Theoretical Study On Thermoelectric Properties Of Several Intrinsically Low Thermal Conductivity Compounds

Since the 20th century,the increasing energy consumption and increasingly severe environmental problems have aroused widespread concern in various countries.It is of great significance to develop sustainable,pollution-free and new energy sources.Thermoelectric materials and devices,which can realize the direct interconversion of thermal energy and electrical energy,are considered as a promising way of energy conversion.However,the low thermoelectric conversion efficiency limits the large-scale commercial application of thermoelectric materials,so the current research focus of thermoelectric materials is to improve their thermoelectric conversion efficiency.The conversion efficiency of the material can be measured by the dimensionless thermoelectric figure of merit ZT value,ZT=S²σT/κ_e+κ_l,the ZT value is related to the Seebeck coefficient S,electrical conductivity σ,thermal conductivity（κ_e+κ_l,where Ke is the electronic thermal conductivity and κ_l is the lattice thermal conductivity）,where the power factor PF=S²σ,the electrical transport performance can be simply characterized by PF.Therefore,the two main avenues to improve the thermoelectric conversion efficiency are to optimize the electrical transport performance and reduce the lattice thermal conductivity,respectively.Due to the mutual coupling of electrical conductivity and Seebeck coefficient,it is more difficult to optimize the electrical transport performance,while the lattice thermal conductivity can be independently adjusted relative to other parameters,so looking for thermoelectric materials with low lattice thermal conductivity and optimizing their thermoelectric performance has become the focus of thermoelectric research.Studies have shown that strong anharmonicity is an important factor leading to low lattice thermal conductivity.Several strategies for inducing strong anharmonicity have been discovered,such as systems containing stereochemically active lone pairs,resonant bonds,or atoms with rattling vibrations.Among them,the rattling vibration behavior refers to the large-scale vibration of weakly bonded atoms or atomic clusters in the lattice,and the acoustic and optical branches in the material phonon spectrum will avoid crossing effects,resulting in softening of the phonon branches,thereby reducing the lattice Thermal conductivity.This dissertation focuses on investigating the physical mechanisms underlying low lattice thermal conductivity and optimizing thermoelectric performance in various systems incorporating the rattling effect.Through the utilization of first-principle calculation methods,several thermoelectric materials exhibiting intrinsic low lattice thermal conductivity,namely TlBiS₂,KCaBi,and KCaSb,were analyzed.The lattice thermal conductivity was evaluated using the ShengBTE software in conjunction with Boltzmann transport theory.Additionally,the electrical conductivity,Seebeck coefficient,and ZT value of these materials were determined.By analyzing the microstructure of these materials,the underlying physical mechanisms responsible for their low thermal conductivity were unveiled.These findings serve as crucial references for the further enhancement of material properties.The main contents of this paper are summarized as follows:1.Through first-principles calculations,the physical mechanism of the low lattice thermal conductivity of TIBiS2（the lattice thermal conductivity at 300 K is only 0.67 W/mK）was explored.The main reason for the low lattice thermal conductivity is the anharmonic rattling vibration of weakly bound Tl atoms and the anharmonic motion of Bi atoms.The anharmonic motion of Tl and Bi is due to the unique electronic structure of this material,which involves cross-gap hybridization among Tl,Bi,and S.This property leads to a high Born effective charge of TlBiS₂,which in turn leads to a large dielectric constant,which is beneficial for high mobility in the presence of point defects.And because the rattling vibration of Tl atoms further leads to the strong anharmonicity of the lattice,so that TlBiS₂ has a high Grüneisen parameter,a low group velocity,and a large scattering phase space,thereby reducing the lattice thermal conductivity.In addition,optical phonons in TlBiS₂ contribute 59%to the accumulation of κ_l,which is mainly due to the fact that these optical phonons have high group velocities,thus making a major contribution to κ_l in TlBiS₂.The thermoelectric performance of TlBiS₂ can be further improved by utilizing intrinsic point defects.The Tl vacancy（VTl）was found by calculation to be the main defect and a shallow acceptor.Therefore,VTl can be used to regulate the Fermi level to further increase the carrier concentration of p-type TlBiS₂.This provides some insights into exploring and studying the application of ultralow lattice thermal conductivity and utilizing intrinsic point defects to improve thermoelectric performance.2.Using density functional theory,this study systematically investigates two isomorphic semiconductors,KCaBi and KCaSb,known for their low-cost and environmentally friendly characteristics.Despite both compounds exhibiting low lattice thermal conductivity at room temperature,KCaBi demonstrates a significantly lower thermal conductivity compared to KCaSb,with KCaBi’s value being only half of that of KCaSb（at 300 K,κ_l=0.95 W/mK for KCaBi and κ_l=2.07 W/mK for KCaSb）.To elucidate the underlying reasons for this substantial difference,the bonding characteristics and phonon properties of these compounds are thoroughly examined,specifically focusing on distinctions in group velocity and the Grüneisen parameter.The analysis reveals an antibonding state below the Fermi level in the Ca-Bi（Ca-Sb）bond,which enhances lattice anharmonicity and diminishes thermal conductivity.The rattling effect of K atoms in KCaBi results in a flattening of its phonon spectrum,with a corresponding reduction in the contribution of these phonons to lattice thermal conductivity and a decrease in group velocity.Therefore,the primary cause of KCaBi’s lower lattice thermal conductivity in comparison to KCaSb can be attributed to the rattling vibrations of K atoms.Additionally,the calculations based on electrical transport theory indicate that KCaBi exhibits favorable characteristics as an n-type thermoelectric material,suggesting its potential application in the field of thermoelectricity.