Carbon nanotube composites in aerospace applications Abstract: This paper gives a brief introduction to the recent progress of carbon nanotube (CNT) technologies, including their synthesis and production, molecular structures, unique properties and the potential applications in varied areas. Specifically, the main focus will be on CNT-reinforced polymer composites, and some technical issues and key research achievements in the area of nanotube reinforced composites for aerospace applications are discussed. Although carbon nanotube composites are very promising in aerospace applications, much R&D work for some key technical issues is necessary before the full potential of CNT-reinforced composites can be realized and implemented.
0 Introduction
Composite materials have become one of the most important trends in the development of new materials. A great progress has been made in high performance composites, especially in carbon fiber reinforced plastics(CFRP). Composite structural parts mainly made of carbon fibers and epoxy resins take a larger and larger share in the aerospace structures. In the recently launched B787 Dreamliner, CFRP parts constitute 50% of the total structural weight, including such primary structures as fuselage, wing and horizontal stabilizer; in the air giant A380 airplane, CFRP parts constitute 25% of the total structural weight. In aerospace applications, the structural materials of light weight and high strength are the everlasting goals, and carbon nanotube reinforced polymer composites become very attractive as a kind of new generation materials.
Since their first study carried out nearly 20 years ago by Iijima, much research work has been done for carbon nanotubes and their applications. The remarkable physical and mechanical properties of carbon nanotubes have been extensively reported, from unique electronic properties, high thermal conductivity to superior mechanical properties where the stiffness, strength and resilience are better than any current materials. Carbon nanotubes offer tremendous opportunities for the development of fundamentally new materials systems. In particular, the nano-composites reinforced with carbon tubes can offer extraordinary specific stiffness and strength, and are very promising for aerospace applications in the 21st century. In this paper, a concise review is made for carbon nanotubes, including the basic molecular structures, unique properties, potential applications, as well as the recent advances in carbon nanotubes composites, and some special technical issues for carbon nanotubes reinforced polymer composites for aerospace applications.
1 What are carbon nanotubes (CNTs)?
Carbon nanotubes (CNTs) are members of the fullerene structural family, as a tube-shaped material, made of carbon, with a diameter of the nanometer scale, typically ranging from less than 1 nm up to 50 nm. A nanometer is one-billionth of a meter, or about one ten-thousandth of the thickness of a human hair. Now nanotubes can be constructed with a very large ratio of length-to-diameter up to 132 000 000:1, significantly larger than any other material at present. That is, the
diameter of a nanotube is of the order of a few nanometers, while the length can be up to 18 cm. For a carbon nanotube, the graphite layer appears somewhat like a rolled-up cylindrical nanostructure with a continuous unbroken hexagonal mesh and carbon atoms at the apexes of the hexagons.
CNTs have many structures, differing in length, thickness, and in the type of helicity and number of layers, which can typically be classified into single-wall nanotubes (SWNTs) and multi-wall nanotubes (MWNTs).
1.1 Single-wall nanotubes (SWNTs)
Single-wall nanotubes (SWNTs) are tube-like nanostructures of graphite, normally capped at the ends, and with a single cylindrical wall. The structure of an SWNT can be visualized as a layer of graphite, a single atom thick, called graphene, which is rolled into a seamless cylinder as shown in Fig. 1.
Most SWNTs typically have a diameter of nearly 1 nm. The tube, however, can be many thousands of times longer.
SWNTs are more flexible but harder to make than MWNTs. They can be twisted, flattened, and bent into small circles or around sharp bends without breaking. SWNTs have remarkable electronic and mechanical properties, which can be used in numerous applications, such as field-emission displays, nano-composite materials, nanosensors, and logic elements. These materials are on the leading-edge of electronic fabrication, and are expected to play a major role in the next generation of miniaturized electronics.
1.2 Multi-wall Nanotubes (MWNTs)
Multi-wall nanotubes can be either in the form of a coaxial assembly of SWNTs similar to a coaxial cable, or as a single sheet of graphite rolled into the shape of a scroll as shown in Fig. 2.
