单壁氮化硼纳米管压缩特性的分子动力学模拟

采用分子动力学方法,分别模拟完好的和含有缺陷的(7,7)扶手椅型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.为验证结果的可靠性,同时针对(7,7)扶手椅型碳纳米管进行对比计算.结果表明,氮化硼纳米管的压缩失效模式同碳纳米管相同,均表现为管壁的局部屈曲.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.     

单、双原子空位缺陷对扶手椅型单层碳纳米管屈曲性能的不同影响

采用分子动力学方法,对完善和含缺陷扶手椅型单层碳纳米管进行轴向压缩的数值模拟,对比研究三种不同的温度环境下单、双原子空位缺陷对碳纳米管轴压变形性能的特殊影响.研究结果表明管壁缺陷显著降低了纳米管低温时的承载能力,由于单原子空位缺陷造成的特殊应力集中效应会引发纳米管过早发生局部屈曲,单原子缺陷管的屈曲强度反而小于双原子管的屈曲强度. 关键词： 分子动力学 碳纳米管 屈曲 缺陷     

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.     

氮化硼、碳化硅、锗纳米管的熔化与拉伸力学特性

该论文采用Tersoff势的分子动力学方法分析了单壁(5,5)氮化硼、碳化硅、锗纳米管的熔化与轴向拉伸力学特性,讨论了三种纳米管熔化与轴向拉伸力学性能的差异.研究表明:氮化硼管熔化后呈现为网状,碳化硅管为疏松的不规则的团状,锗管呈现为紧密排布的近似球状;相同温度下,碳化硅及氮化硼纳米管的熔点、比热容以及熔化热却均远高于锗管,但系统能量却远低于锗管;三种纳米管中,氮化硼管的抗变形抗能力最大,锗管的抗变形与抗载荷能力最小,而氮化硼、碳化硅管的抗载荷能力相当.     

氮化硼、碳化硅、锗纳米管的熔化与拉伸力学特性

该论文采用Tersoff势的分子动力学方法分析了单壁(5,5)氮化硼、碳化硅、锗纳米管的熔化与轴向拉伸力学特性,讨论了三种纳米管熔化与轴向拉伸力学性能的差异。 研究表明：氮化硼管熔化后呈现为网状,碳化硅管为疏松的不规则的团状, 锗管呈现为紧密排布的近似球状;相同温度下,碳化硅及氮化硼纳米管的熔点、比热容以及熔化热却均远高于锗管,但系统能量却远低于锗管;三种纳米管中,氮化硼管的抗变形抗能力最大,锗管的抗变形与抗载荷能力最小,而氮化硼、碳化硅管的抗载荷能力相当。     

率温联合条件下混凝土材料压缩强度的变化规律

混凝土材料的动态压缩强度不仅具有明显的应变率强化(硬化)效应,同时还具有明显的温度弱化(软化)效应。在应变率和温度联合条件下,压缩强度随应变率和温度变化过程中不仅存在清晰的应变率拐折点,拐折点前后压缩强度随应变率变化速率明显不同,而且在不同温度下发生拐折时,其拐折点对应的应变率也存在明显差异。参考近年来相关文献中混凝土材料在率温联合条件下的压缩实验数据,结合理论分析,探讨了在不同温度、不同应变率条件下混凝土材料压缩强度联合效应因子K的变化规律;并对实验数据进行拟合,得到了不同应变率、不同温度下K(T)-■的预测表达式,确定了应变率强化和温度弱化对压缩强度的耦合影响;通过分析应变率拐折点与温度的关系,确定了应变率和温度联合条件下应变率敏感区和不敏感区的率温联合条件界限;建立了率温效应相当(K=1)时的率温等效方程,并确定了混凝土材料的率温等效参数。     

碳纳米管与氮化硼纳米管内铝纳米线的形成及其复合结构抗压特性的模拟研究

采用分子动力学方法分别对管内充以铝原子碳纳米管(CNT)与氮化硼纳米管(BNNT)进行了结构性能研究.优化结果显示:(5, 5) CNT和BNNT内均能形成一束一维铝纳米线(AlNW);(10, 10)管内形成的是多束AlNW,其中(10, 10) CNT内形成的是11束高度轴对称一维AlNW,而(10, 10) BNNT内形成的是5束螺旋结构形状的AlNW.进一步分析表明:CNT内的AlNW具有比BNNT内的AlNW较大的原子分布线密度,但大管径(10, 10)型BNNT内的螺旋状AlNW可以具有比相同管径CNT内纳米线更高的结晶性.通过对其轴向压缩模拟及其能量分析,可以发现AlNW@CNT复合结构的屈曲应变明显大于AlNW@BNNT,且同类型复合结构,屈曲应变随管径增大而减小,故较小管径的AlNW@CNT具有更强轴向抗压能力.能量分析结果表明van der Waals能是维系复合纳米管结构稳定,增大抗压能力的主要原因.     

