The Research Of Properties For Bismuth Telluride Based Thermoelectric Materials By Hot Extrusion

As the earliest studied, most mature and widely used thermoelectric materials, Bi2Te3 compounds have been paid widespread attention. Many studies have demonstrated that zone melting can prepare fine preferred orientation Bi2Te3 based thermoelectric materials, parallel to the crystal growth direction has better thermoelectric performance, the maximum thermoelectric figure of merit (ZT) is about 1. But the Bi2Te3 based thermoelectric materials prepared by zone melting have obvious anisotropy, the direction of good thermoelectric properties have relatively poor mechanical properties, make it not suitable for the production of thermoelectric devices. Therefore, for Bi2Te3 based thermoelectric materials, find a new process to increase its mechanical strength while maintaining its good thermoelectric performance become a new research focus.In this paper, we prepared p-type Bi0.5Sb1.5Te3 and n-type Bi2Seo.21Te2.79 thermoelectric materials by zone melting method. Then the hot extrusion process was carried out using the vacuum hot-pressing furnace. Many theoretically and experimentally studies show that the hot extrusion process can produce large amounts of defects which can cause recrystallization. Recrystallization can lead to grain refinement, obtain high density and excellent mechanical properties. The plastic deformation process can also get some preferred orientation to improve the thermoelectric properties. We used finite element analysis software to simulate the hot extrusion process before the hot extrusion process.We explored the experimental conditions. After hot extrusion experiment, we applied the XRD, SEM and other means to observed and analyzed the Bi2Te3 based thermoelectric materials. Tested the thermoelectric properties and mechanical properties. Studied the relationship between micro structure and properties.The simulation results show that the large chamfer, large extrusion ratio, smaller extrusion angle can effectively reduce the deformation rate for tapered part of the materials, get better surface quality. The experiment results show that better quality could be attained when hot extrusion temperature at 350 to 450℃, and with the extrusion temperature rise, the material properties get better. The p-type zone melting samples get the largest ZT value of 0.98 at 300K, n-type of 0.79. In hot extrusion experiment, when the chamfering is 5mm, extrusion angle is 60°, extrusion ratio is 9:1, extrusion temperature at 450℃, the samples obtained best quality. At 300K p-type samples get the maximum ZT value of 0.94, n-type of 0.74.Because the properties of hot extruded samples are not ideal, we carry out hot calcined treatment to hot extruded samples in SPS equipment. After the treatment the results of the electrical properties indicate a substantial upgrade. When hot calcined temperature is 425℃, pressure is 35MPa the sample obtained the highest power factor at 300K of 4.71×10-3W/mK2, and the sample's relative density is 99.2%, which is more dense then hot extrusion samples.This confirms that the hot calcined treatment has a positive effect on Bi2Te3 based thermoelectric materials. And it is worthy of further study.