| Summary: | 碩士 === 國立成功大學 === 機械工程學系碩博士班 === 93 === Traditionally, thermoelectric devices used the bulk materials as the core of conversion, and the thermoelectric conversion efficiency is only 8.5%. Therefore, the efficiency becomes one of the very critical issues for thermoelectric system development. Nowadays, one can control the materials properties such as thermal conductivity, electric conductivity etc… by adjusting the structure parameters in microcosmic. Thus, the thermoelectric conversion efficiency of the system can be expected to improve about 20%. With the advance in manufacture technology for nano-superlattice the upper limit of particular composites can be expected to raise. The analysis of global effect on the thermoelectric conversion efficiency of nano-superlattice can also be estimated. This research intends to construct an intact theory and computer simulation method in an attempt to probe into the subject of enhancing the thermoelectric conversion efficiency based on the development of a nanotechnology-based system.
In order to improve the figure of merit (ZT) of thermoelectric materials, one can try to reduce the thermal conductivity or to increase the electrical conductivity. In this study, the molecular dynamics and Green Kubo formula were used to calculate the thermal conductivity of the superlattice. In an attempt to obtain the ZT, electrical conductivity of the structure was estimated. Based on the quantum effect in nano-scale, the energy barrier function of the interface was defined. The carrier transmission probability through the ladder-barrier was estimated. The electrical conductivity of the structure was then derived by using the Landauer theory. The results obtained by using the proposed method were found agree well with the experimental data in literature. Therefore, by reducing the thermal conductivity and increasing the electrical conductivity via adjusting structure parameters (such as lattice period length, mass ratio, interface roughness), the thermoelectric conversion efficiency can thus be improved. According to the weight for the effects of structure parameters on ZT, the proposed method was employed to design for the waste-heat recovery of the hybrid car and microcooler. The thermoelectric character analysis proved that the design procedure based on the proposed theory presented in this study is valuable.
By adding the roughness of interface in the thermal conduction model, the thermal conductivity thus obtained was found closer to actual values, and thus the simulation results were more accurate. Moreover, this study employed the MD (molecular dynamics) simulation to obtain the thermal conductivity and other physics quantities, and confirmed that the figure of merit of thermoelectric materials can be actually improved by reducing the thermal conductivity and increasing the electrical conductivity. The design and estimation for the thermoelectric materials can be achieved faster and more precisely by used the implemented theory and computerized simulation package. In this study the ladder-barrier function, that is different from the traditional rectangle-barrier one, was employed to give a more accurate representation for different atoms in superlattice. Finally, it was showed that the proposed theory and computerized simulation procedure are feasible through various case studies, such as the waste-heat recovery of the hybrid car and microcooler thermoelectric character analysis.
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