壁面对纳米管轴向导热影响的MD模拟与分析

纳米管是一种有着广泛应用前景的纳米材料,作为初步研究,本文采用非平衡分子动力学模拟方法和LJ势函数研究了氩结构纳米管的轴向热导率随着纳米管的温度、厚度和长度的变化,并对结果进行了分析。研究表明:在温度为12-60K的范围内,纳米管的轴向导热率随着温度的升高先上升,然后很快下降,再比较缓慢下降。氩纳米管的轴向导热率随着壁厚的增加先是基本不变,然后出现一个极大值。本文分析了产生极值的原因。     

金纳米管力学性能的分子动力学模拟

采用分子动力学模拟方法, 研究了金纳米管沿不同晶向拉伸与压缩载荷下的力学性能, 并分析了金纳米管的半径对其力学行为的影响. 在模拟计算中, 采用镶嵌原子势描述金原子之间的相互作用. 模拟结果表明, 在拉伸及压缩过程中, 不同晶向的金纳米管力学性能相差较大, 在拉伸和压缩载荷下金纳米管<110>向的屈服强度最大; 在三个晶向<100>, <110>, <111>的金纳米管中, <100>晶向的金纳米管其屈服强度和杨氏模量都远远小于其他晶向. 研究结果还发现, 当纳米管的半径小于3.0 nm时, 金纳米管的屈服强度没有大的变化, 而当半径大于3.0 nm后, 随着半径的增大, 其屈服强度明显降低. 关键词： 分子动力学模拟 金纳米管 力学性能     

混凝土材料动态压缩强度的应变率强化规律

混凝土材料的动态压缩强度具有明显的应变率效应,而且在低、高应变率下压缩强度的动态强化因子(Dynamic Increase Factor,DIF)与应变率的关系具有明显的区别。参考近30余年相关文献中大量的混凝土动态压缩实验数据,结合理论分析,探讨了在不同应变率阶段混凝土压缩强度DIF的变化规律,分析准静态压缩强度对压缩强度DIF的影响规律。最后对实验结果进行拟合,得到混凝土材料在不同应变率区间内压缩强度DIF的预测表达式。研究表明:混凝土材料的压缩强度DIF随着应变率的增加呈递增趋势,具有相似的线性关系;压缩强度DIF曲线按照斜率变化分为3个阶段,且在高应变率下斜率最大;不同准静态压缩强度的混凝土DIF随应变率变化的规律有少许区别,但它们之间没有明显的界限,DIF随应变率递增的趋势与准静态压缩强度没有明显的联系。     

Effects of structural defects on the compressive buckling of boron nitride nanotubes

In this paper, we examined the buckling of perfect and defective armchair boron nitride nanotubes with three types of vacancy defects, i.e. B- and N- single vacancy defects and B–N- double vacancy defect, using molecular dynamics simulations. To this end, all systems were modeled with a Tersoff-type potential, which is able to accurately describe covalent bonding of BN systems. We applied external uniaxial compressive forces to the nanotubes in vacuum and derived the critical buckling loads and strains, at room temperature in an NVT-ensemble. Our results showed significant differences between the critical buckling strengths of pristine and defective nanotubes. The resistance to axial buckling decreased with the introduction of one vacancy defect, and the B–N- double vacancy was the most seriously damaged structure, followed by B-vacancy and N-vacancy defects. Furthermore, the B-vacancy was shown to have the most significant effect on the decrease of the critical buckling strain. This can be attributed to the excessive asymmetries and perturbations induced in the structure of the nanotube and the local deformations around the defective site around the B-vacancy, even before loading. Moreover, results show that reduction in the buckling strength of the nanotube due to the presence of more than one B-vacancy defect depends on their distribution. If the two or three defects are close to each other, they act as a single point of weakness and the critical buckling load is only slightly reduced (similar to the existence of only one vacancy defect). However, if the defects are at more distant points, the critical buckling load may experience a higher decrease. Results show that vacancy defects play a critical role in the compressive buckling performance of boron nitride nanotubes and special attention must be paid to the presence of structural defects when designing members against buckling, especially for micro- and nano-electro-mechanical systems. On the other hand, defect engineering is a great means for tailoring the buckling strength of boron nitride nanotubes, in cases where the nanotube is expected to absorb energy through compressive buckling deformation and is not designed against, but for buckling.     

