Design And Thermoelectric Properties Of New Quaternary Diamond-like Compounds

Due to the severe problems of energy shortage and environmental pollution,thermoelectric materials,as a kind of green energy materials that can realize the direct conversion between heat and electricity,have received more and more attention.The thermoelectric properties of materials are usually evaluated by the dimensionless figure of merit(ZT),but the electrical conductivity,Seebeck coefficient and electronic thermal conductivity are strongly coupled,making a challenge in the improvement of ZT value.By contrast,the lattice thermal conductivity is a parameter that can be independently modulated.Therefore,reducing the lattice thermal conductivity of materials or searching for compounds with intrinsically low lattice thermal conductivity have been a common strategy for increasing ZT.Quaternary diamond-like compounds(QDLC)are potential thermoelectric materials with intrinsically low lattice thermal conductivity.However,compared with the state-of-the-art thermoelectric materials,the electrical conductivities of QDLC is relatively low.In addition,the current research involving QDLC is mainly focused on the compounds with cation ordering motifs,and QDLC with high potential for cation disorder are rarely reported.The increase of cation disorder is not only expected to maintain high crystalline symmetry,which is beneficial to electronic transport performance,but also can effectively scatter phonons to reduce the lattice thermal conductivity of materials.Therefore,QDLC with increased cation disorder via a rational design are potential thermoelectric materials with excellent thermoelectric performance.In this work,quaternary ?-?-?-?₃(I=Cu,Ag;?=Zn,Cd;?=In;?=Te,Se)compounds with increased cation disorder were obtained via the rational design of composition and crystal structure,and the effects of composition and crystal structure on their thermoelectric properties were studied.Moreover,their thermoelectric performances were further optimized by different strategies such as doping.The main research contents are as follows:1.Design and thermoelectric performance of new compound.Taking cubic Cd Te and hexagonal Cd Se as prototypes,six quaternary ?-?-?-?₃compounds with high cation disorder are obtained via using cations with different valence(+1,+2 and+3 valence)to replace the Cd atoms in prototypes,where different crystal structures(cubicF4m3,tetragonalI 42dand Hexagonal P6₃mc)are observed.With the variation in cation disorder and chemical composition,the crystal structure of this series changes from cubic(Cu Cd In Te₃and Cu Cd In Se₃)to tetragonal(Cu Zn In Te₃,Cu Zn In Se₃and Ag Cd In Te₃)to hexagonal system(Ag Cd In Se₃).For six new quaternary compounds,the effects of composition and crystal structure of on their electrical and thermal transport performances were studied.The study found that the lattice thermal conductivity is mainly affected by the average cation mass,while the electrical transport performance is closely related to the anion type and crystal structure.Via rational design of composition and crystal structure,the highest ZT values for pristine Ag Cd In Se₃and Cu Cd In Te₃are about 0.87 and 0.5 at 825 K respectively.This work provides a means to search new potential quaternary thermoelectric compounds.2.Optimization of thermoelectric performance.To optimize the electrical transport performance of Cu Cd In Te₃compound,the additional carriers were introduced by Cu doping Cd to improve the electrical conductivity,and the ZT value of Cu_1.3Cd_0.7In Te₃reached 0.37 at 687 K,which is about 40%higher than that of pristine Cu Cd In Te₃.To further reduce the thermal conductivity of Cu Cd In Te₃,the substitution of Ag for Cu is used to introduce mass field fluctuations and stress field fluctuations,which can effectively scatter phonons,and which the lattice thermal conductivity compound at room temperature has been significantly depressed,which is a 30%decrease as compared with that of pristine CuCdInTe₃.