US2189986A

US2189986A - Method of making mosaic electrodes

Info

Publication number: US2189986A
Application number: US198209A
Authority: US
Inventors: Hickok Willard
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1938-03-26
Filing date: 1938-03-26
Publication date: 1940-02-13
Anticipated expiration: 1957-02-13

Description

METHOD OF MAKING MOSAIC ELECTRODES Filed March 26, 1938 amr.

INVEN TOR. WILLRD HICKOK BY www' A TTORNEY.

Patented UNITED STAT METHOD F MAUNG MOSAIC ELECTRODES Delaware Application March 26, 1938, Serial No. 198,209

ZCIaims.

My invention relates to improvements in cathode ray television tubesand particularly to improvedA methods of manufacturing television tubes which have photosensitive mosaic electrodes.

It has been found that in cathode ray tubes with a conventional mosaic electrode having on the front surface of a mica sheet discrete mutually isolated elements or particles of silver photosensitized by exposure to an alkali metal vapor such as caesium vapor the best results are obtained when the tube is treated during exhaust to render the surface conductivity and resultant leakage between particles very low. It has been found that notwithstanding such treatment during exhaust, a certain amount of residual electrical leakage takes place between the photosensitive particles thereby limiting the resolving power of the device, and that further treatment to reduce this electrical leakage makes the photosensitivity of the silver particles considerably less than the maximum obtainable.

The principal object of my invention is to provide an improved cathode ray tube having a mosaic type target electrode of greater photosensitivity and with less electrical leakage between the particles than a conventional type electrode. It is another object to provide an improved and highly sensitized electrode having greater photosensitivity and resolving power than has heretofore been obtainable. It is a further object of my invention to provide a method whereby the photosensitivity and resolving power of a mosaic electrode may be controlled during manufacture.

In accordance with my invention the target electrode having a multiplicity of mutually separated metallic particles is photosensitized in the usual way, such as by exposing the electrode supporting the particles to the vapor of an alkali metal such a-s caesium and subsequently oxidizing or wholly eliminating the metallic caesium which is deposited between the photosensitive particles. This step of eliminating the metal between the particles is preferably performed while the tube is in normal operation. Further in accordance with my invention the electrode from which the free caesium has been removed between the particles is subsequently baked to increase the photosensitivity of the particles.

Other objects, features and advantages of my invention will appear from the following description taken in connection with the accompanying drawing in which:

Figure 1 is a diagrammatic view illustrating (Cl. Z50-27.5)

one form of a television device made in accordance with my invention,

Figure 2 is an enlarged fragmentary sectional view of the photosensitive mosaic electrode during the process of formation and,

Figure 3 is an enlarged fragmentary sectional view made in accordance with my improved method.

A tube made in accordance with my invention is shown in Figure 1 and comprises a highly evacuated glass envelope or bulb I with a tubular arm or neck section enclosing a conventional type electron gun and a cylindrical section enclosing a flat target or mosaic electrode 2 so positioned that its front surface may be scanned by a beam of electrons from the electron gun and also may have projected upon it the optical image to be transmitted. Since the image is produced from an object situated outside the tube, that portion or window 3 of the envelope opposite the electrode 2 is made optically uniform so that the image to be transmitted may be projected upon the electrode with a minimum of distortion by the lens system i.

The electron gun assembly is of the conventional type, and comprises a cathode E from which an electron stream may be drawn, a control electrode 6 connectedto the usual biasing battery, and a rst anode 'I maintained positive with respect to the cathode by a battery 8. The electron stream leaving the first anode 1 is accelerated and concentrated into an electron scanning beam focused on the front surface of the mosaic electrode 2 by a second anode 9, which is preferably a conductive coating on the surface of the bulb I extending into the neck of the bulb but removed from that portion through which is projected the optical image to be transmitted. Conventional deflection means, such as deection coils I0 and II may be used to sweep the beam in a horizontal and Vertical plane, respectively, to scan the target. It is obvious that conventional electrostatic deiiection plates may be substituted for one or both of the deectlon coils if desired. The electrode 2 is connected through the impedance I2 to ground andto the collector electrode or second anode 9, and in operation the current ow in this circuit produces a voltage drop across the impedance I2 which may be impressed on the input of a translating device I3, further amplified and applied to a transmitting network in a manner well known to the art.

