Carbon Nanotubes Prepared By Cool-wall Chemical Vapour Deposition

Because of unique molecular structure and properties, such as high aspect ratio, large surface area, remarkable mechanical strength, carbon nanotubes (CNTs) has been paid wide attention to. This paper described briefly its preparation methods, growth mechanisms and potential applications.The growth of CNTs using CVD was conducted in the cool-wall heating oven on three substrates(monocrystalline silicon, iron, graphite), two catalysts(iron, nickle), two catalyst thickness(20nm, 80nm), three diluted gases(ammonia, hydrogen, nitrogen), three flux ratio(2/1, 10/1, 19/1), under 700℃ and 850℃, respectively. SEM and TEM were used to investigate the growth behaviors as a function of the catalyst, substrate, growth temperature and diluted gas. The main research findings were as follows:Nickel had a higher catalysis than iron in the growth of CNTs, and the thickness of catalyst affected the diameter of CNTs. Compared graphite and iron, when monocrystalline silicon was used as the substrate, CNTs had a higher purity. The involved reason was that monocrystalline silicon had a higher smooth and uniform surface. The flux ratio of diluted gases was hold as 10/1, which could resolve the contradiction of the asynchronism between the product and growth of the atomic carbon. The degree of graphitization and diameter of CNTs increased with elevated temperature, which showed that high temperature improved the catalysis of catalyst, and was attributed to uniform arrangement of atomic carbon, and hindered amorphous carbon. Ammonia played a critical role in the vertical alignment of CNTs, and the possible reason was that in 850℃ the atomic hydrogen decomposed from ammonia reacted with amorphous carbon to form volatile products to keep the metal surface clean, and mechanical leaning against neighboring tubes established a morphology of vertical alignment. The growth process of CNTs using CVD was possibly followed as the based-growth mechanism. The atomic carbon precipitating from the top of the catalysts grewupward, but the catalysts still existed in the bottom of CNTs due to the strong adhesion between catalysts and substrate.