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Metals used in Valve Manufacture

K Spangenberg, Vacuum Tubes (1948). 1001
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Next to tungsten, tantalum has the highest melting temperature of all the metals. Its vapour pressure is very low. It is easily formed and drawn. The metal is expensive as a result of the relatively complicated vacuum processing required to put it into form suitable for vacuum-tube construction. It is extensively used in radiation-cooled transmitting tubes, where the electrodes are often run at a red heat. It has a getter action that causes it to absorb gases, particularly hydrogen, the maximum absorption occurring at 1000°C (cherry red). The gases that have been absorbed are given off again at temperatures of 1300°C and higher. Minimum temperature for getter action is approximately 800°C. Tantalum is also used as an emitter in applications requiring specially shaped cathodes. Its work function is lower than that of tungsten, with the result that its emission is greater at the same temperature. Tungsten can, of course, achieve higher emission because it can be heated to higher temperatures without melting.

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