Sol-gel synthesis and characterisation of nanostructured LaNiO_3-x for thermoelectric applications

Nanostructured LaNiO_3-x (x>0) was prepared at low temperature between 500-800 °C by the sol-gel method. Their potentials as a thermoelectric material were investigated. The minimum temperature to form the LaNiO_3-x phase in air was 600 °C. The samples sintered between 600 and 800 °C had a grain size of 18-31 nm and showed a metallic behaviour with the linear temperature dependence of electrical resistivity and Seebeck coefficient. The electrical conductivity at room temperature varied from 581 to 810 S/cm and the thermal conductivity from 0.580 to 0.886 Wm^-1 K^-1. Negative Seebeck coefficient was observed for all the samples, indicating that the likely charge carrier was electron, which was confirmed by the Hall-effect measurements. The absolute value of the Seebeck coefficient was 12-15 μV/K at 300 K and increased approximately linearly with temperature in the measured range up to 550 K. The attainable ZT so far was 0.034 at 550 K and would rise to 0.15 at 1000 K, which was comparable to the values of other promising n-type oxide thermoelectrics currently under development, such as Nb/La-doped SrTiO₃ and Al-doped ZnO.