Fabrication and performance of P-type ceramic based materials

Abstract

The emerging global energy shortage has intensified the need for clean energy such as biomass, wind power, and solar power. Among the renewable energy focused, thermoelectric materials play an important role which can perform a reversible transformation between heat and electricity directly, which makes high-efficient power regeneration application possible. Excellent thermoelectric performance not only requires high Seebeck value, low electrical resistivity, and low thermal conductivity simultaneously, but also but also should be consisted nontoxic, and abundantly available from nature, with high chemical stability in a wide temperature range, even in the temperature of 800-1200 K. Although the highest figure of merit has been obtained in germanium, tellurium, and antimony based alloy or compound, the instability and toxicity in high-temperature range are the main restrictions for the application. Thus, compared with those compounds mentioned above, oxide materials expand the potential for high-temperature power application due to its superior stability. In this thesis, the thermoelectric properties of complex oxide materials were investigated. Thermoelectric oxide samples Ca3Co4O9+δ and TiO1.56-1.61 were studied and different synthetic methods such as doping, and grain size reducing were used for the thermoelectric enhancement. For Ca3Co4O9+δ, La was selected as the dopant, with the intention of optimizing the thermoelectric performance via doping concentration controlling. Raman spectroscopy was used to study the doping position and the structuralchange in both Co-O layer and rock salt type [Ca2CoO3] layer. The La-doped Ca3Co4O9+δ was further ball milled, for investigating the grain size dependence on the figure of merit value ZT. For the titanium oxide, the high energy ball milling method was used to study the influence of precursor powder size on phase formation and thermoelectric performance. The phase characterization was carried out by X-ray diffraction, and Rietveld analysis was used to investigate the variation of lattice parameters. The purpose of this study was to establish the correlation between doping, grain size reduction, and phase composition changes, and finally acquire a better thermoelectric performance and good understanding on the mechanism.