All materials are composed of atoms which inturn consists of nucleus, made of protons and neutrons, surrounded by electrons. These electrons are distributed in various levels around the nucleus and thus possess different levels of energy. Now, imagine that we start heating a particular material. The thermal energy so supplied increases the kinetic energy of the electrons within the material. This causes them to overcome the force of attraction which exists between them and the protons within the respective nuclei. As a result, they get knocked-out from the material and will be liberated into space surrounding the material (Figure 1). More is the heat supplied, more are the number of electrons ejected. This phenomenon is known as thermionic emission i.e. emission of ions called thermions due to the thermal energy supplied, first observed by Thomas Alva Edison in 1883.
From the discussion presented, it might appear that the number of thermions emitted can be increased upto a large value just by increasing the temperature of the substance in-hand. However this is not entirely true. The fact is that the number of thermions emitted is limited due to the effect of space charge – a phenomenon wherein the liberated thermions surround the electrode forming a shield, preventing the emission of further thermions.
Rate of Thermionic EmissionThe number of thermions emitted per second from the substance considered is known as the rate of thermionic emission. This value depends on
- Nature of the Material In general, every element can be characterized by its electronic configuration i.e. by the distribution of electrons surrounding its nucleus. When we speak of thermionic emission, our particular interest is in the valence electrons (electrons in the outermost shell). This is because, these are the electrons which can be easily freed from the force of attraction so as to enable conduction. However the energy which must be supplied differs from element to element and is regarded to be its threshold energy or work function.
- Surface Temperature Higher is the temperature of the substance, greater is the rate of thermionic emission.
- Surface Area If the surface area of the material considered is larger, then there will be more number of thermions emitted. This means that the rate of thermionic emission is directly proportional to the surface area of the material.
By analyzing these factors, it can be concluded that the substance chosen to be a thermionic emitter should have low work function, larger surface area and high melting point. A few examples of this kind are metals like tungsten, thoriated tungsten, tantallum, etc and coated metals like barium oxide, strontium oxide, etc.