Associate Professor, Center for Atomic and Molecular Technologies, Graduate School of Engineering, Osaka University, Japan

Biography: Mikio Ito is an associate professor in the Center for Atomic and Molecular Technologies, Graduate School of Engineering, Osaka University, Japan. He obtained his master’s (1994) and Doctor’s degrees (1997) in Engineering from Osaka University. His research focuses on development of materials processing, mainly powder processing, for functional materials with excellent performances, such as hard magnetic materials, thermoelectric materials etc. So far, he has produced nearly 140 publications. His resent research also focuses on SPS processing, and he is trying to clarify the effects of directly applied current sintering using SPS on densification behaviors of metal and ceramic powders.

Speech Title: Thermoelectric performance of heterogeneous β-FeSi₂/Si composites synthesized by eutectoid decomposition

Aims: Thermoelectric (TE) is a way of direct and reversible conversion between heat and electricity. Semiconducting β-FeSi₂ is one of the promising TE materials due to its relatively large Seebeck Coefficient, high oxidation resistance, abundant source elements on Earth and low cost. But it possesses large electrical resistivity and high thermal conductivity, which need to be reduced. In this study, to decrease thermal conductivity, distribution of fine Si particles within n type β-FeSi2 matrix was tried through eutectoid decomposition from the α phase, and control of Si sizes towards enhancing its TE properties.

Methods: Mixtures of Fe, Co and Si powders were milled at 300 rpm, for 5 h under Ar gas atmosphere. The mixtures were sintered at 1000℃ for 10 min under 50 MPa. They were annealed at 800℃ and 700℃ for 4h each under vacuum condition. Phase and microstructure were analyzed by XRD and SEM followed by the TE properties measurement.

Results: The microstructure analysis of annealed β-FeSi₂/Si composites showed that Si particles with several hundred microns in size were uniformly dispersed in the Co-doped n-type β-FeSi₂ matrix. It was found that the lower annealing temperature caused finer dispersion of Si. The thermal conductivity of sample annealed at 700℃ decreased compared to the sample annealed at 800℃, indicating that phonon scattering can limit thermal conduction. The values of thermal conductivity were lower than calculated values from the rule of mixture from Maxwell-Eucken equation, which justifies fine dispersion of Si effectively suppressed thermal conduction in the sample.

Conclusions: The n type Co doped β-FeSi₂/Si composites were synthesized through eutectoid decomposition from the α phase. Lower thermal conductivity was obtained from finer Si dispersion, leading to a promising development of TE properties.