US2417461A - Electron tube and method of making same - Google Patents

Description

March 18, 1947. BECKER 2,417,461

ELECTRON TUBE AND METHOD OF MAKING SAME Filed Aug. 51, 1945 Na -e 1195/ r/ IN VEN TOR. George A. Becker ATTORNEY Patented Mar. 18, 1947 UNITED STATES v ginger ATENT OFFICE ELECTRUN TUBE METHOD OF MAKING SAME Application August 31, 1945, Serial No. 613,894

7 Claims. 1

This is a continuation-impart of my copending applications, Serial Nos. 467,455, filed November 30, 1942; 501,721; 501,722; 501,723; and 501,724, filed September 8,1943.

My invention relates to electron tubes and more particularly to improvements in non-emissive electrodes having a surface layer including a refractory metal oxide of the type disclosed in the application of William W. Eitel and Jack A. McCullough, Serial No. 527 ,294,

Itv is among the objects of my invention to provide an improved method of forming a nonemissive coating on a'nelectrode.

Another object is to provide an electrode of the character described having a coating comprising particles sintered together to form a continuous sheath about a core.

A further object is to provide a method of making a coated electrode, which method simplifies the subsequent processing and evacuation of a tube incorporating the electrode.

The invention possesses other objects and features of advantage, some of Which, with the foregoing, will be set forth in the following description of my invention. It is to be understood that I do not limit myself to this disclosure of my invention as I may adopt variant embodiments thereof within the scope of the claims.

Referring to the drawing:

Figure l is a perspective view of a tube having a grid electrode embodying the improvements of my invention; and

Figure 2 is an enlarged cross-sectional View of one of the coated grid Wires.

In terms of broad inclusion, my tube comprises an envelope enclosing a thoriated-cathode and an improved non-emissive electrode. electrode comprises a core of suitable metal coated with finely divided particles including a refractory metal oxide sintered together to form a continuous sheath about the core; the particles preferably comprising at least tWo difierent refractory metals and oxygen combined as an oxide of at least one of the refractory metals. This coating is initially applied in any suitable mani .i." may be applied to other base having prongs is cemented to the lower portions of the envelope.

A plurality of electrodes including cathode 8, plate 9 and the improved grid I I are coaxially disposed within the envelope. Plate or anode 9 has a cap l2 connected by bracket I3 to a lead The latter I4 sealed to the top of the envelope. The plate may be of any suitable metal such as tantalum, and is preferably provided with a dark coating I5 to improve heat radiation. I

Cathode 8 is of the thoriated type, preferably a thoriated tungsten filament welded top and bottom to a pair of leads l7 sealed to stem 3. Flexible conductors l8 connect th'e cathode leads to a pair of base prongs. The thoriated cathode or filament is preferred because my improved grid exhibits special non-emissive properties in such a combination. By the term thoriated cathode I mean a cathode containing thorium as the active emitter material, which thorium may be incorporated either as a pure metal or as a compound.

From the structural standpoint, grid H preferably comprises vertical Wire bars terminating at a base ring 2! supported by brackets 22 on a pair of rods 23 sealed to stem 3. One of these rods functions as a lead and is connected to a base prong by conductor 24. The shape of the grid and the character of its mounting means may be varied within Wide limits.

My improved grid is initially fabricated by making the grid bars of suitable metal, which metal forms the core 25 (Figure 2) upon which my non-emissive coating is applied. The core metal is preferably a refractory metal such as tantalum or molybdenum. An ordinary commerne a y prayi g. Sintering of the coating is cial grade of tantalum or molybdenum Wire may be used. Tantalum is preferred because its ductility lends itself to ready fabrication into a grid structure. After being formed on a suitable mandrel the grid bars are Welded to base ring 2!. The fabricated grid is then cleaned, after which coating 26 is applied.

Both tantalum and molybdenum have been used in the past for making grids. As is well known, such grids are subject to considerable thermionic emission, even at relatively low temperatures, and are receptive to contamination from a thoriated cathode. My improved coating 25, hereinafter described, overcomes this difilculty and renders the grid permanently nonemissive from the standpoint of primary emission at operating temperatures normally contended with. Not only is primary (thermionic) emission largely eliminated, but also secondary emission is brought under control.

The final coating 26 comprises finely divided particles including a refractory metal oxide sintered together to form a continuous sheath about core 25. By the term refractory metal I mean a metal in the class having a relatively high melt ing point and low vapor pressure, the preferred metals in this class being tantalum, tungsten, molybdenum, columbium, titanium, chromium, zirconium, vanadium, and beryllium.

