US2149658A - Thermionic tube - Google Patents

Description

March 7, 1939. l H; H. ARMSTRONG E-r Al. v 2,149,658

' 'THERMIONIC TUBE Filed March 12, 1956 3 1 f5 .1MM

Patented Mar. 7, 1939 UNITED STATES y PATENT QFFQE THERMIONIG TUBE Application March 12, 1936, Serial No. 68,558

2 Claims.

This invention relates to improvements in electronic elements and more particularly to improved thermionic tubes, and is a continuation in part of our prior application Serial No. 48,589, filed November 6, 1935.

The desiderata in this particular eld, that is to say in vacuum tubes, includes such features as high electronic emissivity, thermal resistivity and longevity. The typical vacuum tube of today, as is known, comprises an evacuated envelope containing an electron emitting electrode, a plate and a grid. Usually the electron emitting electrode or filament comprises thoriated tungsten. The elements tungsten and molybdenum are likewise found to be efficacious in the operation of vacuum tubes and, as is known, are employed quite generally in the fabrication of the filament and grid.

The choice of tungsten or thoriated tungsten for the electron emission element is in effect a compromise. It is known that certain substances, such for example as calcium, barium, thorium and the like are better thermionic emitters than metals such as tungsten, molybdenum and so forth. However, while calcium has a low Work fiu'iction and is a good emitter, it hasI a relatively high vapor pressure so that marked evaporation occurs at low temperatures. The use of the thoriated tungsten filament therefore involves a combination of a material having a relatively low emission but which is quite refractory and an element having a high emission. This combination is possible because of the fact that thorium is strongly adsorbed on the surface of tungsten even at the elevated operative temperatures of the vacuum tube.

We have found that improved electronic elements, and particularly thermionic tubes, may be produced by utilizing for the electrodes thereof substances which are provided with a special electrodeposited surface. According to the p-resent invention such surfaces may be widely varied with respect to the components and hence with respect to the electronic or electrical characteristics so as to secure a wide range of desirable results.

More specifically considered, the invention comprehends the utilization for the elements of thermionic tubes of electrodeposited surfacesI which comprise alloys of tungsten, molybdenum, tantalum, thorium and other similar refractory metals having the electrical characteristics which are desirable in this particular use.

As will be seen more fully hereinafter, the percentages of the respective components of the surface may be changed or modied within a wide (Cl. Z50-27.5)

permissive range to correspondingly modify the electrical characteristics of the surface. It will at once be seen that given the possibility of prol ducing a surface including such refractory metals as tungsten, together with metals such as tan- 5 talum, which are not only refractory but which have a desirably low work function, electron emitters of high efficiency are available.

In order more clearly to explain the invention a typical embodiment is shown in the accom- 10 panying drawing, in which:

Figure 1 is a conventionalized illustration of a typical vacuum tube.

Fig. 2 is an Venlarged detail section of the filament. 15

As shown in the drawing, the invention may be embodied in a vacuum tube comprising an evacuated envelope I which is formed with the usual base or ferrule 2 and the contacts 3. The tube may be a hard or soft tube and is producible by 20.- methods well known in the art. The effective operating elements of the tube include the filament d, grid 5 and plate 5. The tube may also be formed with the usual screen l. Such ele-- ments may be connected through suitable insu- 25 lated leads to the contacts 3 in the manner well known to those skilled in the art.

In accordance with the invention one or more of these elements, such for example as the filament, plate or grid, may be formed with special 30 electrodeposited surfaces of tungsten in combination with other metals so as to produce the desired electrical characteristics.

A further important feature of the present invention is the concept of utilizing a metal tube 35 I which is provided, at least on the interior portion,'with an electrodeposited surface 9 of a refractory alloy containing tungsten, and preferably containing tungsten and tantalum.

In carrying out the invention, for example, a 40 tungsten nickel alloy may be electrodeposited upon a suitable base, such as ferruginous or nonferruginous wires or plates in accordance with the process described in our copendingapplication Serial No. 744,566, led September 18, 1934:. 45. Similarly the invention comprehends the employment as electron emitting surfaces of special electrodeposited alloys containing such refractory and effective electron emitting metals as tungsten, tantalum, molybdenum and thorium. 50h Desirable combinations of these metals may be electrodeposited in the form of 'an alloy upon a suitable base metal and utilized as filaments, grids or plates in electronic tubes. Such alloys may be employed either in binary form, such for example as an alloy or tungsten and tantalum, or in polynary alloy form in combination With other desirable metal components, such as thorium, nickel and the like.

