The Research On Heat Transfer Performance Of Sandwich Structures With Strain Engineering And Electric Field Regulation

With the development of science and technology,the studies on two-dimensional（2D）nanomaterials represented by graphene,have attracted extensive attention.Supportting by the rapid development of material preparation technology,two-dimensional materials,such as grapheneg,BN,black phosphoene,etc,have been prepared and wide applied in electronic devices.Noted that the heat transport properties of materials is one of the most improtant factor affecting the performance and lifetime of devices.Thus,it is an important research topic to predict and control the heat transfer performance of two-dimensional materials.In this thesis,by using the first-principles calculations based on density functional theory,the thermal transport performance of sandwich structure with a honeycomb structure similar to graphene is systematically studied with the applying external stress engineering and electric field.The factors infecting phonon transport properties are deeply analyzed in terms of structure,phonon dispersion relation,phonon group velocity,relaxation time,scattering phase space,Grinnesen parameter,electronic structure and size effect,etc.Firstly,the heat transfer properties of the sandwich structures with light carbon atoms at the middle layer and different elements filling the outer-layer atoms are systematically studied.A few novel two-dimensional materials are constructed as study cases,i.e.,Mg₂C,Mg Be C,Be₂C,and Mo₂C.More importantly,during the component design,Janus Mg Be C was predicted as a new two-dimensional material.The low thermal conductivity of the four sandwich structures was 3.74,8.26,14.80 and 5.13 W/m K,respectively.Therefore,the thermal conductivity of the sandwich structure is insensitive to the types of elements.Secondly,the effect of strain engineering on heat transport performances of sandwich structures was studied by applying tensile biaxial stress on Mg₂C and Be₂C.The response trend of Mg₂C and Be₂C to biaxial stress is significantly different.This is also different from the behavior of XN（X=B,Al,Ga）which with the same response trend to tensile biaxial stress.By comparing the electronic structures under various strains,it is found that the uniformity of electronic orbital hybridization can well predict the variation trend of thermal conductivity with biaxial strain.The more homogeneous the orbital hybridization around the Fermi level,the higher the thermal conductivity.Besides,the first derivative of stress with tensile biaxial strain can describe the variation trend of group velocity of materials with applied biaxial strain.Thus,the first derivative of stress versus strain can accurately predict the variation trend of thermal conductivity as a descriptor.Finally,by applying an out-of-plane external electric field to sandwich structure of Mg₂C,the effect of electric field on heat transport performance of sandwich structure was further studied,and make a comparison with silicene and germanene with similar honeycomb structure.The thermal conductivity of silicene and germanene decreases rapidly with the increase of electric field.However,the thermal conductivity of Mg₂C has a relatively stable behvior.Under the electric field ranging from 0 to 0.3 V/（?）,the thermal conductivity of Mg₂C decreases first and then increases lightly with the increase of electric field intensity.Even at the peak of the thermal conductivity,the relative change from the intrinsic value is less than one order of magnitude.It is because the unique sandwich structure of Mg₂C which makes the electronic structure does not change significantly with the electric field.Thus,the thermal conductivity of Mg₂C is stable with the external electric field.This thesis not only predict various low thermal conductivity of sandwich structures,also studied the strain engineering and external electric field regulating the heat transport performance of the materials.It will be of great benefit to the application of electronic devices and further study on heat transport properties.