Thermoelectric Properties Of Copper-based Nanomaterials With Zinc Blende Structure Synthesized By Wet Chemical Method

Thermoelectric?TE?materials can directly convert heat into electricity in solid state,which are of great significance for waste heat recovery and energy utilization efficiency.Devices based on thermoelectric effect have the advantages of no need on gas-liquid recirculation system,no noise and high reliability,but the large-scale application of TE technology is limited at present stage due to their relatively low efficiency.Although researchers have made great progress in binary thermoelectric materials,which exhibit high thermoelectric performance,the focus has extended to ternary compounds because of the diversity of element selection.The ternary Cu-based I₃-V-VI₄ and I₂-IV-VI₃ chalcogenides based on the zinc blende structure have a large number of family members,which attract wide attention from researchers due to their characters of environmental friendliness and earth abundant constituting elements as well as adjustable electrical properties.However,these materials typically have the strong chemical bond between atoms and thus show intrinsically high lattice thermal conductivity,and the reduction in lattice thermal conductivity relying on traditional solid solution method is limited.It is expected to achieve the all-scale heat transport suppression and further reduce the lattice thermal conductivity with the aid of the grain boundaries at micronscale and the precipitates at nanoscale,both of which contribute to enhanced phonon scattering.In this work,Cu₃SbSe₄ and Cu₂SnTe₃ nanocrystals with zinc blende structure are successfully synthesized by a wet chemical method.Furthermore,the electrical and thermal transport properties of these two compounds are investigated and the optimization strategy for thermoelectric performance of them is explored.The results shed light on the subsequent research of the two compounds.The main content of this study is as follows:?1?Microwave-assisted solvothermal method associated by spark plasma sintering is developed for the facile synthesis of?Ag,Sn?co-doped Cu₃SbSe₄ nanocrystals and their thermoelectric properties are investigated.Pure Cu₃SbSe₄ shows a low carrier concentration and Sn-doping on Sb sites dramatically increases the carrier concentration and thus the electrical conductivity.Although the Seebeck coefficient of the samples decreases,the final power factor is greatly improved.On the other hand,Sn-doping barely has any influence on the lattice thermal conductivity of doped samples.The increase in zT value of Sn-doped compounds is mainly attributed to the significantly promoted power factor.Finally,the sample with the composition of Cu₃Sb_0.97Sn_0.03Se₄achieves the maximum zT value of 0.8 at 623 K,which is more than 4 times higher than that of the undoped sample.?2??Ag,Sn?co-doped Cu₃SbSe₄ nanocrystals are rapidly and massively obtained via a facile microwave-assisted solvothermal method,and the relationship between microstructure and thermoelectric properties is investigated.The X-ray diffraction?XRD?and transmission electron microscopy?TEM?images confirm that Cu_2-xSe nanoinclusions emerge in these compounds,which affect the thermoelectric properties.Based on the enhanced electrical properties by Sn-doping on Sb sites,further alloying with Ag on Cu sites strongly suppresses the lattice thermal conductivity,which is close to the glass limit due to strengthened scattering phonon.In all,both point defect caused by Ag substitution and Cu_2-xSe nanoprecipitates serve as additional scattering center for phonons and contribute to the reduced lattice thermal conductivity.Overall,the sample with the nominal composition of Cu_2.8Ag_0.2Sb_0.95Sn_0.05Se₄ reaches a minimum lattice thermal conductivity of 0.27 W m^-1 K^-1 and a maximum zT of 1.18 at 623 K,which is a record high value for the Cu₃SbSe₄-based materials in the same temperature region.?3?Cu₂SnTe₃,an important member of I₂-IV-VI₃ family,could not be readily synthesized by traditional solid-state reaction method,and its thermoelectric properties are unexplored.The Cu₂SnTe₃ nanostructured bulk samples are firstly fabricated by a colloid synthesis method together with spark plasma sintering and the thermoelectric performance is preliminarily evaluated.The scanning electron microscopy?SEM?images indicate that the grain size of the sample is ranging from 10 nm to 100 nm.The electrical transport measurement suggests that the as made Cu₂SnTe₃ sample has extremely high intrinsic carrier concentration,which leads to rather low Seebeck coefficient and low zT values.Although the thermoelectric performance of Cu₂SnTe₃ is not satisfying,the successful preparation of Cu₂SnTe₃ nanocrystals with large quantities by colloid synthesis,which complements the family of I₂-IV-VI₃ system compounds,provides an important implication for the synthesis of other Te-based compounds of this family.Meanwhile,the preliminary exploration on thermoelectric properties of the Cu₂SnTe₃ bulk sample provides guidance for the further optimization of the thermoelectric performance for this compound.