Thermal History of Shock-Compressed Solids

Abstract

Taking the view that observed, shock-induced optical radiation from transparent solids is dependent on defect-controlled (i.e., heterogeneous) properties and processes, we examine an isotropic, heterogeneous, viscous, thermoelastic model of the uniaxially shock-compressed state to determine conditions under which this radiation might be dominantly thermal or nonthermal. Assuming shock compression establishes some distribution of material properties in the initially-compressed state, perturbations in the stress field relax heterogeneously to the hugoniot state by elastic deformation for timescales short and viscous deformation for timescales long relative to the local Maxwell time. Regions of locally high temperatures producing thermal radiation then develop only where the local viscosity is low and Maxwell time short. Alternatively, regions of low elastic moduli and long Maxwell time experience sustained elastic deformation which may result in microfracture and triboluminescence. Consequently, a shock-induced defect structure dominated by long Maxwell times may not develop thermal localization. This may be the case for MgO, which produces no resolvable (≳ 2000K) thermal radiation below 60 GPa. Conversely, fused SiO2 develops locally high temperatures (∼3000K) above 10 GPa (below the stishovite transition), implying that it may develop low viscosity, short Maxwell time regions that persist in the hugoniot state.