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.     

Elastic properties of SWCNTs with curved morphology: Density functional tight binding based treatment

The self-consistent charge density functional based tight-binding method is used to calculate the effect of curvature on the structure, average energy of atoms and Young's modulus of armchair single-wall carbon nanotubes (SWCNTs) under axial strains. We found that as the amount of curvature increases, the average energy of atoms and the Young's modulus decrease and the equilibrium CC distance increases for (7,7) SWCNTs. However, we also found that the average energy of atoms and Young's modulus of (5,5) SWCNTs are weakly affected by increasing the amount of curvature. Our results also show that the average energy of atoms and Young's modulus of smaller diameter armchair nanotubes are smaller than that of the larger diameter ones.     

分子动力学研究表面缺陷对硅纳米线杨氏模量的影响

硅纳米线因受量子尺寸效应与表面效应的影响而具有奇特的力、电及其耦合特性,成为了纳米电子器件的核心构件.然而在硅纳米线的制备过程中,表面产生缺陷不可避免.因此本文采用分子动力学方法着重研究了表面缺陷浓度对不同横截面形状(正方形、六角形和三角形)的[110]晶向和[111]晶向硅纳米线杨氏模量的影响.研究结果表明,当硅纳米线仅有单一表面缺陷时,不同晶向硅纳米线的杨氏模量均随表面缺陷浓度增加而迅速单调减小.当表面缺陷浓度为10%时,杨氏模量的减小幅度在10%-20%之间,减小幅度的差异与硅纳米线的晶向以及横截面形状密切相关.当存在多个表面缺陷时,杨氏模量随着缺陷浓度的增加表现出了不同程度的波动趋势.三角形截面硅纳米线的杨氏模量波动幅度最大,正方形截面的波动较小,即表面缺陷分布的不同对正方形截面硅纳米线的杨氏模量影响较小,这表明表面缺陷的影响与其分布及硅纳米线的横截面形状密切相关.通过与实验结果对比,本文的研究结果揭示了表面缺陷是导致硅纳米线杨氏模量实验值变小的重要因素,因此在表征硅纳米线的力学性能时,需要考虑表面缺陷的影响.     

Predictions of Young's Modulus of Polymer Composites Reinforced with Short Natural Fibers

Polymers reinforced with natural fibers are beneficial to prepare biodegradable composite materials. A new expression for the Young's modulus of short, natural fiber (SNF) reinforced polymer composites was derived based on a micro-mechanical model. The Young's moduli of poly(lactic acid) reinforced with reed fibers and low-density polyethylene (LDPE) reinforced with sisal fibers, from literature data, were estimated in the fiber weight fraction range from 0 to 50% using the equation and both the compounding rule and the Halpin–Tsai equation, and the estimations were compared with the reported measured data. The results showed that the predictions of the Young's moduli by means of the new Young's modulus equation were close to the measured data from the low density polyethylene/sisal fiber composites, as well as the poly(lactic acid)/reed composites at high fiber concentration. Comparing with other Young's modulus equations, the new Young's modulus equation would be more convenient to use owing to the parameters in the equation being easily determined.     

单层石墨烯杨氏模量的理论模型

石墨烯力学性能的研究对其在半导体技术中的应用是十分重要的,本文基于半连续体模型并结合石墨烯纳米结构特性,通过对原子的描述构建了石墨烯形变分量和位移分量的新关系,从而给出了单层石墨烯结构形变能,并计算了不同尺寸单层石墨烯的杨氏模量值.通过对不同方向杨氏模量的分析,讨论了单层石墨烯的手性行为.结果表明:随着尺寸的增加,单层石墨烯两个方向的杨氏模量分别趋于0.746 TPa和0.743 TPa,当尺寸相同时,两方向杨氏模量的最大差值不超过0.003 TPa,此结果与文献报道结果相符.在小应变情况下,单层石墨烯薄膜呈各向同性,且薄膜尺寸变化对该特性影响不大.该计算结果对研究石墨烯的其它力学特性提供一定的参考价值.     

Young's Modulus Characterization of Nano-Level Silicon Nanomembrane

Silicon nanomembrane (SiNM) has drawn great attention for the application in nanoelectrical devices as it shows excellent flexibility and is compatible with the integrated circuit process. The mechanical property measurement of the SiNM with nanoscale thickness is critical. A suspended SiNM (40 nm thick) for mechanical measurements is fabricated by transferring a chemically etched ultrathin monocrystalline silicon film from silicon on insulator wafer to a substrate with a multi-hole array. And then, the atomic force probe is utilized to load force on the free-standing SiNM to obtain a force deflection curve, and then the Young's modulus of such floating SiNM can be directly calculated based on the large deflection plane model. It shows that the Young's modulus of such SiNM is basically consistent with that of the bulk silicon. However, the SiNMs’ floating area significantly affects the results, i.e., the Young's modulus varies with the ratio of the suspended area diameter (i.e., hole diameter) to the film thickness. The Young's modulus is independent of hole diameter when the ratio is greater than 425. According to this relationship, the variation of Young's modulus can be predicted for arbitrary thick SiNMs and any transferable nanofilms.     

