The thermal expansion coefficient as a key design parameter for thermoelectric materials and its relationship to processing-dependent bloating

The coefficient of thermal expansion (CTE) is a key design parameter for thermoelectric (TE) materials, especially in energy harvesting applications since stresses generated by CTE mismatch, thermal gradients, and thermal transients scale with the CTE of the TE material. For the PbTe-PbS-based TE material (Pb_0.95Sn_0.05Te)_0.92(PbS) _0.08 - 0.055 % PbI₂ over the temperature ranges of 293-543 and 293-773 K, a CTE, α_avg, of 21.4 ± 0.3 × 10^-6 K^-1 was measured using (1) dilatometry and (2) high-temperature X-ray diffraction (HT-XRD) for powder and bulk specimens. The CTE values measured via dilatometry and HT-XRD are similar to the literature values for other Pb-based chalcogenides. However, the processing technique was found to impact the thermal expansion such that bloating (which leads to a hysteresis in thermal expansion) occurred for hot pressed billets heated to temperatures >603 K while specimens fabricated by pulsed electric current sintering and as-cast specimens did not show a bloating-modified thermal expansion even for temperatures up to 663 K. The relationship of bloating to the processing techniques is discussed, along with a possible mechanism for inhibiting bloating in powder processed specimens.