Yoshihiko Hirotsu

Magnetic properties and microstructure of nanocrystalline bcc Fe-M-B (M=Zr,Hf,Nb) alloys

Abstract Some nanocrystalline Fe-based soft magnetic alloys have recently been found that exhibit high flux density ( B s ) combined with good soft magnetic properties. Among these alloys, the highest B s with high permeability (μ e ) has been achieved for the Fe-Zr-B ternary alloys, because an amorphous phase with Fe concentrations higher than these of the other nanocrystalline alloys is obtained owing to the high glass-forming ability of Zr and the formation of a nanocrystalline bcc phase without additional nonmagnetic elements of Cu, Nb, or Ge. For example, the B s and μ e (1 kHz) are 1.7 T and 14000, respectively, for Fe 91 Zr 7 B 2 . Furthermore, we have found that a similar nanocrystalline bcc structure is obtained in Fe-M-B (M = Hf, Nb) alloys and these alloys exhibit nearly the same magnetic properties as the Fe-Zr-B alloys. The changes in B s and μ e as a function of annealing temperature for Fe 90 Zr 7 B 3 , Fe 89 Hf 7 B 4 and Fe 84 Nb 7 B 9 alloys are very similar because there is no difference in the crystallization behavior of the three alloys. The highest values of μ e at 1 kHz and B s attained 22000 and 1.63 T for Fe 90 Zr 7 B 3 , 32000 and 1.59 T for Fe 89 Hf 7 B 4 , and 22000 and 1.49 T for Fe 84 Nb 7 B 9 . The core losses at 50 Hz, 0.2–1.4 T and the frequency range 10–300 kHz for these alloys are considerably lower than those for Fe-based amorphous and Fe-3%Si alloys now in use as core materials in various transformers.

Synthesis of BN films by the plasma CVD with various solids: BH3NH3, H3BO3 and NaBH4

Abstract We have studied the formation conditions of cubic boron nitride (c-BN) formed by the dissociation of BH 3 NH 3 , H 3 BO 3 and NaBH 4 in the reative gases H 2 or NH 3 , and also by the plasma excitement of the dissociated gases using r.f. induction and tungsten filament heating. It was found in these deposition processes that the tungsten filament heating greatly contributes to the formation of c-BN as compared with the contribution of r.f. induction. The deposition ratio c-BN/amorphous BN (a-BN) in the films deposited with BH 3 NH 3 −H 2 changes with the change of the emission intensity ratio I (Hα)/ I (H 2 *) in the gas plasma, where the wavelengths of the Hα and H 2 * photoemissions are 656 nm and 603 nm respectively. In particular, when the ratio I (Hα)/ I (H 2 *) becomes maximum, the deposition rate of c-BN also becomes maximum. In the H 3 BO 3 −NH 3 and NaBH 4 −BH 3 systems, it was found that the deposition ratio c-BN/a-BN varies with the emission intensity ratio I (N 2 + )/ I (NH 3 *) and I (N 2 *)/ I (NH 3 *), where the wavelengths of the N 2 + , N 2 * and NH 3 * photoemissions are 391 nm, and 406 nm and 564 nm respectively.

Microstructure of nanocrystalline b.c.c. FeMB(MNb,Hf) soft magnetic alloys

Abstract The magnetic and structural properties of as-deposited and annealed Fe 87.9 Hf 9.8 B 2.3 films produced by a sputtering technique were examined. The nanocrystalline b.c.c. film annealed for 3.6 ks at 873 K exhibits a B s value of 1.4 T and, at 1 MHz, it has a μ e value of 1400. The structure of the annealed film was found by high resolution transmission electron microscopy (HRTEM) to consist of a mixture of a large portion of nearly spherical b.c.c. grains with sizes of 10–15 nm, and a small portion of amorphous layers less than 5 nm wide surrounding the b.c.c. grains. HRTEM and electron diffraction analysis using an electron beam with a diameter of 0.6 nm revealed that the amount of Hf in the amorphous layer was several times higher than that in the b.c.c. grain with a small amount of lattice defects in the film. Furthermore, the structure of a melt-spun Fe 84 Nb 7 B 9 ribbon annealed for 3.6 ks at 873 K (previously known to exhibit high B s values and excellent soft magnetic properties with a mostly singl b.c.c. phase) was clarified to be almost the same as that of the annealed Fe 87.9 Hf 9.8 B 2.3 film; the amounts of Nb and B in the amorphous region were found to be considerably higher than those in the b.c.c. grain for the annealed ribbon. The amorphous region containing a large amount of solute elements is considered to have a high thermal stability and high Curie temperature, to repress the b.c.c. grain growth, and to play an important role in achieving good soft magnetic properties.

High-resolution electron microscopy study of microstructural changes in magnetic FeNbB films in the course of annealing

Abstract The microstructural change in sputter-deposited Fe 79 Nb 8 B 13 films on annealing was studied by high-resolution electron microscopy (HREM) and nanobeam transmission electron analysis. Amorphous structures with atomic medium-range order (MRO) were observed in specimens annealed up to 673 K. At 773 K α-Fe nanocrystals as small as 8–10 nm start to precipitate. The α-Fe nanocrystals grow to a size of 12 nm at 923 K. At this temperature, magnetic properties with high saturation magnetization, the highest permeability, and lowest magnetorestriction and coercivity can be obtained. The lattice parameter of the α-Fe precipitates decreases owing to the decrease in Nb content in the precipitates on annealing. Compounds such as Fe 3 B appear and the soft magnetic properties degrade on annealing at 973 K.

