Real time (ns) determination of liquid phase growth during shock-induced melting*

Although the pressure dependence of melting in solids has been studied extensively for nearly a century, the temporal evolution of the molten phase has eluded measurements because of experimental challenges. To examine this phenomenon, we investigated the time dependent growth of the molten phase in shocked Ge, using the shock front as a fiducial, in combination with x-ray diffraction (XRD) measurements. The target, consisting of a 50 µm thick Kapton ablator bonded to ~33 µm [100] Ge crystals, was ablated using a 500 µm diameter drive pulse generating a 10 ns duration shock wave into the sample. High quality XRD results were obtained in the high pressure (β-Sn) solid phase (below 35 GPa), in the β-Sn/liquid mixed phase (35 to 42 GPa), and upon complete melting (above 42 GPa). The XRD data were obtained at different times (1 to 6 ns) behind the shock front, which quantified the real time growth of the liquid phase at varying peak stresses. Analysis of these results show that the characteristic time for melting in shocked Ge decreases from ~7.2 ns at 35 GPa to less than 1 ns at 42 GPa. These melting kinetics results suggest the need to consider heterogeneous nucleation as a mechanism for shock-induced melting and provide a general approach to measure melting kinetics in shock compressed solids.