Electromechanical buckling analysis of boron nitride nanotube using molecular dynamic simulation

In this paper, the effect of electric field on axial buckling of boron nitride nanotubes is investigated. For this purpose, molecular dynamics simulation and continuum mechanics are used for the first time simultaneously. In molecular dynamics simulation, the potential between boron nitride atoms is considered as Tersoff and Timoshenko beam theory is used in continuum mechanics. In this paper, buckling of zigzag and armchair boron nitride nanotubes are investigated. Here, the effects of the electric field and the length of the boron nitride nanotube on the critical load are investigated and it is shown that the effect of the electric field is different with respect to the arrangement of atoms in the boron nitride nanotubes. In fact, the electric field creates axial and torsional loads on the zigzag and armchair nanotube, respectively. Axial buckling of the zigzag nanotube is dependent on the electric field, whereas in the armchair nanotubes, the electric field changes have no effect on the axial buckling. To better understand the impact of the electric field on axial buckling, these results are compared with the continuum mechanics.     

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.     

Buckling of 2D-FG Cylindrical Shells under Combined External Pressure and Axial Compression

This paper presents the stability of two-dimensional functionally graded (2D-FG) cylindrical shells subjected to combined external pressure and axial compression loads, based on classical shell theory. The material properties of functionally graded cylindrical shell are graded in two directional (radial and axial) and determined by the rule of mixture. The Euler's equation is employed to derive the stability equations, which are solved by GDQ method to obtain the critical mechanical buckling loads of the 2D-FG cylindrical shells. The effects of shell geometry, the mechanical properties distribution in radial and axial direction on the critical buckling load are studied and compared with a cylindrical shell made of 1D-FGM. The numerical results reveal that the 2D-FGM has a significant effect on the critical buckling load.     

First-principle study of nanostructures of functionalized graphene

We present first-principle calculations of 2D nanostructures of graphene functionalized with hydrogen and fluorine, respectively, in chair conformation. The partial density of states, band structure, binding energy and transverse displacement of C atoms due to functionalization (buckling) have been calculated within the framework of density functional theory as implemented in the SIESTA code. The variation in band gap and binding energy per add atom have been plotted against the number of add atoms, as the number of add atoms are incremented one by one. In all, 37 nanostructures with 18C atoms, 3 × 3 × 1 (i.e., the unit cell is repeated three times along x-axis and three times along y-axis) supercell, have been studied. The variation in C–C, C–H and C–F bond lengths and transverse displacement of C atoms (due to increase in add atoms) have been tabulated. A large amount of buckling is observed in the carbon lattice, 0.0053–0.7487 Å, due to hydrogenation and 0.0002–0.5379 Å, due to fluorination. As the number of add atoms (hydrogen or fluorine) is increased, a variation in the band gap is observed around the Fermi energy, resulting in change in behaviour of nanostructure from conductor to semiconductor/insulator. The binding energy per add atom increases with the increase in the number of add atoms. The nanostructures with 18C+18H and 18C+18F have maximum band gap of 4.98 eV and 3.64 eV, respectively, and binding energy per add atom –3.7562 eV and –3.3507 eV, respectively. Thus, these nanostructures are stable and are wide band-gap semiconductors, whereas the nanostructures with 18C+2H, 18C+4H, 18C+4F, 18C+8F, 18C+10F and 18C+10H atoms are small band-gap semiconductors with the band gap lying between 0.14 eV and 1.72 eV. Fluorine being more electronegative than hydrogen, the impact of electronegativity on band gap, binding energy and bond length is visible. It is also clear that it is possible to tune the electronic properties of functionalized graphene, which makes it a suitable material in microelectronics.     

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.     

Al_nMg_m(n+m=6,7;m=1,2)团簇的稳定性与电子结构

用遗传算法结合经验势对AlnMgm(n+m=6,7;m=1,2)的低能结构进行全局搜索,用从头计算方法研究了其稳定性和电子结构.计算表明AlnMgm存在多个能量相近的异构体,在Al Mg团簇中Mg-Mg相互作用远弱于Al-Al作用,稳定结构中Mg原子倾向于形成更多的Mg-Al键.自然键轨道(NBO)电荷分析表明Mg向Al转移电子,Al原子上的电荷大小强烈依赖于相连的Mg原子个数.电子结构分析表明随着原子数目的增大,占据轨道能级略有降低,同尺寸下掺杂一个Mg时能级更低;Al Mg团簇的能隙约为5.30 e V.     

