【作者】 霍俊平；

【导师】 宋怀河；

【作者基本信息】 北京化工大学 ， 材料学， 2007， 博士

【摘要】 碳包覆纳米金属晶(Carbon-encapsulated metal nanocrystals，CEMNs)是一种新型的纳米碳／金属复合材料，其中数层石墨片层紧密围绕纳米金属晶有序排列，纳米金属粒子处于核心位置，形成核壳结构。碳包覆层的存在，可避免纳米金属的氧化和环境降解，还可以提高某些金属与生物体之间的相容性。由于其独特的结构和性能，碳包覆纳米金属晶可望在高密度磁记录材料、铁磁流体、电波屏蔽材料、氧化还原催化剂、核废料处理材料和生物医用材料等方面获得广泛应用。针对目前碳包覆金属纳米晶制备中存在的产物纯度低、形态结构不易控制和难以大量合成等缺点，本论文采用一种新型制备方法一共热解法，通过芳烃物质和有机金属化合物在中温下的共炭化热解制备CEMNs，该方法具有工艺路线简单、反应条件温和、产物组成均匀、结构易于控制及容易实现大量制备等特点。本论文主要以一种精制石油重油为碳源、以二茂铁为金属源，采用共热解法制备碳包覆纳米金属晶。在详细考察制备工艺参数对CEMNs形成、转化及其形态结构影响的基础上，利用TEM、HREM、XRD、SEM、TG／DSC、VSM等测试分析手段研究了材料的形貌、结构和性能，并与制备工艺相关联，实现材料的可控合成；阐明CEMNs的形成机理及其热转化行为。对CEMNs进行氧化和炭化处理，研究此过程中材料形貌和结构的转化行为及机制，为多形态炭纳米材料的研究开发奠定基础。研究结果表明，以芳烃重油为碳源、二茂铁为金属源，采用共热解法，通过调节工艺参数，可以大量制备碳包覆铁纳米颗粒和碳包覆铁纳米棒。在480℃的反应温度下，随着二茂铁添加量的增加(2wt．％至45wt．％)，产物的形貌由碳包覆纳米铁颗粒，逐渐向生成碳包覆纳米铁棒的方向发展。在450℃的反应温度下，随着二茂铁添加量从30wt．％增加到120wt．％，产物的形貌又由碳包覆纳米铁棒，逐渐向生成碳包覆纳米铁颗粒的方向转变。反应温度的升高或保温时间的延长，都使产物形貌发生相同的转化。同时，随着二茂铁添加量的增加，产物中包覆在碳壳中的金属核由单质α-Fe相逐渐转变成碳化三铁，外壳碳层由无定型结构转变成湍层结构。通过原料组成、结构分析和合成工艺参数与产物形貌和结构的关系研究，推断共热解法形成碳包覆纳米金属晶是基于气液两相催化热缩聚的原理，铁纳米颗粒对于芳烃分子的催化作用是基于溶解析出机制。碳包覆纳米金属棒的形成需要两个必要因素：体系中有足够的铁纳米原子簇和适中的反应活性。经惰性气氛下1000℃炭化处理，碳包覆铁纳米晶尺寸变大，大多数纳米铁颗粒从芯部逸出，形成大量碳空心洋葱结构，碳层也在纳米铁核的催化作用下由无定型结构转化为石墨质晶化结构。在250℃氧气气氛中对碳包覆铁纳米棒进行氧化处理，发现：大量铁纳米颗粒从碳层中扩散和逸出，得到少量金属纳米颗粒填充的碳纳米棒，通过进一步的低温和高温炭化处理制备出长度在150-350 nm、直径在30-50 nm的短纳米碳管。磁性能研究表明，碳包覆铁纳米晶及其炭化产物都具有较高的矫顽力和较小的矩磁比值，呈现出一定的铁磁性和顺磁性的综合磁性能。通过原料选择、工艺参数控制和后续氧化炭化处理，由共热解法不仅可以合成出碳包覆纳米金属颗粒和碳包覆纳米金属棒，而且还可以制备出空心碳纳米洋葱和短纳米碳管等多种新型纳米炭材料，说明我们发明的共热解法是一项制备多形态纳米碳材料的平台技术。其深入研究对于碳包覆纳米金属晶的大量制备、促进其实际应用，以及合成、组装或构建碳纳米管及相关新的碳纳米结构等都具有十分重要的理论和实际意义。更多还原

