Self-propagating reaction induced by ball milling in a mixture of Cu 2 O and Al powders
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I. INTRODUCTION
BALL milling has been used to induce chemical reactions and alloying in a variety of powder mixtures. The process is called mechanochemical synthesis when applied to the preparation of inorganic or organic materials.[1–5] An especially promising application is mechanical alloying, whereby oxide-dispersion strengthening, atomic-level alloying, and amorphization are achieved by the application of mechanical energy.[6,7,8] The process is simple, straightforward, and environmentally friendly. If a highly exothermic reaction is possible in the mixture of the reactant powders, a self-sustaining thermal reaction can be set off by the impact of the milling balls. Such mechanically induced self-propagating reactions (MSRs) have been observed in metal-chalcogen systems,[9,10] in metal-metalloid mixtures that form refractory compounds,[11,12] in displacement reactions between a metal oxide and a reductive metal,[13] and in a variety of other systems.[14] Usually, ignition takes place after a certain processing time, as milling reduces the particle size, thoroughly mixes the components, and increases the number of chemically active defect sites. The reaction starts at a hot spot, formed when some powder is compressed between the colliding milling bodies. This incipient reaction sets off a self-sustaining reaction which propagates through the powder charge. The ignition times of MSRs are investigated because they provide information about the activation process. Measurements of the ignition time as a function of the powder mass and as a function of the number and mass of the milling balls have established that the “mechanical dose,” the amount of mechanical energy absorbed by a unit mass of powder, is a proper measure of the level of activation within a broad range of conditions.[10,15] The details of the ignition process depend on the milling conditions as well as on the physicochemical properties of A. BAKHSHAI, Professor and Chairman, and R. PRAGANI, Undergraduate Student, are with the Department of Physics, Goucher College, Towson, MD 21204. L. TAKACS, Associate Professor, is with the Department of Physics, University of Maryland, Baltimore County, Baltimore, MD 21250. Contact e-mail: [email protected] Manuscript submitted December 4, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
the reactant powder. It is generally assumed that a powder lining is formed on the balls and on the inner surface of the milling vial.[16] Every collision between the milling bodies contributes to the activation of the charge, as it mixes the reactant particles on an increasingly fine scale and creates lattice defects that enhance diffusion and serve as chemically active reaction sites. The impacts also create hot spots where a self-sustaining reaction can start. Whether ignition takes place or not depends on the energy balance near the hot spot. The reaction heat produced by the incipient reaction is conducted toward neighboring volume elements, increasing their temperature and, thereby, promoting chemical reactions. The heat tr
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