【作者】 丁兆勇；

【导师】 孙保民；

【作者基本信息】 华北电力大学（北京） ， 热能工程， 2011， 博士

【摘要】 为了设计出有望实现大量、连续、低成本、工业化合成的反应器和实验方案,首先设计出了类棱台型反应器,系统地研究了工艺参数对产物的影响,分析了碳纳米管的合成机理,最后对合成的碳纳米管做了初步的纯化处理。实验得到的主要产物为多壁碳纳米管,也得到了单壁碳纳米管。类棱台型反应器可以避免反应物与空气充分接触,通过调整热解区和合成区之间的距离可以达到热解和合成互不影响,实现原料的热解和碳纳米管的合成在较佳条件下进行。碳源过多(或过少)与催化剂过少(或过多)结果相似,都表现为催化剂被碳源包裹或催化剂迅速长大失活。氢气和氦气的过多或过少表现为吹扫作用过强或过弱,过强将导致碳原子不能附着在催化剂颗粒表面,过弱则不能有效净化管和催化剂颗粒的表面而导致管较粗糙和催化剂颗粒失活。氢气还具有活化催化剂的作用,氦气还起到局部稀释和降温的作用,起携带催化剂的氦气还影响到催化剂的供应速度。取样时间在前60s时未有碳纳米管合成,在第70s时才在颗粒团周围有了管的萌芽,原因在于取样时间较短时基板温度较低,催化剂颗粒和碳原子簇到达基板后被迅速冷却,随着取样时间增加基板的温度升高,到达基板的催化剂颗粒不再被冷却为熔融度较低的状态,具有了催化活性,实验同时根据取样时间对碳纳米管合成的影响得出碳纳米管在增长的同时也在增粗,最初在碳纳米管开始合成前聚集在基板表面的是碳原子簇包裹催化剂颗粒形成的纳米胶囊与催化剂颗粒和碳原子簇的混合物,同时最初生成的聚集物覆盖了整个基板也证明了基板本身成分不能参与催化碳纳米管合成。热解温度对碳纳米管合成的影响主要是温度对一氧化碳热解的影响,影响了碳源的供应速度,而由于五羰基铁在相对较低的温度下即可迅速热解,因此温度对五羰基铁的影响较小。合成温度较高时催化剂颗粒表面的碳原子簇容易被气化,即使有少量碳纳米管生成也很容易被灼烧,温度过低时催化剂颗粒熔融度较低,碳原子簇不能溶入或溶入后扩散缓慢,也不利于碳纳米管合成。雾化为小液滴的五羰基铁做催化剂原料适于在相对温度较低的情况下催化合成碳纳米管,而铁铝氧化物和铁铝钼氧化物适合在相对温度较高的条件下催化合成碳纳米管。研究了不同种类和厚度的取样载体,产物相差较大,分析认为影响表现在载体本身的性能而不是材料元素直接参与催化碳纳米管合成。取样位置对碳纳米管合成的影响也主要体现在合成温度的影响。研究了采用物理法、化学法和综合法对合成的碳纳米管的纯化,单纯的物理法和化学法都不能达到清除掉催化剂颗粒和无定形碳等杂质的目的,石墨片层包裹催化剂颗粒形成的纳米胶囊即使通过综合法也不容易除去。同时研究了在热解区取样合成碳纳米管,这是一个可以实现连续生产的化学气相沉积法的改进,得到了管径较小的碳纳米管,同时进一步证明了五羰基铁做催化剂原料不适于在相对温度较高的情况下催化合成碳纳米管而铁铝氧化物和铁铝钼氧化物适合。本文对碳纳米管的合成机理做了探讨,认为碳纳米管的合成机理符合传统的“溶解-扩散-析出”理论,不同点是碳源在高温下还未与催化剂颗粒接触前就已经分解生成碳原子,同时碳纳米管在长长时管层也在增厚,并且碳纳米管管层的增厚不仅仅局限在管的最外侧,有些在管的最内侧也有层数的增加；也对实验中得到的几种特殊结构的碳纳米管的合成机理做了分析,认为条件的非最佳是导致碳纳米管畸形的主要原因。实验中同时得到了碳纤维、碳纳米洋葱和纳米胶囊等副产物。更多还原

