Effects Of Melt States And Doping On Solidification And Performance Of Bi-Te-Se Thermoelectric Materials

With the environment and energy crisis getting more and more serious, the needs of the R&D of thermoelectric materials become increasingly urgent. Bi2Te3 is the most outstanding thermoelectric materials near room temperature region, but their brittle raises difficulty in machining, which restricts it to obtain the widespread development.The new preparation methods become a research focus,but this kind of new methods almost always need expensive devices and complicated manufacturing process, which are still at the experimental research stage and cannot satisfy the cosmically industry production requires. Based on the current status of research and the correlation of liquid-solid, through a new way of thinking, whichhas happily combine the manipulation of the melt state and conventional coagulation method, to manufacture bulk Bi-Te-Se thermoelectric materials, and explored the melt state and the LiCl doping on the solidification behavior and microstructure, related performance.The behaviors of electrical resistivity vs. temperature of molten Bi2Te3-xSex(x=0.3,0.45,0.6) thermoelectric alloys were explored by DC four-probe method in this paper. Based on the results, by using thermal analysis, metallographic, EDS, XRD and SEM, we explored the effect of melt states and LiCl doping on solidification behaviors,structures and property of Bi2Te3-xSex. The main innovative results and conclusions are summarized as follows:1. The obvious abnormal behaviours appeared on the electrical resistivity vs temperature (p-T) curves of molten Bi2Te3-xSex (x=0.3,0.45,0.6) alloys at the first heating processes, which intuitively revealed that the irreversible temperature-induced melt structure transition (TI-LLST) occurred in the liquid. And the analysis shows, the physical essence of TI-LLST is the different types of clusters (Bi-Te-Se) in the low temperature melt are breaking and forming new atomic cluster, the uniformity and disordered degree of corresponding melt status is higher.2. Thermal analysis data and results of the characterization methods show that the variation of the melt states of the thermoelectric materials during different thermal history plays a vary signification active role in the solidification behaviors and microstructures, and there were two phase existed in the solidification structure of (x=0.3,0.45,0.6)alloys.For sample experienced melt structure transition, the start solidification time delayed, the undercooling degrees of nucleation and growth enlarged, the rates and number of nucleation significantly improved, the solidification time greatly reduced, the solidification structure obviously refined, the thickness of primary phase dramatically reduced, the volume fractions of eutectic phase slightly boosted, the preferred orientation remarkably weakened, the grain distribution became more homogeneous and disordered.3. The electrical performance test results of Bi2Te3-xSex (x=0.3,0.45,0.6) semiconductor materials indicated that doping and melt structure transition had a signification impact on thermoelectric property.In our paper, LiCl is the best donor dopant for n-type thermoelectric alloys, the increase of LiCl content has increases the carrier concentration, the optimal doping content of LiCl is 0.075 wt.% for n-type Bi2Te3-xSex (x=0.3,0.45,0.6) thermoelectric alloys.After the n-type Bi2Te3-xSex (x=0.3,0.45,0.6) thermoelectric alloys had experienced the melt structure transition, the Seebeck coefficient enlarged and the resistivity reduced,thus the power factor promoted 49.2%,48% and 48.7%, respectively.From above conclusions, through manipulating the melt states of n-type Bi2Te3-xSex (x=0.3,0.45,0.6) thermoelectric alloys and with doping, which can effectively optimized the microstructures and properties. This has a new perspective for thermoelectric materials preparation methods, and also comfirmed that simple coagulation method can obtain good performance of thermoelectric materials.