Question

State two properties which a substance used as thermionic emitter should possess.

Answer 1

A cathode of thermionic emitter should have the following properties:
( i ) Low work function. The substance selected as cathode should have low work function so that electron emission takes place by applying small amount of heat energy i.e. at low temperatures.
( ii ) High melting point. As electron emission takes place at very high temperatures (>1500ºC), therefore, the substance used as a cathode should have high melting point. For a material such as copper, which has the advantage of a low work function, it is seen that it cannot be used as a cathode because it melts at 810ºC. Consequently, it will vaporise before it begins to emit electrons.