SOME ASPECTS OF POWDER METALLURGY 429 



spherical, the purity, and the control which can be exercised in particle 

 size. . The method has been used for years in the Mond process for making 

 nickel shot, but, until recently, foreign producers exercised almosta complete 

 monopoly on the manufacture of fine powders from carbonyl. Within the 

 last few years, iron carbonyl powder has been produced on a large scale 

 in this country in several different grades suited to industrial needs. The 

 iron powder is a specialty product commanding a higher price than that 

 produced by most other methods, but because of superior properties it 

 has been used extensively in the electrical industry, particularly in the 

 communications field for various types of magnetic cores. 



6. Condensation of Vapor 



Metals which have low boiling points can be vaporized and the vapor 

 then condensed in powder form. These include zinc, magnesium, and 

 cadmium. The powders so produced are used mainly in the chemical 

 industry. 



7. Reduction of Chemical Compounds 



Metal powders whose characteristics can be varied over a wide range 

 are prepared in large quantities by reduction of compounds of the metal 

 with hydrogen or other reducing gases at temperatures below the melting 

 point. The oxide of the metal is most generally utilized for the purpose, 

 and among the metals produced are copper, nickel, iron, cobalt, molybdenum, 

 and tungsten. The type and shape of the metal powder is governed some- 

 what by the compound from which it is reduced, so that, within limits, 

 these factors are controllable by proper choice of compound. 



8. Electrolytic Deposition 



Metals can be electrodeposited in several ways to obtain powder depend- 

 ing upon the plating conditions. A hard, brittle deposit may be obtained 

 which can be further crushed or ground to small particles, or a soft sponge, or 

 even the metal in powder form can be produced. The pov/der is usually 

 dendritic in shape and requires further treatment for use in molding. This 

 generally comprises some sort of milling or grinding operation, and an 

 annealing treatment to eliminate hydrogen and soften the powder. 



9. Other Methods 



Other methods for the preparation of metal powders include chemical 

 precipitation, granulation, alloy formation and removal of an alloying 

 constituent (such as platinum-arsenic, platinum-mercury, and gold-sulphur 

 previously discussed), and the hydride process^^. The last mentioned 

 method is probably the only one of these which is of more than academic 

 interest for powder metallurgy uses. 



