US2497111A

US2497111A - Electron tube having carburized thoriated cathode

Info

Publication number: US2497111A
Application number: US763574A
Authority: US
Inventors: Paul D Williams
Original assignee: Eitel Mccullough Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1947-07-25
Filing date: 1947-07-25
Publication date: 1950-02-14
Anticipated expiration: 1967-02-14

Description

1 Feb :14 1950 P. D. WILLIAMS 2,497,111

ELECTRON TUBE HAVING CARBURIZED THORIATED CATHODE Filed July 25, 1947 I N VEN TOR. Pad/ 0. Wf/fiams lax M 24 ATTORNEY i atented 14,

ELECTRON TUBE HAVING CARB'URIZE'I) 'rnonm'rao CATHODE Paul D. Williams, Palo Alto, Calif., assignor to Eitel-MoCullough, Inc, San Bruno, Calif., a corporation of California Application July 25, 1947, Serial No. 163,574

My invention relates to electron tubes such as vacuum tubes having a plurality of electrodes including a cathode, grid and anode, and more particularly to tubes in which the cathode is of the thoriated type such as a thoriated tungsten filament.

The use of thoriated tungsten wire for filaments in electron tubes is well known, this material being supplied in the trade to the tube manufacturer as tungsten wire containing around 1% thorium oxide (thoria). During fabrication of the tube the common practice is to carburize the filament so as to convert the outer regions of the tungsten wire to tungsten carbide. This carbide layer materially improves the electron emissive properties of the filament, one of the principal functions of the carbide layer being to provide a source of carbon in the filament for reducing the non-active thoria to the active thorium metal.

From extensive life testing of tubes having thoriated tungsten filaments I have found that the end life of useful electron emission from the filament is determined by exhaustionof carbon from the filament. In other words the tungsten carbide dissociates and the carbon leaves the filament at a certain rate during operation of the tube. This decarburization rate of the filament can be plotted against time, and once established for a given tube can actually be used to predict the emission end life of the filament. I have also found that the decarburization rate is in excess of that which can be accounted for in its reducing action on the thoria, and that there is still an ample supply of thoria left in the filament at the end life. This was interesting because it showed that the useful emission life of the filament was being determined not by the exhaustion of the active agent thorium but rather by a premature exhaustion of the carbon.

- Several theories can be advanced to explain the excessive decarburization rate of the filament. The best theory, confirmed by my experiments, is that oxygen liberated from the grid and anode travels to the filament, robs the carbon from the tungsten carbidelayer and then leaves the filament as carbon monoxide or carbon dioxide.

In View of the foregoing the broad object of my invention is to provide a grid and anode which, in combination with a carburized thoriated cathode, will retard excessive decarburization of the cathode.

The above objective is achieved in my preferred tube by incorporating carbon, preferably as a metallic carbide, in both the grid and anode so 1 Claim. (Cl. 25027.5)

as to satisfy any oxygen contained therein, whereby such oxygen is prevented from robbing carbon from the cathode.

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 claim.

Referring to the drawing:

Figure 1 is an elevational View of an electron tube embodying the improvements of my invention; and

Figure 2 is a fragmentary sectional view showing portions of the filament, grid and anode.

In terms of broad inclusion my improved electron tube comprises a thoriated cathode and an anode and a grid, each of which electrodes contain carbon. The carbon is preferably incorporated in the cathode as tungsten carbide in the usual manner and is preferably incorporated in the grid and anode as a layer of metallic carbide adjacent the surface of the electrodes, the

2 carbide being preferably that of a refractory metal selected from the group consisting of airconium, tantalum, molybdenum, tungsten, titanium and vanadium.

In greater detail, and referring to the drawing, my improvements are illustrated in a triode type of tube, it being understood that the improvements may be incorporated in tubes having more than one grid such as tetrodes or the like. The tube shown comprises an evacuated envelope 2 with a stem 3 carrying an exhaust tubulation i and provided with a base 6 having terminal prongs l. The glass envelope encloses a filamentary cathode 8, tubular anode 9 and a cage type wire grid ll. Anode!) has a cap l2 supported by a bracket I3 on a lead l4 sealed to the upper end of the glass envelope.

Cathode 8 comprises a helix of thoriated tungsten wire welded to a pair of leads ll sealed to stem 3, these leads being connected to a pair of the base prongs 1 by conductors l8. structurally speaking, grid ll comprises vertical wire bars terminating ata base ring 21 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.

This tube structure is merely for purposes of illustration and may be varied widely, it being understood that the improvements may be used in many other tube designs.

