Study On Thermoelectric Transport Properties Of Zn₄sb₃ And Bi₂s₃ Based Materials

Thermoelectric materials have attracted much attentions due to their potential applications in both power generation and refrigeration.Most of sulfides thermoelectric materials are low-costs,environment-fridendly,abundant but have non-ideal thermoelectric properties.Therefore,it is particularly necessary to develop novel sulfide thermoelectric materials as well as improve their properties.Bi₂S₃ has high Seebeck coefficient and low thermal conductivity,and is a promising thermoelectric material in the intermediate temperature range.However,lack of effective dopant to improve its electrical conductivity restricts the improvement of thermoelectric properties.In this work,the effect of doping on the thermoelectric properties of Bi₂S₃ is studied systematically.Meanwhile,the effect of Zn migration on the microstructure and thermoelectric properties of Zn₄Sb₃ is investigated.The main results are listed as below:The crack-free Zn₄Sb₃ samples are successfully fabricated through vaccum melting and plasma activated sintering process.The obvious Zn migration under the direct current applied in the plasma activated sintering?PAS?process is found in Zn₄Sb₃ compounds,and Zn exhibits significantly heterogeneous gradient composition distribution.The T-Zn 58 sample breaks the strong correlation between resistivity and Seebeck coefficient,and its power factor increases obviously.Its lattice thermal conductivity is not decreased,but high power factor is bebfical for achieving high ZT value.As a result,the resulting peak ZT value of 1.2 at 673 K is obtained.Bi₂S₃ samples are successfully fabricated through vaccum melting and plasma activated sintering process.There are several reasons for the low thermal conductivity of Bi₂S₃:1)its low Debye temperature;2)weak chemical bonds;3)strong lattice anharmonicity;4)large molar mass;5)lamellar crystal structure.The effects of anisotropy and I doping on the thermoelectric properties of Bi₂S₃ materials are analyzed.The results show that the ZT values of?pressure direction are higher than those of?pressure direction in the Bi₂S₃ based thermoelectric materials.The electron concentration of Bi₂S₃ samples is improved by I doping,resulting in the enhanced power factor.The introduction of I atoms changes the lattice constant of Bi₂S₃ and causes the fluctuation of mass and stress,which reduces the lattice thermal conductivity.By adjusting the transport properties through I doping,the highest ZT value is 0.58 at 773 K,which is 5 times that of undoped Bi₂S₃?0.11?under the same conditions.The lattice thermal conductivity of the Bi_1.99Al_0.01S₃ sample decreases slightly and the reason of reduction of lattice thermal conductivity is explained from point defect scattering.As a result,the ZT value of Bi_1.99Al_0.01S₃ sample increases slightly,reaching to 0.29 at 773 K.Cu doping can increase electron concentration,decrease lattice thermal conductivity.Density function theory?DFT?calculation shows that Cu doping makes the Bi₂S₃ highly n-type and causes an upward movement of the Fermi level to the conduction band minimum,producing very high electron concentration to increase the?for n-type Bi₂S₃.The introduction of Cu and the presence of S vacancy inhibit the transport of heat-carrying phonons,and the maximum ZT value is 0.62at 723 K.Due to the introduction of SbCl₃,the microstructure of Bi₂S₃ material gradually evolves from the layered-to particle-like structures.The room temperature electrical conductivity sharply increases from 1.3 S/cm for pristine Bi₂S₃ to 418.8 S/cm for Bi₂S₃-1%SbCl₃ sample,which is near two orders of magnitude higher than that of the undoped sample,resulting in a increased power factor of 3.9 Wcm^-1K^-2 at 773 K.The lattice thermal conductivity significantly decreases because of the strong scattering of phonons via grain boundary engineering and in-situ Bi₂S₃ nanoprecipitates.As a result,a peak ZT value of 0.65 at 773 is achieved K for the Bi₂S₃-1%SbCl₃ sample,about six times of that of pristine Bi₂S₃.