Study On The Solvothermal Synthesis And Thermoelectric Properties Of Cu₂S-based Materials

Nowadays,as the global economics has expanded in leaps and bounds,the excessive use of traditional fossil energy and the environmental pollution are becoming serious problems,so it is very important to find and develop new green energy materials with sustainable and high energy utilization efficiency.Thermoelectric material,capable of converting heat and electric energy directly and reversibly,is a kind of clean and pollution-free functional material.Thermoelectric devices have great prospects in the applications of waste heat utilization and refrigeration.Cu₂S-based thermoelectric materials have the advantages of non-toxicity and low price.In the crystal of Cu₂S-based materials,S atoms constitute a rigid network,and Cu⁺has excellent mobility,which not only ensures good electrical transport,but also maintains extremely low thermal conductivity.Therefore,it is a potential high-performance thermoelectric material.However,the thermoelectric performance and conversion efficiency of Cu₂S-based materials remain to be improved,and its synthesis is mainly achieved by high-temperature melting annealing,which is not conducive to its practical application.In this paper,Cu₂S was taken as the research object,and solvothermal method was used as the synthesis strategy.The key synthesis conditions were explored for the controllable preparation of Cu₂S.The influence of stoichiometric ratio and Cu₅FeS₄ compositing on the thermoelectric transport properties was systematically studied,and on this basis,the thermoelectric properties of the material were significantly improved.The research mainly includes the following three aspects:(1)Single-phase Cu₂S nanoparticles were successfully synthesized by solvothermal method and microwave-assisted solvothermal method.The effect of reaction time on the phase and morphology of synthesized products is systematically studied and determined.The key conditions for synthesizing the material are obtained.Then the synthesized powers were sintered into dense bulks by hot pressing,and the phase compositions of the sintered pellets remain unchanged.Thermoelectric performance evaluation results suggest that the thermal conductivity of Cu₂S is extremely low,only 0.36 W m^-1 K^-1 at 815 K,but the electrical conductivity is not high due to its low carrier concentration,so the z T value at 815 K is only 0.25.(2)In order to improve the electrical conductivity of Cu₂S-based material,the molar ratio of Cu Cl:Na₂S in the precursors was reduced from 2:1 to 1.75:1,and a series of Cu_2-xS materials with controllable phase compositions were synthesized.When the molar ratio of Cu Cl:Na₂S is reduced to 1.9:1,the solvothermal product transforms from single-phase Cu₂S to Cu₂S-Cu_1.96S composite.When the molar ratio is reduced from 1.9:1 to 1.75:1,the content of Cu_1.96S in the corresponding solvothermal product increases gradually.Hot pressing can not only densify the solvothermal powders,but also keep the phase composition unchanged in the corresponding sintered pellets.The decrease in the Cu Cl:Na₂S molar ratio leads to the increased carrier concentration in the sintered pellets;therefore,the electrical conductivity is significantly improved.When the molar ratio of Cu Cl:Na₂S in the precursors is 1.75:1,the power factor of the corresponding sintered pellet at 815 K is 8.04?W cm^-2 K^-2,which is about?735%of single-phase Cu₂S.Along with the moderate thermal conductivity,this sample achieves an enhanced z T value of 0.87 at 815 K,representing a?250%increase when compared to stoichiometric Cu₂S.This study demonstrates that phase composition control is an effective strategy to improve the thermoelectric performance of solution-synthesized Cu₂S-based materials.(3)The solvothermal Cu₂S and Cu₅FeS₄ synthesized via colloidal method were ultrasonically mixed according to the molar ratio of 4:1,and then the mixed powders were sintered into a pellet by hot pressing.The introduction of Cu₅FeS₄ increases the carrier concentration of the matrix and leads to the significantly improved electrical conductivity.Because the thermal conductivity of the composite pellet is still low,the z T value of Cu₂S-Cu₅FeS₄ material reaches 0.81 at 683 K,which is much improved compared to that of Cu₂S at the same temperature(0.04).