US2149657A - Thermionic tube - Google Patents

Description

March 7, 1939. H. H. ARMSTRONG E-r A1. y 2,149,657

THERMIONIC TUBE Filed March l2, 1936 Patented Mar. 7, 1939 f UNITED STATES PATENT OFFICE THERM'IONIC 'TUBE Application Marchv 12, 1936, Serial No. 68,557

4 Claims.

'I'his invention relates to improvements in electronic elements and more particularly to irnproved 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 field, that is to say in vacuum tubes, includes such features as high electronic emissivity, thermal resistivity and longevity. 'Ihe 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 lament comprises thoriated tungsten. The elements tungsten and molybdenum are likewise found to be efiicacious 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 thiorated 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 function and is a good emitter, it has 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 present invention such surfaces may be widely varied with respect toy 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 thermicnic tubes of electrodeposited surfaces 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 sur- (Cl. Z-27.5)

face may be changed or modied within a wide permissive range to correspondingly modify the electrical characteristics of the surface. It Will at once be seen that given the possibility of producing a surface including such refractory metals as tungsten, together with metals such as tantalum, which are not only refractory but which have a deslrably low work function, electron emitters of high eillciency are available.

In order more clearly to explain the invention a' typical embodiment is shown in the accompanying drawing, in which: v

Figure l is a conventionallzed illustration of a typical vacuum tube. I

Fig. 2 is an enlarged detail section of the fila ment.

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 methods well known in the art. Theeilective operating elements of the tube include the filament 4, grid 5 and plate 6. The tube may also be formed with the usual screen 1. Such elements may be connected through suitable insulated leads to the contacts 3 in the manner well known to tho'se 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 electrodeposited surfaces of tungsten in combination'with other metals so as to produce the desired electrical characteristics.

In carrying out the invention, for example, a 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 copending application Serial No. 744,566, led September 18, 1934. 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. 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 maybe employed either in binary form, such for example as an alloy of tungsten and tantalum, or in polynary alloy form in combination with other desirable metal components, such as thorium, nickel and the like.

An eiective 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, filed April 2, 1935, or copending application Serial No. 28,187, led June 24, 1935. Similarly, improved results, especially where the elements treated are desired to be used as laments, may be secured by adding to the electrolytic bath a compound having a potentially available thorium ion so as to coplate or codeposit thorium in association with tungsten, tanta um or nickel or combinations' thereof. This ay be done, for example, by adding a predetermined quantity of thorium to the bifluorid bath\as described in copending appncation serial Nt. 28,187. when an e1ectnc current is passed through such a bath it is found that there is plated out on the cathode a metallic surface containing a appreciable quantity of metallic thorium, in co bination or alloyed with tungsten, nickel and tantalum.

It will be appreciated, as has been explained in the copending applicationsreferred to, that by the proper adjustment of temperature, hydrogen ion concentration and current density, the re- A spective quantities of .the several metals which are codeposited may be varied so as to secure any desired combination and thus insure a wide permissive range of electronic characteristics in the resultant plate. It is particularly to be observed that with this method o`f electrodeposition such refractory metals as tungsten, together with highly emissive metals. such as thorium and 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.

As intimated herelnbefore, the elements produced according to the present invention may be employed-for the several elements of a hard or soft tube, such as the filament, grid, or

plate. Depending upon the particular use to which the element 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, tantalum 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 thus impart to the element such features as improved ductility, tensile strength, thermal refractory characteristics and the like. Similarly the physical character of the resultant plate may be further modified by codepositing other metals, such as iron and cobalt, together with the refractory metals described.

It will likewise be appreciated that the special surfaces herein dened may be electrodeposited upon any suitable metal, such as ferrous or nonferrous bases. If desired the special surfaces may be plated upon pure tungsten or tungsten/ alloys, molybdenum or molybdenum alloys, or in fact upon any metal or alloy which functions effectively as a cathode in the electrolytic bath. K

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 lament produced according to the present invention and which 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-l sive characteristics of the surface. It is to be observed that since the effective surface of the electrodes described herein comprise alloyed elements, offsetting 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, during a given plating operation, by modifying the temperature or current density or by modifying both the current density and temperature. In this manner electrodes may be made up having a differential composition in cross-section. Thus, for example, electrodes may be proy thermal or electrical characteristics tting them especially effective for use as emitters, plates or grids.

While the invention has been described with respect to the special surface of the elements of a tube per se. that is to say the filament, grid, plate and the like, it is to be understood that if desired special surfaces may be electrodeposited upon the -envelope itself, where such envelope is constructed of metal.

While preferred modications of the invention have been described, it is to be understood that these are given didactically merely 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:

l. A grid for thermionic tubes which comprisesa suitable metal base having electrodeposited thereon a surface including tungsten and tantalum together with a metal of the iron group.

2. A control grid for thermionic tubes comprising a conductive base member having an adherent electrodeposited surface of an alloy of tungsten and nickel.

3. A control grid for thermionic tubes comprising a conductive base member having an adherent electrodeposited surface containing tungsten, tantalum and nickel.

4. A method of producing improved electronic filaments of tubes which comprises electrode-- -positing on a conductive base a surface con-

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