thermoionic emission Emission of electrons from a surface of a metal or metal oxide at high temperature. The kinetic energy of the electrons at high temperature may be sufficiently high to overcome the potential barrier at the surface called the work function *. Copious emission of electrons can be obtained from pure metals only at temperatures above 2000^oC. Metal oxides produce thermoionic emission at lower temperatures. The theory of thermoionic emission was worked out by Richardson. He showed that the current emitted by the surface per unit area, J is given by,

J = A T² e^{-w /kT} (t3)

where w is the work function, k the Boltzmann constant, and ‘A’ a constant. Thermoionic emission is used in thermoionic valves and electron guns as source of electrons. The most commonly used emitters are tungsten, heated to temperature between 2200 K and 3000 K, thoriated tungsten (thorium and carbon added to tungsten) operated at ~ 1900 K and oxide coated cathodes (barium and strontium oxide), operated at about 1000 to 1150 K.