State two characteristics of a good thermion emitter.

1. Thermion emitter:-

1. When a metal surface is heated to some extent, then free-electron gain the thermal energy which breaks the interaction between the electron and the nucleus due to this result emission of free electron from the metal surface, this type of metal surface is called thermion emitter.

2. Two characteristics of thermion emitter:-

1. Work function of the thermion emitter should be low so that minimum heat energy is required to emit electrons from the metal surface.
2. The melting point of a good thermion emitter should be high so that it does not melt at the high temperature required for thermionic emission.