Oxide Thermoelectric Energy Harvesting Materials

Conventional thermoelectric materials found in many thermoelectric devices have unfavourable properties; they often suffer instability at high temperatures and contain toxic metals which pose a hazard to the environment. Oxide thermoelectric materials are stable, less toxic and could eventually replace conventional materials. The thermoelectric performance of oxide materials currently do not match conventional materials however, there is potential for improvement through doping and altering the microstructure and chemistry through modification of the processing conditions. This project aims to examine the doping and processing conditions and the effect this has upon the thermoelectric behaviour of oxide based thermoelectric materials. Zinc oxide (ZnO) has been investigated as an oxide thermoelectric material and doping of ZnO with aluminium (Al) and antimony (Sb) by mixed oxide synthesis was investigated. Al2O3 and Sb2O3 were used as aluminium (Al) and antimony (Sb) dopant sources for ZnO, which were reacted with ZnO at temperatures of 1000°C-1300°C. Al was found to incorporate effectively into the ZnO system and was shown to produce n-type behaviour. The Sb doped ZnO material was also found to display n-type behaviour which is intriguing as Sb is considered a p-type dopant in the ZnO system; at low levels <1.0at.%, Sb incorporates onto the Zn site rather than the O site as expected, which leads to n-type behaviour. The addition of Sb dopant leads to the formation of secondary phase of Zn7Sb2O12, which appears to increase the Seebeck coefficient by an energy filtering effect with higher levels of dopant leading to higher levels of secondary phase. Grain size and porosity also play a significant role in both the Al and Sb doped systems with small grains and higher levels of porosity leading to higher values of Seebeck coefficient up to -100µV.K-1 for Al (0.5at.%) and - 115µV.K-1 for Sb (0.8at.%). The ZT figure of merits were found to be highest for materials sintered at 1300°C with values of 6×10-5 and 2×10-10 for Al and Sb doped ZnO respectively, these values are low compared to literature values, which are in the region of 0.01. This is due to high electrical resistivities of the synthesised samples, which is linked to porosity. A better understanding of the effects that microstructure plays on thermoelectric behaviour has been developed and procedures to isolate the contributions from grain size, and degree of dopant incorporation to the thermoelectric properties have been conducted.