444 BELL SYSTEM TECHNICAL JOURNAL 



tungsten, molybdenum, nickel, or graphite can be retained, while good 

 electrical conductivity may be obtained with copper and silver"-^--^-. 



Another type of material with good spark quenching properties is the 

 combination of silver and cadmium oxide, which, because no alloying 

 results, also has high electrical conductivit>'^^. 



The contact materials may be made by any of the suitable powder tech- 

 niques. One method is to press and sinter the powder composition sought, 

 with or without final sizing or shaping of the part. Another method that 

 is utilized for making tungsten-copper compositions consists in pressing 

 a bar from tungsten powder and sintering at 1300° C. in hydrogen. The 

 tungsten thus forms a strong porous structure which can then be impregnated 

 with copper. This may be accomplished by placing the part in a graphite 

 boat with copper, heating above the melting point of the latter, and allowing 

 the voids to be filled by capillary action.^^ 



No single contact material is satisfactory for all purposes, and a number 

 of dififerent combinations have been developed. These include silver- 

 tungsten, copper-tungsten, silver-graphite, silver-molybdenum, cemented 

 tungsten carbide, and copper-nickel-tungsten. They are used in many 

 installations such as circuit breakers, welding machines, relays, and many 

 types of industrial control equipment. 



7. Alnico magnets 



Many Alnico magnets of small size have been produced commercially 

 by powder methods''^'^^. Magnets made in this manner are fine grained 

 in contrast to the relatively coarse grained material obtained by casting 

 methods. The material is uniform throughout with no cold shuts, cracks, 

 blow holes or grain boundary segregation so that a more uniform flux 

 density is obtained. Of particular interest are the close dimensional 

 tolerances which can be maintained in the powder method and the small 

 amount of grinding required in finishing. The composition can be held 

 much more closely than for the cast alloy. 



The process is limited economicalh' to the production of small samples. 

 Large samples can be prepared by conventional methods at a cost that 

 would not allow sintered products to compete. 



The presence of a highly oxidizable element (9-13 per cent of aluminum) 

 presented difficulties when attempts were first made to prepare Alnico by 

 sintering pressed compacts. To overcome this oxidation, the aluminum 

 is added in the form of alloy powder of 50 aluminum-50 iron composition 

 prepared by crushing and ball milling a casting of the brittle material''^. 

 In such form, there is practically no oxidation of the aluminum under the 

 sintering conditions which prevail. 