【Abstract】 Carbon-encapsulated metal nanocrystals （CEMNs） are a new kind of carbon/metal nanocomposite, in which graphite layers arrange around metal nanocrystals located in the center to form core-shell structure. The carbon layers provide the oxidation resistance of bare metal nanoparticles and prevent them from environment degradation, and can endow some metal nanoparticles with biocompatibility. CEMNs, especially the magnetic nanomaterials, possess special structures and properties, and thus might have important applications in areas such as high-density magnetic data storage, ferrofluids, microwave absorption materials, oxidation-reduction catalyst, handling materials of radioactive waste and bio-medical materials.For the preparation of CEMNs, the present techniques, such as the arc-discharge technique, chemical vapor deposition, liquid-impregnating carbonization of non-graphitizing carbon and etc., show some disadvantages, e.g., the morphology and the structure of the resulting nanocrystals are not easy to control, and it is difficult to produce high-purity nanocrystals in large quantities. While here we proposed a novel method for the preparation of CEMNs by co-carbonization of aromatics with metal compounds, i.e., co-pyrolysis method. It featured in simplicity, low temperature, good controllability and high yield.The research focused on the preparation of CEMNs by co-pyrolysis of a refined aromatic heavy oil and ferrocene. The effect of the synthesis parameters on the formation and transformation of CEMNs were investigated in detail. The morphology, structure and particular properties of the product were characterized via TEM, HREM, XRD, SEM, TG/DSC and VSM measurements, and the relationship between them and synthesis parameters were studied to realize the controllable preparation of the product. The formation mechanism and thermo-transformation behavior of CEMNs were elucidated. In addition, the oxidation and further carbonization treatments on CEMNs were carried out and some interesting phenomenon and new results were obtained, which will provide a novel strategy for the research and development of various carbon nanomaterials.The results show that large amount of carbon-encapsulated iron nanoparticles （CEMPs） and nanorods （CEMRs） were obtained by co-pyrolysis of an aromatic heavy oil and ferrocene. With the increase of ferrocene content from 2 wt. % to 45 wt. % under the reaction temperature of 480℃, the morphology of the product had a transformation from CEMPs towards CEMRs. While with the increase of ferrocene content from 30 wt. % to 120 wt. % at 450℃, the morphology of the product were transformed from CEMRs to CEMPs. And the same morphology transformation took place with the enhancement of reaction temperature or the elongation of soaking time. Meanwhile, with the increase of ferrocene content, the encapsulated metal core in the product were changed from singleα-Fe to iron carbides （Fe₃C）, and the carbon shells were turned from amorphous into turbostratic structure.By the analysis of raw feedstock and the investigation of the relationship among the morphology, structure of CEMNs and the synthetic parameters, it was concluded that the formation of CEMNs by co-pyrolysis method was based on the principle of condensation and polymerization in a vapor-liquid bi-phase by the aid of catalysis under pressure, and the catalysis of iron nanoparticles was based on the dissolution-precipitation mechanism. It is assumed that two essential factors must be met for the formation of the nanorods, including the sufficient amounts of iron clusters and moderate reaction activity in the system.By carbonization at 1000℃under N₂ atmosphere, the size of CEMNs became larger and the carbon shells were changed from the disordered into graphitic structure under the catalysis of nano-sized iron core. At the same time a large amount of hollow onion-like carbon nanoparticles emerged due to the ejection of iron nanoparticles. After oxidation of as-grown nanorods at 250℃, large amounts of ferric nanoparticles diffused and ejected from carbon shells, leading to the formation of carbon nanotubes with several nanoparticles trapped inside, and short nanotubes with diameter of 30-50 nm and length of 150-350 nm were generated by further carbonization at low and high temperatures.Both as-grown CEMNs and the carbonized product had a high coercivity and a small ratio of remanent magnetization （Mr） to saturation magnetization （Ms）. It is implied that CEMNs show a combined magnetic characteristic of ferromagnetic and paramagnetic materials.The above research afforded a novel and effective method for the preparation of various carbon nanomaterials. It can be seen that, via the selectivity of raw materials, the controllability of synthesis parameters and the post-oxidation and carbonization treatments, not only CEMNs including nanoparticles and nanorods but also empty carbon onions and short carbon nanotubes were obtained by co-pyrolysis method. Moreover, it had a great significance for the promotion of the practical applications of CEMNs and for the synthesis, assembly and construction of carbon nanotubes and related nanostructures.更多还原