Shock-wave-induced optical emission from sapphire in the stress range 12 to 45 GPa: Images and spectra

We took short-duration exposure images and spectra of the optical emission from shock-compressed single-crystal sapphire (α-Al₂O₃) for three common crystal orientations: c-plane {0001}, a-plane 1120, and r-plane 1102. The images show that this emission is not homogeneous, but is spatially localized. For a given shock stress, the r-plane orientation consistently produces less emission intensity than the other two orientations. The emission spectra were fit to a gray-body function. Apparent temperatures range between 4700 and 5400 K, consistent with earlier spectroscopic work. The value of the apparent emissivity in the spectral fits is very low, between 2×10^-3 and 6×10^-3. We attribute the emission source to be shear banding which is not associated with twinning or slip on {0001}.