US1670463A - Electron-emission material and method of preparation - Google Patents

Description

Patented May 22,1928. I

UNITED STATES,PATENTTOFFICE.

JOHN -WESLEY MABDEN, OF EAST ORANGE, NEW JERSEY, ASSIGNOR TO WESTING- HOUSE LAMP COMPANY, A CORPORATION OF PENNSYLVANIA.

ELECTRON-EMISSION MATERIAL AND METHOD OF PREPARATION.

No Drawing.

This application is a continuation-impart of my copending application Serial No. 504,154, filed Sept. 29,1921, production of rare metals and alloys thereof and assigned to the \Vestinghouse Lamp C0mpany.

This invention relates to electron-emlssion material and the preparation thereof and more particularly to the em loyment of alloys of aluminum and meta s such as thorium, uranium, zirconium and the like for activating such material and for other purposes.

An object of my invention is the preparation of activated filamentary material by 1 ncorporating :efiicient metallic electron-em ssion material into refractory metal, said electron-emission material being first combined with another metal as a carrier and protector of the electron-emission material. Another object of my invention is the employment of alloys of aluminum with refractory rare metals for activating material for electron-emission purposes and the like.

A further object of my invention is the activation of metals of the tungsten class including tungsten, molybdenum, platinum, tantalum and the like by incorporatmg therewith an alloy of aluminum and an electron-emission material such as thorium, uranium, titanium, zirconium or the like and subse uently eliminating the aluminum there rom by vaporization.

A still further object of my invention is the preparation of alloys of aluminum and refractory rare metals such as thorium, ura-, nium, zirconium, titanium and the like for application to an element of an electron device, whereby activation of an electron-emitting filament therein may be accomplished by the rare metal, simultaneously with a clean-up action by the aluminum.

Other objects and advantages of the invention will become apparent as the description proceeds. I

In the copending application of Marden and Rentschler Serial No. 583,377, filed August 21, 1922."rare metal alloys and method of pre aration and assigned to the Westinghouse llamp Company, is disclosed a method of preparing activated molybdenum by incorporating a small proportion of metallic thorium therein prior to pressing, sintering and working the same to the desired form. The method disclosed. has given ood results, but it entails the use of metallic thorium Application filed April 11, 1923. Serial No. 631,366.

which is not only difficult and expensive to prepare, but readily oxidizes and is easily contaminated by moisture, .nitrogen and other materials.

In the copending application of C. T. Ulrey, Serial No. 577,021, filed July 24, 1922,

manufacture of electron-emitting devices.

.is satisfactory, but. likewise entails the useof metallic thorium which, as before noted, is expensive, difficult to prepare andreadily contaminated by other substances.

According to my invention, I accomplish the results obtained according to the aforementioned applications, but instead of using metallic thorium, I propose to use an alloy, solid-solution, mixture or compound of thorium or other material with high electronemitting properties, such as uranium, zirconium or the like, with aluminum, which serves as a protector, of the thorium or other rare metal, from contamination and holds it in a form in which it may be conveniently utilized for the purposes desired. The alloy of the electron-emitting metal and aluminum maybe prepared in a manner similar to that disclosed in my copendin'g application before referred to, Serial No. 504,154, filed September 29, 1921, production of rare metals and alloys thereof and assigned to the Westinghouse Lamp Company.

After preparing the desired aluminum alloy, according to said application, the same may be powdered and mixed in the desired proportion with the' selected refractory metal to be activated, for example, molybdenum. The mixture may then be pressed into the form of a cake or slug,

tered in a vacuum or heat-treated to cause the particles to coalesce to form a homohigh enough tempreferably square in cross sectlon. It may then be sin-' Instead of producing activated material in such manner, an electron-emitting cathode may be activated in place in a radio tube or the like, by applying to an adjacent plate or anode in said tube, a coating of powdered thorium aluminide or other alloy or mixture of aluminum with a metal having good electron-emissivity.

Said material may be applied by mixing the same with a suitable binder, such as an amyl-acetate solution of nitrocellulose, and painting the mixture upon the anode or, if desired, directly on the cathode. Instead of applying the alloy by painting, the same may be formed of a strip or clip and fastened to said plate or anode in any suitable manner, as by pasting, crimping or welding.-

' process may be employed for preparing acpared tivated tungsten, or other metal of the tungsten class.

