First-principles Studies On New Binary And Ternary Thermoelectric Materials

Nowadays,the highlight of environmental issues has led people to develop green energy and new energy technologies.Thermoelectric conversion technology,which can achieve directly conversion between thermal energy and electrical energy,can greatly improve the efficiency of energy utilization and reduce environmental pollution.However,the efficiency of thermoelectric conversion is mainly subject to the thermoelectric figure of merit.In order to achieve new breakthrough in figure of merit,people developed so called electronic band engineering to improve power factor,and tried to break the lower limit of lattice thermal conductivity by low-dimensional nanostructures.Besides,people are also committed to looking for new bulk thermoelectric materials with high figure of merit.Theoretical calculations and predictions are helpful to understand and discover the behavior of electronic and phononic transport in different thermoelectric systems,providing favourable guidance for experiments.In this thesis,based on first-principle calculations and Boltzmann transport equations,we deal with some frontier problems in thermoelectric field,and predict promising thermoelectric performance in some new binary and ternary systems.Our calculations help to further understand the thermoelectric mechanisms and provide theoretical reference for experiments.At first,we study in detail the positive effect of electronic band engineering on improving the performance of thermoelectric transport.Based on PbTe,distinct from the previous application of band engineering to optimize the p-type thermoelectric properties,we use elements to replace the Pb site and successfully control its conduction band degeneration near the Fermi level.As a result,the n-type power factor is significantly enhanced.Our results indicate that lightly doping on Pb site can effectively control the degeneracy of conduction bands and improve the n-type thermoelectric performance.It was found that some bulk layered materials possess a separation of pz and px,y orbits at the valence band edge due to the crystal field splitting.Taking ZrS2 as an example,we successfully control the degeneracy of its two separate orbits at valence band edge by applying biaxial strains,leading to a double enhancement of its figure of merit.The results indicate that it is feasible to design high-performance layered thermoelectric materials by oribital degeneracy.Next,we predict the promising thermoelectric performance of 1 T-CdI2 type transition-metal dichalcogenides monolayer ZrSe2 and HfSe2.The enhanced phonon scattering rates in nanostructures can lead to the decrease of lattice thermal conductivity and the increase of figure of merit.Recently,thermoelectric transport in two-dimensional systems have attracted much interest,and monolayer transition-metal dichalcogenides is one of the typical examples.Distinct from previous 2H-MoS2 type monolayers,we find 1T-CdI2 type monolayer ZrSe2 and HfSe2 exhibit low lattice thermal conductivity and promising figure of merit.The low lattice thermal conductivity is attributed to the lower phonon velocity and higher phonon scattering rates.Our results highlight 1T-CdI2 type monolayers as promising two-dimensional thermoelectric materials.Finally,we also investigate the electronic and phononic transport of MPtBi(M=Sc,Y,La)and TIBiSe2 topological insulators.The close relationship between thermoelectric materials and topological insulators makes it possible to find better thermoelectric properties from some of the novel topological insulators.Our calculations indicate that the n-type electronic transport performance of half-Heusler topological insulator LaPtBi is very close to that of Bi2Te3,and it also exhibits low lattice thermal conductivity compared to traditional half-Heusler alloys.As a ramification of the Bi2Se3 family,TIBiSe2 exhibits comparable electronic and phononic transport performance with Bi2Te3.Its in-plane power factors are comparable to that of Bi2Te3 while the cross-plane power factors are much higher than Bi2Te3.The lattice thermal conductivity at 300K along in-plane and cross-plane directions are 0.84 W/mK and 0.87 W/mK,respectively.The mirror anisotropy of TIBiSe2 makes it more favourable for real thermoelectric applications.Comparing the electronic transport between T1BiSe2 and Bi2Te3,topological band with the characteristic of multi-valley degeneracy has better electronic transport performance.Generally,it is possible to find new bulk thermoelectric materials from topological family.