Theoretical study on the structural,mechanical, electronic properties and QTAIM of CrB_4 as a hard material

Using the first-principles calculations based on spin density functional theory(DFT), we investigate the structure,elastic properties, and electronic structure of Pnnm-CrB4. It is found that Pnnm-CrB_4 is thermodynamically and mechanically stable. The calculated elastic properties such as the bulk modulus, shear modulus, Young's modulus, and Poisson's ratio indicate that CrB_4 is an incompressible material. Vicker's hardness of Pnnm-CrB_4 is estimated to be 26.3 GPa, which is in good agreement with the experimental values. The analysis of the investigated electronic properties shows that PnnmCrB_4 has the metallic character and there exist strong B–B and Cr–B bonds in the compound, which are further confirmed by Bader's quantum theory of atoms in molecules(QTAIM). Thermodynamic properties are also investigated.     

Mechanical and Thermal Performance of High-Density Polyethylene/Alumina Nanocomposites

High-density polyethylene (HDPE) nanocomposites reinforced with pristine and vinyltrimethoxysilane (VTMS)-treated alumina nanoparticles of 2, 4, and 6 wt% were melt-compounded in a twin-screw extruder followed by injection molding. Their structure, thermal and mechanical behaviors were studied. Fourier transform infrared (FTIR) spectra showed that VTMS was successfully covalently grafted to the alumina nanoparticles. The X-ray diffraction (XRD) patterns indicated that the alumina nanoparticle additions broadened the characteristic peak width of HDPE, indicating that they reduced the crystallite size of HDPE. The heat deflection temperature and thermogravimetric analyses demonstrated that the dimensional and thermal stability of HDPE were enhanced markedly by adding pristine and silane-treated alumina nanoparticles. The alumina nanoparticle additions were also beneficial in enhancing Young's modulus and yield strength of HDPE. The reinforcing effect was particularly apparent in the silane-treated nanocomposites due to improved filler–matrix interactions.     

The influence of grain size and texture on the Young's modulus of nanocrystalline nickel and nickel–iron alloys

Hardness and Young's modulus were measured by nanoindentation on a series of electrodeposited nanocrystalline nickel and nickel–iron alloys. Hardness values showed a transition from regular to inverse Hall–Petch behaviour, consistent with previous studies. There was no significant influence of grain size on the Young's modulus of nanocrystalline nickel and nickel–iron alloys with grain sizes greater than 20?nm. The Young's modulus values for nanocrystalline nickel and nickel–iron alloys for grain sizes less than 20?nm were slightly reduced when compared to their conventional (randomly oriented) polycrystalline counterparts. The observed trend with decreasing grain size was found to be consistent with composite model predictions that consider the influence of intercrystalline defects. However, there was some notable variability of the measured values when compared to the model predictions. Three theoretical relationships were used to characterise the anisotropic elastic behaviour of these materials. As a result, texture was also considered to have an influence on the measured Young's modulus and used to explain some of the observed variability for the entire grain size range (9.8–81?nm).     

Reverse analysis in depth-sensing indentation for evaluation of the Young's modulus of thin films

This paper presents an approach to reverse analysis in depth-sensing indentation of composite film/substrate materials, which makes use of numerical simulation. This methodology allows the results of experimental hardness tests, acquired with pyramidal indenter geometry, to be used to determine the Young's modulus of thin film materials. Forward and reverse analyses were performing using three-dimensional numerical simulations of pyramidal and flat punch indentation tests to determine the Young's modulus of the thin films. The pyramidal indenter used in the numerical simulations takes into account the presence of the most common imperfection of the tip, so-called offset. The contact friction between the Vickers indenter and the deformable body is also considered. The forward analysis uses fictitious composite materials with different relationships between the values of the Young's modulus of the film and substrate. The proposed reverse analysis procedure provides a unique value for the film's Young's modulus. Depending on material properties, the value of the Young's modulus of the film can be more or less sensitive to the scatter of the experimental results obtained using the depth-sensing equipment. The validity of the proposed reverse analysis method is checked using four real cases of composite materials.     

