Microstructure Control And Performance Optimization Of Cu₂Se-Based Thermoelectric Materials

In recent years,with the development and progress of the economy,the consumption of energy is increasing day by day,which has led to a serious energy crisis and environmental pollution problems and caused many adverse effects on the development of human society and life.Thus the search for new energy technologies is urgent.Thermoelectric materials are a new type of energy conversion materials that can convert heat and electricity into each other,with the advantages of no pollution,no noise,long life,etc.However,due to their low conversion efficiency,they are mostly used in high-end fields such as aerospace.They are difficult to be used in large-scale commercialized.So the search for novel high-performance thermoelectric materials and the improvement of the thermoelectric figure of merit（ZT value）of the existing thermoelectric materials provide unlimited possibilities to solve these problems.The Cu₂Se thermoelectric material is a typical liquid-like material with low lattice thermal conductivity and high thermoelectric properties.In addition,it also has many advantages such as abundant raw material reserves,low price,non-toxicity and simple preparation process.Therefore,in this paper Cu₂Se is selected as the research object to explore the effects of doping,introduction of nano-second phase and nano-holes on its thermoelectric properties.All the required target samples in this paper were prepared by hydrothermal and hot pressing methods,and their physical phase structures as well as electrical transport properties and thermal transport properties were analyzed.First,Pb doping and Lu doping were performed on Cu₂Se to study the effects of element doping on its thermoelectric properties.Next,on the basis of Cu₂Se and Cu_1.98Li_0.02Se materials,the zero-dimensional carbon nanomaterial carbon dots（CDs）were introduced as the second phase to explore their effects on thermal and electrical transport properties.Finally,Cu₂Se powder was mixed with ground C₈H₈ and NH₄HCO₃ organic powders,respectively.Then nano-pores were introduced by high temperature sintering.The effects of nano-pores on the thermoelectric figure of merit ZT value of Cu₂Se were explored.By analyzing the experimental data and results,it was found that although the Pb doping reduced the carrier concentration and mobility,and the power factor,it increased the point defects,depressed the lattice thermal conductivity,improved the thermal transport properties.Finally,the thermoelectric properties of the material were improved under the combined effect.Lu doping deteriorated the electrical transport properties,and the final ZT value did not increase.The introduction of CDs could refine the grains,enhance the phonon scattering,and reduce the lattice thermal conductivity,which was beneficial to the enhancement of the material ZT value despite the decrease of the electrical transport properties due to the carrier mobility.The method of introducing the number of nano-pores to enhance the thermoelectric properties of Cu₂Se was very effective and convenient.The introduction of holes could effectively scatter phonons,reduce the lattice thermal conductivity of the material,and improve the electrical transport properties sometimes.But,it was worth that the size and uniformity of nano-pores affected the thermoelectric properties too,and related experiments were need to do further.Ultimately,for the Pb doped Cu₂Se series samples,the ZT value of Cu_1.97Pb_0.03Se reached 1.6 at 973 K,which was about 34.7%improvement over the pure sample.The electrical transport properties of the Lu-doped series samples were improved by up to 33.3%.The higher ZT values for CDs dispersion obtained in the nominal compositions of Cu₂Se+0.8 wt%and Cu_1.98Li_0.02Se+0.015 wt%were 1.98 and1.67,respectively,at 973 K.