The diameters of MWNTs are typically in the range of 5～50 nm. The interlayer distance in MWNTs is close to the distance between graphene layers in graphite. MWNTs are easier to produce in high volume quantities than SWNTs. However, the structure of MWNTs is less well understood because of its greater complexity and variety. Regions of structural imperfection may degrade its desirable material properties.
The challenge in the production of SWNTs on a large scale as compared to that of MWNTs is reflected in the prices of SWNTs, which currently are higher than those of MWNTs.
SWNTs, however, enjoy a performance of up to ten times better, which is a very important factor for some specific applications.
Both single-wall carbon nanotubes (SWCNTs) and multi-wall carbon nanotubes (MWCNTs) can be produced by a variety of different processes, which can broadly be divided into two categories: high-temperature evaporation using arc-discharge or laser ablation,, and various chemical vapour deposition (CVD) or catalytic growth processes. In the high-temperature methods, MWCNTs can be produced from the evaporation of pure carbon, but the synthesis of SWCNTs requires the presence of a metallic catalyst. The CVD approach requires a catalyst for both types of CNTs but also allows the production of carbon nanofibers (CNFs).
2 Properties of carbon nanotubes
CNTs were much studied with respect to their mechanical and transport properties. For example, the extremely high elastic modulus, greater than 1 TPa (the elastic modulus of diamond is 1.2 TPa) and reported strengths 10～ 100 times higher than the strongest steel at a fraction of the weight. This means that carbon nanotubes may bring about an entirely new class of advanced materials. In addition to the exceptional mechanical properties, carbon nanotubes also offer superior thermal and electrical properties: thermal stability up to 2 800 ℃ in vacuum, thermal conductivity about twice as high as diamond, electrical current carrying capacity 1 000 times higher than copper wires.
The following tables (Table 1 and Table 2) compare these properties with other engineering materials.
Overall, carbon nanotubes show a unique combination of stiffness, strength, and tenacity as compared to other fiber materials which are usually inferior in one or more of these properties. Thermal and electrical conductivities are also very high, comparable to other conductive materials.
3 Potential applications for carbon nanotubes
Carbon nanotube technology can be used for a wide range of applications: Conductive plastics;
Structural composite materials;
Flat-panel displays;
Gas storage;
Antifouling paint;
Micro- and nano-electronics;
Radar-absorbing coating;
Technical textiles;
Ultra-capacitors;
Atomic force microscope (AFM) tips;
Batteries with improved lifetime;
Biosensors for harmful gases;
Extra strong fibers.
As a very important field, CNT composites are very promising in aerospace applications because of their superior mechanical properties combined with the low density, fiberlike structure and high aspect ratio. Many studies were carried out in improving mechanical properties of composites by adding CNTs. For example, the tensile modulus and yield strength are increased with the addition of SWNT into a polyimide SWNT composite, a much bigger increase than that for film samples (cast without preferred SWCNT orientation), but much less increase than what would be expected from an oriented discontinuous fiber reinforced polymer composite, which is likely due to inefficient and incomplete dispersion. With an improved dispersion, significant reinforcing effects of the aligned fibers on the mechanical properties are anticipated.
If the composites made of carbon-nanotube/polystyrene were added by 1% of MWNT in weight (about 0.5% in volume), the elastic stiffness was shown to
increase by up to 36%～42%, and the tensile strength by 25%.
Significant toughening of polymer matrices through the incorporation of CNT was reported. A loading of 1% (wt.) MWNT, randomly distributed in an ultra-high molecular weight polyethylene was shown to increase the strain energy density by about 150% and the ductility by about 140%. For secondary crystallites, nucleated from the MWNT, a higher mobility was attributed and hence the increase in strain energy. A similar effect was found in aligned MWNT/polyacrylonitrile. Fibers containing 1.8%(vol.) MWNT were found to be related with an approximately 80% increase in energy to yield and energy to break.
Significant improvements in the mechanical properties of the epoxy/SWNT nano-composites were reported with a 50.8% increase in the storage modulus as illustrated in Fig. 3
Nano-composites consisting of double-wall carbon nanotubes (DWCNT) and an epoxy matrix were shown to have a significant increase of strength, Young’s modulus and strain to failure at a nanotube content of only 0.1%(wt.).
