Hiroaki Abe

Carbon nanotubes prepared by spinning and carbonizing fine core-shell polymer microspheres

Carbon nanotubes prepared from core-shell polymer particles are reported. As depicted in the Figure, polymethylmethacrylate (PMMA) core/polyacrylonitrile (PAN) shell microspheres are blended with a PMMA matrix, spun, and elongated. Then the shell is stabilized and finally carbonized to produce contaminant-free carbon nanotubes.

Role of irradiation spectrum in the microstructural evolution of magnesium aluminate spinel

Abstract In-situ observations of the nucleation and growth of defect clusters in MgAl2O4 were made in transmission electron microscope-accelerator facilities using concurrent irradiation with ions and electrons to obtain insight into the role of the irradiation spectrum. Various kinds of 30 or 300 keV ions (He+, O+, Mg+, Ar+ and Xe+) and 200 keV or 1 MeV electrons were used to provide a wide range of nuclear and ionizing stopping powers as well as a various effects of displacement cascades. Dislocation loops were formed both inside and outside the electron beam at 870 K under concurrent irradiation with 30keV ions and 1 MeV electrons. We observed a variety of microstructures that formed during concurrent irradiation with 300 keV ions and 200 keV electrons. These include preferential formation of cavities, suppression of dislocation loop formation and preferential formation of loops at the periphery of the focused electron beam. We discuss the irradiation spectrum effects in terms of the ionizing, displaci...

Effect of damage cascades on the irradiation-induced amorphization in graphite

Abstract In-situ observation of the irradiation-induced amorphization and measurements of the critical amorphization dose in graphite under irradiation with ions and electrons have been performed to reveal the effects of damage cascades. The critical amorphization dose ( D c ) increases exponentially with temperature and shows an apparent critical amorphization temperature ( T c ). T c increases with increase in mass of projectiles and with decrease in energy of ions. Dose rate dependence of D c and T c was also observed. No amorphization was detected above 860 K. Accumulation of vacancies and/or interlayer carbon molecules is attributable to the amorphization, while annealing of amorphous regions is dominated by kinetics of lattice defects. Defect annealing is associated with recombination of interlayer carbon molecules with vacancies. Damage cascades leave high density of interlayer molecular clusters ((C 2 ) n , n = 1, 2, 3…) and stable di-vacancies, resulting in stable amorphous regions at high temperature.

Amorphization in aluminum oxide induced by ion irradiation

Abstract Amorphization in sapphire induced by irradiation has been observed for years but it is still in discussion whether displacements or implanted ions have an important role for the amorphization. In this work, we perform irradiation with high-energy heavy ions in thin film alpha-alumina below room temperature, so that the effects of atomic displacements, especially of damage cascades, can be revealed. Amorphization was detected at temperatures from 90 K to 230 K under irradiation with 600–900 keV xenon and krypton ions, while no amorphization was detected under irradiation with argon nor oxygen ions. The critical energy density to induce amorphization is estimated. Four stages on the crystalline-to-amorphous transformation was observed from the temperature dependence of the dose-to-amorphization. The effect of damage cascades on the irradiation-induced amorphization and defect kinetics resulting in an increase of the dose-to-amorphization will be discussed.

Formation mechanisms for carbon onions and nanocapsules in C+-ion implanted copper

Copper substrates were implanted with carbon ions at temperature ranging from 570 to 973 K. Implantation microstructures were investigated using transmission electron microscopy and high-resolution electron microscopy. Carbon onions and nanocapsules were observed together with amorphous carbon layers. Most of the nanocapsules were found to be hollow and rarely included copper nanoparticles. The encapsulating of Cu nanoparticles with graphene layers, the gradual shrinkage of the encapsulated clusters, and finally the disappearance of the clusters (leaving behind hollow nanocapsules) were observed under electron irradiation at 783 K. Statistics of cluster size as a function of ion fluence, implantation temperature, and substrate crystallinity gave insights into the nucleation processes of onions and nanocapsules. One process involves the formation of graphene layers on grain boundaries to encapsulate copper particles. The other process is the nucleation of graphene cages, probably fullerenes, due to high co...

