THERMOELECTRICITY AND MODERN TRENDS IN ITS STUDIES

Abstract

The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice versa. A thermoelectric device creates voltage when there is a different temperature on each side. Conversely, when a voltage is applied to it, it creates a temperature difference. At the microscopic level of understanding one can say that an applied temperature gradient causes charge carriers in the material to diffuse from the hot side to the cold side. We will start our discussion from the discovery of the phenomenon of thermoelectricity by the Estonian physicist Thomas Johann Seebeck in 1821 and its early manifestations. Today the term “thermoelectric effect” encompasses three separately identified effects: the Seebeck effect, Peltier effect, and Thomson effect. Application of magnetic field considerably increases the variety of possible manifestations of thermoelectricity. The most known among them is the Nernst effect which is nothing else as a thermoelectric effect observed when a conducting sample is subjected to a magnetic field and a temperature gradient perpendicular to each other. The crossed electric and magnetic fields should lead to the drift of a charged particle in the direction perpendicular to both of them. In the case of broken circuit condition such motion of the carriers is prevented by appearance of the temperature gradient in corresponding direction, what is the essence of the Nernst-Ettingshaus effect, reciprocal to the Nernst one. The theory of thermoelectric and thermomagnetic phenomena in metals and semiconductors, based on the quantum theory of solids, was developed in the middle of XX century. It was found that in metals these effects are negligibly small (for Bi the Seebeck coefficient is maximal and is of the order of 7μV/K). The magnitudes of thermoelectric signals considerably increase in semiconductors what allows to use them as the working elements of thermoelectric generators (solid state devices that convert heat flux (temperature differences) directly into electrical energy), for studies of the scattering mechanisms in semiconductors, etc. Today the interest to the thermoelectricity is very high, especially in view of the possibility to design new artificial materials with tuned high thermoelectric properties: graphene, new generation of superconductors, conducting polymers, electrolytes and ferrofluids. Their non-trivial properties will be reviewed in the second part of our presentation.