PM Nickel Alloys for Thermionic Emitters

Abstract

An introduction explains the principles for substrates of oxide thermionic emitters. A method of fabrication, alternative to the traditional fusion processes, is described, employing roll compaction of mixed metal powders, with the recognized advantages over conventional ingot pressing and sintering for medium-scale production with modest equipment investment. Especially valuable is the capability to sinter continuously, rapidly, and at relatively low temperatures, which confers extra control over contents of volatile and reactive additives. Protection of these additives during sintering and preliminary fabrication is effected by prealloying and coating the alloy powders with a layer of the matrix metal-nickel. The equipment and process is described with attention to controlling uniformity of thickness and density transversely and along the strip length, which is essential in minimizing edge cracking during the early rolling stages. Purity and compositional control is held to much tighter tolerances than for cast alloys, but a microheterogeneity obtains: a method for demonstrating this feature is illustrated. This microinhomogeneity modifies the diffusion characteristics of the additives and is a factor in restricting grain growth during fabrication and operation. PM/0240