宏量可控制备碳纳米管阵列

附件2论文中英文摘要格式作者姓名：张强论文题目：宏量可控制备碳纳米管阵列作者简介：张强，男，1984年3月出生，2004年9月师从于清华大学魏飞教授，于2009年7月获博士学位。

中文摘要化学工程为现代高新技术产业的发展提供了最基本的生产手段与技术。

近年来，纳米技术的蓬勃发展为新型工业开拓了科学和工程技术应用空间。

作为一种具有一维管状结构的纳米材料，碳纳米管在力学、热学、电学、光学、声学等方面表现出优异的性能，成为纳米领域中最受关注的对象之一。

随着应用研究的深入，人们发现，相互缠绕的聚团状碳纳米管往往作为一种化工原材料添加到最终产品中，这样就难以充分发挥碳纳米管的优异性能。

相关研究表明，碳纳米管的取向和排列可以显著影响其作为宏观材料的性能。

如果能够将碳纳米管做成高度规整的定向阵列结构，那么阵列本身就是带有功能的产品——超级弹簧、定向薄膜、过滤器、电池电极、场发射体等；碳纳米管阵列也可以经进一步加工形成人工合成的超强纤维、电子器件、高性能复合材料，从而极大地提高材料的性能；由于碳纳米管阵列的易分散性，即使破坏碳纳米管阵列的排列，将其单分散后应用于导电、导热、力学增强复合材料时仍表现出比聚团状单壁和多壁碳纳米管更为优异的性能。

所以，碳纳米管阵列是诸多类型碳纳米管材料中的高端产品。

虽然高度规整的碳纳米管阵列已经开发出很多应用。

但是，时至今日，基于碳纳米管阵列的所有应用还不能够看到明确的实用前景。

其重要原因是可控、大规模宏量制备应用研究所需的宏量规模的定向碳纳米管阵列样品仍然非常困难。

众多研究者采用高纯硅片作为基板，每批次仅获得几毫克碳纳米管阵列样品。

碳纳米管阵列的市售价格可高达2000美元/片硅片。

因此，要实现纳米技术为人类造福的目标，首要的问题就是能够探索出可在工业规模上大量生产碳纳米管阵列的方法。

而探索宏量可控制备碳纳米管阵列的科学以及开发工业生产碳纳米管阵列的技术是化学工程发展中的新问题。

本文研究了碳纳米管阵列所涉及到的各个尺度上的科学问题，并应用纳米过程工程的基本方法分析了碳纳米管阵列的宏量制备，探索制备过程中的技术和控制手段，为碳纳米管阵列的宏量制备提供一个切实可行的技术路线，进而为碳纳米管阵列的实用化铺平道路。

本文在理解碳纳米管团聚结构特点的基础上，对碳纳米管阵列的生长机理进行了探索。

采用时空分析方法，指出在原子/分子层次上，碳纳米管的生长遵循气液固生长机制。

此模型较好地解释单根碳纳米管如何在纳米金属催化剂生长，以及碳纳米管的直径控制，但是并未解释碳纳米管如何有效组装形成有序阵列。

我们通过催化剂标记的方法，发现在单根尺度上，碳纳米管阵列遵循底部生长机制，即碳纳米管的生长点在其与基板结合的根部，但没有解释碳纳米管的组装行为。

进一步在整体尺度上采用扫描电子显微镜和Raman光谱对分层生长技术获得四层碳纳米管阵列进行表征，发现本研究获得的碳纳米管阵列中存在两种类型的碳纳米管：类型一直管和类型二弯管。

通过Raman光谱中G峰峰位偏移指出碳纳米管阵列中存在应力。

在应力的作用下，弯管的形貌发生了显著的变化，其曲折因子随着碳纳米管距其顶部的距离增加而下降。

我们推测，碳纳米管阵列的生长是一种协同作用自组织生长的过程：催化剂颗粒均匀分散到基板表面，当引入碳源后，直管在基板表面先长出，形成了无序网络，弯管受到直管的空间位阻而被迫垂直生长；直管受到弯管的推动也垂直定向生长；直管和弯管协同作用形成有序阵列结构。

