US2855536A - Cathode - Google Patents

Description

Oct. 7, 1958 INVENTOR, EUGENE 1v WYLEH By FRANCIS a T000 AT TOR/VE).

United States Patent CATHODE Eugene N. Wyler, Worthington, and Francis C. Todd, Amlm, Ohio, assignors to the United States of America as represented by the Secretary of the Army Application October 12, 1954, Serial No. 461,945

1 Claim. (Cl. 313-341) The principal object of the present invention is to provide an improved cathode for electron discharge tubes.

This invention relates to cathodes for electron discharge devices and more particularly to cathodes for electron discharge devices of the magnetron type.

An object of the invention is to provide an improved cathode which has the ability to withstand extremely high operating temperatures without structural failure.

The primary object of the present invention is to generally improve electron emissive tubes and particularly to eliminate fracture of the cathode due to temperature changes.

The present invention provides an improved cathode support characterized by long life and mechanical rigidity.

To accomplish the foregoing and other objects which will hereinafter appear, our invention resides in a support for a ceramic-metallic sleeve type cathode which provides improved mechanical strength and improved electrode contact.

The specification is accompanied by a drawing in which the single figure represents a cross-sectional View of the cathode support assembly.

In cathodes for power applications it is desirable to have the highest possible emission current. To accomplish this, extremely high operating temperatures are required, however, at these temperatures shorting of turns of the tungsten filament occurs. A mixture of 80% thorium oxide and 20% molybdenum powder produces a conducting mixture which has several advantages over mixtures of thoria with powdered tungsten. Further, this new mixture withstands extremely high temperatures without excessive evaporation. However, my invention overcomes the major difliiculties encountered with the cathode sleeve of this type by providing suitable electrical contact with the ends of the cathode sleeve and by supporting the sleeve to improve its mechanical strength. The attachment of tantalum to the ceramic-metallic sleeve results in cracking because radial expansion and contraction of the soldered tantalum apparently produces tension on the ceramic-metallic sleeve. Ceramic-metallic, as well as ceramic bodies, are extremely weak in tension. By resting the ceramic-metallic cathode on powdered molybdenum, radial expansion and contraction are resisted less than with the tantalum foil. Molybdenum powder is efiective until it sinters into a solid mass, then failure always occurs by cracking the cermet. Our improved supporting means is capable of withstanding temperatures 2,855,536 Patented Oct. 7, 1958 in the order of 1900 C. without injury to the sleeve and which produces D. C. emission substantially equal to that of pure thoria.

Referring to the drawing, the cathode sleeve 16 is supported in position by cup shaped end members 18 and 19. Molybdenum powder 15, included in the cups, coats the end surfaces of cathode sleeve 16 and the cup walls to provide a low impedance electrical contact to the ends of the sleeve. Contact is established to the lower plug 19 by means of support rod 10 which passes through the length of the sleeve 16. The rod is insulated at 20 from the top member 18 and also from the sleeve 16. The cathode is held under compressive force by a tungsten spring 13 which is insulated from member 18 by an insulating sleeve 14. A second lead 12 is joined to end plate 18. This type of construction permits both leads from the heater to be brought out of the same side of the sleeve and thus minimizes the distortion of the electrical field around the emitting surface of the cathode. The cathode is easily assembled since after the components are properly positioned on rod 10, a spot weld of the spring 13 to the rod 10 at point 11 firmly aflixes the components in place. The close fit and the compressive force between the cathode sleeve 16 and the support members 15 retain the powder in position.

In a specific case we employ a .040 inch tantalum rod 10, a 0.14 inch tungsten spring. The end members 18 and 19 are machined from tantalum and the insulating sleeve 14 is thorium oxide. With this support, a cermet sleeve of 20% molybdenum power and thorium oxide is capable of withstanding temperatures in the order of 1800 C. for long periods of time without failure.

While there has been shown the preferred embodiment of this invention, it will be apparent to those skilled in the art that many changes and modifications can be made without departing from the scope of the invention.

What is claimed is:

A cathode structure comprising a molybdenum-thorium oxide cylindrical cathode sleeve, an upper cup shaped sup port member engaging the upper edge of said sleeve; a lower cup shaped support member engaging the lower edge of said sleeve; molybdenum powder coating the surfaces between the sleeve and the support members; compression means urging said upper support and sleeve toward said lower support; an insulating sleeve interposed between said compression means and upper support; a rod centrally positioned passing through said insulating sleeve, upper cathode support and cathode sleeve; said rod aflixed to said lower support; said compression means consisting of a spring attached at the upper end to said rod with the lower end adjacent said insulating sleeve, and an electrical connection to said upper support.

References Cited in the file of this patent UNITED STATES PATENTS 1,614,462 Dowsett Jan. 18, 1927 2,269,081 Felsner Jan. 6, 1942 2,468,129 Spencer Apr. 26, 1949 2,473,550 Spencer June 21, 1949 2,477,601 Hanson Aug. 2, 1949 2,682,511 Cronin June 29, 1954

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