Hint: To answer this question, concentrate on the word emission. The term thermionic emission itself explains that it is related to emission of something when we provide sufficient thermal energy to any metal. It is defined as the emission of electrons when sufficiently high levels of thermal energy is provided. Thermal emission occurs when the thermal energy provided to the charge carrier overcomes the work function of the material. It was first observed by Thomas Alva Edison in 1883.
Complete answer: Thermionic emission can be defined as the liberation of electrons from the surface of the metal when it is heated. These electrons are known as thermions. We can also define it as when the large amount of external energy in the form of heat is supplied to free electrons in the metal, the electrons are emitted. The number of thermions emitted per second from the substance is called the rate of thermionic emission. It depends upon the following factors: 1. Nature of the material 2. Surface temperature 3. Surface area It also depends on the amount of heat applied to the metal and the work function of the metal. A substance can be considered as a thermionic emitter only if it has low work function, larger surface area and high melting point. Substances with these factors include tungsten, barium oxide, thoriated tungsten, etc.
Note: Students must remember that the thermionic emission is possible only in metal surfaces. The emission varies from metal to metal with varying intensity of thermal energy provided. Greater the melting point of the metal, higher is the rate of emission of electrons. The classical example of thermionic emission is that of electrons emitted from a hot cathode into vacuum. This example is also known as thermal emission or the Edison effect in a vacuum tube.