Thermionic systems are best suited where a low weight, silent operation, high heat rejection temperature, no moving parts, and no maintenance are desirable. In such a system electrons are ' boiled off an emitting electrode, the cathode, by heating it, and collected by a second electrode, the anode, placed nearby. The electrons return to the cathode via an external electrical circuit. Power densities of up to several tens of watts per square centimeter of electrode area are theoretically attainable in such thermionic diodes.
Process of Thermoionic conversion:
- The emission of an electron from a metal surface is opposed by a potential barrier equal to the difference between the energies of an electron outside and inside the metal. Therefore, a certain amount of energy has to be spent to release the electron from the surface. This energy is called surface work function (f).
- The maximum electron current per unit area emitted from the surface is given by the following Richardson Dushman equation:
- The kinetic energy of the free electrons at absolute zero would occupy discrete energy levels from zero up to some maximum value defined by the Fermi energy level, Ɛf. Each energy level contains a limited number of free electrons.
- Above absolute zero temperature, some electrons may have energies higher than the Fermi level.
- The energy that must be supplied to overcome the weak attractive force on the outermost orbital electrons is the work function, f, so that the electron leaving the emitter has an energy level f Ɛf.
- When emitter is heated, some high energy free electrons at the Fermi level receive energy equal to emitter work function fc, and escape the emitter surface. They move through the gap and strike the collector.
- The K.E. (Ɛfa) plus the energy equal to collector work function fa is given up and this energy is rejected as heat from the low temperature collector.
- The electron energy is reduced to the Fermi energy level of the anode Ɛfa. This energy state is higher than that of the electron at the Fermi energy level of cathode Ɛfc. Therefore, the electron is able to pass through the external load from anode to cathode. The cathode materials are selected with low Fermi levels as comprised to anode materials which must have higher Fermi level.