US2389060A - Refractory body of high electronic emission - Google Patents

Description

Filed Aug. 13, 1943 I N V EN TOR. (44405 KW? 72 A TTORNEY Patented Nov. 13, 1945 2,38%060 REFBAOI'ORY BODY OF HIGH ELECTRONIC EMISSION Jacob Kurtis, Teaneck, N. 3., asslgnor to Callite Tungsten Corporation, Union City, N. 1., a corporation of Delaware '7 Application August l3, 1943, Serial No. 495,582 7 6 Claims. (01. 250-27.5)

The present invention .relates to refractory bodies of high electronic emission and to methods of manufacturing the same. More particularly, it relates to such bodies as are intended the ordinary cathode of this type. Assuming,

for instance, that the cathode of the present invention is tubular or sleevelike in form and that it has a diameter and length no greater than that of the type of such cathodes at present in for use as cathodes of radio tubes and similar use, it will have a considerably greater effective electron discharge devices, surface forthe reason that it is porous or foram- It is quite old in the art to employ cathodes inate in structure and that not only the external that depend on coating a surface with electron surface of the tube or sleeve carries a coating of emitting oxides such as thoria or the oxides of the electron emitting material, but this coating the alkaline earth metalsbarium, strontium 10 extends to the surfaces of the pores or foramina and calcium. Such cathodes may be heated within the metal of the sleeve itself so that there either directly or indirectly and the value of the is a considerable reservoir of electrons capable of emission is directly proportional to the extent being drawn p Ough a very long P od of of the surface coated with the oxides. operation. The effective surface of the sleeve of A directly heated cathode is generally a filathe present invention, therefore, includes not ment of thoriated tungsten or it may be a nickel o y the e e al Surface thereof but e Surfaces wire or ribbon having alkaline earth oxides coatof the pores or foramina extending throughout ed thereon. Such filaments are great improvethe whole structure, The electron emitting maments over the pure tungsten filaments for as a matter of fact, is carried within and whereas a pure tungsten filament may be heated th h the metal composing the sleeve. to incandescence and even then give very small The single figure of the drawing is a perspecemission of electrons, a thoriated tungsten filatiVe Viewment will operate at about 1700 C. and will give The structure thus described is not comparable a considerably larger emission at that temperato the structure of a thoriated tungsten filament, ture. A nickel wire or ribbon coated wit th for instance, where also the electron emitting alkaline earth oxides can be operated at about material is contained Within the y of the 700 to 750 C. and gives a very copious supply merit, for in this latter case the electron emitof electrons, ting material is located in the crystal boundaries An indirectly heated cathode generally conmaking p e fi ament, and it Only slowly dissists of a thin metal sleeve or tube coated exter- 11s to the surface d gives rise to a y limited many with the electron emitting material, Withemission of electrons. In the case Of the cathode in the sleeve is a heater which usually consist of the present invention, the electron emitting of a wire of tungsten or tungsten alloy, Th materials are already on the surface and this heater is used only for the purpose of raising the surface extends into and throughout the y cathode sleeve and the coating thereon to an f the cathode in such a manner that it is at electron emitting temperature, and no reliance all times available for the emission of electrons is placed on the heater itself for the emission of in a large volume and continuous fiow. electrons. Where electron emitting materials have here- 'I'he cathodes above described which are in tofore been incorporated intoatungsten cathode, general use in the art at th present ti are 40 it has always been considered that it is essential short lived and have a tendency to fall off in that electron emitting material should be introemission after a short period of use. In the case duced into the n s n bef e it is sint r d. and of indirectly heated cathodes, the effective surit is generally added to the tungsten Oxide face adapted to carry the electron emitting mafore reduction to metal powder and before formteriai is also very small, and it is larg ly for th ing the ingot which is later mechanically worked reason that the capacity for emitting electrons drawnis very limited and is exhausted within a com- The method of the present invention departs paratively short time, radically from this procedure. Refractory metal It is an object of the present invention to propowder, for examp tungsten o y u vide a cathode of refractor metal such as tungp w r, is mixed with a binder such as p sten or molybdenum that depends for it 1 fin dissolved in carbon tetrachloride in a manner tron emission on the surface coating of electron hereinafter described, Dressed in e (316 W t emitting materials, but which has an effective form Of a rod, and then heated in a furnace 1n coated surface many times greater than that of an phere of hydrogen at l w temperature but sufilciently high completely to volatilize and The temperature is then raised suiilciently to" cause partial sintering or pre-sintering of the body. The result is a rod which is extremely porous and this body is then soaked in an aqueous solution of the nitrate of one or more of the alkaline earth metals or thorium nitrate, until the limit of absorption is reached. The piece is then dried in the air to evaporate the water and the result is a porous body of tungsten having nitrate uniformly distributed through the mass. This body is then heated in a hydrogen iumace at a temperature Just high enough to reduce the nitrate to the oxide, and when this process is complete, the "temperature is then raised to a temperature of approximately 1500* to 2000" C. During this heat treatment a considerable shrinkage in volume takes place and the result is a strong, dense, refractory body, thoroughly impregnated with the emissive oxides. The material may also be formed into a tube, sleeve. or other shape according to the form of cathode desired or according to whether a directly heated cathode or indirectly heated cathode is to be employed.

