the electron velocity or energy (E) increases, according to a relation of the following equation:

R=CE

where C and n are constants depending on the system involved. The constant n is between 1 and 2.

At sufficiently high electron energies, the luminescent cross section and hence the brightness starts to decrease. This decrease, of course, takes place only after the electrons have sufiicient energy to penetrate the phosphor. As long as the electrons are completely stopped in the phosphor layer, they give all of their energy to the solid and the luminescent brightness is directly proportional to the energy. s

It is an object of the present invention to provide a method for forming new luminescent screens which are useful-in cathode ray tubes for the production of color by providing a tube which has transparent layers of phosphorescent film, each layer producing a different color. In the transparent layers there is no light diffusion by Hence, the sharpness of the image is and by making the interior ofthe tube light absorbing, there is no reflection and daylight viewing is greatly improved. By using transparent layers of different colored phosphorescent film, colored images can be easily and simply produced.

The general 'idea' of a multilayer chromatic screen is that electrons enter the screen and strike the first phosphor This luminescence is viewed through the remaining phosphor layers and glass substrate. If the electron energy is low and the electron is stopped in this first layer, then the colorviewed will be only that-due to the phosphor in the first layer. As the electron energy is increased, the electrons will begin to penetrate to the second layer and excite the second images. It isalso an object to provide a'method for forming new luminescent screens which contain a thin transparent film or layer of luminescence-activated zinc silicate and a thin transparent film or layer of luminescence-activated magnesium silicate which differ in the color of the light emitted by them on excitation by electron bombardment.

The above and other objects will become apparent and the invention understood from a reading of a following description taken in conjunction with the accompanying drawing in which:

FIGURE 1 is a fiowsheet in illustration of the method of the present invention, and

FIGURE 2 is a sectional view in illustration of a luminescent screen which may be made by the method of the invention. 1

The method of the invention for forming the lumines-- 15, 1961. Laminated screens for the above said purpose may be made by forming a screen of thin layers on top of one another in the manner described for the single layer. Each film is fired before another one is placed on top. In forming these laminated screens,'the lumines-, cent layer which requires the highest firing or forming temperature must be placed on the base first. This is followed by the screen which requires the next highest firing temperature and so on until the required number of layers are deposited. This order of application and firing is necessary to prevent deterioration of previously applied layers when subsequent layers are fired.

Transparent luminescent screens for application in cathode ray tubes can be obtained by vaporizing zinc silicate or magnesium silicate and a luminescence activator material and depositing a thin film of the vapors on a transparent refractory base, e.g., silica glass such as quartz .1 would normally be a refractory glass.

glass and Vycor glass and subjecting the thin film and base to baking in air at a temperature between about 1000 and 1200 C., preferably at about 1100 C., for a period of between about 15 minutes and an hour to form a luminescent film of the metal silicate. The baking is conducted until the film is activated to luminesce in the color characteristic of the activator-silicate metal combination thereof. With manganese as activator, this color is green for zinc silicate films and red for magnesium silicate films.

The initial films for producing the luminescent screens of my invention are formed by an application of the known vacuum evaporation-deposition process. In the application of this process, the metal silicate and the luminescence activator material are vaporized and the vapors condensed on the transparent substrate within a high vacuum, e.g., of the order of 10- to 5X mm. Hg, the process being controlled to deposit thin films of the order of thickness of about 0.5 to 5 microns on the substrate. The initial film on the substrate contains the solid components of the metal silicate, but not the metal silicate as such. The solid components of the metal silicates are the metal or metal oxide, silicon or the silicon oxides. The necessary oxygen for formation of the metal silicate from the solid components in the film is supplied from the air in the subsequent air baking of the film.

The solid components of the metal silicates can be used in place of the metal silicates as starting materials for forming the initial film on the substrate, for example, Zinc or zinc oxide and silicon or silica for forming a luminescent zinc silicate screen. The vaporization of the solid components is conducted from separate vessels within the i J high vacuum and controlled so as to deposit the vapors of the solid components on the substrate in the correct proportions for forming the metal silicate therefrom in the subsequent baking of the film in the presence of =2 oxygen (air).