这一从无序缠绕到有序组装的过程可利用液晶形成相关的Onsager virial理论进行描述。

高催化剂颗粒密度(>1010/cm2)分布在较小曲率(曲率半径>1 μm)的基板上有利于碳纳米管阵列的生长。

以此为基础，通过控制催化剂的尺寸分布和方差，缩小催化剂活性的差异，形成超顺排可抽丝的碳纳米管阵列。

通过缩小碳纳米管生长区域、减弱协同作用以及扩散限制，维持其自由生长，可以获得长度高达14.5 mm的超长碳纳米管阵列。

为克服平整基板上碳纳米管阵列生产工程化遇到的比表面积有限的瓶颈，提出了颗粒表面辐射生长碳纳米管阵列的方法，并依次采用比表面积约为2.0×102 cm2/g的陶瓷球、1.0×104 cm2/g的石英纤维、2.4×105 cm2/g的层状双羟基金属氢氧化物（LDH）片状颗粒作为生长基板，在其表面大量辐射生长碳纳米管阵列。

这些新型颗粒状基板具有高比表面积、低成本、易流动、在反应器内可连续输运等优点。

通过化学手段在这些新型颗粒基板表面形成高密度纳米金属催化剂颗粒，放入反应器内加热到碳纳米管阵列生长温度，通入如乙烯、丙烯、液化石油气、环己烷或二甲苯等烃类物质作为碳源，即可以在其表面高效率的生长碳纳米管阵列。

当采用直径为0.7 mm的陶瓷球作为生长基板，可以在球表面辐射生长长度至1.2 mm，直径为10-50 nm的多壁碳纳米管阵列；采用FeMgAl-LDH作为生长基板，通过还原促进LDH骨架所含金属离子转变为金属催化剂颗粒，通过化学气相沉积，可在LDH表面法向生长长度至50 μm，直径约为4-7 nm的双壁碳纳米管阵列。

为解决碳纳米管阵列形貌在宏量制备过程中阵列易损坏问题，提出了在片层材料中插层生长碳纳米管阵列的方法。

选用自然界中存在的天然片层材料，如蛭石、云母等具有层状结构和可交换性层间离子的一类粘土化合物作为碳纳米管生长基板，通过离子交换将金属催化剂活性组分插层到片层中，然后进一步煅烧和还原，形成活性催化剂颗粒高度分散在层间的碳纳米管阵列生长催化剂。

通过化学气相沉积过程，在一颗厚度约为3 mm的层状蛭石颗粒中，约有3000层垂直定向的碳纳米管阵列插层生长。

在每两片片层中间，碳纳米管阵列相向生长，使层状颗粒在轴向方向上进一步膨胀；在每一片粘土片层上，碳纳米管阵列都垂直于片层生长。

在所得的阵列产物中，碳纳米管的直径约为7-13 nm，长度可在0.10-100 μm范围内调变。

针对碳纳米管阵列产量有限的现状，提出采用Fe/Mo/蛭石片层催化剂在流化床中生长。

在流化床中，气体通过包括催化剂和生长的碳纳米管的固相颗粒层，使固体颗粒处于悬浮运动状态进而进入流化状态。

当进行化学气相沉积时，一方面，流化床反应器提供了定向碳纳米管生长足够的空间；另一方面，颗粒运动强化了热量和质量传递，提供了充足的物质交换和热量交换，进而可以连续稳定地大量生长碳纳米管。

采用厚度在50-100 μm，直径为100-300 μm 的片层状可流化的颗粒作为碳纳米管阵列生长催化剂，当气速高于起始流化速度之后，片层催化剂处于流化状态。

通入碳源之后，碳纳米管阵列在片层催化剂中插层生长，定向生长行为也不会受颗粒之间的碰撞影响。

据此，在保证催化剂可流化的前提下，采用碳源空速与催化剂比表面积比值不变的原则，成功实现了碳纳米管阵列的工程放大。

采用直径为500 mm的流化床反应器作为工业中试，以片层状催化剂载体，通过流化床化学气相沉积过程，实现了产量为3.0 kg/hr的碳纳米管阵列宏量制备。

这种依靠化学气相沉积过程自组织形成的上万层的垂直自组织生长的一维、二维复合纳米结构材料具有极好的抗冲击性能；长度为25 μm的碳纳米管阵列插层复合物的能量吸收值高达149 kJ/kg，远高于传统的泡沫材料(约36 kJ/kg)。