An effective method of producing elements of a thermionic tube comprises the utilization of an electrolyte, such for example as described in copending application Serial No. 14,372, led April 2, 1935, and copending application Serial No, 28,187, filed June 24, 1935. Similarly, im` proved results, especially Where the elements treated are desired to be used as filaments, may be secured by adding to the electrolytic bath a compound having a potentially available thorium ion so as to coplate or co-deposit thorium in association with tungsten and tantalum or tung-` sten and nickel. This may be done, for example, by adding a predetermined quantity of thorium to the bifluorid bath as described in copending application Serial No. 28,187. When an electric current is passed through such a bath it is found that there is plated out on the cathode a metallic surface containing an appreciable quantity of metallic thorium, in combination or alloyed with tungsten, nickel and tantalum.

It will be appreciated, as has been explained in the copending applications referred to, that by the proper adjustment of temperature, hy-

drogen ion concentration and current density,

the respective quantities of the several metals which are codeposited may be varied so as to secure any desired combination and thus insure 'a Wide permission range of electronic characteristics in the resultant plate. It is particularly to be observed that with this method of electrodepose iting such refractory metals as tungsten, together with highly emissive metals', such as thorium or such metals as tantalum which are not only refractory but which also present a low work function, the electronic characteristics of an element may be advantageously modified or varied so as to secure a novel composite electrode having the characteristics desired for any particular use.

'AAs intimated hereinbefore, the elements produced according to the present invention may be employed for the several elements of a hard or soft tube, such as the lament, grid, or plate. Depending upon the particular use to which the elementl is to be put, the electrolytic bath or the plating conditions may be modified so as to increase or decrease the respective quantity of any particular component. By this process of coplating nickel, for example, with tungsten and tantalum, or of an alloy of nickel, tungsten, tan-1 talum and thorium, and by the potentially variable composition of the plate, the physical characteristics of the elements may be controlled within a relatively wide range to Vthus impart to j the element such features as improvedduotility, tensile strength, thermal refractory characteristics and the like.

Y acter of the resultantplate may be further modisive" characteristics of the surface.

Similarly the physical charf,

fact upon any metal or alloy which functions effectively as a cathode in the electrolytic bath.

Likewise it is Within the contemplation of the invention to specially treat the plated cathode prior to use as an element in a thermionic tube. For example a filament produced according to the present invention andwhich presents a surface containing such metals as tungsten, thorium or tantalum may be flashed and heat treated at elevated temperatures so as to improve the emis- It is to be observed that since the .effective surface of the electrodes described herein comprise alloyed elements, osettin'g is materially diminished.

It is to be observed that special electrodes, of differential cross-sectional characteristics, may be readily produced under the operations hereindescribed. As has been pointed out in the prior applications referred to, the respective quantities of the several components of the alloys may be varied,fduring a given plating operation, by modifying the temperature or current density or by modifying both the temperature and current density. In this manner electrodes may be made up, having a differential composition in crosssection. Thus,.for example, electrodes may be produced by electro-codepositing nickel, thorium and tantalum upon a suitable base, such as a tungsten base plate, and thereafter there may be electrodeposited on the plate on alloyof tungsten, tantalum and nickel. 'I'he element may be thereafter heat treated to insure the controlled diffusion of thorium through the element.

As noted above, operating under the principles of the present invention, elements for electronic tubes may be produced having a Wide variety of thermal and electrical characteristics fitting them especially effective for use as emitters', plates or grids.

As noted above also, the present invention comprehends the concept of utilizing metal tubes with special interior surfaces. Thus, as explained, a metal envelope I may be employed, at least the interior surface of Which is plated withV alloys of the type described. It Will be understood that this type of construction insures improved operation of thermionic tubes. When, for example, 4the surface 9 comprises an alloy of tungsten and tantalum, and'preferably one relatively high in tantalum, the getter function of r `these are given didacticallymerely for the purpose of explaining the underlying principles of the invention and not as limiting the invention to the specific illustrations set forth.

We claim: Y l. A plate for thermionic tubes having an electrodeposited surface of a tungsten nickel alloy thereon. i

2. iA thermionic tube comprising a metal enve1ope,a plate, grid kand filament enclosed with-y in the envelope,v the interior surface of the envelope being formed of an electrodeposited alloy of tungsten and nickel. Y

HARRY HOWARD ARMSTRONG. ARTHUR BURLEY

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