Effect of CVD Process Temperature on Activation Energy and Structural Growth of MWCNTs
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THE discovery of nanotubes in 1991 revolutionized many aspects of the materials research. The incredible CNTs structures have an array of fascinating thermal, magnetic, electronic, and mechanical properties. CNTs are at least 100 times stronger than steel, therefore can be used to strengthen almost every material. The electrical and thermal conductivity of CNTs is far better than silver and copper. CNTs can be added to polymers to enhance their conductivity or anti-static packaging. Since their discovery, CNTs are being produced using various methods including laser ablation, electric arc discharge, and chemical vapor deposition (CVD).[1–5] However, majority of the research efforts are focused on the methods that offer low cost and controllable CNTs production routes.[3,4] The fluidized bed chemical vapor deposition (FBCVD) allows continuous growth of the nanotubes at relatively low cost and therefore it is regarded as the most favorable technique for large-scale production of CNTs.[4,5] However, a major challenge S. SHUKRULLAH, Research Officer, N.M. MOHAMED, Professor, Director MOR, M.S.M. SAHEED, Lecturer, and M.I. IRSHAD, Ph.D. Candidate, are with the Center of Innovative Nanostructures and Nanodevices, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, Perak, Malaysia. Contact e-mail: [email protected] M.S. SHAHARUN, Senior Lecturer, is with the Department of Fundamental and Applied Sciences, Universiti Teknologi PETRONAS. Manuscript submitted April 4, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
associated with FBCVD process is the growth of relatively pure and well-structured nanotubes. A number of parameters such as the process temperature, precursor pressure, reactor size, carbon precursor, activation energy, and catalyst can significantly influence the quantity and quality of the carbon product.[6] The process temperature at which tube nucleation takes place is an important parameter for the study of morphology, diameter distribution, growth rate, and structures of MWCNTs. Muataz et al.[7] studied the effect of the process temperature on MWCNT structures. Benzene and ferrocene were used as carbon precursor and catalyst, respectively. They pointed out that MWCNTs growth starts at the process temperature higher than 773.15 K (500 °C). The highest number of walls with minimum surface defects was obtained at the process temperature of 1123.15 K (850 °C). They revealed a linear increase in average tube diameter with the process temperature. At higher process temperatures, the tube diameter was also increased and some of MWCNTs were composed of non-tubular carbon and nanofibers. Similar results were reported by some other researchers.[8–10] At higher process temperatures, the metal catalyst changes from its solid to a quasi-liquid state by providing large surface area for CNTs nucleation, which results in thick tubes of larger diameters. However, there are still some inconsistencies in the past reports investigating the true role of the process temperature in MWCNTs growth process. For example, u
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