这是由于蛭石片层和25 μm的碳纳米管阵列尺寸匹配较好，从而使较硬的无机片层能够把应力传递给耐压缩的碳纳米管阵列。

即使经过二十次压缩，碳纳米管阵列仍保持较好的取向，进而表现出优异的抗冲击循环稳定性。

这说明碳纳米管阵列在复合材料、缓冲抗震等大规模工业民用领域有突出的性能和广泛的应用前景。

关键词：碳纳米管阵列；化学气相沉积；催化剂；宏量制备；流化床。

Mass Production of Vertically Aligned Carbon Nanotube ArraysZhang QiangABSTRACTChemical engineering is one of the advanced technologies for the development of modern society. In recent years, the rapid development of nanotechnology has opened up new intriguing industrial applications. Carbon nanotubes (CNTs), which are cylindrical tubes composed of wrapped graphene sheets, possess extremely high tensile strength, high modulus, large aspect ratio, low density, good chemical and environmental stability, and high thermal and electrical conductivities. Various applications based on CNTs have been demonstrated. However, it is noticed that the agglomerated CNTs showed limited improvement on products, and the extraordinary performance of CNTs were not fully illustrated. The alignment of CNTs greatly affects the performance of CNT in macroscopical applications. If the CNTs can be aligned to form ordered CNT arrays, the as-grown arrays can be directly used as nano-spring, anisotropic conductive materials, multi-functional membranes, filters, battery electrodes, and field emitters. Aligned CNTs can also be further processed into strong fibers, electronic components, and high-performance composites. Even with the loss of the original alignment, longer and straighter multi-walled CNTs from the aligned CNT arrays are found to be better than randomly aggregated multi-walled or even single-walled CNTs in improving the electronic, mechanical, and thermal properties of a polymer. Vertically aligned CNT arrays were one of the advanced materials to demonstrate the fascinating properties of nanomaterials.Up to now, many intriguing applications based on aligned CNTs have been proposed. However, not all of them are with clear practical-prospects. The major reason is that the availability of aligned CNTs in large amount is still very limited. Usually, expensive silicon wafers were used as the growth substrate, and only several milligrams of aligned CNTs samples can be obtained in a batch of chemical vapor deposition. The price of aligned CNTs was estimated to be 2,000 U.S. dollars/piece. If the goal of nanotechnology were for human benefit, one of the urgent tasks is the ability for mass production of high quality CNT arrays. Exploring the science and developing the technology for industrial production of aligned CNT arrays becomes one of cutting-edge issues in chemical engineering. In this thesis, the scientific problems of aligned CNT growth are discussed,and the mass production of aligned CNT arrays is analyzed by nanoscale process engineering methods. Based on the novel scientific understanding, a practical route to the mass production of aligned CNT arrays is proposed and realized to promote the applications of aligned CNTs.Based on the understanding of CNT array structure, the growth mechanism of CNT arrays was proposed. On the atomic scale, the vapor-liquid-solid model explains how a single CNT grows out from the catalyst, and the diameter of CNTs in the arrays can be easily modulated through nucleation and sintering of the metal catalyst particles during the heterogeneous catalysis process. The growth sites of individual CNTs in the array were at the bottom of the array. To illustrate the synchronous growth of CNTs, a multi-layered aligned CNT array grown by a four-step interim reactant supply reaction was characterized by the scanning electron microscopy and Raman spectra. These were used to deduce that the pristine stress was present in the CNT arrays. A model of the stresses induced by space limitation and different growth rates was applied to explain the development of a synchronous growth of the CNT arrays. Large amount of CNTs were self organized into array structure based on the interaction among CNTs, which can be described by the Onsager virial theory. The CNT arrays can be easily synthesized on a low curvature substrate with high density active metal particles (>1010/cm2). By controlling the formation of uniform catalyst with a narrow size distribution, spinnable aligned CNTs were obtained by an improved floating catalyst chemical vapor deposition method. Moreover, if CNTs were synthesized in a small area, they can grow freely into super long CNTs with a length of 14.5 mm.To overcome the limited surface area for the growth of CNT arrays on flat substrates, radial growth of CNT