Mechanical Properties Of Boron Nitride And Its New Heterostructure Nanosheets

Boron nitride nanosheets are a new type of two-dimensional nanomaterials with a graphene-like structure.They have attracted widespread attention because of their excellent properties and potential.It is well known that research on the mechanical properties of nanomaterials is the basis for their applications,and the mechanical behavior of boron nitride nanosheets will directly affect its applications as material reinforcement,nanodevices,nanoelectronic components,etc.The purpose of this study is to obtain an analytical solution of the elastic properties of nanosheets through theoretical expression based on the continuum model,and to study the size and temperature effect on the elastic properties of hexagonal honeycomb structure of boron nitride nanosheets.Combined with the theory of Winkler's foundation spring model,the effect of interlayer van der Waals forces on the mechanical properties of heterostructures is analyzed using molecular dynamics simulation to study the tensile behavior of graphene/hexagonal boron nitride nanosheet heterostructures.The mechanical response of boron nitride nanosheets with defects is studied with the help of molecular dynamics simulation,finite element and continuum mechanics theory.Molecular dynamics simulations were carried out using the Tersoff-type potential to simulate the tensile behavior of boron nitride nanosheets at different sizes and temperatures.And based on the beam model theory of molecular mechanics,the models of molecular dynamics simulation can be introduced into finite element simalations.The so-called “StickSpiral” model is improved.Analyzing geometric deformation under external loading of hexagonal honeycomb structure,the relationships are obtained among bond length,out-ofplane displacement,temperature and the elastic properties.Introducing wrinkle coefficient,the expression of Young's modulus can be thus deduced.And the results are in good agreement with those obtained by finite element simulations and molecular simulations.Employing the expression of “Stick-Spiral”,size and temperature effect of Young's modulus of boron nitride nanosheet is investigated.The results show that the edges of boron nitride nanosheet share a larger elastic constant than the interior.So with the size increasing,the Young's modulus decrease as the edge proportions are less.The lower out-of-plane displacement is,the greater elastic constant is.Therefore,as temperature increases,the Young's modulus decrease since the out-of-plane displacement increases.Tensile behavior of graphene/hexagonal boron nitride nanosheets heterostructure is simulated by molecular dynamics simulations using Tersoff-type potential and Lennard-Jones potential.Young's modulus,critical stress,and critical strain are obtained with various sizes and at different temperature.The results show that Young's modulus of heterostructure decrease with the increase of size and temperature,which are corresponding to the analysis of size and temperature effect of Young's modulus using “Stick-Spiral” model.By analyzing the tensile behavior of heterostructure,the deformation laws of linear,nonlinear,failure and residual failure stages are summarized.The van der Waals forces among nitrogen,carbon,and boron atoms are computed.Based on the theory of Winkler foundation spring model,the influences of spring stiffness on the elastic properties of layered materials are concluded.Due to the constraint of van der Waals force on the thickness direction,the graphene layer of heterostructure can be induced to “double elastic deformation” at room temperature of 300 K at 6nm×6nm.Molecular dynamics simulation of boron nitride nanosheets with atomic vacancy defects was carried out,and the mechanical properties of boron nitride nanosheets with different defect rates and different temperatures were obtained.The results show that Young's modulus,the critical stress,and critical strain are obvious dependent on the defect rate.The defect will reduce these mechanical properties,and its influence is temperature-sensitive.The Young's modulus of boron nitride nanosheet with vacancy defect was computed using the molecular mechanics expression by introducing the defect coefficient.It is found that the results of molecular dynamics simulation,molecular mechanics,and finite element simulation are in good agreement,which proves the models are reasonable.By calculating the damage time of boron nitride nanometer sheet with different defect rates,to some extent,it is revealed that defects can increase the ductility of boron nitride nanosheets.