US2497110A - Method of making electrodes - Google Patents

Description

m m in Feb, M, 195% P. D. WILLIAMS METHOD OF MAKING ELECTRODES Original Filed Feb. 4, 19 .6

[01 re veo r/a INVENTOR. Paul D. WlY/l'a ATTORNE Y Patented Feb. .14, 1950 METHOD OF MAKING ELECTRODES Paul D. Williams, Palo Alto, Calif., assignor to Eitel-McCullough, Inc., San Bruno, Calif., a corporation of California Original application February 4, 1946, Serial No. 645,443. Divided and this application February 8, 1947, Serial No. 727,409

k This is a division of my copending application Serial No. 645,443 filed Feb. 4, 1946.

My invention relates to electron tubes, and more particularly to an improved method of mak-' ing an electrode for such tubes.

In electron tubes there are certain electrodes whose operation is seriously afiected by their electron emission properties. For example, the proper functioning of a control grid in a triode depends upon the absence of excessive primary emission, namely, thermionic emission of electrons from the grid clue to elevated grid tempera ture. It also depends upon the amount and stability of secondary emission, namely, emission of electrons due to bombardment of the grid by electrons from the cathode. Of these the efi'ects of primary grid emission are most serious, especially in power tubes having thoriated filaments because the grids run quite hot and because thorium from the filament tends to activate the grid.

The broad object of my invention is to provide a method of making an electrode which exhibits negligible primary emission and has stable secondary emission.

Another object is to provide an electrode whose emission properties are not appreciably influenced by thorium contamination from a cathode.

Another important object of my invention is to provide a grid material which may be prepared in the wire state prior to fabrication of the wire into a grid.

Still another object is to provide a processed grid material in the form of wire or the like which may be readily welded together for fabrication purposes.

Further objects include the provision of an electrode of the character described which is strong mechanically, inexpensive, and easy to manufacture.

The invention possesses other objects and fea: tures of advantage, some of which, with the foregoing, will be set forth in the following descrip- Figure 2 is an enlarged view of portions of one of thegrid wires.

In terms of broad inclusion. my improved 1 Claim. (01. 204-29) method of making an electrode material such as grid wire comprises carburizing a metallic core and then applying an outer layer of a metal in group 8 of the periodic table. In my preferred process only the outer portions of the core are carburized so as to provide a carbide layer lying intermediate the -outer layer and the inner core metal. If desired, however, the entire core material may be converted to the carbide. The outer layer of platinum or the like metal is preferably applied as finely divided particles, preferably electrodeposited from a bath. The product is then heated to fuse the particles together into a sheath about the core. My above treatment of the electrode material may be done either before or after the material is fabricated into an electrode. Treatment of a grid wire, for example, before welding the wire together into grid formation has certain advantages from the standpoint of grid fabrication.

In greater detail, and referrin to the drawing, my improvements are illustrated by a triode tube structure comprising an envelope 2 having a stem 3 carrying an exhaust tubulation 4 and provided with a base 6 having prongs l. The envelope encloses a filamentary cathode 8, tubular anode 9 and grid ll. Anode 9 has a :cap l2 supported by bracket [3 on a lead I4 sealed to the upper end of the envelope. If desired, a coating l5 may be provided on the anode to improve its heat dissipation properties.

Cathode 8 comprises a helix of thoriated tungsten wire welded to a pair of leads i1 sealed to stem 3, these leads being connected to a pair of base prongs l by conductors l8. Structually speaking, grid H is of the cage type comprising vertical wire bars terminating at a base rin 2| supported by brackets 22 on a pair of rods 23 sealed to stem 3. One of these rods functions as a grid lead and is connected by a conductor 24 to a base prong.

This tube structure is merely for purposes of illustration and may be varied within wide limits, it :being understood that my improvements may be incorporated in many other tube designs.

The improved grid electrode l l embodying my method comprises a core 26 of a metal having a high melting point and low vapor pressure such as molybdenum, tantalum or tungsten, these metals being generally classified in the art as refractory metals. Molybdenum is especially well suited as a core material and is preferred. The usual procedure in fabricating grids of the cage type is to wind 9. wire on a mandrel and fasten it to the basering 2i. This same procedure may be followed in making my grid, the wire thus wound comprising the core 26 of my electrode as is shown in Figure 2. After fabrication the grid is cleaned in a suitable manner, as by electrolyzing in a sulfuric acid bath. As previously mentioned the grid wire may be treated either before or after the grid is made, the latter procedure being first described.

The grid is treated to form an intermediate, barrier layer 21 of a metallic compound for isolating the core from the surface portions of the grid. I have used a metallic carbide in layer 21 with excellent results. This. layer is prepared by carburizing the outer portions of the core metal. thus forming a layer of molybdenum carbide in in several ways, as by heatlng'the electrode in; a hydrocarbon atmosphere or by heating it in a pot containing powdered carbon to a tempera"- ture sufficiently high to promote the formation of the. carbide. For convenience, however,;and; toestablish better control over the carburizin step,-,I prefer to apply carbon as a coating and then heat the coated electrode. This is done by depositing a thin coating of finely divided carbon particles on the grid, the particles being pref-1 erably electrodeposited from a bath containing the particles in suspension, The coated grid is then heated to say 1850 C. for about five minutes inan oxygen free atmosphere oi?- an inert gas or in vacuum. During this heating the carbon combines with the core metal to forma metallic carbide, as will be readily understood. Theresulting grid at this stage has a layer 2'1- of the forming a carbide layer 21 as illustrated in the drawing, the entire core metal maybe converted to the carbide. Carburizing the electrode by heating it in a pot of powdered carbon, assuggested above, accomplishes this result. In either case the desired end of providing a carbid base for the outer layer 28 is secured.