3 Technical issues limiting the advancement of nanotube-reinforced composites The extraordinary stiffness and specific tensile strength of CNTs makes them well-suited for use as reinforcing elements in polymer composites. The incorporation of carbon nanotubes can greatly increase the strength and stiffness of a polymer matrix with minimal increases in weight. One may also expect its ability in protecting against vibration and flame. There are a number of technical difficulties that must be overcome before carbon nanotube reinforced-polymer composites can become commercially available.
Perhaps the most obvious obstacle is that of cost. Pure, high quality carbon nanotubes are definitely very expensive. However, the price of nanotubes has dropped dramatically over the past several years as a result of several efforts focused on realizing the mass production of CNTs. This trend is expected to continue as nanotube synthesis techniques are refined and more manufacturing facilities are brought online.
For CNTs reinforced polymer composites, four critical technical issues will be dealt for effective fiber reinforcement of composite materials: large aspect ratio, interfacial stress transfer, good dispersion, and alignment.
While carbon nanotubes typically have very high aspect ratios, their absolute lengths still remain short, which makes them difficult to be controlled and processed. Most CNTs are only of an order of microns in length, although individual CNTs of some centimeters in length have been synthesized. However, long nanotubes cannot be synthesized in bulk quantities at present.
In order to increase the length of nanotube-based reinforcing fibers, a number of methods for producing longer fibers from shorter nanotubes have been developed. Woven CNT composite fibers up to 100 meters in length have been demonstrated in a thin, continuous, transparent sheet of nanotubes. While its focus is on the development of conductive fibers for wearable electronics, these materials also are well-suited for inclusions within composite materials.
Nanotube sheets, in particular, have been used to fuse together thermally two polymer sheets in a transparent and seamless fashion. Due to their unique
electrical and structural properties, carbon nanotubes tend not to bond strongly with their host matrix or with one another. As a result, the potential increases and improvements in the mechanical properties of a nanotube composite are limited by the degree of interfacial stress transfer that can be achieved. A great deal of study shows that it can be done through chemical functionalization and surface modification of carbon nanotubes. Ion beam irradiation has also been shown to promote crosslinking and improve binding within bundles of nanotubes.
CNT dispersion within a matrix is important to achieve efficient and effective load transfer to the nanotubes. This helps to ensure uniform stress distributions and to minimize the effect of stress concentration. The two primary difficulties associated with dispersion are to separate individual CNTs from each other and then to uniformly mix them with the polymer matrix. Sonication of CNTs within a solvent is one of the most common methods and can deal with both difficulties simultaneously. Shear mixing and magnetic stirring are also commonly used to mix nanotubes within a polymer.
The alignment of CNTs within a matrix is probably the least critical of the four issues for nanotube composites, as the alignment requirements are often dictated by the intended application. Highly-oriented fibers will provide the greatest increase in strength in the fiber direction, but little to no improvement in the transverse direction. In comparison, randomly-oriented fibers will result in isotropic mechanical properties, but at reduced levels of improvement.
5 Summary and conclusions
The exceptional mechanical and physical properties of carbon nanotubes, combined with their low density, make this new form of carbon materials an excellent candidate for the next generation of composite reinforcement. However, there are many economic and technical issues currently to prevent carbon nanotube-reinforced polymers from being applied to large-scale composite structures, especially in high-tech aerospace applications.
The high cost and relatively short lengths of CNTs, combined with the inability to effectively disperse and align them within a host matrix, become the main limits to develop composite structures that could supplement or replace conventional aerospace materials.
However, a great number of studies focus on these issues. Investigations worldwide aim to develop advanced synthesis processes capable of large-scale production of CNTs in macroscopic lengths, and to combine shorter CNTs into longer and more useable composite fibers.
Functionalization and irradiation of polymer-embedded nanotubes and nanotube fibers also have been shown to be able to enhance dispersion and strengthen nanotube-matrix interactions, therefore to improve further the mechanical properties of CNT-reinforced composites.
Despite these studies, much further R&D work is still needed before the full potential of CNTs reinforcing composites can be realized and implemented.
