The search for mechanically stable PbTe based thermoelectric materials

The search for alternative energy sources is nowadays at the forefront of applied research. In this context, thermoelectricity for direct energy conversion from thermal to electrical energy plays an important role, in particular, for the exploitation of waste heat [G. J. Snyder and E. S. Toberer, Nat. Mater. 7, 105 (2008); M. S. Dresselhaus, Adv. Mater. (Weinheim, Ger.) 19, 1043 (2007)]. Materials for such applications should exhibit thermoelectric potential as well as mechanical stability. PbTe based alloys have been considered for many years as state of the art thermoelectric materials for miderature power generation (500-900 K), with efficiency values that are still being improved by both alloying [P. F. P. Poudeu, Chem., Int. Ed. 45, 1 (2006); J. R. Sootsman, Chem. Mater. 18, 4993 (2006); P. F. P. Poudeu, Chem. Soc. 126, 14347 (2006); J. Androulakis, Adv. Mater. (Weinheim, Ger.) 18, 1170 (2006); K. F. Hsu, Science 303, 818 (2004)] and doping [Y. Gelbstein, Physica B (Amsterdam) 363, 196 (2005); Y. Gelbstein, Physica B (Amsterdam) 396, 16 (2007)] optimizations. However, the mechanical properties of PbTe based materials are highly dependent on the conductivity type (n or p) and carrier concentrations [Y. Gelbstein, Scr. Mater. 58, 251 (2008)]. This paper puts forward the mechanical durability of thermoelectric materials and, in particular, of PbTe as a dominant factor that is nondetachable from the transport properties, which should be considered in the search for high quality thermoelectric materials. Here we discuss the microhardness enhancement of p -type PbTe alloys with hole concentrations higher than 5× 1018 cm-3. This anomaly is obtained while all the other investigated n -type (up to 1020 cm-3) and p -type (up to 1018 cm-3) compositions maintained a constant microhardness value of ∼30 HV. The origin of this microhardness enhancement is not yet understood on a fundamental level, however two possible mechanisms are discussed. One deals with the elastic interaction between dislocations and impurities with higher covalent radius than the sublattice vacancy. The other is correlated with the existence of a second valence band of heavy holes in PbTe, which begins to fill up at the same concentration where a hardness enhancement was observed. These mechanisms correlating between mechanical end electronic properties of PbTe based alloys can serve as guidelines for the search for potential candidates, obtaining both thermoelectric potential and mechanical stability for thermoelectric applications.