US2417458A - Electrode - Google Patents

Description

ELECTRODE Original Filed July 28,: 1945 wail/amid INVENTORS MLL/AM w. E/TEL. 1 BY c/ACK ,4. ME Cuu. UGH

THEu ATTORNEY Patented Mar. 18, 1947 UNITED STATES PATENT OFFICE ELECTRODE William W. Eitel, Woodside, and Jack A.

McCullough, Millbrae, Calif., assignors to Eitel- McCullough, Inc., tion of California San Bruno, Califi, a corpora- Original application July 28; 1943, Serial No. 496,686. Divided and this application March 20, 1944, Serial No. 527,294 i 4 Claims. (Cl. 250-177) an electrode, such as a grid, from which primary,

and secondary emission is eliminated.

Another object is to provide an electrode of the character describedwhich retains its non-. emissive properties at high temperatures.

Stillanother object is to provide a tube embodying an electrode which will not become emissive by contamination with thorium from a thoriated filament.

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

claims,

Referring to the drawing, the figure is a perspective view of a, tube embodying the improvements of our invention.

One of the most serious problems in vacuum tubes for transmission purposes is thermionic (primary) emission from the grid. Primary emission results in a reverse grid current which not only makes the tube unstable and erratic in its operation, but limits the output and prevents use of the tube in certain types of service. The problem is particularly aggravated in tubes where the grid normally runs at a relatively high temperature. Under these conditions usable 7 grid materials are limited to refractory metals having high melting points, such as tungsten, tantalum, molybdenum and the like. These metals unfortunately are subject to contamination and exhibit undesirable amounts of primary emission. There are two factors mainly responsive for primary grid emission. One is thermionic emission from the grid material per se, which depends upon the work function of the material used in making up the grid in the first instance. The other and most troublesome factor is lowering of the initial work function by contamination of the grid with material of higher emissivity such as thorium from a thoriated filament. Thus, even if a grid starts out as a non-emitter it ends up with high primary emission due to activation by thorium sputtered or otherwise deposited on it.

We have discovered an electrode material which exhibits substantially no primary emission,

even at high temperatures. Furthermore, the. electrode does not become emissive by contamination with thorium. The improved electrode. material is quite stable at high temperatures and may be operated in elevated temperature regions Where ordinary electrode materials would quickly fail.

Another feature of our improved electrode material is that it exhibits no secondary emission (electrons knocked out by impact). This absence of secondary electrons from an electrode is highly desirable in many cases.

In terms of broad inclusion, our improved nonemissive electrode comprises a metallic core having a stable surface layer including a refractory metal oxide. An alloy of refractory metals is preferably used for the core to facilitate formation of the desired surface composition. This electrode is enclosed in an evacuated envelope together with a thoriated cathode. While we have shown a tube embodying the improvements in the grid, it is understood that this is merely for purposes of illustration and that other electrodes may be rendered non-emissive in a like manner.

In greater detail, and referring to the drawing, the tube chosen to illustrate the invention comprises an envelope 2 of glass or the like having a reentrant stem 3 carrying an exhaust tubulation 4. A suitable base 6 having prongs l is cemented to the lower portions of the envelope. The triode illustrated contains a cathode 8, plate 9 and the improved grid II, all coaxially disposed in the envelope. Plate 9 has a cap I2 connected by bracket 13 to a lead l4 sealed to the top of the envelope. The plate may be of any suitable material, such as molybdenum or tantalum, and, if desired may be provided with a coating l5 of zirconium or the like to improve the gettering and heat radiating properties. v

Cathode 8 is of the thoriated type, preferably a thoriated tungsten filament, comprising a coil welded at top and bottom to a pair of leads I! sealed to stem 3. Flexible conductors l8 connect the cathode leads with a pair of base prongs. The thoriated cathode is preferred because our improved grid exhibits special non-emissive properties in such a combination. It is understood that the filament may assume other shapes than the spiral type illustrated.

structurally speaking, grid II is preferably of the cage type comprising vertical Wire bars terminating at a base ring 2| supported by bracket 22 on a pair of rods 23 sealed to stem 3. One of these rods functions as a lead and is connected 3 to a base prong by conductor 24. The shape of the grid and its mounting means may be varied within wide limits. The improved grid is initially fabricated from an alloy of at least two refractory metals, preferably a tantalum-tungsten alloy comprising a major proportion of tantalum and a minor pro-.

portion of tungsten. A tantalum base alloy ineluding about 93% tantalum and about 7% tungflame. Because the core includes tantalum and" tungsten, oxides of' both of these metals are formed on the surface; Furthermore, since the; core' is an alloy the oxides are thoroughly intermixed and exist in a finely divided state of 'subdivision. The degree and length of heating mayj bevaried within wide limits, depending upon thedepth of'oxid'e layer desired.