The mosaic electrode as best shown in Figure 2 comprises an insulating foundation sheet or lasl a signal electrode from which the picture signals may be obtained. The other side of the sheet of mical has on its surface a plurality of mutually isolated metallic particles Il.

In accordance with my invention the metal particles are photosensitised during the evacuating process of the tube in which the electrode is used by oxidizing the metal particles and sub- Jecting the electrode and oxidized particles to the vapor of an alkali metal such. as caesium which deposits not: only on the metal particles Il but also upon the base Il between the particles which metal between the particles is converted into an electrically non-conducting medium, by subjecting the electrode assembly to a controlled quantity of dry oxygen which reacts with the free alkali metal on the base between the particles to form an electrically nonconducting material.

In making the mosaic electrode 2, I select a sheet of mica or other good insulating material having a thickness of* approximately .002" and an area sunlcient to satisfy the operating requirements of the devicein which the electrode is to be used. One surface of the mica sheet is dusted with a finely divided metal compound, such as silver oxide, and the dusted sheet is rapidly heated in an oven at 800 C. until the silver oxide is reduced to silver of which individual and minute portions are drawn up by surface tension to form a multiplicity oi' microscopic particles I8 spaced from each other, the number and average size of the particles in a imit area being sunlcient to satisfy the operating requirements in the way of detail of picture reproduction. The mosaic surface may be formed, however, in a variety of ways, thus for example. instead of using the metal compound which is applied to the mica base and subsequently reduced and` drawn up by surface tension into individual particles, a thin continuous nlm of silver may be deposited on the base, which nlm is then broken up into microscopic particles by suitable heat treatment.

The mica sheet or base Il carrying the silver particles Il is then mounted in the tube, as shown in Figure l, the tube evacuated and the silver particles oxidized by admitting oxygen to the envelope to form an oxide coating I'I on each of the silver particles. The excess oxygen is removed from the bulb and the oxidized particles are photosensitized by exposing them to vapor of an alkali metal such as caesium. A conventional way of performing the sensitizing step is to provide a source oi' caesium, such as a reduoible compound of caesium Il in theannulus In, which annulus following the photosensitizing step is sealed oil and removed from the bulb I at the point 2l.

During the exposure of the particles to caesium vapor some of the caesium condenses on the surface of the mica base as a metallic nlm 2| which covers both the oxidized particles Il and the base Il between the particles thereby forming an electrical leakage path between the particles and consequently reducing the electrical resistance from particle to particle. A portion of this caesium nlm 2| which is between the par ticles may be removed during the evacuating process. However, if suilicient caesium is deposited on the particles to provide optimum phoj:

tosensitisstion, the residual caeshnn between the insulatignubetw l, the depositionofcaesimthetubeisbaked andsimultaneously pumped until maxlminn photosmsitivity is obtained and during this baking and pumping some of the free caesium previously condensed-as the nlm-2| on the mica base between the particles is removed. The baking is discontinued when the photosensitivlty has increasedtoamaximumandhasdecreasedvery slightly at which Point the leakage between the particles due to the presence of caesium between the particles may be still too excessive to satisfy the operating requirements of the device.

In accordance with my invention, the electrodes of the tube are connected, as shown in Figure l, an optical image of an object projected on the mosaic electrode 2, and the mosaic electrode scanned with the cathode rarbeam. The

