A New Mechanism for the Catalytic Growth of Carbon Filaments and Metal Dusting Phenomenon
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A New Mechanism for the Catalytic Growth of Carbon Filaments and Metal Dusting Phenomenon Zuotao Zeng and K. Natesan Argonne National Laboratory, Energy Technology Division Argonne, IL 60439, USA ABSTRACT Raman scattering and X-ray diffraction were used to study the mechanism of the catalytic crystallization of carbon and metal dusting. The following new mechanism is proposed for metal dusting and the growth of carbon filaments. Carbon cannot crystallize well by deposition from carburizing gases at low temperature without catalytic activation because of its strong C-C bonds and high melting temperature. The poorly crystalline carbon has higher free energy than that of good crystalline carbon. To form good crystalline carbon, carbon atoms have to dissolve, diffuse through metal particles, and crystallize on a proper lattice plane that can act as a template to help the epitaxial growth of carbon crystals. Metal particles are liberated from the pure metal and alloys in this process, which leads to metal dusting attack. The decrease of free energy from highly disordered to well crystalline carbon is the driving force for metal dusting and carbon filament growth through metal particles. INTRODUCTION Recently, the growth of carbon nanotubes has attracted much attention since they have many potential applications in electronic devices and as a hydrogen storage medium. To grow carbon filaments, nanoparticles of Ni and Fe are used as catalysts. In 1972, Baker proposed a mechanism for the growth of carbon filaments assisted by metal particles [1]. This mechanism involves the diffusion of carbon through the metal particle from the hotter leading face, on which exothermic decomposition of the hydrocarbon occurs, to the cooler trailing faces, at which carbon is deposited from solution. The temperature gradient is the driving force for the carbon diffusion through the metal particle and the growth of carbon filaments. But this mechanism fails to explain the carbon filament growth by pyrolysis of methane [2], an endothermic decomposition. In another research field, metal dusting is also involved in carbon filament growth. Metal dusting is a metal loss process that occurs in a strong carburizing gas at high temperature. During this process, metals or alloys corrode, accompanied by the formation of carbon filaments, fine metal carbide, and/or pure metal. Significant problems with the metal dusting of steel components have occurred in the petroleum, petrochemical, and other industries. Hochman proposed a mechanism for the metal dusting of iron [3]. The steps involve the following: (a) carbon deposits on the metal surface and dissolves in the metal; (b) Fe3C forms as carbon diffuses into the iron, and the metal becomes oversaturated; and (c) metastable Fe3C decomposes to Fe and graphite. According to this mechanism, the final product is graphite and iron. However, it is not known whether the iron phase is present in every case of metal dusting and what drives the formation and decomposition of Fe3C under the same conditions o
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