缺陷对石墨烯摩擦性能影响的分子动力学研究

采用分子动力学方法模拟硅探针在空位缺陷和Stone-Wales(SW)缺陷石墨烯上的滑移过程,研究空位缺陷和SW缺陷对石墨烯摩擦力的影响.研究结果表明:两种缺陷石墨烯摩擦力大于完美石墨烯,空位缺陷使石墨烯界面势垒增大导致能量耗散增加,摩擦力增大;SW缺陷使石墨烯表面形成凸起,阻碍探针滑移,摩擦力增大.空位缺陷石墨烯平均摩擦力随缺陷浓度的增加而增加,Y向空位缺陷石墨烯平均摩擦力大于X向,这都是由空位陷处能量势垒和缺陷与探针切向作用距离共同决定的.SW2型缺陷石墨烯摩擦力大于SW1型,X向SW2型缺陷石墨烯摩擦力大于Y向SW2型,因为存在相邻五边形碳原子环结构的石墨烯表面更容易产生凸起,摩擦力较大.以上研究结果完善了缺陷石墨烯的摩擦机制,对设计和开发石墨烯微纳器件提够了理论依据和指导.     

石墨烯辐照损伤及力学性能研究

石墨烯的加工和掺杂是其工程应用和性能开发的重要手段,离子辐照技术是实现上述目的的有效途径.利用分子动力学方法建立了硅离子辐照石墨烯和辐照后拉伸的数值模型.考虑辐照剂量、辐照能量和辐照角度这3个主要影响因素,研究了不同辐照条件下石墨烯的缺陷类型和数量,并分析了在辐照剂量影响下的拉伸破坏.结果表明:当辐照能量较小时,入射粒子会吸附在石墨烯表面.随着辐照能量的增大,入射粒子会穿透石墨烯而形成缺陷,当辐照能量到达一定值时,再无吸附原子.随着辐照剂量的增加,溅射原子和缺陷数目均增多,且缺陷类型以空位缺陷为主,其拉伸力学性能随着缺陷数量的增加而减小,二者近似成线性关系.辐照后石墨烯的拉伸破坏机理与完美石墨烯的有所不同,应力强化阶段明显缩短,缺陷带决定其起裂位置和断裂走向.      

六方氮化硼韧脆转变性能的理论研究

基于六方氮化硼原子间Tersoff势函数,本文建立了手性依赖的六方氮化硼“杆-簧”模型,得到了其在大变形下的非线性力学行为。结果表明六方氮化硼断裂模式从扶手椅型(Armchair,手性角为0o)到锯齿型(Zigzag,手性角为30o)由韧性断裂转变为脆性断裂。通过分析拉伸过程中键长键角变化情况,键角变化所起作用超过键长变化导致韧性断裂,而键长起主导作用时则为脆性断裂。探讨不同手性氮化硼非线性力学行为,获得两种断裂模式的分界点大约为手性角15o。相同条件下,石墨烯在拉伸过程中键长变化起支配作用,而氮化硼除了键长变化的影响之外,键角的变化对整个过程也有着至关重要的作用。最后,与分子动力学模拟结果对比显示本文的理论结果是可靠的。本研究对理解六方氮化硼优异的力学性能及其在微纳器件领域的应用提供了理论支撑。     

原位微拉曼测试技术在碳纳米管纤维和薄膜材料力学性能研究中的应用

利用宏观应力联合原位微拉曼测试技术对双壁碳纳米管(Carbon Nanotube,CNT)纤维和薄膜材料的力学性能进行了实验分析,探讨了拉伸加载期间纤维和薄膜内CNT的载荷响应及其与宏观力学性能的关联,揭示了两种材料力学性能差异性的微观机理。实验分析表明,CNT纤维和薄膜的拉伸变形呈现弹性、强化和损伤断裂三个阶段,但其内的CNT只发生弹性变形,没有塑性形变,且没有明显的损伤或键的断裂,纤维和薄膜呈现阶段性拉伸变形的原因可归结为滑移。纤维的弹性模量显著高于薄膜,是薄膜的4.7倍,原因是弹性阶段纤维中CNT的轴向伸长对宏观应变的贡献较大。纤维和薄膜的拉伸强度相差较小,原因是强化阶段薄膜内不断有大量CNT进入承载队伍,这也使得薄膜具有比纤维更高的韧性。     