Accumulation and annihilation processes of cascades in metallic and nonmetallic crystals under irradiation with ions and/or electrons

Abstract In situ observation has been performed under dual-beam irradiation with heavy ions and fast electrons in the HVEM-accelerator facility at Kyushu University. The objectives of the present study are to understand the structure and the accumulation process of cascade damage and the synergistic effect of electronic excitation and/or free point defects on the behavior of cascade damage in metallic and nonmetallic crystals, such as Cu, graphite, SiC, Si, Ge, Ge-20at%Si, MgO, α-Al 2 O 3 , MgAl 2 O 4 and others. The HVEM-accelerator facility has been confirmed to be extremely useful for observing the behavior of cascade damage: (1) Cascade damage is directly produced without the help from other collision cascades in Cu. (2) Most collisions themselves in Si, Ge and Ge-20at%Si induce invisible structural change through electron microscopy, but they are converted into visible ones by the impact from other collision cascades. (3) No visible cascade damage is formed in covalent crystals with light masses such as graphite and SiC and in ionic crystals such as MgO, α-Al 2 O 3 and MgAl 2 O 4 . (4) Free interstitials suppress the evolution of visible cascade damage and assist the nucleation and the growth of interstitial loops around collision cascades in metallic and nonmetallic crystals.

Effects of annealing and quenching treatments on reconstruction of rutile thin films on sapphire substrates

Morphology change in rutile TiO2 thin films on sapphire substrates prepared by pulsed laser deposition under reduced oxygen environment was investigated as a function of film thickness, temperature and cooling treatments with atomic force microscopy, X-ray diffraction and scanning electron microscopy equipped with X-ray spectroscopy (SEM/EDX). The deposited TiO2 was determined as epitaxially grown rutile films whose crystallographic correlation with substrates was (1 0 0)rutile//(0 0 0 1)sapphire. As increasing thickness of TiO2 films, smooth surface changed to island structure. In addition, the morphology of TiO2 film on α-Al2O3(0 0 0 1) varied drastically by annealing treatment from 973 to 1123 K. In case of ∼5 nm thickness films, morphology strongly depended on annealing and cooling treatments. We found interesting order structure of TiO2 islands at annealing temperature (∼1073 K) and subsequent quenching (∼1.3 K/s). Formation process of TiO2 particles on α-Al2O3(0 0 0 1) substrates is modeled based on instability of substrate at elevated temperatures.

Nucleation of carbon onions and nanocapsules under ion implantation at high temperature

In-situ and ex-situ TEM observation was performed in copper implanted with carbon ions at temperatures from 570 to 973 K. Carbon onions (concentric graphitic spheres) and nanocapsules (concentric graphitic spheres with cavities) were observed together with amorphous carbon layers. Statistics of cluster size as a function of ion fluence, implantation temperature and substrate crystallinity gave insights into the nucleation processes of onions and nanocapsules. One is the formation of graphitic layers on grain boundaries to encapsulate copper particles. The other is the nucleation of graphitic cages, probably fullerenes, due to both high concentration of carbon atoms and high amount of radiation damage. Simultaneous observation of microstructural evolution under implantation revealed that onions were formed inside the substrate, not surface and that they segregate at the surface due to radiation-enhanced evaporation.

Accumulation process of cascades in ceramics under ion and/or electron irradiation

Dual-beam irradiation and simultaneous observation with 30 keV Xe+ ions and 250 or 1000 keV electrons have been performed for understanding the structure and the accumulation process of cascades and for attaining insight into the synergistic effect of electron and/or free point defects on the properties of cascades. Covalent crystals such as Si, Ge and Ge20 at% Si show contrast in transmission electron microscopy through the overlap of cascades, which stabilizes high concentration of point defects and induces an amorphous-like phase in covalent crystals. In ionic crystals such as MgO, Al2O3 and MgAl2O4, recombination of Frenkel pairs is of predominance and a small number of point defects are rather homogeneously distributed even when heavy ion impacts are introduced. The effects of electrons are to suppress the formation of vacancy clusters and to assist the nucleation of interstitial loops under the dual-beam irradiation.

Formation and growth process of defect clusters in magnesia under ion irradiation

Abstract In order to understand the effect of damage cascades on the formation and growth process of dislocation loops in MgO under ion irradiation, in-situ observations of microstructural evolution have been performed with the use of a 400 keV TEM combined with two sets of ion accelerators. In-situ observations under 1000 keV electrons have been also done with the use of high voltage electron microscopy. The 1 2〈110〉{110} type of interstitial dislocation loops are formed under irradiation with various kinds of particles. The minimum necessary dose at which the loops are able to be observed in TEM and their formation and growth rates, strongly depends on the irradiation particles. Such dependences of between kinetic processes on irradiation particles are considered to be influenced not only by the distribution of point defects but also by their stability, that is, the higher density of point defects inside damage cascades or at their periphery results in suppressing the growth of loops due to the enhancement of loop nucleation.