快速生长超长定向碳纳米管阵列的制备及机理

 采用一种无模板的化学气相沉积法裂解金属有机物,以二茂铁为催化剂,二甲苯为碳源,利用单温炉加热装置在100 min内成功制备了2.7 mm超长定向碳纳米管阵列,生长速率高达27 μm·min^-1。运用扫描电子显微镜、透射电子显微镜、拉曼光谱对定向碳纳米管阵列进行形貌观察和表征,结果表明:制得的碳纳米管阵列具有优越的定向性和管结构,并且石墨化程度高。给出了快速生长超长定向碳纳米管阵列的优化制备条件,结合表征结果讨论了碳纳米管阵列的生长机制,认为超长碳纳米管阵列采用的是一种催化剂固定不动的开口生长方式,碳源和催化剂的连续供应保证了超长碳纳米管阵列的快速生长。     

基于HRTEM和Raman分析定向多壁碳纳米管阵列的生长机理

以二茂铁、二甲苯为前驱体,石英为衬底,在850 oC的管式炉内采用化学气相沉积法制备出了定向碳纳米管阵列. 高分辨透射电子显微镜和拉曼光谱的结果表明：碳纳米管阵列具有良好的定向性和多壁管状结构,石墨化程度高,并且只在表面存在少量单壁碳纳米管.定向多壁碳纳米管阵列的生长模式为“底部”生长模式,即在生长的初期,当催化剂颗粒较小时,析出的碳原子生成了单壁碳纳米管或与其性质类似的多壁碳纳米管(一般层数小于5层);催化剂颗粒逐渐长大后,大量的碳原子析出后生成了普通的多壁碳纳米管,从而形成了单壁碳纳米管只存在于碳纳米管阵列膜表面和多层碳纳米管膜表面与界面的现象.     

超长定向含铁多壁碳纳米管阵列的制备

采用无模板化学气相沉积法,以二茂铁为催化剂,二甲苯为碳源,利用单温炉加热装置制备了定向碳纳米管阵列。运用扫描电子显微镜、透射电子显微镜、拉曼光谱和X射线衍射仪等对定向碳纳米管阵列的形貌、成分和物相进行细致的分析和表征。结果表明:制得的碳纳米管阵列具有良好的定向性和多壁管状结构,并且石墨化程度高;碳纳米管中除碳元素外,管中包含有少量以纳米颗粒和纳米线形式存在的铁及其化合物,主要成分是铁和碳化铁。结合碳纳米管的制备和透射电子显微镜分析表征结果,认为超长碳纳米管阵列的生长模式为底部生长方式,即经历催化剂分解、催化、成核、长大、中毒、凝聚成粒和连接成线的循环过程,正是由于碳源和催化剂的连续供应促成了碳纳米管阵列的快速定向生长。     

多孔氧化铝模板催化制备定向碳纳米管阵列

在不加过渡金属做催化剂的前提下,利用化学气相沉积法在二次阳极氧化法制得的多孔氧化铝模板中制备沉积了定取向碳纳米管阵列。考察了不同沉积温度以及退火处理对沉积结果的影响。温度达到600℃以上时,能得到开口的、定取向的多壁碳纳米管阵列；当沉积温度降至550℃左右时,沉积结果中存在碳纳米管、纳米纤维以及类似于弯曲的竹节状的结构；温度继续降低至500℃以下时,不能得到碳纳米管或碳纳米纤维；从实验结果中可以得出,在氧化铝模板中沉积碳纳米管过程中氧化铝起到了催化乙炔裂解以及催化沉积的碳石墨化成碳纳米管两种作用。另外,退火处理虽然能够提高沉积的碳纳米管的石墨化程度,但是也可能会引入新的缺陷。     

