The effect of an electric field on self-sustaining combustion synthesis: Part I. modeling studies
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I.
INTRODUCTION
THE effect of an electric field on a variety of physical and chemical processes has been the focus of numerous investigations. The influence of an externally imposed field on dislocation mobility and mass transport has been dealt with in a recent review by Munir and Schmalzried. tl] Effects of electric fields on such processes as evaporation, [2,3,4j crystallite growth and orientation, t5,6] grain boundary motion, t7] and others have been reported. In recent investigations, an effect of an electric field on the dynamics of a self-propagating combustion wave has been observed. [s,9J In this article, results of modeling studies that account for the influence of a field on self-propagating high-temperature synthesis (SHS) reactions are reported, and in the following article, ~176 details of an experimental investigation on the effect of a field on the self-propagating synthesis of/3-SIC are provided. The feasibility of formation of a large number of single and multiphase materials by SHS reactions has been demonstrated, and the process itself has been described in several recent reviews, tH-14] As will be described in more detail in this section, the existence of a selfsustaining reaction (or combustion) wave is controlled by a balance between rates of heat generation and heat dissipation. The former is provided by the chemical reaction to form the product phase, and the latter is governed by heat conduction and heat loss. Under adiabatic conditions, both convective and radiative heat losses can be ignored, and the process becomes dependent on heat generation and thermal conduction. The heat generation rate depends on the magnitude of the enthalpy of the reaction and the rate of conversion of the reactants to the product, i.e., Q(d~l/ot), where Q is the reaction heat, ~7 is the fraction reacted, and t is time. Thus, an important condition for the existence of SHS reactions is a high (exothermic) heat of reaction, and only in such cases have self-sustaining reactions been observed. Since the adiabatic combustion temperature, Ta, is directly related to the enthalpy of formation of the product, it has been used as an empirical guide for the existence of SHS reactions, till Experimental observations show that reactions A. FENG, Postgraduate Researcher, and Z.A. MUNIR, Professor and Associate Dean, are with the Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616. Manuscript submitted April 26, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS B
with T, -< 1800 K cannot become self-sustaining. Since Ta for a variety of important materials (e.g., SiC, B4C, WC, intermetallic, etc.) is less than this value, attempts have been made to activate the SHS reaction by preheating the reactants. If preheating is carried out up to a sufficiently high temperature, another form of combustion reaction can take place, one that is referred to as simultaneous combustion or thermal explosion. In this case, the reaction is not in the form of a self-sustaining wave but occurs simul
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