The effect of temperature,defect and strain rate on the mechanical property of multi-layer graphene: Coarse-grained molecular dynamics study

In this work, we investigate the effect of temperature, defect, and strain rate on the mechanical properties of multi-layer graphene using coarse-grained molecular dynamics (CGMD) simulations. The simulation results reveal that the mechanical properties of multi-layer graphene tend to be less sensitive to temperature as the layer increases, but they are sensitive to the distribution and coverage of Stone-Wales (SW) defects. For the same number of defect, there is less decline in the fracture stress and Young's modulus of graphene when the defects have a regular distribution, in contrast to random distribution. In addition, Young's modulus is less influenced by temperature and defect, compared to fracture stress. Both the fracture stress and Young's modulus have little dependence on strain rate.     

Fracture analysis of monolayer graphene sheets with double vacancy defects via MD simulation

Carbon nanostructures such as carbon nanotubes (CNTs) and graphene sheets have attracted great attention due to their exceptionally high strength and elastic strain. These extraordinary mechanical properties, however, can be affected by the presence of defects in their structures. When a material contains multiple defects, it is expected that the stress concentration of them superposes if the separation distances of the defects are low, which causes a more reduction of the strength. On the other hand, it is believed that if the defects are far enough such that their affected areas are distinct, their behavior is similar to a material with single defect. In this article, molecular dynamics (MD) is used to explore the influence of separation distance of double vacancy defects on the mechanical properties of single-layered graphene sheets (SLGSs). To this end, critical stress and strain of SLGSs containing double vacancy with different distances are determined and the results are compared with those of perfect SLGSs and graphene sheets with single vacancy defect. The results reveal that the ultimate strength of the SLGS with double vacancy tends to the one with a single vacancy when the separation distance becomes further. In this regard, the threshold distance beyond which double defects behave like a single one is examined. Finally, Young’s modulus of perfect, single and double vacancy defected graphene sheets with different separation distances is determined. It is concluded that this property is slightly affected by the separation distance.     

温度、应变率和空位缺陷对氮化硼纳米管压缩性能的影响

采用分子动力学方法,分别模拟了完好的和含有缺陷的氮化硼纳米管的轴向压缩过程。原子间的相互作用采用Tersoff多体势函数来描述。结果表明,同尺寸的锯齿型氮化硼纳米管的临界轴向压缩强度高于扶手型氮化硼纳米管,这与碳纳米管的研究结果一致。发现纳米管的压缩强度,如临界轴向内力在低温下受温度影响明显,并且和应变率的大小有关。然而,应变率对纳米管的弹性变形没有影响。另外,还发现空位缺陷降低了纳米管的力学性能。与完好的纳米管相比,含有缺陷的纳米管轴向压缩强度对于温度的影响并不敏感。     

缺陷位置对单层石墨烯拉伸形变的影响

采用AIREBO势函数,对含有多种空位缺陷的单层石墨烯进行分子动力学拉伸模拟,计算不同空位缺陷的位置和排布结构对单层石墨烯应力-应变的影响.结果表明：石墨烯拉伸过程中空位缺陷对其力学性能影响较大.石墨烯内缺陷位置、缺陷排布对拉伸过程中发生的撕裂现象有不同程度的影响.对比分析发现,远离石墨烯边界的空位缺陷对其力学稳定性影响较大.通过控制石墨烯中缺陷的位置,可实现对其力学性能的调控.     

Effects of vacancy defects and axial strain on thermal conductivity of silicon nanowires: A reverse nonequilibrium molecular dynamics simulation

Thermal conductivity of silicon nanowires (SiNWs) is evaluated using the reverse nonequilibrium molecular dynamics simulation. The Stillinger–Weber (SW) and Tersoff interatomic potentials are employed to simulate thermal conductivity of SiNWs. In this work, the influence of random vacancy defects, axial strain, temperature and length on thermal conductivity and effective mean free path of SiNWs is investigated. It is found that by raising the percent of random vacancy defects, thermal conductivity of SiNWs decreases linearly for the results obtained form SW potential and nonlinearly for those obtained from Tersoff interatomic potential. Dependence of the thermal conductivity on axial strain is also studied. Results show that thermal conductivity increases as compressive strain increases and decreases as tensile strain increases. Influence of temperature is also predicted. It is found that the thermal conductivity of SiNWs decreases with increasing the mean temperature. Most of the simulations are performed for 4 UC×4 UC×40 UC silicon nanowires using ssp boundary condition.     

