In-situ observation of temperature dependent microstructural changes in HPT-produced p-type skutterudites

To build thermoelectric generators, leg materials with a high figure of merit, ZT, are essential. Skutterudites are promising candidates because besides being environmentally friendly, the starting material is available and cheap and they can be used in a wide temperature range. To enhance ZT, severe plastic deformation via high-pressure torsion, HPT, was successfully applied on ball-milled and hot-pressed skutterudites as well as to directly densify skutterudite powder. Severe plastic deformation introduces many defects, mainly dislocations, into the sample and in parallel the crystallite size is significantly reduced. During measurement-induced heating these defects anneal partially out, and the grains grow. It was observed that while heating HPT processed material from room temperature to about 850 K, changes of the temperature-dependent physical properties, most of all the electrical resistivity, the density, and the thermal expansion occur more or less simultaneously around 600 K. For the first time we have combined in situ TEM observations as well as in situ measurements of the elastic modulus and hardness in order to get a deeper insight into the microstructural behavior of a p-type skutterudite, DD0.7Fe3CoSb12 (DD = didymium) during increasing temperature from 300 K to 823 K. HPT-DD0.7Fe3CoSb12 is a high quality thermoelectric material with 1.2 < ZT < 1.4 at 750 K (hot-pressed reference sample: ZT similar to 1.2 at 810 K).