含缺陷双层石墨烯的纳米压痕模拟研究

石墨烯具有独特的力学、电学性能,被誉为是具有战略意义的新材料,具有广泛的应用前景. 目前生产的石墨烯含有各种缺陷,相较于完美石墨烯,其仍有较大应用价值. 因此有必要研究和掌握缺陷对石墨烯性能的影响,以便在目前的生产技术下,推动其工业化应用. 采用Tersoffff 势来模拟C—C 共价键的相互作用,Lernnard-Jones 势来模拟非成键碳原子之间相互作用力,基于分子动力学模拟了金刚石压头压入含缺陷双层石墨烯的纳米压痕过程,讨论了Lernnard-Jones 势函数的截断半径最佳值以及得到了典型的载荷-位移曲线. 重点探讨了Stone-Thrower-Wales、空位(包括单空位和双空位缺陷) 以及圆孔缺陷当位置不同和数目不同时对石墨烯力学性能的影响. 得出结论:薄膜中心存在缺陷时,破坏强度下降幅度特别明显. 空位缺陷在压头半径范围内存在时,临界载荷与缺陷与薄膜中心的距离成线性关系;缺陷数目越多,其杨氏模量、破坏强度等就越低. 圆孔缺陷数目在压头范围外达到一定浓度后会使石墨烯的力学性质显著降低. 本文结论也说明石墨烯结构稳定,对小缺陷不敏感,缺陷石墨烯仍具有较好的性能和使用价值.      

褶皱与晶界偶合作用对石墨烯断裂行为的影响

由CVD方法制备的石墨烯含有大量的晶界,通常还带有许多褶皱,本文通过分子动力学方法研究了具有褶皱和晶界的石墨烯平面拉伸断裂行为,结果显示,在垂直晶界方向,褶皱能够显著提高小角度双晶石墨烯的断裂应力,断裂应力增幅最大约为50%,褶皱对断裂应力的影响随晶界角的增大减弱,导致双晶石墨烯断裂应力对晶界角不敏感,只略低于单晶石墨烯,和实验结果完全吻合;在沿晶界方向,褶皱对双晶石墨烯断裂应力影响不明显.另外,褶皱可以显著提高双晶石墨烯的断裂应变,增幅最大约为100%.增强机制归纳主要如下:通过面外变形,褶皱可以部分释放晶界5-7环中C—C键的预拉伸变形,提高双晶石墨烯的断裂应力;褶皱可以降低相邻5-7环之间相互作用,导致断裂应力对晶界角不敏感;在拉伸作用下,褶皱被部分拉平,这可以显著降低C—C键面内拉伸变形,导致断裂应变显著增大.本研究为准确理解多晶石墨烯断裂行为提供重要帮助.     

石墨烯与柔性基底界面载荷传递的拉曼光谱实验

本文研究了CVD制备的大尺寸石墨烯与柔性PET基底在拉伸变形过程中切向界面载荷传递的问题,采用原位拉曼光谱实验给出了加载过程中石墨烯的正应变、正应力以及界面切应力的分布曲线。分析表明,石墨烯与PET基底间的载荷传递存在四个阶段,分别是初始阶段、粘附阶段、滑移阶段和界面脱粘破坏阶段。在此基础上,本文对50μm、140μm、270μm和600μm四种尺寸石墨烯试件的界面力学性能进行测量,得到了不同尺寸石墨烯试件的界面力学性能参数,并初步给出了基底变形引起的石墨烯切向界面粘接能的变化,同时分析了试件尺寸对石墨烯界面力学性能的影响。实验结果表明,石墨烯材料和柔性基底最大切应力与临界脱粘切向界面粘接能等界面强度指标受到尺寸的显著影响,尺寸越小切向界面强度越高,反之,尺寸越大则越低。     

