γ辐照对a-C: H薄膜微观组织、力学性能及摩擦学性能的影响

近年来随着核能及其核装备的发展,辐照环境下高能粒子对润滑材料服役行为的影响受到越来越多的关注. 本研究利用自行设计研制的磁控溅射系统制备a-C: H润滑薄膜,并对其进行伽马 (γ) 辐照处理. 考察γ辐照康普顿效应对a-C: H薄膜微观组织、力学性能和摩擦学性能的影响. 结果表明:经γ辐照后a-C: H薄膜存在由sp2杂化C原子结构向sp3杂化C原子结构转变的趋势,且辐照使得C-H键发生断裂,薄膜内H原子的键合能降低. 伽马辐照使得a-C: H薄膜的纳米机械性能显著提高,辐照样品的残余应力也随辐照剂量呈增加趋势. 此外,γ辐照也使得a-C: H薄膜的摩擦系数和磨损率轻微增加. 综合分析可知,γ辐照在测试剂量范围内对a-C: H薄膜的摩擦性能影响有限,但辐照诱发应力的增加是限制其在核环境中应用的主要因素.      

金属材料力学性能的辐照硬化效应

开展金属材料力学性能的辐照硬化研究对抗辐照材料的设计及工程应用具有重要意义.材料辐照损伤效应主要包括材料原子移位产生的辐照缺陷以及由核反应产生的氢、氦等气体杂质对材料性能的影响.金属材料的辐照效应主要包括辐照硬化、辐照脆化和辐照蠕变等.该文主要综述在低温(T<0.3 T_m,T_m是材料的熔点温度)和低辐照剂量下,由原子移位损伤产生的辐照缺陷所导致的辐照硬化行为,即受辐照缺陷的影响,材料的强度会升高.材料的晶粒     

金属材料力学性能的辐照硬化效应

开展金属材料力学性能的辐照硬化研究对抗辐照材料的设计及工程应用具有重要意义. 材料辐照损伤效应主要包括材料原子移位产生的辐照缺陷以及由核反应产生的氢、氦等气体杂质对材料性能的影响. 金属材料的辐照效应主要包括辐照硬化、辐照脆化和辐照蠕变等. 该文主要综述在低温(T < 0.3 T_m, T_m 是材料的熔点温度) 和低辐照剂量下, 由原子移位损伤产生的辐照缺陷所导致的辐照硬化行为, 即受辐照缺陷的影响, 材料的强度会升高. 材料的晶粒尺寸、晶界以及温度等因素对多晶材料的辐照硬化具有重要影响. 金属材料力学性能的辐照硬化研究是个多尺度问题, 其宏观力学性能既取决于微观尺度上辐照缺陷导致晶粒内部结构的变化, 也取决于细观尺度上晶粒间的相互作用. 该文从实验结果、数值模拟和理论模型三方面综述金属材料力学性能的辐照硬化研究进展. 在此基础上, 展望了该领域中存在的主要科学问题.      

γ射线辐照PTFE的摩擦磨损性能研究

在干摩擦条件下,利用M200型环 -块磨损试验机对聚四氟乙稀在γ射线辐照后的摩擦磨损性能进行了实验研究,结果表明,在辐射剂量从 0增至 200KGy时,PTFE材料经γ辐照后,其耐磨性显著提高;至辐射剂量达 300KGy时,其磨损量降至最低,降幅达 94. 4%;其后,随辐射剂量的进一步增加,其磨损量未再有显著的变化。其耐磨效果与在PTFE基体中填充无机 "硬粒子"的效果相当,但与填充无机粒子相比,辐射PTFE对摩擦对偶面没有 "研磨 "损伤。辐照后其摩擦系数略有变化,当辐射剂量从 0增加到 80KGy时,摩擦系数从 0. 214增至 0. 220,但辐射剂量进一步增至 400KGy时,摩擦系数缓慢降至 0. 184。磨屑的显微分析表明,随着辐射剂量的增加,磨屑逐渐从粗长的片状变为细小的纤维状。辐照后其硬度有所增加。文章还讨论了PTFE辐照后的耐磨机理。     

Stone-Wales拓扑缺陷对石墨烯拉伸力学性能的影响

采用Tersoff势对含Stone-Wales(SW)拓扑缺陷的单层石墨烯薄膜的单向拉伸力学性能进行了分子动力学模拟,分别研究了SW拓扑缺陷对扶手椅型和锯齿型石墨烯拉伸力学性能及变形机制的影响.研究结果表明,单个SW缺陷对两种手性石墨烯薄膜的杨氏模量几乎无影响,而对薄膜的强度、应变等力学性能和变形破坏机制的影响与手性有...     

