New Ways of Looking for Phase Transitions at Multi-Megabar Dynamic Pressures

摘要

Shock wave techniques have provided much of our information about very high pressure phase transitions. However, because of the specific thermodynamic region accessible to simple shock wave experiments, phase transitions with very small density changes are difficult to detect. Those phase changes with a density decrease at constant pressure, such as normal melting, are virtually impossible to detect in a shock experiment. Three new techniques have been applied to thermodynamic measurements behind a shock wave, with the result that several previously undetected phase boundaries have been measured. First, optical pyrometry has been used to measure the temperature of shocked SiO sub 2 . A temperature anomaly was found, indicating a phase change, probably melting, between 0.6 and 1.1 Mbar. Secondly, magnetic probes have been used to measure the velocity of sound in shock compressed metals. The loss of a longitudinal elastic component of the sound wave is indicative of melting. Finally, a novel optical analyzer technique, exploiting the sensitivity of thermal radiation to small temperature changes, has also been used to measure the sound velocity in shock compressed metals. With the extra sensitivity provided by this derivative measurement of an equation of state surface, both solid-solid and melting transitions of iron above 2 Mbar have been detected. (ERA citation 06:030930)