Compressive creep behavior of hot-pressed GeTe based TAGS-85 and effect of creep on thermoelectric properties

Thermoelectric materials often operate at high homologous temperatures where they can be subjected to sizeable internal and external stresses, making them prone to creep deformation over long periods of use. Hot-pressed (GeTe)(85)(AgSbTe2)(15) (TAGS-85) exhibits a fine grain size averaging similar to 8 mu m - of concern for creep resistance - with 70% of the grains below 10 mu m and 30% between 10 and 150 mu m; second phase Ag8GeTe6 particles, similar to 2 mu m in size, are also present at grain boundaries. Strain rates of TAGS-85 under a series of stresses are measured at 375-425 degrees C. By fitting the creep data to a power law creep equation, the stress exponent n and activation energy Q for creep are determined to be n = 2.6 +/- 0.1 (<12 MPa applied stress) or n = 3.7 +/- 0.4 (>12 MPa applied stress) and Q= 157 +/- 2 kj/mol, respectively. Thermoelectric measurements show 15% lower electrical conductivity and similar to 30% higher lattice thermal conductivity after creeping to 10% strain. The evolution of ambient-temperature electrical conductivity and thermal conductivity for a single sample crept at intervals of 2% strain is consistent with the expectation that dislocations accumulate during primary creep while annealing effects may reduce subgrain boundaries that scatter phonons. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.