Thermoelectric material is a kind of functional materials that can directly convert heat energy to electricity. In order to compete with traditional techniques for power generation and refrigenation, the method to improve the thermoelectric efficiency for power generation and refrigenation is a key subject in a long period of time. It is of importance to confirm the validity of improving the performance of thermoelectric materials by changing their macrostructure, and put forward feasible approach for enhancing the thermoelectric properties of foamed materials. In our previous work, it was found that the thermoelectric performance of SiC foam is much better than that of SiC bulk, which indicated that changing the macrostructure of SiC may be a new effective way to improve the properties of thermoelectric materials. In this dissertation, the influences of the macrostructure, strut layer structures and pore size of SiC foam on the thermoelectric performance, which were fabricated by controlled melt-infiltration reaction sintering, were investigated. The main results obtained are as follows:
The influence of the macrostructure on the thermoelectric property of SiC foam was studied. The results show that: (1) SiC foam exhibits a much greater Seebeck coefficient than SiC bulk. (2) When the macrostructure of SiC ceramics changes from bulk to foam, the thermal conductivity decreases more deeply than the electrical conductivity does, which results in an improvement of the thermoelectric performance of SiC foam. (3) Compared with 5wt% Si addition to SiC bulk, SiC foam has a larger figure of merit, which confirms the improvement of the thermoelectric property of SiC ceramics by the foam structure. Therefore, changing the macrostructure for enhancing the performance of thermoelectric materials is a feasible way.
The influence of the strut layer structures on the thermoelectric properties of SiC foam was discussed. The results show that: (1) the Seebeck coefficient and the electrical conductivity increase with the numbers of the strut layers increase. (2) The power factor of SiC foam with a three-layer strut structure (Si layer/SiC layer/Si core) is the largest. It is advantageous for improving the thermoelectric performance by adding the strut layers in SiC foam.
The effects of Si content on the thermoelectric properties of SiC foam were investigated. The results show that: (1) At 250℃, the Seebeck coefficient of SiC foam increases as Si content increases. (2) The electrical resistivity of SiC foam decreases drastically as Si content increases. In the whole temperature range, the power factor and the thermoelectric performance of SiC foam with 60% Si addition is the best.
The effects of pore size factor on the thermoelectric properties of SiC foam were studied. The results show that: (1) When the volume fraction of SiC foam increases from 30% to 50%, the seebeck coefficient and the electrical resistivity decrease, while the themal conductivity increases. However, the influence of volume fraction on the thermoelectric figure of merit of SiC foam is not strong when the seebeck coefficient, the electrical resistivity and the themal conductivity were considered together at lower temperature. When the temperature increases over 450℃, the figure of merit of SiC foam ceramics with the volume fraction of 30% is the largest, which shows the best thermoelectric property. (2) As the pore diameter decreases, the Seebeck coefficient, the electrical conductivity and the thermal conductivity all increase. When the temperature increases over 450℃, the thermoelectric figure of merit of SiC foam with 0.1mm pore diameter is the largest, which results in the best thermoelectric performance.
修改评论