Formation of Magneli Phase and Its Influence on Thermoelectric Properties in Preparation of TiO₂-TiB₂ Thermoelectric Compacts Fabricated by Spark Plasma Sintering

Abstract

Powders of TiO₂ mixed with TiB₂ of mmol% (m = 0, 1, 2, 3, 4, 5, 10, 12, 20) were prepared, and sintered by using an spark plasma sintering apparatus, the chemical reaction between TiO₂ and TiB₂ in the sintering process was investigated. In the case of m = 2 to 20, it was found TiO₂ was reduced by TiB₂, to form and the Magneli phases (Ti_nO_2n−1 (n = 3, 4, 5, 6)). The maximum value of the dimensionless figure of merit ZT was 0.18 at 800°C for m = 3, and almost the same value as that of the TiO₂-TiN sintered bodies was obtained. The effective carrier concentrations in the TiO₂-TiB₂ sintered bodies were estimated from Heikes’s formula was about 2.4 times larger than those of the TiO₂-TiN, TiO₂-TiC, TiO₂-VC sintered bodies. The electron mobility μ tends to increase as m increases. The thermal conductivity was dominated by the lattice thermal conductivity at m = 0, while the electronic thermal conductivity was suggested at m = 4 and 5. It was confirmed that the formation of the magneli phase contributes to the improvement of ZT.