US2682511A

US2682511A - Thermionic cathodes

Info

Publication number: US2682511A
Application number: US201219A
Authority: US
Inventors: Leo J Cronin
Original assignee: Raytheon Manufacturing Co
Current assignee: Raytheon Co
Priority date: 1950-12-16
Filing date: 1950-12-16
Publication date: 1954-06-29
Anticipated expiration: 1971-06-29

Description

Patented June 29, 1954 THERMEONIC CATHODES Leo J. Cronin, Natick, Mass., assignor to Raytheon Manufacturing Qompany, Newton, Mass, a corporation of Delaware Application December 16, 1950, Serial No. 201,219

Claims.

This invention relates to electron-emitting structures, and more particularly to cathodes of the directly-heated therminoic-emission type.

The invention discloses an improvement of the directly-heated cathode disclosed in Patent No. 2,477,601, issued August 2, 1949, to John F. Hanson. This cathode is made from a mixture of powdered refractory metal such as molybdenum and thoria, said mixture being compressed into the desired cathode shape and then heated to an elevated temperature. I

It has been found that cathodes constructed in this manner are extremely rugged, and are copious electron emitters. However, such cathodes have the disadvantage that their resistance decreases during the cathode life. For example, a cathode of this type which has an initial heater voltage of ten volts for a heater current of eighty amperes will usually drop to a heater voltage of six to seven volts for a heater current of eighty amperes during the processing and operation of the magnetron in which the cathode is used.

This invention discloses a means whereby the change in heater voltage during the processing of the tube may be sharply reduced. Briefly, this invention discloses the addition of a small quantity of ceria (cerium oxide) to the mixture of thoria and refractory metal from which the cathode is made.

Other and further objects and advantages of this invention will be apparent as the description thereof progresses, reference being had to the accompanying drawing, wherein:

Fig. 1 illustrates a longitudinal, cross-sectional view of a cathode and cathode support embodying this invention; and

Fig. 2 illustrates a transverse, cross-sectional view of the cathode and cathode support structure shown in Fig. 1, taken along line 2-2 of Fig. 1.

Referring now to the drawing, there is shown a cathode I 0 comprising a hollow, cylindrical member made of a compressed sintered mixture of thoria, refractory metal and ceria. The. mixture may be made up, for example, as follows: approximately sixteen to thirty per cent, by weight, of a refractory metal, metal'compound or metal alloy such as molybdenum, molybdenum carbide, molybdenum cobalt, molybdenum nickel, tungsten, tungsten carbide, tungsten cobalt or tungsten nickel, point five per cent, by Weight, of ceria, and the remainder of thoria. This mixture is formed into the hollow, cylindrical form of cathode H), for example,

by means of a mold under pressure which may be, for example, in excess of two hundred thousand pounds per square inch. The compressed, cylindrical member is then sintered by being heated, for example, to within the temperature range from thirteen hundred to twentyfive hundred degrees centigrade for from a few minutes to a number of hours depending upon the characteristics desired.

As shown here, the cathode cylinder I0 is held in a cathode support comprising a lower cylindrical member I I having a recess 12 in the upper end thereof adapted to engage the lower end of the cylinder I0. An upper support member I 3 has a recess I 4 therein adapted to engage the upper end of cylinder [0. A central conductor I5, shown here by way of example, extends through cylinders I and I0, and is attached to upper support member [3. Thus, it may be seen that cathode cylinder 10 is rigidly held between support members l3 and H in such a manner that, by the application of a potential between the central conductor l5 and the lower support cylinder I l, a current will flow from central conductor l5 through upper support member l3, cathode cylinder l6, and lower support member II. Since members 15, II and I3 are metal, they present substantially no resistance to the current flow, while cathode cylinder l0, being of a mixture of metal, thoria and ceria presents a substantial resistance to current flow, and, as a result, is heated by said current flow to electron-emitting temperature.

It has been found that a cathode made in accordance with this invention will have a relatively small resistance change during the processing and life of the tube. For example, cathodes of this type requiring an initial heater voltage of ten point one volts to maintain a current flow of eighty amperes therethrough will retain a heater voltage requirement in excess of nine point eight volts for a current flow of eighty amperes through the cathode throughout the processing and life of the tube.

While the precise mechanism of the action of ceria in the cathode is not known, it is believed that ceria produces a bond that is more stable electrically, and, as a result, the resistance of the bond remains relatively stable throughout the life of the cathode. While, in its preferred form, the cathode mixture comprises point five per cent, by weight, of ceria, good results may be obtained with mixtures comprising point one per cent to ten per cent, by weight, of ceria.

This completes the description of the particular embodiment of the invention described here in. However, many modifications thereof will be apparent to persons skilled in the art without departing from the spirit and scope of this invention. For example, many difierent shapes of cathodes may be used, other well-known electron-emitting materials could be substituted for thoria, and the invention is not necessarily limited to directly-heated thermionic cathodes, since it could also be used with indirectly-heated cathodes. Therefore, it is desired that this invention be not limited to the particular details of the species described herein, except as defined by the appended claims.

What is claimed is: a

1. A directly-heated thermionic cathode comprising a compressed body consisting of thoria, substantially sixteen to thirty per cent, by weight, of a refractory metal, and less than ten -per cent, by weight, of ceria.

ing of thoria, substantially sixteen to thirty per cent, by weight, of molybdenum and substantially zero point five per cent, by weight, of ceria.

4. A directly-heated thermionic cathode comprising a compressed body consisting of greater than sixty per cent, by Weight, of thoria, less than ten per cent, by Weight, of ceria, and the balance of a refractory metal.

5. A directly-heated thermionic cathode comprising a compressed body consisting of greater than sixty per cent, by Weight, of thoria, substantially zero point five per cent, by weight, of

ceria, and the valance of a refractory metal.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 568,780 Lowenberg Oct. 6, 1896 1,716,545 Mackay June 11, 1929 2,473,550 Spencer June 29, 1949 2,501,089 Pomerantz Mar. 21, 1950 FOREIGN PATENTS Number Country Date 388,161 France May 23, 1908 3,112 Great Britain Feb. 5, 1913

Claims

1. A DIRECTLY-HEATED THERMIONIC CATHODE COMPRISING A COMPRESSED BODY CONSISTING OF THORIA, SUBSTANTIALLY SIXTEEN OF THIRTY PER CENT, BY WEIGHT, OF A REFRACTORY METAL AND LESS THAN TEN PER CENT, BY WEIGHT, OF CERIA.