Thermoelectric Performance Optimization Of Bi₂S₃-Based Materials

Thermoelectric(TE)materials could directly interconvert thermal energy to electrical energy with each other,which can be used for refrigeration,microelectronic devices,waste heat utilization and other fields,Thermoelectric technique occupies an important position in China's medium-and long-term energy strategy.The conversion efficiency of thermoelectric materials is determined by the dimensionless thermoelectric figure of merit ZT,defined as ZT=S²?T/?,where S,?,?,T are the Seebeck coefficient,electrical conductivity,thermal conductivity and absolute temperature,respectively.High performance thermoelectric materials are very important for the practical application of energy conversion technology.Bi₂S₃-based materials have been considered one of the alternative materials of Bi₂Te₃ due to the advantages of non-toxicity,low cost,abundance and feasible potential for good thermoelectric properties(the high Seebeck coefficient and low thermal conductivity).However,the high resistivity seriously impedes the practical process of Bi₂S₃ materials.Therefore,it is of great significance to develop a new preparation technique and to systematically study the optimization mechanism of microstructure engineering and composition regulation on electrical/thermal transport properties of Bi₂S₃-based materials,so as to effectively improve its thermoelectric performance.In this thesis,the influence of different preparation methods on the thermoelectric transport performance of Bi₂S₃ materials is investigated,and the regulation mechanism of different chemical doping on its thermoelectric transport performances is explored,provides a new route to achieve high thermoelectric performance in n-type polycrystalline Bi₂S₃.The specific research contents are as follows:1.Regulation mechanism of different preparation methods on microstructure and thermoelectric properties of Bi₂S₃ materials.Bi₂S₃ samples were prepared by different preparation methods(mechanical alloying-MA,high temperature and high pressure-HPHT,spark plasma sintering-SPS).The Bi₂S₃ samples prepared by mechanical alloying+spark plasma sintering(MA+SPS)showed a large number of grain boundaries and micropores(100 nm?1?m),which led to low electrical conductivity and lattice thermal conductivity.Bi₂S₃ samples by mechanical alloying+high temperature and high pressure(HPHT)method exhibited layered structure,with high crystallinity and large size(10?100?m magnitude)grain.The morphology by HPHT could significantly improve the carrier mobility and thus improve the electrical conductivity of the Bi₂S₃ sample,but the weak phonon scattering led to high lattice thermal conductivity.Bi₂S₃ samples by mechanical alloying+spark plasma sintering+high temperature and high pressure(MA+HPHT+SPS)method had abundant multi-scale grains and microstructures,with relatively high electrical conductivity and power factor.Meanwhile,the abundant micro-nano structures reduced the lattice thermal conductivity.The maximum power factor was 381?Wm^-1K^-2 at 300 K and the highest ZT value reached 0.37 at 773 K.Therefore,the combination of HPHT and SPS techniques can realize the synergic optimization of electrical and thermal properties of Bi₂S₃materials.2.Regulation mechanism of Cu Cl₂ doping on thermoelectric properties of Bi₂S₃materials.Based on research content 1,Bi₂S₃+x mol%Cu Cl₂(x=0,0.25,0.5,0.75,1.0)samples were prepared by MA+HPHT+SPS method.After Cu Cl₂doping,the carrier concentration increased from 1.05×10¹⁹ cm^-3 to 1.09×10²⁰ cm^-3.The electrical properties of Bi₂S₃ were effectively improved by Cu Cl₂ doping.The power factor of all Cu Cl₂-doped samples reached more than 450?Wm^-1K^-2 at 773K,and the highest ZT value of 0.75 at 773 K was obtained for Bi₂S₃ sample with0.5mol%Cu Cl₂ doping.Bi₂S₃+0.25 mol%Cu Cl₂ sample exhibited good thermal stability after three testing cycles(300?773 K)with the maximum ZT value of0.71±0.01 at 773 K.3.The comparison of doping effects of Cu,Bi Cl₃ and Cu Cl₂ doping on thermoelectric performances of Bi₂S₃ samples.Based on research content 2,the optimization effects of Cu,Bi Cl₃ and Cu Cl₂ doping on the thermoelectric performance of Bi₂S₃ were compared.The electrical conductivity of all Bi₂S₃samples with different chemical doping was increased to different degrees,From the perspective of improving the electrical properties of Bi₂S₃ materials,the doping effects decreased from Cu Cl₂,Bi Cl₃ to Cu.Meanwhile,the lattice thermal conductivity for Cu,Bi Cl₃ and Cu Cl₂ doped samples was reduced compared to that of pure Bi₂S₃ sample.The maximum ZT values of 0.62,0.60 and 0.70 at 773 K were achieved for 0.25mol%Cu,0.17mol%Bi Cl₃ and 0.25mol%Cu Cl₂ doped samples,respectively.