428 BELL SYSTEM TECHNICAL JOURNAL 



to prevent the flakes from welding together, this type of powder is not 

 greatly used for molding metal parts. The grease or other lubricant 

 interferes with proper sintering, and there is an additional disadvantage 

 of flakes in that low strength laminated or layered structures result in the 

 pressing operation. The flake powders are more generally used as pigments 

 in the paint industry where their flat surface is an asset for good coverage. 

 A special type of mill, the Eddy Mill, can be used for malleable metals 

 to give particles of suitable shape, fineness, and purity for the manufacture 

 of sintered briquettes. Essentially, the mill consists of a chamber wherein 

 are mounted two fans facing one another and operating at high speeds 

 in opposite directions. The metal is introduced into the chamber in rel- 

 atively small pieces, (e.g. \-^ inch lengths of 0.05 inch diameter wire) 

 which, by collision with one another in the fan blasts, become very finely 

 pulverized. The process can be accurately controlled and a variety of 

 shapes, angular, flake, or pebble, can be produced as desired. 



3. Shotting 



Metal shot can be prepared by dropping the molten metal from a small 

 opening through air or an inert atmosphere into water. If the method is 

 controlled properly, a fairly fine shot can be produced. On the whole, 

 however, this process in powder metallurgical work is confined largely 

 to preparing intermediate size particles for further reduction by other 

 methods. 



4. Atomization 



For metals having relatively low melting points, atomization provides 

 a convenient method of producing fine particles. The molten metal is 

 forced through a small nozzle orifice and broken up by a stream of com- 

 pressed air, steam, or inert gas. The process can be controlled rather 

 closely by proper choice of nozzle, pressure and temperature of the gas 

 used, and the rate of metal flow. As a rule, it is applied to metals melting 

 below 700° C. such as lead, lead alloys, zinc, and aluminum; but copper, 

 having a much higher melting point, has also been successfully treated 

 in this manner. The product can be drawn off and collected in standard 

 dust collector systems, and is suitable for many tj^es of powder compacting. 



5. Carhonyl Process 



Both nickel and iron under suitable temperature and pressure conditions 

 will react with carbon monoxide to form the respective carbonyls^. From 

 these carbonyls, the metals can be obtained by a reverse of the process, 

 decomposing the compound to the metal and the monoxide. The virtue 

 of the process lies in the shape of particle, which appears to be almost 



