The search for mechanically stable PbTe based thermoelectric materials

Abstract

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 et al., 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 mid-temperature power generation (500–900 K), with efficiency values that are still being improved by both alloying [P. F. P. Poudeu et al., Chem., Int. Ed. 45, 1 (2006); J. R. Sootsman et al., Chem. Mater. 18, 4993 (2006); P. F. P. Poudeu et al., Chem. Soc. 126, 14347 (2006); J. Androulakis et al., Adv. Mater. (Weinheim, Ger.) 18, 1170 (2006); K. F. Hsu et al., Science 303, 818 (2004)] and doping [Y. Gelbstein et al., Physica B (Amsterdam) 363, 196 (2005); Y. Gelbstein et al., 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 et al., 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.

Micro- and Macromechanical Properties of Thermoelectric Lead Chalcogenides

“…21 Gelbstein et al found that n-type PbTe is much stronger in its bending strength and more ductile than p-type PbTe. 22 They also discussed the microhardness enhancement of p-type PbTe alloys with hole concentrations higher than 5×10 18 cm -3 , 23 which agrees well with Zlatanov's experimental observation. 24 Cui et al prepared the binary alloys (PbTe) 1-x (SnTe) x over the whole composition range and found that alloys with mole fraction x ranging from 0.2 to 0.6 have the highest compression strengths (310 -330 MPa) and hardness (HV95).…”

Hardening of (Pb,Cd)Te Crystal Lattice with an Increasing CdTe Content in the Solid Solution

“…The first papers dedicated to the hardness determination for this compound appeared almost fifty years ago [1,2]. In single n-type PbTe bulk crystal grown by the Bridgman method the microhardness is almost a constant value of H ≈ 300 MPa for various electron concentration whereas in p-type material this value strongly increases with the hole concentration and can be as high as H ≈ 700 MPa [3,4]. Due to the relatively low PbTe microhardness value a layer of this compound is so soft that it can be scratched easily.…”

Inelastic X-Ray Scattering Studies of Phonon Dispersion in PbTe and (Pb,Cd)Te Solid Solution

“…On the other (1251) hand, a modification of the heat transport in this material by high doping or mixing/alloying with a variety of compounds seems to be a hot topic for the last few years. The search for mechanically stable PbTe-based thermoelectric materials by the crystal doping with Sn, I, or Na [4], improved mechanical properties of PbTe containing Ge [5], Na [6] and Ca [7] are just a few examples of papers related to the problem mentioned above. In particular, the properties of (Pb,Cd)Te solid solution formed by PbTe alloying with CdTe -well-known and widely investigated compound of II-VI type -were the subject of several reports in the past.…”