Si表面间水平碳纳米管束的分子动力学模拟研究

本文采用分子动力学模拟方法研究了Si表面间单壁水平碳纳米管束SWCNT (10,10)的变形和摩擦特性.系统在弛豫平衡后,首先对碳纳米管束施加压力至碳纳米管或Si表面结构破坏.之后在无压力和高压力两种情况下使上表面沿水平方向做剪切运动以研究碳纳米管束的摩擦特性.结果表明,由于碳纳米管的柔韧性,碳纳米管束在加载过程中出现明显变形,但直至3.8 GPa高压下并无结构破坏.系统无压力时SWCNT (10,10)在原地轻微随机滚动,压力为3.8 GPa时,碳纳米管束出现了整体的轻微滑动,同时伴随无规律的轻微滚动, 关键词： 碳纳米管束 摩擦 分子动力学模拟     

碳纳米管束中的正电子理论

采用中性原子叠加模型和有限差分方法(SNA-FD)计算了大范围内不同管径的单壁碳纳米管束中的正电子情况，发现对于单壁碳纳米管束，正电子的主要湮没区域，湮没对象和正电子寿命随碳纳米管管径的不同而发生规律性变化.计算得到管径范围在0.8—1.6nm的碳纳米管束的正电子寿命范围为332—470ps，与实验测得的394ps符合较好.     

TEM纳米云纹法测量纳米碳管变形

提出了透射电子显微镜(TEM)纳米云纹法的新技术,首次将该方法用于单根单壁碳纳米管的残余变形测量。纳米云纹由计算机显示器扫描线与碳纳米管束TEM图像干涉而成。该方法具有纳米级空间分辨率,可直接测量碳纳米管的力学性能。对TEM纳米云纹法的原理进行了详细的阐述,并利用不同管径的单壁碳管束产生了云纹。对直径为7.5nm的弯曲碳管束的残余变形进行测量,直接得到了其中一根直径为1.0nm的单壁碳管的残余变形场。实验结果证明了该方法的可行性。该方法为纳米尺度的碳管力学性能测量提供了新途径。     

碳纳米流体强化传热研究

在去离子水中分别添加了单壁、多壁碳纳米管材料,通过配比一定质量的亲水性分散剂,经超声波振荡试制了水基碳纳米流体。测试分析了不同碳纳米管材料质量分数下的纳米流体热导率和动力粘度等重要热物性参数。测试结果表明:碳纳米管粒子能够强化基液工质导热性能,随着其质量分数的增加,水基单壁、多壁碳纳米流体热导率均明显提高;单壁碳纳米流体粘度显著增加,多壁碳纳米流体粘度无明显变化,多壁碳管更适用于纳米流体强化传热。     

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．     

碳纳米流体相变特性实验研究

通过"两步法"制备分散均匀、稳定的单壁碳纳米管/水、多壁碳纳米管/水碳纳米流体,通过差示扫描量热法(DSC)测试分析了碳纳米流体的凝固、熔化相变过程,实验结果表明在凝固相变过程中随着碳纳米管质量分数的增加,碳纳米流体的起始凝固温度及凝固温度逐渐升高,单壁碳纳米流体的凝固温度显著提高,在熔化相变过程中,碳纳米流体的质量分数对于纳米流体的熔点无明显影响,但对于碳纳米流体的潜热影响较大,碳纳米流体的潜热随着质量分数的增加而明显减小。     

单壁碳纳米管的SERS研究

本文通过改进实验方法,将单壁碳纳米管的乙醇溶液与金胶混合并制作成固体薄膜,使单壁碳纳米管夹裹在金纳米粒子之间,保证了吸附的紧密性,获得了高质量的单壁碳纳米管SERS光谱。不但观测到文献中报道的径向呼吸振动模(RBM)和C-C正切拉伸模(GM)的增强,还在1100-1500cm-1区域观测到一组新峰,其峰形完整并有相当的强度。这些峰在现有的文献中几乎没有报导。文章对这组新峰的出现进行了初步的分析。     

单壁碳纳米管受压屈曲行为的数值模拟

采用以Tersoff Brenner势函数来描述碳纳米管中碳原子间的相互作用的分子动力学方法,模拟了单壁 碳纳米管(SWCNTs)的受压屈曲行为.计算结果表明,单壁碳纳米管的杨氏模量随着管径的增大而减小;碳纳米 管屈曲的临界应力和临界应变与碳纳米管细长比有关,不同的细长比决定了碳纳米管结构不同的屈曲模态;碳纳 米管的受压屈曲机理和连续介质力学中柱体壳的受压屈曲理论随细长比的不同而存在一些异同.     

