Citrate gel process and thermoelectric properties of Ge-doped In2O3 bulk ceramics

Indium oxide ceramics doped with different doping levels of germanium (from 0 to 10 at.%) were prepared with nano-sized powders obtained by citrate gel process. Ge for In substitution in the In2O3 bixbyite structure below the solubility limit (about 0.5–1 atom%) leads to a large decrease in the electrical resistivity and the absolute value of the Seebeck coefficient. X-ray diffraction and scanning electron microscopy show the presence after sintering of well dispersed secondary phases In2Ge2O7 when the Ge solubility in the In2O3 matrix is passed. The thermal properties measurements confirm that the decrease in the lattice thermal conductivity observed in these composite materials can be attributed to the presence of insulating In2Ge2O7 phases. Furthermore, preliminary microwave sintering experiments have been tested in order to keep the nanoscale after sintering. Particular nano–microstructures were obtained due to minimal grain growth induced by this rapid sintering process.