US2339392A

US2339392A - Cathode

Info

Publication number: US2339392A
Application number: US460964A
Authority: US
Inventors: Lloyd P Garner
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1942-10-06
Filing date: 1942-10-06
Publication date: 1944-01-18
Anticipated expiration: 1961-01-18
Also published as: GB568962A

Description

Jan. 18, 1944. P GARNER 2,339,32

CATHODE Filed 0G12. 6, 1942 PEA-Am c roxy ffm/ BASE Pal/v7- POWDEREO METHLL/Q TUNGSTEN ON Pffwncropy BASF AND .5l/V751? Patented Jan. 18, 1944 Lloyd P. Garner, Princeton,

dio Corporation o! Ame Delaware N. J., asslgnor to Rarlca, a corporation of Application october s, 1942, serial'No. 460,964

(ci. 25o-27.5)

9 Claims.

My invention relates to cathodes for electron discharge devices and the like.

All metals and many of their inorganic compounds are electron emissive at elevated temperatures. The metals most widely used for pure metal electron emitting cathodes are tungsten and tantalum. Both of these metals have relatively high work functions and require large heating powers to maintain them at a suiiiciently high temperature to produce useful electron emission. The pure metal emitters have the 'advantage of being relatively insensitive to poisoning of emission by contaminating lms and resi'dual gases resulting from and/or surviving the preparation and pumping. The pure metal cathode has the further advantage that when emitting high electron current, there is no associated decomposition of compounds or alloys to evolve vapors or gases to contaminate the electrode or spoil the vacuum, although there may be some evaporation of the cathode metal. 'I'he pure metal cathodes are limited to use in high power electron discharge devices.

Of the inorganic compounds and mixtures used .as emitters, the most common are barium and strontium oxides mixed and applied to a refractory metal base, usually nickel containing small quantities of reducing agents such as titanium, magnesium, carbon and tantalum. 'I'he greatest advantage accruing to this barium oxide cathode is long life and low operating temperature and its greatest disadvantage is sensitiveness to contamination or poisoning and its inability to sustain large electron emissions Without the destruction of the emitting oxide coating and without evolution of vapors, sublimates, and gases. The cathode, further, is limited to low temperature operation, and consequently low emission current, because at elevated temperatures the barium evaporates from the cathode, condenses on the grid, and causes excessive grid emission." The emitting materials, also, seem to have such high ohmic cross resistance that elevated emission currents passing through the material cause excessive heating which destroys the emitting material. These cathodes nd wide use in radio receiving tubes and the smaller electron discharge devices.

'I'he next most widely used electron emitting cathode is the thoriated tungsten cathode. This cathode is tungsten mixed with a small amount, usually less than 1.5%, of thorium oxide, ThOz. The ThOz powder is mixed with the tungsten powder and then the tungsten is pressed into ingots and sintered and swaged and drawn er of a refractory metal into wire. Since tungsten, for technological reasons, cannot readily be rolled into large sheets. thoriated tungsten type cathodes are confined to lamentary cathode shapes. An advantage of this cathode is the relatively lower operating temperature (between pure metals and barium oxides), long life, and ability to sustain great emission currents without destroying itself. Its principal disadvantages are: it is applicable only to lamentary cathode shapes, is extremely sensitive to some contaminations, and is destroyed or deteriorated by gas ionization.- These cathodes are widely used in power devices of intermediate sizes.

The object of my invention is to combine the several desirable features of the pure metal, the oxide coated, and the thoriated tungsten cathodes.

More specically, the object of my invention is a cathode that has high emission current, a relatively low temperature of operation, that is insensitive to contamination and gas ion boml bardment, and that can be constructed in a great variety of shapes.

The'objects of my invention have been realized in practice with a cathode comprising a refractory metal base to which is sintered a laypowder of substantial thickness. Refractory metals which I have found to be suitable are tantalum, tungsten, and molybdenum. The surface layer of the cathode then constitutes a matrix of discrete metal particles arranged much like the oxide particles of the conventional oxide coated cathode. Into the recesses of the metal matrix is worked, and secured by sintering, small particles of thorium oxide. Such a cathode is easy to make, operates at a temperature several hundred degrees versely through the coating, called cross resistance, is very low compared to the cross resistance of the coating of conventional oxide coated cathodes.

ufacture of one cathode of my invention are outlined in the block diagram of the accompanying drawing.

Tantalum is one metal found to have the desirable physical and thermal properties for a.

good cathode base. Tantalum is malleable and may be easily worked into thin sheets, is refractory, does not sag nor evaporate at operating temperatures and has high hot strength. Tantalum may be formed into narrow or wide filament ribbon or rolled into cathode sleeves for direct or indirect heating. While tungsten has a high melting point and is otherwise desirable for my cathode core, it cannot be readily worked into thin sheets and can only be used as drawn filament wire. In tubes where the evaporation of molybdenum is not deleterious, this metal also may be used as the base or core for my cathode.

