Nucleation of the Fe 3 C in reaction of methane with nanocrystalline iron
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A. Pattek-Janczyk Faculty of Chemistry, Jagiellonian University, 30-060 Krakow, Poland (Received 17 March 2004; accepted 22 October 2004)
The carburization process of nanocrystalline iron in a flow of CH4/H2 mixture under atmospheric pressure at 580 °C in a differential reactor–thermobalance was studied. The course of reaction was followed by thermogravimetry, and the phase composition of the samples carburized to different degrees was determined by x-ray diffraction (XRD) and Mössbauer spectroscopy techniques. The XRD method was also used for calculating the mean crystallite size of unconverted iron after reaction at different time intervals. An unexpected relation between the average size of iron crystallites and the degree of conversion was found. The nucleation mechanism of the nanocrystalline iron carbide in the kinetic area of the reaction, limited by the dissociative adsorption of methane, has been suggested. According to this mechanism, iron crystallites are carburized successively, from the smallest to the largest.
I. INTRODUCTION
Considering the reaction taking place at phase boundaries in gas–solid systems, several elementary partial stages should be taken into account. If the reaction results in solid production, the formation of the new phase nuclei is one of these stages. The studies concerning the nucleation process at the surface of solids are described in reference books.1–4 The theory of the nucleation based on thermodynamics, has been developed by Gibbs and then further studied by Volmer and Weber.5 One of the principal tenets of the theory is that when liquid droplets are formed from the vapor there is a stationary distribution of embryos of a size smaller than that of the critical radius of the droplet. If the new phase is stable, the distribution of embryos is the same as the equilibrium distribution. The classical Volmer–Weber theory was improved by many authors, e.g., Becker and Döring,6 who proposed a kinetic theory of nucleation or Frenkel,7 who developed the classical theory for other phase transformations. The above studies looked into the case when the formation of a nucleus is considered as a homogeneous process, occurring in the same way in all the assembly. The situation is more complicated when the nucleation is heterogeneous, when
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0043 386
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 2, Feb 2005 Downloaded: 17 Nov 2014
it occurs on the grain boundaries or as a result of a chemical reaction. In the schema presenting the classification of transformations, given in a recently published book about the transformations in metals,8 the case of discontinuous reactions is accompanied by the note “diffusion plus interface controlled?” The reaction in a gas–solid system first leads to the crystallization of the product and next to the growth of crystals on the surface of solid. When the product is polycrystalline or amorphous, the acronym used for the description of the m
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