Deformation and fracture in graphene with divacancies of the 555–777 type

The deformation and fracture of graphene sheets containing 555–777 defects have been investigated by molecular dynamics simulations. Each such defect is a divacancy forming a localized configuration of three pentagonal and three septangular cells of carbon atoms in a hexagonal graphene lattice. An emphasis is made on the influence of 555–777 defects in graphene on its mechanical characteristics (stress–strain curve, uniaxial tensile strength, and maximum elastic strain).     

多晶石墨烯拉伸断裂行为的分子动力学模拟

采用分子动力学模拟方法研究了不同晶界对石墨烯拉伸力学特性及断裂行为的影响. 定义了表征晶界能量特性的新参量缺陷能, 并以此为基础分析了晶界结构的能量特性. 探讨了晶界对弹性模量和强度极限等的影响以及强度对晶界能量特性的依赖关系. 结果表明: 晶界能量特性可以间接反映晶界强度; 同时, 晶界中缺陷会使实际承载碳键数量小于名义承载碳键数, 从而在较大范围内影响弹性模量. 分析了不同晶界的断裂过程, 发现了裂纹扩展方向的强度依赖性: 低强度晶界主要是以碳键直接断裂为主要方式的沿晶断裂, 而高强度晶界通常是碳键直接断裂和Stone-Wales翻转过程交替进行下的穿晶断裂. 研究结果可为石墨烯器件的设计制造提供理论指导.     

Molecular dynamics study of the shear strength and fracture behavior of nanoporous graphene membranes

We perform molecular dynamics (MD) simulations to study the structural response and fracture characteristics of nanoporous graphene (NPG) membranes subjected to shear loading. The effects of porosity, temperature, and shear velocity on the mechanical responses of NPG membranes are examined. The results show that the wrinkling of the membrane becomes more obvious with increasing strain. Fractures occur around holes on the long diagonal of the NPG parallelogram, and fracture stress in the NPG membrane decreases with increasing porosity. In addition, the effect of shear velocity on the shear modulus decreases with increasing porosity. The fracture strain of NPG membranes with different porosities obviously decreases with increasing temperature. The results enhance our understanding of the shear mechanical properties of NPG membranes and are helpful for the design and application of high-performance NPG membranes.     

Atomistic simulations of mechanical properties of graphene nanoribbons

The mechanical behavior of graphene nanoribbons (GNRs) was investigated by molecular dynamics (MD) simulations. The simulation results showed that GNRs behave nonlinear elastically under tensile loads. When the strain exceeds 18%, the stress stiffening causes an increase in the Young's modulus. The width of a GNR has slight effects on the Young modulus and fracture strength. The maximum strain at which fracture occurs can reach 30.26% with an intrinsic strength of about 0.175 TPa. The excellent flexibility is attributed to the elongation of the CC bonds and the variations of the bond angles.     

Effect of triangular vacancy defect on thermal conductivity and thermal rectification in graphene nanoribbons

We investigate the thermal transport properties of armchair graphene nanoribbons (AGNRs) possessing various sizes of triangular vacancy defect within a temperature range of 200–600 K by using classical molecular dynamics simulation. The results show that the thermal conductivities of the graphene nanoribbons decrease with increasing sizes of triangular vacancy defects in both directions across the whole temperature range tested, and the presence of the defect can decrease the thermal conductivity by more than 40% as the number of removed cluster atoms is increased to 25 (1.56% for vacancy concentration) owing to the effect of phonon–defect scattering. In the meantime, we find the thermal conductivity of defective graphene nanoribbons is insensitive to the temperature change at higher vacancy concentrations. Furthermore, the dependence of temperatures and various sizes of triangular vacancy defect for the thermal rectification ration are also detected. This work implies a possible route to achieve thermal rectifier for 2D materials by defect engineering.     

