Thermoelectric Performance Enhancement of Magnéli Phase TinO2n−1 Compacts by In Situ Reduction of TiO2 with Charcoal Powder via Spark Plasma Sintering.

Magnéli phase Ti_nO_2n−1 compacts were prepared by in situ reduction of TiO₂ with charcoal powder via spark plasma sintering, and their thermoelectric performance was investigated. All the prepared compacts showed a significant decrease in electrical resistivity due to the enhanced carrier concentration originating from the introduced oxygen vacancy defects. The microstructures of the compacts were characterized by scanning electron microscopy, which revealed that grain refinement due to the dry-mixing method promoted the reduction of TiO₂ and formed a greater number of oxygen vacancy defects, leading to a further decrease in electrical resistivity of the MA30-5-50 MPa-10 min compact. Moreover, while the absolute Seebeck coefficient decreased as the number of oxygen vacancy defects increased, the power factor was steadily enhanced by a further decrease in the electrical resistivity at elevated temperatures. The x-ray diffraction results suggest that the n index of the Magnéli phase Ti_nO_2n−1 mainly depended on the TiO₂ reduction and the sintering time, and the formed Magnéli phase Ti₆O₁₁ and Ti₇O₁₃ contributed significantly to a higher power factor. The highest power factor of 7.31 × 10^–4 W m⁻¹ K⁻² was achieved in the C30-5-50 MPa-60 min compact at 973 K, and the highest ZT value reached 0.32 at 973 K, indicating an excellent n-type thermoelectric performance. [ABSTRACT FROM AUTHOR]