褶皱与晶界耦合作用对石墨烯断裂行为的影响

由CVD方法制备的石墨烯含有大量的晶界,通常还带有许多褶皱,本文通过分子动力学方法研究了具有褶皱和晶界的石墨烯 平面拉伸断裂行为,结果显示,在垂直晶界方向,褶皱能够显著提高小角度双晶石墨烯的断裂应力,断裂应力增幅最 大约为50%,褶皱对断裂应力的影响随晶界角的增大减弱,导致双晶石墨烯断裂应力对晶界角不敏感,只略低于单晶石墨 烯,和实验结果完全吻合;在沿晶界方向,褶皱对双晶石墨烯断裂应力影响不明显. 另外,褶皱可以显著提高双晶石墨烯的断 裂应变,增幅最大约为100%. 增强机制归纳主要如下:通过面外变形,褶皱可以部分释放晶界5-7环中C---C键的预拉伸变形, 提高双晶石墨烯的断裂应力;褶皱可以降低相邻5-7环之间相互作用,导致断裂应力对晶界角不敏感;在拉伸作用下,褶皱被部分 拉平,这可以显著降低C---C键面内拉伸变形,导致断裂应变显著增大. 本研究为准确理解多晶石墨烯断裂行为提供重要帮助.      

褶皱与晶界耦合作用对石墨烯断裂行为的影响

由CVD方法制备的石墨烯含有大量的晶界,通常还带有许多褶皱,本文通过分子动力学方法研究了具有褶皱和晶界的石墨烯平面拉伸断裂行为,结果显示,在垂直晶界方向,褶皱能够显著提高小角度双晶石墨烯的断裂应力,断裂应力增幅最大约为50%,褶皱对断裂应力的影响随晶界角的增大减弱,导致双晶石墨烯断裂应力对晶界角不敏感,只略低于单晶石墨烯,和实验结果完全吻合;在沿晶界方向,褶皱对双晶石墨烯断裂应力影响不明显.另外,褶皱可以显著提高双晶石墨烯的断裂应变,增幅最大约为100%.增强机制归纳主要如下:通过面外变形,褶皱可以部分释放晶界5-7环中C—C键的预拉伸变形,提高双晶石墨烯的断裂应力;褶皱可以降低相邻5-7环之间相互作用,导致断裂应力对晶界角不敏感;在拉伸作用下,褶皱被部分拉平,这可以显著降低C—C键面内拉伸变形,导致断裂应变显著增大.本研究为准确理解多晶石墨烯断裂行为提供重要帮助.     

利用数字散斑相关法测定聚酰亚胺/SiO2合成薄膜的力学性能

聚酰亚胺 /SiO2 合成薄膜是一种具有优良力热光电性能的薄膜材料,在MEMS工艺中具有广阔的应用前景,其力学性能测量的研究具有非常重要的意义。本文将数字散斑相关技术和微拉伸试验相结合,对厚度为 37μm的聚酰亚胺(polyimide) /二氧化硅(SiO2 )合成薄膜进行了力学性能测量,获得了比较满意的结果。文章给出了测得的弹性模量和泊松比。为了解决数字散斑相关法不能直接测量较大变形的缺陷,本文提出了多级相关算法,并利用亚像素搜索和双线性插值进行了数据处理。     

