Direct Evidence Of Chemical Reactions Induced By Shear-Strains
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Direct Evidence Of Chemical Reactions Induced By Shear-Strains John J. Gilman Materials Science and Engineering University of California at Los Angeles Los Angeles, CA 90095 ABSTRACT After a brief review of the history of mechanochemistry, and of the theoretical principles of chemical reactivity, four examples are described of reactions that demonstrate the importance of elastic shear strains compared with hydrostatic compressive strains (volume changes). Techniques for separating shear strains from volume changes, and for isolating elastic strains from plastic deformation are described. The latter (isolation) is achieved simply by localizing a strained region making it too small for dislocation nucleation. Shear strain acts by reducing the chemical hardness (activation energy) of a reactant. The four examples are: (1) “hammer chemistry” in which physisorped methane is struck by argon atoms with enough kinetic energy to cause chemisorption; (2) enhanced oxidation of silicon at stressed crack tips; (3) selective dissolution of crystals at screw dislocations; and (4) increased rates of catalyzed reactions when surface acoustic waves are passed through the catalyst.
INTRODUCTION Since the time of Carey Lea nearly 140 years ago, the induction of solid-state chemical reactions by elastic shear-strains has been established [1]. Lea’s evidence was that mechanical straining of a set of reactants gave different products than heating the reactants. For example, he found that grinding silver nitrate in a mortar caused decomposition into silver oxide plus nitrous oxide, while heating simply melted the compound [2]. Lea’s general interest was photographic chemistry. He became interested in mechanochemistry when he observed that photographic emulsions turn black when they are rubbed firmly; and that light rubbing produces a latent image that can be chemically developed. He then showed that grinding various solid chemicals with a mortar and pestle caused them to react, while simply pressing on them had little, or no, effect. Examples are: AgCl, AgBr, Ag2C2O4, NaClAuO, HgCl2, and several more. The behavior of mercuric chloride is noteworthy. It is unchanged by about 70 kbar of applied pressure, but decomposes under a few bars of shear. The factor of 10,000 difference is impressive. Unfortunately, the states of strain present in Lea’s experiments, and the role of crystal defects, is not accurately known. Mechanical effects are not easily associated with quantitatively defined parameters. There are at least four cases to be considered: elastic, inelastic, dilatation, and shear. The experimental evidence is that the effect of dilatation, and its corresponding pressure, is small compared with the effect of elastic shear strain. But it is not easy to apply pure states of elastic shear-strain to solid specimens. Dilatation is almost always mixed with the shear. Also, the reactants may contain numerous cracks, or, if they are plastic, they may contain high dislocation densities. Therefore, some of the best evidence of shear effe
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