The thorium-aluminum alloy may be preaccording to my aforementioned application Serial No. 504,154, ofwhich this application is a continuation-in-part or, briefly, as follows.

A halide salt of thorium, for example, thorium-ammonium-chloride is prepared, preferably by the recess disclosed in my copending application Serial No. 498,397, filed September 3 1921, preparation of metals and their halides and assigned to the Westinghouse Lamp Company. -That process involves the treatment of the hydrated salts of the rare metals with a displacing substance, for example, ammonium chloride added to a solution thereof. The solution may then be evaporated to dryness to eliminate the water and part of the ammonium chloride.

The residue, which is thorium-ammoniumchloride, may be placed in a graphite crucible and covered with a previously fused and cooled mixture of sodium and potassium chloride or fluoride or other halogen alkali compounds. The crucible, with the mixture therein, may be placed in a furnace and the temperature raised until the mixture is fused to a clear liquid.

In order to prevent oxidation, 9. slow current of carbon tetrachloride vapor or other desired manner to recover the thorium-aluminum alloy or aluminidc. It is deemed preferable to purify the powder by treating with a fairly strong solution of caustic soda to dissolve out the excess of aluminum not alloyed with the thorium.

The residue, which is thorium aluminide (Th Al may then be washed with water and treated with dilute acid, for example, hydrochloric acid, to remove all the caustic soda and acid-soluble materials from the alloy. After filtering and drying, the aluminide may be used for activating purposes, as will be subsequently described, for hardenlng aluminum by formin aluminum alloys containing'small proportlons of refractory rare metals or for any desired purpose.

The powdered alloy or aluminide may be mixed in the desired proportion with pow-- dered molybdenum or the like. In order to repare activated material or material havmg high electron-emissivity, it is only necessary to use a small proportion of the' thorium alloy with a molybdenum, tungsten or other refractory metal powder which may be used. For example, from V; to 1%, by weight, of the alloy 1s sutlicient to give good activation. Such mixture may then .be pressed into a cake or slug, preferably square in cross section, and considerably longer than the cross sectional dimensions thereof. Slugs for experimental work 3 inches long and inch square in cross section, may be used, but, in practice, it is preferable to use slugs about inch to inch square and 8 inches long.

The slug may be sintered to a high temperature in a high-frequency induction furnace, that is, for example, the pressed slug may be placed on a sheet of molybdenum or other refractory metal and inserted in a sintering furnace of the type described in the copending application of Marden and Rentschler Serial No. 488,229, filed July 28, 1921, a process of m anufacturin metals and their uses, and assigned to the estinghouse Lamp Company.

Such sintering is optional and 1s recommended for slugs of small size only, or where such have been formed too weak for the heat treatment, later described, without sintering. If the material has been sufiiciently sintered and' the of the same maintained, by causin the'sintering to beaccom-' plished by a hig vacuum, the current may then be shut offend the resultant product permitted to cool within the furnace while maintaining the high vacuum. v r

A pressed slug either sintered,',or the optional sintering operation having been emitted, maythen be heat-treated, at a tem- 'perature nearly high enough to melt it, in a specially-constructed furnace, for example,

i such as that described and claimed in the copending application of Rentschler and Marden Serial No. 488,230, filed Julyv 28, 1921, furnaces and assigned to theWestinghouse Lamp Company. Any furnace wherein the slug may be heatedin an inert environment, such as a high vacuum, to nearly the melding point thereof, may be employed.

The resulting alloywill be found to be strongly coherent, dense, tough and less liable to crack on-unequal heating and cooling and to have lost entirely all of its brittleness, so that, when it is heated red-hot, it-

may be readil swaged, rolled, hammered or drawn to ren er it ductile. The slug may be rolled to size or swaged hot until round in cross-section and then drawn down to the desired size of wire, in a manner well known for working molybdenum, if such is used, or for workmg tungsten or other refractory metal, if such is used, instead of molybdenum. r

The sintering or heat treatment previously described may be conducted at such high temperature as to eliminate the aluminum content by distillation or vaporization thereof. The thorium aluminide, or other alloy used, will not dissociate until the slug has been substantially entirely rid of gases, which prevents the thorium,- after the aluminum has been vaporized therefrom, from gas contamination, whereby it is leftin a metallic condition alloyed withthemolybdenum, tungsten or whatever refractory metal, the thorium-aluminum alloy has been added to. Wire formed from such an alloy, is an eflicient electron-emission material, in fact as.

good as pure thorium, even though the thormm content may be ver small. The aluminum content has a uri ying and scavenging action on the pro uct, whereby an alloy is obtained which is more free from oxides and other deleterious impurities than those heretofore employed.

