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Metals used in Valve Manufacture

K Spangenberg, Vacuum Tubes (1948). 1001
    
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Tungsten

Reference has already been made to some of the numerous applications of tungsten in vacuum-tube construction; as an emitter and filament wire and in some lead-sealing applications it has virtually no substitute. Its high melting temperature makes it especially useful in some vacuum-tube construction processes. It is used as a filament wire for silver-soldering operations. It is likewise used as a filament heater in numerous metal-evaporation processes. It is one of the few metals that can be used as a target in X-ray tubes. Numerous gauges and control devices make use of its large change of resistance with temperature.

Tungsten is not readily drawn or formed. It must be hammered or swaged into shape. As a result, it is principally available in wire or rod form. Tungsten has a pronounced crystalline structure, which is accentuated by heating.

Tungsten filaments therefore become brittle if overheated for appreciable periods of time. Tungsten is relatively inactive chemically, which reduces contamination problems. It is sometimes alloyed with molybdenum (W/Mo = 49:51) to give a material that is more workable than tungsten itself and yet retains a high melting temperature.

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