Impact of high pressure torsion on the microstructure and physical properties of Pr 0.67Fe 3CoSb 12, Pr 0.71Fe 3.5Ni 0.5Sb 12, and Ba 0.06Co 4Sb 12

Abstract

Both p- and n-type skutterudites (Pr 0.67Fe 3CoSb 12, Pr 0.71Fe 3.5Ni 0.5Sb 12 and Ba 0.06Co 4Sb 12) have been deformed by high pressure torsion (HPT) with 2 GPa resulting in a lamellar shaped nanograined structure. The crystallite size distribution and dislocation density are evaluated using X-ray powder diffraction data, revealing a crystallite size of 47 nm and a dislocation density of 7 × 10 14 m −2 for Ba 0.06Co 4Sb 12. Whilst at T < 5.6 K the electrical resistivities of HPT processed Pr 0.67Fe 3CoSb 12 and Pr 0.71Fe 3.5Ni 0.5Sb 12 do not indicate long-range magnetic order, the temperature dependent susceptibility elucidates antiferromagnetic ordering after HPT although the anomaly at the phase transition becomes washed out. The effective magnetic moments are 4.18 μ B and 4.07 μ B for Pr 0.67Fe 3CoSb 12 before and after HPT, revealing a non-zero effective moment on the Fe 3CoSb 12 framework. Metamagnetic transitions at μ 0H = 0.9 (before HPT) and 0.8 (after HPT) are clearly seen in isothermal magnetization curves. In comparison with the microstructures of milled and hot pressed samples, those after HPT exhibit markedly smaller although lamellar grains, and also amorphous aggregates. The thermal conductivity of HPT samples is smaller, but the electrical resistivity is markedly higher than that of milled material, which in sum results in a lower figure of merit ZT. The increase of resistivity is caused by the high density of microcracks observed in the HPT samples, which may be avoided by suitable modification of the HPT processing parameters.