Improvement of Thermoelectric Performance of Intermetallic Compound Phases by Reduction of Thermal Conductivity through Phonon Scattering

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We have focused on half-Heusler MNiSn (M=Hf, Zr) as potential candidates for n-type thermoelectric materials used at high temperatures around 1000 K. It is possible to achieve large Seebeck coefficient and low electrical resistivity in single-phase alloys fabricated by directional solidification using the optical floating zone melting method (OFZ-DS). To further improve thermoelectric properties through lowering the lattice thermal conductivity, we proposed a new fabrication process combining OFZ-DS method with hot-pressing (HP) aiming to produce composite microstructures. It is advantageous to introduce interfaces for reducing the lattice thermal cnductivity by phonon scattering. Monolithic half-Heusler alloy powders were prepared from OFZ-DS ingots, and sintered by HP under various conditions. In HfNiSn/ZrNiSn composite, continuous solid solution (Hf, Zr) NiSn phase plays a role to reduce the lattice thermal conductivity by the solid solution effect. A composite of fine ZrO_2 particl … More es homogeneously dispersed in the HfNiSn matrix was also prepared aiming to reduce the lattice thermal conductivity. We anticipated that electrical properties would be sacrificed by introduced interfaces, though, excellent power factors can be achieved. Thermal diffusivity can be apparently reduced by introducing interfaces while heat capacity is affected by interfaces and impurity phases formed during HP process. It is hard to suppress the formation of impurity oxide phases during HP process. Considering the oxide formation in the sintering process, HfNiSn/ZrNiSn composite was sintered under the Ar-4%H_2 deoxidizing atmosphere using compact powders without applying load. We found that deoxidizing atmosphere is quite effective to suppress the oxide formation and that thermal conductivity can be lowered by controlling density, i.e. porosity, of sintered specimens. It indicates that the optimization of density by controlling the applied load is effective to achieve the best balance of low thermal conductivity and excellent electrical properties, which leads us to maximize thermoelectric performance of half-Heusler based thermoelectric materials. Less