Tomitsugu Taguchi

In this study, ion irradiation induced microstructural development of polycrystalline double-thick-walled (DTW) silicon carbide (SiC) nanotubes are investigated. In-situ transmission electron microscopy (TEM) observations of two kinds of polycrystalline DTW SiC nanotubes with a wide and narrow spacing between the outer and inner nanotubes are carried out under the ion irradiation with 200-keV Si+ ions at room temperature. The critical doses of amorphization for both DTW SiC nanotubes are larger than those of SiC bulk materials in the previous studies. Amorphous DTW SiC nanotubes are successfully synthesized for the first time via the ion irradiation of both polycrystalline DTW SiC nanotubes. From in-situ TEM observation results, all of outer, inner diameters and length of both inner and outer nanotubes increase with increasing the irradiation dose till around complete amorphization of SiC crystals, and then decrease. After ion irradiation of more than 20 displacement per atom, polycrystalline DTW SiC nanotubes with a wide and narrow spacing between the outer and inner nanotubes are transformed to amorphous SiC nanotube and SiC nanowire, respectively.

Natural monoclinic zirconia (baddeleyite) was irradiated with 340 MeV Au ions, and the irradiation-induced nanostructures (i.e., ion tracks and nanohillocks) were observed using transmission electron microscopy. The diameter of the nanohillocks was approximately 10 nm, which was similar to the maximum molten region size calculated using the analytical thermal spike model. Ion tracks were imaged as strained regions that maintained their crystalline structure. The cross-sections of most of the ion tracks were imaged as rectangular contrasts as large as 10 nm. These results strongly indicated that the molten region was recrystallized anisotropically, reflecting the lattice structure. Furthermore, low-density track cores were formed in the center of the ion tracks. The formation of low-density track cores can be attributed to the ejection of molten matter toward the surface. A comparison of the ion tracks in the synthetic zirconia nanoparticles and those in larger natural zirconia samples showed that the interface between the strained track contrast and the matrix was less clear in the former than in the latter. These findings suggest that the recrystallization process was affected by the size of the irradiated samples.

Amorphizable ceramics (LiNbO₃, ZrSiO₄, and Gd₃Ga₅O₁₂) were irradiated with 200 MeV Au ions at an oblique incidence angle, and the as-irradiated samples were observed by transmission electron microscopy (TEM). Ion tracks in amorphizable ceramics are confirmed to be homogenous along the ion paths. Magnified TEM images show the formation of bell-shaped hillocks. The ion track diameter and hillock diameter are similar for all the amorphizable ceramics, while there is a tendency for the hillocks to be slightly bigger than the ion tracks. For SrTiO₃ (STO) and 0.5 wt% niobium-doped STO (Nb-STO), whose hillock formation has not been fully explored, 200 MeV Au ion irradiation and TEM observation were also performed. The ion track diameters in these materials are found to be markedly smaller than the hillock diameters. The ion tracks in these materials exhibit inhomogeneity, which is similar to that reported for non-amorphizable ceramics. On the other hand, the hillocks appear to be amorphous, and the amorphous feature is in contrast to the crystalline feature of hillocks observed in non-amorphizable ceramics. No marked difference is recognized between the nanostructures in STO and those in Nb-STO. The material dependence of the nanostructure formation is explained in terms of the intricate recrystallization process.

In this study, a compact fiber-optic laser-induced breakdown spectroscopy (LIBS) system was developed using a microchip laser (MCL) with a monolithic Nd:YAG/Cr:YAG composite ceramic, for remote analysis of hazardous environments, such as nuclear reactor cores. Short duration laser pulses exhibiting a near-Gaussian beam profile were obtained. The output properties of the laser, such as pulse energy, repetition rate, temporal shape, and beam profile, were measured in view of their applicability to LIBS analysis and were found suitable for the purpose of this research. Spectra of zirconium metal were obtained, and signal intensity was further enhanced by applying multi-burst mode irradiation to the target. The results of this study reveal that the fiber-optic microchip-laser induced breakdown spectroscopy system is advantageous for efficient remote analysis of hazardous environments and is suitable for analyzing the inside of nuclear reactor cores.

Ion-track-etched capillaries containing nanoparticles of precious metals (e.g., Pt, Au, and Ag) can be applied to plasmonic absorber materials. The precipitation of homogeneous and highly dispersed precious metal nanoparticles inside capillaries represents a key process. Ion-track-etched capillaries (diameter: ~500 nm, length: ~25 μm) were created in polyimide film by 350 MeV Xe irradiation (3 × 10⁷ ions/cm²) and chemical etching (using a sodium hypochlorite solution). The films with capillaries were immersed in an aqueous solution containing 0.1–10 mmol/L H₂PtCl₆ and 0.5 vol% C₂H₅OH, and then irradiated with a 2 MeV electron beam up to a fluence of 1.4 × 10¹⁶ e/cm². The Pt particles inside the capillaries were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The precipitation of Pt nanoparticles and isolated aggregates inside the capillaries was confirmed by TEM. The Pt nanoparticles tended to aggregate under increasing concentrations of H₂PtCl₆ in the aqueous solution; meanwhile, no changes in nanoparticle size were noted under increasing electron beam fluence. The results suggest that the proposed method can be used to form metal nanoparticles in nanosized capillaries with a high aspect ratio.