Molecular dynamics study on resonance frequency change due to axial-strain-induced torsions of single-walled carbon nanotubes

We investigated, via the classical MD simulation method based on Tersoff-Brenner potential, the fundamental resonance frequency changes of single-walled carbon nanotube (SWCNT) resonators originated from the purely mechanical coupling of the axial-strain-induced torsion (ASIT) response. The fundamental frequency changes were also negligible where the ASIT responses were negligible in achiral SWCNTs whereas those were explicitly found under both compression and tension for the chiral SWCNTs with the obvious ASIT responses. Specially, for SWCNT with the chiral angle of π/12, where the highest ASIT response can be found, the fundamental resonance frequency changes were highest. The fundamental resonance frequencies under the tensioning increased almost linearly with increasing the axial strain whereas they rapidly decreased under compression with increasing the compressive strain.     

结构和变形对单壁碳纳米管力学性能的影响

使用分子动力学方法模拟了单壁碳纳米管的拉伸变形行为和泊松比,并从单壁碳纳米管晶胞单元的结构特征角度,系统分析了管径、螺旋性和应变对力学性能的影响．模拟结果显示,单臂性碳纳米管(8,8)-(22,22)和锯齿性碳纳米管(9,0)-(29,0)的拉伸弹性变形可以分别达到35％-38％和20％-27％,拉伸条件下这些碳纳米管的弹性模量随管径的增大从960 GPa下降到750 GPa,并且锯齿性碳纳米管的弹性模量比单臂性碳纳米管的弹性模量要高．通过对三根具有相同直径和不同螺旋性的碳纳米管(9,9),(12,6)和(16,0)分别在拉伸和压缩条件下的模拟发现,随着变形的增大,碳纳米管的泊松比将减小;在相同的拉伸应变下,碳纳米管的泊松比随其螺旋角的减小而减小,而在相同的压缩应变下,碳纳米管的泊松比随其螺旋角的减小而增大．     

Consideration of mechanical properties of single-walled carbon nanotubes under various loading conditions

In this article, mechanical properties of single-walled carbon nanotubes (SWCNTs) with various radiuses under tensile, compressive and lateral loads are considered. Stress–strain curve, elastic modulus, tensile, compressive and rotational stiffness, buckling behaviour, and critical axial compressive load and pressure of eight different zigzag and armchair SWCNTs are investigated to figure out the effect of radius and chirality on mechanical properties of nanotubes. Using molecular dynamic simulation (MDS) method, it can be explained that SWCNTs have higher Young’s modulus and tensile stiffness than compressive elastic modulus and compressive stiffness. Critical axial force of zigzag SWCNT is independent from the radius, but that of armchair type rises by increasing of radius, also these two types show different buckling modes.     

Strain and chirality effects on the mechanical and electronic properties of silicene and silicane under uniaxial tension

We present first principles theory calculations on the mechanical and electronic properties of silicene and silicane structure under uniaxial tensile strain along different directions. Chirality effect is more significant in the mechanical properties of silicene than those of silicane. Different failure mechanisms are identified. A small band gap (up to 0.8 eV) is developed from zero with silicene structure under uniaxial tension and vanishes before the structure reaches its in-plane ultimate strength. However, a pre-existing band gap (2.39 eV) exists with silicane structure and decreases to zero with the increasing tensile strain without chirality effects.     

Elastic behavior of bilayer graphene under in-plane loadings

Due to its many superior properties, bilayer graphene is expected to serve as a proper candidate in various applications, and further provokes intensive research on how it deforms. Based on atomistic simulations, the elastic behavior of bilayer graphenes, including fracture under tension and buckling under compression, is investigated under in-plane loadings. The elastic property, e.g. Young's modulus and fracture strain, of either armchair or zigzag graphene is sensitive to both chirality and loading direction when tension is applied. However, the armchair-zigzag bilayer graphene with mixed chirality has no dependency on loading direction and its tensile rupture process is in a step-by-step manner. Under different loading histories, the bilayer graphene also exhibits quite different mechanical response. These results are useful for both further investigation and potential application of graphene in nano-electromechanical systems.     

Piezoelectricity of the ice nanotube with odd side faces

The molecular dynamics (MD) simulations have been utilized to investigate the strain effect on the polarization distribution and piezoelectric coefficient of the ice nanotube (ice-NT) with odd side faces. It is found that the polarization of the system increases under the compressive strain, and decreases with the tensile strain. The piezoelectric coefficient is about 1.3 C/m² for a single 〈5, 0〉 ice-NT with its diameter of 4.34 Å, which is superior to BaTiO₃ nanowire, the ferroelectricity of the latter with the same diameter has been vanished owning to the depolarization effect. We have also considered the axial strain energy under the different strains, and found that most of the strain energy in the compressive process seems larger than that of the stretching case.     