AlnMgm (n+m=6,7;m=1,2)团簇的稳定性与电子结构

用遗传算法结合经验势对AlnMgm(n+m=6,7; m=1,2)的低能结构进行全局搜索,用从头计算方法研究了其稳定性和电子结构。计算表明AlnMgm存在多个能量相近的异构体,在AlMg团簇中Mg-Mg相互作用远弱于Al-Al作用,稳定结构中Mg原子倾向于形成更多的Mg-Al键。NBO电荷分析表明Mg向Al转移电子,Al原子上的电荷大小强烈依赖于相连的Mg原子个数。电子结构分析表明随着原子数目的增大,占据轨道能级略有降低,同尺寸下掺杂一个Mg时能级更低;AlMg团簇的能隙约为5.30 eV。     

考虑介质膨胀速率的锂离子电池管状电极中扩散诱导应力及轴向支反力分析

硅作为锂离子电池阴极材料相对于传统负极材料具有高比容量,价格低廉等优势.本文针对充电过程中锂离子电池中电极建立力学模型和扩散模型,并在扩散模型引入考虑介质膨胀速率的影响.以硅空心柱形电极为例,分析了恒流充电下介质膨胀速率对电极中扩散诱导应力分布的影响,并研究了不同内外半径比、充电速率、材料参数以及锂化诱导软化系数(lithiation induced softening factor,LISF)对轴向的支反力达到临界欧拉屈曲力所需时间的影响.结果表明,随着电极中锂浓度上升,介质膨胀速率对应力分布的影响增大,对轴向的支反力影响较小.弹性模量和应力成正比,但其与轴向的支反力达到临界欧拉屈曲力所需时间无关;扩散系数与所需时间成反比;偏摩尔体积增大时,达到临界屈曲力所需时间减少;随着LISF绝对值增大,完全锂化时轴向力降低.     

Prediction of compressive post-buckling behavior of single-walled carbon nanotubes in thermal environments

In the present investigation, the axial buckling and post-buckling configurations of single-walled carbon nanotubes (SWCNTs) are studied including the thermal environment effect. For this purpose, Eringen’s nonlocal elasticity continuum theory is implemented into the classical Euler–Bernoulli beam theory to represent the SWCNTs as a nonlocal elastic beam model. A closed-form analytical solution is carried out to analyze the static response of SWCNTs in their post-buckling state in which the axial buckling load is assumed to be beyond the critical axial buckling load. Common sets of boundary conditions, named simply supported–simply supported (SS–SS), clamped–clamped (C–C), and clamped–simply supported (C–SS), are considered in the investigation. Selected numerical results are given to represent the variation of the carbon nanotube’s mid-span deflection with the applied axial load corresponding to various nonlocal parameters, length-to-diameter aspect ratios, temperature changes, and end supports. Moreover, a comparison between the post-buckling behaviors of SWCNTs at low- and high-temperature environments is presented. It is found that the size effect leads to a decrease of the axial buckling load especially for SWCNTs with C–C boundary conditions. Also, it is revealed that the value of the temperature change plays different roles in the post-buckling response of SWCNTs at low- and high-temperature environments.     

碳纳米管封装Si20团簇的热稳定性研究

用分子动力学方法模拟一种特殊结构Si20 (表面异构的硅十二面体结构)填充到不同管径的扶手椅型碳纳米管中组成复合结构的热稳定性。通过能量分析和定量统计缺陷多边形数量等方法来研究这种结构在碳纳米管中的稳定性和结构演变情况。研究发现Si20的热稳定性和碳纳米管的直径关系密切;其在CNT(15, 15)中的热稳定性最好,当管径逐渐增加时,其热稳定性呈下降趋势;直至管径增加到CNT(21, 21),碳纳米管对Si20的空间限制作用变得很小,以至于不足以维持Si20的稳定。此外,Si原子因热振动替换碳纳米管中C原子而形成十二边形缺陷,这对碳纳米管的热稳定性有着明显的降低作用。     

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.     

