Shock Compression of Diamond Crystal

Two shock wave experiments employing Lau• technique. The normal axis to the sample inclined mirrors have been carried out to surface of both crystals were very close to each determine the Hugoniot elastic limit (HEL), final other and inclined 15.4 ñ 0.6 ø from <111>, or 29.6 shock state at 191 and 217 GPa, and the post-shock ñ 0.6 ø from <110> in approximately the (110) state of diamond crystal, which is plane. The normal axis to the sample surface was shock-compressed along the intermediate direction actually inclined 1.5 ñ 0.2 ø from the (110) plane. between the <111> and <110> crystallographic axes. Longitudinal sound velocity measured at ambient The HEL wave has a velocity of 19.9 ñ 0.3 mm/•sec pressure and temperature using a pulse and an amplitude of 63 ñ 28 GPa. An alternate transmission technique was 18.13 ñ 0.41 mm/•s. interpretation of the inclined wedge mirror streak Using McSkimin and Andreatch's [1972] elastic record suggests a ramp precursor wave and then moduli, the calculated longitudinal sound velocity another HEL value. The maximum post-shock density in the same crystallographic direction as these achieved upon release from the ~200 GPa shock samples was 18.57 mm/•s, which was in reasonable state is ~3.95 Mg/m 3, which compares to the agreement with the observed value. initial density 3.52 Mg/m 3. This result suggests The two samples were mounted on OFHC copper and an elastic unloading effect or shock-induced tantalum driver plates, 0.6 mm thick, transition to a denser (possibly metallic) phase. respectively. Four flat mirrors and inclined wedge-mirrors were mounted on the sample Introduction assemblies in order to measure shock wave and free surface velocities [Ahrens, 1980]. The wedge The high pressure equation of diamond is of mirrors with appropriate angles for shot #106 and geophysical nterest because of the likelihood #108 were placed at the inclined angle of 9.62 ø that the proposed post-diamond metallic phase of and 6.24 ø , respectively. Self-shorting pins were carbon [see e.g. Bundy, 1980] is stable at lower also mounted on the sample assemblies to measure mantle and core pressures implying high shock-wave velocity using an oscilloscope. The solubilities of C in the earth's iron core. target assembly for shot #106 and #108 samples Previously, Pavlovskii [1971] reported four were respectively impacted by Cu and Ta flyer Hugoniot data for single crystal diamond to 590 plate imbedded on a lexan sabot accelerated by a GPa and five data for porous samples (O = 1.90 25 mm bore diameter two-stage light-gas gun. The Mg/m•), whereas McQ•een andMarsh [19687 reported projectile v locity was measured by a flash X-ray the Hugoniot data for pressed diamond (O 0 = shadowgraph technique [Jeanloz and Ahrens, 1977]. 3.191). Other investigations of the high pressure Shock and free-surface velocities were measured by phase diagram of carbon have utilized using an image converter streak trace. The time shock-compression studies on graphite or carbon calibration of the writing rate was obtained using with various densities [Grover, 1979, and Bundy, a Pockel's-cell modulated Argon laser beam with 1980]. The object of the present work was to 30.0 MHz timing marks [Jeanloz and Ahrens, 1977]. carry out a reconnaissance study of the dynamic Self-shorting pins were connected to a RC pulse yielding stress and search for evidence of the circuit, which utilized a tilt-pin technique metallic phase transition. [Kondo et al., 1981] in order to identify which pins were activated. Experimental Results and Discussion The two natural, industrial diamond samples (4.7 carat and 8.9 carat for shot #106 and shot The streak camera records for both experiments #108, respectively) used in this study were suggest a double-wave structure in diamond sample. transparent but contained cracks and brown The streak trace of the inclined mirror clearly inclusions. Bulk densities of the samples, which detects the first shock arrival. The slope of the were measured by an Archimedean method using inclined mirror image is used to obtain the toluene at 21.6øC, were 3.516 ñ 0.005 and 3.519 ñ accompanying free-surface velocity. The particle 0.003 Mg/m •, respectively, which closely agrees velocity behind the first wave was obtained using with the X-ray density of 3.515 Mg/m •. The the free-surface approximation [Wackerle, 1962]. crystallographic directions of the sample surface The reflectivity at the flat arrival mirror is not for both crystals were determined by the X-ray completely lost upon the first shock-wave arrival. At the second (final) shock-wave arrival, the reflectivity of the arrival mirror is completely 1Now at Research Laboratory of Engineering destroyed. The self-shorting pin also recorded Materials, Tokyo Institute of Technology, this wave arrival. The bending point of the Nagatsuta, Midori, Yokohama 227, Japan inclined mirror streak trace, taking into account of the free-surface motion due to the first shock Copyright 1983 by the American Geophysical Union. arrival, is consistent with both the arrival mirror and shorting-pins result (within an Paper number 3L0262. experimental error). The impedance-match method 0094-8276/83/003L-026253-00 was used to obtain the final shock state,