Study of the Concentration and Mobility of Carriers in Nanostructured PbTe- and GeTe-Based Thermoelectric Materials

A promising approach to increasing the thermoelectric figure of merit ZT is the development of nanostructured thermoelectric materials, including those based on lead telluride PbTe and germanium telluride GeTe. To optimize the nanostructuring technology, it is important to know the mechanisms of heat and electricity transfer in thermoelectric materials, which depend on the charge-carrier concentration and mobility. In this study, nanostructured middle-temperature n-PbTe- and p-GeTe-based thermoelectric materials are obtained using the developed technique, which involves synthesis by the direct fusion of components, grinding of the synthesized PbTe and GeTe into nanopowders in a planetary ball mill, and their consolidation into a bulk material by spark plasma sintering. To perform electrical measurements, ohmic contacts are formed on the thermoelectric material samples by the electrochemical deposition of nickel. The temperature dependences of the thermoelectric parameters of the nanostructured thermoelectric materials are examined. The concentration and mobility of majority carriers are measured by the van der Pauw method. It is found that the majority-carrier concentrations in the thermoelectric materials obtained by hot pressing and spark plasma sintering lie in the range of 10¹⁹–10²⁰ cm^–3, which is optimum for them. It is established that the mobility of majority carriers in PbTe is much higher than in GeTe. The majority-carrier mobility in nanostructured PbTe decreases by 36% as compared with the value for PbTe obtained by hot pressing. However, this does not lead to a noticeable decrease in the electrical conductivity due to an increase in the majority-carrier concentration. As a result, the maximum ZT values are obtained for nanostructured PbTe and GeTe: higher by 14% and 13%, respectively, than in the thermoelectric materials formed by hot pressing.