US2188940A - Electron discharge device - Google Patents

Description

1940- B. A. DIGGORY ET AL 2,138,940

ELECTRON DISCHARGE DEVICE Filed Dec. 2, 1937 H20 mpon B. A. D/GGORV INVENTORS- 61K. TEAL A 7' TORNE V Patented Feb. 6, 1940 1 NI ED STATES Fries v Benjamin A. mil-my, Plainfield, N. 3., acerdon K. Teal, NewxYork, N. Y., assignors to f Bell Telephone Laboratories, Incorporated, I New York, N. Y., a corporation of New York Application December 2,1937, Serial No. 177,660

11 Claims. (01'. 250 275),

This invention relates to methods of oxidizing metallic elements and to electrode structures, and more specifically toelectrode structures in special types of electron discharge devices and to methods of oxidizing the electrodes inysuch devices. j '1 X In certain electron discharge devices such as, for example, photoelectric tubeselectroni'c cam era tubes for television, ande'lectron multipliers, U electrodes having photosensitive surfaces are pro vided. In the preparation 'of these surfaces a metal face'of the electrode is oxidized and then, in many teases, treated with a metal, such as silver. followed by photosensitization with an alkali metal. I 3

In the preparation of some'types of standard photoelectric tubes, the oxidation of the cathode secondary emission. devices; Asymmetries' in the 30 electrode structure of aydevice are reflected in asymmetries in the electricfields and, where the oxidation is accomplished by means of charged particles, nonuniformity of oxidation of they electrodes may follow. In a device in which the electrode is insulated and to which an electrical connection cannot bemade, 'it is impossible vto pass a continuous current to it as is done in the discharge method described. above. If the electrode to be oxidized is a thin metal film on glass,

W a continuous discharge passed directly to it usually tears-it ofi the glass. Also, it is sometimes. desirable use metals other than silverfas' a base of a photoelectric or secondary emitting matrix and difficulty may then be experienced in oxidizing the metal by passing adischarge current to it in an oxygen atmosphere. For'example, copper cannot be; oxidized, at least not more than.- very superficially, by an electric discharge. v so It is an object of this invention to provide novellelectron'discharge structures utilizing electrodes which behave differently in. the presence of an oxidizing atmosphere.

It is aiurther object of this invention to provide anovel method of oxidizing metal strucmeans such as, for example, by means of a spark Such conditions, however, cannot be tures which is particularly suitable for use in the preparationof electrode structures and photosensitive targets such as, for example, those used in television electronic cameras and electron v multipliers. i In the method according to this invention as hereinafter described in detail by way of example, ozone [(03) is generated by any suitable coil and the gas allowed to flow through water 0 metallic electrode, the tube structure-is baked out and evacuated at about 250 C. Photosensitizaticn of the oxide film follows thisbake-out.

Theinvention will be more readily understood from the following description taken in connection with the accompanying drawing" forming a part thereof in which:

Fig. 1v shows an electronic camera such as is used in television transmission in which the method of thisinvention may be applied;

Fig, 2 is'an; enlarged cross-sectional view of the mosaic screen or target of the tube shown in Fig.1; I Fig. 3 shows apparatus for preparing the ozone 3 used in the method of this invention; and

Fig. 4 shows an electron multiplier tube in which the methodof this inventioncan be used.

Referring more particularly to the drawing,

Fig. 1 shows an electronic camera tube used as a pick-up device in t'elevisiontransmission com,-

prising'anevacuated container l 0' enclosing an electron "gun assembly ll for producin a moving beam ofelectrons and. for accelerating this beam toward'a screen S at the end of the tube 40 1939, which/shows and describesan electron gun f comprising a cathode and two accelerating anodes for iocussing. a beam of electrons upon a target, one of these anodes being'a conducting cu coating on the inside wall of the tube A simi-' lar. coating is shown in Fig. l of this application and is designated by the reference numeral l2.

Either electrostatic or electromagnetic means. (not shown) can .be used for deflecting) the beam so that it scans every elemental area in turn of the screen S. Both of these means are well known to the art.

Reference will. now be made to Fig. '2 for a more detailed description of the mosaic screen structure S. This screen structure has as a base a glass member 20 in the form of a very thin center portion 2| and a thicker edge portion 22. On the face of the glass member remote from the electron beam is evaporated a thin coating 23 of a suitable electrical conducting material, such as aluminum. On the surface of the center portion 21 nearer the electron beam is coated a discontinuous layer 24 of photosensitized metallic globules which isfprepared by sputtering the center portionwith a layer of metallic material such as silver, which is heated to make it discontinuous. The discontinuous globules are oxidized and caesium is'introduced and the tube is baked to make the surface 24 photo-emissive. p

