US1698850A - Activation of refractory metal filaments - Google Patents

Description

Patented Jan. -15, 1929.

. UNITEDSTA Joan wnsnnx mnnmor EAST m2 comm,-va

ORANGE, NEW JERSEY, ASSIGNOR TO WESTINGHOUSE CORPORATION OF PENNSYLVANIA.

nc'rrvarroiw or nnrnac'ronx METAL FILAMENTS.

No Drawing.

This invention relates to the activation of material for electron-emission.purposes and more particularly to the useof ahalide of thorium or the like and aluminum'for such 6 purposes.

An object of my invention is the preparation of filamentary material coated with metallic thorium, whereby good electron emissivity is imparted thereto. 7

Another object of my invention is the'application of a mixture of halide of thorium and finely powdered aluminum, whereby when the same is heated, metallic tho: rium is developed thereon to give good activation.

A further object of my invention is the use of thorium fluoride or an anhydrous double fluoride of thorium and an alkali metal,

mixed with powdered aluminum, for activa- 2 tion purposes, either by applying the mixture directly to a filament or cathode for electron-emission or by application to a component p ple, the plate or anode of a radio tube and transfer of thorium developed thereon by heat, to an adjacent filament.

A still further object of my invention is the utilization of a halide of a refractory metal, such as thorium, uranium, zirconium or the like, preferably the fluoride or double fluoride with an alkali metal, mixed with powdcred aluminum for-the preparation of the respective refractory metal for activation or other purposes.

Other objects'andtadvantages of the in-v vention will be made apparent upon reading the following description.

Various methods have been employed for developing thorium on filamentary material in evacuated vessels, for electron-emission purposes. a I p In the copending application of C. T. Ulrey, Serial No. 577,021, filed July 24'. 1922, manufacture of electron-emitting devices and inghouse Lamp Company, is. disclosed a method of applying thorium to the e ectronemitting filament in a radio tube or the like by applying a. iece of metallic thorium 'tothe plate thereo applying finely divided thorium art of an electron device, for exam-.

' he-likeand assigned to the Westthe plate, a mixture of thoria,

be emp hpplieationflled Harch 31, '1923. Serial No.629,190. I

in the tube by highly heating the plate in any by making desired manner, for example, by electron,

bombardment or high freqHency induction, to cause a transfer of thorium from the plate to the filament.

' In my copending application Serial'No. 629,189, filed March 31, 1923, electron devices and their manufacture, and assigned to the \Vestinghouse Lamp Company, is disclosed a method of activation, similar to that of the Ulrey application above referred to,

except that, instead of using free thorium on or other thorium compound, and carbon may be applied thereon,'with or without a clean-up agent, such as aluminum or magnesium. :Such a mixture on the plate is subsequently heated to cause a. liberation of thorium and transfer thereof, at least in part, to the electron-emitting filament. I

-Both of the aforementioned methods are desirable but the former entails the use of metallic thorium which is expensive and difficult' to keep in an unoxidized or uncontaminated condition. The latter obviates the use of expensive metallic thorium but entails the use of such material as carbon which results the liberation of gaseous carbon compounds which are undesirable as part of the residual atmosphere in the bulb and necessitate an extra quantity of clean-up materlal "for their elimination. v According to my present invention, I

avoid the use of expensive metallic thorium and the generation of undesirable gaseous carbon compounds by using a salt of thorium which is reducible by aluminum'or other desirable metal. The aluminum may serve the double purpose of reducing the thorium compound used and cleaning up residual gases in the bulb.

' It has been found that the class of salts of thorium most suitable for the purpose are the halidesyof which the fluoride is preferred.

Although thorium tetra-fluoride: (ThFg may oyed, for certain reasons a ouble fluoride of thorium and an alkali metal, for example, potassium-thorium fluoride, having the formula KThF or K ThF is preferred. Such material, preferably mixed with an excess of finely divided aluminum, may be applied or gettered on a tungsten wire, for example, in any desired manner, using cryolite mixed therewith as a binder, if desired, or painted on or attached to the plate for a radio tube and sealed into a bulb. After exhausting the bulb, the wire or filamentary cathode therein may be activated by heating the same, if the mixture was applied directly thereto, or if the mixture was merely applied to the plate adjacent thereto, by heating said plate until thorium is liberated therefrom and transferred to the filament.

