Yoshitsugu Tomokiyo

Radiation-induced amorphization and swelling in ceramics☆

Irradiation-induced amorphization and swelling of ceramics including graphite have been studied through in-situ observation of electron microscopy, electron energy loss spectroscopy and convergent beam electron diffraction. Amorphization occurs below 500 K for highly-oriented pyrolytic graphite (HOPG) and below 300 K for SiC under electron irradiation. The amorphization fluences of HOPG and SiC required for full amorphization show no flux dependence. The displacement threshold energy is determined to be 12 eV for HOPG from the electron energy dependence of the amorphization fluence which is equivalent to 1 dpa. An atomistic model for describing the irradiation-induced amorphization and swelling of graphite is proposed. A high concentration of C2 molecules and their clusters between basal planes induce abrupt swelling along the c-axis within 4 × 10−3 dpa and fade the periodical structural image at 4 × 10−2 dpa. The concentration of vacancies increases gradually up to 1 dpa at which amorphization is completed.

Room-temperature epitaxial growth of ferromagnetic Fe3Si films on Si(111) by facing target direct-current sputtering

Ferromagnetic Fe3Si thin films with an extremely smooth surface morphology can be epitaxially grown on Si(111) at room temperature by facing target direct-current sputtering. The epitaxial relationship is Fe3Si(111)‖Si(111) with Fe3Si[11¯0]‖Si[1¯10]. By the application of the extinction rule of x-ray diffraction, the generated Fe3Si was confirmed to possess a B2 structure and not a DO3 one. The film showed a saturation magnetization value of 960emu∕cm3, which was slightly lower than that of bulk DO3-Fe3Si. It was observed that the magnetization easy axis was along the [11¯0] direction in the film plane.

Semiconducting nanocrystalline iron disilicide thin films prepared by pulsed-laser ablation

Amorphous iron silicide was reported to be semiconducting as well as β-FeSi2, and it has received considerable attention from both the physical and engineering points of view. However, there have been few studies and its basic properties are still unknown. We could grow the semiconducting nanocrystalline iron disilicide thin films by pulsed-laser deposition using an FeSi2 target. They consist of crystallites with diameters ranging from 3 to 5 nm. The carrier density and the mobility at 300 K were 1.5×1019 cm−3 and 35 cm/V s, respectively.

Microstructures of MgB2/Fe tapes fabricated by an in situ powder-in-tube method using MgH2 as a precursor powder

Microstructures of MgB2/Fe tapes fabricated by an in situ powder-in-tube method using MgH2 as a precursor powder have been studied by means of x-ray diffraction and analytical transmission electron microscopy combined with a focused ion beam microsampling technique. The overall microstructures in the tapes are characterized as densely crystallized MgB2 areas with 10–200 nm grain size, uncrystallized areas mainly containing MgO and amorphous phases enriched with B, and a number of holes and cracks. The crystallized MgB2 areas increase upon doping with SiC nanoparticles. Si and C atoms decomposed from SiC nanoparticles during heat treatment exhibit different spatial distributions: the Si atoms are inhomogeneously distributed, forming silicides such as Mg2Si with grain size of 5–20 nm, while the C atoms tend to be uniformly distributed in the MgB2 matrix. A significant difference in distribution of O atoms between the SiC-doped and non-doped specimens was observed. The processes of formation of these microstructures and their relationships with the critical current density under magnetic fields have been discussed.

Strain contrast of coherent precipitates in bright-field images under zone axis incidence

Abstract It has been proposed that bright-field imaging of coherent spherical precipitates under zone axis incidence would give improved resolution in the strain contrast compared with the weak-beam dark-field imaging. We observed the strain contrast from coherent Co-precipitates in Cu-Co alloys, and simulated the images on the basis of many-beam dynamical electron diffraction, taking into account the elastic anisotropy. Zone axis incidence such as [001] or [001] considerably reduces the effective extinction distance of the transmitted beam, and suppresses the intensity oscillation with foil thickness, in comparison with a conventional two-beam condition. The contrast in the bright-field images is straightforwardly related to the precipitate geometry. Individual precipitates are well resolved in the image when the number density of precipitates is fairly high. It has been demonstrated that the imaging under zone axis incidence is quite useful to understanding of microstructures in phase transformation in ...

