The First-Principles Prediction Of The Low Lattice Thermal Conductivity Of Two-dimensional Materials Considering Four-phonon Scattering

In recent years,two-dimensional materials have attracted extensive attention all over the world because of their excellent physical properties.Its excellent thermophysical properties make two-dimensional materials an ideal thermoelectric material or heat dissipation material in the future.Thermoelectric materials with excellent performance can efficiently and directly convert waste heat into electric energy.In the context of energy crisis and global warming,the search for thermoelectric materials with excellent performance is very important for the sustainable development of energy.At the same time,the development of new electronic devices to micro nano scale forces people to provide effective management solutions for their heat dissipation problems.These problems need to be solved urgently.Therefore,the study of phonon heat transport characteristics of two-dimensional materials is of great significance for the application of two-dimensional materials to solve the problems of thermoelectric conversion and heat management.In addition,although there are many works related to the research on the thermophysical properties of two-dimensional materials and the regulation of thermophysical parameters at present,in terms of theoretical calculation,the commonly used methods to study the thermophysical properties of two-dimensional materials can only consider the influence of the second-order and third-order scattering on the lattice thermal conductivity(κL).At present,there is a lack of research on the influence of fourth-order and even higher-order scattering on the heat transport of two-dimensional materials.However,in the face of the existing phenomenon that the theoretical calculation overestimates the thermal conductivity of materials,it is particularly important to study the phonon thermal transport characteristics of materials by considering the influence of fourth-order or even higher-order scattering on the κL.Therefore,in this thesis:Firstly,we studied the thermophysical properties of monolayer orthogonal ScX2(o-SCX2)(X=C and N).Previous work has shown that monolayer o-SCC2 and o-SCN2 have great potential to become excellent anode materials for sodium ion batteries.From the dispersion relationship between the two materials,we noted that two-dimensional orthogonal ScX2(X=C and N)compounds also have the potential to play a role in the thermoelectric field.Different from most materials containing C or N atoms,which are generally accompanied by high κL,the two-dimensional o-SCX2(X=C and N)exhibits rather low and anisotropic κL.The κL3(the κL including the effect of three-phonon scattering and isotope scattering)of o-ScC2 along the X and Y directions are 2.79 W/mK and 1.55 W/mK,and those of o-ScN2 are 1.57 W/mK and 0.56 W/mK.Through regular residual analysis,we determined that four phonon scattering plays an important role in the κL of the two materials.By calculating the fourth-order interatomic force constants,we obtained the κL3+4 with the additional four-phonon scattering effect.The κL3+4 of o-ScC2 along the X and Y directions are 1.26 W/mK and 0.76 W/mK,and those of o-ScN2 are 1.35 W/mK and 0.49 W/mK.Combining with phonon analysis,we found that the number of four-phonon scattering channels and the four-phonon scattering intensity play different roles in different frequency ranges for monolayer o-SCX2(X=C and N),which is a very interesting phenomenon.Furthermore,it can be noticed the low lattice thermal conductivities of monolayer o-SCX2(X=C and N)are the results of many factors,e.g.,heavy atom doping,the strong anharmonicity caused by the vibration of Sc atoms in the out-of-plane direction and C(N)atoms in the in-plane direction,important four-phonon scattering and strongly polarized covalent bonds between C(N)atoms and Sc atoms.Moreover,it is interesting to find that the thermal transport properties of monolayer o-SCX2(X=C and N)are leaded by different phonon mechanism,e.g.,the different κL of monolayer o-SCC2 and o-SCN2 are determined by the anharmonic characteristic,and the harmonic characteristic plays a more important role in the anisotropy of monolayer o-SCX2(X=C and N).In general,our research can be expected to provide important guidance for the application of monolayer o-SCX2(X=C and N)in the thermoelectric field.Secondly,the κL of monolayer honeycomb BX(h-BX)(X=P,As,Sb)is calculated by first-principles calculations combined with Boltzmann transport equation and phonon spectral energy density method.Recently,cubic boron arsenide has attracted global attention due to its higher thermal conductivity,which is comparable to diamond,and excellent thermal properties.Can cubic boron arsenide achieve the leap in κL after transforming its structure from three-dimensional to two-dimensional like diamond to graphene?The existing studies have only investigated the κL considering three-phonon scattering and isotope scattering,and the calculated results are diverse.Therefore,we calculated the κL of monolayer h-BX(X=P,As,Sb)by considering three-phonon scattering,four-phonon scattering and multi-phonon scattering respectively.We firstly calculated second-order interatomic force constants and third-order interatomic force constants to iteratively solve the Boltzmann transport equation and to obtain the κL3 of monolayer h-BX(X=P,As,Sb)considering only three-phonon and isotope scattering and explain the monotonous change different from previous studies whereκL3 decreases with an increase in average atomic mass(mavg).The corresponding κL3 of monolayer h-BX(X=P,As,Sb)are 278.2 W/mK,205.7 W/mK,20.2 W/mK at room temperature.Subsequently we used regular residual analysis to determine the necessity of including four-phonon scattering when calculating the κL of monolayer h-BX(X=P,As,Sb).By calculating the fourth-order interatomic force constants,we obtained the κL3+4 of monolayer h-BX(X=P,As,Sb)including four-phonon scattering.The values of κL3+4 at room temperature are 61.12 W/mK,37.99 W/mK and 5.73 W/mK,which are highly consistent with the κL∞ of monolayer h-BX(X=P,As,Sb)as predicted by the phonon spectral energy density method.The phonon spectral energy density method considers all-order scattering,and gives values of 54.05 ± 21.48 W/mK(h-BP),18.20 ± 4.47 W/mK(h-BAs)and 2.46±0.34 W/mK(h-BSb),respectively.Our results show that the influence of four-phonon scattering on the κL of monolayer h-BX(X=P,As,Sb)is significant,and that the κL still undergoes a monotonic change after including four-phonon scattering.The main factors that determine the low(ultra-low)κL of monolayer h-BAs(h-BSb)are large mavg and weaker bonding strength,the existence of intermediate frequency ZO and scattered acoustic branches,the strong anharmonicity caused by the in-plane vibrations of As(Sb)atoms,and four-phonon scattering.Our work is expected to end the differences on the numerical simulation of the κL of monolayer h-BAs,and demonstrated the potential of monolayer h-BSb in thermoelectric field applications.