Chemical reactions can be triggered by mechanical forces in solids because, unlike gases and liquids, solids support 'shear strains. Shear changes the symmetry of a molecule or solid and is therefore more effective in stimulating reactions than is simple isotropic compression. Sheared spheres become ellipsoids, cubic symmetry becomes tetragonal, and so on. Such symmetry breaking destabilizes the electronic structure of bonding and makes the solid prone to chemical reaction. Such mechanochemical phenomena are found in a variety of processes--friction and wear, detonations, solid-state synthesis, and mechanical alloying.
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