The activator may be supplied in the films by using commercial activated zinc or magnesium phosphor powders for making of the films. It may be also supplied by separately and simultaneously vaporizing the activator material and the metal silicate or its solid components and condensing the vapors on the substrate within the high vacuum. Manganese is a preferred activator for the production of the luminescent screens of the invention. However, titanium or other activators can be employed.

In place of forming the film layers directly on top of one another, it is usually desirable to separate the layers by a non-luminescent film. This causes the pure colors to be more pure for given voltages. Likewise, the layers may be separated by non-luminescent conducting layers which provide a means of controlling the luminescentcolor.

In the luminescent screen of FIGURE 2, a transparent refractory glass base is shown at 1, having thereon a thin transparent luminescent zinc silicate layer 3, a thin transparent non-luminescent layer 4 and a thin transparent: luminescent magnesium silicate layer 5. The luminescent zincsilicate layer luminesces in one color,green as shown, and the luminescent magnesium silicate. layer in a different color, red as shown, on excitation by a beam of electrons.

Each luminescent layer is deposited and baked one at a time and one on top of the other on a substrate, which In the baking process, to activate or form the luminescent layers, each succeeding layer must be of a kind which is formed 0 activated at a lower temperature or at the same temperature as the preceding layer. Preferably each luminescent layer is separated from each other and from the substrate .by a non-luminescent transparent layer.

For example, a three layer screenrof zinc silicate, magnesium silicate, and calcium tungstate, each activated by manganese, would'be formed in the following manner. First the layer of zinc silicate with the manganese is vapor deposited on the transparent base and then baked in an oxygen atmosphere at a temperature of between about 1000 C. and l200 C. preferably at about ll0O C. for a period sufficient to form or activate the layer in the manner described in the said copending application Serial No. 650,958, now Patent No. 2,998,323. This baking step shall be called firing. The layer of manganese activated magnesium silicate is vapor deposited on top of the zinc silicate layer according to the steps outlined in the said copending application Serial No. 650,958,

now Patent No. 2,998,323. It is desirable to place two layers of magnesium silicate over the zinc silicate. The first layer is fired only to a temperature sufiicient to clear the film of magnesium silicate which is condensed in a black opaque state. A second layer of magnesium silicate is then condensed from the vapors in the vacuum on top of this composite screen. The entire screen isthen baked in an oxygen atmosphere at ll00 C., for a time sufficient to form or activate the magnesium silicate luminescent layer. The third layer of tungsten activated calcium tungstate is vapor deposited on top of the first two layers and then baked in air at a temperature of about 550 C., for a time sulficient to form or activate the luminescent film of calcium tungstate in the manner described in the said copending application Serial No. 650,958, now Patent No. 2,998,323. A conducting transparent layer is formed on top of the lastevaporated film. This layer may be composed of tin oxide, for example, or formed of a thin metallic film in a well known manner. The non-luminescent transparent layers separating the luminescent layers from each other and from the glass substrate are formed by evaporating and depositing a layer of silicon dioxide, for example, in a well known manner between the evaporating and firing of the luminescent layers. Such a layer would be non-conducting. The screen thus .formed is useful in a cathode ray tube to produce colored images as in a color television system.

A further use of the screen is an electronic display which appears to have depth by the stereoscopic effect. A further use is for high resolution electronic display wherein the amount of information is not limited by optical scattering from particles comprising the screen.

In addition to uses with cathode ray tubes, it is obvious that the luminescent layers can also be excited by other forms of radiant energy, for example, ultraviolet, X-ray,

gamma ray, etc. They also may be excited by an electric field. The thin light transmitting laminated screen thus can be employed for electroluminescent illumination, and can be further employed in combination with other films such as photoconductive surfaces for solid state display devices.

In the screens formed by the method of the invention the order of occurrence of the transparent luminescent silicate layers in respect to the glass base is first the zinc silicate layer and then the magnesium silicate layer. This order of occurrence of the luminescent silicate layers in the screen insures purity of the characteristic color of the light emitted by the particular luminescent silicate layer on bombardment with electrons. The transparent nonl-uminescent separating layer between the zinc and magn'esium silicate luminescent layers does not give this efiect'.

