Question

Name a substance used as cathode for thermionic emission. State its work function and the temperature to which it is to be heated for electron emission.

Answer 1

Tungsten. It was the earliest material used as a cathode and has a slightly higher work function (4.52 eV). The important factors in its favour are : high melting point (3650 K), greater mechanical strength and longer life. The disadvantages are : high operating temperature (2500 K), high work function and low emission efficiency. Therefore, it is used in applications involving voltages exceeding 5 kV e.g. in X-ray tubes.