碳纳米管复合材料在航空航天应用
摘要：本文简要介绍了碳纳米管（CNT ）的技术，包括它们的合成和生产，分子结构，独特的性质和在不同领域的潜在应用。具体而言，主要的重点是碳纳米管增强聚合物复合材料，对于航空航天领域的碳纳米管增强复合材料的一些技术问题和主要研究成果会进行讨论。虽然碳纳米管复合材料在航空航天领域的应用是非常有前途，但在认识和实施碳纳米管增强复合材料的全部潜力之前，对于一些关键技术问题的许多研发工作是有必要的。
介绍
复合材料已成为新材料发展最重要的趋势之一。高性能复合材料已取得了很大的进展，尤其是在碳纤维增强塑料（CFRP ）方面。以碳纤维和环氧树脂为主体的复合材料结构在航空航天结构中的比重越来越大。在最近推出的B787客机上，碳纤维复合材料部件占构成总结构重量的50％，包括机身，机翼和水平尾翼等主要结构; 空中客车A380，由碳纤维构成的部分占总结构重量的25 ％。在航空航天应用中，重量轻、高强度的结构材料是永远的目标，碳纳米管增强聚合物复合材料作为一种新一代材料非常具有吸引力。
由于饭岛爱他们的第一项研究进行了近20年，很多关于碳纳米管及其应用的研究工作已经完成。
碳纳米管卓越的物理和机械性能已经被广泛报道，从独特的电子特性，高的热传导性到优异的机械性能，其刚度、强度和韧性优于当前任何材料。碳纳米管为新材料体系根本上的发展提供了巨大的机会。特别地，用碳管增强的纳米复合材料可以提供非凡的比刚度和强度，并在21世纪的航空航天应用上非常有前途。在本文中，对碳纳米管有一个简明的回顾，其中包括基本分子结构，独特的性能，潜在的应用，以及碳纳米管复合材料的最新研究进展，还有纳米管增强聚合物复合材料在航空航天应用中的一些特殊的技术问题。
1碳纳米管（CNTs ）是什么？
碳纳米管（CNT ）是富勒烯结构家族中的成员，是一种由碳制成的、以纳米尺度为直径的（通常从小于1nm 至50nm ）管形材料。一纳米是一米的十亿分之一，或约人的头发丝的万分之一的厚度。现在可以建造一个非常大的、长度与直径比为高达132 000 000:1且明显大于现今任何其他材料的碳纳米管。也就是说，纳米管的直径为几纳米，而长度可以达至18厘米。对于碳纳米管，石墨层看起来有点像一个卷起来的圆柱形纳米结构体，它是连续完整的六角网络状，碳原子在六角形的顶点。
碳纳米管具有许多结构，不同的长度、厚度、螺旋结构和层的数量，通常可以分为单壁碳纳米管（SWNTs ）和多壁碳纳米管（多壁碳纳米管）。
1.1单壁碳纳米管（SWNTs ）
单壁碳纳米管（SWNTs ）是筒状纳米结构的石墨，通常在一个单一的圆柱形壁两端封端。单壁碳纳米管的结构可以是可视作为石墨的层，一个原子厚的，称为石墨，单壁碳纳米管被卷成无缝圆筒，如图1.
