Study On Thermoelectric Properties Of?-?High Performance Thermoelectric Materials Fabricated By Melt Spinning

Nowadays,the deteriorating environmental issues are more serious due to the massive consumption of carbon-based energy sources.People have realized that the arrival of the energy crisis will jeopardize human life.Therefore,researchers from all over the world are committed to the development of new green energy sources to replace the fossil fuels.Thermoelectricity,enabling the direct conversion between heat and electricity,is one of the solutions.Owing to the advantages of zero emission,long steady-state operation period,no moving parts,and capability of operating in extreme situations,the prospect of thermoelectric devices is promising in terms of power generation and refrigeration.However,at present,it only has preliminary applications in aerospace,microelectronics and other niche fields and no large scale application is attainable,which is limited by the fact that the conversion efficiency of thermoelectric devices is still lower than the requirements of industrial applications.It is still a difficult problem for thermoelectric researchers to develop high-performance thermoelectric materials.In the mid-temperature region,the representative candidates are the PbTe family,CoSb3 skutterudites,and Mg-based alloys,all of which are featured with multiple sub-bands with small energy offset??E?near the bandgap?E_g?.Among known thermoelectrics,p-type IV-VI semiconductors have led most of the advancements and demonstrated the best performance above room temperature.Most thermoelectric IV-VI compounds crystallize in a cubic structure,and the high zT achieved is well known to be due to the convergence of the L and low-lying?bands at high temperatures.Among the IV-VI semiconductors,PbTe and its related alloys have long been well known as good thermoelectric materials for their excellent performance,but the high toxicity of Pb limits its potential applications.Hence,as its analog,germanium telluride?GeTe?and tin telluride?SnTe?with the rock-salt crystal structure have been inspired with great interests.Herein,in this study,we focus on GeTe-based and SnTe-based thermoelectric materials,using homemade melt spinning apparatus to synthesize Ge_0.9Sb_0.1Te_1+x+x and Sn_1-x-yBi_xIn_yTe compounds,respectively.Then on base of these,we discuss the influence of excess Te content on the thermoelectric properties of p-type GeTe-based compounds.Besides,the inherent relationship between microstructures and electrical and thermal properties is fully investigated.Thereafter,the effects of different experimental parameters of melting spinning on microstructure and thermoelectric properties of SnTe-based samples are elucidated.Then we demonstrate that thermoelectric performance of SnTe can be greatly improved by Bi and In co-doping and the physical reasons are discussed.Finally,we determine the optimum doping level for these compounds,which exhibit and high thermoelectric performance.The contents and results of this study are as follows:1.We demonstrate an enhancement of thermoelectric performance for p-type Ge_0.9Sb_0.1Te_1+x+x by a unique melt spinning method combined with hot pressing?MS-HP?,through which increased Seebeck coefficients and reduced thermal conductivities are achieved simultaneously.Moreover,the MS technique greatly gives rise to a variety of microstructures,such as the fine Ge precipitates,twins and nano-domains,all of which contribute to the significantly reduced thermal conductivities.Finally,the Ge_0.9Sb_0.1Te_1.03.03 reaches a maximum zT value of¹.9 at 760 K and a giant average zT of¹.2 between 300 K and 820 K,which is comparable to the reported results for GeTe based materials.Combined with the rapid fabrication,Ge_0.9Sb_0.1Te_1+x+x prepared by melt spinning process is an attractive p-type material for thermoelectric power generation application.2.We adopt a non-equilibrium melt spinning technology combined with hot pressing to rapidly synthesize SnTe compounds in less than 1 hour and contrast with different rotating speeds to get the best speed.We investigate that the refined microstructure generated by melt spinning significantly contribute to the reduction of thermal conductivity.Melt-spun undoped SnTe sample reveals a 15%lower thermal conductivity⁶.8 W/m K at room temperature and a 10%higher zT⁰.65 at 900 K,compared with the undoped SnTe samples prepared by traditional melting and long-term annealing.3.To further improve the electrical transport properties of the SnTe system,Bi and In are introduced as dopants.It is found that Bi and In co-doping can enhance Seebeck coefficients in a broad temperature range via optimizing carrier density and creating resonant state,respectively.Besides,point defects and nanoparticles from Bi and In alloying and microscale grains from melt spinning lead to a remarkably decreased thermal conductivities via the enhanced phonon scattering.Finally,a significant enhancement of the thermoelectric performance of SnTe melt spinning samples over a broad temperature is realized with a peak of zT value of¹.26 at 900 K and a average zT of⁰.48 for Sn_0.9675Bi_0.03In_0.0025Te through synergistic effect of Bi and In co-doping.This work indicates melt-spinning combined with co-doping provides a new strategy to improve thermoelectric properties of SnTe.