Electronic properties of a silicon carbide nanotube under uniaxial tensile strain: a density function theory study

The electronic properties of an armchair (4,4) single-walled silicon carbide nanotube (SWSiCNT) with the length and diameter of 22.4 and 6.93 Å, respectively under different tensile strains are investigated by density functional theory (DFT) calculation. The change of highest occupied molecular orbital and lowest unoccupied molecular orbital (HOMO–LUMO) gap of the nanotube has been observed during the elongation process. Our results show that the gap will significantly decrease linearly with the increase of axial strain. Two different slopes are found before and after an 11% strain in the profiles of the HOMO–LUMO gap. The radial buckling has been performed to investigate the radial geometry of nanotube. The partial density of states (PDOS) of two neighboring Si and C atoms of the nanotube are further studied to demonstrate the strain effect on the electronic structure of SiC nanotube. The PDOS results exhibit that the occupied states of Si atom and the unoccupied states of C atom are red-shifted and blue-shifted under stretching, respectively. Mulliken charge analysis reveals that Si and C atoms will become less ionic under the larger strain. The electron differences of silicon carbide nanotube (SiCNT) on tensile loading are also studied.     

杂质浓度对Zr替位掺杂γ-TiAl合金的结构延性和电子性质的影响

以Zr替代Ti(或Al)掺杂γ-TiAl体系为研究对象, 掺杂浓度(摩尔比)分别为1/54, 1/36, 1/24和1/16. 采用基于密度泛函理论的第一性原理方法, 计算研究了Zr掺杂γ-TiAl体系的晶体结构及其稳定性、延性和电子性质等. 结果显示, Zr替位掺杂, 可以改变γ-TiAl基合金的结构对称性. 计算的形成能表明, Zr替代Ti原子会使体系的形成能降低, 而Zr替代Al原子会使体系的形成能增加. 因而, 在掺入γ-TiAl时, Zr更倾向于替代Ti 原子, 但是Zr替代Al原子也具有一定的可能性, 从而会产生多样的掺杂体系, 对于改善合金的性质具有重要意义. 对各个体系轴比的计算与分析表明, 当掺杂浓度为1.85 at%–6.25 at% 时, Zr替代Al原子会使体系的轴比减小、接近于1, 从而改善合金的延性效果明显. 能带结构显示各个Zr掺杂γ-TiAl体系均具有金属导电性. 对电子态密度和布居数的分析表明, Zr替代Al原子后, Zr与其邻近Ti原子的共价键结合强度大为降低, 导致合金体系中的Ti-Al(Zr)键的平均强度明显减弱, 金属键增强, 这是改善γ-TiAl合金延性的重要因素.     

Density-functional study of the CO chemisorption on bimetallic Pd–Sn(1 1 0) surfaces

We study the ordered PdSn c(2 × 2), (2 × 1), and PdSn₂ (3 × 1) overlayers deposited on Pd(1 1 0) by using first-principles density-functional calculations. It appears that the two PdSn structures are almost degenerate in the energy. Pd–Sn surfaces we consider do not display the marked buckling with Sn atoms displaced towards vacuum that is common for Pt–Sn surfaces. Low-coverage CO chemisorption at these overlayers and on analogous surface structures on Pd₃Sn is considered. It is shown that inclusion of an empirical correction to the CO adsorption energy changes the stable adsorption site from the long-bridge to the top one in most cases. The adsorption energy decreases with the number of Sn atoms in the vicinity of the adsorption site, and this property correlates well with the position of the centre of gravity of the local Pd d-electron band, and also with the variation of the local density of d-electron states at the Fermi level. The centre-of-gravity value is used to assess the core-level shifts for Pd atoms in various geometries. Most of the calculated data compare rather well with the recent measurements on Pd–Sn overlayers at Pd(1 1 0) as well as with other data on related bimetallic systems.     

First-principles study of structural evolution of medium-sized Si<Subscript>N</Subscript> clusters (41 N 50) within stuffed fullerene cages

The stuffed fullerene structures of medium-sized Si_N ( ) clusters have been systemically studied using the all-electron density functional theory with gradient correction. For each cluster size, fullerene cages with different topologies and filled by different number of atoms were constructed and optimized to find the lowest-energy structure. The core atoms filled in the fullerene cages tend to form cage-like structures that resemble structural character of bulk diamond fragments. The size-dependent physical properties such as binding energies, electronic gaps, and ionization potentials have been discussed. Si₄₅ exhibits relatively higher stability that can be associated with the low chemical reactivity observed by experiment.     

