Synthesis of Nanocarbon Materials by Carburization of Nanocrystalline Iron
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Synthesis of Nanocarbon Materials by Carburization of Nanocrystalline Iron U. Narkiewicz1, W. Arabczyk1, I. Kucharewicz1, M.J. Wozniak2, H. Matysiak2 and K.J. Kurzydlowski2 1 Institute of Chemical and Environment Engineering, Technical University of Szczecin, 10 Pulaskiego Str., 70-322 Szczecin, Poland 2 Faculty of Materials Science and Engineering, Warsaw University of Technology, 141 Woloska Str., 02-507 Warsaw, Poland ABSTRACT Two products are formed in the process of carburization of iron with hydrocarbons: iron carbide (Fe3C, cementite) and in the next stage - carbon deposit. This paper deals with the formation of carbon deposit on nanocrystalline iron from ethylene decomposition and the reduction of its product with hydrogen. The carburization process was controlled using spring thermobalance. The samples after carburization contained cementite and some amount of carbon deposit in the form of carbon nanofibers and carbon nanotubes. In the next step iron carbide had been reduced under hydrogen flow at 400 – 500°C and pure iron was obtained. Some carbon was hydrogenated and the morphology of the remaining carbon deposit was changed – thicker carbon nanofibers were eliminated and thinner carbon nanotubes remained. The samples after carburization and reduction processes were characterized using XRD (X-rays Diffraction) and HRTEM (High Resolution Transmission Electron Microscopy) methods. INTRODUCTION A number of preparation methods of nanomaterials were developed during the last decade. One of the simplest and most inexpensive methods is chemical vapor deposition (CVD) which uses transition metal catalysts [1-11]. The preparation conditions in the method are not difficult and the process is easy to control. The method can also be up – scaled to industrial relevance. During a chemical vapor deposition process different carbon structures can be obtained, some of them having very interesting properties, as e.g. carbon nanotubes. However, the materials produced by CVD methods contain impurities, such as amorphous carbon, graphite particles or particles of metal catalysts. The process of separating nanotubes and catalytic particles is most often carried out by means of treating them with hydrochloric acid [12, 15, 17], hydrofluoric acid [13 - 14] or nitric acid [16]. Amorphous carbon is eliminated by oxidation in air flow [12 - 15, 17] or by oxidation in a liquid phase through potassium permanganate in acidic solution [13 - 14]. Carbon contamination can also be eliminated by means of hydrogenation process [18] at 900ºC lasting for 4.5 hrs. Comparing the reactivity of different carbon structures to hydrogen, it can be concluded that the reaction rate of amorphous carbon is significantly higher than that of graphite, which effectively means that their separation is a possibility. In another paper [19] reporting investigation of hydrogenation process the author concludes that the obtained results were similar to those obtained as a result of oxidation. Because the rate of hydrogenation is much lower than that of
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