Optimization Of Thermoelectric Properties And Stability Of Binary Copper Chalcogenide

Recently,liquid-like Cu_2-xSe compounds have attracted considerable interest for their excellent thermoelectric performance,abundant element reserves and low toxicity.However,the phase transition behavior of Cu_2-xSe compounds with increasing temperature and Cu precipitation phenomenon caused by the long-range migration of Cu ions under external field that give rise to instability,hindering its practical applications.The results show that a certain Cu vacancy can inhibit the phase transition and enhance the service stability of the materials.However,the excess copper vacancies generate overhigh carrier concentration that departuring from the optimal range for excellent ZT.In view of this,this paper takes Cu_2-xSe with the cubic structure at room temperature as matrix material and tries to decouple the relationship among phase structure,service stability and thermoelectric performance via doping and alloying.The main research contents are as follows:(1)Phase analysis shows that the cubic structure is successfully introduced in Cu_2-xSe(0.15?x?0.2)compounds around room temperature and it remains stable with the temperature increased.Simultaneously,the increase of Cu contents can promote the optimization of ZT value.Thereby,Cu_1.85Se(x=0.15)based compounds naturally are more outstanding in thermoelectric performance as well as good stability among them,and selected as the matrix material in this paper.Through orthogonal experiments,the optimal preparation conditions of Cu_1.85Se are obtained as follows:ball milling for 10 h,hot pressing pressure for 8125 N,and it has the highest ZT that achieves 0.2 at 760 K.In addition,the excellent stability of the Cu_1.85Se material is proved by high-temperature XRD and critical voltage experiment.(2)Whereas,beyond the improved stability,the large Cu off-stoichiometry leads to overhigh hole concentration,which deviates off the optimal range for high ZT.Therefore,Ag,Bi Sn and Li elements with different valence electron numbers are doped/filled to reduce the carrier concentration of the matrix material and optimize its thermoelectric properties.The Ag,Bi,Li and a little Sn doping does not change the cubic structure of the matrix material.Besides,the optimization effect of Ag doping is not obvious due to the same extranuclear electrons between Ag atom and Cu.Sn and Bi doping can obviously reduce the carrier concentration,and the latter effect is particularly prominent.In addition,Li filling can also optimize the ZT value.At 760 K,the ZT value of Cu_1.81Bi_0.04Se and Li_0.03Cu_1.85Se materials are 0.49 and 0.31,respectively,indicating that both single-element doping and filling can optimize the thermoelectric performance,but the optimization range is limited.(3)Therefore,through Li filling and Bi doping,the obtained samples maintain high structural stability and service stability,and at the same time,the carrier concentration in the matrix material is greatly reduced.The carrier concentration of Li_0.03Cu_1.81Bi_0.04Se is reduced to 1.15×10²¹cm^-3,and the power factor is increased to 10.8?Wcm^-1K^-2,which is higher than that of most Cu_2-xSe-based compounds.However,the high thermal conductivity of the material leads to the low ZT value.Hence,through the electronegativity stronger S instead of Se,the thermal conductivity of the Li_0.03Cu_1.81Bi_0.04Se_0.9S_0.1further reduces to 0.93 Wm^-1K^-1.Eventually,the ZT value is 0.9of Li_0.03Cu_1.81Bi_0.04Se_0.9S_0.1at 760 K,up 350%compared with the matrix material.This work provides a new idea for the study of liquid copper(silver)chalcogenides with high stability and high thermoelectric properties.