While in the above description reference has been made to a specific embodiment of the invention, such 'is" not i .comprises forming a thin transparent non-luminescent,

non-conducting refractory layer on a transparent refractory glass base, condensing the vaporized solid components of zinc silicate and a vaporized luminescence .activator on said tnansparentnon-luminescent refractory layer within a high vacuum to form thereon a thin layer containing said solid components in the correct proportions for forming the zinc silicate therefrom on baking of said layer in the presence of oxygen, subjecting said layer to baking'in the presence of oxygen at a temperature and for a time sufficient to form zinc silicate and' activate the same, forming a thin transparent non-luminescent refractory layer on said zinc silicate layer, condensing the vaporized solid components of magnesium silicate and a vaporized luminescence activator on said transparent non-luminescent refractory layer within a high vacuum to form thereon a thin layer containing said solid com ponents in the correct proportions for forming magnesium silicate therefrom on baking of said layer in the presence of oxygen and a luminescence activator capable of causing the formed magnesium silicate to emit light of a color different from that emitted by said luminescent zinc silicnte when excited lay-bombardment with electrons, and subjecting said layer to baking in the presence of oxygen at a temperature and for a time sutficient to form magnesium silicate and activate the same.

2. A method of making a luminescent screen as defined in claim 1, wherein the temperature for baking of the layers containing the solid components of the respective metal silicate is from about 1000 to 1200 C.

References Cited in the file'ofthis patent UNITED STATES'PATENTS 2,312,229 Anderson Feb. 23, 1943 2,590,018 Koller et al Mar. 18, 1952 2,600,579 Ruedy et al. June 17, 1952 2,887,401 Cusano May 19, 1959 2,980,550 Seats Apr. 18, 1961 2,998,323 Feldman Aug. 29, 1961

Claims (1)

1. A METHOD OF MAKING A LUMINESCENT SCREEN WHICH COMPRISES FROMING A THIN TRANSPARENT NON-LUMINESCENT, NON-CONDUCTING REFRACTORY LAYER ON A TRANSPARENT REFRACTORY GLASS BASE, CONDENSING THE VAPORIZED SOLID COMPONENTS OF ZINC SILICATE AND A VAPORIZED LUMINENESCENCE ACTIVATOR ON SAID TRANSPARENT NON-LUNINESCENT REFRACTORY LAYER WITHIN A HIGH VACUUM TO FORM THEREON A THIN LAYER CONTAINING SAID SOLID COMPONENTS IN THE CORRECT PROPORTIONS FOR FORMING THE ZINC SILILCATE THEREFROM ON BAKING OF SAID LAYER IN THE PRESENCE OF OXYGEN, SUBJECTING SAID LAYER TO FOR A TIME SUFFICIENT TO FORM ZINC SILICATE AND ACTIVATE THE SAME, FORMING A THIN TRANSPARENT NON-LUMINESCENT REFRACTORY LAYER ON SAID ZINC SILICATE LAYER, CONDENSING THE VAPORIZED SOLID COMPONENTS OF MAGNESIUM SILILCATE AND A VAPORIZED LUNINESCENCE ACTIVATOR ON SAID TRANSPARENT NON-LUMINESCENT REFRACTORY LAYER WITHIN A HIGH VACUUM TO FORM THEREON A THIN LAYER CONTAINING SAID SOLID COMPONENTS IN THE CORRECT PROPORTIONS FOR FORMING MAGNESIUM SILICATE THEREFROM ON BAKING OF SAID LAYER IN THE PRESENCE OF OXYGEN AND A LUMINESCENCE ACTIVATOR CAPABLE OF CAUSING THE FORMED MAGNESIUM SILICATE TO EMIT LIGHT OF A COLOR DIFFERENT FROM THAT EMITTED BY SAID LUMINESCENT ZINC SILICATE WHEN EXCITED BY BOMBARDMENT WITH ELECTRONS, AND SUBJECTING SAID LAYER TO BAKING IN THE PRESENCE OF OXYGEN AT A TEMPERATURE AND FOR A TIME SUFFICIENT TO FORM MAGNESIUM SILICATE AND ACTIVATE THE SAME.

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