The oxidized grid is'preferably next heated in a reducing atmosphere such as hydrogen, The time and temperature of firing may be varied, satisfactory results being obtained; by heating to around 1000 C. for about 3 minutes in an excess of hydrogen. .The'loss of weight involved points to at least a partial reduction of the tungsten oxide, this oxide either going to metal or to a lower order of oxide. Less reduction ofthe tan-' talum oxide is apt to occur because of the greater affinity of tantalum for oxygen. From the loss in weight measurements it is apparent that considerable oxygen is retained, hence the final composition includes the three elements: tantalum and tungsten and oxygen; the latter being combined as an oxide of at least one of these meta s.

It Will be noted that one of the refractory metals has a greater affinity for oxygen than the other, resulting in a selective reduction. In other words,

there is at least a partial reduction oi'the oxide of that metal having the lesser aflinity for oxygen, Because. the oxides initially formed from the. alloy are in a very finely dividedand'inti mately comingled statev the resulting composition forms a dense, closely knit layer.

The appearance of the final coating is an enamel-like layer. It is probable that the metallic oxides or mixture of oxides and, finely divided metal, by reason of their relative solubilities under the temperature conditions involved, combine to form a glass-like material of the nature. of an intermetallic compound or a metalloid.

Stability of the surfacing layer is very important from the standpoint of tube life and where high temperature operation is involved. We have made a surfacing of tantalum oxide alone, (formed by oxidizing a plain tantalum core) and found that our preferred surface layer exhibits non-emission properties for a materially longer time and under much higher temperatures.

Other combinations of refractory metals may be employed in the alloy core of the electrode. By the term refractory metal we mean'metals in the-class having a high melting point and rela- 'tively low vapor pressure, such as tungsten, tantalum and molybdenum. We have'used a molybdenum-tungsten alloy with satisfactory results. In this case the oxides formed during the oxidation step are those of molybdenum and tungsten. During the reductionstep some oxygen is lost, but there is not a complete, reduction of both oxides. As in the example first described, the final coating composition includes at least two refractory metals and oxygen combined as an oxide of at least one of the metals. In both cases the core is surfaced with a layer composed of the partial reductionproduct from oxides of two refractory metals.

After thegrid has been treated it is sealed into the envelope along with the plate and cathode; the. latter being carbonized in accordance with the usualpracticepf making thoriated tungsten filaments. The: tube-to be exhausted is connected to a suitable vacuum pump through tubulation 4.

' During'exhaust the electrodes are heated'by suitable means, preferably by electron bombardment from the filament." This-exhaust-' procedureis well understood by. those skilled in the art. 'Aften exhaust the envelope issealed-off the" pump and base 6 is applied,

If desired the step of firing thegrid in hydrogen may be eliminated,-andtheheating or the grid in vacuum during exhaust or the-tune relied' upon to en'ect reduction. Heating:the -g-rid. aboveaoout NW C; cluring evacuation accomplishes the desired purpose.

tube, The. principleyof operation underlying the improved grid is not fully understood Itisappar-- ent however thatthe work function of the treated grid is increasedso thatthe electrode-material itself exhibits less primary emission at a given pointed out, but this is not inconsistent witlrthe theory because thorium has a-tremendous-afiinity for oxygen. The-tungsten component'of the'surface layer may enter into this-by some catalytic acti0n,-i-n addition-to performingthefunction of keeping the'material stable at high" temperatures by lowering its vapor pressure.

Whatever the correct explanation may be, we

have conclusively; demonstrated that the iniproved grid exhibits negligible'primary' andsecondary emission andthat'thegridretains its-non= emissive propertiesunder' high temperatures.

We claim:

1. A non-emissive electrodefor'electron' tubes, said electrode having a surface layer consisting; essentially of oxygen, tantalum and tungsten, the. oxygen being-combined as an oxide of at least one of said metals. 1

2. A non-emissive electrode having" a surface layer consisting essentially. of oxygen, tantalum and tungsten, the oxygen beingcombined asian oxide of tantalum;

3: An electrontube having'athoriated cathode and a non-emissiveelectrode', said electrode hav-- ing a surface layer consisting essentially oftantalum and tungsten and oxygen combined as an oxide of at least one of'said metals, forinhibiting primary emission on contamination. of the'elec trode'with thorium: J

4. An electron tube having a thoriatedjcathode and a non-emissive electrode; said electrode'hav- Prefiring in a reducing atmospnere is preferred however asthereie less chance of contaminatingother electrodes in the 5 I 6 N ing a surface layer including in intimate admix- UNITED STATES PATENTS ture two refractory metals and oxygen combined Number Name Date as an oxide of at least one of said metals, for 1 926 846 Giard Sept 12 1933 inhibiting primary emission on contamination of 1871'363 Fritz 1932 the electmde with thmum- 5 1:558:961 Bullimore Oct. 27: 1925 WILLIAM EITEL- 2,012,339 Edwards et a1 Aug. 27, 1935 JACK MCCULLOUGH- 2,238,596 Mouromtseff et a1. Apr. 15,1941 REFERENCES CITED 1,929,661 Von Wedel Oct. 10, 1933 The following references are of record in the 10 file of this patent:

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