The porosity of the refractory metal body, up to the time that it has been impregnated with the electron emitting materials, is a feature of great importance in the invention. I may, however, form this porous body of the hard dense alloy disclosed in copending application Serial" No. 460,226 of Kurtz and Williams. According to the method therein described and claimed, the hard dense tungsten alloy of the aforesaid application is formed into a porous or foraminate body in the following manner:

Purified refractor metal powder, such as tungsten, having a particle size of approximately from 1 to 25 microns, is combined with a small but effective amount of powdered metals that result in the alloying, bonding and densifyin of the ultimate body to be formed without mechanical working. The alloying, bonding and densifyins metal mixture is first prepared by thoroughly mixing and ball milling approximately equal proportionsby weight of pure finely divided nickel powder and pure finely divided platinum powder. Platinized nickel powder containing approximately 50% by weight of platinum, made by shaking nickel powder in a solution of chlorplatinic acid, may be used. I

Then approximately /2 by weight of this prepared mixture is added to finely divided tungsten powder of a particle size of 1 to 25 microns, and is thoroughly mixed therewith. It may be ball milled for a period of several hours or at least long enou h to insure the uniform distribution of the small amount of alloying powders throughout the mass of tungsten powder. This mixture of metal powders is then thoroughly mixed with a bonding material, such as paraffin dissolved in carbon tetrachloride, and the whole is then formed into the shape desired for the final cathode body in smile or other known manner. When so formed, the body is heat treated to a temperature suflicientto volatilize the bonding material and drive it out of the body without leaving any carbon residue.

The temperature is then increased to' a presintering temperature of about 800 to 1200 C;

and the result is an exceedingly hard, dense metal structure of foraminate or sponge-like type characterized by numerous intercommunicating' channels or pores. This foraminate b dy is then soaked in an aqueous solution of thorium nitrate or of nitrates of one or more of the alkaline earth metals such as barium, strontium and calcium, or a mixture of two or more of these.

The invention will be more clearly understood by reference to the drawing which represents a wire of the invention, greatly enlarged, with a portion cut away to show the foraminate structure.

In the drawing, I represents the wire or filament. In the interior of the filamentary body,

' passages 2 are shown intercommunicating with each other and coming out on the surface as holes or pores 3. In the finished body the electron emitting oxides are deposited within the foramina or passages of the interior as well as on the surface thereof.

Instead or seeking the body in the nitrate solution, the nitrate may be added by slowly pouring the solution drop by drop upon this porous intermediate body until a desired measured amount has been completely absorbed or until the porous tungsten body hasabsorbed the nitrate solution to its full capacity. The piece is then dried in the air to evaporate the water and at this stage the nitrate is distributed throughout the pores of the entire piece'as'a dry film. The body is then heated in a hydrogen furnace at a temperature lust sumcient to decompose the nitrate to the oxide, and when this conversion is complete the temperature is raised to a temperature of approximately 1500 1.92000 C. A very considerable shrinkage takes place and the body becomes a strong, porous body of hard dense metal, thoroughly impregnated with the electron emitting oxides.

Instead of the above described tungsten, nickel. platinum alloy, an alloy consisting of 99% of metal of the group consisting of tungsten and molybdenum, 0.25% of nickel, 0.25% of beryllium and 0.50% of platinum may be employed, the same being formed into the porous or foraminate body in the same manner as above described. It will be further understood that in specific examples above given which illustrate the invention by the tungsten metal specifically, molyb-, denummay be used instead of tungsten, the procedure and temperatures specified being the same.

It will be apparent that in the manner hereinabove described, electron emitting bodies either of the directly heated type or of the indirectly heated type may be formed as desired.

Having thus described my invention, what I claim is:

1. Refractory body having high electronic emission when heated consisting of refractory metal of the group consisting of tungsten and molybdenum and their alloys of foraminate structure coated with electron emitting oxides both on the external surfaces and on the surfaces of the foramina of said body.

2. Refractory body having high electronic emission when heated consisting of tungsten metal of foraminate structure coated with electron-emitting oxides of alkaline earth metals both on the external surfaces and on the surfaces of the foramina of said body.

3. Refractory body having high electronic emission when heated consisting of tungsten metal of foraminate structure coated with thorium oxide both on the external surfaces and on the surfaces of the foramina of said body.

4.-Refractory body having high. electronic emission when heated consisting of hard dense tungsten alloy of ioraminate structure coated 6. Refractory metal body having high elecwith electron emitting oxides both on the extronic emission when heated which comprises ternal surfaces of said body and on the surfaces an alloy consistingoi! 99% of metal of the group of the foramina within the same. tungsten and molybdenum, 0.25% nickel, 0.25% "'5. Refractory metal body having high elec- 5 beryllium, and 0.50% of platinum; said alloy body tronic emission when heated which comprises having a foraminate structure and a coating of an alloy consisting of.99.5% of metal of the group electron emitting oxides both on the xternal consisting of tungsten and molybdenum, 0.25% surfaces of said body and on the surfaces of the of nickel and 0.25% of platinum; said alloy body Ioramina within said body.

having a foraminate structure and a. coating of 10 JACOB KURTZ.

electron emitting oxides both on the external surfaces 0! said body and on the surfaces of the ioramina within said body.

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