Bulletin of the American Physical Society
22nd Biennial Conference of the APS Topical Group on Shock Compression of Condensed Matter
Volume 67, Number 8
Monday–Friday, July 11–15, 2022; Anaheim, California
Session P04: Shock Compression of Geophysical Materials IIFocus Recordings Available
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Chair: Jamie Kimberley, New Mexico Institute of Mining and Techn Room: Anaheim Marriott Platinum 2 |
Wednesday, July 13, 2022 11:00AM - 11:30AM |
P04.00001: The Role of Graphite Crystal Structure and Microstructure on the Shock-induced Graphite to Diamond Phase Transformation Invited Speaker: Travis Volz Due to diamond’s scientific utility, the formation of diamond from shock-compressed graphite has been of broad interest since the 1960s. Despite continued research, studies on the shock-induced graphite to diamond phase transformation have reported a broad range of seemingly contradictory findings. In this work, we performed plate-impact shock compression experiments to better understand the effects of initial graphite crystal structure and microstructure on the shock-induced graphite to diamond phase transformation. Three graphite types studied were, listed by decreasing internal order: ZYB-grade highly oriented pyrolytic graphite (HOPG), ZYH-grade HOPG, and as-deposited pyrolytic graphite (PG). Wave profile transmission experiments showed rapid transformation for all graphite types though the PG transformation stress (46 GPa) was more than twice that of HOPG (22 GPa). Real time in-situ X-ray diffraction measurements during shock compression highlighted a likely cause for the different observed transformation stresses—additional c-axis compression was required to overcome the internal crystallographic disorder of PG and induce transformation to the cubic diamond structure. HOPG instead transformed to the rare hexagonal diamond polymorph, likely following a martensitic mechanism. Front-surface impact experiments showed that shock-formed hexagonal diamond has a larger longitudinal modulus than shock-formed or single crystal cubic diamond. These results suggest that hexagonal diamond is likely stronger than cubic diamond, making it a potential ultra-hard alternative to cubic diamond for numerous scientific and industrial applications. |
Wednesday, July 13, 2022 11:30AM - 11:45AM |
P04.00002: Mechanical and optical response of magnesium fluoride single crystals shock compressed along the c-axis Anirban Mandal, Brian J Jensen Magnesium fluoride (MgF2), which occurs naturally as the mineral sellaite, is of significant interest to geophysics because its ambient rutile-type structure is isomorphic to SiO2 (stishovite). Magneisum fluoride is also used extensively as an optical material. In this work, plate impact experiments were carried out to study the mechanical and optical response of single crystal MgF2 samples shock compressed to 130 GPa along the c-axis. Resulting wave profiles and shock velocities through the samples were measured using laser velocimetry (PDV). Shocked MgF2 exhibits a nonlinear elastic response up to ~10 GPa, beyond which a two-wave structure was observed. A single overdriven wave was recorded for peak longitudinal stresses above 95 GPa. In the absence of x-ray diffraction data, it was not possible to determine whether the observed two-wave structure resulted from inelastic deformation or due to phase transitions reported under static high-pressure loading (rutile → orthorhombic → distorted fluorite → cotunnite). Our experiments also show that c-axis MgF2 remains optically transparent to 1550 nm wavelength light for the entire stress range examined here and therefore, it can be used as an optical window in shock wave experiments. LA-UR-22-22137 |
Wednesday, July 13, 2022 11:45AM - 12:00PM |
P04.00003: Shock response of [100] MgF2 single crystals to 120 GPa Ian K Ocampo, Thomas S Duffy, Michael Winey Under static compression, the high-pressure polymorphism of rutile-type difluorides can be characterized by a common sequence from rutile CaCl2 HP-PdF2 cotunnite. Relative to dioxides, the reduced valence and ionic radius of the F- anion in rutile-type MgF2 (sellaite) results in decreased transition pressures, making it an ideal analogue for stishovite (SiO2) which has been predicted to undergo the HP-PdF2 to cotunnite-type transformation at pressures in excess of 600 GPa. In plate impact experiments conducted at the Institute for Shock Physics, laser interferometry (VISAR and PDV) was used to measure wave profiles for [100]-oriented MgF2 single crystals shock compressed to 24 – 120 GPa. At low stresses (24-44 GPa), we observe wave profile features consistent with elastic-inelastic response, followed by a phase transformation. Peak stress-density states in this stress range are consistent with the modified fluorite-type (HP-PdF2) structure. At higher stresses (69-91 GPa), we observe a two-wave structure with peak stress-density states showing good agreement with the cotunnite-type structure. At 120 GPa, only a single wave structure is observed, indicating that the wave profile features observed at lower stresses are overdriven. |
Wednesday, July 13, 2022 12:00PM - 12:15PM |
P04.00004: Soda-lime Glass Revisited: Applying an Energy-Dependent Grüneisen Model to Shock Velocity, Temperature, and Sound Speed Data Paul D Asimow, Jinping Hu Shock experiments constrain the thermal equation of state (EOS) of planetary materials, giving pressure (P), volume (V), internal energy (E), temperature (T), and sound velocity in Hugoniot and off-Hugoniot states. The functional form of the EOS must be adequate to cover the P-V-T range of planetary interiors with sparse data and avoid erroneous predictions. When an EOS is needed at lower (or higher) energies than the Hugoniot, the thermal pressure term is critical. Most often γ≡V(∂P/∂E)V is taken to obey γ=γ(V) (the Mie-Grüneisen approximation), e.g. using (γ/γo)=(V/Vo)q (3 parameters). This is justified for solids, where q~1. Thermal pressure in liquids behaves differently; γ increases with decreasing V (q<0). But few studies have questioned the Mie-Grüneisen approximation for liquids. Our simulations and experiments on a basaltic analogue melt showed that γ=γ(V, E); we proposed a new four-parameter γ function. We extend this form to melts of soda-lime glass, an analogue for felsic natural melts. Our shock velocity, release, sound speed, and T data up to ~120 GPa are fit by a 3rd-order Birch-Murnaghan isentrope with γ(V, E). T is lower than previous results, indicating that heat capacity CV increases on compression in this melt. |
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