Takamichi Kobayashi

COLLAPSING CARBON NANOTUBES AND DIAMOND FORMATION UNDER SHOCK WAVES

Abstract In order to investigate the mechanical properties of carbon nanotubes, dynamic shock wave pressures (⩽50 GPa) were applied on arc-discharge-generated carbon nanotubes containing polyhedral nanoparticles. High-resolution transmission electron microscopy (HRTEM) studies of the shock-recovered samples reveal that: (i) layers of the outer shells of the nanotubes break and transform into curled graphitic structures and (ii) the inner tube walls and bulk material display structural defects. Further X-ray powder diffraction and HRTEM analyses exhibit the presence of diamond nanocrystals which are produced after the shock-wave compression of polyhedral particles (present in the starting material).

Yield properties, phase transition, and equation of state of aluminum nitride (AlN) under shock compression up to 150 GPa

Inclined-mirror Hugoniot measurements were performed on pure AlN polycrystals in the pressure range up to 150 GPa to study the yield properties, phase transition, and equation of state. The Hugoniot-elastic limit (HEL) stress was approximately 9.4 GPa. Above the HEL, the Hugoniot data converged to a static compression curve despite the high thermal conductivity, which indicated that the thermal property is not an important factor in determining the shock yield property. The phase transformation from wurtzite-type (B4) to rock salt-type (B1) structure took place at approximately 19.4 GPa, and was completed by about 75 GPa. The corrected transition pressure at 298 K was 19.2 GPa. Shock velocity (Us) versus particle velocity (Up) relation of the final phase was given by Us=3.27+1.81Up km/s. The Birch–Murnaghan fitting curve of the calculated isothermal compression curve of the B1-type phase roughly coincided with the recent static x-ray diffraction data up to over 100 GPa. The Gruneisen parameter, bulk modul...

Shock compression behaviors of boron carbide (B4C)

Hugoniot measurements on the highly dense, pure B4C polycrystal were performed by the inclined-mirror method to study the elastoplastic transition and to search phase transition. In inclined-mirror streak photographs, the smoothly jagged structure was observed at the free-surface shape in the plastic region. The Hugoniot-elastic limit (HEL) has been determined to be approximately 19.5GPa. In the plastic region, a kink was observed at a particle velocity of around 1.26km∕s. The shock velocity (US)–particle velocity (UP) Hugoniot relations in the plastic region were given by US=3.7+5.4UPkm∕s in the Up range of 0.54–1.26km∕s and US=9.61+0.73UPkm∕s in the Up range of 1.26–4.3km∕s. The S value (0.73) in US=C0+SUP above UP=1.26km∕s is significantly small compared with the result of Vogler et al. [J. Appl. Phys. 95, 4173 (2004)], and was much smaller than those of many oxides and nitrides. This material behaved as an elastoisotropic solid above the HEL and showed a large and linear change in the pressure-density...

High pressure behavior of titanium–silicon carbide (Ti3SiC2)

The dynamic high-pressure behavior and phase stability of titanium–silicon carbide (Ti3SiC2), a unique ceramic having metal-like properties, was investigated in this study. Time-resolved measurements of the Hugoniot equation of state, employing a plate impact geometry, were conducted on the Ti3SiC2 samples in the pressure range of 50–120 GPa using a two stage light gas gun. At pressures around 90–120 GPa, Ti3SiC2 was found to transform to a more compressed state. Shock-recovery experiments were also performed on Ti3SiC2 powders at impact velocities of 1.5–2 km/s using a single capsule geometry, with and without the addition of copper powder to vary the shock-loading pressure (calculated to be 22–58 GPa) and temperature (calculated to be up to 3250 °C) in the sample. No evidence of shock-induced decomposition was observed in these recovery experiments performed on the Ti3SiC2 powders.

