US2456761A - Rugged cathode - Google Patents

Description

Dec. 21, 1948. P. D. wnJJM/lsv Y RUGGED CATHQDE l Filed April 19, 1947 INVENTR Paul Williams BY M 2 ATTU/QNEY Patented ec. 21,* i943 RUGGED CATHODE Paul D. Williams, Palo Alto, Calif., assignor to Eitel-McCullough, Inc., San Bruno, Calif., a corporation of California Application April 19, 1947, Serial No. 742,489

Filamentary type thermionic catho'des are usually made of shaped into coils, `hairpins or the like depending upon the filament structure used. Thoriated tungsten wire has thorium oxide (thoria) incorporated in the tungsten metal, and is supplied as such to the tube manufacturer. After being fabricated into a filament,V the wire is carburized (sometimes called carbonized) by a processinvolving heating the filament in a hydrocarbon atmosphere. In this step carbon is combined in the filament as tungsten carbide, the carbide lying in a layer or ring in the outer regions of the lament wire. The carbon so introduced functions as areducing agent for gradually reducing the thoria to metallic thorium during the life of the filament, which reduction is required to yield copious electron emission in the operation of a thoriated tungsten filament. In accordance with past practice, the carburized filamentis sealed directly into an envelope andthe tube then evacuated.

While `carburizing the lament is important from the standpoint of electron emission it has the serious disadvantage of making the filament quite brittle and fragile. A filament processed in the usual manner above described constitutes one of the weakest elements in the tube and is responsible for much loss due to breakage, particularly under conditions where the tube is subjected to shock and vibration. Weakness of the filament is due largely to the changes in crystal structurev taking place in the outer regions of the filament wire during formation of the tungsten carbide. These changes involve a crystal growth in the carbide layer, producing well `defined planes of weakness along the interfaces of the crystals. In addition to the crystal growth the situation is further aggravated by thermal shocks occurring in the carburizing process, causing enlarged crevices or iissures to open up along the cleavage planes. Such cleavage planes frequently extend completely through the carbide layer, which is a substantial portion of the wire diameter, and often a slight shock is all that is necessary to make the lament part at that point.

The broad object of the present invention is to improve the strength Vof carburized thoriated tungsten filaments. i v

A further object is to achieve the increased strength without sacrifice of eelctron emission efficiency.

Still another object is to provide a method of lament of the character described ruggedizing a which does not interfere with or obsolete the claims. (c1. 25o- 275) thoriated tungsten `wire suitably Apractices or equipment heretofore used in the fabrication and carburizing of filaments.

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 thatf I do not limit myself to this disclosure of species of my invention as I may adoptvariant embodi` ments thereof within the scope 'of the claims.

Referring to the drawing:

Figure 1 is an elevational view of an electron` tube embodying the improvements of my invention, portions of the anode being broken away to show the internal structure more clearly; and

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

In terms of broad inclusion, my method of improving the strength of a thoriated tungsten cathode comprises modifying the crystalline structure of the cathode to reinforce the structurey along cleavage planes between the crystals. This is done by introducing a bonding agent between the interfaces of the crystalline structure, the agent used being preferably an element selected from the group consisting of vanadium, boron and rhodium. The nal cathode therefore, comprises carburized thoriated tungsten together with the added bonding agent. In my preferred process the cathode or filament is fabricated and `carburized in the usual manner, after which thev filament is heated in the presence of the bonding agentv l f In greater detail, and referring to Figure 1 of the drawing my improved cathode as embodied ina lamentary structure of an electron tube comprises a helical wire 2 Welded top Vand bottom Y toa pair of leads 3 sealed to stem 4 of envelope 6. l Filament wire 2 may be formed into hairpins or other suitable shapes, the helix being shown for simplicity. Other components of the tube illustrated include an anode l and a grid 8, it being understood that the improved cathode o1- lilament may be incorporated in a diode or a multigrid tube, either of the internal or external` `anode type.

supportedby a bracket 9 on lead i l sealed to In the triode shown, anode l is the upper end of envelope 6. Grid 8 is supported by brackets I3 on rods of these rods serving as a grid lead. A base IS cemented to the lower end of the envelope car-` ries prongs leads.