The carburized grid is next treated to'forman outer layer is of a material: having the desired electron emission properties. I prefer to use an- Quter layer composed mainly of a. metal in group a of theperiodic table,such as iridium, ruthenium or platinum; the latter being preferred. Layer 28 is... formed by depositing on the electrode a thin coating of finely divided platinum particles, and then heating the electrode to fuse or si'nter the particles together and to the underlying layer. The platinum particles are preferably electrodeposited from a chlorplatinic acid solution, and the gridthus coated is preferably heated to say 1650, C. for about five minutes in an oxygen free atmosphere of an inert gas or in vacuum. Heating in vacuum is preferred because it also serves tooutgas the electrode. This heating step-sintersthe particles into a continuous sheath'and fuses the'outer layer to intermediate layer 2?. Also, a certain amount of metallic carbide from the-underlying layer becomes mixed with the platinum during the sinterin operation, so thatthe final layer 28 comprises a mixture of a major-proportion of platinum and a minor proportion of the metallic carbide.

A grid electrode prepared in themanner above described has excellent properties from the stand-- point of both primary and secondary emission. The grid exhibits an initial low order of primary emission, and the amount of primary emission does'not increase upon contamination of the gridwith thorium. The suppression ofprimary-cruis the preferred electrode. Carburizing may be done metallic carbide, the nature of the carbide depending upon the core metal used. Insteadof" 4 sion is due to the composition of outer layer 28 and also to the fact that this layer is isolated by the barrier layer 21. V

The particular kind of layer 28 which is formed by fusing or sintering together finely divided particles of platinum or the like ismuch superior from the standpoint of suppressing primary emission than platinum coatings applied by plating or by drawing a platinum sheath on the wire.

My improved results are therefore derived from two'things, first the particular nature of the outer metallic layer, and second, the fact that the outer layer lies on a metallic carbide instead of on the 'core metal.

Furthermore, the amount of secondary emis- .sion from my improved grid is subject to control "that the platinum is deposited as discrete particles,- subsequently fused together, enables one to achieve a predetermined surface-texture, either rough or smooth, depending upon the size of particles depositedand upon the temperature of sintering. sired a rougher surface is produced. I have heated the grids to temperatures: of from 1400 C. to W00" (3;. to produce surfaces ranging from yery rough to quite, smooth, depending upon the amount of secondary grid emission desired in a giver-1 tube design. the grid is heated to higher, 7 temperatures in the final firing step there an:

parently is a greater inclusion of the metallic carbide into the, outer layer 2-8, which increased carbide content also modifies the secondary emission properties. In any event, once a desired secondary emission property is established, I have found that: such emission from my grid .isa istable andndzependazble characteristic. After the finalizing step the :grid is readyfor assembly in a tube. The processed grid is rugged and: no special car in handling is required prior'to mounting in the tube. Thoriated tungsten filament 18 is :carburized in the usual manner, and the tube is evacuated in accordance w-ith'acommonpraictice.

While :I "have described my improvements'in connection with the controlrgridof a -triode,.i't is understood that the improvements may be incorporated in other electrodes where electron emission is a problem. For example, I have had good results in a tetrode where both the control and screen grids were made in accordance with thete'achin'gs of invention.

My improvements are not :to be confused-with grids made in the past having a :coating of platinum on a wire-core. Such grids have been a used with some rn'easure of success; but eventual 1y develop-a high order of primary grid-emission due to an interaction or alloying that takes place between the platinum and :core metal. I -have eliminatedthis difiiculty by providing metallic carbide underthe platinum layer. A still further advantage of my electrode is that very li ttle or the expensive platinum is required since only a verythinouterlayer 284s needed-to obtain the desiredemission properties. This small 'platinum requirement, compared to platinum coated grids used in the past', -is due-to'the presence of the under lying metallicxcar bide. My improvements-are also to -be-distinguished"from the old practice of carburizing the outer-platinum surface of-'--a grid, Such psaotice-doesnot' produce thenim proved results I secure by providing ametallic carbideunder the platinum layer.

If fewer secondary electrons are 618".

As earlier mentioned, the electrode material or grid wire may be processed before the material is fabricated into an electrode. In such event the raw material is first carburized. For example, in the case of grid wire a roll of molybdenum wire may be carburized by packing the roll in a pot of powdered carbon and then heating it in a suitable furnace. The outer layer 28 of platinum may then be applied and fired. This pretreated wire is then ready for fabrication into a grid. I have found molybdenum wire so treated spot welds together more readily than ordinary molybdenum wire.

I claim:

The method of processing molybdenum wire for a grid of an electron tube, said wire exhibit ing, when employed as a grid, negligible primary emission and stable secondary emission, which comprises carburizing the wire by heating the wire in the presence of carbon to a temperature sufiicient to form a layer of molybdenum carbide, then coating the carburized wire by electrodepositing thereon finely divided particles of a metal selected from the group consisting of iridium, ruthernium, and platinum, and then heating it to a temperature suflicient to fuse the particles together and to the underlying molybdenum carbide layer,

PAUL D. WILLIAMS.

REFERENCES CITED The following references are of record in the file of this patent:

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