Elastic modulus and biaxial fracture strength of thermally fatigued hot pressed LAST and LASTT thermoelectric materials

An application area for thermoelectric materials is the harvesting of waste heat from heat sources. Such applications can lead to thermal fatigue of the thermoelectric materials due to the thermal transients that arise during the heat-up and cool-down of the heat source. Thermal fatigue can in turn result in the accumulation of macrocrack and/or microcrack damage in the thermoelectric elements. For this study, two PbTe based thermoelectric materials, n-type LAST (Lead-Antimony-Silver-Tellurium) and p-type LASTT (Lead-Antimony-Silver-Tellurium-Tin) were thermally fatigued for up to 200 thermal cycles between 50°C and 400°C. All LAST specimens had surface inclusion that averaged 50--70 microns in diameter and spalled during thermal cycling; also 21 of 90 LASTT specimens had blisters on average of 7.5 mm in diameter and 2.5 mm in height as a result of thermal cycling. LAST and LASTT specimens had a relatively constant Young's modulus and Poisson's ratio, with a Young's modulus decrease no greater than 1 GPa and the largest decrease in the Poisson's ratio of 2.2% over 200 thermal cycles. A LASTT fracture strength band between 25 MPa and 40 MPa occurred for specimens fatigued up to 200 thermal cycles, where a band of fracture strengths between 15 MPa and 38 MPa occurred for LAST specimens. The LAST and LASTT specimens appeared to be unaffected by microcracking during the thermal fatigue cycling.