Ultra rapid heating by spontaneous mixing reactions in metal-metal multilayer composites
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The mixing of multilayers of two pure metals or alloys A and B with strong negative heat of mixing AHm can result in rapid heat release provided that the mixing time is short compared to the time to dissipate AHm in the environment, which is the case for sufficiently thin layers. The mixing reaction sets off the heat release when the composite is heated up to temperatures near or equal to the melting temperature of the metal layer with the lower melting point. Experimental results are given for the mechanisms and for the kinetics of the exothermic reaction. Possible applications of these materials are given.
I. INTRODUCTION
During mixing of two elements A and B, A-A and B-B nearest neighbor atom pairs are replaced by A-B nearest neighbors and if heteroatomic interactions are attractive, mixing results in the release of significant amounts of heat, usually referred to as the heat of mixing AHm = Han — Hin, corresponding to the enthalpy difference between the final mixed state and the initial unmixed state. Other terms are usually preferred for mixing reactions that result in the formation of ionic or molecular substances (i.e., heat of oxidation, heat of formation, etc.). The heats released by oxidation (burning) and various organic reactions are commonly used as power or energy sources. The present work uses the heat of mixing of metals for the same purpose.1 While the metallic character of the bonding is maintained upon alloying of various metals, depending on the interactions between their outer-electron shells, AHm (J/mole) released during their mixing can be very negative and of the order of heats released during combustion of various substances. AHm can often be calculated to a good approximation using the semi-empirical model of Miedema.2 For example, AHm is particularly strong in the case of mixing of nickel, palladium, or platinum with most of the elements of groups IIA, IIIB, IVB of the periodic table and with the rare earths. For a self-sustained exothermic mixing reaction to occur at or below the melting temperature Tf of the B atom layers that melt first in an A-B multilayer composite, the following two criteria must hold: (1) The heat released, AHm < 0, must be well in excess of the heat of fusion AHf > 0 needed to melt the sample (i.e., AHm + AHf < 0). The excess heat can superheat the molten mixture by about AT = {AHm + AHf)/Cp where Cp (J/mole • K) is the alloy's specific heat. (More details and calculations were given in a previous paper.3) 1656
http://journals.cambridge.org
J. Mater. Res., Vol. 5, No. 8, Aug 1990
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(2) The mixing reaction time tm must be much shorter than the time td required for the dissipation of AHm in the environment {tm < td or else the selfsustained reaction will not occur). From a structural point of view the diffusion distance and therefore the thickness per layer in the composite must be low enough to allow short mixing times. Depending on the composition, an amorphous structure can be obtained if the molten reaction product is cooled rapidly.
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