Study On Structure And Thermoelectric Properties Of Cu₂Se With Doping And Second Phase

Due to the increasingly serious energy crisis,thermoelectric materials are widely concerned by researchers in the fields of materials,physics and chemistry.Cu2Se has the characteristics of"ionic liquid-electron crystal" and exhibits excellent ZT value.It is a promising green low-cost thermoelectric material.In this thesis,Ag-doped and Cu2Se composite Pt,Si nanoparticles and regular octahedral SnTe single crystal samples were prepared by hydrothermal synthesis,vacuum melting and SPS sintering.XRD,SEM,Hall test,DSC and thermoelectric performance tests were used.Means study the effects of doping and second relative Cu2Se phase structure and thermoelectric properties.In this paper,?-Cu2-xSe flaky micro-nano powders were prepared by hydrothermal method.The hydrothermal reaction process can be divided into two stages:CuSe formation and Cu2-xSe transformation.The CuSe reaction barrier is low and the kinetic process is fast.The Cu2-xSe reaction barrier is higher and the kinetic process is slower.The higher the reaction temperature,the more regular the hexagonal Cu2-xSe.The increase of copper metering ratio is prone to Cu2O.Increasing the reaction temperature and increasing the holding time can inhibit the appearance of oxidation products.Pt-?-Cu1.6sSe composite bulk was prepared by physical vapor deposition and spark plasma sintering.Part of Pt entered the lattice of the matrix,causing lattice expansion,causing lattice distortion,increasing carrier mobility and optimizing power factor.Point defects and lattice distortion significantly reduce the lattice thermal conductivity of the material.The maximum ZT of the 40s Pt-Cu1.65Se composite is 0.67,which is 110%higher than that of the pure sample.Cu2-xAgxSe series samples were prepared by vacuum melting technique.When the doping content is 1%,Ag enters the crystal lattice mainly in the form of Ag+,and Ag+replaces Cu+,which introduces point defects and causes lattice distortion,increases copper ion diffusion channels,improves conductivity,and simultaneously carriers and phonons.The increased scattering reduces the lattice thermal conductivity.At 773 K,the thermoelectric figure of merit reaches 1.07,which is 123%higher than that of the undoped sample.As the amount of Ag doping further increases,?-Cu2Se gradually becomes a stable phase at room temperature,accompanied by the appearance of the second phase of AgCuSe.The introduction of the second phase increases the conductivity but the Seebeck coefficient decreases,resulting in a deterioration of the PF,an increase in the electron thermal conductivity leading to a decrease in the total thermal conductivity,and an excessive Ag doping resulting in a significant decrease in the ZT value.The Si-Cu2Se composite block was prepared by magnetron sputtering,vacuum melting and spark plasma sintering.The SEM energy spectrum was used to test the growth time of Si from the nanoparticle to the particle aggregation.The introduction of Si nanoparticles mainly increases the new scattering center,causing lattice distortion,and increasing the scattering reduces the lattice thermal conductivity.The ZT value of the 1 min Si-Cu2Se composite sample reached 0.61 at 773 K,which was 27%higher than that of the pure sample.The Pt-Cu2Se composite block was prepared by vacuum melting,vapor phase physical deposition,and discharge plasma sintering.During the sintering process,Pt enters the crystal lattice,and the crystal lattice expands,causing lattice distortion.The physical deposition of Pt recombination increases the scattering center,the lattice distortion reduces the lattice thermal conductivity,and the ZT value of the composite sample is improved.The highest ZT value of the 10 s Pt-Cu2Se sample is 0.57(773 K),which is 21%higher than that of the Cu2Se sample.In this paper,a single crystal SnTe octahedron with {111} surface was successfully synthesized by a hydrothermal synthesis route without surfactant.The effects of reaction temperature,reactant concentration and stoichiometric ratio on the phase of the phase and the morphology of the single crystal were discussed.The results show that high temperature is favorable for the formation of pure phase,and excessive Sn inhibits the appearance of SnTe with octahedral morphology.Lower reactant concentrations facilitate the preparation of uniform SnTe octahedrons by selective growth.The crystallographic characteristics of SnTe octahedrons were investigated by focused ion beam(FIB),electron backscatter diffraction(EBSD)and transmission electron microscopy(TEM).Through the corresponding three Euler angle diagrams,Kikuchi diffraction pattern and selected area diffraction,it is concluded that the surface of the octahedral SnTe is composed of {111} crystal planes with an average size of 1-3 ?m.The a-Cu2Se powder was subjected to SnTe recombination by a physical method,and a bulk sample was prepared by SPS sintering.During the sintering process,the Te2-substituted portion Se2-,Se2-and Sn2+ form a second phase SnSe.Due to the incorporation of Te,the conductivity of the sample increases,and the PF is enhanced.At the same time,the lattice distortion and the second phase increase the phonon scattering,the lattice thermal conductivity decreases,and the total thermal conductivity decreases.In particular,the 1%SnTe-Cu2Se sample exhibited the highest ZT value of 0.95 at 773 K.