Mechanical properties of defective single-layered graphene sheets via molecular dynamics simulation

In this paper, the effects of two main types of structural defects, i.e. Stone–Wales and single vacancy, on the mechanical properties of single-layered graphene sheets (SLGSs) are investigated. To this end, molecular dynamics simulations based on the Tersoff–Brenner potential function and Nose–Hoover thermostat technique are implemented. The results obtained have revealed that the presence of defects significantly reduces the failure strain and the intrinsic strength of SLGSs, while it has a slight effect on Young’s modulus. Furthermore, the examination of loading in both armchair and zigzag directions demonstrated that SLGSs are slightly stronger in the armchair direction and defects have lower effect in this direction. Considering the fracture mechanism, the failure process of defective and perfect graphene sheets is also presented.     

Mechanical Properties of Ni-Coated Single Graphene Sheet and Their Embedded Aluminum Matrix Composites

The effects of Ni coating on the mechanical behaviors of single graphene sheet and their embedded Al matrix composites under axial tensionare investigated using molecular dynamics (MD) simulation method. Theresults show that the Young's moduli and tensile strength of grapheneobviously decrease after Ni coating. The results also show that the mechanical properties of Al matrix can be obviously increased by embedding asingle graphene sheet. From the simulation, we also find that the Young'smodulus and tensile strength of the Ni-coated graphene/Al composite isobviously larger than those of the uncoated graphene/Al composite. Theincreased magnitude of the Young's modulus and tensile strength ofgraphene/Al composite are 52.27 and 32.32 at 0.01 K, respectively,due to Ni coating. By exploring the effects of temperature on the mechanicalproperties of single graphene sheet and their embedded Al matrix composites, it is found that the higher temperature leads to the lower critical strain and tensile strength.     

Mechanical and fracture behaviors of defective silicon nanowires: combined effects of vacancy clusters,temperature, wire size,and shape

Abstract The coupled effects of vacancy clusters (VCs), temperature, wire size, and geometry on the mechanical and fracture behaviors of defective silicon nanowires (Si NWs) were investigated using molecular dynamics modeling with Tersoff potential. The formation energies (E _v ) of a monovacancy (3.933 eV) and a tetrahedron vacancy (10.189 eV) obtained in this study agree well with experimental results and ab initio calculation. Simulation results show that the slip deformations of defective Si NWs are triggered at the wire’s surface and edge due to the number of dangling bonds on the wire’s surface being much greater than that inside a vacancy defect. VC defects barely affect to the value of Young’s modulus, but substantially weaken the ultimate strength of wires with a small cross-sectional size. With decreasing wire size and increasing operation temperature, significant reductions in Young’s modulus and fracture strength were observed. The average Young’s modulus for square NWs was about 3.7 % higher than that of wires with a circular shape due to the surface facet effect. A brittle-to-ductile transition (BDT) occurred for [001]-oriented Si NWs with a lateral size≤5.43 nm and an operation temperature T≥300 K.     

Anharmonicity induced thermal modulation in stressed graphene

Thermal properties are essentially decided by atomic geometry and thus stress is the most direct way for manipulating. In this paper, we investigate stress modulation of thermal conductivity of graphene by molecular dynamics simulations and discuss the underlying microscopic mechanism. It is found that thermal conductivity of flexural-free graphene increases with compression and decreases with strain, while thermal conductivity of flexural-included graphene decreases with both compression and strain. Such difference in thermal behavior originates from the changes in the anharmonicity of the interatomic potential, where the wrinkle scattering is responsible for the thermal conductivity diminishment in flexural-included graphene under strain. By comparing the results obtained from the Tersoff and AIREBO potentials, it is revealed that the degree of the symmetry of interatomic potential determines the thermal conductivity variation of graphene. Our results indicate that the symmetry of interatomic potential should be taken into careful consideration in constructing the lattice model of graphene.     

