K. G. Nakamura

Nanosecond time-resolved Raman spectroscopy on phase transition of polytetrafluoroethylene under laser-driven shock compression

Nanosecond time-resolved Raman spectroscopy is performed on polytetrafluoroethylene (PTFE) under laser shock compression at approximately 1 GPa. Blueshifts (8 cm−1) of a C–C stretching and redshifts (−8 cm−1) of a CF2 twisting in Raman spectrum due to a high-pressure phase (phase III) are observed. High-pressure phase transition to phase III of PTFE is suggested to occur within 10 ns.

Time-resolved Raman spectra of ring-stretching modes of benzene derivatives under laser-driven shock compression at 1 GPa

Raman spectra were collected from benzene, toluene and nitrobenzene, under laser-driven shock compression at approximately 1 GPa. Raman peaks attributed to the ring-breathing and ring-vibrational mode showed a blue shift. The magnitudes of the frequency shift show a systematic dependence on substituents. The magnitude of the shift of the ring-breathing mode decreased in the order of benzene, nitrobenzene, toluene, while that of the ring-vibrational (distortion along an axis) increased.

Impact of built-in potential across LaFeO3/SrTiO3 heterojunctions on photocatalytic activity

Polar-nonpolar interfaces between insulating LaFeO3 (LFO) and semiconducting SrTiO3 (STO) were investigated to elucidate effects of built-in potential on photocarrier dynamics during water oxidation reactions. The LFO films were grown on the TiO2- and SrO-terminated (001) Nb-doped STO substrates by using pulsed-laser deposition. The photocurrent-voltage curves varied depending on the interface termination. Potential profiles across the interface were established from these curves as well as capacitance-voltage curves. The presence of a depletion (accumulation) region near the SrO- (TiO2-) terminated interface facilitates (suppresses) the extraction of photocarriers generated in STO. On the other hand, the difference in the built-in potential in LFO barely reflects the magnitude of the photocurrent.

Shock-induced intermediate-range structural change of SiO2 glass in the nonlinear elastic region

We study shock compressed fused quartz in the nonlinear elastic region using single-shot time-resolved x-ray scattering measurements. The first sharp diffraction peak (FSDP) of fused quartz shifts to the high Q region under shock compression. In contrast, the short-range order structure does not change around 3.5 GPa. The nanosecond FSDP shift provides clear evidence of intermediate-range order structural changes in the nonlinear elastic region. Because the intermediate-order structure is too short to produce the final structural state in the nonlinear elastic region, the FSDP shift is lower compared with hydrostatic experiments.

TEM OBSERVATION OF DISPROPORTIONATION OF MULLITE AND SILLIMANITE UNDER SHOCK COMPRESSION

Shock recovery experiment on sillimanite was conducted using a two‐stage light‐gas gun to clarify origin of the peculiar nano‐frgmentation observed in mullite. Recovered sillimanite was examined by X‐ray powder diffraction (XRD) and transmission electron microscopy (TEM). Shock recovered sillimanite showed amorphization by XRD measurements as seen in mullite. However, microtexture observed by TEM showed characteristic planar deformation structures and following amorphization, which are considerably different from those observed in mullite. Importance of oxygen vacancies in crystal structure of mullite for the nano‐fragmentation was inferred from comparison to the present result on sillimanite.

Intergrowth microstructures of MnF2 subjected to shock compression

Intergrowth microstructures of MnF2 subjected to shock compression at 4.4, 9.0 and 21.6 GPa were examined using transmission electron microscopy (TEM). Intergrowth microstructures consisting of rutile- and α-PbO2-type phases were observed in samples shock-loaded to 4.4 and 9 GPa. The sample subjected to 21.6 GPa consisted of a twin structure with stacking faults, with a rutile-type but not the α-PbO2-type phase. In the 9.0-GPa shocked sample, the phase ledge structure originating from a phase transition is directly captured by high-resolution transmission electron microscopy.

Liquid‐Solid Phase Transition of Benzene under Shock Compression Stuided by Time‐Resolved Nonlinear Raman Spectroscopy

The liquid‐solid phase transition of benzene has been studied under laser‐shock compression up to 4.2 GPa by using nanosecond time‐resolved nonlinear Raman spectroscopy. A shock wave is generated by irradiation of 10‐ns pulsed laser beam on the plasma confinement target and its pressure is estimated from the particle velocity, which is measured by a velocity interferometer. The ring‐breathing mode shows blue shift under shock compression. Nanosecond time‐resolved nonlinear Raman spectra show a rapid phase transition from liquid phase to solid phase under shock compression.

Phase Transition of MnF2 by Shock Compression up to 33 GPa

Rutile type MnF2 was shock‐loaded to 3.6 – 33.4 GPa by impact of flyer plates accelerated by a propellant gun. Recovered samples were studied by X‐ray powder diffraction and transmission electron microscope (TEM). The phase transition of rutile type to the α‐PbO2 type structure was found in the recovered sample. The yield of α‐PbO2 phase in the recovered sample increases up to about 10 GPa, and decreased at high pressures. The XRD line width of rutile phase (110) also has maximum at pressure of 10 GPa, and that of α‐PbO2 phase (111) is broader than rutile phase and almost constant against shock pressure. The lamellae pattern, which consists of two crystal phases (rutile and α‐PbO2 phase), was observed in the shock recovered sample from 8.3 GPa by TEM.