The preparation. of the glass member'ZEl for use in the screen S will now be described. A piece of glass, preferably about .005 inch thick is etched by total immersion in a suitable etching solution, such as hydrofluoric acid, the edges of the glass being protected by a coating of a suitable substance which is not affected by the hydrofluoric acid, such as, for example, beeswax, so that the film of glass produced by the etching process has a. border of glass of the thickness of the original glass plate by which it may be supported. For amore complete description of the method of preparing these glass screen members, reference maybe made to the above-mentioned Teal patent wherein the process is described in then cooled. The heating causes the silver to break up into discrete globules. This process may be repeated one or more times in order to obtain a film of silver of the desired. thickness. A

platinum film'is then formed on the entire back surface of the blank by sputtering. The blank is then mounted in the tube and the tube baked out and evacuated so that all the elements in the tube are thoroughly degassed. Thetube isthen ready for the oxidation process.

Referring now to Fig. 3 which shows a preferred form of apparatus for preparing moist ozone used in the oxidizing process, oxygen is introduced through the inlet 38 and passes between the electrodes 3i and 32 which are connected to a spark coil (not shown). The inside electrode 32 is an aluminum cylinder sealed into the pyrex glass tube 33 while the outside electrode is of copper foil attached to the external surface of the glass tube 33. The ozone formed in the silent discharge in the condenser chamber 36 is swept through the glass. tube 35 and thesintered glass filter 3t and is saturated with water vapor by passing through the water in the reservoir 37.

The moist gas isfcollected in the tube 33 and spray is removed from it by passage through the.

ing the latter to about 50 '0. Failure to do this 1 results in a 'non-uniformfilm of silver oxide;

The excess ozone delivered from the,electronic. f

through a glass tube heated to about 550? C. (not shown). :1 l r The degree of oxidation of the silver film mayv be followed by determining its resistance as a function of time. When theresistance reaches a j limiting value it is assumed that oxidation of the, silver is approximately complete. Measurements made on the approximate ratio of silver to oxygen in silver films oxidized with moist ozone in-Y dicated approximate agreement with the formula I AgzOL This indicates thatthe chief reaction I occurring maybe represented by the equation J" Moisture :is necessary to catalyze the reaction; without it, the oxidation proceeds with extreme,

slowness.

. After oxidation of the silver film, the electronic camera tube is then baked out and'evacuate'd at about 250 C. This temperature is suflicicnt to decompose the silver oxide completely if mainestablish equilibrium. conditions. I However, the

approach to these conditions is rather slow and 'tained for a sufficient length, of time to fully l i the amount of decomposition is made negligible I by limiting the time of baking. If thaozone method of oxidation as described abov'ejis used, the metal portions of the tube should be made of materials not attacked readily by ozone. Aluminum metal, platinum, and chromium-plated nickel -have been found, to be fairly satisfactory in-this respect. 1 I

,At the conclusion" of the: oxidation process, excess gas is removed by evacuation and a known amount of caesium is emitted in the bulb by flashing a caesium pill. The pill is fiashedin a side tube or in the bulb so that caesium .vapor passesinto the bulb to photosensitize the globules. of the screen S. Fora description of a process of photos'ensitizing with a caesium pill' and the composition of such a' pill, reference may be made to British Patent381606 to George R. I,

Stilwell and Charles H. Prescott, Jr., complete accepted-October 10; 1932. .The tube may then be J bakedat a temperatureof about 200 to 225" .C.

for varying degrees of time to remove all gases. For a further description ofa tubehaving .a

mosaic photosensitive target of the type def scribed herein together with a method of operaj tion of a tube of this type, reference may be'made to the above-menti'oned'Teal patent.

Fig. 4 shows an electron multiplier tube similar to that described byP. T.;Farnsworth in the I Journal .of the Franklin Institute, volume 218,. page-411 .(1934). This electron multiplier com-f prises anenVe-lope tfienclosing an anode .51 and a from apress 53 by support members 5d and 55.-

The photo-cathode 52 is made by sputtering a very thin film of nickel on the inside of the glass cylinder 56 and then asemitransparentfilm of I silver on the nickel film. Heavy rings 51 and 58 of platinum are sputtered on the inside at the ends of the cylinder to make contact with the silver film. The silver film is oxidized and" then photosensitized. Several attempts using various other methods of oxidizing the silver were made prior to the use of the meth'od according to this invention. An attempt. was made toox'i'dize the I silveri-by passage of a direct electric current using the method of this invention.

cylinder 56 and theanode 5|. In both cases difiiculties were encountered because of the silver film being torn away from theglass. The oxida- I tion was satisfactorily accomplished, however, by,

The ozone vapor is admitted to the tube at the inlet cc and passes from the tube through outlet 6|; Tube'lil is sealed on at .the conclusion of the oxidation but tube 60 is used to seal the envelope to the vacuum station. The proc- "limited to the preparation or electronic camera I I tubes or electron multipliers, but is of appl ication in photoelectric tubes or in any other cases ess is similar to that described above in connection with the, electronic camera." Itwill be understood that this method is not where it is desired to produceuniiornroxidation and it is impossible-to make electric contact to. electrodes in the tube or other containing ves I sel. While the method has been described in connection with oxidizing silver it is to be understood that it is suitable also in connection with the oxidation of certain, other metals, such as,

for example, copper. The heating temperatures in the process where copper is used are likely. to be higher than when silver is used due to the tact that copper oxide ismore stable than silver oxide but otherwise the processesare similar. the invention may obviously be made without departing from the spirit of the invention, the

scope or which is defined by the appended claims.