In the following description, my invention will be explained more in detail.

It is known in metallurgy that thorium chloride, for example, may be reduced by sodium but neither thorium chloride nor sodium can be exposed to the air without con-- tamination, nor can they be gettered or applied to a wire or filament in any known manner without complete oxidation of both the sodium and the thorium chloride.

In my copending' application Serial No. 498,397, filed September 3, 1921, the preparation of metals and their halides and assigned to the Westinghouse Lamp Company, is disclosed a method for the production of the anhydrous halides of refractory metals, includin" thorium, by the treatment of a hydrous salt of such a metal with an agent, for example, ammonium chloride, which will displace the water from such salt. Such method is particularly adapted for producing thorium chloride, which may then be used for the production of other halides of thorium, more particularly adaptable for practicing the present invention. For example, thorium chloride, produced in any desired manner or according to the method disclosed in my referred-to'application, may be fused with a small excess of potassium fluoride to produce potassium-thorium fluoride (KThF or K,ThF according to one of the following equations The exact proportions in which these salts are fused, is not essential, as a small excess of potassium fluoride makes no difference. After complete fusion, the mass is cooled powdered and the soluble constituents thereof dissolved i'iutby water. The excess of potassiuni fluoride'dissolves away, leaving the 'dou'- ble thorium salt desired, which'is' insoluble in water and dilute acids. This should then be ground, as finel; as possible, and mixed with aluminum, which is preferably in a finely divided condition and in excessof that 1 ne d to cmnuletelv reduce the thorium C ntained in the compound according to the fol lowing equation V rectly. This salt may be prepared by -pre cipitation from a water solution of the chloride, but is not always free from oxide when made in this way. It is, however, suitable for activation purposes.

The double halide of thorium and an alkali metal or the halide of thorium which, as heretofore mentioned, it is desired to use, may then be mixed in the form of a dry powder with finely divided aluminum and applied either to a filament or wire to be activated or to a-plate intended to be used in cooperation therewith. The proportion of aluminum and the halide salt used, does not hate to be exact, but it is preferable that there shall be an excess of aluminum over that needed to reduce the thorium in said salt to the metal, for the purpose of cleaning up residual gases in the bulb, although, of

course, if such excess is not employed, some other clean-up agent, for example, magnesium, may be used instead.

The mixture of potassium-thorium fluo ride and aluminum, for example, may then be applied to a filament or wire of'tungsten or the like, which it is desired to use for electron-emitting purposes, either as an aqueous suspension or paste or in the form of a paint with some suitable vehicle or hinder, such as an amyl acetate solution of nitrocellulose or with cryolite, in a manner similar to that, for holding thoria and the like ona filament, and described and claimed in the copendin application of MacRae and Richardson, Se 8, 1922, getters and the application thereof and assigned to the Westmghouse Lamp Go. The filament may then be sealed into a bulb and heated to incandescence until the thorium is reduced, as may be determined by noting the increase in the thermionic current therefrom, in any desired manner, or in the manner disclosed in the application of C. 'T. Ulrey, before referred to. v

Instead of applying the mixture directly to the filament, the same may be applied instead, to a plate or anode intended to be used in the bulb in cooperation with, and adjacent to said filament. 'After sealing both the filament and the coated plate into a bulb and evacuatin said bulb, the filament may be activated by heating the plate in any desired manner, for example, by electron bombardment orhizzh-freguency indueiiom until the rial No. 566,847, filed Junev IUD - asaforedescribed, the

filament becomes activated to a suitable degree by a transfer of liberated thorium thereto from the plate in a manner similar to that disclosed in the-aforementioned Ulrey application.

The excess of aluminum serves .to clean up residual gases in the bulb, making it possible to use a cold exhaust as disclosed in the co-' gending application of H. C. Rentschler, erial No. 582,261, filed August :16, 1922, vacuum devices-and method of exhausting the same and assigned to the Westinghouse Lamp Company.