picture signals derived from the signal electrode Il are amplined by a series of amplifiers, such as the ampliner Il, and the optical image projected on the mosaic electrode -2 is recreated on a viewing tube (not shown). The resolution of the recreated image, which may be denned as the ability to distinguish two lines of an image which are close together, is noted. Oxygen is then liberated within the tube and in the presence of 'the mosaic electrode. the liberation of oxygen being continued until the resolving power of the mosaic electrode which determines the resolution of the recreated image has passed through a maximum. This indicates that the insulation between the particles has become greater. the metal nlm 2| being converted into an electrically non-conducting material such as the oxide of the metal. .At the same time the operating photosensitivity of the device likewise increases. Care should be taken that just sunicient oxygen is liberated to bring the resolution and the photosensitivity to a maximum' as it has been found that following the point at which maximum photosensitivity is obtained, the photosensitivity rapidly decreases with the admission of additional oxygen. Care should likewise be taken that the oxygen is dry as it has been found that if any moisture is present the photosensitization will be decreased I prefer to provide a source of oxygen within the tube capable of liberating oxygen in the amount required. For this purpose I provide a source of oxygen such as the nickel nlament 22 which contains absorbed oxygen and` which is located on the inside wall of the bulb I to the rear of the mosaic electrode. I have found that a nickel'ribbon nlament having a length of 1.5",

, which combines with the caesium on .the mica.

base between the particles. This caesium! is converted to caesium oxide which is a relatively poor electrical conductor. 'I'he caesium nlm 2I shown in Figure 2 is thus converted into a nlm of non-conducting material such as the caesium oxide nlm 24 shown in Figure 3.

Following the liberation of oxygen to the proper amount, as indicated by the operation of the device, the photosensitivity of the mosaic electrode is higher than that which is observed prior to the liberation of the oxygen, but may be subsequently increased to a still higher value by baking the tube at a temperature of 200 C. for approximately 7 to 10 minutes or until maximum photosensitivity is obtained.

Television transmitting tubes when made in accordance with my invention have shown higher sensitivities to light and higher resolution than tubes not made in accordance with my new and improved method. Tubes made in the manner disclosed are more uniform with respect to photosensitivity and resolving power and it has been found that tubes low in photosensitivity and resolving power may be improved considerably by following the teachings of my invention.

While I do not wish to be limited by any particular theory to explain the improved sensitivity and increased resolution of a television device made in accordance with my invention, it seems probable that the oxygen which is liberated within the tube first reacts with the metallic caesium between the photosensitized metal particles, oxidizing this caesium to caesium oxide which is a relatively poor electrical conductor. It is also believed that the oxygen reacts with part of thev free caesium present on the oxidized silver particles, thereby increasing the photosensitivity. During the subsequent baking operation it is believed that free caesium which has been adsorbed by the oxidized silver particles is driven to the surface of the particles, but that this caesium does not migrate to the intervening areas of the base member because the resolution following the baking step is as high as or higher than the resolution prior to baking.

d While I have indicated the preferred embodilimited to the exact forms illustrated or the use indicated, but that many variations may be made in the particular structure used and the purpose for which it is employed without departing from the scope of my invention as set forth in the appended claims.

I claim:

l. The method of photosensitizing a mosaic electrode having a plurality of mutually separated metallic particles on an insulating base supported within a cathode ray tube wherein a cathode ray beam for scanning is developed, which comprises depositing a lm of alkali metal on said metallic particles and said base, thermally removing a portion of the alkali metal from areas of the base intermediate the particles, projecting an optical image on said mosaicelectrode, tracing the developed cathode ray beam across the said particles to produce signals representative of the surface condition of said electrode, liberating oxygen within said envelope and limiting the amount of liberated oxygen to the point of maximum photosensitivity and resolving power of the mosaic electrode in accordance with observations of the signals resulting from said scanning.

2. The method of photosensitizing a mosaic electrode having a plurality of mutually separated metallic particles on an insulating base supported within a cathode ray tube wherein a cathode ray beam for scanning is developed, which comprises depositing a lm of caesium on lsaid metallic particles and said base, thermally removing a portion of the caesium from areas of the base intermediate lthe particles, projecting an optical image on said mosaic electrode, tracing the developed cathode ray beam across the said particles to produce signals representative of the surface condition of said electrode, liberating oxygen within said envelope and limiting the amount of liberated oxygen to the point of maximum photosensitivity and resolving power of the mosaic electrode in accordance with observations of the signals resulting from said scanning, and subsequentlyheating the mosaic electrode to a temperature of approximately 200 C. for a predetermined period of time to photosensitize said metallic particles.

WIILARD HICKOK.