Study On The Solution Synthesis And Thermoelectric Property Modulation Of Cu₂Se Nanostructures

At present,the energy crisis facing mankind is increasingly serious.To improve energy efficiency,developing new green energy and effectively promoting the sustainable development of economy and society belong to a major issue facing the whole world.Thermoelectric materials can directly convert heat into electricity and achieve solid refrigeration,without any pollution emissions or vibration parts of the outstanding characteristics,providing a promising solution for the world energy crisis,which has been widely concerned.The ideal thermoelectric material needs to have excellent electrical properties and low thermal conductivity to achieve high thermoelectric conversion efficiency,and it also needs to be environmentally friendly and pollution-free,and the material elements of the Earth's crust are abundant.Cu₂X(X=S,Se,Te)is one of the most excellent thermoelectric materials and has been the focus of research in recent years.In particular,Cu₂Se has two allotropys,namely,the low-temperature monocline phase structure(?-Cu₂se)and the high-temperature cubic phase structure(?-Cu₂Se),and the phase transition temperature is?400 K.It is an excellent high-temperature thermoelectric material.However,the high cost and time consuming disadvantages of traditional solid phase synthesis technology limit the further development of Cu₂Se.In this work,Cu₂Se material was prepared by wet chemical method,and its thermoelectric performance enhancement strategy was studied,and the following results were achieved:(1)Multiscale Cu₂Se nanostructured materials were synthesized by solvothermal method.In general,the solution synthesis is usually obtained as the cubic structure of?-Cu₂Se rather than monoclinic crystal of?-Cu₂Se,which is usually produced by the solid-phase melting method.In this paper,we have achieved the controlled solvothermal synthesis of?-Cu₂Se nanostructures and revealed the evolution process from?-Cu_2-xSe to?-Cu₂Se with time and temperature.On this basis,the microwave-assisted solvothermal route was successfully used to synthesize?-Cu₂Se nanostructures,which greatly improved the synthesis efficiency.We found that,the Cu₂Se bulk materials after the spark plasma sintering(SPS)shows different thermal conductivity properties in parallel and vertical of the sinter pressure direction,and the thermal conductivity in the parallel direction is much lower.When T=?770 K,thermal conductivity reaches the lowest 0.58 W m^-1 K^-1 in the direction parallel to the pressure,eventually harvesting a peak z T 1.4 at 823 K.Compared with the melting method synthesis of?-Cu₂Se material,the thermoelectric performance realizes a great improvement.(2)Through the solution strategy,we controlledly synthesized?-Cu₂Se/reduced graphene oxide(r GO)-x(x=0.1,0.15,0.2,0.3 wt%)nanocomposites with different graphene mass fractions.Characterized by electron microscopy,it was found that the graphene oxide sheets were embedded between the Cu₂Se nanocrystals,forming many Cu₂Se/r GO interface structures.The thermoelectric performance measurement shows that,compared with Cu₂Se itself,the incorporation of r GO nanosheets with a mass fraction of 0.1-0.2 wt%into the Cu₂Se matrix can achieve a significant reduction in the thermal conductivity of the sintered pellets,which is due to the strong interface phonon scattering.As a result,the thermoelectric performance of the material has been improved.When the graphene recombination exceeds 0.2 wt%,the thermoelectric performance begins to deteriorate.Cu₂Se/r GO-0.15 is the optimal sample,a low thermal conductivity of?0.44 W m^-1 K^-1 is obtained at 723 K(a 26%reduction compared to Cu₂Se),and a high thermoelectric figure of merit of 1.58 is achieved at 823K(An increase of approximately 12%),the average z T at 423-823 K is 0.94(an increase of approximately 40%).This study shows that the nanocrystallization of Cu₂Se through solution synthesis and the construction of a Cu₂Se/r GO nano-interface structure by incorporating a suitable amount of reduced graphenen in the Cu₂Se single-phase compound can effectively reduce the thermal conductivity of the material,enhancing the figure of merit,which is a facile and effective material thermoelectric performance improvement strategy.