Thermal Decomposition

Of the group of thermally decomposed powders, those produced by thermal decomposition of carbonyls are the most important. Both iron and nickel are produced by decomposition of the respective carbonyls.

Processing Conditions. Carbonyls are obtained by passing carbon monoxide over spongy metal at specific temperatures and pressures. Iron pentacarbonyl, Fe(CO)5, is a liquid at room temperature, boiling at 103 °C (217 °F). Nickel tetracarbonyl, Ni(CO)4, boils at 43 °C (109 °F). When the pressure is reduced to 1 atm and the temperature is raised correspondingly, both of these carbonyls decompose to re-form the metal and carbon monoxide. The latter is recycled to form more carbonyl and to continue the process. These reactions are expressed as follows:

Powder is produced by boiling the carbonyls in heated vessels at atmospheric pressure under conditions that allow the vapors to decompose within the heated space and not on the sides of the container. The powder is collected and sieved and may be milled, followed by an anneal in hydrogen. The chemical purity of the powders can be very high (over 99.5%), with the principal impurities being carbon, nitrogen, and oxygen. Particle size can be controlled very closely. Iron carbonyl powder is usually spherical in shape and very fine (less than lO/'m), while the nickel powder is usually quite irregular in shape, porous, and fine. Detailed descriptions of the iron and nickel carbonyl processes are given in the articles "Production of Nickel-Base Powders" and "Production of Iron Powders" in this Volume.

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