Young's modulus measurement of the radius bone using a shearing interferometer with carrier fringes

A technique for measuring Young's modulus of the radius bone by using electronic speckle-shearing pattern interferometry is proposed. The technique is based on the introduction of carrier fringes and the use of a phase unwrapping algorithm to obtain a phase map corresponding to the out-of-plane displacements of a radius bone without sign ambiguities. The carrier fringes are obtained by the correlation algorithm of two consecutive speckle fields with a translation of the illuminating beam between each recording, then the spatial synchronous detection (SSD), an unwrapping phase algorithm and integration process along shear direction are successfully applied for obtaining a displacement phase map. Next, analysis of the radius bone as a cantilever allows us to obtain Young's modulus. The study was performed with speckle interferometry and with the finite element method for comparing the results. Advantages and disadvantages of the proposed method are discussed.     

Temperature-dependent elastic moduli of lead telluride-based thermoelectric materials

In the open literature, reports of mechanical properties are limited for semiconducting thermoelectric materials, including the temperature dependence of elastic moduli. In this study, for both cast ingots and hot-pressed billets of Ag-, Sb-, Sn- and S-doped PbTe thermoelectric materials, resonant ultrasound spectroscopy (RUS) was utilized to determine the temperature dependence of elastic moduli, including Young's modulus, shear modulus and Poisson's ratio. This study is the first to determine the temperature-dependent elastic moduli for these PbTe-based thermoelectrics, and among the few determinations of elasticity of any thermoelectric material for temperatures above 300 K. The Young's modulus and Poisson's ratio, measured from room temperature to 773 K during heating and cooling, agreed well. Also, the observed Young's modulus, E, versus temperature, T, relationship, E(T) = E ₀(1–bT), is consistent with predictions for materials in the range well above the Debye temperature. A nanoindentation study of Young's modulus on the specimen faces showed that both the cast and hot-pressed specimens were approximately elastically isotropic.     

The composition-dependent mechanical properties of Ge/Si core–shell nanowires

The Stillinger–Weber potential is used to study the composition-dependent Young's modulus for Ge-core/Si-shell and Si-core/Ge-shell nanowires. Here, the composition is defined as a ratio of the number of atoms of the core to the number of atoms of a core–shell nanowire. For each concerned Ge-core/Si-shell nanowire with a specified diameter, we find that its Young's modulus increases to a maximal value and then decreases as the composition increases. Whereas Young's modulus of the Si-core/Ge-shell nanowires increase nonlinearly in a wide compositional range. Our calculations reveal that these observed trends of Young's modulus of core–shell nanowires are essentially attributed to the different components of the cores and the shells, as well as the different strains in the interfaces between the cores and the shells.     

Combining Brillouin spectroscopy and laser-SAW technique for elastic property characterization of thick DLC films

Surface Brillouin spectroscopy (SBS) has been widely used for elastic property characterization of thin films. For films thicker than 500 nm, however, the wavelength of surface acoustic wave in the frequency range available for SBS is smaller than film thickness, and the SBS measures only the Rayleigh wave of the film. The laser-SAW technique, on the other hand, measures only the low-frequency portion of the surface acoustic wave dispersion and can estimate only one elastic modulus of the film (typically Young's modulus). In this work, we have combined the two methods to determine both Young's modulus and Poisson's ratio of a diamond-like carbon (DLC) film. It was found that reasonable estimates can be obtained for the longitudinal wave velocity, shear wave velocity, and Young's modulus of the film. The Poisson's ratio, however, still has a relatively large measurement error.     

Surface waves and mode conversion at a surface coated with an elastic heavy layer

A surface wave of frequency lying within bulk band of transverse waves is found in an elastic medium coated with a thin layer endowed with a surface mass density, surface Young's modulus and surface bending modulus. The wave is a particular case of surface resonance with infinite lifetime. In materials with negative Poisson's ratio (auxetics) the wave exists even for coating material with zero bending modulus, whereas with positive Poisson's ratio it requires the surface bending modulus to be larger than the surface Young's modulus. The manifestation of this wave in the reflection coefficient seems promising for fabrication of devices showing monochromator properties.     