Phase transformation of germanium nitride (Ge3N4) under shock wave compression

The phase transformation behavior of hexagonal germanium nitride, including both α- and β-Ge3N4, has been studied under shock wave compression. The shock compressed quenched samples indicate phase transformation from hexagonal into a cubic spinel structure (γ-Ge3N4). This transformation is completed with increasing shock pressure up to 40–46 GPa (temperature of 1300–1500 K). The lattice constant of γ-Ge3N4 is measured to be 0.820 63±0.000 19 nm, and the crystal density 6.581 g/cm3, by the powder x-ray diffraction. The stability of γ-Ge3N4 also has been investigated under shock wave compression. It is found that the spinel structure is very stable, and up to at least 63 GPa (temperature of ∼1700 K) there is no indication of the formation of a postspinel phase.

Cubic Si6−zAlzOzN8−z (z=1.8 and 2.8) spinels formed by shock compression

Abstract We present the formation of cubic Si6−zAlzOzN8−z (z=1.8 and 2.8) spinels by shock compressions from the corresponding β-sialon. Formed sialon spinels coexist with considerable amounts of the amorphous phase. The amorphization is considered to be a solid–solid reaction due to the sluggish phase transition under shock compression. The lattice parameter of the spinel increases and the density decreases with increasing z parameter. Shock-synthesized sialon spinels are nanocrystals with most grain sizes less than 30 nm and consist of main units of SiN4, SiN6 and AlO6.

Aluminum oxynitride at pressures up to 180 GPa

Hugoniot equation-of-state data of shock compressed aluminum oxynitride (AlON), consisting of 64.1 mol% Al2O3⋅35.9 mol% AlN with a density of ∼3.68 g/cm3, have been determined to 180 GPa. The relationship between shock velocity (Us) and particle velocity (Up) is expressed by a straight line: Us(km/s)=8.08+0.761Up(km/s). Although there is no evidence of phase transition in the data, the determined Hugoniot of AlON has been compared with those of oxide spinels such as MgAl2O4 and Fe3O4. We discuss the systematics of high pressure phase transitions of spinels that indicate a phase transition to CaTi2O4-type phases. The phase transition to CaTi2O4-type structures implies that the recently discovered Si3N4 spinel also may be transformed into a CaTi2O4-type phase with increasing pressure.

Shock equation of state of basalt

Detailed wave profiles for Kinosaki basalt at pressures up to 25 GPa are measured using a laser velocity interferometer in order to determine the dynamic properties. The results indicate a Hugoniot elastic limit of ∼2 GPa and a relationship between shock velocity (Us) and particle velocity (Up) approximated by Us (km/s) = 3.5 + 1.3Up (km/s) in the low-pressure plastic region (Up below ∼4 km/s). These data are compared with the known data for rocks with basaltic compositions, and tensile strength of the basaltic rocks was found to be about one tenth of that of compression strength.

Shock-induced phase transitions among SiC polytypes

A series of recovery experiments was conducted using a propellant single-stage gun on starting materials of both α-SiC and β-SiC. X-ray examination on the recovered samples indicated that obvious polytype transformations among 3C, 6H, and 15R took place. To the α-SiC starting material, 15R tends to increase and 6H tends to decrease, while a small amount of α-SiC form transforms to 3C type, along with increasing the shock temperature and pressure. X-ray diffraction analysis showed that the β-SiC polytype is transformed into rhombohedral forms. From results of both types of SiC samples, rhombohedral polytypes seem to be the favored shock modification. The effects of shock pressure and shock temperature and their heterogeneous distribution on these polytype transitions are discussed in detail. Analysis showed that these polytype transitions resulted from the stacking sequence changes of SiC atom layers.

Accurate measurement of the velocity history of a laser-driven foil plate with a push–pull-type VISAR

We describe a technique that provides high time resolution and high accuracy in the velocity-history measurement by coupling an electronic streak camera with a push-pull-type velocity interferometer system for any reflector. This technique shows strong potential for the study of the dynamic process associated with a rapid velocity change, such as the acceleration of a foil plate driven by a pulsed laser beam. Also, by using a micrometer-size spot optical probe, we demonstrate the acceleration histories of Al 10-mum-thick foil plates at laser intensities ranging from 30 to ~400 GW/cm(2) with a subnanosecond time resolution and within a 1-2% error for peak velocity.