US2061755A - Electron emission device - Google Patents

Description

Patented Nov. 24, 1936 ELECTRON EMISSION DEVICE Hugh S. Cooper, Cleveland,

Ohio, assignor, by

mesne assignments, to Union Carbide and Carhon Corporation, a corporation of New York No Drawing. Application March 18, 1931, Serial No. 523,651

1 Claim.

The invention relates to improvements in electron emission devices" and has for its main object a more economical construction for certain of the metallic parts of these devices.

Molybdenum is commonly used for some of the elements of thermionic tubes which are subjected to the great distorting forces and to the high temperatures which are necessary in the preparation or operation of the tubes. Grids of most types of tubes are composed of molybdenum. In power tubes the plates, supports and other parts are generally composed of molybdenum because of the freedom of this metal from warpage at the high temperatures which are reached during the manufacture and use of the tubes. Molybdenum is technically satisfactory as far as distortion at high temperatures is concerned, but the stiffness of the metal, its tendency to oxidize when heated, and other difficulties encountered in shaping and molding the metal, makes the use of molybdenum elements costly.

Attempts have been made to use a metal which is easier to work than molybdenum, and nickel has been tried. Nickel can be used where strength and stiffness are not required at high temperatures. Where temperatures of about 9% C. or more are applied in preparing or operating the evacuated tube, nickel sags, stretches, warps or breaks. Nickel as the base metal of oxide coated filaments has the desirable property of giving the oxide coating a higher electron emission than molybdenum and tungsten base metals, but filaments composed of nickel have the disadvantage of stretching and sagging and therefore leave much to be desired. Nickel grids are entirely unsatisfactory since they warp and sag. Supporting wires for tube elements and plates of low power tubes can frequently be composed of nickel, but the best results cannot be obtained because the temperature at which nickel warps or fails is near the operating temperatures or the treating temperatures which are necessary for degassing.

According to the invention, the metal portions of one or more than one of the elements of the tubes are made of compacted cobalt. It has been found that compacted cobalt is much easier to work than molybdenum. When subjected to high temperatures, such as about 900 C. cobalt wire or plate is not deleteriously affected and elements composed of cobalt compare favorably in strength, stiffness and freedom from warping with similar elements composed of molybdenum. The filamentary metal of oxide-coated cathodes which are composed of cobalt resemble nickel in that they produce high electron emission but at high temperatures the cobalt filaments do not stretch and deform. Cobalt wire was found to have one and onehalf times the strength of nickel wire at 900 C., and cobalt plates, screens and supporting wires do not warp in service at the temperatures which warp similar nickel parts.

Compact homogeneous cobalt may be obtained by compacting or working a cobalt rod, for example by forging, rolling or swaging, but any convenient method may be used to obtain compact homogeneous metal. A convenient method of preparing the parts consists in compressing cobalt powder into briquettes. The briquettes are heated to about 1350 C. in a dry hydrogen atmosphere and swaged into bars of compact homogeneous metal. By repeatedly heating and swaging, the metal is worked into the form of plates or rods which may be further reduced to sheets or wires by rolling or drawing with appropriate annealing in hydrogen. are formed, coiled or woven into the desired shapes in accordance with practices well known in the art.

A high degree of purity is desirable in order to work down the metal of those parts which are reduced to the smallest sizes. Wires for filaments or grids should contain about 99.75% cobalt but a higher purity would not be harmful. The remainder of the metal may consist of elements which form solid solutions with cobalt and do not volatilize at tube operating temperatures, such as nickel, iron, manganese, and chromium. More massive structures may contain up to about 1% of these elements but all structures may desirably be composed of pure cobalt. The term, substantially pure cobalt, as used in the specification and claim refers to pure cobalt as well as to cobalt containing the above-mentioned impurities. OX- ides and sulphides, other substances which readily produce gases when moderately heated in a vacuum, and carbon are excluded. The term, parts, as used in the specification and claim refers to the metal of the supporting wires or of the tube elements, such as grids, filaments, screens or plates.

I claim:-

An electron emission device comprising an evacuated envelope enclosing an element which possesses high strength, stiffness, and which is free from warpage when subjected to elevated temperatures in the neighborhood of 900 C., said element consisting wholly of homogeneous compressed powdered pure cobalt in the form of a compacted briquette, the powder particles being superficially fused.

HUGH S. COOPER.

The tube parts