单壁碳纳米管束针尖增强近场拉曼光谱探测实验研究

针尖增强近场拉曼光谱术是最近发展起来的光谱技术。金属探针在获得样品纳米局域表面形貌的同时,受激光激发,在针尖附近产生增强电磁场,得到与形貌位置精确对应的针尖增强局域拉曼光谱,形貌和光谱的结合实现了纳米局域的光谱指认。文章建立了一套针尖增强近场拉曼光谱测量装置,并用此装置对电弧法合成的单壁碳纳米管进行了近场拉曼光谱探测。测量了直径为100 nm单壁碳纳米管束的针尖增强拉曼光谱,进一步得到至多3根单壁碳纳米管的近场拉曼光谱,实现了超衍射分辨光谱探测。通过与远场拉曼光谱比较发现,针尖增强近场拉曼光谱的增强因子大于230倍。实验证明,同时具有超衍射空间分辨和拉曼光谱信号增强能力的针尖增强近场拉曼光谱术将是纳米材料和纳米结构表征的一种重要方法。     

利用径向呼吸模及其倍频模的共振特性精确测定单壁碳纳米管的电子跃迁能量

提出一个根据拉曼基频模及其倍频模的斯托克斯和反斯托克斯拉曼成分的不同共振行为来探测样品与激光共振的系统能级的方法.此方法被应用到不均匀单壁碳纳米管束样品中某一径向呼吸模频率为219波数的金属型碳纳米管.通过分析呼吸模及其倍频模和切向模的共振行为,获得了该碳纳米管的电子跃迁能量,并获得纳米管C-C最近邻重叠积分因子为2.80 eV.此数值可以很好的解释单壁碳纳米管径向呼吸模的共振行为. 关键词： 单壁碳纳米管 呼吸模 共振拉曼散射 电子跃迁能     

The Fourier Raman spectra of HiPCO single-wall carbon nanotubes under high pressure

The Raman spectra of single-wall carbon semiconducting nanotubes of the HiPCO type were investigated upon excitation at 1.06 μm at pressures up to 3 GPa. A method of purification of the initial material from iron is proposed and the diffraction patterns of purified nanotubes are considered. The diameters and chiralities of the nanotubes manifesting themselves in the spectra are determined. Pressure dependences of the frequencies are obtained and the Gruneisen parameters and their relationship with interatomic interactions in nanotubes are discussed. Original Russian Text ? V.D. Blank, V.A. Ivdenko, A.S. Lobach, B.N. Mavrin, N.R. Serebryanaya, 2006, published in Optika i Spektroskopiya, 2006, Vol. 100, No. 2, pp. 282–289.     

Mechanical properties of carbon nanotubes

A variety of outstanding experimental results on the elucidation of the elastic properties of carbon nanotubes are fast appearing. These are based mainly on the techniques of high-resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM) to determine the Young’s moduli of single-wall nanotube bundles and multi-walled nanotubes, prepared by a number of methods. These results are confirming the theoretical predictions that carbon nanotubes have high strength plus extraordinary flexibility and resilience. As well as summarising the most notable achievements of theory and experiment in the last few years, this paper explains the properties of nanotubes in the wider context of materials science and highlights the contribution of our research group in this rapidly expanding field. A deeper understanding of the relationship between the structural order of the nanotubes and their mechanical properties will be necessary for the development of carbon-nanotube-based composites. Our research to date illustrates a qualitative relationship between the Young’s modulus of a nanotube and the amount of disorder in the atomic structure of the walls. Other exciting results indicate that composites will benefit from the exceptional mechanical properties of carbon nanotubes, but that the major outstanding problem of load transfer efficiency must be overcome before suitable engineering materials can be produced. Received: 17 May 1999 / Accepted: 18 May 1999 / Published online: 29 July 1999     

On the limiting physical adsorption of hydrogen in carbon materials

The specific features of hydrogen adsorption (and adsorption of other gases) at supercritical temperatures (specifically, the absence of capillary condensation and polymolecular adsorption and the appearance of a maximum in the adsorption isotherm in the pressure range 1–10 MPa) are discussed. Hydrogen adsorption decreases by an order of magnitude as the temperature increases from the critical temperature to the room value. The experimental adsorption isotherms in the supercritical range found in the literature are used to deduce a criterion of limiting hydrogen adsorption at various temperatures. Carbon adsorbents of different types (individual single-wall nanotubes, bundles of such nanotubes, multiwall nanotubes, and carbon fibers) are considered. A model of single graphite plane shows that the limiting hydrogen adsorption is 5 wt % at 77 K and 1 wt % at 293 K. These values can only be approached by adsorption in a material made of individual single-wall nanotubes. Methods to increase the adsorption are proposed.     

