Study On Process And Properties Of CoSb₃ Nanomaterials Prepared Via Microwave Hydrothermal/solventothermal Method

With the development of aero-hypersonic vehicles,their heat dissipation needs continue to climb.CoSb₃-based thermoelectric materials are capable of directly converting waste heat generated in supersonic vehicles into electrical energy for utilization,and are of interest because of their high carrier mobility and good electrical conductivity.However,its high lattice thermal conductivity limits its application.The strategies to enhance the thermoelectric properties of CoSb₃ are mainly nanosizing,doping,and composite engineering,etc.In this paper,we adopt the strategy of nanosizing combined with composite to improve the thermoelectric properties of CoSb₃ materials.Firstly,CoSb₃ nanopowders were synthesized by microwave hydrothermal/solventothermal method,and high purity CoSb₃ nanopowders were prepared by adjusting the process parameters such as reaction temperature,dwelling time,and raw material ratio.Subsequently,2%graphene oxide was reduced to graphene,and CoSb₃/graphene nanopowders were prepared by one-step in situ composite by microwave solvothermal method.Finally,the synthesized CoSb₃ powder and CoSb₃/graphene composite powder were sintered and molded via High-Temperature and High-Pressure,and their thermoelectric properties such as thermal conductivity,electrical conductivity,and Seebeck coefficient were measured.The results show that:（1）Compared with the traditional hydrothermal method,the microwave hydrothermal synthesis requires much shorter holding time.In addition,the dwelling temperature had the greatest impact on the CoSb₃powders,and it was difficult to synthesize high-purity CoSb₃powders below 260℃.Besides,the addition of bromine salt ionic liquid could greatly accelerate the reaction product nucleation speed.（2）The optimal process for the preparation of CoSb₃ nanopowders by microwave solvothermal method:The sintering temperature is 280℃,n(Co²⁺):n(Sb³⁺)=1:3.5,the ionic liquid addition amount is 3 g and the holding time for 20 min.The CoSb₃ phase content was95%by Rietveld refinement for quantitative analysis.The crystals of the CoSb₃ samples were in the shape of small grain spheres with uniform grain distribution,the average grain size was33 nm and the crystal structure was cubic crystal with space group of Im3,which showed isotropic.（3）The CoSb₃/graphene composite powder was synthesized in one step using microwave solvothermal method.Throughout the synthesis process graphene oxide was reduced to graphene,and isometric spherical CoSb₃particles were grown in situ on the graphene lamellar structure,and the average grain size of CoSb₃ matrix was 33 nm.CoSb₃ was cubic crystal with high density of grain boundaries.Raman and XPS results showed that the oxygen-containing functional groups of graphene oxide were reduced to C-C bonds,which restored the graphene conjugated system.（4）CoSb₃ and CoSb₃/graphene composite blocks were sintered by High-Temperature and High-Pressure sintering.The CoSb₃ matrix material and graphene formed a highly dense composite block,and the high-temperature and high-pressure sintering effectively inhibited CoSb₃ grain growth,and the grain size grew slightly to 41 nm.The thermoelectric properties of the CoSb₃/graphene bulk showed a large improvement compared to the CoSb₃ bulk,with the conductivity increasing from 2456.6 Sm^-1 to 9191.7 Sm^-1 at room temperature.The Seebeck coefficient increased from 49.8μVK^-1 to 59.5μVK^-1.The thermal conductivity of CoSb₃/graphene is only 0.9 Wm^-1K^-1 at room temperature,and the introduction of graphene can reduce the lattice thermal conductivity of CoSb₃ matrix.The nanosizing and compounding methods not only improve the electrical conductivity and Seebeck coefficient of CoSb₃material,but obtain more interfacial scattering which reduces the lattice thermal conductivity of the system to improve the thermoelectric merit of CoSb₃.