A metal can be considered as the collection of a conglomeration of crystal with various shapes and sizes. Each crystal consists of a nucleus and orbits surrounding nucleus. The nucleus can be considered as the positive charged portion and in the orbits, electrons are revolving. Since electrons have negative charge, we can consider orbits with negatively charged electrons revolving with a velocity of light. The valence electrons, ie, the electrons in the outermost orbits decide the chemical behavior of an atom. When we brought similar atoms close to each other, the electrons in the metal try to move from one atom to another. In a random way, the valence electrons with high potential energy will move very freely from atom to atom. These electrons which can move freely in an atom are called as “free electrons”. When the valence electrons reach the surface of metal, it encounters a potential energy barrier; the kinetic energy of such electrons will get reduced to zero and is turned back into the body of the metal.
If the energy is greater than zero, it emits from the metal surface. The “work function” of the metal can be defined as this minimum amount of energy required at absolute temperature to make some electrons to escape from the metal.
The electron emission can be classified as,
1. Thermionic Emission
2. Secondary Emission
3. Photoelectric Emission
4. High Field Emission
From the name itself, the thermionic emission deals with the effect of heating. We know that when a metal is heated, its temperature increases and the kinetic energy of some of the electrons in the metal may increase beyond the fermilevel so as to surmount the potential energy barrier of the surface. These electrons can escape from the metal and yields to a type of emission called ‘Thermionic Emission’. Thermionic emitters are of two types,
1. Directly heated Emitter
2. Indirectly Heated Emitter (Oxide Coated Emitter)