Transitional failure of hybrid carbon nanotubes under multiaxial loads

Transitional failure envelopes of hybrid single-walled carbon nanotubes functionalized by functional groups and filled with butane molecules under combined tension–torsion are predicted using classical molecular dynamics simulations. The observations reveal that while the tensile failure load decreases with combined torsion, the torsional buckling moment increases with combined tension. As a result, the failure envelopes under combined tension–torsion are definitely different from those under pure tension or torsion. In such combined loading, there is a multitude of failure modes (tensile failure and torsional buckling), and the failure therefore exhibits the feature of transitional failure envelopes. In addition, the functionalization by functional groups decreases both tensile failure load and torsional buckling moment, while filling with butane molecules increases only the torsional buckling moment. Consequently, the transitional failure envelopes of functionalized and filled nanotubes are absolutely different relative to what is predicted for pristine nanotubes.     

Torsion of cylindrically anisotropic nano/microtubes from seven-constant tetragonal crystals. Poynting’s effect

In the paper presented, we continue the research in the elastic properties of seven-constant tetragonal crystals and nano/microtubes. Our previous study concerned tension of this type of structures, and here we are dealing with their torsion, Poynting’s effect or axial extension under torsion with no tensile force, and torsional stiffness. It is demonstrated that there exists inverse Poynting’s effect: the tubes experience torsion under tension without applying a torque.     

Effect of misfit strain on the electrocaloric effect of P(VDF-TrFE) copolymer thin films

Based on the phenomenological Landau-Devonshire theory, we investigate the effect of misfit strain on the electrocaloric effect of P(VDF-TrFE) copolymer thin films. Theoretical analysis indicates that the compressive misfit strain reduces the working temperature to a great extent where the electrocaloric effect is maximized, which is different from the result of the conventional ferroelectric thin films, such as BaTiO₃. Although the compressive or tensile misfit strain does not change the maximum of the electrocaloric coefficient, the compressive misfit strain decreases the maximum of the adiabatic temperature change and the tensile misfit strain results in the opposite effect. Consequently, control of the misfit strain provides potential means to vary the working temperature for use in cooling systems.     

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.     

激光诱导2024铝合金表面动态应变检测

采用PVDF贴片传感器对脉冲激光作用下2024铝合金表面的动态应变进行了测量,分析了动态应变曲线的特性。结果表明,PVDF贴片传感器在动态应变测量中动态响应快,灵敏度高,可有效应用于脉冲激光诱导材料表面动态应变的实时测量。脉冲激光作用过程中,2024铝合金冲击光斑周围材料先受挤压,后压应变减小。脉冲激光作用结束后,2024铝合金冲击光斑周围材料表面粒子在卸载稀疏波和表面稀疏波的作用下不断往复运动,冲击光斑周围材料甚至受到了拉应变的作用。最后随着时间的推移,材料表面粒子的动态响应经反复震荡后逐渐衰弱形成最终的稳定状态。     

单壁碳纳米管拉伸变形的原子尺度模拟

采用分子动力学方法对单壁碳纳米管的拉伸变形行为进行了模拟,结果表明,碳纳米管具有较高的断裂应变.在结构产生缺陷之前,碳纳米管表现出弹性变形的特征.通过对能量变化的分析可以看出,能量分布的不均匀是导致结构失稳产生缺陷的主要因素.通过对含初始结构缺陷的碳纳米管在拉伸变形过程中的构型变化进行分析,发现在缺陷附近原来相邻的两个六边形蜂窝结构,随着拉伸变形的发展转变成5 7结构(Stone Wales转变),能量产生突变,应变能的释放使系统能量降低.分析也表明,较少数目的初始缺陷对碳纳米管的力学性质并不会有太大影响.     

Effect of the temperature and way of preliminary torsion in the austenitic state on the shape memory effect in TiNi alloy

The effect of temperature and direction of preliminary torsion in the austenitic state on the degree of strain recovery upon heating of a TiNi alloy has been investigated. It is shown that an increase in the preliminary deformation temperature from 500 to 700 K leads to an increase in the degree of shape recovery upon heating of the material studied. In particular, a 20% strain at a temperature of 500 K decreases the recovery coefficient by 20%, whereas the same preliminary strain at 700 K deteriorates the shape recovery by only 4%. It is established that, applying preliminary torsion in the austenitic and martensitic states in opposite directions, one can obtain an increase in the shape memory strain with an increase in the preliminary plastic strain. Thus, at some plastic strains (λ _pl > 10%), the strain recovered upon heating may even exceed the strain set in the martensitic state.     