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.     

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.      

基于分子动力学的氧化石墨烯拉伸断裂行为与力学性能研究

与石墨烯相比,氧化石墨烯(graphene oxide, GO)的亲水性、分散性和反应活性更好,更易于作为增强材料而研发生成性能超常的复合材料,但另一方面,由于其电子结构较为复杂,致使目前有关力学方面的研究存在一定差异.本文利用分子动力学方法,建立了羟基、羧基和环氧基等官能团随机分布的GO原子模型;通过单向拉伸模拟,分析了其断裂行为,结果表明,远离羟基和羧基的环氧基对断裂具有"诱导"作用,并从化学成键、体系能量和应力分布三个角度对其机理进行了阐释;此外,进一步研究了拉伸应力-应变曲线、极限强度、极限应变等力学性能与含氧官能团覆盖度间的关系,结果表明,极限强度、极限应变均随含氧官能团覆盖度的增大而呈减小趋势.分析认为,主要原因是官能团的出现对石墨烯面内的sp~2杂化形式造成了破坏,进而使得原子间键合能弱化,随着含氧官能团的覆盖度的增大,被弱化的键合能的数量和程度将越大,从而使得GO的极限强度、极限应变等越低.研究结果可为GO的基础研究和工程应用提供参考.     

基于分子动力学的氧化石墨烯拉伸断裂行为与力学性能研究

与石墨烯相比,氧化石墨烯(graphene oxide, GO)的亲水性、分散性和反应活性更好,更易于作为增强材料而研发生成性能超常的复合材料,但另一方面,由于其电子结构较为复杂,致使目前有关力学方面的研究存在一定差异.本文利用分子动力学方法,建立了羟基、羧基和环氧基等官能团随机分布的GO原子模型;通过单向拉伸模拟,分析了其断裂行为,结果表明,远离羟基和羧基的环氧基对断裂具有"诱导"作用,并从化学成键、体系能量和应力分布三个角度对其机理进行了阐释;此外,进一步研究了拉伸应力$\!$-$\!$-$\!$应变曲线、极限强度、极限应变等力学性能与含氧官能团覆盖度间的关系,结果表明,极限强度、极限应变均随含氧官能团覆盖度的增大而呈减小趋势.分析认为,主要原因是官能团的出现对石墨烯面内的sp$^{2}$杂化形式造成了破坏,进而使得原子间键合能弱化,随着含氧官能团的覆盖度的增大,被弱化的键合能的数量和程度将越大,从而使得GO的极限强度、极限应变等越低. 研究结果可为GO的基础研究和工程应用提供参考.      

不同高应变率冲击荷载作用下混凝土试样的能量耗散分析

针对混凝土材料在冲击荷载作用下能量耗散和破碎程度关联性难确定的问题,采用分离式霍普金森压杆(SHPB)完成了混凝土试块冲击试验,通过对冲击后碎块的筛分试验和应力波传播过程中应变的测试,得到了试块的能量耗散规律和破碎特征。结果表明,(1) 入射应力波的加载时间约为380 μs,入射能、反射能和吸收能随冲击气压的增大而增大;同一气压下,反射能最早达到平衡; (2) 平均应变率越大,入射能和吸收能也越大;当平均应变率为300/s左右时,能量利用率为最低值; (3) 随着入射能的增加,吸收能的增长率越来越大,当入射能达到2100 J时,吸收能近乎线性增长; (4) 随冲击气压增大,试样由拉裂破坏逐渐转移为压碎破坏,且随着材料吸收能的增大,平均破碎尺寸越来越小。当吸收能大于700 J时,吸收能对试样的平均破碎尺寸减小的影响较小。     

Effects of vacancy defect on the tensile behavior of graphene

Graphene is the strongest material but its performance is significantly weakened by vacancy defects. We use molecular dynamics simulations to investigate the tensile behavior of a graphene which contains a single vacancy defect. Our results suggest that because of the single vacancy, the fracture strength of graphene losses about 17.7%. The stress concentration around the vacancy defect leads to the destruction of nearby six-member rings structure, which forms the initial crack. The propagation direction of this crack in defective graphene is at an angle of 60° to the tensile direction initially, but then becomes perpendicular to the tensile direction.     

