Studey On Doping Optimization And Performance Of Bi₂S₃ Based Thermoelectric Materials

The contradiction between energy supply and demand becomes more and more serious due to exhaustion of conventional fossil fuels.Therefore,exploring and developing new energy materials plays an important role in alleviating the energy crisis.Thermoelectric materials can transform electric energy into heat by internal carrier transfer vice versa,being used for solid state power generator and heating/cooling to address environmental issues.Most of sulfides-based thermoelectric materials are low-costs,enviroments-friendly,abundant,which have aroused extensive attention from scholars.However,there are fewer sulfur-based thermoelectric materials with high thermoelectric performance.Therefore,it is particularly significant to develop new sulfur-based thermoelectric materials and to improve their properties.Bi₂S₃ has a band gap E_g of 1.3 eV,which is considered to be a promising thermoelectric material in the intermediate temperature range due to high Seebeck coefficient and low thermal conductivity.Nevertheless,lack of effective dopant to improve its electrical conductivity restricts the improvement of thermoelectric properties.In this paper,the N-type Bi₂S₃ thermoelectric material is doped with Ag-I,Cu-Cl or Ag-Cl to improve its thermoelectric properties.The main results and conclusions are listed as below:Bi₂S₃ thermoelectric materials are successfully fabricated through vaccum melting and spark plasma sintering process.There are several reasons for the low thermal conductivity of Bi₂S₃:?1?weak chemical bonds and Vander Vaals in the structure;?2?lamellar crystal structure;?3?large molar mass.The electron concentration of Bi₂S₃ samples is improved by Ag-I co-doping,resulting in an enhanced conductivity.In addition,the Ag-I co-doping not only changes the lattice parameters but also produces lattice defects,which reduces the lattice thermal conductivity.The ZT value for the undoped Bi₂S₃ is only 0.15 at 723 K,while the ZT value of0.62 is obtained for Ag_0.0075Bi₂S₃ sample doped with 1 mol%BiI₃.The results of electrical transport tests show that the Cu-Cl co-doping Bi₂S₃ based thermoelectric material improves free electrons which can enhance electrical conductivity and decrease Seebeck coefficien.Besides,the Cu-Cl co-doping introduces impurity levels in the band structure of Bi₂S₃ and reduces the band gap of Bi₂S₃.The PF value for Cu_0.02Bi₂S₃ sample doped with 1 mol%BiCl₃ reaches 5.24?Wcm^-1K^-2 at 723 K.The nanostructures introduced by Cu doping and the presence of S vacancies inhibit the transport of heat-carrying phonons,as a result of that the ZT value of 0.66 is obtained for Cu_0.02Bi₂S₃ sample doped with 1 mol%BiCl₃ at723K.The Ag-I co-doping significantly increases the conductivity of Bi₂S₃ and increases the carrier effective mass?m*?which can enhance the density of state near the Fermi surface,resulting in an ideal Seebeck coefficient.The peak PF value of 5.77?Wcm^-1K^-2 is obtained for Ag_0.01Bi₂S₃ sample doped with 1 mol%BiCl₃ at 723K.Thermal transport tests reveal that the lattic thermal conductivity decreases because of the enhanced phonon scattering via high-density grain boundaies and point defects.As a result,a peak ZT value of 0.94 at 773 K is achieved for Ag_0.01Bi₂S₃ sample doped with 1 mol%BiCl₃,which is 5 times higher than that of undoped Bi₂S₃sample.