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CNTs consisting of a single carbon layer or multiple coaxial carbon layers seemingly are ideal perfect structures [16] with ultra high strength, high electrical and thermal conductivity, etc [17, 18]. However, there are many defects when the nanotubes are prepared in a practical process [19-22]. These defects, including vacancies of atoms, holes, Stone–Wales defects, and even line defects, etc [23-25] create heterogeneous structures which harm the symmetry of the carbon layer, hence reduce its mechanical and electrical properties, and in consequence, seriously influence their potential application in such areas as nanoelectronics, nanoelectro-mechanical systems (NEMS) and nanocomposites [26-29]. Defects in CNTs can arise from various causes. Chemical defects consist of atoms/groups covalently attached to the carbon lattice of the tubes such as oxidized carbon sites [30, 31]. Topological defects can be defined as the presence of rings other than regular hexagons and mainly as Stone–Wales (pentagon–heptagon) defects [32, 33]. Very often in the structure, incomplete bonding defects like vacancies are caused by exposure to high temperature for a long time during the manufacture, impact with high energy electrons in the transmission electron microscopy environment and damage resulting from the harsh oxidative purification process [34]. Theoretically it is known that CNTs have high tensile modulus, around 1 TPa, and tensile strength, around 300 GPa [35, 36]. On the other hand, using molecular dynamic simulations based on the empirical interatomic potential for carbon, Yakobson et al [37] suggested that the breaking strain of CNTs could be as large as 55%. In this context, some experimental results are worth noting [38-41]. Two sets of measurements [38, 39] have been reported for the fracture of ropes of single-walled CNTs (SWCNTs). The observed maximum failure strains were less than 6%, tensile strengths were between 13 and 52 GPa, and the Young’s modulus ranged from 0.32 to 1.47 TPa. Demczyk et al [40] reported results for the fracture of single multi-walled CNTs (MWCNTs) that contained 1–5% boron, which indicated a Young’s modulus of 0.91 TPa, a failure strain of 4–7% and a failure stress of 150±45 GPa. The most extensive set of CNT fracture measurements for 19 MWCNTs placed under tensile load was reported by Yu et al [41]. The observed failure strains ranged from 2 to 13%, the Young’s modulus ranged from 0.27 to 0.95 TPa and the failure stress ranged from 11 to 63 GPa, with an average value of 28 GPa. Another study reported that MWCNTs with outer diameter in the range of 12–30 nm possessed a Young’s modulus in the range 0.1–1.6 TPa [42]. Such observations conflicted with the theoretical and numerical analysis outcomes of [35-37]. However, this conflict between the theoretical and experimental results can be explained by the existence of defects in the CNT structures and erroneous measurement systems. While the error in measurement systems and the related calculation can be corrected by sophisticating and repeating the experimental procedure, the defects in the structures should be taken into account. Much importance has been given to the Stone–Wales defects so far in the literature, as Stone–Wales defects play a key role in the formation of CNT caps, intermolecular junctions, and variation of diameter and chirality, though it is not obvious how a Stone–Wales defect can occur in the CNT structure. The effect of Stone–Wales defects on the mechanical properties and their nucleation under loading as well as degradation of load carrying capacity in the defected zone has been reported in [33]. Recently, the effect of random distribution of Stone–Wales defects on the mechanical properties has been reported in [43, 44]; the mean value of the stiffness, strength and ultimate strain is found to decrease as the average number of defect increases. One-atom or two-atom vacancy defects, which are expected to occur during most synthetic schemes and other processes, reduce the failure strength of CNTs by as much as 26% [45, 46] and the expected failure strains by as much as a factor of two. These reductions are much larger than those caused by Stone–Wales defects reported in [33, 43] and [44]. Though degradation in properties of CNTs was observed due to the presence of defects in the structure, the reason for the scatter in mechanical properties found in the literature is still unanswered. |
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wshbinzhang
铁杆木虫 (知名作家)
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168yjh(金币+10,VIP+0):请继续翻译完 2-16 19:47
168yjh(金币+10,VIP+0):请继续翻译完 2-16 19:47
| 碳纳米管是由一个碳层或多个同轴碳层组成的近似理想的完美的结构,具有超高强度，高电气和热导率等.然而，碳纳米管在实际制备过程中会产生许多缺陷.这些缺陷，包括原子空位，空穴，石威尔士缺陷 甚至线缺陷等, 导致结构混乱和损害对称性，从而降低其机械和电学性能, 甚至严重影响其在纳米电子学,纳米电-力学系统和纳米复合材料方面的应用.导致碳纳米管中缺陷的原因很多. 化学缺陷是原子/团体由共价键连接到碳管的晶格，如被氧化的炭格点.拓扑缺陷可以被定义为非通常的六边形链接,主要是石威尔士（五边）缺陷结构中不完整的健合,例如缺陷,经常是由于在制备过程中和高温接触过长时间，在透射电子显微观察时受高能电子的冲击或者在氧化纯化过程中产生的. |
2楼2009-02-16 17:35:26
wshbinzhang
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168yjh(金币+40,VIP+0):谢谢！ 2-17 18:16
168yjh(金币+40,VIP+0):谢谢！ 2-17 18:16
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理论上碳纳米管具有高拉伸模量(1 TPa )和拉伸强度(300 GPa).另一方面，利用经验原子势的分子动力学模拟， Yakobson等人认为破坏碳纳米管的应变可达到55 ％。在这方面，一些实验结果值得注意。两组实验测量了单壁碳纳米管绳的断裂强度。观察到的最高失效应变小于6 ％ ，拉伸强度为13至52 GPa，杨氏模量为0.32至1.47 TPa。Demczyk等人报道了单个含有1-5 ％硼的多壁碳纳米管的断裂结果，杨氏模量为0.91 TPa失效应变为4-7 ％，失效应力为150 ± 45 GPa。最广泛的认可的一个实验是yu等人完成的，他们测量了19根多壁碳纳米管的拉伸强度。观察到的失效应变为2 〜 13 ％ ，杨氏模量为0.27至0.95 TPa，失效应力介于11至63 GPa，平均值为28 GPa。另一项研究报告说，外径为12-30纳米的多壁碳纳米管，其杨氏模量介于0.1-1.6 TPa。这些结果与理论和数值分析结果相矛盾。 然而，理论和实验结果之间的矛盾可以由碳纳米管结构缺陷的存在和系统测量误差来解释。 虽然系统的测量误差和相关的计算可以由精密的实验和重复实验得到纠正，但是结构的缺陷是应该考虑到的。 目前为止，文献一直重视石威尔士缺陷，认为其在形成碳纳米管帽，分子间连接，直径和手性的变化中发挥关键作用，虽然它并没有明显在结构中显现出来。 已经有人报道了石威尔士缺陷对力学性能及行核的影响，以及缺陷区承载能力的降低。 。 最近，有人报道了随机分布的石威尔士缺陷对力学性能的影响：刚度，强度和极限应变随缺陷增加而降低。 绝大部分的制备过程中会产生单原子或两个原子的空位缺陷，可降低断裂强度26 ％和失效应变a factor of two。 这些降低远大于文献[ 33 ， 43 ]和[ 44 ] 报道的由石威尔士缺陷所造成的降低。 虽然已经观察到由于结构缺陷的存在导致碳纳米管性能降低，力学性能结果的分散还没有很好的回答。 |
3楼2009-02-16 21:07:51