US2497109A - Electrode for electron tubes - Google Patents

Description

Ebb. M E95@ P. D. WILLIAMS ELECTRODE FOR ELECTRON TUBES Filed Feb. 4, 1946 [171 ro ved r/a INVEVNTOR. Paul D. M/i/lmmfi ATTORNEY electron emission properties. proper functioning of a control grid in a triode Patented Feb. 14, 195) UNITED STATES PATENT OFFICE 2,497,109 ELECTRODE FOR ELECTRON TUBES Paul D. Williams, Palo Alto, Calif., assignor to Eitel-McCullough, Inc., San Bruno, Calif., a corporation of California Application February 4, 1M6, Serial No. 645,443

8 Claims. (omen-477) My invention relates to electron tubes, and more particularly to an improved electrode for such tubes.

In electron tubes there are certain electrodes whose operation is seriously affected by their 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 effects of primary grid emission are most serious, espe- For example, the

cially 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 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.

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

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 species of my invention as I may adopt variant embodiments thereof within the scope of the claims.

Referring to the drawing:

Figure 1 is an elevational View of a tube embodying the improvements of my invention, portions of the anode being broken away to show the inner electrodes; and

Figure 2 is an enlarged view of portions of one of the grid wires.

In terms of broad inclusion, my improved electrode comprises a metallic core, an outer layer of a primary electron emission inhibiting material, and an intermediate layer of a metallic compound isolating the outer layer from the core. In my preferred electrode the outer layer iricludes a metal in group 8 of the periodic table, and the intermediate layer comprises a metallic carbide. The method of making the electrode preferably comprises the steps of fabricating the electrode of metal, carburizing the electrode, and

then applying the outer layer by electrodeposition and sintering.

In greater detail, and referring 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 I2 supported by bracket 53 on a lead l4 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 I! sealed to stem 3, these leads being connected to a pair of base prongs I by conductors l8. structurally speaking, grid II is of the cage type comprising 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 grid lead and is connected by a conductor 24 to a base prong. v

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 ll 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 awire on a mandrel and fasten it to the base ring 2|. This same procedure is 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.

The grid is next 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 the preferred electrode. Carburizing may be done in several ways, as by heating the electrode in a hydrocarbon atmosphere or by heating it in a pot containing powdered carbon. For convenience, however, and to establish better control over the carburizing 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 preferably electrodeposited from a bath containing the particles in suspension. The coated grid is then heated to say 1850 C. for about five minutes in an oxygen free atmosphere of an inert gas or. in.vacuum.. During thisheating the carbon combines withi the core metalto". form a metallic carbide, as will be readily understood. The resulting grid at this stage has a layer 21 of the metallic carbide, the nature of the carbide depending upon the core metaluse'di The carburized grid is next treated to. form an outer layer 28 of a material having the-desired electron emission properties. outer layer composed mainly of a metal in group 8 of the periodic 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 sinter the particles together and to the underlying. layer. The platinum particles. arev preferably. electrodeposited from a chlorplatinic. acid solution, and the grid thus coated is preferably heatedto say 1650" C. for. about five minutes-in an oxygeniree atmosphere of an inert gasor in vacuum. Heating in vacuum is preferred'because italso. serves to outgass the electrode. This heating step sinters the particles. into. a continuous sheath and fuses the outer layer to intermediate layer 21. Also, a certain amount of metallic carbidelfrom the underlying layerv becomes. mixed with the I prefer to use an platinum during the sinteringoperatiomso that point. of. both primary and secondaryemission.'

The grid exhibits aninitiallow order of primary emission, and the amount of primary emission does. not increase-uponcontamination of. the grid with thorium. The suppression oi primar emission is due to the composition. of outer layer- 28 and also to-the fact thatthislayer is isol'ated by thebarrier layer 21.

Furthermore, the amount. ofsecondary emission from my improved grid is subjecttocontrol and; once established, is stable. Thecomposition of outer layer 28 and itssurface texture determine its secondary emission properties. The. fact-that the platinum is deposited. asdiscrete particles, subsequently fused. together,. enables one to achieve. a. predetermined surface texture, either rough or. smooth, depending upon thesizeof par-'- ticles deposited and uponthe temperatureof sin tering. If fewer secondary electrons. are desired arougher surface is produced. L'haveheatedthe grids to temperatures. of froml400." C. to 1700 C. to produce surfaces'ranging from very rough to quite smooth, depending upon theamountof secondary grid emission desired ina giventube design. As the grid is heated to higher. temperatures in the final firing step there apparently is a greater inclusion of. the metallic carbide. into the outer'la yer 28, which increased carbide content also modifies the secondary emission-proper.- ties. In any event, once a desired secondary emission property is established,.I have found that such emission-from'my' grid is a stable and dependable characteristic.

Afterthe final firing step the grid is readyfor assembly in a tube. The. processed grid'is' very rugged and no special care in Ii'andlin'g'ls're- 1 mum on a wire'core.

good results in a tetrode where both the control and screen grids were made in accordance with the teachings of my invention.

My improvements are not to be confused with grids made in thepast having a coating of plati- Such grids have been used with some measure of success, but eventually develop-a' high order of primary grid emission due to an interaction that takes place between the platinum and core metal. I have eliminated this difficulty by the interposed barrier layer 21-. Also, the inclusion of 'a' metallic carbide in my outer layer 28 helps to control the emission properties. Furthermore, the smooth surface coatings of pure platinum heretofore used do not have the ad'vant'age of regulating secondary emission as is possible in my improved electrode. A still further advantage of my electrode is that very little of the expensive platinum is required since only a very thin outer layer 28 is needed to obtain the desired emission properties. This small'plati'num requirement, compared to platinum coated grids usedin the past, is du tothepr'esence ofba'rri'er layer" 27 and also to the fact that part of outer layer 28 is made up of the metalliccarbid'e.

I claim 1'. An electrode for. an electron tube comprising a metallic core, an outer layer'of'a primary electron emission inhibiting material, and an inter mediate layer of a metallic carbide isolating the outer. layer from said core.

2. An electrodefor an electron tube comprising a metallic core, an outer layer including a metal selected from the group consisting of platinum, iridium, and ruthenium, and an intermediate layer of metallic carbide.

3.. An electrode for an electron tube comprising ei-m'etallic core, an outer layer comprising amixtureof a metallic carbide and a metalselected from the group consisting, of platinum, iridium, and. ruthenium. and an intermediate layer of metallic carbide.

4. An electrode for anelectron tube comprising'a' metallic core,.an outer layer including platinum, and an intermediate layerof metallic carbide.

5. An electrode for anelectron tube comprising a core of molybdenum, an' outer layer. including platinum, and an intermediate layer of molybd'enum' carbide.

6. An electron t'ubehaving a thorlated cathode and a grid, said grid comprising a metallic core, an-outer'layer including'a'metal'selected from the group consisting of, platinum, iridium; and ruthenium, and an intermediate" layer ofmetallic carbide.

'7. A grid'material for an-electron tubecomprlsingan inner layer or metallic carbide, and an outerlayer'overlyingsaid carbide layerand including a metal selected from the group consisting of platinum, iridium,.andruthenium.

8. Agridma-terial-for an electron tube comprising an inner layer of metallic carbide, and an outer layer overlying-said carbide layer and comprising finely dividedpartioles of a metalselected from the group consisting of platinum, iridium,

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

Number UNITED STATES PATENTS Name Date Marden July 6, 1926 Hyde Nov. 23, 1926 Long Nov. 20, 1934 Taylor May 5, 1942 Holdaway et a1 Oct. 24, 1944

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