arrays on the surface of various particles was proposed. Spherical, fibrous, and flaked layered double hydroxide particles with a specified surface area of 2.0×102, 1.0×104, and 2.4×105 cm2/g, respectively, were used as substrates for large scale growth of aligned CNTs. These powder substrates exhibit large specified surface area, low cost, good flowability, and can be transferred easily in the reactor. When high density of metal catalyst particle distributed on their surface, with introduction of various carbon sources (such as ethylene, propylene, liquified petroleum gas, cyclohexane, and xylene) at growth temperature, aligned CNTs were efficiently synthesized on the surface of those particles. For instance, multi-walled CNTs in array form with a diameter of 10-50 nm and a length of 10-1200 μm were radially synthesized on ceramic spheres. Double-walled CNTs in array form with a diameter of 4-7 nm and a length of 50 μm were grown on FeMgAl layered double hydroxide flakes.To avoid the damage caused by the collisions among CNT arrays during th e process of transport and fluidization, a strategy of intercalated growth of CNT arrays among layered catalyst was proposed. The layered vermiculite catalyst with dispersed active particles among the layers was prepared by an ion-exchanged method. Various natural clays, such as vermiculite, mica, were with a lamellar structure and exchangeable ions were used as growth substrates. The clays were impregnated with catalyst seeds in solution to exchange active metal phase among the layers. After calcination and reduction in a reducing atmosphere, metal catalyst nanoparticles were formed in a high density among clay layers. Through chemical vapor deposition method, aligned CNTs were intercalated grown among the natural clay particle. For example, about 3000 layers of aligned CNT films were intercalated into one vermiculite block of 3 mm thickness. Each interlayer consisted of two aligned CNTs that were grown from opposite clay surfaces and joined in the middle. The as-grown CNTs exhibited uniform length and good alignment, and the vermiculite sheets along the perpendicular direction could be distinguished. Vertically aligned CNT arrays with a diameter of 7-13 nm and a length of 0.10-100 μm, were intercalated among inorganic layers.Large amount of CNT arrays can be produced in a fluidized bed reactor on the lamellar catalysts. In a fluidized bed, the solid particles (such as catalyst and the as-grown products) transformed into a fluid-like state through suspension in gas. The fluidized bed reactor has great advantages in terms of enough growth space, excellent diffusion and heat transfer, easiness in scaling up and continuous operation for CNT production. Multilayer structured lamellar particles with a thickness of 50-100 μm and a diameter of 100-300 μm were considered as fluidizable catalysts, which exhibit dense phase expansion after minimum fluidization velocity. The aligned CNTs can be intercalatedly grown within a single lamellar particle, and the collisions among CNT arrays during growth can therefore be avoided. The CNTs grown among vermiculites through the fluidized bed chemical vapor deposition possess good alignment. By keeping the ratio of carbon source space velocity to catalyst surface area a constant and maintaining the well fluidization state, the synthesis of aligned CNT arrays was scaled up. A 3.0 kg/hr CNT array productivity was realized in a fluidized bed reactor with a diameter of 500 mm. The CNTs in the as grown arrays were with good alignment, and can be easily purified.The introduction of strong yet flexible CNTs makes the self-organized CNT arrays intercalated composites highly compliant and resilient, in contrast to bare inorganic compounds that are rigid and brittle. The CNT-intercalated composite can be repeatedly compressed at high strains, with an energy absorption capacity of 149 kJ/kg when thelength of CNTs was 25 μm, which is much higher than CNTs in cushioning polystyrene foams (36 kJ/kg). The enhanced mechanical properties can be attributed to the combination of aligned CNTs with inorganic layers. The CNTs still keep good alignment after 20 cycles of compression. The obtained aligned CNT arrays intercalated composites are with superior mechanical properties as energy absorption, dumping, shock absorbing materials, which are highly necessary for aircraft, container, car, and so on.Key words: carbon nanotube array; chemical vapor deposition; catalyst; mass production; fluidized bed.。