Evolution of mechanical response and dislocation microstructures in small-scale specimens under slightly different loading conditions

In small dimensions, the flow stress of metallic samples shows a size-dependence such that smaller is stronger, even in nominally strain gradient-free loading conditions. However, the role of the boundary conditions in miniaturised tension or compression tests on the mechanical response and dislocation structure has not been studied in detail. In simulations performed with a three-dimensional discrete dislocation dynamics tool, initial, well-defined dislocation microstructures are loaded in tension with different boundary conditions including superimposed torsion moments. The influence of the loading conditions on details of the evolving dislocation microstructure was investigated by using identical starting configuration. An additional torsion moment significantly influences the dislocation activity since forest-dislocations are generated, but size effect of the flow stress is found to be unchanged.     

轴压和扭转复合载荷作用下氮化硼纳米管屈曲行为的分子动力学模拟

采用分子动力学方法模拟了氮化硼纳米管在轴压和扭转复合荷载作用下的屈曲和后屈曲行为.在各加载比例下,给出了初始线性变形阶段和后屈曲阶段原子间相互作用力的变化,确定了屈曲临界荷载关系.通过对屈曲模态的细致研究,从微观变形机理上分析了纳米管对不同外荷载力学响应的差异.研究结果表明,扶手型和锯齿型纳米管均呈现出非线性的屈曲临界荷载关系,复合加载下的屈曲行为具有强烈的尺寸依赖性.温度升高将导致屈曲临界荷载的下降,且温度的影响随加载比例的变化而变化.无论在简单加载或复合加载中,同尺寸的碳纳米管均比氮化硼纳米管具有更强地抵抗屈曲荷载的能力.     

Strain engineering for ZnO nanowires: First-principle calculations

We employ density-functional theory to investigate the strain engineering for infinitely long [0001] ZnO nanowires with rectangular cross sections. The structural and electronic properties of ZnO nanowires with uniaxial, lateral and shear strain are systemically calculated. The results show that the band-gaps of ZnONWs will decrease (increase) with increasing (decreasing) tensile (compressive) uniaxial strain. The tensile (compressive) lateral strain on {10 1̅0} surfaces will improve (reduce) the band-gaps for ZnONW with clearly nonlinear characteristic, while the change trend of band-gaps for ZnONW with lateral strain on {1 2̅10} surfaces is basically opposite. When we enhance shear strain on ZnONWs, the band-gaps are reduced. The increasing shear strain along [10 1̅0] direction will sharply reduce the band-gap and the curve is nonlinear, while the band-gap decreases nearly linearly with the increase of shear strain along [1 2̅10] direction.     

Giant Rashba spin splitting in Bi2Se3:Tl

First‐principles calculations are employed to demonstrate a giant Rashba spin splitting in Bi₂Se₃:Tl. Biaxial tensile and compressive strain is used to tune the splitting by modifying the potential gradient. The band gap is found to increase under compression and decreases under tension, whereas the dependence of the Rashba spin splitting on the strain is the opposite. Large values of α_R = 1.57 eV Å at the bottom of the conduction band (electrons) and α_R = 3.34 eV Å at the top of the valence band (holes) are obtained without strain. These values can be further enhanced to α_R = 1.83 eV Å and α_R = 3.64 eV Å, respectively, by 2% tensile strain. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Molecular dynamic investigation of mechanical properties of armchair and zigzag double-walled carbon nanotubes under various loading conditions

Using molecular dynamic simulation (MDS), effects of chirality and Van der Waals interaction on Young's modulus, elastic compressive modulus, bending, tensile, and compressive stiffness, and critical axial force of double-walled carbon nanotube (DWCNT) and its inner and outer tubes are considered. Achieving the highest safety factor, mechanical properties have been investigated under applied load on both inner and outer tubes simultaneously and on each one of them separately. Results indicate that as a compressive element, DWCNT is more beneficial than single-walled carbon nanotube (SWCNT) since it carries two times higher compression before buckling. Except critical axial pressure and tensile stiffness, in other parameters zigzag DWCNT shows higher amounts than armchair type. Outer tube has lower strength than inner tube; therefore, most reliable design of nanostructures can be attained if the mechanical properties of outer tube taken as the properties of DWCNT.     

Chirality effect of mechanical and electronic properties of monolayer MoS2 with vacancies

We present first principles theory calculations about the chirality and vacancy effects of the mechanical and electronic properties of monolayer MoS₂. In the uni-axial tensile tests, chirality effect of the mechanical properties is negligible at zero strain and becomes significant with the increasing strain, regardless of vacancies. The existence of vacancies decreases the Young's modulus and ultimate strength of the MoS₂ structure. During the uni-axial tensile tests, the band gap decreases with the increasing strain, regardless of chirality and vacancies. The band gap is reduced with the intermediate state brought by the existence of vacancies. No chirality effect can be observed on the band gap variations of perfect MoS₂. Chirality effect appears to the band gap variation of defected MoS₂ due to the local lattice relaxation near the vacancies.     

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。