大多数单壁碳纳米管通常具有近1纳米的直径。但是这种管需要花费几千倍的时间。 单壁碳纳米管更灵活，但比多壁碳纳米管更难制作。他们可以被扭曲，被抚平，被弯曲成小圆圈或不被破坏而向周围弯曲。
单壁碳纳米管有着显著的电子和机械性能，在许多应用中，它可以用来作为场致发射显示器，纳米复合材料，纳米传感器和逻辑元件等。这些材料在电子制造上具有领先优势，并预期将在下一代微型电子领域发挥重要作用。
1.2多壁碳纳米管（多壁碳纳米管）
多壁碳纳米管既可以是类似的同轴电缆的同轴电缆组件的单壁碳纳米管形式，也可以作为单一的石墨片被卷成如图2所示的涡旋形状的同轴组件形式。
多壁碳纳米管的直径通常是在5〜 50nm 的范围内。多壁碳纳米管的层间距离，与石墨烯层之间的距离接近。
多壁碳纳米管比单壁碳纳米管更容易批量生产。然而，该结构的多壁碳纳米管不太好理解，因为其更大的复杂性和多样性。而地区的结构性缺陷，可能会降低其理想的材料性能。 相比较于多壁碳纳米管的大规模生产，单壁碳纳米管的批量生产所面临的挑战体现在单壁碳纳米管的价格上，目前它高于多壁碳纳米管的价格。
然而，单壁碳纳米管享受高达十倍的更好的性能，在某些特定的应用中，这是一个非常重要的因素。
无论是单壁碳纳米管（单壁碳纳米管）还是多壁碳纳米管（MWCNT ），都可以通过各种不同的过程制得，这些过程大致可分为两类：使用电弧放电或激光烧蚀而高温蒸发；或各种化学气相沉积或催化的生长过程。在高温的方法下，多壁碳纳米管可以利用纯碳的蒸发来生产，但单壁碳纳米管的合成需要金属催化剂的存在。 CVD 方法可以为这两种类型的碳纳米管提供催化剂，也可以生产碳纳米纤维（碳纳米纤维）。
2碳纳米管的性能
碳纳米管的机械和运输性在被很大程度上被研究了。例如，非常高的弹性模量，大于1 TPA （金刚石的弹性模量为1.2 TPA ）还有报道过在重量的一小部分强度为最强的钢的10〜 100倍以上的长处。这意味着，碳纳米管可能会带来一个全新的先进材料。
碳纳米管除了具有优异的机械性能，还有着优异的热和电性能；在高达2 800℃在真空中依然具有热稳定性，热传导率是金刚石的两倍，电流承载能力是铜导线的1000倍。 下面的表（表1和表2 ）是碳纳米管在这些属性上与其他工程材料的比较。
总体而言，碳纳米管显示出独特的刚度，强度和韧性的组合，而其他的纤维材料，通常在这些属性中只具有一个或多个。与其他导电材料相比，其导电导热性也很高。 3碳纳米管的潜在应用
碳纳米管的技术，可用于广泛的应用：
1导电塑料;
2结构复合材料;
3平板显示器;
4储气库;
5防污涂料;
6微型和纳米电子学;
7雷达吸收涂层;
8技术纺织品;
9超级电容器;
10原子力显微镜（ AFM ）的提示;
11电池寿命与改进;
12有害气体的生物传感器;
13超强纤维。
作为一个非常重要的领域， CNT 复合材料在航空航天应用领域非常有前途，因为它具有优越的机械性能，低密度，纤维状结构和高宽比。通过加入碳纳米管以提高复合材料的力学性能的研究也进行了许多。例如，有单壁碳纳米管加入的聚酰亚胺复合材料的拉伸弹性模量和屈服强度增加了，比薄膜样品（没有优先选投SWCNT ）有一个更大的增长，但比预期的取向不连续纤维增强聚合物复合材料的增长的要小得多，这可能是由于低效和不完整的分散。有了改进的分散体，对齐纤维机械性能的显著增强效果是可以预见的。
如果由碳纳米管/聚苯乙烯制成的复合材料中加入由1 ％的多壁碳纳米管的重量（约
0.5％ ），结果表明弹性刚度增加高达36％ 〜42％和25％的拉伸强度。
通过CNT 的加入使聚合物材料的显著增韧被报道了。将1％ （重量）的多壁碳纳米管加载到超高分子量聚乙烯上，并且随机分布在超高分子量聚乙烯，结果显示了应变能量密度增加了约150％，延性增加了约140%。对于从多壁碳纳米管成核的二次晶粒，由于其较高的迁移率，因此应变能增加。在对齐的多壁碳纳米管/聚丙烯腈中发现了类似的效果。含