Microstructural change in the course of decomposition of an amorphous Pd82Si18 alloy

Abstract The medium range order (MRO) and its relation to the structural decomposition on annealing in PdSi amorphous alloys have been studied by high resolution electron microscopy, nano-beam electron diffraction and nano-beam elemental analysis. Amorphous Pd 82 Si 18 thin film specimens were prepared by Ar-beam sputtering. The f.c.c. MRO domains were observed in the as-sputtered films. On annealing, observation frequency and the size of the MRO domains increased. The f.c.c. MRO domains grew to nano-precipitates of α-Pd after annealing at 573 K. At this stage, the second halo diffraction ring tends to split into two in the selected area electron diffraction patterns. The Si atoms are thought to occupy octahedral interstices in the small precipitates and diffuse out to the matrix with increasing the precipitate sizes. The Si composition in the nano-precipitates decreases in the course of growth. It is concluded that the atomic free volume is annihilated by the atomic rearrangement in the formation and development of MRO domains, and that chemical ordering is developed in the surrounding matrix by the local compositional change. The structural relaxation must be corresponding to these local atomic structural changes.

Synthesis of cubic BN from the gas phase by a new plasma chemical vapour deposition method using r.f. waves and a tungsten filament

Abstract Several types of plasma chemical vapour deposition (CVD) apparatus have been developed which enable cubic BN (c-BN) films to be formed. In the development, it was essential to introduce a high energy into the reaction space electromagnetically and thermally for plasma excitation. A recently developed plasma CVD apparatus is composed of a Knudsen-cell-type crucible in which a tungsten filament is mounted. The crucible is covered by a cap with a small pinhole for the high energy gas plasma to jet out. An r.f. coil is placed around the crucible. With this apparatus the yield of c-BN formation has been increased remarkably and a BN film of about 100% c-BN (volume fraction) was formed in the optimum range of conditions. It was found that tungsten filament heating contributes to c-BN formation by excitation of the plasma gas to create high energy ions, radicals and atoms. The atomic hydrogen created was found to etch the deposited turbostratic BN structure considerably to give a high volume fraction of c-BN in the BN films. A mechanism for c-BN formation is discussed qualitatively in relation to the atomic potentials with sp 2 and sp 3 electron orbitals.

In-plane orientations and grain boundaries of YBa2Cu3O7-x thin films on (001)MgO substrates grown by metalorganic chemical vapor deposition

Growth orientations and structures of YBa2Cu3O7-x (YBCO) thin films on (001) MgO substrates formed by metalorganic chemical vapor deposition (MOCVD) were investigated by means of X-ray diffraction, and plan-view and cross-sectional transmission electron microscopy (TEM). Less than half of the films are epitaxially grown (001)-oriented films, and the rest are films composed of epitaxial grains and in-plane misoriented grains with specific misorientation angles. Cross-sectional TEM revealed that the in-plane misoriented grains are grown on a flat MgO surface, while the epitaxially oriented grains are grown on MgO with surface atomic steps as large as 1 nm. The misoriented films are composed of large grains several µm in size with large-angle grain boundaries (LAGBs), and each grain contains several small grains less than 1 µm in size with slight mutual misorientations. A clear Josephson effect was observed from the films with the LAGBs. From the lattice imaging, it was found that the LAGBs tend to form lower Σ structures. A TEM microbeam analysis revealed an oxygen deficiency at the LAGBs.

High resolution electron microscopy of medium-range order in amorphous alloys

Abstract Under suitable defocus conditions, structures with atomic medium-range order (MRO) extending 1–2 nm in space can be observed in amorphous alloys as clear lattice images by means of high-resolution electron microscopy (HREM). Using the PdSi system, it was demonstrated that the MRO structure and morphology depend on the cooling rate and alloying composition. In PdSi alloys, the volume fraction and size of the MRO domains were found to increase as the cooling rate decreased. MRO with f.c.c.-like structure appears in a eutectic composition range, between approximately Pd 84 Si 16 and Pd 80 Si 20 . We also performed detailed HREM observations and nano-beam analysis of the local structural change on annealing in Pd 82 Si 18 alloy. It is concluded that the atomic free volume is annihilated by the atomic rearrangement for the formation and development of MRO domains.

High-resolution electron microscope observation of medium-range atomic ordering in amorphous alloy

Abstract Amorphous Pd82Si18 thin films were prepared by the argon-beam sputtering technique. The specimens were deposited on NaCl substrates with a substrate temperature of 293 K. An investigation of the medium-range order (MRO) and its relation to the structural relaxation and decomposition on annealing has been made by means of high-resolution electron microscopy, nano-beam diffraction and nano-beam elemental analysis. Domains of fcc MRO were observed in the as-sputtered film. At the initial stage of annealing, the number of observed MRO domains increases and they grow with increasing annealing temperature. This stage corresponds to that of structural relaxation. The large grown fcc MRO domains become nano-precipitates which may be called ‘nuclei of the α-Pd precipitates’. The lattice parameter of the nano-precipitates becomes closer to that of pure α-Pd with increasing size of precipitates. The Si composition in the (nano-)precipitates starts to decrease when the annealing temperature is increased to 523 K.

Electron Diffraction and High-Resolution Electron Microscopy Study on the Structure of Solid C60

The structure of solid C60 was studied by means of electron diffraction and high-resolution electron microscopy. From both the plan-view and sectional observations of platelike specimens, the solid C60 was concluded to have the fcc structure. The weak reflections, which have been viewed as those from the hcp structure, were found to be due to the effect of the Ewald sphere cutting of streaked intensity regions caused by the stacking disorder in the fcc structure.