Fluoride-doped MWCNT/Si3N4 composite with improved mechanical and structural properties

The addition of carbon nanotubes (CNT) in ceramic composites has stimulated a substantial interest due to their high mechanical, thermal and electrical properties. This approach used fluoride additives (AlF₃ and MgF₂) to prepare multi-walled carbon nanotubes/silicon nitride (MWCNT/Si₃N₄) composite densified at 1700 °C for 1 h by hot press (HP) sintering. The microstructural analyses of MWCNT/Si₃N₄ composites indicate that the fluoride additives have substantially improved densification and the transformation of α-Si₃N₄ to β-Si₃N₄. As observed, the mechanical properties, i.e. flexural strength, fracture toughness, Young's modulus and hardness of MWCNT/Si₃N₄ composites are improved with an increasing concentration of MWCNT. These results attributed to the highly dense composites, strong interfacial interaction and the pull-out mechanism of MWCNT and β-Si₃N₄. The maximum values of fracture toughness flexural strength, Young's modulus, and hardness were 12.76 ± 1.15 MPa.m^0.5, 883 ±46 MPa, 260 ±9 GPa, and 26.4 ± 1.3 GPa, respectively. The improved mechanical properties also ascribed to the synergistic strengthening and toughening influence of MWCNT and β-Si₃N₄.     

Fracture of polycrystalline graphene membranes by in situ nanoindentation in a scanning electron microscope

Failure of polycrystalline graphene grown by chemical vapor deposition was investigated by nanoindentation in a scanning electron microscope. Circular graphene membranes were subject to central point loads using a nanomanipulator combined with an atomic force microscope cantilever as a force sensor. The grain boundaries of the polycrystalline graphene were visualized by Raman spectroscopy coupled with a carbon isotope labeling technique. Graphene membranes without any grain boundary had a failure strength of 45.4 ± 10.4 GPa, compared to 16.4 ± 5.1 GPa for those with grain boundaries when a Young's modulus was assumed to be 1 TPa. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Study of the symmetry of single-wall nanotubes by electron diffraction

Determining precisely the atomic structure of single-wall carbon nanotubes is essential since it tailors electronic properties of this new carbon material. Here, we present a quantitative electron diffraction study of electric-arc produced single-wall carbon nanotube bundles, combined with simulations based on the kinematic theory and with real-space images. We stress the importance of the twist of the bundle in the interpretation of our data and we analyze both packing lattice parameters and chirality distribution. We show that, within a given bundle, no chirality is favoured whereas SWNT diameters are almost uniform. Received 5 February 1999     

单壁纳米管的弹性性质

基于第一性原理计算了一系列单壁碳纳米管(椅型、锯齿型)和氮化硼锯齿型纳米管的杨氏模量.用实际的数值结果显示了杨氏模量与纳米管管径之间的依赖关系.还讨论了各类纳米管卷曲形变能以及平衡基矢长度a随管型的变化. 关键词： 单壁纳米管 平衡基矢长度 杨氏模量 形变能     

Novel superhard carbon: C-centered orthorhombic C8

A novel carbon allotrope of C-centered orthorhombic C(8) (Cco-C(8)) is predicted by using a recently developed particle-swarm optimization method on structural search. Cco-C(8) adopts a sp(3) three-dimensional bonding network that can be viewed as interconnected (2,2) carbon nanotubes through 4- and 6-member rings and is energetically more favorable than earlier proposed carbon polymorphs (e.g., M carbon, bct-C(4), W carbon, and chiral C(6)) over a wide range of pressures studied (0-100 GPa). The simulated x-ray diffraction pattern, density, and bulk modulus of Cco-C(8) are in good accordance with the experimental data on structurally undetermined superhard carbon recovered from cold compression of carbon nanotube bundles. The simulated hardness of Cco-C(8) can reach a remarkably high value of 95.1 GPa, such that it is capable of cracking diamond.     

Carbon Nanotubes in Arrays: Competition of van-der-Waals and Elastic Forces

Doklady Physics - The van-der-Waals interaction between carbon nanotubes leads to the formation of agglomerates of bundles and strands. In such a self-assemblage, identical nanotubes are assembled...