Makoto Hirabayashi

Crystal Structures of YBa2Cu3-δAδO9-γ (A=Ni, Zn and Co)

A structural study of orthorhombic YBa2Cu3O6.94, YBa2Cu2.8Ni0.2O6.85 and YBa2Cu2.7Zn0.3O6.75, and tetragonal YBa2Cu2CoO7.16 by means of X-ray and time-of-flight type (TOF) neutron diffractometers was carried out to determine the locations of the dopant atoms, Ni, Zn and Co, in the 1-2-3 (Y/Ba/Cu) oxide. The nickel atoms occupy only the Cu2 site 2q(00z) with z=0.36. The zinc atoms occupy the Cu1 site 1a(000) and the Cu2 site with occupancies of 0.20(5) and 0.05(2). In the tetragonal YBa2Cu2CoO7.16, cobalt atoms were found at both the Cu1 and Cu2 sites with occupancies of 0.83(2) and 0.08(1).

A Study of Microstructures of Grain Boundaries in Sintered Fe77Nd15B8 Permanent Magnet by High-Resolution Electron Microscopy

The microstructures of the grain boundaries in an Fe77Nd15B8 permanent magnet have been examined by high-resolution electron microscopy, in samples quenched from the sintering temperature of 1350 K and in samples quenched and then annealed at 870 K. The coercivities of the quenched and the quenched-and-annealed samples were 500 and 1000 kA/m, respectively. Thin layers of a bcc phase were observed over the surfaces of most of the Fe14Nd2B grains in both samples. In the quenched sample, the interfaces between the bcc phase and the Fe14Nd2B grains were not so distinct, and many thin platelets of the bcc phase were found to extend from the interfaces to the inside of the Fe14Nd2B grains of which the surrounding lattice is compressed locally. In the annealed sample, the platelets disappear completely and the interfaces between the bcc phase and the Fe14Nd2B grains become very sharp and distinct. It is concluded that the morphology of the bcc phase, particularly the microstructure of its interface with Fe14Nd2B grains, controls the coercivity mechanism of Fe77Nd15B2.

Highly Ordered Icosahedral Quasicrystal of Al-Cu-Fe Alloy Studied by Electron Diffraction and High-Resolution Electron Microscopy

An icosahedral quasicrystal in a conventionally solidified Al-Cu-Fe alloy has been examined with electron diffraction and high-resolution electron microscopy. Diffraction and Kikuchi patterns taken with the incident beam exactly parallel to the 5- and 3-fold symmetry axes show strict 5- and 3-fold rotational symmetry. Any systematic shift of diffraction spots, which often appears in rapidly solidified quasicrystals, was not observed at all. A high-resolution electron micrograph taken with the incident beam parallel to the 5-fold axis shows no existence of phason strains in the icosahedral quasicrystal.

Metallographic and structural observations in the pseudo-binary section Ni3Si-Ni3Ti of the Ni-Si-Ti system

Abstract The metallographic and structural features of the alloys in the pseudo-binary section between Ni 3 Si (L1 2 ) and Ni 3 Ti (D0 24 ) were investigated. The L1 2 phase (γ′) extended along a pseudo-binary line between the two phases with Ti solubility of about 11 at.%. Also, the L1 2 phase expanded up to about 80 at.% Ni at high Ti concentration. Consistent with previous observations, the addition of the Ti elements into the Ni 3 Si alloy led to congruent melting of L1 2 phase. Based on the observations of X-ray diffraction and an electron channeling technique aided with EDX (Energy Dispersive X-ray spectrometer), it was shown that the alloying element, Ti, substituted for the component element of Si the ternary alloy Ni 3 (Si, Ti) was highly ordered. The solubility limit of boron in Ni 3 (Si, Ti) was shown to be very low, i.e. less than 50 ppm.

