US2373752A - Picture transmitter tube mosaic screen - Google Patents

Description

April 17, 1945. E. FLORY 2,373,752

PICTURE TRANSMITTER TUBE MOSAIC SCREEN Filed Jan. 25, 1a41 Fi e. 1.

nnentor Leslie Eil 'io'r Patented Apr. 17, 1945 PICTURE TRANSMITTER TUBE MosAIo SCREEN Leslie E. Flry,"0aklyn, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application January 25, 1941, Serial No. 375,895

2 Claims (Cl. 250167) My invention relates to cathode ray tubes de-.

signed for picture transmission and particularly to tubes of this type having therein a novel screen structure to be scanned by the cathode ray.

An object of the present invention is to provide an improved cathode ra tulbe for picture transmission and in particular to provide an improved screen structure or mosaic for such tubes.

A further object of the invention is to provide an improved method of making a screen Structure or mosaic for cathode ray tubes,

In accordance with the present invention the necessity for evaporating a'film of silver on a layer of powder is avoided by mixing a silver compound, such as silver oxide or silver carbonate, with an insulating powder, such'as china c1ay,'and then depositing this mixture of pow- Figure 3 is a view which is referred to in de-. scribing our improved screen structure, and.

Figure 4 is view of a combination image tube and cathode rayzpicture transmitter tube embodying our invention.

Referring to Fig. .1, the invention is shown applied to a cathode ray picturetransmitter tube of a well known type. It comprises an evacuated envelope I having therein a electron gun. come ders upon the metallic surface which is to function as the signal plate of the tube.

The quantity of silver compound which is mixed With the insulatingpow'der is made small enough so that the conductivity of the powder layer is not appreciablyincreased as compared withthe conductivity of a layer of china clay alone,

After the mixture of powders has been deposited on the signal plate, preferably by means of cataphoresis, the particles of silver compound at the'surfa'ce of the powder layer may be suitably photosensitized. It will be understood that it is only surface particles that are; photosensitizcd, the powder layer Ming so dense that the caesium or other photosensitizing' material Will not penetrate the powder layer.

i The final mosaic, when made as just described, functions the same as the well known iconoscope mosaic, that is, each'photosensiti zed particle is insulated froma'djacent photosensitized particles and from the signal plate whereby'each photosensitive particle may acquire a' certain charge and storethis charge. For some tube de-' signs, it is not necessary to make the surface particles of silver or the like photosensitive, it being sufficient if they emitsecondary electrons when an electron image is projected upon the mosaic. i

The invention will be better understo'od'from the following description, taken in connection with the accompanying drawing in which Figure 1 i a view of a cathode ray picture transmitter tube embodying our'invention,

Figure 2a is an enlarged view, in section, ofv the screen structure of mosaic used in thetube shown in Fig; 1,

Fig. 2b is 'afragmentary view of said screen structure,

prising an indirectly heated cathode 2, a control electrode 3, a first anode 4, and a second anode 6 and a screen structure I. A light image is projected upon the screen 1 -.whereby a corresponding electrostatic image isformed thereon. This is scanned by the cathode rayproduced by the electron gun wherebypicture signals appear in the output conductor 8 4 s In accordance withmy invention, the screen l is made as illustrated inFig. 2a, .It comprises a conducting signal plate 9 which may be a pol-. ished metal plate or it may be .a glass plate with a metal coating. If a glass plate is used, the metal surface may be formed by evaporating aluminum or some. other-metal upon the glass surface, or a metal such as platinum may be deposited chemically upon the glass},

The layer l l is a mixture of an insulating pow-.

' der such as china clay and a metallic compound such as silver oxide whichhas been deposited upon themetal surface of the signal plate 9 as previously described. 7

The percentage of silver oxide, silver carbonate or the like, which is mixed with the chin clay or other insulating powder is not critical, the important requirement being that enough mate-' rial is included to providea reasonable numberof silver particles on the surface of the powder, layer to permit the formation of a good mosaic surface, while at the same time the percentage is, kept low enough to avoid. making the powder layer conductive enough to impair the storage action of the mosaic. In practice, mosaics made. with a mixture of china clayand silver oxide,,with the silver oxide 6% by weight of the china clay, have although one of the silvercompounds ispreferred. vAlso, while china clay has been'mentioned as a preferred insulating powder, there are other powdered materials that are satisfac tory. Such materials, for example, are powdered mica, aluminum silicate, powdered quartz, titanium oxide and MgO.

The powder layer Il may be put on the plate 9 by various methods as by spraying it on, by set tling it on by the process will known in making fluorescent screens, by evaporatin a liquid in which the powder is suspended or by cataphoresis. The latter method is preferred and is practiced as illustrated in Fig. 3. In any case, a method is used which results in a dense adhering layer of the mixed powders which will not bepenetrated any substantial amormt by the caesium or other photosensitizing material. The powder particles must be small to produce a layer of the desired density. In one mosaic, for example, a majority of the particles were less than 0.002 millimeter in diameter, the average diameter of all particles being about 0.001 millimeter.