含冷却孔镍基合金次级取向效应的应变梯度晶体塑性有限元研究

单晶镍基合金具有优异的耐高温、高强、高韧等性能, 这些力学性能受制造过程引入的次级取向和冷却孔的影响. 已有研究大多关注单孔薄板的变形机理和力学性能, 而工程中应用的往往是多孔薄板, 当前亟需阐明多孔的塑性滑移带变形机理、次级取向效应以及冷却孔引起的应变梯度效应. 文章采用基于位错机制的非局部晶体塑性本构模型对含冷却孔镍基单晶薄板的单拉变形进行了数值模拟. 此模型基于塑性滑移梯度与几何必需位错的关系引入了位错流动项, 因此可有效刻画非均匀变形过程中的应变梯度效应. 为了全面揭示含孔镍基薄板的次级取向效应, 系统研究了[100]和[110]取向(两种次级取向)下镍基薄板的单拉变形行为, 并重点探究了在两种次级取向下冷却孔数量对薄板塑性行为的影响. 此外, 还分析了镍基合金板变形过程中各个滑移系上分切应力变化、主导滑移系开动以及几何必需位错密度的演化过程, 并讨论了塑性滑移量及其分布特征对不同次级取向镍基合金板强度的影响. 研究表明, 单孔和多孔的[110]薄板抗拉强度均低于[100]薄板, 多孔薄板的塑性变形过程比单孔薄板更为复杂且受次级取向影响更大, 并且发生滑移梯度位置主要位于冷却孔附近以及塑性滑移带区域. 研究结果可为工程中镍基合金的设计和服役提供理论指导.      

Young’s modulus prediction of hexagonal nanosheets and nanotubes based on dimensional analysis and atomistic simulations

The present work investigates Young’s modulus of hexagonal nanosheets and nanotubes based on dimensional analysis and molecular mechanics. Using second derivatives of the strain energy density revealed from molecular dynamics simulations at 0 K (i.e., molecular mechanics) with harmonic potentials for various combinations of force constants, Young’s modulus have been computed for single-walled armchair and zigzag nanotubes of different radii. This parametric study with the aid of dimensional analysis allows explicitly establishing Young’s modulus of (n, n) armchair and (n, 0) zigzag nanotubes as functions of the force constants, bond length and chiral index n. Proposed formulae are applied to estimate Young’s modulus of graphene, boron nitride, silicon carbide sheets and their nanotubes. The accuracy of the proposed formulae are verified and discussed with available data in the literature for these three sheets and their nanotubes.     

EFFECTS OF SI,N AND B DOPING ON THE MECHANICAL PROPERTIES OF GRAPHENE SHEETS

Molecular dynamics(MD) simulations were performed to stretch the rectangular graphene sheets doped with silicon, nitrogen or boron atoms. Young's modulus, ultimate stress(strain) and energy absorption were measured for the graphene sheets with the doping concentration(DC) ranging from 0 to 5%. The emphasis was placed on the distinct effects of each individual dopant on the fundamental mechanical properties of graphene. The results indicated that incorporating the dopants into graphene led to an almost linear decrease in Young's modulus. Monotonic reductions in ultimate strength, ultimate strain and energy absorption were also observed. Such doping effects were found to be most significant for silicon, less pronounced for boron, and small or negligible for nitrogen. The outputs provide an important guidance for the development and optimization of novel nanoscale devices, and facilitate the development of graphene-based M/NEMS.     

An analytical molecular structural mechanics model for the mechanical properties of carbon nanotubes

An analytical molecular structural mechanics model for the prediction of mechanical properties of defect-free carbon nanotubes is developed by incorporating the modified Morse potential with an analytical molecular structural model. The developed model is capable of predicting Young’s moduli, Poisson’s ratios and stress–strain relationships of carbon nanotubes under tension and torsion loading conditions. Results on the mechanical properties of single-walled carbon nanotubes show that Young’s moduli of carbon nanotubes are sensitive to the tube diameter and the helicity. Young’s moduli of both armchair and zigzag carbon nanotubes increase monotonically and approach Young’s modulus of graphite when the tube diameter is increased. The nonlinear stress–strain relationships for defect-free nanotubes have been predicted, which gives a good approximation on the ultimate strength and strain to failure of nanotubes. Armchair nanotubes exhibit higher tensile strength than zigzag nanotubes but their torsion strengths are identical based on the present study. The present theoretical investigation provides a very simple approach to predict the mechanical properties of carbon nanotubes.     