Axial-strain-induced torsion in single-walled carbon nanotubes

Using classical molecular dynamics and empirical potentials, we show that the axial deformation of single-walled carbon nanotubes is coupled to their torsion. The axial-strain-induced torsion is limited to chiral nanotubes-graphite sheets rolled around an axis that breaks its symmetry. Small strain behavior is consistent with chirality and curvature-induced elastic anisotropy (CCIEA)-carbon nanotube rotation is equal and opposite in tension and compression, and decreases with curvature and chirality. The large-strain compressive response is remarkably different. The coupling progressively decreases, in contrast to the tensile case, and changes its sign at a critical compressive strain. Thereafter, it untwists with increasing axial strain and then rotates in the opposite direction, i.e., the same sense as under tension. This suggests that the response is now dictated by a combination of nonlinear elasticity and CCIEA.     

Buckling analysis of carbon nanotube bundles under axial compressive,bending and torsional loadings via a structural mechanics model

A structural mechanics model is employed for the investigation of the buckling behavior of carbon nanotube bundles of three single-walled carbon nanotubes (SWCNTs) under axial compressive, bending and torsional loadings. The effects of van der Waals (vdW) forces are further modeled using a nonlinear spring element.The effects of different types of boundary conditions are studied for nanotubes with various aspect ratios. The results reveal that bundles comprising longer SWCNTs exhibit lower critical buckling load. Moreover, for the fixed-free boundary condition the rate of critical buckling load reduction is highest, while the lowest critical buckling load occurs. Simulations show good agreement between our model and molecular dynamics results.     

Influence of temperature change on column buckling of multiwalled carbon nanotubes

Based on theory of thermal elasticity mechanics, an elastic multiple column model is developed for column buckling of MWNTs with large aspect ratios under axial compression coupling with temperature change. In this model, each of the nested concentric tubes is regarded as an individual column and the deflection of all the columns is coupled together through the van der Waals interactions between adjacent tubes. The thermal effect is incorporated in the formulation. Following this model, an explicit expression is derived for the critical buckling strain for a double-walled carbon nanotube. The influence of temperature change on the buckling strain is investigated. It is concluded that the effect of temperature change on the buckling strain is dependent on the temperature changes, the aspect ratios, and the buckling modes of carbon nanotubes.     

Adsorption of mercaptopurine drug on the BN nanotube,nanosheet and nanocluster: a density functional theory study

ABSTRACT

We have investigated the interaction of mercaptopurine (MP) drug with BN nanotube, nanosheet and nanocluster using density functional theory calculations in the gas phase, and aqueous solution. We predicted that the MP drug tends to be physically adsorbed on the surface of BN nanosheet with an adsorption energy (E_ad) about ?3.2?kcal/mol. The electronic properties of BN nanosheet are not affected by the MP drug, and this sheet is not a sensor. But the electronic properties of BN nanotube and nanocluster are significantly sensitive to this drug in both gas phase, and aqueous solution. The BN nanocluster suffers from a long recovery time (8.8?×?10⁸?s) because of a strong interaction (E_ad?=??28.6?kcal/mol), and this cluster is not a proper sensor for MP detection. But the BN nanotube benefits from a short recovery time about 49.5?s at room temperature, and may be a promising candidate for application in the MP sensors. The water solvent decreases the strength of interaction between the BN nanotube, and MP drug, but it does not affect the electronic sensitivity of the nanotube sensibly.     

温度对单壁碳纳米管端口结构的影响

利用紧束缚势分子动力学模拟方法，研究了温度在2000—3500 K之间单壁碳纳米管端口结构的变化趋势.研究表明，温度对整个管端口结构起关键作用，计算表明温度在3000K和3500K下碳管两端口在15ps时间尺度内依次闭合，温度高易于使理想单壁碳管端口封闭，且端口封闭导致碳管系统能量的降低.由于Armchair型碳纳米管与相同半径的Zigzag型碳纳米管相比有相对低的应力能，导致Armchair型碳纳米管更易形成端口封闭的结构. 关键词： 碳纳米管 紧束缚势     

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.