To the base metal is applied a thin layer of powdered metallic tungsten which lis made to adhere to the base metal by heating for a few minutesat sintering but below fusing temperature in vacuum. The thickness of the tungsten layer is not critical and may be applied by mixing with a liquid vehicle and painting or spraying. Several successive applications and firings of tungsten powder are found advantageous if one application does not leave a coating sufficiently thick. The tungsten powder coating should completely cover the base metal yet need not be thicker than about .0005 to .002 inch. Next, to the sintered layer of tungsten is applied a coating of powdered thorium oxide, the thorium oxide, like the metal tungsten powder, preferably being mixed with a liquid vehicle and painted or sprayed over the tungsten layer. Some of the oxide particles arel probably carried by the liquid vehicle into and are more or less uniformly dispersed throughout the interstices of the tungsten powder. The amount of thorium oxide deposited is not critical, but little advantage could be found in applying more thorium oxide than would correspond to a film .0005 to about .002 inch thick. After degassing and vacuum firing to x the coatings in place, the cathode is ready for use. At operating temperatures between 1900 and 2100 K., as measured by an optical pyrometer, continuous current emissions of 6 to 7 amperes per square centimeter of cathode area are obtained. Higher temperatures will produce higher currents.

Good results have been obtained in the manufacture of power tubes designed for delivering 50 kilowatts or more of high frequency power by using a tantalum ribbon 0.010 thick and 0.8" wide and 21/2" long. Four of these ribbons are each folded at their centers, as shown in the patent to Smith et al. 2,256,297, September 16, 1941, and screwed to current supply bus bars. After degassing and vacuum firing, the tantalum ribbon is coated with a layer of powder. The powder and a nitro-cellulose binder, mixed to the consistency of thin paint, is brushed on the side of the tantalum ribbon which is to face the anode. The filament is then sealed into an envelope and fired in vacuum for about thirty minutes to a brightness temperature of 2500 K. If the surface of the tantalum is not completely covered, a second or a third coating of the tungsten paint may be applied, each application being fired as the rst. The number of applications will depend upon the consistency of the paint, the wetting properties of the liquid used, the technic of application, and such factors as are known in thorium oxide powder is applied. The oxide is mixed with the nitrocellulose binder and is coated over the sintered tungsten layers and is fired in vacuum for several minutes at about 2300 K. Two or more applications of the oxide may metallic tungsten` the painting art. Next the:4

be made to completely cover the tungsten. The cathode is then finally sealed in its envelope and is degassed for a few minutes at about 2500 K. The filament temperature may then be to 1900 or 2000 K., the anode voltage applied, and the tube operated in the usual manner.

The high emission of my cathode may be obtained, according to a further feature of my invention, at a slightly lower temperature. After the tungsten powder has been red to the base, a lm of powdered metallic molybdenum of about the particle size of the tungsten is applied and sintered in place by firing in vacuum at 2100 K. for thirty minutes, whereupon the thorium oxide powder is applied as described.

Experience with my metal matrix-thorium oxide shows that the cathode will emit more than seven amperes per square centimeter throughout a normal cathode life of more than one thousand hours. This characteristic of my cathode is particularly desirable since full emission is obtained at temperatures as low as 1900o K. to 2050 K. compared to an operating temperature of over 2900 K. for pure tungsten -to obtain the seven amperes per square centimeter. Pure tungsten at 2050 K. will emit only two milliamperes per square centimeter. The .high current, low temperature emission of my cathode'may be due, I believe, to the thorium oxide particles being held in an open metal lattice or matrix on the surface of the cathode as distinguished, for example, from the conventional thoriated tungsten cathode where the thorium oxide is embedded below the smooth impervious surface of a solid tungsten body. The discrete but sintered metallic tungsten particles serve the triple function of decreasing the electrical cross resistance of the surface films, of slowly reducing the admixed thorium oxide, and of physically holding thel thorium oxide particles in place.

It is apparent that the tungsten particles are not melted at the sintering temperature of 2500 K., but it is probable that there is incipient fusion between the metal particles and the tantalum base. There may be, however, a tungstentantalum alloy formed at the points of contact between the tungsten particles and the tantalum base. Because oi' the relatively open structure of the tungsten powder layer, the tantalum is partially exposed to the thorium oxide. This partial exposure probably accelerates the reduction of the thorium oxide and accounts for the higher level of emission at the relatively low operating temperature of 2050 K.. This exposure o f tantalum, however, is so controlled and limited by the particles of tungsten metal that the thorium oxide reduction is not too rapid for long life. I found that when the thorium oxide was placed directly on the tantalum base, the emission at 1900 K. was initially high, but was exhausted in about two hours. Further evidence of the controlled thorium oxide reduction according to my invention was found when the thorium oxide powder was mixed with the metallic tungsten powder before application to the tantalum base. Here again tests show that the initial emission was high but the life was short, indicating a too intimate contact between thorium oxide and tantalum and a too rapid reduction ofthe oxide.