Referring to Fig. 3, the powder mixture is held in suspension in a liquid which may be in either an insulating or a metal container I4. he insulating powder and silver compound preferably are thoroughly mixed by grinding them together before being suspended in the liquid. The signal plate 9 is placed in this liquid and opposite the plate 9 is placed an electrode l6. Byapplying a suitable D. C. potential between the plate 9 and the electrode IS the suspended powder mixture is caused to move over to the plate 9 and deposit thereon as a uniform coating, this movement of the powder mixture to an electrode being due to a difference in the dielectric constants of the powder and the liquid.

Whether the powder goes to the negative electrode or to the positive electrode depends upon the particular powder and liquid used. The china clay and silver oxide mixture goes to the positive electrode, at least in the cases where it is suspended in either acetone or water.

Generally a dispersing agent of some kind. should be employed to prevent the suspended powder from collecting in lumps or coagulating.

With china clay in acetone, about 1% of 1% nitrocellulose may be added for this purpose. With china clay in water, a few drops of ammonia may be added.

The insulating powder should have the following properties, all of which are found in the above-mentioned powders: 1) it should be very fine grained; (2) it should be chemically inert under the condition required for activating or processing the tube: (3) it should have a low vapor pressure: and (4) it should have a high resistance.

The screen 1 preferably is sensitized in the as sembled tube as follows:

After the powder layer has been deposited on the signal plate the mosaic is sealed in the oathode ray tube envelope and the .tube is then baked to at least 400 C. to out gas the tube and to reduce the silver compound after which the su face is oxidized by admitting ox gen to the tub and producing an electric discharge. If silver oxide is used. it is slightly reoxidized after the baking step the same as one of the other silver compounds.

A photosensitive material such as caesium is admitted after the oxygen has been pumped out and the tube is then baked. The resultin surface on the powder includes a multitude of very small discontinuous photosensitive elemen s.

Referring to the tube shown n Fig.4, it comprises an evacuated envelope 2| havin therein an electron gun comprising an indirectly heated cathode 22, a control electrode 23, a first anode 24 and a second anode 26, and a screen structure 21.

The screen 21 consists of a conducting signal plate 28 and a mixed powder layer 29, the elements 28 and 29 corresponding to the elements 9 and II, respectively, of Fig. 2a.

The envelope 2| also includes an image tube section comprising a transparent photosensitive cathode 3|, and electrostatic focusing ring electrodes 32 and 33 and the accelerating electrode 34. When a light image is projected upon the cathode 3|, electrons are released therefrom in accordance with the light intensity and an electron image is focused upon the screen 21 by means of the ring electrodes 32 and 33. An image tube of this general desi n is described and claimed in Patent No. 2,189,321 entitled Electrooptical devices, issued February 2, 1940, to G. A. Morton. It will be understood that the electron image may be focused upon the screen 21 by means of electromagnetic focusing, if desired, or by another suitable electron lens.

In a combination tube of this type, it is not necessary to sensitize the surface of the powder layer 29 as a separate step because the surface of layer 29 will become sensitized during the process' of sensitizing the cathode 3| as a result of the caesium or other photoelectric sensitizing metal goingover to the layer 29. The introduction of oxygen during the process of sensitizing the cathode 3| also plays a partin sensitizing the layer 29.

The process employed in sensitizing the oathode 3| preferably is the same as that described in Patent No. 2,189,322, issued February 6, 1940, t Leslie E. Flory. This process is carried out after the tube has been completely assembled. Briefly, the process comprises evacuating the tube, evaporating silver upon a transparent supporting surface, admitting oxygen and oxidizing the silver by a glow discharge, againevacuating the tube, admitting caesium or the like, and then baking the tube. Preferably a'very thin final layer of silver is evaporated upon the cathode 3| after the caesiating step and the tube is again baked as the final step.

It will be understood that if during the time an electron image is projected on the screen by the image tube, the surface of the screen 21 is scanned by the cathode ray produced by the electron gun, picture signals appear in the output conductor 36 leading from the signal plate 28.

The mixed powder layers of screens 1 and 21 do not include any binder in the preferred embodiments, although when some powders are used the finished layer may include a binder. In most cases, however, if the liquid in which the mixed powder is suspended during the process of forming the powder layer is a binder or contains a binder, the binder is evaporated out of the finished product. Thus, if the powder is china clay and silver oxide, the layers and 29 consist of china clay'and silver oxide only, there being no binder included.

It may be noted that in Figs. 2a and 4 of the drawing, the thickness of the layer of insulating powder is greatly exaggerated since the true thickness of the powder cannot readily be illustrated. The thickness of the mixed powder layer is not critical as long as the capacity remains a reasonable value and the layer does not get so thin that the signal plate is exposed. but it may be mentioned, by way of example, that a thick ness of two mils will be found satisfactory.

emissive elements capable of holding an electric charge.

2. The screen structure for a cathode ray tube which comprises a conducting surface having thereon a dense adhering layer of powder which is substantially non-conducting, said powder being a mixtureof a silver compound and an insu- I lating powder which remains insulating after processing of the tube, said layer having on its surface sufllc'ient photosensitizing material to increase the electron emissivity of said surface while retaining non-conductivity laterally along said surface whereby the resulting electron emissive elements on said surface are capable of holding an electric charge.

' LESLIE E. FLORY.

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