The mechanism and kinetics of the niobium-carbon reaction under self-propagating high-temperature synthesis-like conditi
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The mechanism and kinetics of the chemical reaction between Nb(s) and C(s) under self-propagating high-temperature synthesis (SHS)-like (or combustion synthesis-like) conditions have been studied. Experiments were designed and conducted in order to produce a transport-resistance-free reaction between Nb and C under time-temperature conditions that are characteristic of the combustion synthesis process. To do so, a reaction couple, consisting of carbon and either a thin niobium foil or a fine niobium wire, was used. The effects of the temperature history and the formation of a liquid phase on the reaction were studied. In addition, theoretical experiments of the reaction were also conducted. The results showed that at high temperatures, layered niobium carbide phases formed in a direction that was parallel to the original carbon-niobium interface. As might be expected, local melting played a very significant role in the reactions. The mechanism and kinetics of these reactions provide a fundamental understanding of the manner and rate by which a powder-based Nb/C SHS process takes place, and, by extension, to a large, general class of solid-solid material synthesis processes that are based on the SHS (or combustion synthesis) process.
I. INTRODUCTION The combustion synthesis [or self-propagating hightemperature synthesis (SHS)] process is an efficient method for producing a wide variety of ceramic, metallic, and composite materials.'~ 4 Once the sample has been ignited by an external energy source, the highly exothermic reaction progresses as a reaction front that propagates through the pelletized reactant-powder sample. Combustion synthesis can typically be carried out in either a solid-solid or a gas-solid system. The process involves not only chemical reactions, but also a variety of transport phenomena involving energy, mass, and momentum. A number of fundamental studies have been conducted on the sample-level in powder-based reactions, including the observation of the propagation of the combustion-wave-front and of the structure of the combustion wave.5 However, in order to account for all of the essential details of the SHS process thoroughly and quantitatively, it is also necessary and important to obtain particleor intraparticle-level information concerning the local chemical reaction that takes place between the reactants during the combustion synthesis process. Unfortunately, this is a less well understood area. The difficulties arise from the fact that constituent particles are very fine and may possess complicated geometrical shapes, and that a very large number of particles are involved. This makes direct experimental observation and analyJ. Mater. Res., Vol. 10, No. 11, Nov 1995 http://journals.cambridge.org
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sis virtually impossible. Some research results have been reported in which reaction couples with a welldefined starting geometry were used. Vadchenko et al.6 conducted experiments on the reaction between Ti and Ni as well as between Al and Ni, in which wires of either Ti
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