Thermionic emission refers to the phenomenon that electrons escape from the surface of a metal or other solid materials when heated under vacuum or filled with inert gas.
Thermionic emission current density strongly depends on the properties of the material itself and the heating temperature. During use, the temperature should not be too high to prevent excessive evaporation of the material.
There is a force on the metal surface that prevents electrons from escaping. The escape of electrons needs to overcome the resistance to perform work, which is called work function. At room temperature, only a very small amount of electrons have kinetic energy exceeding the work function, and very few electrons escape from the metal surface.
Generally, when the metal temperature rises above 1000K, the number of electrons whose kinetic energy exceeds the work function increases sharply, and a large number of electrons escape from the metal, which is thermionic emission. If there is no external electric field, the escaped hot electrons accumulate near the metal surface and become space charges, which will prevent the hot electrons from continuing to emit.