Study On Preparation,Thermoelectric Properties And Thermal Stability Of Cu₂Se Thermoelectric Materials By NaCl Flux Method

With the increasingly serious environmental problems in China and the aggravation of global energy crisis,high temperature thermoelectric technology is playing an increasingly important role in seeking sustainable and environment-friendly clean energy supply.Thermoelectric technology can directly convert heat energy into electric energy when the temperature gradient is formed on the thermoelectric junction.It has the advantages of high reliability,no moving parts,quiet operation and no pollution.So far,thermoelectric technology has been successfully applied in space power mission,waste heat collection of power plant,automobile exhaust system and other industrial processes.High performance thermoelectric materials are the basis for obtaining high performance thermoelectric devices.The current market and safety regulations have led the research and development of thermoelectric materials to lead-free,environmental friendly and cheap materials made of rich elements.Therefore,the thermoelectric material Cu₂Se,which is cheap,environmentally friendly and rich in earth elements,has attracted extensive attention of researchers.As the most typical"phonon liquid crystal electronic crystal"high performance liquid crystal like thermoelectric materials,Cu₂Se compounds have been improved to more than 2.0 in recent years due to their excellent thermoelectric properties.Although Cu₂Se thermoelectric materials have attracted more and more research interests due to its high ZT value,its practical application is limited due to its poor stability and oxidation resistance.In this paper,nano layered Cu₂Se thermoelectric materials were prepared by NaCl flux method,and the effects of component content and different element doping on the structure,crystal quality,thermoelectric properties,thermal stability and mechanical properties of the materials were studied.The results are as follows:In this study,nano layered Cu₂Se thermoelectric materials with excellent crystal quality and high ZT value were prepared by NaCl flux method based on the stoichiometric ratio of Cu₂Se_1+x（NaCl）_2.5（x=0,0.01,0.02,and 0.03）.With the increase of Se content,the Cu vacancy increases,which effectively regulates the carrier concentration of Cu₂Se and reduces the lattice thermal conductivity of Cu₂Se.The increase of Se content can effectively adjust the carrier concentration and increase pf from 0.8×10^-3Wm^-1K^-2to 2.0×10^-3Wm^-1K^-2.By adjusting the chemical composition of Cu₂Se_1.03,the ultra-low lattice thermal conductivity of 0.1 Wm^-1K^-1was obtained.The results of TG-DTA show that the volatilization of Se in cu2se prepared by NaCl flux method is inhibited.Finally,the ZT value of Cu₂Se_1.03 sample is～1.77.The results show that the proportion of elements can effectively adjust the structure of Cu₂Se.This study shows that Zn doping in Cu₂Se is a simple and effective way to improve the power factor and thermoelectric properties at the same time.The sharp increase of carrier concentration leads to the significant improvement of power factor and thermoelectric performance.Especially when the Zn content is 4%,the power factor of Cu_1.94Zn_0.06Se is up to 16.99×10^-3 Wm^-1K^-2 at 700K.The maximum ZT of Cu_1.96Zn_0.04Se is 1.75,which is five times of that of undoped Cu₂Se at the same temperature.The current stress test results show that the stability of Zn doped Cu₂Se is significantly improved compared with that of undoped Cu₂Se.Based on the stoichiometry of Cu_2-xGa_xSe（x=0、0.01、0.02和x=0.03）,nano layered Cu₂Se thermoelectric materials with high ZT value were prepared by NaCl flux method.XRD results show that the diffraction peaks of the samples completely correspond to the monoclinic structureα-Cu₂Se phase.When x>0,the impurity phase of gase is detected in the sample.TEM showed that Cu₂Se/Ga Se nanocomposites were formed in Cu₂Se matrix.In addition,with the increase of Ga content,the carrier concentration decreases and the carrier mobility increases.Finally,the sample with x=0.01 has the best thermoelectric performance,and the maximum ZT value is 1.67 at700 K.The results show that the thermoelectric properties and thermal stability of nano layered Cu₂Se thermoelectric materials can be further optimized by doping appropriate amount of Ga.