Surface Tension and Shear Strain Contributions to the Mechanical Behavior of Individual Mg-Ni-Phyllosilicate Nanoscrolls

Some phyllosilicate compounds have the ability of spontaneous scrolling because of the size mismatch between the covalently bounded metal oxide and silica sheets. Their unique structure and high theoretically predicted Young's modulus (around 210–230 GPa) induce phyllosilicates’ application as reinforcing fillers. However, previous nanomechanical experiments with individual phyllosilicate nanoscrolls are in poor agreement with theory. The main reason for this is the low accuracy of experiments, which leads to large measurement errors compared to measured average values. Here, the study of the mechanical properties of synthetic (Mg_1–xNi_x)₃Si₂O₅(OH)₄ phyllosilicates is reported by testing a suspended nanoobject (a nanobridge) with an atomic force microscope (AFM). The Young's modulus of corresponding phyllosilicate model layers is also calculated by means of the density functional theory (DFT). The original AFM approach makes it possible to account for the probe slipping off the nanobridge and determine its boundary conditions. The measured Young's modulus values are considered within the models of surface tension and shear strain contributions. The shear strain appears to have a decisive impact on the measured Young's modulus (from 150 ± 70 GPa to 200 ± 210 GPa) and its spread.     

Nanoindentation of gold nanoparticles functionalized with proteins

The hardness and Young's modulus of 10 and 20 nm gold nanoparticles (Au NPs) modified with bovine serum albumin and streptavidin were measured using a nanoindenter. The Au NPs were immobilized on a semiconductor surface through organic self-assembled monolayers. Changes in mechanical properties occurred when the Au NPs were immobilized on the surface. The hardness and Young's modulus were dependent on the size of the NPs, and the proteins on the particles showed highly plastic and elastic behavior compared to flat surfaces modified with self-assembled monolayers.     

Ca0.5Sr0.5TiO3弹性和热学性质的第一性原理研究

利用能量最小原理, 确定了Ca_0.5Sr_0.5TiO₃晶体中4c位置的Ca/Sr原子对称分布, 建立了Ca_0.5Sr_0.5TiO₃稳定的晶体结构, 在此基础上利用基于密度泛函理论第一性原理的平面波超软赝势方法, 采用局域密度近似和广义梯度近似函数, 计算了Ca_0.5Sr_0.5TiO₃的晶格参数、弹性常数、体弹模量、剪切模量、杨氏模量、泊松比, 并基于Christoffel方程的本征值研究了平面声波的特征, 基于Cahill和Cahill-Pohl模型研究了最小热导率的特征. 计算结果表明: Ca_0.5Sr_0.5TiO₃晶格参数和实验值很接近, 体弹模量大于剪切模量, [100], [010], [001]晶向的杨氏模量、泊松比、普适弹性常数(A^U)以及杨氏模量三维图均显示了弹性各向异性; 平面声波在(010), (001)平面呈现各向异性, 在(100)平面呈现各向同性, 平面声波大小与平均横波和平均纵波的数值很接近. Cahill模型最小热导率在各平面呈现各向同性, Cahill-Pohl模型最小热导率在高温时趋于恒定. 准谐德拜模型下Ca_0.5Sr_0.5TiO₃ 晶体的摩尔热容和热膨胀系数与CaTiO₃晶体的接近, 并且高温下具有稳定的热膨胀性能. 计算所得禁带宽度为2.19 eV, 导带底主要是Ti-3d与O-2p态电子贡献; 由电荷布居和电荷密度图理论证实Ca_0.5Sr_0.5TiO₃具有稳定的Ti-O八面体结构.     

FeCrNiCoCu纳米压痕性能的分子动力学研究

纳米压痕是研究金属特性最广泛的方法之一.因此,本文采用分子动力学方法研究了晶粒数、压痕半径和压痕速度对FeCrNiCoCu压痕性能的影响.结果表明,晶粒数从4增加到16,杨氏模量和硬度值逐渐减小,呈现反Hall-Petch现象;随着压头半径的增加,杨氏模量增大,硬度受接触面积的影响较大而减小,较大的压头半径有利于模型内部位错的产生和扩展;压入速度对杨氏模量和硬度的影响微弱,压入速度越快,位错密度越低,位错传播速度越慢.本工作以期为FeCrNiCoCu的研究提供理论指导.     

The influence of tilt grain boundaries on the mechanical properties of bicrystalline graphene nanoribbons

The mechanical properties of bicrystalline graphene nanoribbons with various tilt grain boundaries (GBs) which typically consist of repeating pentagon–heptagon ring defects are investigated based on the method of molecular structural mechanics. The GB models are constructed via the theory of disclinations in crystals, and the elastic properties and ultimate strength of bicrystalline graphene nanoribbons are calculated under uniaxial tensile loads in perpendicular and parallel directions to grain boundaries. The dependence of mechanical properties is analyzed on the chirality and misorientation angles of graphene nanoribbons, and the experimental phenomena that Young's modulus and ultimate strength of bicrystalline graphene nanoribbons can either increase or decrease with the grain boundary angles are further verified and discussed. In addition, the influence of GB on the size effects of graphene Young's modulus is also analyzed.