It is obvious closure of means into a vacuum device, device containing an inert gas pressure, sired purpose. For example,- if it is desired to introduce calcium or any element, for that matter, a mixture of a compound or salt thereof with a reducing agent, such as aluminum, may be applied to the plate'of a for introducing a metal or even an evacuated at reduced ample, or even upon an auxiliary plate m a lamp. The mixture is subsequently heated,

and the desired metal, for example, is liberated for accomplishing the purpose, that be the activation of an electron-emitting filament, the cleaning-up of residual gases for combination with which'it is particularly adapted, or the purification of an inert gas filling.

Although thorium has been-mentioned as;

the preferred activatin material, electronemitting filaments may %e activated by other metals, having good electron-emission properties, for example, uranium, zirconium and the like, in the manner disclosed for activating by means of thorium and Ido not wish to be restricted entirely to thorium, but

desire that it be' understood that such modifications as fall within the scope of the appended claims pertain to my mvention. What is claimed is: v 1. The method of activating, in a. vacuum,

a refractory filament for electron-emission purposes comprising placing adjacent thereto, a mixture of a halide of thorium and powdered aluminum and heating said mixture to cause metallic'thorium to be liberated and form on 'the filament.

2. The method of emissivity ofa filamentary comprising placing adj ace ture of a double halide alkali metal with finely divided aluminum and heating said mixture to cause a reduction of said thorium compound and a formation of thorium on the cathode.

3. The method of activating a refractory increasing the electronmetal cathode thereto, a mix.-

filament for electron-emission purposes comprising applying thereto, a mixture of a halide of a metal of good electron-emission metal on the filament.

that the foregoing'is a disfor activation or for any 0 her deblndlng material, enclosing the same n an thorlum, uranlum, zirconium,

radio tube, for .ex-

compound is reduced whether ated envelope into double function of thorium and an properties and powdered aluminum, enclosing the same in an evacuated vessel and heating the filament to cause a reaction in the applied mixture with a liberation of the free 4. The method of activating a refractory metal filament for electron-emission purposes, comprising applying thereto, a mixture of a fluoride of thorium and-aluminum with "a suitable binding material, enclosing the same in an" evacuated envelope and heating the filament to cause areduction and liberation of free "metallic thorium thereon.

5. The method of activating a refractory metal filament for electron-emission purposes comprising applying thereto, a mixture of p'otassiumthorium fluoride and powdered aluminumheld thereon by means of a suitable evacuated envelope and heating the filament to cause an interaction between the thorium compound and the aluminum with the liberation of free metallic thorium on the filament.

.6.- An electron device comprising an evacuated vessel. having sealed therein a cathode, an anode and a mixture of a halide of thorium and aluminum applied to at least one of said eleme ts.

n electron devlce comprising an evacuwhich is sealed a filament of which has been applied a metalsuitable for a metallic and a plate, to one a mixture of a halide of electron-emission purposes, and. reducing agent.

8. An electron ated-envelope enclosing a cooperating therewith and ad'acent. thereto and a mixture aluminum applied to one of said elements. 9. An electron device comprising an evacudevice comprising an evacuated vessel enclosing an electron-emitting cathode and a cooperating anode, one of said elements having applied thereto, a mixture of potassium-thorium fluoride and anexcess of aluminum, whereby said aluminum serves the of reducing the thorium'salt to liberate thorium and to clean. up residual gases in the vessel. 4

10. An electrondevice comprising an evacuated'envelope into wh1ch is sealed a cathode and an anode, and a mixture ofa fluoride .of j

'1 filament and a plate of a fluoride o thorium 'and' thorium and powdered aluminum applied to saidcathode." u

11. An electron device.'comprising an evacuated envelope enclosing a filamentary cathode coated with .a mixture of potassiumthoriu'm' fluoride and an excess of aluminum, and an anode cooperating therewith.

'12. An electron device comprising an evacuated vessel enclosing a filament and a cooperating electrode coated with a mixture of thorium fluoride and aluminum and held thelreon by means of a suitable binding mate r1a 13. The-method of introducing thorium into an evacuated device comprising mixing potassium thorium fluoride with powdered aluminum, applying said mixture to an element in the device and heating to cause a reaction With the liberation of free thorium therein.

14. An evacuated device comprising a vessel enclosing a mixture of potassium thorium fluoride and aluminum, said mixture being applied on a conductiveelement in the device whereby the same may be heated to effect a reaction in the mixture.

In testimony whereof, I have hereunto subscribed my name this 30th day of March, 1923.

JOHN WESLEY MARDENI