SiO2 thermal histories during phase transitions and release paths

Crystalline quartz and amorphous fused silica (SiO2) undergo a number of solid phase transitions prior to melting as well as metastable superheating past the equilibrium melt curve. Using near infrared (NIR) and streaked visible spectrometry (SVS) spectral radiance data from the UC Davis Shock Compression Laboratory, we observe thermal histories that indicate different decompression paths for material that is shocked to a solid phase, within the superheated region, and in the liquid phase region. Material that is shocked into the solid, below the equilibrium melt curve, has a multi-step cooling process upon release where the temperature is first buffered by the melt curve, reaches a local minimum, then is buffered by the liquid-vapor phase boundary. Material that is shocked beyond the superheating region melts during compression resulting in an immediate drop in temperature with no change of phase that is buffered by the vapor dome. Finally, material shocked within the superheated region has an immediate drop in temperature upon decompression, more similar to material that is shock melted compared to material shocked within the solid. This difference between solid and superheated solid cases may be due to solid phase changes and changes in elastic plastic strength on decompression. These thermal histories provide insight to the differences in release paths for silica and a better understanding of how to incorporate these complex decompression paths into multi-phase EOS models for crystalline and amorphous silica.