Molecular Mechanics Modelling And Physical Mechanical Properties Study In Low-dimensional Nanomaterials

Since the rapid development of the nano science and technology,and the proposal of mechanisms of various novel nano devices,new nano materials and new physical properties of present materials have attracted most attention.The low-dimensional nanomaterials,such as nanowires,nanotubes and nanoribbons,have unique size-dependent and chirality-dependent mechanical and physical properties due to the quantum confinement effect.They are expected to be the potential candidates for the functional materials in future nano electromechanical systems(NEMS).The basic properties of the common nano materials have already been systematically researched in recent years,but the studies of the unique mechanical and multi-field-coupled physical properties of individual materials still remain in phenomenology,lack of guidance of systematical theories.The electronic and magnetic properties have been extensively investigated through quantum mechanics calculations;however,quantum mechanics numerical methods are extremely time-consuming for studying mechanical,thermodynamic and electromechanical behaviors.Thus,to establish multi-scale methods based on classical molecular mechanics(MM)and molecular dynamics(MD)is necessary for systematical researches of these complex behaviors and helpful for the engineering applications of functional nanomaterials.In this thesis,based on the molecular mechanics,the continuum mechanical theory and the first principles calculations,we systematically investigate the variation of structures,mechanical and electromechanical properties of low-dimensional nano materials.(1)Molecular mechanics study on size-dependent elastic properties of single-walled boron nitride nanotubes.A molecular mechanics model is established for studying the axial mechanical properties of single-walled armchair and zigzag boron nitride nanotubes.On the basis of the molecular mechanics approach,we present an analytical ‘‘stick-spiral'' model for single-walled boron nitride(BN)nanotubes to investigate their size-dependent elastic properties.We obtained closed-form expressions for Young's modulus,Poisson's ratio and surface shear modulus of BN nanotubes subjected to axial loadings at small strains,as functions of lateral size of the nanotubes.The solutions are in reasonable agreement with available results of ab initio calculations.This study is a first effort to introduce molecular mechanics analytical methods to binary nanomaterials,and provides theoretical directions for studies of mechanical properties and engineering applications of compound nanomaterials.(2)Analytical solution for electromechanical coupling in boron nitride nanotubes.An analytical non-linear electroelastic model is developed for binary hexagonal nanocrystals,based on molecular mechanics approach.By introducing Morse potential into conventional molecular mechanics model for nanotubes,and incorporating electroelastic theory,the analytical electroelastic constitutive relationship of boron nitride nanotubes is derived in a large strain range.The electric-field-induced deformation is obtained in explicit expressions of the strength of electric field.Our model showshighly size-dependent and chirality-dependent electroelastic behaviors,comparable with the existing first principles calculations of the piezoelectric effect for boron nitride nanotubes.The predicted anisotropic electroelastic behaviors are the combination of the strains attributed by both intrinsic and induced bond dipoles.Our model is the very first attempt to incorporate electroelasticity into the framework of molecular mechanics,proposing a possible way to analytically and systematically investigate the electromechanical coupling effect of nanomaterials in a large scale range.(3)Analytical solutions for elastic binary nanotubes of arbitral chirality.We have developed a set of molecular mechanics solutions for the chirality-and size-dependent elastic properties of single-walled binary nanotubes by modifying the ‘stick-spiral' model.An out-of-plane inversion term is introduced to characterize the effects of bond polarity and buckling structures on the elastic properties of binary nanotubes.Parameters and analytical solutions have been obtained for all binary nanotubes that have already synthesized in laboratory.The surface elastic properties,i.e.,longitudinal and circumferential Young's modulus and Poisson's ratio are explicitly presented as functions of the tube chirality and diameter.The obtained inversion force constants are negative for all types of binary nanotubes,leading to the lower stiffness of the nanotubes than that by our former ‘stick-spiral' model without considering the inversion term.For different binary nanotubes,the force constants are very sensitive to the bond length and the effective charges,leading to significant dependence of elastic properties on the elements constituting the nanotubes.For all kinds of binary nanotubes,the Young's modulus increases with increasing tube diameter,while the Poisson's ratio decreases,in all chiralities.The predictions are in good agreement with available numerical calculation results.(4)Molecular mechanics study on the neutral surface strain effect in bending wurzite nanowires.We demonstrate that “stick-spiral” model can be applied not only to the grapheme-like materials,but also to the wurzite nano structures consisting of VIII-group transitional metals(TM)and VIA-group nonmetallic elements.An analytical molecular mechanics model for wurzite nanowires,e.g.Zn O nanowires,is established for studying their elastic behaviors under axial loading and pure bending.An additional axial strain is characterized as a linear function of the bending strain gradient.The bending-strain-gradient induced axial strain distributes equally on the lattices including the neutral layer,which is distinct from bulk materials.We predict the band modification near the neutral layer as a linear function of the bending strain gradient,by comparing with the first principles calculation results of nanowires under axial stress.Our analytical mechanics model for the first time proposes a systematical explanation of the bending strain gradient effect in nanomaterials,and establishes the theoretical basis for the similar effects in other materials.(5)First principles energetic study on edge kinetics of graphene nanoribbons with unique zigzag edges.Narrow zigzag graphene nanoribbons are found to have exceptional electronic and magnetic properties,but difficult to fabricate.Here the edge growth kinetics and a new fabricating approach of graphene nanoribbons with unique zigzag edges are proposed by first principles energetic calculations.First,we explore the growth kinetics of unique zigzag edges and the energy-optimized structure during graphene CVD growth on Cu(111)surfaces by density functional theory(DFT)calculations.We find that the energy barrier of forming hexagonal rings from pentagonal rings adsorbed by vaporized carbon atoms on Cu substrates is lower than in vacuum,due to the effect of copper atoms.The zigzag epitaxial growth on original zigzag edges is an energy-decreasing process.Next we show by density functional calculations that oxygen atoms have strong energetic favorability to adsorb on small-diameter carbon nanotubes along the direction of minimum angle to the tube axis.Along the same direction,unzipping the carbon nanotube by C–O–C epoxy chain is also nanoribbons could be fabricated by oxidization of small-diameter carbon nanotubes with arbitrary chiralities according to the minimum energy principle.These works theoretically propose a new fabrication approach of graphene nanoribbons with unique zigzag edges.