First-principles Studies On Heat Transport Properties Of Layered Thermoelectric Materials

With the shortage of energy and the environmental crisis,thermoelectric materials have attracted widespread attention in the material field because of their characteristics of being able to directly convert thermal energy into electrical energy.After decades of exploration,many thermoelectric materials have been studied and used in the field of aerospace and industrial power generation.Even so,the weaknesses of high cost and low conversion efficiency have limited the further development and promotion of thermoelectric materials.Therefore,finding thermoelectric materials with high conversion efficiency is an important goal for current researches in the field of materials science.It should be noticed that the layered thermoelectric materials can obtain excellent thermoelectric properties by artificial modulation because of their unique crystal structures.Based on density functional theory and Boltzmann transport theory,we have studied the thermal transport properties of layered ZrTe5,HfTe5,and Na2MgSn,Na2MgPb by numerical calculation,such as their lattice thermal conductivities and sound velocities,providing references for the exploration of high performance thermoelectric materials.The main contents and achievements of the research are as follows.1.The layered transition-metal pentatellurides ZrTes and HfTes have high electrical conductivity(>105 ?-1 m-1)and Seebeck coefficient(>100 ?V/K)at room temperature,so these two materials are expected to be good thermoelectric materials.However,existing reports show that ZrTes and HfTe5 have relatively high thermal conductivities,which are very detrimental to the thermoelectric properties of these materials.ZrTes and HfTes have the similar anisotropic crystal structures,leading to their anisotropic thermal conductivities.According to our calculations,the thermal conductivities of ZrTes and HfTe5 are in the range of 5-8 W/m·K,which is well consistent with the experimental values.Through the figures of three-phase space and cumulative thermal conductivity of ZrTes and HfTe5,we can find that several kinds of structural engineering at atomic or mesoscopic scale,such as alloying,doping,nanostructures and polycrystalline structures,could be used to reduce their thermal conductivities and make them good thermoelectric materials.2.Based on the density functional theory and Boltzmann equation,the theoretical study on the lattice thermal conductivity and thermoelectric properties of the layered intermetallic compounds Na2MgSn and Na2MgPb shows that both materials exhibit very low and isotropic intrinsic lattice thermal conductivities.Although the mass density of Na2MgSn is very low and its crystal structure is quite simple,its lattice thermal conductivities along a-and c-axes are only 1.77 and 0.81 W/m·K respectively at room temperature.When Sn is replaced by heavier element Pb,its lattice thermal conductivities decrease significantly to 0.56 and 0.31 W/m·K respectively at the same temperature.We find that the low lattice thermal conductivities in the two materials are mainly due to their ultra low phonon lifetime and the phonon mean free path.Combined with the limited experimental and our theoretical data,it is found that Na2MgPb is metal,so it is not a good thermoelectric material.While Na2MgSn is a semiconductor and we estimiate that its ZT is about 0.36 at 300 K.By comparison with other good thermoelectric materials such as SnSe,SnS,PbTe,we propose that Na2MgSn is a potential good thermoelectric material at room temperature.In summary,the thermal transport properties of Zr/HfTe5 and Na2MgSn are systematically studied based on the density functional theory and Boltzmann equation.For Zr/HflTe5,it is predicted that the out-of-plane lattice thermal conductivity can be effectively reduced by structural engineering,thereby improving the thermoelectric properties.For Na2MgSn,its thermoelectric properties could be further improved by different methods,such as growing single crystal Na2MgSn,doping Pb or other ions in Na2MgSn.Our work reveals the thermal transport mechanism in the two types of layered materials,and can motivate further experimental and theoretical work on the related thermoelectric materials.