CONTROLLING THE TYPE OF CONDUCTIVITY BY ALLOYING AND NANO-STRUCTURING IN MATERIALS FOR THERMOELECTRIC APPLICATIONS

Thermoelectric energy based on a direct conversion of heat (or thermal gradient) into electricity is one possible solution for the problem of affordable, environmentally clean energy. A number of compounds have been investigated in the search for potential thermoelectric materials, including the half-Heusler (HH) and PbTe-type alloys, capable of operation at elevated temperatures with adequate chemical, structural and mechanical stability. Due to relatively easy techniques of alloying of these materials with different elements substituting for its constituents, new ways are considered to manage their conductivity properties. The perspective way to improve the desired properties is also in using the nanostructured materials. The concept of nano-structuring of thermoelectric materials has drawn much attention primarily due to the prediction that quantum confinement of charge carriers in low dimensional structures within a given matrix could drastically increase Seebeck coefficient together with significant decrease of thermal conductivity of the composite leading to enhancement of the power factor.

We report the results of ab initio calculations aimed at developing the consistent scheme for determining the role of impurities that may change the type of conductivity in these two classes of materials. It is demonstrated that alloying of PbTe with small amount of Na substituting for Pb leads to p-type conductivity, while Cl substituting for Te makes PbTe an n-type material. Similar calculations for TiNiSn demonstrate that alloying with Cu makes the material of n-type, and alloying with Fe leads to p-type conductivity.

It is shown also that for nano-grained materials the n-type conductivity should be observed. The effect of impurities segregating to the grain boundaries in nano-structured materials is also discussed.