The Preparation Of Cu-S Based Thermoelectric Materials And The Effect Of Transition Element Doping On Its Thermoelectric Properties

At present,due to the inexpensiveness,nontoxicity and ultralow thermal conductivity,the copper sulfide thermoelectric materials have attracted boosting attention and been predicted as promising thermoelectric materials with great application prospects.However,most researches on copper sulfide are focused on Cu₂S and Cu_1.8S,while the thermoelectric properties of Cu_xS with other Cu/S ratio have rarely been studied.In addition,as a typical liquid-like material,the crystal structure of copper sulfide is sensitive to the Cu/S ratio,leading to the complicated crystal structure.In view of the above problems,this work tries to find out high property thermoelectric Cu_xS sample,and increases its thermoelectric property via Ag doping and Ni doping.The following are the experimental research results:（1）A series of Cu_xS bulk samples are successfully fabricated by an easy and environment-friendly method of ball milling and spark plasma sintering（SPS）.Through XRD analysis and thermoelectric property testing,it’s indicated that Cu_1.9S sample has the highest thermoelectric performance.（2）After a small amount of Ag doping,the thermoelectric performance of Cu_1.9S is efficiently improved.According to the SEM and XPS result,we demonstrate that Ag has been successfully doped into Cu_1.9S and all elements are homogeneously distributed in Cu_1.9-xAg_xS bulk samples.Besides,the TEM characterization images show that the line defect,stacked fault and high-angle boundaries are exsited in Cu_1.9-xAg_xS sample,which can be considered as extra-scattering center to strengthen the phonon scattering,leading to the decease of lattice thermal conductivity and enhance ZT value.Through the optimization of thermal conductivity,the highest ZT can reach to 0.7 for Cu_1.885Ag_0.015S at773K.In order to further improve the thermoelectric properties of Cu_1.9S,Ni is used to optimize the thermoelectric properties by doping.From the experimental results,we find that Ni doping can reduce thermal conductivity significantly,including carrier thermal conductivity and lattice thermal conductivity,while it have little impact on PF.On one hand,the Hall carrier concentration decrease extremely with Ni increasing,which leads to the reduction of carrier thermal conductivity;on the other hand,Ni doping not only can introduce the nanopores into Cu_1.9-xNi_xS,but also make the Cu_1.9-xNi_xS crystal structure to be more complex,which is origin from the phase transition of the second phase.The exsit of nanopores and complex crystal structure all can contribute to strengthen the phonon scattering and reduce lattice thermal conductivity remarkably.As a consequence,the ZT of Cu_1.88Ni_0.02S sample can reach to 0.9 at 773K,in the meantime,its ZT_ave is about 0.39 from323K-773K,demonstrates that Ni-doping Cu_1.9S is a pleasurable candidate for thermoelectric application.