Optical emission of shocked magnesium oxide single crystals: Heat-conduction and orientation effects

Abstract

To investigate light emission properties of shocked high-conductivity single crystals during plastic deformation, [100] and [110] magnesium oxide (MgO) single crystals are shock-compressed up to 64 GPa via a planar impact. [111] gadolinium–gallium garnet (GGG) and [100] Y-stabilized zirconate (YSZ) single crystals, respectively, with intermediate and low conductivities are also examined for comparison. Emission spectral radiance and velocity histories are simultaneously measured with a fast, high-sensitivity pyrometer and a Doppler pin system. While velocity histories of [100] MgO, [111] GGG, and [100] YSZ show typical two-wave structures at elastic–plastic transition, abnormal three-wave structures are observed for [110] MgO, consistent with literature reports. Correspondingly, spectral radiance histories of [100] MgO, [111] GGG, and [100] YSZ under each tested stress demonstrate a typical “single-wave” structure, while a “two-wave” structure is observed for [110] MgO shocked to 33–37 GPa. The peak intensities of spectral radiance for both [100] and [110] MgO are much lower than those of GGG and YSZ shocked to similar plastic strains as a result of different heat conductivities. A model, taking into account heat conduction from a shear band into its surrounding bulk areas and two-zone emission, is thus developed and describes well the measurements. Our observations make a concrete connection between shear banding and localized heating/cooling as proposed previously.