中子辐照金属材料的脆化模型研究

金属材料的辐照脆化问题一直以来都是核能安全领域亟待解决的关键问题之一.为了更准确地预测金属材料的辐照脆化行为,基于Johnson-Cook本构模型,将未辐照金属材料的断裂真应力取作辐照材料的断裂真应力,建立了通过辐照退火态金属材料屈服强度就能够预测其整个真应力$\!$-$\!$-$\!$应变曲线,以及断裂真应变的辐照脆化模型.实验研究了不同中子剂量辐照退火态高纯铝的准静态拉伸真应力$\!$-$\!$-$\!$应变曲线、断裂真应力和断裂真应变随辐照剂量的变化规律.结果表明,辐照剂量越高,高纯铝的屈服强度越高,断裂真应变越低,但断裂真应力几乎不变.通过TEM显微分析获得了高纯铝内部辐照缺陷的尺寸和数密度随辐照剂量的变化规律,结果表明,辐照剂量越高,孔洞的尺寸和数密度越高,但位错环尺寸和数密度始终很小,难以准确统计.由辐照高纯铝实验数据拟合得到了辐照脆化模型所需参数,并检验了该模型的预测效果.结果表明,无论是通过实验还是显微分析得到辐照高纯铝的屈服强度,模型的预测结果均能够与实验结果较好地吻合,且模型对退火态高纯铝临界中子剂量的预测值也与文献结果一致.      

Molecular dynamics simulations of ion-irradiation induced deflection of 2D graphene films

Ion-irradiation induced surface stress generation and the resulting deflection of 2D cantilever graphene films is studied using molecular dynamics (MD) simulations. The simulation results show that the free-end deflection is strongly dependent on the kinetic energy of the incident ions. At low incident energies (?10 eV), the graphene film bends towards the irradiated side (upward deflection in our simulations); a transition from bending towards the irradiated side (upward deflection) to bending away from the irradiated side (downward deflection) occurs when the incident energy is ～10 eV; the downward deflection peaks at ～50 eV. Further increases of the incident energy cause the magnitude of downward deflection to decrease. The evolution of free-end deflection with respect to the number of incidences is also dependent on the incident energy. The dependence of the deflection behavior of the graphene films on the incident energy revealed by our atomistic simulations suggests the generation of intrinsic stress of different levels in the growing films. Such behavior may be attributed to competing mechanisms of production and annihilation of interstitial- and vacancy-like defects in the growing film. Understanding the dependence of thin film deflection on the incident energy provides guidelines for controlling thin film shapes at the nanometer scale using ion-beam machining.     

多层氧化石墨烯的三维弹性常数及氧化度对力学性能的影响

宏观氧化石墨烯膜由多层石墨烯组成,其法向拉伸和层间剪切性能远比面内性能低。本文视多层氧化石墨烯为一种特殊的三维正交各向异性材料——横观各向同性材料,通过建立羟基和环氧基在石墨烯表面随机分布的多层氧化石墨烯三维模型,采用分子动力学方法模拟多层氧化石墨烯的面内拉伸、法向拉伸和层间剪切行为,分别得到了多层氧化石墨烯材料的全部五个独立弹性常数E2、E3、μ12、μ32和G23,进而确定了三维弹性矩阵（柔度矩阵和刚度矩阵）,并进一步分析了氧化度对弹性常数和强度的影响规律。结果表明：随着氧化度R逐步增大,多层氧化石墨烯面内杨氏模量E2和拉伸强度σ2max逐步降低,法向杨氏模量E3和拉伸强度σ3max、层间剪切模量G23和剪切强度τ23max均逐步增大,而对泊松比的影响较小;拉伸和剪切断裂破坏位置由氧化基团(羟基和羧基)与碳原子结合键能大小所决定。     