Filament 2 which constitutes the electron emitter is composed of thoriated tungsten wire I1 connected to the grid and lament of the kind ordinarily `used in the filaments of I4 sealed to stem il, one,

electron tubes. This wire usually contains a small amount of thorium oxide (thoria), say 1/2% to 1%, incorporated in the tungsten. After being mounted on its stem the lament is carburized by heating in a hydrocarbon atmosphere. Any conventional carburizing procedure may be followed, such as by heating the filament to say 21D0 C. in a reduced pressure atmosphere containing a suitable hydrocarbon gas such as benzene. The carbon so introduced is combined with the tungsten as tungsten carbide.

The carbide usually lies in a layer or ringl surrounding an uncarburized core portion I9 of te wire (see Figure 2), the depthsof the carbide layer being controlled by such factors as time, temperature and the density of the hydrocarbon atmosphere employed during the ca-rburizing process. The amount of carbon introduced is usually measured by the drop in filament current during carburizing, a current drop of about 20% being common practice. In such a filament the thickness of the tungsten carbide layer i8 is equal to about half the wire radius.

From this point on the usual procedure has been to seal the carburized filament in the envelope together with the other electrodes and then evacuate the tube, the various electrodes being heated during evacuation for `outgassing purposes. My improvements involve an additional treating step after the carburizing process for reasons hereinafter described.

Thoriated tungsten filaments treated in the conventional manner have quite good electron emission properties but are Very weak mechanically. From a study of microphotographs of cross sections of ordinary filament wires both before and after carburizing it becamerapparent to me that certain changes took place in the crystal structure of the wire during'the carburizing procedure which accounted for the mechanical weakness. For example, there was pronuonced crystal growth in layer I8 during formation of the tungsten carbide, some of the larger crystals being coextensive with the thickness-of the carbide layer, producing cleavage planes across thel layer. It Was also observed that these planes of weakness along the interfaces of the crystals actually opened up to produce crevices or fissures through the carbide layer under the thermal shocks occurring during the oarburizing process.

In order to obviate this weakness I introduce a bonding agent to cement the crystals together along .the interfaces. This is illustrated by the bonding agent lying in veins 2l along the crystal boundaries in Figure 2, which View has been exaggerated to illustrate the structure being described. The bonding agent used preferably comprises an element or combination of elements selected from the group consisting of vanadium, boron and rhodium.

The bondingagent may be included in several Ways but I prefer to incorporate it after carburization of the filament. Asimple way is to apply the agent as a coating on the filament and then heat the filament to absorb the agent.

Wh'en vanadium is used as the bonding agent itmay be `applied either as a substantially pure metal or as a compound such as vanadium oxide.

In the case of the pure metal I prefer to usev finely divided vanadium powder. This is mixed with a vehicle such as amyl acetate to a fluid consistency and applied to the filament by spraying. A coating sufiicient to completely cover the surface of the wire is satisfactory. The coatedfilament is then heated in an inert atmosphere, preferably in vacuum, .by passing current through the filament. Heating to about 2200 C. for 10 to 30 seconds is sufiicient to cause penetration of the coating material. The penetration appears to occur as soon as the coating melts, indicating that the surface tension and wetting properties of the vanadium are such as promote its travel into the intercrystalline structure of the tungsten carbide. Also, since the vanadium bonds the tungsten carbide crystals tightly together it is apparent that the adhesive energy of Yvanadium with respect to tungsten carbide is high.