What is claimed is: I I

I 1. The method of o'xidizingan electrode structure to produce "on. said structure a uniform oxide layer of a thickness within predetermined limits, :which comprises the steps of subjecting. the electrode structure to moist ozone, and simultaneously heating the electrode structure to ,a

I temperaturesufficient to prevent condensation of water vapor thereon;

2. The method of; oxidizing an electrode structure to, produce on said. structure a uniform oxide'layer, of a thickness within predetermined limits, which comprises the steps of subjecting,

the electrode structureto moist ozone, and there-' after baking the structure. I I 7 3. The method of oxidizingtansilver electrode structure tov produce on saidstructure a uniform oxide layer of a'thickness within predeter- I mined limits, which-comprisesthe steps of sub: jectin g the silver electrode structure to moist ozone at a temperature of about '50 C., and

thereafter ba "g the structure to a temperature of approximately 250 C.

4. The" method of *oxidizing a metallic electrode structure to produce on'said structure a uniform oxide layer of a thickness within predetermined limits comprising the steps [of introducing oxygen into, a chamber wherein an electric Jdischargeisproduced to form therein I ozone, passing the ozone through a reservoir having water therein to add water vapor to said ozone, removing 'the sprayfrom saidm'ixture by passage through-atrap, and passing the resultant mlxturefwhich comprises oxygen, --ozone and water vapor to theelectrode structure.

Other modifications and applications of ofelectrode elements therein, one of said elements being of material which oxidizes in the presencefof moist ozone and another of material which .d'oesnot so oxidize when subjected to moist ozone at the same temperature, means for producing a mixture of ozone and water vapor, and means for conducting said mixture to the interior of said container to produce an oxide coating on said element which oxidizes in the presence oi moist ozone.

6. The combination with a gas-tight container, of an electrode of silver and a cooperating electrode of a material which does not readily oxidize in the presence of water vapor and ozone, means for producing a mixture of ozone and water, vapor, and means for conducting said mixture to the interior of said container to produce an oxide coating on said silver electrode.

The method of oxidizing a plurality of electrode elements in a gas-tight container, oneof said elements being of material which oxidizes -in the presence of moist ozone to produce an oxide which is unstable when subjected to temperatures ordinarily employed in baking electrodes, in electronic current discharge devices] and another-of said elements being of material which 'does, not so oxidize when subjected to moist ozone at the same temperature, which comprises the steps of mixing water vapor with ozone, conducting said mixture to the interior of said container and simultaneously therewith The combination with a gas-tight container,

maintaining said electrodes at a temperature at *which one of said elements will oxidize while the other of said electrode elements willnot oxidize to produce an oxide coating on one of said electrodes. I 8. The method of oxidizing an electrode elemeritv in a gas-tight container which electrode element is of a material which oxidizes in the presence of moist ozone, comprising the steps of mixing ozone with. water vapor, conducting the mixtureto the interior of the containerto form an oxide coating on said element, and applying heat to said element while said mixture is being conducted into the tubesufficient to prevent con- Idensation of water vapor thereon. I I 9. The method of oxidizing a metallic electrode structure to produce thereon a uniform oxide layer comprising the steps of passing ozone through water to add water vapor to said ozone,

removing the spray fromsaid mixture by passage through a trap, and directing the remaining mixture to the electrode structure.

10. The method of oxidizing .a metallic electrode structure of a material which readily oxidizes in the presence of moist ozone to produce. on said. structure a uniform oxide layer of a thickness within predetermined limits, comprising the steps of producing ozone, mixing the ozone with water vapor, and passing the resultant mixture in controlled amounts to the electrode structure. I

I 11. The method of oxidizing ametallic electrode structure of a material which readily oxidizes "in the presence of ozone to'produce on said structure a uniform oxide layer of a thickness within predetermined limits, comprising the steps of introducing oxygen into a I chamber wherein an electric discharge is produced to form therein ozone, passing the ozone through a reservoir having water therein to add water vapor to saidozon'e, and passing the resultant mixture to the electrode structure,

BENJAMIN A. DIGGORY.

GORDON I

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