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Abstract
The effects of different dopants (Hf, Ga, Lu and Lu/Sn) on the thermoelectric performance of In2O3 have been investigated. These materials were synthesized by using a co-precipitation method followed by spark plasma sintering (SPS) process or conventional sintering method. The total thermal conductivity consists of two parts: lattice thermal conductivity and electronic thermal conductivity. Lattice thermal conductivity was found to dominate thermal conductivity in these systems; however when the magnitude of electronic thermal conductivity increase exceeded some threshold, point defect scattering would be suppressed and thermal conductivity would be increased, which were found in Hf doped In2O3 and Lu/Sn co-doped In2O3. In the former system, the highest zT value was obtained in x=0.002 Hf doped In2O3 at 973K (~0.2), almost 2 times higher than that of In2O3. ZT values of this sample from different synthesis methods indicated density would not change the thermoelectric performance of this compound. In Lu/Sn co-doped system, thermoelectric performance of In2O3 could be improved by manipulating the ratio of Sn/Lu. However, co-doping Lu/Sn did not have an advantage in improving the thermoelectric property of In2O3 compared with single Sn doping in this study. In the isovalent Ga and Lu doping systems, opposite change of thermoelectric performance was observed; namely that Ga doping slightly increased zT of In2O3 especially in the high temperatures (T>850K) while Lu largely decreased thermoelectric performance of In2O3. This was attributed to charge carrier concentration increasing in Ga doped In2O3 while dropping in Lu doped system. The Doping-Induced Strain Model was established to explain the phenomenon. The relation between charge carrier concentration and crystal structure change has been roughly quantified, indicating dopant with smaller radius than that of In would increase the charge carrier concentration while the one with larger radius would decrease it. Moreover the secondary phases in Ga doped In2O3 (GaInO3) and Hf doped In2O3 decreased thermal conductivity and improved thermoelectric property.
Another In-based oxide In5SnSbO12 was then investigated in this dissertation. The thermal conductivity of pristine In5SnSbO12 was much lower than that of In2O3 due to its more complex crystal structure. The zT of this compound was comparable to that of In2O3, indicating In5SnSbO12 was a promising thermoelectric material. Ga doping in In5SnSbO12 resulted in the formation of secondary phase (Ga2In6Sn2O16), which significantly increased Hall mobility and helped improve the relative density of this compound from 60% to 90%.