Effect of Doping and Defect Structures on Thermo Physical Properties of Thermoelectric Materials
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Development of thermoelectric materials to date has focused on materials that can operate at lower temperatures. However; there is now an increased need to develop materials for higher temperature applications. In this research, medium to high temperature oxide and non-oxide thermoelectric materials were fabricated and characterized. For oxide thermoelectric materials, La4Ti4O14 and Sr4Nb4O14 were chosen. These compounds are members of the homologous A4B4O14 series and possess perovskite-like layered structure (PLS). PLS compounds have low thermal conductivity due to a layered structure compared to the perovskite materials (e.g. SrTiO3). These atomic scale layers help to reduce the thermal conductivity of PLS compounds. Doping in PLS materials also creates atomic scale disorders. The effect of acceptor-donor doping and oxidation-reduction on the thermal conductivity of PLS ceramics were investigated in relation to mass contrast and compositional non-stoichiometry. High resolution TEM and XPS revealed that acceptor doping of La4Ti4O14 produced nanoscale intergrowth regions of n=5 layered phase inside n=4 layered phase, while donor doping produced nanoscale intergrowth regions of n=3 layered structure. As a result of these nanoscale intergrowths, the thermal conductivity value reduced by ~ 20% compared to the theoretical value. Pure La4Ti4O14 has a thermal conductivity value of ~ 1.1 W/m.K which dropped to a value of ~ 0.98 W/m.K in Sr doped La4Ti4O14 and ~ 0.93 W/m.K in Ta doped La4Ti4O14. Pure Sr4Nb4O14 has a thermal conductivity value of ~ 1.05 W/m.K which dropped to ~ 0.6 W/m.K after La doping. The factors influencing the thermal conductivity of PLS compounds were also discussed.For non-oxide ceramics, CoSb3 was chosen due to its cage-like structure and ideal for the application of Phonon Glass Electron Crystal Concept. The cage like structure gives room to engineer its electrical and thermal properties without affecting the other. For the first time, CoSb3 stuffed with Yb and substituted with Te (YbyCoSb3-xTex) was synthesized by mechanical alloying and spark plasma sintering. The electrical and thermal properties were characterized for pure and doped material. A Seebeck coefficient value of ~ 160 μV/K was obtained at ~ 600-800 K for Yb0.075CoSb2.85Te0.15. The electric resistivity dropped from ~ 1000 μΩm for pure CoSb3 to ~ 9 μΩm for Yb0.075CoSb2.85Te0.15. Lattice thermal conductivity was significantly reduced to a very low value of 1.17 W/m.K by the addition of Yb atoms into CoSb2.85Te0.15 without significantly affecting its Seebeck coefficient and electrical resistivity. This value is comparable to those produced by the costly processing of nanostructured materials. A zT value of ~ 0.70 was obtained at 600 K. This research has shown that by engineering the defect chemistry of thermoelectric materials, it is possible to significantly reduce their thermal conductivity without compromising their electrical properties.
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