| Summary: | 博士 === 國立清華大學 === 材料科學工程學系 === 106 === Thermoelectric materials that enable the direct conversion of thermal energy into electricity are promising for applications of waste heat harvesting and vibrationless cooling. However, poor conversion efficiency and high manufacturing costs appear to be major limiting factors for large-scale thermoelectric applications. In recent years, some intriguing breakthroughs in thermoelectric development have been reported with the new transport theories proposed and the advancement of material processing techniques. These groundbreaking works suggest that microstructrual manipulation is effective in modulating transport properties of thermoelectric materials. Among various processing techniques, current-assisted sintering is commonly employed to prepare thermoelectrics. This study intends to investigate the effect of high-density electric current on the evolution of microstructure and transport properties of thermoelectric material. A current-assisted hot-press system is designed and used to prepare hot-pressed and electrically-sintered Bi0.4Sb1.6Te3 samples. The samples sintered under low direct-current density exhibit enhanced carrier mobility and maintain low lattice thermal conductivity. According to Mayadas-Shatzkes model, the boundary in electrically-sintered sample reveals a smaller reflection coefficient, indicating that the boundary is beneficial for carrier transport with less hindering. The retained low lattice thermal conductivity suggests that the boundary prepared remains effective in blocking phonon transport in the electrically-sintered samples. Alternatively, the samples sintered with high pulse-current density also possess enhanced mobility and low lattice thermal conductivity. However, they reveal obvious grain growth and nanoscale Sb precipitates at grain boundaries. According to the modified Callaway model, these Sb precipitates tend to scatter phonons preferentially and lead to reduction in lattice thermal conductivity. Therefore, low lattice thermal conductivity is remained when electrical conductivity is increased due to grain growth. In conclusion, a high-density electric current indeed modulates the boundary structure and motivate Sb nano-precipitation in Bi0.4Sb1.6Te3 compounds, leading to increased electrical conductivity and reduced lattice thermal conductivity simultaneously.
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