ICT2014 Abstracts

For the thermoelectric materials design, not only high performance but also high durability and reliability are important factors especially for high temperature applications. Eco-friendly β-FeSi₂ is a thermoelectric material which can be used stably at high temperatures around 1073 K. Fabrication process of β-FeSi₂ has always been a problem to be overcome since β-FeSi₂ is formed by the peritoctoid reaction between α-FeSi₂ and ε-FeSi, but formed directly from the liquid phase. The formation of β-FeSi₂ requires very long term heat treatments depending on solidification microstructure of α and ε phases. We have developed powder metallurgy methods in two routes using microstructure control based on phase equilibria. One is aiming at nearly single-phase β-FeSi₂ alloys using rapid solidification methods. The other is focusing on new composite type thermoelectric alloys consisting of β-FeSi₂ and SiO₂, for which the eutectoid decomposition of α into β-FeSi₂ and Si was combined with the redox reaction involving iron oxides addition. Phase constitution of β-FeSi₂-base thermoelectric alloys was identified by X-ray diffractometry, and microstructure was observed and analyzed in detail by scanning transmission electron microscopy. Thermoelectric properties of nearly single-phase β-FeSi₂ alloys and β-FeSi₂/SiO₂ composite alloys were measured and compared in a temperature range from 300 K to 1073 K. The additions of Co and Mn were conducted as n-type and p-type doping, respectively, to the intrinsic semiconductor β-FeSi. Nearly single-phase alloys show excellent electrical power factor with a good balance between Seebeck coefficient and electrical conductivity, while electrical power factor is obviously lowered in composite alloys. Nevertheless, the dimensionless figure of merit ZT of composite alloys is comparable to that of nearly single-phase alloys since the lattice thermal conductivity can be effectively reduced by dispersed SiO₂ particles and introduced interfaces in composite alloys.