1.8 ％ （体积）的多壁碳纳米管纤维的能源产量和能源突破被发现有了约80 ％的增长。 环氧树脂涂料/ 多壁碳纳米管的纳米复合材料的力学性能显着改善和储能模量增加50.8 ％被报道了，就像图3表示的那样。
由双壁碳纳米管（DWCNT ）和环氧基体组成的纳米复合材料显示出强度有一个显着的提高，杨氏模量和应变失败碳纳米管含量仅为0.1％（重量）。
3限制了碳纳米管增强复合材料进步的技术问题
碳纳米管非凡的刚度和比抗拉强度使得它们非常适合于用作增强聚合物复合材料中的组成成分。以最小的重量增加，碳纳米管的掺入可以大大增加聚合物基体的强度和刚度。我们也可以预见其防止振动和火焰的能力。在纳米管增强聚合物复合材料可以成为上市销售之前，有许多技术上的困难必须克服。
也许最明显的障碍是成本。纯净，高品质的碳纳米管肯定是相当昂贵的。然而，在过去几年碳纳米管的价格已经大幅下降，那是因为有几次尝试专注于实现大规模生产碳纳米管。这一趋势预计将因为碳纳米管合成技术的完善和更多的生产设施联机而继续。
对于碳纳米管增强聚合物复合材料而言，有效的纤维增强复合材料有四个关键技术问题将被解决：大长径比，界面应力转移，良好的分散性和对齐处理。
虽然碳纳米管通常具有非常高的长径比，它们的绝对长度仍短，这使得它们很难进行控制和处理。虽然有些厘米长的单根碳管被合成出来，但大多数的碳纳米管以微米长排列。然而，长的碳纳米管目前不能被大宗合成。
为了增加基于纳米管的增强纤维的长度，从较短的碳纳米管到更长的纤维的制造方法一已经被开发。由薄、连续、透明的薄片纳米管编织的CNT 复合纤维已被证明有100米长。虽然它的重点是可穿戴式电子产品的导电纤维的发展，但是这些材料也非常适合作为在复合材料内的夹杂物。
碳纳米管片材，特别是，已被用一个透明和无缝的方式融合在一起的两个热聚合物片材。由于其独特的电气和结构特性，碳纳米管往往和主体或另外一个没有强烈的结合。其结果是，通过界面应力转移的程度，碳纳米管复合材料机械性能的潜在增加和改进受到限制。大量的研究表明，可以通过对碳纳米管进行化学修饰和表面改性来实现。离子束照射也已表明促进了交联，并改善了碳纳米管束内的约束力。
为实现高效和有效将载荷转移到纳米管，CNT 分散在基体中是很重要的。这将有助于确保应力分布均匀，并尽量减少应力集中的效果。与扩散相关联的两个主要的困难是将单个碳纳米管一一分开，然后让它们与聚合物基体均匀地混合。超声处理在溶剂中的碳纳米管是最常用的方法之一，可以同时解决两个困难。也常用剪切混合和磁力搅拌将碳纳米管混合到聚合物内。
对于碳纳米管复合材料来说，碳纳米管在基体中的对齐方式可能是四个问题中最关键的一个，因为对齐的要求，往往取决于预期的应用。高度定向的纤维将提供在纤维方向强度的最大增加，但在横向方向上几乎没有改善。相比较而言，随机取向的纤维将导致各向同性的机械性能，但是降低了改善程度。
5总结和结论
碳纳米管特殊的机械和物理性能，结合其低密度，使这种新形式的碳材料成为下一代增强复合材料的一个出色的候选人。然而目前也有许多经济和技术问题，阻止了碳纳米管增强
聚合物应用于大型复合材料结构中，尤其是在航天高科技的应用。
高成本和长度相对短的碳纳米管，结合其无法有效地分散和对齐它们内部的基质，成为了其发展成为可以补充或取代传统航空航天复合材料结构的主要限制。
然而，大量的研究集中在这些问题上。全球范围内的目标是开发先进的合成工艺，能够在宏观长度上实现碳纳米管的大规模生产，并将较短的碳纳米管结合到较长和更多的可用复合纤维上去。
嵌入聚合物的纳米管和纳米管纤维的功能化与照射也已被证明能提高分散性和加强纳米管 - 基质之间的相互作用，因此也进一步提高了CNT 纤维增强复合材料的机械性能。 除了这些研究，为实现和落实充分发挥碳纳米管增强复合的潜力，进一步的R ＆D 工作仍被需要。