生长参数对碳纳米管阵列制备的影响

利用化学气相沉积法在沉积铁纳米颗粒的硅衬底上制备了垂直方向高度有序的碳纳米管阵列．扫描电子显微镜的观测发现，碳纳米管阵列的形貌受到若干生长参数的影响，包括催化剂颗粒大小、反应温度和反应气体的分压等．研究发现，当反应温度升高，或反应气体中碳源气体含量增加时，碳纳米管变粗、变短．当催化剂薄膜的厚度减小时，碳纳米管的直径随之减小而纳米管阵列的高度则先增后减，有一个最大值．这些结果表明，碳纳米管的直径和阵列的高度可通过选择合适的反应温度、匀胶机转速和反应气比率来调节．     

化学气相沉积法制备定向碳纳米管阵列

以二茂铁和二甲苯分别作为催化剂和碳源，采用一种无模板的化学气相沉积法，使用单温炉设备，成功地制备了高度定向的碳纳米管阵列.分别用扫描电子显微镜、透射电子显微镜和电子能量散射谱、拉曼光谱对碳纳米管阵列进行形貌观察和表征, 并研究了不同工艺参数对碳纳米管阵列形貌的影响.结果表明：在生长温度为800℃，催化剂浓度为0.02g/mL，抛光硅片上容易获得高质量的定向碳纳米管阵列，在此优化条件下生长的定向碳纳米管的平均生长速率可达25μm/min.     

碳纳米管阵列拉曼光谱的对比研究

利用热化学气相沉积技术制备碳纳米管阵列,并对不同工艺下获得的一系列定向碳纳米管阵列进行了拉曼光谱的对比研究。研究发现：碳纳米管阵列一阶拉曼光谱的G峰中心和D峰中心都会向低波数方向发生红移。并且阵列中碳管的一致性、准直性越好,红移的波数就越多。除了谱峰以外,D线和G线的积分强度比I_D / I_G也能够反映所研究的碳材料的有序度和完整性。I_D / I_G越低,说明该碳纳米管阵列的石墨化越好,无定形碳杂质越少。     

金镍复合膜上碳纳米管的定向生长及生长过程中金的作用

以金镍复合膜作催化剂，在96%的高氢气浓度下实现了碳纳米管的定向生长，并对其生长过 程进行了深入探讨.结果表明，高氢气浓度下碳纳米管生长的实现与本实验所选用的催化剂 ——金镍复合膜有密切关系.催化剂中金的参与，促进了碳在催化剂中的扩散，提高了碳在 催化剂中的活度.与催化剂中没有金的情况相比较，金的参与有利于镍吸收气氛中的碳，从 而使镍更容易达到碳饱和，有利于在高的氢气浓度下实现碳纳米管的定向生长. 关键词： 金镍复合膜 高氢气浓度 原子氢 碳活度     

面向热界面应用的多壁碳纳米管阵列生长优化

由于杰出的导热和柔韧性能,碳纳米管被认为是提升大功率微电子系统热耗散性能的理想热界面材料。多壁碳纳米管阵列能实现低成本的大规模生产,成为制备热界面材料的最佳选择。但是多壁碳纳米管阵列的生长过程的最优因素尚未廓清。本文从实验上系统研究了催化剂的升华温度对浮动催化化学气相沉积法制备出的多壁碳纳米管阵列结构及热输运的影响规律,得到了最佳的催化剂升华温度,为实现最优导热性能的多壁碳纳米管阵列热界面材料提供实验指导。     

初始温度及碳源对碳纳米管气相爆轰法合成的影响

改变初始温度以及分别使用甲烷和乙炔气体作碳源时气相爆轰合成碳纳米管,研究了初始温度与不同碳源对碳纳米管的影响。利用X射线衍射(XRD)、透射电镜(TEM)、拉曼(Raman)光谱等对碳纳米管进行表征。结果表明,随着初始温度的升高,所合成的碳纳米管的产量减少且石墨化程度降低,但管壁会变得光滑且管径有所增加。当使用乙炔时,所合成的产物中没有碳纳米管,而是合成了石墨化程度较高的无定形碳,随着催化剂量的增加,产物中碳包覆颗粒增多且包覆层清晰可见,但存在结构缺陷。当初始温度在110~130 ℃时,使用甲烷气体运用气相爆轰的手段是合成碳纳米管的较佳方案。     

