Ramp compression of iron to 273 GPa

Abstract

Multiple thickness Fe foils were ramp compressed over several nanoseconds to pressure conditions relevant to the Earth's core. Using wave-profile analysis, the sound speed and the stress-density response were determined to a peak longitudinal stress of 273 GPa. The measured stress-density states lie between shock compression and 300-K static data, and are consistent with relatively low temperatures being achieved in these experiments. Phase transitions generally display time-dependent material response and generate a growing shock. We demonstrate for the first time that a low-pressure phase transformation (a-Fe to e-Fe) can be overdriven by an initial steady shock to avoid both the time-dependent response and the growing shock that has previously limited ramp-wave-loading experiments. In addition, the initial steady shock pre-compresses the Fe and allows different thermodynamic compression paths to be explored.

X-ray diffraction of molybdenum under ramp compression to 1 TPa

“…We consider this unreasonable for two reasons: first, the density would be significantly higher than that expected along the 293-K isotherm whereas ramp-loaded samples are expected to have densities intermediate between the room-temperature isotherm and the shock Hugoniot (Fig. 6b) 46 . Second, such a large volume change is unlikely for phase transitions at multi-megabar pressures where phase changes typically involve only small volume changes 35,36 .…”

“…Calculated densities are also shown for possible assignment of the measured diffraction line above 900 GPa to the FCC, HCP, dHCP, and ω phases by assigning the measured Mo diffraction line to the indicated (hkl) reflection for each structure (see legend). For hexagonal phases (HCP, dHCP, and ω ), the c/a ratios were taken to be 1.633, 3.154 and 0.622 46,[51][52][53][54] respectively. Uncertainties are shown for BCC only and are similar for other structures.…”

“…It remains very challenging to reach such multi-megabar pressures with DAC technology 40 . Along the Hugoniot, Mo melts near 390 GPa 2,7,8 precluding the study of the solid phase to higher pressures by shock compression.…”

“…Ablation of the surface due to the absorbed laser energy leads to the propagation of a ramped loading wave into the sample. Recent developments in laser-based ramp compression allow pressures up to TPa (10 million atmosphere pressure) conditions to be reached for compression over nanosecond timescales [42][43][44][45][46][47][48][49] . The development of nanosecond (or faster) x-ray diffraction diagnostics provides a new means to probe the lattice compression and structural state of materials under these extreme conditions [49][50][51][52][53][54] .…”

“…Mo is an important material in high-pressure science. It has been studied extensively under shock compression [1][2][3][4][5][6][7][8] and has been used as a pressure calibrant in diamond anvil cell experiments [9][10][11][12] . Static compression experiments have been used to explore its equation of state and phase stability [13][14][15] , melting behavior 16,17 , thermoelastic properties [18][19][20] and shear strength 21 .…”