The preferred final coating comprises at least two refractory metals and oxygen combined as an oxide of at least one of the metals. I have had good results from a coating prepared from abase mixture of tantalumpentoxide and tungsten trioxide. The tantalum pentoxide (TazOs) used is preferably formed-by burning 50 to 500 mesh tantalum powder in air until the whitish color characteristic of the pentoxide is produced. The tungsten trioxide (W03) employed is formed by burning in air tungsten powder, of a fineness similar to that of the tantalum powder, until the yellowish color characteristic of this oxide is produced. These finely dividedoxide powders are then thoroughly mixed; in the dry state. 'The preferred mixture comprises about 90% by weight of tantalum pentoxide and by weight of tungsten trioxide. These proportions may be varied within wide limits, depending upon the characteristics desired in the final grid.

In my. preferred process the base mixture is next fired in a reducing atmosphere, preferably hydrogen; Reduction may be accomplished by other means,- as by heating in an atmosphere of carbon monoxide, or by firing in vacuum. The time and temperature of firing may be varied, good results having been obtained'by heating to around 1000 C. for about 3 minutes in an excess of hydrogen. During this heating treatment the mixture loses some weight and darkens materially in color, rangingfrom grey to black depending on the temperature and time of firing. From the practical standpoint it is immaterial if treatment is stopped after the powderhas turned grey or whetherit is allowed to darkenfurther. This fired mixture is still a powder, the particles retaining their finely divided granular form.

The fired powderis then mixed with a suitable binder to cause adherence of the coating on the grid, a nitrocellulose binder being preferred, using amyl acetate as a, thinner. Sufiicient amount of the liquid binder or vehicle is mixed with the powder to produce a fluid mixture having the proper consistency for spraying. An ordinary spray gun is preferably employed for applying the coating. On a grid made of .012! wire the coating applied is preferably about .002, thick. The binder functions to temporarily hold the particles on the grid. prior to further treatment.

, The coated grid is then prefired by heating in an oxygen-free atmosphere, preferably in vacu um, prior to mounting it in the envelope. I have preheated coated grids in an evacuated furnace to about 1700 C. (brightness temperature) for about minutes with good results, utilizing heat radiatedfrom an adjacent heating element. This preheatingis important for sintering of the sintering operation also knits the particles sumciently to insure formation of a. continuous sheath about the core, and providesa hard sur face layer so that the grid may be safely handled during assembly into the tube Without danger of dislodging the coating. Sintering the particles to effect bonding has the further advantage of making it unnecessary to have any foreign binder material in the final coating. In my process the temporary binder is volatilized during the firing operation.

H bombardment from the filament.

of exhausting tubes is well known; comprising. connecting the electrodes in a suitable circuit,

A. An oil diffusion pump is preferred, capable of reducing the pressure to a low value.

During exhaust the electrodes are heated in situ by suitable means preferably-by electron This method energizing the filament and applying positive potentials to the plate and grid Heating of the plate and grid is caused by the kinetic energy dissipated when the fast moving electrons are suddenly stopped, the temperature depending upon the potentials applied. Other systems of heating the electrodes in the tube, such as the high frequency induction method, may be emcoating particles together .and to the core, it being understood that up to this point the particles are merely held by the temporary binder. The

ployed. After exhaust, the envelope is sealed oil the pump and base 6 is applied.

The step of preheating thegrid in vacuum prior to mounting it in the envelope has several advantages. Itnot only accomplishes the desired sintering of the particles, but it also functions to outgas the coating. Considerable I gas is trapped between the particles when the coating is applied, and some gas is occluded in the solid particles. Unless this gas is removed from the coat priorto evacuating the tube it cancause much damage. For one thing, gas liberated during the exhaust of the tube can poison the filament. Also, excessive gassing of the coated electrode produces undesirable ionization in the envelope, and prolongs the time required to exhaust the tube. In my process of firing the electrode in vacuum the coating is thoroughly outgassed 7 first described is preferred however, because the results are better and more consistent.

Just what happens to the mixture of oxides during the firing step is not fullyunderfstood.

The change in color and loss of Weight points to at least a partial reduction of at least one of the oxides. The fact that the oxides are in intimate admixture during reduction appears to be im-' portant. In thecase of the preferred mixture of tantalum and tungsten oxides there is probably at least a partial reduction of the tungsten trioxide and also possibly of the tantalum pentoxide to the lower order of oxides of these metals.

From the loss of weight however, it is evident that considerable oxygen is retained so that the final mixture includes the threeelements; tantalum and tungsten and oxygen, the oxygen being combined as an oxide of. at least one of these metals.

Combination of two or more refractory metal oxides other than those of tantalum and tungsten may be used in the basic mixture for the coating. For example, some of the combinations I have used with especially good results include: molybdenum and columbium oxides; tungsten and columbium oxides; molybdenum and tungsten oxides; molybdenum and tantalum oxides; tantalum and columbium oxides; tungsten and chromium oxides; tantalum and titanium oxides; titanium and Zirconium oxides; tantalum and zirconium oxides; and zirconium and beryllium oxides.

