Reactive Templated Grain Growth and Thermoelectric Power Factor Enhancement of Textured CuFeO₂ Ceramics

The delafossite-type CuFeO₂ (CFO) is a promising p-type thermoelectric oxide reported to show a large Seebeck coefficient, but the lack of a texturing method for CFO ceramics limits their thermoelectric performance. Here, we show that textured CFO ceramics with an enhanced thermoelectric power factor can be obtained by a facile process based on the reactive templated grain growth (RTGG) method. Nanorods of α-FeOOH were synthesized by a hydrothermal reaction and used as templates for the RTGG process. Textured CFO ceramics with a high c-axis orientation were obtained by ordinal uniaxial pressing of a mixed powder of the α-FeOOH nanorods and Cu₂O particles and subsequent reaction sintering process. The best CFO ceramic with the highest orientation degree (Lotgering factor) of 0.71 was obtained at a sintering temperature of 1020 °C. It was found that textured CFO ceramics were formed through a topotactic conversion process via an intermediate α-Fe₂O₃ phase. The textured CFO ceramic showed a high in-plane electrical conductivity, leading to an enhanced thermoelectric power factor of 1.4 × 10^-4 W m^-1 K^-2 at 700 K, which is the highest value ever reported for undoped CFO ceramics.