Fabrication of aligned carbon nanotubes on Cu catalyst by dc plasma-enhanced catalytic decomposition

Aligned multi-walled carbon nanotubes (ACNTs) are deposited using copper (Cu) catalyst on Chromium (Cr)-coated substrate by plasma-enhanced chemical vapor deposition at temperature of 700 °C. Acetylene gas has been used as the carbon source while ammonia is used for diluting and etching. The thicknesses of Cu films on Cr-coated Si (100) substrates are controlled by deposition time of magnetron sputtering. The growth behaviors and quality of ACNTs are investigated by scanning electron microscopy (SEM) and transmission electron microscopy. The different performance of ACNTs on various Cu films is explained by referring to the graphitic order as detected by Raman spectroscopy. The results indicate that the ACNTs are formed in tip-growth model where Cu is used as a novel catalyst, and the thickness of Cu films is responsible to the diameter and quality of synthesized CNTs.     

Study on effects of substrate temperature on growth and structure of alignment carbon nanotubes in plasma-enhanced hot filament chemical vapor deposition system

Alignment carbon nanotubes (ACNTs) were synthesized on silicon substrate coated with Ni catalyst film and Ta buffer layer by plasma-enhanced hot filament chemical vapor deposition using CH₄, NH₃, and H₂ as the reaction gas, and they were investigated by scanning electron microscopy and transmission electron microscopy. It is found that the diameter of the bamboo-structured ACNTs is increased from 62 to 177 nm when the substrate temperature was changed from 626 to 756 °C. Their growth rate is enhanced by the substrate temperature in a range of 626-683 °C and it is reversely reduced with the substrate temperature after the substrate temperature is over 683 °C. Beginning with wetting phenomenon, the effects of the substrate temperature on the structure and growth rate of the ACNTs are analyzed.     

The effect of Fe and Ni catalysts on the growth of multiwalled carbon nanotubes using chemical vapor deposition

The effect of Fe and Ni catalysts on the synthesis of carbon nanotubes (CNTs) using atmospheric pressure chemical vapor deposition (APCVD) was investigated. Field emission scanning electron microscopy (FESEM) analysis suggests that the samples grow through a tip growth mechanism. High-resolution transmission electron microscopy (HRTEM) measurements show multiwalled carbon nanotubes (MWCNTs) with bamboo structure for Ni catalyst while iron filled straight tubes were obtained with the Fe catalyst. The X-ray diffraction (XRD) pattern indicates that nanotubes are graphitic in nature and there is no trace of carbide phases in both the cases. Low frequency Raman analysis of the bamboo-like and filled CNTs confirms the presence of radial breathing modes (RBM). The degree of graphitization of CNTs synthesized from Fe catalyst is higher than that from Ni catalyst as demonstrated by the high frequency Raman analysis. Simple models for the growth of bamboo-like and tubular catalyst filled nanotubes are proposed.     

Effect of Carbon Source with Different Graphitization Degrees on the Synthesis of Diamond

Using three kinds of graphites with different graphitization degrees as carbon source and Fe-Ni alloy powder as catalyst, the synthesis of diamond crystals is performed in a cubic anvil high-pressure and high-temperature apparatus （SPD-6 × 1200）. Diamond crystals with perfect hexoctahedron shape are successfully synthesized at pressure from 5.0 to 5.5GPa and at temperature from 1570 to 1770K. The synthetic conditions, nucleation, morphology, inclusion and granularity of diamond crystals are studied. The temperature and pressure increase with the increase of the graphitization degree of graphite. The quantity of nucleation and granularity ofdiamonds decreases with the increase of graphitization degree of graphite under the same synthesis conditions. Moreover, according to the results of the M6ssbauer spectrum, the composition of inclusions is mainly Fe3 C and Fe-Ni alloy phases in diamond crystals synthesized with three kinds of graphites.     

