The Preparation And Thermoelectric Performance Of Chromium Selenide Based Materials

Fossil energy,such as coal and oil,has promoted the rapid development of the global economy and brought people a high-quality modern life,but also led to the depletion of some non-renewable resources,leading to a serious environmental crisis.In order to cope with the challenge of energy crisis and environmental pollution,the development of new green low-carbon energy materials has attracted more and more attention from all over the world.Thermoelectric material is a new kind of green functional material which uses Seebeck effect and Peltier effect to realize the mutual conversion of thermal energy and electric energy.It has shown great application potential in saving energy and reducing greenhouse gas emissions.Traditional thermoelectric materials,such as PbTe and Bi₂Te₃,have excellent thermoelectric properties,but the rarity of Te and toxicity of Pb restrict the application of these materials to a certain extent.In recent years,researchers have been looking for new thermoelectric materials,Cr-Se based materials,as a new type of thermoelectric materials with low price,have attracted the attention of researchers.The crystal structure of Cr-Se based materials is complex and it has the characteristics of intrinsic low thermal conductivity,so it is a potential thermoelectric material.At present,there is little research on the thermoelectric properties of Cr-Se based materials,therefore,it is of great significance to study the basic physical properties of materials and their electrothermal transport mechanisms and explore the possibilities and ways of optimizing the thermoelectric properties by means of solid solution and doping.In this paper,Cr₂Se₃ based materials and SbCrSe₃ based materials are taken as the research object.The problems of long preparation time,poor thermoelectric performance of Cr₂Se₃ based compounds were studied.The optimum technological parameters for the preparation of single-phase Cr₂Se₃ based compounds were explored by solid state reaction combined with plasma activation sintering technology.On this basis,the effects of S doping on Se site,transition metal elements?Ag,Nb,Ni,Mn?doping on Cr site and Mn and S double doping on the phase composition,microstructure,electron motion and phonon motion of Cr₂Se₃ based compounds were investigated.In order to solve the problems of low electrical conductivity and the transition of p type to n type of SbCrSe₃ based compounds,the effects of Sn doping on Sb site and Ti doping on Cr site on the phase composition and thermoelectric properties of SbCrSe₃ based compounds were studied.The main work and results of this paper are as follows:Single phase Cr₂Se₃ compounds were prepared by solid-state reaction combined with PAS sintering.Cr₂Se₃ compound has weak orientation,and the thermoelectric properties in the vertical sintering pressure direction are better than those in the parallel sintering pressure direction.In order to avoid the influence of orientation on the thermoelectric properties,in the follow-up study,the vertical sintering pressure direction of the samples is selected to test the thermoelectric properties.Because of the low boiling point of Se,it is easy to volatilize in the preparation process.The effect of Se excess on the phase composition and thermoelectric properties of the material is discussed.The results show that the excess Se in Cr₂Se₃ compound can not improve the thermoelectric properties of the material.The volatilization of excessive Se in the sintering process of PAS will pollute the instrument cavity,so the Cr₂Se₃ which does not deviate from the stoichiometric ratio is selected as the best proportion for further study.S doping at the Se site is carried out of Cr₂Se₃ compound,and the doping limit of S in Cr₂Se₃ is about 10%.With the increase of S doping content in Cr₂Se_3-3xS_3x?x=0?0.1?compound,the band gap increases and the Seebeck coefficient increases.The lattice thermal conductivity and bipolar thermal conductivity decrease.The ZT value of Cr₂Se_2.7S_0.3 compound was 0.29 at 623 K,which was 32%higher than that of intrinsic Cr₂Se₃ compound.The transition metal elements?Ag,Nb,Ni,Mn?were doped at the Cr site of Cr₂Se₃ compound,in which the Ag atom and Nb atom tend to replace the 6c Wyckoff position,while the Ni atom and Mn atom tend to replace the 3a Wyckoff position.The difference of the intrinsic outer charge of the transition metal elements and their preferential occupation in different Wyckoff positions make the materials behave differently in the thermoelectric properties.The carrier concentration of the material is basically unchanged after Ag element doping,while the carrier concentration of the material is decreased by Nb element doping,and the carrier concentration of the material is significantly increased by Ni element doping and Mn element doping.The carrier concentrations of Ni_0.04Cr_1.96Se₃ and Mn_0.04Cr_1.96Se₃ samples at room temperature are 2.92×10²⁰ cm^-3 and 3.14×10²⁰ cm^-3,respectively,which are three times higher than those of intrinsic Cr₂Se₃ compounds at 1×10²⁰ cm^-3.With the increase of carrier concentration,the intrinsic excitation initiation temperature is effectively shifted to higher temperature.The doping of transition metal elements significantly enhances the alloying scattering,and the lattice thermal conductivity of the material is greatly reduced.The minimum lattice thermal conductivity of the sample obtained at 573 K decreases from 1.51 Wm^-1K^-1 of the intrinsic Cr₂Se₃compound to 0.91 Wm^-1K^-1 of the Nb_0.04Cr_1.96Se₃ sample.Due to the reduced bipolar thermal conductivity,lattice thermal conductivity and optimized electrical properties of the samples doped with transition metals,the samples doped with Mn elements show excellent thermoelectric properties,and the maximum ZT value of Mn_0.04Cr_1.96Se₃sample is 0.28 at 823 K.Compared to the intrinsic Cr₂Se₃,the maximum ZT value is0.22 at 623 K,and the maximum ZT value is increased by 30%.At the same time,the temperature corresponding to the maximum ZT value shifts to higher temperature by200 K.A compound of Mn_0.04Cr_1.96Se_3-3xS_3x?x=0?0.1?was prepared by combining Mn doping on Cr site and S doping on Se site.Co-doping of Mn and S can increase the forbidden band width of the material,improve the Seebeck coefficient,reduce the bipolar thermal conductivity;increase the carrier concentration,increase the electrical conductivity,inhibit the intrinsic excitation;enhance the alloy scattering,and reduce the lattice thermal conuctivity to realize the synergistic optimization of material electrical and thermal transport properties.The ZT value of the Mn_0.04Cr_1.96Se_2.7S_0.3sample reached a maximum value of 0.33 at 823 K,and the maximum ZT value obtained by the intrinsic Cr₂Se₃ compound at 623 K is 0.22,which is increased by 50%,and the temperature corresponding to the maximum ZT value is shifted to higher temperature by 200 K.Single phase SbCrSe₃ compounds were prepared by solid-state reaction combined with PAS sintering process.According to the calculation of orientation factor and the observation of microstructure,SbCrSe₃ compound has weak orientation.By testing the thermoelectric properties in different directions,the parallel sintering pressure direction is finally selected as the direction to test the thermoelectric properties.Sn doping at Sb site successfully improves the electrical conductivity of the material,eliminates the transition of p type to n type,increases the alloy scattering,and reduces the lattice thermal conductivity.The maximum ZT value of Sn_0.03Sb_0.97CrSe₃ sample at 873 K is0.14,which is 28 times of that of intrinsic SbCrSe₃ compound at 873K?0.005?.When Ti element is doped at Cr site,Ti doping inhibits the p type to n type transition,enhances the alloy scattering,and reduces the lattice thermal conductivity.The maximum ZT value of SbTi_0.05Cr_0.95Se₃ compound at 873 K is 0.04,which is 8 times higher than that of intrinsic SbCrSe₃ compound at 873 K?0.005?.