Binary Chalcogenides As Thermoelectric Materials:High-Throughput Computations And Property Studies

Thermoelectric materials are one of the important functional materials that can directly generate electricity from heat.They play many promising roles in solving problems of energy and environmental crises.However,the energy conversion efficiencies of the currently discovered thermoelectric materials are still low,many of them contain either toxic or expensive elements and suffer from the mechanical instability.These limit their global commercialization.Binary chalcogenides have been used in many functional materials,including the thermoelectric applications.However,due to the complexities of theoretical computations and the difficulties of experimental measurements,the thermoelectric properties of a large number of binary chalcogenides have not yet been studied theoretically and experimentally.Therefore,in this dissertation,we attempt to establish an efficient methodology to evaluate the thermoelectric properties,use the high-throughput computational method to screen new promising thermoelectric materials within all binary chalcogenides,and understand the physical mechanisms in the predicted new thermoelectric materials.The followings are the descriptions of our studies:1.Evaluating anharmonicity and lattice thermal conductivity using the elastic properties:The difficulties in calculating the lattice thermal conductivity(?l)limit the applications of high-throughput computations to screen high-performance thermoelectric materials.To overcome this problem,based on the definition of anharmonicity(the Gruneisen parameter,y),we have proposed a method to efficiently evaluate ? and ?l using the computational feasible elastic properties.For 39 different compounds,the theoretically calculated thermal conductivities using the method are comparable to those experimental values.The agreement verifies the reliability of the devolved method.This method is simple,flexible and efficient.Thus,it could be used to efficiently screen new promising thermoelectric materials with(ultra)low lattice thermal conductivity in high-throughput computations.2.Screening promising thermoelectric materials in binary chalcogenides through high-throughput computations:Based on our developed methods(using rigid band model and elastic properties to evaluate the electrical properties and anharmonicities,respectively),we have established two simple thermoelectric descriptors(?and ?).With these two descriptors,we have high-throughput evaluated the thermoelectric properties of all 243 binary semiconductor chalcogenides,and successfully screened 50 promising thermoelectric materials,in which the thermoelectric properties of 16 compounds have not yet been reported theoretically and experimentally.Furthermore,through systematically analyzing of the relationship between thermoelectric properties and space groups and molecular formulas of compounds,we have provided a useful preliminary judgment for the promising thermoelectric materials.Our work provided not only new promising thermoelectric candidates for experiments but also reliable evaluation methods for high-throughput screening high-performance thermoelectric materials.3.Physical mechanism of a new thermoelectric material(ZnSe2):Among the predictions of the high-throughput computations,we have noticed ZnSe2 as a Pyrite-type compound,because it has a high-symmetry simple crystal structure and contains earth abundant elements.Using the detailed electronic and phonon properties,we have obtained its thermoelectric properties based on solving the corresponding Boltzmann transport equations.The results show that the complex isoenergy surfaces near both VBM and CBM lead to the promising electrical transport properties of both p-type and n-type ZnSe2.Furthermore,the localized Se-Se dimers with strong covalent bond in ZnSe2 result in the strong anharmonicity and low lattice thermal conductivity.Our study provided a distinct method to search new high-performance thermoelectric materials containing localized dimers or trimers.4.Influences of the non-metal dimers on the lattice thermal conductivities of Pyrite-type compounds:Inspired by the promising thermoelectric properties of Pyrite-type ZnSe2,we have focused on the thermoelectric properties of other Pyrite-type compounds.Through comparing the phonon properties of 4 Pyrite-type compounds(ZnS2,CdS2,CdSe2 and FeS2),we found that the bonding strengths of the non-metal dimers have an important influence on the anharmonicities and the lattice thermal conductivities of Pyrite-type compounds:only the strong non-metal dimers can contribute to the strong anharmonicity,low lattice thermal conductivity and promising thermoelectric properties.This work paved a way for searching and tuning the thermoelectric properties of Pyrite-type compounds.