纳米碳管的电化学贮锂性能

用透射电镜、高分辨透射电镜、X射线衍射和拉曼光谱表征了用催化热解法制备的纳米碳管的结构,研究了纳米碳管的电化学嵌脱锂性能。以纳米级铁粉为催化剂热解乙炔气得到的纳米碳管石墨化程度较低,结构中存在褶皱的石墨层、乱层石墨和微孔等缺陷,具有国交高的贮锂容量,初始容量为640mAh/g,但循环稳定性较差。而以纳米级氧化铁粉为催化剂热解乙烯得到的纳米碳管结构比较规则,循环稳定性较好,但贮锂容量较低,初始容量为282mAh/g。讨论了纳米碳管的结构对其温度特性和不同电流密度下的充放电容易的影响。     

PAN基碳纤维在石墨化过程中的拉曼光谱

采用激光拉曼光谱研究了PAN基碳纤维在石墨化(2 000～3 000 ℃)过程中的结构变化;比较了石墨化前后纤维表面和断面拉曼光谱特征。结果表明：高温石墨化后,碳纤维的一级拉曼光谱有3个峰(D,G和D′),表征碳纤维结构有序程度的拉曼参数主要有D和G峰的半高宽(FWHM)、G峰的拉曼位移和D与G峰的积分强度比R(I_D/I_G)。随着热处理温度的提高,D和G峰的半高宽、G峰的拉曼位移和R值均逐渐减小,即使经过3 000 ℃高温处理后,D峰仍然存在,R值为0.19,说明纤维中仍存在无序结构。另外,R值与纤维中石墨微晶的基面宽度L_a成反比,石墨化后纤维取向性的增加使得表面和断面的拉曼光谱有明显的差异。因此,可利用激光拉曼光谱来定量表征碳纤维的石墨化程度和取向。     

Synthesis and characterization of graphenated carbon nanotubes on IONPs using acetylene by chemical vapor deposition method

The graphenated carbon nanotubes (G-CNTs) were synthesized on monodisperse spherical iron oxide nanoparticles (IONPs) using acetylene as carbon precursor by simple chemical vapor deposition method. The reaction parameters such as temperature and flow of carbon source were optimized in order to achieve G-CNTs with excellent quality and quantity. Transmission electron microscopy (TEM) clearly illustrated that the graphene flakes are forming along the whole length on CNTs. The degree of graphitization was revealed by X-ray diffraction (XRD) analysis and Raman spectroscopic techniques. The intensity of D to G value was less than one which confirms the obtained G-CNTs have high degree of graphitization. The optimum reaction temperature for the IONPs to form metallic clusters which in turn lead to the formation of G-CNTs with high carbon deposition yield is at 900 °C. The TEM shows the CNTs diameter is 50 nm with foiled graphene flakes of diameter around 70 nm. Our results advocate for IONPs as a promising catalytic template for quantitative and qualitative productivity of nanohybrid G-CNTs. The produced G-CNTs with high degree of graphitization might be an ideal candidate for nanoelectronic application like super capacitors and so on.     

Causes of different catalytic activities of metals in formation of single-wall carbon nanotubes

When single-wall carbon nanotubes (SWNTs) were formed by pulsed Nd:YAG laser ablation or arc discharge, the yield depended on the metal catalyst: NiCo> Ni∼NiFe≫Co∼Fe>Pd∼Pt. It appears that an effective catalyst for SWNT growth must satisfy three conditions: it must be a good graphitization catalyst, have low solubility in carbon, and have a stable crystallographic orientation on graphite. NiCo, Ni, and NiFe satisfy these three conditions. The poor catalytic activities of Co, Fe, Pd, and Pt for SWNT formation would be explained by the ineffectiveness of Pt and Pd as graphitization catalysts, crystallographic orientation instability of Co crystals on graphite, and high solubility of Fe in graphite. Received: 29 October 2001 / Accepted: 7 November 2001 / Published online: 23 January 2002