US2420014A - Cathode - Google Patents

Description

Patented May 6, 1947 ZAZMM CATHODE GeorgeffT. Rayel'chiSan BrunofCalifI, assgnorto .A

Inc., SanBrunofCalyifd, a

- corporation of California A"Application-Ap1'ilv18, 1944, SerialNo. 531,631 4Clams.. (Cl. Z50-i275) :My invention relates tofa cathode fon-vacuum tubes It? isamong theobjects of my inventiontogprovide a cathode having improved electron emissive characteristics and superior physical properties.

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 a vertical sectional view showing an electron tube having a filamentary cathode embodying my improvements; and

Figure 2 is an enlarged cross-sectional view of the improved filament wire.

Figure 3 is an enlarged sectional View of an indirectly heated cathode.

In terms of broad inclu-sion, my cathode comprises a core of material having good electron emissive properties, such as thorium, clad with a refractory metal, such as tantalum; the refractory metal sheath being preferably carburized. The method comprises first forming the cathode of the improved material, and then carburizing it. The cathode is mounted in an envelope with other suitable electrodes, the envelope being evacuated in any suitable manner.

In greater detail, and referring to the drawing, my cathode 2 is illustrated as lamentary type comprising a helical wire welded top and bottom to a pair of leads 3 sealed to stem ll of envelope 6. It is understood however that the cathode may be of any suitable shape or structure, either directly or indirectly heated, as will be apparent to those skilled in the art.

Other components of the tube chosen for purposes of illustration include a tubular anode or plate 1 and a cage-type grid 8. The plate is supported on a lead 9 sealed to the upper end of the envelope by a bracket Il secured to plate cap I2. Grid 8 is supported on rod I3 sealed to stem 4 by brackets I4, one of these rods serving as a lead. A base I5 cemented to the bottom of the envelope carries prongs Il connected to cathode leads 3 by conductors i8 and `to grid lead I3 by a conductor I9.

While I have shown my cathode in a triode combination, it is understood that it may be embodied in other tube types such as a diode or multi-grid tubes.

Now going to the cathode or thermionic emitter element from which electrons are evaporated. As.

, f2. shown-1 in Figure l2, the ,-lam'ent .wire.comprises a composite strand havingaeorelclad'withia: sheath 2 2; .Corezliisof .a material havinghgood electron emissivity, which would include :generallythos-e materials havingv la. g Alow f work. function such as thorium, uranium and bariumythorium being preferred. In the filament illustrated the core is a solid rod composed essentially of metallic thorium, preferably having a diameter about half that of the overall wire diameter. This has the advantage of being able to draw the final composite wire in an ordinary wire machine.

If desired however the core may be of powdered metallic thorium compressed into the tubular outer sheath. Also, the core may be a composition or alloy of metallic thorium with another metal, or a compound such as thorium carbide. In any case however the core is a distinct homogenous body forming the central zone of the wire, and provides within the lament wire an ample reservoir of the highly emissive or active material.

Sheath 22 is of a refractory metal such as tantalum or molybdenum. Tantalum is preferred for several reasons, one being that it i-s ductile enough to be readily drawn as a sheath over the core in a wire machine. Tantalum also provides a good medium through which the thorium is able to migrate to the surface of the wire, from which surface the electrons are emitted.

, mosphere containing a ordinary thoriated tungsten Sheath 22 is also preferably carburized. Ihis is accomplished by heating the cathode in an athydrocarbon, -similar to the process of carburizing an ordinary tungsten filament, whereby carbon is taken up by the sheath in the form of tantalum carbide. This carburization further improves the emissive properties of the cathode.

My improved cathode is superior to the usual thoriated tungsten filament wherein nely divided thoria (thorium oxide) is distributed throughout a tungsten wire. In making this wire it is diicult to control the distribution of the thoria, resulting in a bad manufacturing problem and nonuniformity in the final product. This is overcome in my cathode element. Furthermore my improved emitter element provides a reservoir of the active agent, insuring longer life of the filament. Still further, my improved lament has greater mechanical strength and is less brittle than an ordinary th'orlated tungsten filament. This is very important because many tubes are lost during manufacture and subsequent operation because of filament breakage.

improved emitter element is also adaptable for use in an indirectly heated type oi cathode. As shown in Figure 3, the element is formed as a sheet with a core layer 26 of metallic thorium interposed between sheath layers 21 of a refractory metal. Tantalum is particularly preferred as the sheath in this case because of its ductility in rolling and otherwise fabricating the composite sheet. The sheet may then be shaped as a tube or cylinder for encircling a heater 28.

My improved cathode is not to be confused with composite elements in the past wherein a layer of material containing barium or strontium oxide is formed on the surface, or where the inner portion of the element is of a refractory metal having thorium oxide distributed in it.

I claim:

1. A cathode for electron tubes comprising a thorium core clad with tantalum.

2. An indirectly heated cathode comprising a tubular emitter elemento! sheet material having a core layer of thorium interposed between sheath layers of tantalum. 3. 'A sheet material for cathodes comprising a core layer composed essentially of metallic thorium interposed between sheath layers of a refractory metal.

4. An indirectly rheated cathode for electron tubes comprising a, tubular emitting element consisting essentially of tantalum and thorium, and a heater disposed within said element.

GEORGE T. RAYFIELD.

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

UNITED STATES PATENTS Number Name Date 1,663,547 Gero Mar. 27, 1928 1,695,819 ONeill Dec. 18, 1928 1,961,122 Lowry May 29, 1934 1,917,991 McCullough July 11, 1933 K 1,894,059 Ruiiley Jan. 10, 1933 2,091,554 Mendenhall Aug. 31, 1937 1,948,445 Lederer Feb. 20, 1934 1,957,486 Becker May 8, 1934 2,140,534 Mahoney Dec. 20, 1938 2,107,945 Hull et al Feb. 8, 1938 2,175,345 Gaidies et al. Oct. 10, 1939

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