Propagation behavior of elastic waves in one-dimensional piezoelectric/piezomagnetic phononic crystal with line defects

Using a stiffness matrix method, we in- vestigate the propagation behaviors of elastic waves in one-dimensional （1D） piezoelectric/piezomagnetic （PE/PM） phononic crystals （PCs） with line defects by calculating energy reflection/transmittion coefficients of quasi-pressure and quasi-shear waves. Line defects are created by the re- placement of PE or PM constituent layer. The defect modes existing in the first gap are considered and the influences on defect modes of the material properties and volume fraction of the defect layers, the type of incident waves, the location of defect layer and the number of structural layers are discussed in detail. Numerical results indicate that defect modes are the most obvious when the defect layers are inserted in the middle of the perfect PCs; the types of incidence wave and material properties of the defect layers have important effects on the numbers, the location of frequencies and the peaks of defect modes, and the defect modes are strongly de- pendent on volume fraction of the defect layers. We hope this paper will be found useful for the design of PE/PM acoustic filters or acoustic transducer with PCs structures.     

Harmonic model of graphene based on a tight binding interatomic potential

Like in many other materials, the presence of topological defects in graphene has been demonstrated to modify its behavior, thus enhancing features aimed at several technological applications, more specifically, its electronic and transport properties. In particular, pristine defect-free graphene has been shown to be of limited use for semiconductor-based electronics, whereas the presence of individual or cluster defect rings along grain boundaries hinders electron transport and introduce a transport gap, unveiling the possibility of novel electronic device applications based on the structural engineering of graphene-based materials. In this work, we present an atomic bondwise force-constant model from the tight binding potential by Xu et al. (1992), that accounts for the electron-mechanical coupling effects in graphene. First we verify that this computational scheme is capable of accurately predicting the defect energies and core structures of dislocation dipoles based on the theory of discrete dislocations of Ariza and Ortiz (2005). In order to demonstrate our ability to characterize the effect of patterned distributions of structural defects on the electronic structure of graphene, we present the electronic band structures and density of states curves of several defective graphene sheets.     

An atomistic and non-classical continuum field theoretic perspective of elastic interactions between defects (force dipoles) of various symmetries and application to graphene

Force multipoles are employed to represent various types of defects and physical phenomena in solids: point defects (interstitials, vacancies), surface steps and islands, proteins on biological membranes, inclusions, extended defects, and biological cell interactions among others. In the present work, we (i) as a prototype simple test case, conduct quantum mechanical calculations for mechanics of defects in graphene sheet and in parallel, (ii) formulate an enriched continuum elasticity theory of force dipoles of various anisotropies incorporating up to second gradients of strain fields (thus accounting for nonlocal dispersive effects) instead of the usual dispersion-less classical elasticity formulation that depends on just the strain (c.f. Peyla, P., Misbah, C., 2003. Elastic interaction between defects in thin and 2-D films. Eur. Phys. J. B. 33, 233-247). The fundamental Green's function is derived for the governing equations of second gradient elasticity and the elastic self and interaction energies between force dipoles are formulated for both the two-dimensional thin film and the three-dimensional case. While our continuum results asymptotically yield the same interaction energy law as Peyla and Misbah for large defect separations (∼1/rⁿ for defects with n-fold symmetry), the near-field interactions are qualitatively far more complex and free of singularities. Certain qualitative behavior of defect mechanics predicted by atomistic calculations are well captured by our enriched continuum models in contrast to classical elasticity calculations. For example, consistent with our atomistic calculations of defects in isotropic graphene, even two dilation centers show a finite interaction (as opposed to classical elasticity that predicts zero interaction). We explicitly find the physically consistent result that the self-energy of a defect is equivalent to half the interaction energy between two identical defects when they “merge” into each other. The atomistic, classical elastic and the enriched continuum predictions are thoroughly compared for two types of defects in graphene: Stone-Wales and divacancy.