The Effect of Nb Doping on the Thermoelectric Properties of Indium Oxide

To optimize the performance of indium oxide (In2O3)-based thermoelectric materials, this study prepared Nb-doped In2O3 via mechanical alloying and spark plasma sintering (SPS), investigating its regulatory mechanism on the crystal structure, as well as on thermoelectric and mechanical properties. X-ray diffraction (XRD) showed that Nb was incorporated into the In2O3 lattice; with increasing Nb doping, the lattice constant decreased due to the ionic radius difference between Nb (64 pm) and In3+ (80 pm) plus Nb’s strong polarization effect. In electrical properties, Nb doping significantly improved conductivity: pure In2O3 had ~53.42 S/cm, reaching 272.07 S/cm (over 5x increase) at x = 0.005, which is attributed to Nb releasing free electrons and increasing the carrier concentration; though carrier mobility slightly decreased, carrier concentration growth dominated conductivity improvement. The absolute Seebeck coefficient decreased (from −185.24 μV/K to −120.12 μV/K at x = 0.005), but electrical conductivity increased and far exceeded the decrease in the square of the Seebeck coefficient, leading to a high-temperature power factor of 5.10 μW/(cm·K2). In terms of thermal properties, Nb doping reduced thermal conductivity and lattice thermal conductivity. Collectively, the x = 0.004 sample achieved a ZT value of 0.302 at 800 K, which is over 5 times higher than that of pure In2O3 (0.055 at 973 K); meanwhile, Nb doping enhanced the Vickers hardness, realizing the optimization of thermoelectric and mechanical properties.