【Abstract】 In order to realize industrialization synthesis of carbon nanotubes which is large scale, continuous, and low cost, a pyramid shaped reactor and experiment method were designed first. Then systematically studied the influence of process parameters on product and analyzed the synthesis mechanism of carbon nanotubes, and made the preliminary purification processing in the last.In this experiment, the main product is muiti-walled carbon nanotubes, single-walled carbon nanotubes also was synthesized. Pyramid shaped reactor can preclude the reactant from meeting the air. By adjusting the distance between the pyrolysis area and synthesis area, pyrolysis and synthesis do not affect each other can realized, and the temperature of pyrolysis and synthesis is at the best situation. Carbon sources too much (or too less) and catalyst too little (or excessive) are similar, all show that the catalyst is packaged by carbon source or catalyst deactive rapidly. Too much or too little hydrogen and helium show purging too strong which can bring about that carbon can not adsorb on catalyst or too weak which can not cleanse tubes and catalyst particles. In addition, hydrogen can make catalyst active, helium has local dilution and cooling effect, since the helium also carry catalyst into reactor which affect the supply speed of catalyst. There were not carbon nanotubes synthesized at first 60s of sampling time, only tube shape buds around particle group at 70s. The reason is that when the sampling time is short, the temperature of substrate is so low that catalyst particles and carbon atom clusters will be cooled as soon as they reached. With the increases of sampling time, the temperature of substrate rise, catalyst particles are no longer be cooled to a lower molten degree when they reach the substrate and with the catalytic activity. Experiments also conclude that carbon nanotubes grow thick with the time from the research on sampling time. Before carbon nanotubes can synthesis, the accumulation which coated on the surface of substrate is the mixture of carbon atom clusters, catalyst particles, and nanocapsules which carbon packaged catalyst particles forming. The accumulation covering the entire substrate also proved that the substrate can not participate in catalytic synthesis of carbon nanotubes. Pyrolysis temperature mainly influence the pyrolysis of carbon monoxide, and little to iron pentacarbonyl because iron pentacarbonyl can rapidly decompose at low temperature. Too high synthesis temperature easily gasify the carbon atom clusters from the surface of catalyst, and burn the synthesized tubes if some can. At low temperature, the molten degree of catalyst is too low to allow carbon atom clusters to dissolve into, or diffuse slowly. Small droplets iron pentacarbonyl as catalyst source are suitable to catalyse synthesis of carbon nanotubes at relative low temperature, while Fe-Al-O and Fe-Al-Mo-O compound are opposite. By the different sample carrier, products were difference. It is concluded that the influences is the capability of the carrier itself rather than material elements directly involved in to catalyse synthesis of carbon nanotubes. The influences of sampling location to synthesis of carbon nanotubes are mainly reflected in the synthesis temperature. Physical, chemical and comprehensive methods were used to purify carbon nanotubes. Only physical or chemical method can not remove all of the impurity, even if comprehensive method can not easily remove the nanocapsules. Sampling at pyrolysis area is a improvement of the traditional chemical vapor deposition which can realize continuous production and get small diameter carbon nanotubes. And further proved that iron pentacarbonyl as raw materials is not suitable in a relatively high temperature while Fe-Al-0 and Fe-Al-Mo-O compound can. In this paper, the synthesis mechanism of carbon nanotubes also be discussed. It is concluded that synthesis mechanism of carbon nanotubes accord with the traditional" solution-diffusion-precipitation " theory, but the difference is that the carbon source has decomposed at high temperatures before contact with the catalyst particles. Carbon nanotubes are also being thicker while growing, and their being thicker is not limited to the outermost, some of the most inside tube layers also increase. The synthesis mechanism of some special structure carbon nanotubes also be analyzed, that the main reason is the non-optimal conditions. Carbon nanofibers, carbon nanoonions, and nanocapsules also be produced at experiment.更多还原