多晶石墨烯拉伸力学性能及其影响因素的灵敏度分析

晶粒尺寸、温度和应变率等对纳米材料的力学性能有重要影响。本文通过分子动力学（MD）数值模拟,分析了不同晶粒尺寸多晶石墨烯在不同温度、拉伸应变率下的杨氏弹性模量、极限应力、极限应变等拉伸力学性能。结果表明,晶粒尺寸、温度和拉伸应变率对拉伸力学性能有较大影响。利用正交实验法,分别分析了杨氏弹性模量、极限应力和极限应变对晶粒尺寸、温度和拉伸应变率的敏感程度。结果表明,杨氏弹性模量和极限应力对影响因素的敏感程度由大到小依次为晶粒尺寸、温度和拉伸应变率;极限应变对影响因素的敏感程度由大到小依次为晶粒尺寸、拉伸应变率和温度。研究结果可为多晶石墨烯的理论研究和工程应用提供参考。     

An accurate molecular mechanics model for computation of size-dependent elastic properties of armchair and zigzag single-walled carbon nanotubes

In this paper, an analytical solution based on a molecular mechanics model is developed to evaluate the mechanical properties of armchair and zigzag single-walled carbon nanotubes (SWCNTs). Adopting the Perdew–Burke–Ernzerhof (PBE) exchange correlation, the density functional theory (DFT) calculations are performed within the generalized gradient approximation (GGA) to evaluate force constants used in the molecular mechanics model. After that, based on the principle of molecular mechanics, explicit expressions are proposed to obtain surface Young’s modulus, Poisson’s ratio and surface shear modulus of SWCNTs corresponding to both types of armchair and zigzag chiralities. Based on the DFT calculations, it is found that the flexural rigidity of graphene is independent of the type of chirality which indicates the isotropic characteristic of this material. Moreover, it is observed that for the all values of nanotube diameter, surface Young’s modulus for the armchair nanotube is more than that of zigzag nanotube. It is shown that the trend predicted by the present model is in good agreement with other models which confirms the validity as well as the accuracy of the present molecular mechanics model.     

Loading direction-dependent shear behavior at different temperatures of single-layer chiral graphene sheets

The loading direction-dependent shear behavior of single-layer chiral graphene sheets at different temperatures is studied by molecular dynamics(MD) simulations.Our results show that the shear properties(such as shear stress–strain curves, buckling strains, and failure strains) of chiral graphene sheets strongly depend on the loading direction due to the structural asymmetry. The maximum values of both the critical buckling shear strain and the failure strain under positive shear deformation can be around 1.4 times higher than those under negative shear deformation. For a given chiral graphene sheet, both its failure strain and failure stress decrease with increasing temperature. In particular, the amplitude to wavelength ratio of wrinkles for different chiral graphene sheets under shear deformation using present MD simulations agrees well with that from the existing theory. These findings provide physical insights into the origins of the loading direction-dependent shear behavior of chiral graphene sheets and their potential applications in nanodevices.     

STABILITY CHARACTERISTICS OF SINGLE-LAYERED ZINC OXIDE NANOSHEETS UNDER UNIAXIAL LOADING

This paper is aimed to propose a three-dimensional model which would be used for investigation on the mechanical behavior of single-layered zinc oxide nanosheets. To develop this model, molecular mechanics is coupled with the density functional theory. Simulating the hexagonal lattices of nanosheets as a hexagonal mechanical structure composed of structural beam elements, the buckling behavior of zinc oxide nanosheets is studied. Effects of different parameters on the stability of armchair and zigzag nanosheets are examined. It is shown that the buckling forces of zigzag nanosheets are slightly greater than those of armchair ones. However, with increasing size of nanosheets the effect of atomic structure on the stability of nanosheets diminishes.By studying the effect of end conditions on the buckling behavior of nanosheets, it is shown the stability of nanosheets is affected significantly by boundary conditions.