Thermoelectric materials have the ability to convert thermal energy into electrical energy. This offers the opportunity to use waste heat from engines for conversion into electricity. This reduces energy losses and improve their efficiency.
In 1821 Thomas Johann Seebeck discovered that a temperature difference between the two ends of a metal rod causes an electric voltage. This was the first time that heat could be converted into electricity. The voltage U, depending on the difference in temperature (Delta-T), is then defined by the Seebeck coefficient (also called thermopower) and is derived by S=U/Delta-T. The magnitude of S is solely a property of the material to which the temperature gradient is applied. However, the efficiency of thermoelectric conversion is not only determined by S but also by the thermal conductivity (lambda) and the electrical resistivity (rho) of this material. Therefore, the "figure of merit" (Z) needs to be investigated to get useful information about the suitability of the material for converting heat into electrical energy. It is expressed by the formula Z = S^2/(lambda*rho). Consequently, when searching for materials with large S-values, lambda and rho should be as small as possible. As shown by the formula above, promising materials need to have a large Seebeck coefficient but low thermal conductivity and low electrical resistivity to be effective. Nature, however, does not favour such a combination since large thermopower requires usually materials with low charge carrier concentration. In order to improve the thermoelectric performance of clathrates, substitution and doping is a promising method. Clathrates are compositions of elements that form large cages in their crystal structure in which other elements are caught as in a trap. The interaction of this trapped element with the crystal lattice influences the transport coefficients (lambda, rho, S) of the material.
As the elements in the samples are changed, the transport properties are, of course, changed as well. This might then lead to an improvement of the figure of merit.
Consequently, the change of elements and their concentrations in clathrates by substitution and doping can improve the efficiency of thermoelectric processes and therefore increase the figure of merit.
This diploma thesis focuses on the investigation of Ba8-based clathrates with copper, zinc and cadmium as variable elements.
Furthermore, the concentration of cadmium is altered. As the results of the measurements will show, some of these compositions offer an outstanding figure of merit and, thus, promise to have useful applications in technical processes.