US 2438732 A
Description (OCR text may contain errors)
March 30, 1948. P, w s 2,438,732
ELECTRON TUBE cATHoDE Filed March 15, 1947 I INVENTOR. Pau/ 0. Williams ATTORNEY [m raved Caf/roae Patented Mar. .30, 1948 I 2,438,732 7 ELECTRON TUBE eA'rnonE.
Paul D. Williams, Palo Alto, Calif., assignor to Eitel-McCnlongh, Inc., San Bruno, Calif., a corporation of California Application March 15, 1947, Serial No. 134.923
My invention relates to cathodes for electron 1 tubes, and more particularly to cathodes including thorium oxide.
In working with cathodes wherein the emitter comprises a body or layer of thorium oxide I have found that the electron emission properties of such cathodes are improved by the incorporation of certain other materials. An emitter body consisting of sintered thorium oxide exhibits reasonably good electron emission if it is operated at a sufficiently high temperature. The emission mechanism involved here is that enough of the active thorium metal is released by thermal dissociation high temperatures to support thermionic emission. Operational and life test data, however, show that these cathodes do not have satisfactory emission characteristics at normal cathode operating temperatures, say around 1600 C.,.and that they do not yield sustained emission with prolonged life.
Other limitations of these cathodes are that they have relatively poor thermal properties and that the emitter body is not a good electrical conductor.
The broad object of my invention is to improve cathodes of the character described by overcoming the disadvantages above mentioned.
The invention possesses other objects and features of advantage, some of which, with the foregoing, will be set forth in the following descrip- I tion 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; and
Figures 2.3, 4 and 5 are sectional views of four different types of cathodes illustrating the invention.
In terms of broad inclusion, my improvements comprise incorporating a reducing agent such as carbonin the cathode to promote reduction of the thorium oxide at lower operating temperatures. I also preferably include metallic particles with the thorium oxide to further enhance its electron emission characteristics and improve its electrical conducting and thermal properties.
These features are preferably achieved by incorporating a metallic compound, such as a metallic carbide, in the cathode.
I In greater detail, and referring to Figure 1 of the drawing,.a tube showing a filamentary type of my cathode comprises an envelope 2 having a stem 8 carrying an exhaust tubulation I A base 8 with prongs l is provided at the lower end of the tube. The envelope encloses a plurality of electrodes including a helical filament or cathode 8 which is of a special composition hereinafter described.
Other electrodes include anode 8 and grid II,'
the anode having a cap if supported by bracket l3 on lead it sealed to the envelope. If desired, a suitable coating i8 may be provided on the anode to improve its heat dissipation properties. Grid II has a base ring 2| supported'by brackets 22 on rods 28, one of which serves as a grid lead connected to a base prong I by conductor 24. The filament is secured top and bottom to a pair of leads i'l sealed to stem 8 and connected to a pair of the base prongs by the conductors I8.
The above described tube structure is merely for purposes of illustration and may be varied within wide limits.
My improvements are better illustrated in connection with Figure 2 which is an enlarged sectional view of filament 8. The improved cathode comprises a body 21 of thorium oxide. In the filamentary type cathode being first described this body is in the form of a coating layer sintered on a metallic core wire 28, which core may be of any suitable refractory metal. By the term refractory metal" I mean a metal having a high melting point and low vapor pressure such as tungsten, molybdenum or tantalum.
An important feature of my invention is that the cathode includes an agent to facilitate reduction of thorium oxide to metallic thorium. A reducing agent such as carbon is preferred, the requirements of the agent being that it be capable of performing the reducing function hereinbefore mentioned under normal operating temperatures.
The filamentary cathode shown in Figures 1 and 2 is preferably made by first winding a tungsten core wire 28 about a suitable mandrel to shape the filament. The filament core wire is then mounted on the filament stem by welding the wire to leads ll. Core wire 28 is next coated with a mixture of thorium oxide and carbon, the carbon being incorporated as finely divided carbon particles 28. The amount of carbon in cluded with the thorium oxide is not critical.
The mixture may be coated on core 28 by any suitable means, as by spraying or by cataphor-' .013 inch in diameter having been found satisfactory. After coating, the filament is heated to a temperature of from 1800 C. to 2800 C. in
I an oxygen free atmosphere such as in vacuum or in an inert atmosphere. This heat treatment may be eifected either before or after the filament stem is sealed into envelope 2. During this heating step the thorium oxide particles are sintered into a solid body or layer 21 throughout which carbon particles 28 are uniformly distributed. By the term "sintered" I mean sufficient fusion between adjacent particles in the mass to hold the particles together in a relatively rigid body.
emissionproperties at lower operating temperatures because of the action of the reducing agent, as hereinbefore explained. I have been able to still further improve the cathode, however, by the further inclusion of finely divided particles of a refractory metal such as tungsten, tantalum, molybdenum, zirconium or titanium. This metal powder is preferably uniformly distributed throughout body 21 along with the carbon particles 28. I believe that the further improvement in the emission properties of the cathode, thus secured, is due to the fact that metal surfaces are provided in body 21 upon which the metallic thorium can deposit. Other advantages of including the refractory metal particles are that the thermal properties of the cathode are improved and that the electrical resistance of body 21 is decreased.
Instead of adding a mixture of the carbon and 40 An alternative procedure is to apply the carbon as a coating on the core underlying the coating of thorium oxide. In this case, it pure carbon is applied, at least part of the carbon will combine as a carbide with the core metal during the initial heat treatment. I This is indicated in Figure 3 as a metallic carbide layer 29 underlying the thorium oxide layer 21. With a core wire of tungsten, this layer would, of course,-be tun sten carbide.
My improvements may be incorporated in cathodes other than the filamentary type. Figure 4 shows an indirectly heated cathode wherein the operating temperature of the cathode is maintained by a separate heater 3|. The cathode illustrated comprises a cylindrical core 32 of a refractory metal such as tantalum, carrying a coating or body.33 of sintered thorium oxide.
' An underlying layer 34 of metallic carbide is refractory metal particles, 9. similar result can be 1 secured by incorporating particles of a refractory metal carbide, such as tantalum carbide, molybdenum carbide, zirconium carbide or titanium carbide. These carbides supply the necessary carbon for reducing the thorium oxide, and the residue particles of refractory metal perform the in a layer contiguous with the body, such as in a layer on the surface of the body. For example, I have made cathodes by first applying a layer of thorium oxide and then applying a surface layer of carbon. The cathode was then heated to a temperature of from 1800 C. to 2800 C. as first described.
shown, similar to that described in connection with Figure 3, but it is understood that any of the other cathode compositions hereinbefore discussed may be used.
Figure 5 shows a cathode comprising a cylindrical self-supporting body 35 of sintered thorium oxide in which heater wires 31 are embedded to maintainthe cathode at operating temperature. The reducing agent is illustrated as particles 38 distributed throughout the body 36.
1. A cathode for an electron tube comprising a body of sintered thorium oxide, and a metallic carbide incorporated in the cathode.
2. A cathode for an electron tube comprising a metallic core, an outer layer of thorium oxide. and carbon combined as a carbide with the core metal.
3. A cathode for an electron tube comprising a metallic core. an outer layer of thorium oxide, and an intermediate layer including carbon.
4. A cathode for an electron tube comprising a metallic core, an outer layer of thorium oxide, and. an intermediate layer of metallic carbide.
5. A cathode for an electron tube comprising a metallic core, an outer layer of thorium oxide, and an intermediate layer of tungsten carbide.
PAUL D. WILLIAMS.
REFERENCES CITED The following references are ofrecord in the me of this patent:
UNITED STATES PATENTS