Carbon nanostructures in silica aerogel composites
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A new method of preparing carbon nanotubes and their derivatives using silica aerogels as a matrix for the deposition of carbon is repeated. We present results of observations of graphite tubes and rings including nested structures in nanometer dimensions using high resolution transmission electron microscopy. Furthermore, we propose a model for the growth of carbon nanotubes in three steps including nucleation, growth, and closure of tubes.
Since Iijima1 first discovered novel carbon tubes of nanometer dimensions under High Resolution Transmission Electron Microscopy (HRTEM), it has greatly stimulated studies in the field of carbon fiber growth.2"5 The large-scale synthesis of carbon nanotubes was developed by Ebbesen and Ajayan,6 based on an arc discharge method similar to the synthesis of fullerenes. Some researchers 78 have tried to form a second phase into carbon nanotubes or their derivative structures. However, up to now, no other methods to form carbon nanotubes have been reported in the literature. The purpose of this paper is to present the HRTEM results on carbon nanostructures in silica aerogel composites. In addition, we propose a model for the growth of carbon nanotubes based on the observations from HRTEM. Silica aerogels are highly porous materials prepared using sol-gel processing and supercritical solvent extraction.9 The first step is to mix tetraethylorthosilicate (TEOS), water alcohol, and catalyst (usually a base, e.g., ammonia) in appropriate proportions. TEOS undergoes several steps of hydrolysis and condensation reactions, leading to the formation of a soft material called an alcogel. Then liquid carbon dioxide is used to replace the fluid in the alcogel and removed above the supercritical temperature and pressure to make an aerogel. Silica aerogels have a high porosity (90% or higher) and high surface area (800-1000 m 2 /g), an ideal matrix for the chemical vapor infiltration of a second solid material on an extremely fine scale. The deposition of carbon was carried out in a tube furnace by the decomposition of acetylene at 550 °C and post-treated in an inert atmosphere at 670 °C for 0.5 h. The carbon deposition was found to be homogeneous throughout the aerogel volume and the weight of the composite material increased by about 8.7% after a deposition of
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Permanent address: National Laboratory for Fine Ceramics and Structure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, China. J. Mater. Res., Vol. 10, No. 2, Feb 1995 http://journals.cambridge.org
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1 h. In another similar experiment, both carbon and iron particles were simultaneously deposited in the aerogel by the decomposition of ferrocene at 120 °C for —20 h in a helium atmosphere (—200 Torr). The as-deposited sample was further treated in an inert atmosphere at 700 °C for about 1 h. More detailed information on the deposition process and characterization of the aerogel composites will be published elsewhere.10 HRTEM on the carbon-doped silica aerogel was performed using a JEM-200CX
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