Formation and structures of multiply-twinned nanoparticles with fivefold symmetry in chemical vapor deposited boron nitride

Abstract Formation and structures of multiply-twinned nanoparticles with fivefold symmetry in chemical vapor deposited boron nitride (BN) were investigated by transmission electron microscopy, electron diffraction and molecular orbital calculation. Sizes and nanostructures of the BN nanoparticles were strongly dependent on the deposition temperatures, total gas pressure and synthesis gas. Hexagonal- and rhombohedral-BN were produced at temperatures above 1700 °C and below 1600 °C, respectively. A three-dimensional model for the fivefold BN nanoparticles is proposed; the fivefold symmetry showed some distortion due to the geometrical arrangements along the fivefold axis.

Study on the Ordered Alloy Ag3Mg

One-dimensional anti-phase domain structure of the ordered alloy Ag 3 Mg has been confirmed by X-ray diffraction study using a single crystal as well as polycrystals and by electron diffraction study with evaporated thin films. The measurements of specific heat and electrical resistivity have also been done, and these results have been qualitatively discussed in connection with the structure of this alloy.

Structural study on high-Tc superconductor Bi2−x(Ca, Sr)3Cu2+xO9−y

The crystal structure of the high-Tc Bi2-x(Ca, Sr)3Cu2+xO9-y with x=0.2 and y=0.78 was studied by means of neutron and X-ray powder diffraction. Since the unit cell parameters were relatively large for powder diffraction analysis, an averaged structure was refined, assuming an orthorhombic sub-cell (a=5.39 A, b=5.39 A and c=30.37 A) which corresponds to 1/5 of the unit cell: This is a derivative of Bi4Ti3O12. The double perovskite unit interleaved by the double bismuth layers is B-type, i.e. a CuO6 octahedron is surrounded by eight alkaline earth metal ions. The pyramidal CuO5 configuration is found in this oxide. The copper valency is estimated at about 2.3 on the basis of the determined chemical composition.

High Resolution Electron Microscopy of Grain Boundaries in Sintered Fe77Nd15B8 Permanent Magnets

Grain boundaries of sintered Fe77Nd15B8 permanent magnets with different heat-treatments have been examined by high resolution electron microcopy. A b.c.c. phase with the lattice constant of a=0.29 nm was found at most of the boundaries of Fe14Nd2B grains. The b.c.c. phase and Fe14Nd2B grains are joined with smooth interfaces in the samples annealed at 870 K, which have a coercivity as high as 1000 KA/m. In samples quenched from 1350 K with a coercivity of about 500 KA/m, however, thin plates of the b.c.c. phase extend from the interfaces to the inside of the Fe14Nd2B grains, and deform the lattice spacings of (001) planes of the Fe14Nd2B matrix. These thin plates are considered to act as nucleation centers of reverse magnetic domains which reduce the coercivity of the specimen.

High-resolution electron microscopy of Al−Mn−Si icosahedral and Al−Mn decagonal quasicrystals

Al‐Mn‐Si icosahedral and Al‐Mn decagonal quasicrystals have been studied by high‐resolution electron microscopy. High‐resolution micrographs of the Al‐Mn‐Si icosahedral quasicrystal taken with the incident beam along the five‐, three‐ and two‐fold symmetry axes are well interpreted on a three‐dimensional Penrose tiling model. An image of the Al‐Mn decagonal quasicrystal with the incident beam parallel to the ten‐fold symmetry axis shows a pattern of an aperiodic tiling with two types of rhombs which can be distinguished from the Penrose tiling using matching and inflation rules. The Al‐Mn decagonal quasicrystal examined coexists coherently with that of icosahedral symmetry exhibiting a definite crystallographic orientation relationship.

Dislocations in an Al–Mn–Si Icosahedral Quasicrystal Observed by High-Resolution Electron Microscopy

We report for the first time experimental evidence for the existence of dislocations in an Al-Mn-Si icosahedral quasicrystal. By many-beam electron microscopy, the Burgers vectors on the plane perpendicular to the five-fold symmetry axis have been determined as the displacement of lattice planes or bright-dot alignments around the dislocation cores. The observed results are briefly discussed in comparison with a theoretical prediction by Levine et al.