Synthesis And Thermoelectric Properties Of Cu₂SnSe₃-Based Compounds

With the rapid development of the global economy,people’s demand for energy is increasing,and fossil fuels,as a traditional energy form,are increasingly unable to meet people’s needs.Thermoelectric materials,as a new type of environmentally friendly materials that use thermoelectric conversion technology to realize the conversion of thermal energy and electrical energy,have attracted extensive attention of researchers.However,currently common thermoelectric materials often contain toxic or rare materials,which is not conducive to the large-scale use of thermoelectric materials.Cu₂SnSe₃ compounds,as green,environmentally friendly high-performance thermoelectric materials,have received extensive attention from researchers in recent years.In the Cu₂SnSe₃ compound,Cu-Se bonds form a three-dimensional network structure.In this structure,Cu andSe are used as framework atoms to provide channels for the transport of carriers.Electricity price balance and structural stability.Cu₂SnSe₃compounds have two phase structures,cubic phase and monoclinic phase.However,the previous studies mostly focused on doping to improve the electrical and thermal transport properties,and little attention was paid to the effect of the structural changes of Cu₂SnSe₃ compounds on their electrical and thermal transport properties.In this study,the Cu₂SnSe₃ compound was used as the matrix to explore the influence of the solid solution of ZnSe,CdSe and Ge on the structure and thermoelectric properties of the material.In addition,on the basis of solid solution,In doping was performed to control the carrier concentration,and the effect of In doping on the electrothermal transport properties of the material was explored.The specific research contents and conclusions are as follows:（1）The samples with the stoichiometric ratio of Cu₂Sn Zn_xSe_3+x,Cu₂Sn_1-yIn_yZn_0.1Se_3.1 and Cu₂Sn_1-yIn_yZn_0.2Se_3.2 were prepared by melting-annealing-plasma activated sintering method.When the solid solution amount of ZnSe is 0≤x<0.2,the sample is cubic phase,when 0.2≤x≤0.5,the sample transforms into monoclinic phase,and when 0.5<x≤1,the sample transforms into tetragonal phase.With the increase of the solid solution amount,the conductivity of the material first increases and then decreases,and theSeebeck coefficient shows an opposite trend to the conductivity.The carrier concentration of the sample after solid solution of ZnSe has been significantly improved,from 6×10¹⁷ cm^-3 of the intrinsic sample to 2×10²⁰ cm^-3,but at the same time,the carrier scattering is enhanced,the carrier mobility of the sample after solution is reduced compared with that of the intrinsic sample.Calculations show that the effective mass of carriers changes significantly after solid solution,and the solid solution of ZnSe changes the energy band structure of the material.The solid solution of ZnSe has no obvious effect on thermal conductivity,and the final sample Cu₂Sn Zn_0.2Se_3.2 achieves the maximum ZT value of 0.55 at 823 K.For the components with a solid solution of x=0.1 and 0.15,the In doping was carried out respectively.After In doping,the carrier concentration at room temperature of the sample was optimized.Which increase from 2.4×10²⁰ cm^-3 and 2.1×10²⁰ cm^-3 to5.6×10²⁰ cm^-3 and 5.2×10²⁰ cm^-3,respectively.Cu₂Sn_0.99In_0.01Zn_0.1Se_3.1 and Cu₂Sn_0.97In_0.03Zn_0.2Se_3.2 samples obtained the maximum conductivity of 4.30×10⁴ S m^-1and 5.46×10⁴ Sm^-1at 300 K,respectively.The samples Cu₂Sn_0.98In_0.02Zn_0.1Se_3.1 and Cu₂Sn_0.995In_0.005Zn_0.2Se_3.2 obtained maximum ZT values of 0.57 and 0.64 at 773 K and840 K,respectively.（2）The samples with stoichiometric ratios of Cu₂Sn Cd_xSe_3+x,Cu₂Sn_1-yIn_yCd_0.1Se_3.1 and Cu₂Sn_1-yIn_yCd_0.15Se_3.15 were prepared by melting-annealing-plasma activated sintering method.When the solid solution amount is lower than x=0.1,the sample is a cubic phase,when the solid solution amount is 0.1≤x<0.6,the sample is a monoclinic phase,and when the solid solution amount reaches 0.6≤x≤1,the sample exhibits a tetragonal phase.As the solubility increases,the lattice of the material expands.The conductivity increases first and then decreases with the increase of the solid solution amount,and theSeebeck coefficient shows the opposite trend.With the increase of the solid solution amount at room temperature,the carrier concentration of the sample first increases and then decreases,and the lattice thermal conductivity of the material first decreases and then increases with the increase of the solid solution amount.The final sample Cu₂Sn Cd_0.15Se_0.15 obtained a maximum ZT value of 0.63 at823 K.The samples with the solid solution of x=0.15 and 0.2 were respectively doped with In,and the doping of In resulted in a significant change in the effective mass of the carriers,indicating that the In doping changed the energy band structure of the samples.The electrical conductivity has been significantly improved with the doping of In.The final samples Cu2Sn0.97In0.03Cd0.1Se3.1 and Cu2Sn0.975In0.025Cd0.15Se3.15obtained maximum ZT values of 0.62 and 1.06 at 800 K and 873 K,respectively.（3）The samples with the stoichiometric ratio of Cu₂Sn_1-xGe_xSe₃ and Cu₂Sn_0.85-yIn_yGe_0.15Se₃ were prepared by melting-annealing-plasma activated sintering method.When the solid solution amount of Ge is small,a small amount of SnSe second phase can be observed in the backscattering image of the sample,and the second phase disappears with the increase of the solid solution amount.The conductivity increases first and then decreases with the increase of the solid solution amount of Ge.TheSeebeck coefficient shows the opposite trend to the conductivity.The solid solution of Ge makes the thermal conductivity of the sample increase first and then decrease.The sample Cu₂Sn_0.5Ge_0.5Se₃ obtained the lowest thermal conductivity 0.71 Wm^-1K^-1 at 773 K,and the final sample Cu₂Sn_0.5Ge_0.5Se₃ obtained the maximum ZT value of 0.52 at 773 K.The Cu₂Sn_0.85Ge_0.15Se₃ sample was further selected as the matrix for In doping.The doping of In significantly increased the carrier concentration,from 2.2×10¹⁹ cm^-3 to3.1×10²⁰ cm^-3 when undoped.The sample Cu₂Sn_0.845In_0.005Ge_0.15Se₃ obtained a maximum conductivity of 3.28×10⁴ Sm^-1 at 300 K,and the final sample Cu₂Sn_0.82In_0.03Ge_0.15Se₃ obtained a maximum ZT value of 0.51 at 773 K.