Optimizing The Thermoelectric Performance Of Bi₂S₃ Material By Halides Doping

Thermoelectric materials can directly convert heat to electricity or conversely,have been potentially used in power generation and solid-state cooling and are expected to be an important method to improve the energy efficiency and alleviate the environment pollution.Most sulfide thermoelectric materials have the characteristics of low cost,environmental friendliness and abundant reserves.They have great potential for commercial application in the future and have been widely concerned in recent years.Bismuth sulfide(Bi2S3)is a potential medium temperature thermoelectric material with low thermal conductivity and high Seebeck coefficient,but its intrinsic low conductivity limits the improvement of thermoelectric performance.In this paper,Bi2S3 material is selected as the research object.In the first part,Bi2S3 bulks were prepared by melting combined with spark plasma sintering technology,and the thermoelectric properties were improved by halides doping.We doped ZrF4 into Bi2S3 material to introduce electrons and point defects to synergically optimize the electrical and thermal transport properties.A ZT value of?0.33 was achieved at 723 K,which is approximately 2.5 times that of the pristine Bi2S3 sample(?0.13).Further Cu doping can achieve the increase of carrier concentration and mobility simultaneously,consquently a ZT value of?0.53 was achieved at 773 K,revealing that Cu doping can effectively improve the thermoelectric performance of Bi2S3 material.CuI doping in Bi2S3 material can realize the significantly enhancement of power factor.Meanwhile,the introduction of point defects and the precipitated Bi19S27I3-secondary phases with low intrinsic thermal conductivity further reduced the lattice thermal conductivity of Bi2S3,which is close to the theoretical minimum value of Bi2S3 system at high temperature(0.36 W/mK),thereby yielding an enhanced ZT value of?0.55 at 773 K.BiBr3 doping in Bi2S3 material can greatly increase the carrier concentration from 7.6×1017 cm-3 to 1.3×1020 cm-3,which significantly improves the power factor.The room temperature power factor reached 517 ?W/mK2,which is about 11.5 times that of the pristine Bi2S3 sample.As a result,a ZT value of?0.64 at 773 K and an engineering ZT value of?0.26 in the temperature range of 323-773 K were achieved.In the second part,based on the chain crystal structure of Bi2S3,we prepared the oriented Bi2S3 bulk via temperature gradient cooling method.The cooling rate was precisely controlled to adjust the intrinsic defects of the sample,and the electrical transport properties were further optimized by adjusting the melting temperature and the content of S element.When the melting temperature is 1103 K,the Bi2S3.02 component achieved a significant increase in power factor over the entire temperature range,reaching 167 ?W/mK2 at 773 K,which is much higher than the pristine Bi2S3 sample prepared by other methods.It shows the application potential of temperature gradient cooling method in the preparation of high-performance Bi-S-based thermoelectric materials.