Study Of Hot Forging Process For Bi₂Te₃-based Thermoelectric Materials

Bismuth-telluride-based alloys are widely recognized as the best commercial thermoelectric (TE) materials used near room temperature. Commercial Bi2Te3-based ingots with high performance are generally fabricated by traditional unidirectional crystal-growth methods. However, these zone-melted ingots always show bad mechanical properties and poor machining performance, which greatly limit their device service. We have performed a comprehensive investigation on the hot deformation and achieved leading results to improve the thermoelectric properties of bismuth telluride based alloys. This thesis focuses on the research of hot forging process and discusses the effects of hot deformation process on the point defects and micro structures. Furthermore, the potential application of hot deformation process is explored.1. We fabricate p-type Bi0.3Sb1.7Te3 alloys by repetitive hot deformations and study the changes of point defects during hot forging. The power factor is improved after hot deformation because the recovery process weakens donor-like effect. Meanwhile, the grain size grows during repetitive hot deformations and consequently the lattice thermal conductivity increase. As a result, the zT values of p-type Bi0.3Sb1.7Te3 remain unchanged. We also investigate the stability of donor-like effect under annealing with different time. It is shown that the electrical properties of hot forging samples don’t change much after low temperature annealing.The deviation is no more than 5%.2. We also investigate the operating parameters of hot forging to control and tune the micro structures. It is found that the (00l) preferred orientation can be formed during the hot forging process. A semi-quantitative method is used to calculate the texture intensity. The calculated results demonstrate that the texture intensity is related with the hot forging process parameters, especially the deformation strain and pressure. The orientation factor F reaches its maximum value 0.39 when the ε=0.75 or the pressure is 60 MPa. The deformation strain and pressure have little influence on zT values of the alloys. The zT values of all the samples are near 0.9.3. We prepare large bulk Bi0.3Sb1.7Te3 alloys by powder metallurgy sintering and hot forging. The distribution rules of thermoelectric properties in the large bulk are studied. We also prepare a massive bulk bismuth telluride based alloy by hot forging, which weighs about 600 g. The property measurement shows that the thermoelectric properties of different parts from the massive bulk show small differences, which are less than 5% and the maximum value of zT reaches 1.2. In addition, it is also found that the Bio.3Sb1.7Te3 alloys prepared by hot forging show higher zT than commercial zone-melted materials. Based on the properties of Bio.3Sb1.7Te3 alloys by hot forging, we design a single-stage thermoelectric generation device which works at low temperature. Excellent conversion efficiency～4.3% is obtained, about 55% higher than that of the device made of zone-melted materials.