MA法により作製したβ-FeSi₂の熱電特性

抄録

Mechanical Alloying (MA) in Fe-(Mn)-Si-(Al) system with methanol as an agent was performed by using a horizontal ball mill under Argon atmosphere. For the nominal composition of Fe₃₃Si₆₇ and Fe₃₀Si₇₀, ε-FeSi and β-FeSi₂ were formed after 1800ks of MA, respectively. The powder milled for 720ks was consolidated by hot pressing (1373K, 1.8ks, 28MPa) and followed by β annealing (1033K, 360ks). The volume fraction of ε phase formed by hot pressing the MA powder is higher than that expected from the equiliblium phase diagram. This is considered to be due to the small molar volume of ε phase compared to those of α and β phase. The temperature dependence of transformon rate from (α+ε) to β was measured using 720ks milled MA powder of Fe₃₀Si₇₀. Two maxima were observed at 1153K and 1033K. The figure of merit, Z=α²/(ρ⋅K), where α: seebeck coefficient, p : specific electrical resistivity, is : thermal conductivity, of MA sample is higher than that prepared from ingot metallurgy (IM). This improvement of Z is caused by the reduction of is ^L(lattice thermal conductivity). But, ρ is about twice of IM sample. The Al₂O₃ particles which may form during MA process by the reaction of Al with oxygen in methanol may raise the electrical resistance of MA sample. To improve the thermoelectric properties, it is important use an additive with high electrical conductivity which act to prevent grain growth during sintering.