Referring to Figure 2, which shows fragmentary portions of the cathode, grid and anode at one side of the tube center line 25, the filamentary cathode 8 has a core portion 28 of uncarburized thoriated tungsten and a layer 2'! adjacent the surface of tungsten carbide. This carbide layer is preferably formed in the conventional manner by heating the mounted filament in a carburizing atmosphere of hydrocarbon gas such as benzene. The usual practice is to heat the oathode in the carburizing atmosphere by passing current through the filament until the drop in filament current due to the increased filament resistance indicates a suificient penetration of the tungsten carbide layer. Adrop infilament current of about 15% of its initial value is common practice in carburizing filaments of this kind.

Grid il preferably has a wire core 28 of a refractory metal such as molybdenum, tantalum or tungsten, molybdenum being preferred. A layer 29 containing carbon is provided adjacent the surface of the grid. The carbon-is preferably incorporated as a metallic carbide in the layer surrounding core 28, the carbide being preferably that of a metal selected from the group consisting of zirconium, tantalum, molybdenum, tungsten, titanium and vanadium. With a core of. molybdenum the layer 29 is preferably of molybdenum carbide, since it can be readily formed by merely heating the grid in a carburizing atmosphere so as to convert the outer regions of the core wire to the carbide, If the carbide layer is to be of a metal different from thatof the core material, the preferred procedure is to coat the core wire with finely divided particles of preformed metallic carbide and then sinter the coated grid in an oxygen free atmosphere to a temperature sufficient to'fuse the carbide particles into an adherent layer onto the core. A layer of zirconium carbide thus formed on a molybdenum core makes a good combination.

Others of the carbides specified, or a mixture of those named, may. likewise be applied on any suitable core material. The thickness of this carbide layer is not critical, my practice being to provide a layer of suffcient thickness to comfpletely cover the core material. As indicated earlier herein the function of the carbon in the grid is to satisfy anyoxygen occluded or otherwise present in the grid, and'the particular way inwhich the carbon'is incorporated or. applied is of'secondary importance. Instead of using a metallic carbide, a layer of pure carbon or' a mixture of carbon and metallic carbide may be employed. Carbon combined in the form of a carbide of the metals named is preferred, however, because so combined it is stable and adherent enough to be retained under the operating conditions involved and yet be available, for combination with any oxygen present."

The carbide layer 29 may either'be used alone as a surface coating on thefgrid or in combination with additional materials supplied for other purposes. For example, an outer layer 3| of platinum or other metal in group 8 of the periodic table may be provided. Such a multi-layer grid is preferred because it also has the additional advantage of supressing undesired electron emission from the grid as disclosed in my copending application Serial No. 645,443.

Anode 9 preferably has a cylindrical core or body 32 of sheet metal such as molybdenum,

tantalum or tungsten, molybdenum being preferred. The carbon in this case is also preferably incorporated in a layer 33 adjacent the surface of the electrode, and while the carbon may be in the nature of pure carbon, it is preferably in the form of a carbide of one or more of the metals specified for the grid carbide layer 29. Anode layer 33 may conveniently be applied by coating the anode with finely divided particles of preformed metallic carbide and then sintering the coated anode in an oxygen free atmosphere. In order to gain additional advantages such as gettering (gas absorbing) and improved heat dissipation properties, I prefer to use an anode coating containing zirconium carbide such as disclosed in my copending application Serial No. 742,773, which issued as Patent No. 2,447,973 on August 24, 1948.

Whatever specific compositions are used in the grid and anode, however, the important feature of my present invention is that a grid containing carbon and an anode containing carbon are both used together and in combination with a carburized thoriated cathode in order to accomplish the result of retarding excessive decarburization of the cathode. The desired result is not achieved if carbon were supplied in the anode alone or in the grid alone because the oxygen leaving the electrode not containing the carbon would be suificient to decarburize the cathode. If a tube having more than one grid is constructed in accordance with the teachings of my invention it is understood that all the grids as well as the anode are provided with carbon.

It is further understood that the electrodes may be of shapes and composition different from those described. For example, the anode could be oblong instead of cylindrical and could be solid carbon or graphite instead of having a metal body.

In any event, I have shown by extensive life testing that thoriated tungsten cathode tubes having carbon provided in both the grid and anode exhibit a decidedly lower cathode decarburization rate and consequently sustain useful electron emission for a much longer life period.

I claim:

An electron tube comprising a thoriated tungsten cathode having, a layer adjacent its surface comprising tungsten carbide, an anode having a layer adjacent its surface comprising zirconium carbide, and a grid having a plurality of layers adjacent its surface, one of said grid layers containing platinum and another grid layer containing the carbide of a metal selected from the group consisting of zirconium, tantalum, molybdenum, tungstein, titanium and vanadium.

PAUL D. WILLIAMS.

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

UNITED STATES PATENTS Number Name Date 1,852,865 Upp Apr. 5, 1932 2,144,249 Allen Jan. 1'7, 1939 2,232,083 Strohfeldt Feb. 18, 1941 2,282,097 Taylor May 5 1942 2,400,893 Thurber et al. 1 May 28, 1946 2,459,792 Chevigny Jan. 25, 1949