Vanadium being rather hard to get in the pure state I have used rthe more readily available vanadium pentoxide with good results. The

`vtreatmenthere is'the same as that first described except that the coated filament is heated in a reducing atmosphere, preferably an atmosphere of cleandry hydrogen. In this atmosphere the vanadium loxide is at least partially reduced. The inclusion ofsome oxygen does notzappearto be harmful and may actually help inthe penetration and cementing properties of the rVanadiurn.

vMyivanadiumized filaments are much stronger than ordinary filaments. For example, a helical lament about 11-0 of an inch inidiameter and 1A@ inch lon'goi .0035 inch diameter wire in a tube of the character illustrated was used forcomparative testingin a standard high impact machine supplied by the Navy Department. 'Such tubes with filaments treatedin the ordinary manner broke at an average impact of .200 G. Similar n tubes having vanadiumized filaments vbroke at iof an average impact of375 G. This constitutes .an average improvement of over whichis a decided improvement when it is considered'that it almost doubles the strength of the'older type filaments.

If boron is used as thebonding agent-it may also be applied tothe filament either inthe pure state or as a compound such as boron carbide. Both boron and boroncarbide are handled ina manner similar to vanadium metal, namely, the finely divided'boron or boron carbide powder is mixed with a vehicle and coatedon the filament, after vwhich the filament is heated in an'inert atmosphere to absorb the coating material Application of -the coating may be by spraying or by deposition from a bath by cataphoresis. The boronized filament shows an improved strength approaching that of the vanadiumized filament.

In my process, particularly when boron or boron carbide is used, -l prefer to surround the ends -of' the coated filament with heat shieldsduringthe heating step. Small cylinders of molybdenum slightly larger 'than the filament and 'temporarily fastened to the filament structure during the heating'step work very well. These'heat shields are preferably of a length lsufficient to extend over the last several convolutions ofthe filament helix. The shields compensate for the end cooling that normally takes place and :provides an evener temperature along the filament length. This insures a more uniform penetrationfof the coating material.

-If-rhodium is used as a bonding ag'ent'it may be coated on Ithe filament by ordina-ry electroplating methods. The filament so plated is'then heated in vacuum or other inert atmosphere :to about 2100o Cfbrightness temperature as in lthe preceding treatments.

An important feature of my `invention is 'that the `filzufnent'is -ruggedized Without sacrifice of lthe desirable velectron emission properties which characterize .an ordinary carburized thoriated tungsten filament. Many tubes made by my process and life tested for hundreds of hours show that electron emission from the cathode sustains itself with life as well or better than tubes with cathodes treated in the usual manner. Another feature of the invention which will be appreciated by the electron ltube industry is that my process merely involves an added `step between the usual carburizing and pumping procedure, without interfering with or obsoleting any of the practices'or equipment heretofore used in the fabrication and carburizing of filaments.r

I am vaware of the fact that vanadium has been previously incorporated in some types of cathodes such as disclosed in .the patents to Laise 1,569,095 and to Marden et al. 1,787,694; the former de' scribing a cathode in which vanadium and metallic thorium are -alloyed with tungsten, and the latter disclosing the substitution of vanadium for thorium as the active material in .the cathode. These prior inventors therefore did not solve the problems involved in thoriated tungsten cathodes and do not teach the inclusion of a lbonding material such as vanadium in such cathodes.

I claim:

1. A carburized thoriated tungsten cathode hav-ing .a layer of tungsten carbide and a bonding agent lcementing the crystalline structure together in said layer. i

2. A ca-rburized thoriated tungsten cathode having a layer of tungsten carbide and a bonding agent cernenting the crystalline structure together in said layer, said agent comprising an element se lected from the group consisting of vanadium, boron and rhodium.

3. A thoriated tungsten cathode including carbon and vanadium.

4. A thoriated tungsten cathode including carbon and boron.

5. A lthoriated tung-sten cathode including carbon and rhodium.

PAUL D. WILLIAMS.

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

UNITED STATES PATENTS Number Name Date 1,695,819 ONeill Dec. 18, 1928 1,990,277 Fuessner et al Feb. 5, 1935 2,247,755. Hensel et al. July 1, 1941 2,392,318 Gustn et al Jan. 8, 1946

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