Effect Of Grain Boundary Geometry On Thermal Conduction Of Nanocrystalline ZnO

Due to the particularity of the structure,nanomaterials have great differences in light,heat,electricity,magnetism,chemical and mechanical properties from conventional materials,exhibiting quantum size effects and macroscopic quantum tunneling effects.Therefore,it is widely used in many fields such as microelectronics,optoelectronics,molecular assembly,detector sensors,biomimetic materials,and hydrogen storage materials.Among them,the nano-materials have special thermal conductivity characteristics,making them a hot topic in the field of thermal insulation and thermoelectric conversion.In order to further study the thermal transport properties of nanocrystalline materials,this article will take nanomaterials ZnO as a research object,focusing on the influence of the grain boundary geometry on the thermal conductivity of the materials.The abstract geometrical structure of the grain boundary surface is divided into several special shapes.In the case of single or multiple geometrical mixing,an effective grain boundary surface roughness calculation model is established.Meanwhile,according to Snell's law and SB.Soffer's research results,the calculation model of the specular reflectance of the grain boundary is improved by correcting the effective incident angle and incident area of the phonon.Based on the theory of molecular dynamics and the computational model of specular reflection at grain boundaries established in this paper,the thermal conductivity of materials related to grain size is calculated.At the same time,the thermal conductivity of the nanocrystalline ZnO lattice was obtained by solving the Boltzmann transport equation using an iterative method.In the end,the simulation results were combined with the theoretical calculation results.The effects of phonon incident angle,grain boundary surface roughness,specular reflectance and grain size on the thermal conductivity of nanocrystalline ZnO materials were analyzed.The results show that:(1)the decrease of the grain boundary surface roughness or the increase of the phonon incident angle will increase the grain boundary specular reflectance;(2)when the phonon specular reflection occurs at the grain boundary,the outgoing phonon will interact with Other phonons continue to exchange energy,and the phonon energy and momentum will not decrease.This process is similar to the phonon-phonon collision during the phonon heat transport process,but the diffuse reflection will generate thermal resistance,so nanocrystalline ZnO materials The thermal conductivity of ZnO increases with the increase of specular reflectance;(3)The thermal conductivity of nanocrystalline ZnO materials shows a strong size effect,and the size effect decreases with the increase of grain size.