Adsorption on the graphene surface of carbon nanotubes and their energy spectrum

The conditions of formation of local states in the energy spectra of semi-infinite carbon nanotubes with regularly arranged atoms adsorbed on the outer surface are studied in the π-electron approximation. The influence of the adsorption type (physical and chemical), the donor-acceptor properties of adsorbed atoms, their concentration on the graphene surface, and the nanotube diameter on the characteristics of the local states that arise is considered. It is shown that both physical and chemical adsorptions cause a decrease in the band gap separating the upper filled energy band and the lower vacant band. This effect can significantly change the electrical and optical properties of the nanotubes under consideration in comparison with the initial “pure” tubulene.     

Density functional investigation of structural and electronic properties of small bimetallic silver–gold clusters

Structural and electronic properties of bimetallic silver–gold clusters up to eight atoms are investigated by the density functional theory using Wu and Cohen generalized gradient approximation functional. By substitution of Ag and Au atoms, in the optimized lowest energy structures of pure gold and silver clusters, we determine the ground state conformations of the bimetallic silver–gold ones. We reveal that Ag atoms prefer internal positions whereas Au atoms prefer exposed ones favoring charge transfer from Ag to Au atoms. For each size and composition, binding energy, HOMO–LUMO gap, magnetic moment, vertical ionization potential, electron affinity and chemical hardness were calculated. On increasing the size of the cluster by varying number of Ag atoms with fixed number of Au ones, vertical ionization potential and electron affinity show obvious odd–even oscillations consistent with the pure Ag and Au clusters. Au atoms inclusion in the cluster increases the binding energy and vertical ionization potential, indicating higher stability as the number of Au atoms grows. The variation of chemical hardness with the composition in a cluster with the same size shows peaks when the number of Ag atoms is greater than or equal to Au ones, corresponding to transition from planar to tri-dimensional structures. For clusters with even number of atoms, the peaks indicate that the clusters with the same number of Ag and Au atoms are the most stable ones. Analyzing the density of states, we found that increasing the concentration of Ag atoms affects the energy separation between the HOMO and the low lying occupied states.     

On a series of Ir(III) complexes with various numbers of fluorine atoms

In this paper, a series of acetylacetonate derived ligands with various numbers of fluorine atoms, as well as their corresponding Ir(III) complexes, are fabricated and studied. We investigate their single crystal structure, thermal stability, electrochemical property and photophysical performance of these Ir(III) complexes in detail. The effect of fluorine substituent effect on these Ir(III) complexes is also investigated. It is found that the introduction of fluorine atoms into acetylacetonate derived ligand has slim effect on emissive energy and absorption character due to the small contribution of acetylacetonate derived ligand to frontier molecular orbitals. But the introduction of fluorine atoms can improve emissive characters and thermal stability of Ir(III) complexes obviously, including spectral blue shift, excited state lifetime decrease and proper thermal stability. On the other hand excess fluorine atoms compromise above parameters on the contrary.     

碳纳米管封装表面异构Si20融化的结构演变

采用分子动力学方法模拟一种硅的特殊结构（表面异构的硅十二面体结构）填充到扶手型单壁纳米管组成的复合结构的加热过程,通过可视化,键角分布,二面角分布等分析方法来研究这种结构在碳纳米管中的稳定性和结构演变情况。研究发现这种结构在碳纳米管中是非常稳定的,并且随着温度的升高,硅纳米团簇的四面体结构开始减少,近邻原子数目有所增加,但不超过8个。该复合结构是由二十个四面体组成的正十二面体,通过模拟分析可知这种结构具有相当高的稳定性,一部分原因是正四面体的单臂纳米管比较稳定,对十二面体结构的硅起了一定的保护作用;另一部分原因是Si20的正十二面体本身具有较高稳定度,这一点我们通过可视化软件发现这种团簇是缩成一团而并不是从中间裂开观察得到。     

Study of X-ray emission of dense plasma focus device in the presence of external magnetic field

The X-ray emission from a 3.3 kJ dense plasma focus device filled with argon in the presence of an external axial magnetic field has been quantitatively measured for the first time with the help of a diode X-ray spectrometer. The X-ray energy and electron temperature estimated from signals are found to be lower with application of the external axial magnetic field. The variation of the electron temperature with pressure shows a maximum at 80 Pa and a decrease both with increase or decrease of the filling gas pressure.