The Preparation And Thermoelectric Properties Of Cu₂SnSe₃ Based Composites Thermoelectric Materials

Thermoelectric?TE?materials are energy materials which can directly convert waste heat into electrical based on the TE effects.The advantages of small volume,no pollution,no noise,no mechanical rotation endow TE materials the enduring development potential.TE materials have an irreplaceable role in the application of TE refrigeration and power generation.Ternary copper based Cu₂SnSe₃ material with diamond-like structure has become one of the potential TE materials,because of its low thermal conductivity and the adjustable electrical conductivity.In this research,the Cu₂SnSe₃ was prepared by vacuum melting and quenching.The possibility and the preparation parameters by this method were explored.In addition,the thermal stability of Cu₂SnSe₃ was investigated at high temperature.In order to further optimize the TE properties of Cu₂SnSe₃,the nano-ZnO,TiO₂,WO₃ particles and graphene were introduced into Cu₂SnSe₃ by ball milling.The Spark Plasma Sintering?SPS?technology was employed to fabricate the Cu₂SnSe₃ based bulk composites.The X-ray diffraction?XRD?,field emission scanning electron microscope?FE-SEM?and transmission electron microscope?TEM?were used to study the phase and microstructure of the materials.Electrical resistivity,Seebeck coefficient and thermal conductivity measurement were applied to analysis the thermoelectric properties.The influences of the second phase on the Cu₂SnSe₃ materials were comprehensively researched in this thesis.The main results are as follows:The ternary Cu₂SnSe₃ compound has been successfully prepared following the process: annealing at 1279 K for 12 h,then quenching and annealing at 923 K for 2 days.According to the TG and DTA analysis,the Cu₂SnSe₃ had better thermal stability below 973 K.The ZnO/Cu₂SnSe₃ composites were prepared and their TE properties were studied.It was shown that,the electrical conductivity of the composites was improved in the whole temperature.For the sample composed with 1.8 vol.% ZnO,the electrical conductivity was significantly improved due to the large carrier mobility,but the improvement of electrical conductivity was not enough to offset the reduction of Seebeck coefficient,as a result,the power factor?PF?was seriously deteriorated.Due to the high carrier thermal conductivity,the composites have high thermal conductivity.The highest ZT value of 0.179 was obtained for the composites with 0.4 vol.% or 1.0 vol.% nano-Zn O at 700 K,which was 8% higher than that of the matrix Cu₂SnSe₃.The TiO₂/Cu₂SnSe₃ composites were fabricated and analyzed.The analysis result showed that the nano-TiO₂ particles which mainly distributed on the grain boundary effectively restrained the growth of grain and formed new interfaces with Cu₂SnSe₃ matrix.For the composite with 1.4vol.% nano-TiO₂,the electrical conductivity was improved whereas the Seebeck coefficient was lower than that of pure Cu₂SnSe₃.For the other composites,the Seebeck coefficient was improved meanwhile the electrical conductivity was reduced.The thermal conductivity of the composite keeps lower value than that of the matrix can be attributed to the lower carrier conductivity.The composite with 0.4 vol.% nano-TiO₂ obtained a maximum ZT of 0.296,which was 16.5% higher than that of the pure Cu₂SnSe₃ at 700 K.The WO₃/Cu₂SnSe₃ composites with different content of nano-WO₃ were designed and fabricated.The microstructure of the composites revealed that the size of the Cu₂SnSe₃ grains become smaller with the increase of the additive nano-WO₃.The introduction of nano-WO₃ could help to enhance the value of electrical conductivity while made little contribution to reduce the thermal conductivity.When the content of nano-WO₃ was no more than 0.8vol.%,the power factor of the composites was efficiently enhanced.The composite with 0.4vol.% nano-WO₃ exhibited a maximum PF of 5.33×10-4Wm-1K-2 at 700 K.The high PF for the composite directly resulted in an enhancement in ZT value.The maximum ZT reached 0.177 at 700 K,which was improved by 6.2% compared with the pure Cu₂SnSe₃ bulk material.The microstructure and the TE performance of graphene/Cu₂SnSe₃ composites were detected.It was shown that graphene embedded on the grain boundary and the existence of graphene inhibited the growth of Cu₂SnSe₃ grain.The size of Cu₂SnSe₃ particles decreased with the increase of the content of the graphene.The electrical conductivity of all the composites was significantly enhanced.Moreover,the lattice thermal conductivity of the composites was reduced.When the content of graphene was 1%,the ZT reached 0.21 at 700 K,and increased by 27% compared with the pure Cu₂SnSe₃.