六角相硼碳氮化合物的合成研究

 以三聚氰胺和氧化硼为原料,利用真空热处理和高温高压技术,对BCN化合物的形成及结构进行了研究。在真空条件下,原料经1 100 K热处理得到非晶BCN的前驱物,将前驱物在5.0 GPa、1 500 K条件下高温高压热处理30 min,合成出了单相的六角BCN晶体。采用Materials Studio软件的Reflex模块对样品的X射线衍射图谱进行分析,结果表明,得到的样品为纯的六角相,晶格常数为a=0.250 5 nm,c=0.665 9 nm。对样品进行了透射电镜（TEM）分析,得到了样品的形貌和电子衍射,同样证实了样品为六角BCN晶体,晶粒尺寸约200 nm。对样品进行了XPS表征,确定了样品中存在C—C、C—N、C—B、N—B键,表明B、C、N三元素之间达到了原子级化合,样品的组分含量通过EDX进行标定。     

The effects of vacancy defects and nitrogen doping on the thermal conductivity of armchair (10, 10) single-wall carbon nanotubes

The influence of vacancy defects and nitrogen doping on the thermal conductivity of typical armchair (10, 10) single-walled carbon nanotubes is investigated using molecular dynamics (MD) simulation. The second-generation reactive empirical bond order potential and Tersoff potential are used to describe the interatomic interactions and the thermal conductivities are calculated using the Müller-Plathe approach (also called non-equilibrium MD simulation). Vacancy defects decrease the thermal conductivity whereas the substitution of nitrogen at vacancy sites improves the thermal conductivity. Quantum correction of the calculated results produces a thermal conductance temperature dependence that is in qualitative agreement with experimental data.     

Thermomechanical properties of graphene: valence force field model approach

Using the valence force field model of Perebeinos and Tersoff (2009 Phys. Rev. B 79 241409(R)), different energy modes of suspended graphene subjected to tensile or compressive strain are studied. By carrying out Monte Carlo simulations it is found that: (i) only for small strains (|ε| 相似文献   

Dislocation density-based constitutive model for the mechanical behaviour of irradiated Cu

Performance degradation of structural steels in nuclear environments results from the formation of a high number density of nanometre-scale defects. The defects observed in copper-based alloys are composed of vacancy clusters in the form of stacking fault tetrahedra and/or prismatic dislocation loops that impede the motion of dislocations. The mechanical behaviour of irradiated copper alloys exhibits increased yield strength, decreased total strain to failure and decreased work hardening as compared to their unirradiated behaviour. Above certain critical defect concentrations (neutron doses), the mechanical behaviour exhibits distinct upper yield points. In this paper, we describe the formulation of an internal state variable model for the mechanical behaviour of such materials subject to these (irradiation) environments. This model has been developed within a multiscale materials-modelling framework, in which molecular dynamics simulations of dislocation–radiation defect interactions inform the final coarse-grained continuum model. The plasticity model includes mechanisms for dislocation density growth and multiplication and for irradiation defect density evolution with dislocation interaction. The general behaviour of the constitutive (homogeneous material point) model shows that as the defect density increases, the initial yield point increases and the initial strain hardening decreases. The final coarse-grained model is implemented into a finite element framework and used to simulate the behaviour of tensile specimens with varying levels of irradiation-induced material damage. The simulation results compare favourably with the experimentally observed mechanical behaviour of irradiated materials.     

Thickness and in-plane elasticity of graphane

The thickness and in-plane mechanical properties (Young?s, shear modulus, Poisson?s ratios) of fully hydrogenated graphene (graphane) sheets are predicted using a molecular mechanics approach. The equilibrium lengths and bond angles distortions used for the graphane models are obtained from Density Functional Theory (DFT) simulations. Our models compare well with existing data on the uniaxial properties of graphane and graphene sheets from first principle and Molecular Dynamics (MD) simulations, highlight a special orthotropic mechanical behaviour for graphane, and identify thickness and shear stiffness values which are peculiar of hydrogenated graphene.     

Nitrogen doping and curvature effects on thermal conductivity of graphene: A non-equilibrium molecular dynamics study

In this study, the effects of nitrogen atom substitution and curvature on the thermal conductivity of graphene are studied using non-equilibrium molecular dynamics (NEMD) simulations. Using the optimized Tersoff potential proposed by Lindsay and Broido [L. Lindsay, D.A. Broido, Phys. Rev. B 82 (2010) 205441], the predicted thermal conductivity of graphene is close to the experimental range. It was observed that only 1% concentration of nitrogen doping in graphene decreases the thermal conductivity of graphene by more than 50% and removes its chirality dependency. Our simulations also show that graphene is a high flexible structure and suggest limited curvature effects on its thermal conductivity.