Ramp Compression of Copper and a Pressure Standard to 450 GPa

Abstract:

Diamond anvil cell pressure standards such as copper, tungsten, gold, and platinum are calibrated by reducing Hugoniot data to an isentrope or isotherm using a model for the thermal pressure. At pressures below the bulk modulus of the sample, the correction for the thermal pressure is relatively small and therefore the uncertainties in the thermal model are not significant. However, as stresses in diamond anvil cells are achieving pressures of 4-10 Mbar, reducing Hugoniot data to an isotherm requires a tremendous thermal pressure correction and uncertainties in the reduced isotherm are unconstrained. Here we present ramp-wave compression experiments at the Sandia Z-Machine that we use to constrain the equation of state of copper to a stress state of nearly 5 Mbar. We use the iterative Lagrangian analysis technique, developed by Rothman and Maw, to determine the stress-strain path. We correct for the plastic work heating and the deviatoric stress contribution to the stress-density measurement to obtain an isentrope. Our measured isentrope compares well with our shock-wave reduced isentrope at low pressures and provides an accurate pressure standard for diamond anvil cells at extreme conditions.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.