Mechanical properties and thermal stability of nanostructured thermoelectric materials on the basis of PbTe and GeTe

Bulk nanostructured n-type PbTe (0.2 wt% PbI2; 0.3 wt% Ni) and p-type Ge0.96Bi0.04Te were prepared by grinding the synthesized materials in a planetary ball mill followed by compaction by spark plasma sintering. Hardness, Young's modulus and stiffness of the synthesized and nanostructured thermoelectric materials were investigated by nanoindentation. Nanostructured thermoelectric materials have significantly higher mechanical properties than synthesized materials, and grinding time have small effect on the hardness, Young's modulus and stiffness. The efficiency of nanostructured thermoelectric materials in the entire range of operating temperatures is 10–14% higher than that of the materials obtained by hot pressing. The maximum values of dimensionless thermoelectric figure of merit for nanostructured thermoelectric materials on the basis of PbTe and GeTe are equal to 1.34 and 1.43, respectively. The increase in the efficiency is explained by the scattering of phonons by inhomogeneities of the nanocrystalline structure. Thermal treatment at a temperature of 750 K for 72 h reduced dimensionless thermoelectric figure of merit by not more than 2% and 3% for GeTe and PbTe nanostructured thermoelectric materials, respectively. Repeated differential scanning calorimetry showed that nanostructured thermoelectric materials are thermally stable up to 850 K. The grinding time does not affect the thermal stability of materials. According to thermogravimetry sublimation of thermoelectric materials begins above 850 K. To ensure long-term use of thermoelectric materials on the basis of PbTe and GeTe in the temperature range of 850–900 K it is necessary to use protective Si3N4 or SiO2 coatings.