In any case, an electrode surfacing layer is produced which inhibits primary emission. The presence of two refractory metals with the combined oxygen gives the coating electrical con-- ducting properties, which is desirable, particularly on grids, as will be readily understood. Another feature of an electrode coating formed by my process is that the step of prefiring the coated grid to sinter the particle may be controlled to give either a rough or smooth surface, thereby controlling the secondary emitting properties of the grid. Thus, if more secondary electrons are desired from a particular grid, a smoother coating is produced. If little or no secondary emission is desired, a rough coating is provided. The smoothness of the surface depends upon the degree of sintering, which is determined by the temperature of firing. For a rough surface the sintering operation is carried only far enough to fuse adjacent surfaces of the particles, leaving the coating with a clinker-like appearance. For a smooth surface the sintering temperature is raised to a point where the particles of the coating more completely fuse or fiow together. For example, an electrode coated with a mixture of 50% tantalum pentoxide and 50% titanium dioxide, and fired to 1850 C. in vacuum, produces a smooth enamel-like surface having fairly good secondary emitting properties. This sintering at higher temperatures to get a smooth surface does not adversely affect that property of the coating which inhibits primary emission.

I have also used coatings made from single oxides instead of using the mixed oxides. Grids made in this way tend to contaminate more readily, have a higher resistance surface, and therefore coatings made of mixed oxides are preferred.

My improved electrode exhibits remarkable non-emissive properties, particularly in combination with a thoriated cathode. Primary emission is of a low order and does not increase upon contamination of the electrode with thorium from the cathode.

While I have described my improvements in connection with a grid, it is understood that other electrodes may be so treated to suppress emission. For example, a plate coated in accordance with my invention makes an excellent anode for a rectifier tube where elimination of reverse current is highly desirable.

I claim:

1. A non-emissive electrode having a coating comprising finely divided particles sintered together, said particles including two refractory metals and oxygen combined as an oxide of :at.

least one of said metals, formed by reduction-0f a mixture of oxides of said metals said metals being selected from the group consisting of tantalum, tungsten, molybdenum, columbium, vanadium, chromium, titanium, zirconium and beryllium,

2. An electron tube comprising a thoriated cathode and a cooperating electrode having a coating comprising finely divided particles sintered together, said particles including two refractory metals and oxygen combined as an oxide of at least one of said metals, formed by reduction of a mixture of oxides of said metals said metals being selected from the group consisting of tantalum, tungsten, molybdenum, columbium, vanadium, chromium, titanium, zirconium and beryllium.

3. The method of making an electrode for an electron tube, which comprises coating the electrode with a mixture of finely divided oxides of two refractory metals selected from the group consisting of tantalum, tungsten, molybdenum, columbium, vanadium, chromium, titanium, zirconium and beryllium, and then heating the coated electrode in vacuum at a temperature sufiicient to effect reduction of the mixture and to form particles comprising the metals and oxygen combined as an oxide of at least one of said metals, said reduction being efiected prior to placing the electrode in the tube.

4. The method of making an electrode for an electron tube, which comprises coating the electrode with a mixture of finely divided oxides of two refractory metals selected from the group consisting of tantalum, tungsten, molybdenum, columbium, vanadium, chromium, titanium, zirconium and beryllium, and then heating the coated electrode in an atmosphere from which free oxygen is excluded at a temperature sufficient to efiect reduction of the mixture and to form particles comprising the metals and oxygen combined as an oxide of at least one of said metals, said reduction being effected prior to placing the electrode in the tube.

5. The method of making an electrode for an electron tube, which comprises mixing together finely divided oxides of two refractory metals selected from the group consisting of tantalum, tungsten, molybdenum, columbium, vanadium, chromium, titanium, zirconium and beryllium, heating the mixed oxides in a reducing atmosphere at a temperature sufiicient to effect reduction of said oxides and to form particles comprising the metals and oxygen combined as an oxide of at least one of said metals, and then coating the electrode with the mixture.

6. The method of making an electrode for an electron tube, which comprises mixing together finely divided oxides of two refractory metals selected from the group consisting of tantalum,

tungsten, molybdenum, columbium, vanadium,

chromium, titanium, zirconium and beryllium,

heating the mixed oxides in a reducing atmosphere at a temperature sufficient to effect reduction of said oxides and to form particles comprising the metals and oxygen combined as an oxide of at least one of said metals, then coating the electrode with the mixture, and then heating the coated electrode in vacuum prior to mounting it in the tube.

'7. In the manufacture of an electron tube wherein an electrode is coated with finely divided particles of primary emission inhibiting material comprising two refractory metals and oxygen'combined as an oxide of atleast one of said metals, formed by reduction of a mixture of oxides of said metals, the step of heating the. coated electrode to a temperature sufiicient to completely fuse and sinter said particles together 5 2,213,558

Number into asmooth-surface coating prior to mounting 2,343 045 it in the tube, to enhance secondary emission from said electrode.

' GEORGE A. BECKER. Number REFERENCES CITED The following references are of record in the file of this patent:

German July 21, 1932

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