442 BELL SYSTEM TECHNICAL JOURNAL 



The brushes can be pressed around pigtail conductor wire inserts to insure 

 good contact for the lead wire and eliminate attachment problems. 



Commutator segments, resistance rings, and rotor bars in squirrel-cage 

 motors, have been successfully produced from copper by powder met- 

 allurgy methods^^. 



4. Refractory Metals 



Because of high melting points, the refractory metals, of which tungsten, 

 molybdenum and tantalum are the most important, are prepared by powder 

 methods. The preparation of each is similar, with the technique differing 

 only in certain details where the characteristics of the individual metals 

 require it^^. 



With tungsten^^, the ore is treated by chemical methods to yield pure 

 tungstic oxide which is then reduced by hydrogen at 650°-950° C. to give 

 tungsten powder with a particle size range from 0.5 to 8 microns. As 

 with other metal powders, care is exercised throughout to maintain high 

 purity. After proper mixing and blending, the powder is compressed and 

 the briquette given a pre-sintering treatment at 1000°-1200° C. to give suffi- 

 cient strength for further handling. The resulting bar is then clamped 

 in electrodes in a suitably designed hydrogen chamber, where acting as 

 a resistance heater, heavy electric current is passed through it. The 

 compact shrinks, the density increases, and a relatively solid bar results 

 which can then be hot-worked. During the swaging or rolling, the working 

 temperature can be gradually decreased until there is sufficient ductiUty 

 by control of grain size to draw the material cold. 



For tungsten used in lamp filaments, certain additions such as thorium 

 oxide, or compounds of sodium or potassium mixed with such relatively 

 non-volatile materials as SiOo, AI2O3, or Th02 are intimately- mixed with 

 the tungstic oxide prior to reduction. These additions are effective in 

 controUing grain growth and insuring proper grain boundary orientation 

 for producing ''non-sag" coiled filament. Essentially, the sodium and 

 potassium compounds promote large grain growth while the others, such 

 as thorium oxide, inhibit grain growth under the conditions of wire fabri- 

 cation. When the material is drawn in wire form, the thoria particles 

 form elongated stringers in the direction of drawing and tend to prevent 

 grain growth across the wire while allowing exaggerated growth along the 

 axis of the wire. The resulting structure of long grains with boundaries 

 forming acute angles with the longitudinal axis of the wire is ideally suited 

 for the coil t>^e of lamp filament. 



Molybdenum and tantalum are prepared in much the same manner as 

 tungsten, although tantalum sintering and annealing must be conducted 

 in high vacuum because of the ability of the metal to absorb and retain 

 gases at high temperatures. 



