Combustion synthesis of LiGa and LiAl intermetallic alloys
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I.
INTRODUCTION
COMBUSTION synthesis or self-propagating hightemperature synthesis (SHS) is a new technique being used to produce a variety of ceramics, composites, and intermetallic compounds because it uses less energy compared with other conventional methods, such as carbothermic reduction, sintering, or melting and casting. t~-4~The most significant feature of combustion synthesis is a sudden, rapid release of exothermic heat at a characteristic temperature known as the ignition temperature, T~g, as schematically presented in Figure 1. The saving in process energy arises from the fact that the reactants need only to be heated to the ignition temperature for the synthesis reaction to be initiated. Combustion synthesis reactions can be conducted in two general modes. In the "propagating" mode, applicable to highly exothermic systems, a local heat source initiates the reaction which then propagates as a combustion reaction wave through the sample. The second mode, known as the "thermal explosion," "simultaneous combustion," or "bulk" mode, is used for systems with low exothermic heats of reaction which, therefore, may not be self-sustaining. The SHS reaction is initiated in this latter mode by rapidly heating the entire sample to the ignition temperature. It may be useful here to examine the underlying thermochemistry of combustion synthesis reactions, t2] Consider an exothermic SHS reaction of a green reactant powder mix at an initial temperature, To, which is ignited under adiabatic conditions in the propagating mode at T~g. In order for the reaction to ignite at Tig, the reactants need to be heated from To to Tig. Therefore, the S.R. PRITCHETT, formerly graduate student with the Kroll Institute for Extractive Metallurgy, is with K-T Feldspar Corporation, Spruce Pine, NC 28777. B. MISHRA, Research Associate Professor and Associate Director of the Kroll Institute for Extractive Metallurgy, and J.J. MOORE, Head of the Department of Metallurgical and Materials Engineering, are with the Colorado School of Mines, Golden, CO 80401. Manuscript submitted May 18, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS B
amount of heat, H(R), needed to do this is given by the following:
~ig
H(R) =
Z niCe(R~)dT + Z
niL(Ri)
[1]
To- T~
where ni, C e ( R i ) , and L(Ri) are the reactants, stoichiometric coefficient, heat capacity, and phase transformation enthalpy (if the reactants go through a phase change, e.g., solid to liquid), respectively. The thermochemistry can be presented in the form of an enthalpy (H)-temperature (T) plot for both the reactants and products, as shown in Figure 2 and in which the value of H(R) is indicated. Because the SHS reaction is initiated at Tig, the heat of the reaction under these conditions is given by AH(Tig), also indicated on the H-T plot in Figure 2. At a certain distance from the heat source in the reactant mixture, a steady state is reached in which the heat of the reaction, AH(Tig), is used only to heat the adjacent reactant layer from To to Tig, i.e., there is no influence of the h
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