Relationship between microstructure and Jc property in MgB2/α-Al2O3 film fabricated by in situ electron beam evaporation

A transmission electron microscopy (TEM) study has been carried out on an MgB2/?-Al2O3 film that exhibits the typical property of critical current density (Jc) under magnetic fields. The MgB2 layer of 300?nm in thickness was grown on a (001)?-Al2O3 substrate using an in situ electron beam evaporation method. Jc of the film takes significantly high values when the applied magnetic field is perpendicular to the film surface. The MgB2 layer consists of fine columnar MgB2 crystals 20?30?nm in size. The columnar MgB2 crystals grow almost perpendicular to the substrate surface and have no crystallographic orientation relationship with the ?-Al2O3 substrate because of an amorphous layer formed first on the substrate. A high density of columnar grain boundaries within the MgB2 layer may be effective for the enhancement of the flux-pinning under the perpendicular magnetic field.

Dynamical diffraction effect on HOLZ-pattern geometry in Si-Ge alloys and determination of local lattice parameter

Abstract Higher-order Laue zone (HOLZ) patterns are observed in convergent-beam electron diffraction (CBED) patterns of Si-Ge alloys and the dynamical effect on HOLZ lines is discussed in relation to the kinematical approximation to simulate the HOLZ lines. Double lines often appear from one HOLZ reflection due to the excitation of the first and second branches of the zeroth-layer dispersion surface. The magnitude of the systematic line shift from the line positions predicted by the kinematical simulation changes sensitively with specimen thickness and alloy composition depending on which branch is dominantly excited. The kinematical simulation is applicable to accurate determination of lattice parameters with a proper correction δ E : a difference between the effective electron energy and the actual microscope operating voltage. Usefulness of the kinematical simulation is demonstrated in the investigation of the segregation in a Si-Ge alloy.

Evidence of charge stripes, charge-spin-orbital coupling and phase transition in a simple copper oxide CuO

There is a great current interest in understanding the role of charge stripes in high- T c superconductivity. We found that static charge ordering and alignment of charge-ordered domains analogous ...

Defects occurring at or near surfaces in α-Al2O3 during electron irradiation

Abstract A new type of defect introduced during electron irradiation in an α-Al 2 O 3 single crystal has been investigated through in situ high-resolution electron microscopy (HREM). The defects are observed as a black line contrast parallel to the (006) lattice plane in HREM images along the [110] direction but they are invisible in images along [001]. Line contrasts of the defects can also be observed in a bright-field or dark-field image under excitation of the systematic row of (006) reflection. Electron-diffraction patterns show streaks and extra spots suggesting the presence of planar defects inside the specimen as well as the surface. The defects do not appear during irradiation at 200 kV, but do appear at 400 kV after irradiation of several minutes. The defects occur in a thick area as well as in a thin area of specimen edge and some of them grow or disappear abruptly during irradiation. Examples of multi-layer and complex-layer defects are also shown.

Analysis of Local Lattice Strains Around Plate-Like Oxygen Precipitates in Czochralski-Silicon Wafers by Convergent-Beam Electron Diffraction

Convergent-beam electron diffraction (CBED) is used to study lattice strain around plate-like oxygen precipitates in Czochralski (CZ)-grown silicon. Local lattice strain determined from higher-order Laue zone (HOLZ) patterns shows that compressive and tensile stress fields exist near the precipitates. The spatial variation of local lattice strain and lattice rotation is visualized in a defocused large angle CBED disc, or a convergent-beam imaging (CBIM) disc.