The particle size of the metallic tungsten and of the thorium oxide is not critical. The particular powder with which good results have been obtained, appears under a microscope, with a magnification of 700, to consist predominantly of tungsten particles which vary in size from less than 1/ micron in diameter to 3 or more microns in diameter. Dispersed through the tungsten powder, however, are some larger particles, varying between 25 and 30 microns in diameter, which apparently were aggregates or agglomerated groups of the smaller tungsten particles. The aggregates donot break up when mixed with a cellulose vehicle and painted on the base. After firing, the tungsten appears, when magnified '700 diameters, to lay in a thin smooth layer much as seaside beach sand, but from which protrudes large rough lumps. It is probable the smooth surface is composed of the 1/2 to 3 micron particles, while the lumps are the 25 to 30 micron aggregates.

The thorium oxide particles used vary between 1 and 3 or 4 microns in diameter, and, like the tungsten, formed lumps or aggregates 25 to 30 microns in diameter. The oxide aggregates are transparent to light and outlines of the 1 to 4 micron particles in the aggregates could be observed. While the absolute as well as the relative sizes of the tungsten and thorium oxide particles on my cathode are not critical, it is probable that the tungsten particles should not be so large and the thorium oxide particles so small as to permit the oxide particles to settle freely through the tungsten onto the tantalum base, where the oxide could be reduced and evaporated after a short life. .Ey using tungsten and thorium oxide particles of about the same size, the particles become uniformly dispersed, one in the other, and when the tungsten is applied rst the thorium oxide particles are to a large extent held in the tungsten matrix out of contact with the tantalum.

Any of the commercial binders, such as used for coating barium-strontium oxides, may be used for mixing with the tungsten and thorium oxide powders of my cathode. For example, 64.5 grams of nitrocellulose mixed with 19.5 cc. of Diatol and 850 cc. of diethyl oxalate may be used.

My improved cathode has an emission emciency higher than that of the conventional thor-lated tungsten, can sustain steady emission currents of several amperes per centimeter, can withstand the ravages of gas ion bombardment and can be made in a variety of shapes easily and inexpensively.

I claim:

1. A cathode comprising a base of coherent refractory metal, a layer of powdered refractory metal particles sintered to the surface of said base, and particles of thorium oxide on and intermingled with the sintered particles of said layer.

2. A cathode comprising a refractory metal base selected from the group consisting of tungsten, molybdenum and tantalum, an incipiently fused surface layer of particles of metal selected from the group consisting of tungsten and molyb denum on said base and, a coating of thorium oxide on said layer, the oxide particles being held in place by said metal particles incipiently fused to the surface of-said base.

3. A cathode comprising a. coherent refractory metal bas'e, a matrix of discrete tungsten particles incipiently fused to the surface of said base said matrix consisting of tungsten particles which are predominantly of sizes from about one-half to 3 microns in size and of some tungsten particles about 20 to 30 microns in size dispersed through said matrix, and thorium oxide particles embedded in the interstices of said matrix.

4. A cathode comprising a tantalum core, mixed particles of metallic tungsten and thorium oxide adhering to the surface of the core.

5. A cathode comprising a tantalum core, a coating of mixed particles of metallic tungsten and thorium oxide on said core, said coating being .0005 to .004 inch deep.

6. A cathode comprising a refractory metal core, a coating of mixed particles of metallic tungsten and metallic molybdenum adhering to said core, and particles of thorium oxide on and dispersed through the coating -of metallic particles.

7. The method of making a cathode comprising coating a metal base with athin layer of metallic tungsten particles, heating said base to the sintering temperature of the base and particles,

then coating the roughened sintered surface with thorium oxide particles and heating in vacuum to a temperature at which the oxide particles adhere to the tungsten particles.

8. The method of makinga cathode comprising coating a tantalum base with a thin layer of tungsten particles, sintering said vtungsten particles to said base, then coating a thin layer of molybdenum particles on the sintered tungsten layer and heating in vacuum to the sintered temperature of the molybdenum, and finally applying a layer of thorium oxide particles to the sintered tungsten-molybdenum surface and heating in vacuum.

9. The method of making a cathode which comprises a refractory base metal with a thin layer of refractory metallic particles, heating said I base to a temperature below the melting point of said base or of said particles and to a temperature at which said particles sinter and form a rough surface layer, coating the rough sintered surface i with thorium oxide particles, and heating the coated base in vacuum to a temperature at which the thorium oxide particles adhere to said refractory metallic particles.

LLOYD P. GARNER.