. Activation by means of thorium aluminide or other thorlum-aluminum alloy may be effected, in an electron tube or the like, by

applyin the same upon the plate or anode therein, y mixin the same in a powdered form with a suite le vehicle or binder, such as an amyl-acetate solution of nitro-cellu lose, and painting the mixture upon the plate. An alternative method may consist in applying the aluminide to the plate or plate may then be heated to a bi h temperature in any convenient manner, or example, as described in the aforementloned applicatlon is tran'sferre or sputtered from the plate to the electron-emitting filament, to cause the same to become activated. The aluminum content, in this instance, may serve a double, function of preserving the thorium from contamination until it is desired to use anodein the form of a clip or strip fastened thereto in any suitable manner.

of C. T. Ulre whereby the thorium content the same for activating purposes and, also metal is usedmerely as a hardener and strengthener in. the aluminum, togive the same desirable properties for castings which could be used in automobiles, aeroplanes and other devices needing light metals of high tensile strength. and where the parts could be cast.

The-desired alloy may be prepared as herealuminum is usedso that the proportionof the rare metal, for example, zirconium, is

tofore described, although preferably more relatively small. Aluminum holds about 8% of zirconium, after which crystals, havingabout 30% aluminum in them and corresponding to the formula Zr,Al separate from the mass. An 8% zirconium-aluminum alloy has a Brinellhardness of about 54, a

Shore hardness of about 12, an elastic limit of 13,500 pounds per -s uare inch and a tensile strength of about'2 ,000 pounds per square inch. This is about the same tensile strength and hardness as a copper-aluminum alloy, having perhaps about 10 to 12% copper. A 4% zirconium-aluminum mixture or alloy has-a hardness and tensile strength cor, relsponding to about a 6% copper-aluminum a 0y.

Although .I have described what I now consider to be the preferred methods of practicing my inventlon, in connection. with certain specific alloys or mixtures, it is to be noted that the-alloys mentioned are merely illustrative and that my invention is adapted for alloys of aluminum with the refractory rare metals for electron-emission purposes or 7 general use.

What is claimed 1s:

1. The process of activating refractory metal for electron-emission urposes com prising incorporating a sma proportion of a powdered aluminum alloy of electrona desired proportion to cause the particles thereof to be firmly welded together, said heat-treating operation being conducted in a vacuum.

3. The process of alloyin or. forming a coherent ductile alloy of thorium and mol bdenum comprising mixing powdered t 10- rium-aluminide and powdered molybdenum in the desired proportion, pressing the same in the form of a slug and heat-treating until the aluminum is eliminated therefrom and the particles of thorium and molybdenum become firmly welded together.

4. The method of making alloys or mixtures comprising mixing an alloy of thorium and aluminum in powdered form with powdered molybdenum, pressing the mixture into the form of a slug and heat-treating to vaporize the aluminum therefrom and cause the particles of the remaining alloy to weld firmly together said heat-treating operation being conducteinan inert environment.

5. The method of producing refractory I alloys for electron emission purposes whic comprises activatmg a filament b .the addition of an alloy of aluminum an a metallic electron emission materialand purifying by heating the filament in a vacuum to eliminate the aluminum therefrom.

6. The method of activatin refractory metal filaments consisting in a din thereto an alloy of thorium and a'vaporizable metal and heating the filament to eliminate the vaporizable metal.

7 .The method of activating refractory metal filaments comprising associating therewith, an alloy of thorium and aluminum and heating the filament to eliminate the aluminum therefrom.

In testimony whereof, I have hereunto subscribed my name this 10th day of April 1923. i

' J OHN WESLEY MARDEN.