Katsuyuki Matsunaga

Tersoff Potential Parameters for Simulating Cubic Boron Carbonitrides

We have developed Tersoff potential parameters for boron in order to simulate cubic boron carbonitride systems by molecular dynamics. Combined with parameters for C and N available from the literature, our parameters are shown to reproduce the lattice parameters and bulk moduli of boron nitride and boron carbonitride (C0.33(BN)0.67) with good accuracy. By simulating several systems of formula (Cx(BN)1-x) over a wide range of carbon contents (x=0 to 1), we observed the same trends in the deviation from ideal mixing as found experimentally. We attribute this deviation to the relatively longer C-N bonds distributed randomly throughout the intermediate C content systems.

Mechanism of incorporation of zinc into hydroxyapatite.

The atomic level mechanism of incorporation of Zn(2+) into hydroxyapatite (HAp), which is a potential dopant to promote bone formation, was investigated, based on first principles total energy calculations and experimental X-ray absorption near edge structure (XANES) analyses. It was found that Zn(2+)-doped HAp tends to have a Ca-deficient chemical composition and substitutional Zn(2+) ions are associated with a defect complex with a Ca(2+) vacancy and two charge compensating protons. Moreover, first principles calculations demonstrated that Zn(2+) incorporation into HAp can take place by occupying the Ca(2+) vacancy of the defect complex. The Ca(2+) vacancy complex is not only the origin of the calcium deficiency in HAp, but also plays a key role in the uptake of trace elements during mineralization.

Structures and energetics of Ga2O3 polymorphs

First-principles calculations are made for five Ga2O3 polymorphs. The structure of ?-Ga2O3 with the space group Pna 21 (No.?33, orthorhombic), which is sometimes called ?-Ga2O3 in the literature, is consistent with experimental reports. The structure of ?-Ga2O3 is optimized within 14 inequivalent configurations of defective spinel structures. Phonon dispersion curves of four polymorphs are obtained. The volume expansivity, bulk modulus, and specific heat at constant volume are computed as a function of temperature within the quasi-harmonic approximation. The Helmholtz free energies of the polymorphs are thus compared. The expansivity shows a relationship of ?<?<?<?, while ?<?<?<? for the bulk modulus. The formation free energies have the tendency ?<?<?<?<? at low temperatures. With the increase of temperature, the difference in free energy between the ?-phase and the ?-phase becomes smaller. Eventually the ? phase becomes more stable at above 1600?K.

Dislocation-enhanced ionic conductivity of yttria-stabilized zirconia

The ionic conductivity of yttria-stabilized zirconia (YSZ) single crystals (10 mol %Y2O3–ZrO2) was enhanced by introducing high density of dislocations. YSZ single crystals were systematically deformed by compression tests at 1300 °C, and their electrical conductivity was measured by an AC impedance method. It was confirmed that a great number of dislocations were introduced in YSZ by the high-temperature deformation, and the density reached up to more than 8×1012 m−2, depending on the compression strains imposed on the samples. The electrical conductivity of the deformed samples was found to be larger than that of undeformed samples. Furthermore, samples with larger strains exhibited higher electrical conductivity, which indicates that dislocations generated by the plastic deformation play an important role to enhance the ionic conductivity of YSZ.

First-principles study of substitutional magnesium and zinc in hydroxyapatite and octacalcium phosphate.

First-principles calculations are performed for Mg(2+) and Zn(2+) substitution in hydroxyapatite (HAp) and octacalcium phosphate (OCP), because the foreign ions are known to play an important role for bone formation. In order to study their possible location in the system of HAp in contact with the aqueous solution, OCP is considered as a structural model of the transition region between HAp and the solution. It is found that, when the foreign ions substitute for Ca sites, the surrounding oxygen ions undergo considerable inward relaxation, due to their smaller ionic sizes than Ca(2+), which results in the smaller coordination numbers with oxygen as compared with those of Ca in bulk HAp and OCP. From the calculated defect formation energies, it is likely that the substitutional foreign ions are quite difficult to dissolve into HAp whereas can be more easily incorporated in OCP. In particular, Zn(2+) can more favorably substitute for the specific Ca site of OCP, as compared to Mg(2+), which is attributed with covalent bond formation between Zn and the surrounding oxygen ions. It is thus considered that zinc may play its role to promote bone formation by being incorporated into the transition region between HAp and the surrounding solution.

Direct measurements of grain boundary sliding in yttrium-doped alumina bicrystals

The behavior of grain boundary sliding in pure and yttrium-doped Al2O3 was directly measured at a high temperature, using bicrystal experiments. For this purpose, we fabricated Al2O3 bicrystals containing a random grain boundary with or without yttrium ions. High-resolution transmission electron microscopy observations and energy dispersive x-ray spectroscopy analyses showed that bicrystals were successfully joined at an atomic scale, and doped yttrium ions segregated along the grain boundaries. It was found by compressive creep tests that the grain boundary sliding rate was restrained by two orders of magnitude due to yttrium addition, as compared to that of undoped bicrystals.

Atomic and electronic structures of Cu/α-Al2O3 interfaces prepared by pulsed-laser deposition

Abstract Interfacial atomic structures of Cu/Al2O3(0001) and Cu/Al2O3 systems prepared by a pulsed-laser deposition technique have been characterized by using high-resolution transmission electron microscopy (HRTEM). It was found that Cu metals were epitaxially oriented to the surface of Al2O3 substrates, and the following orientation relationships (ORs) were found to be formed: (111)Cu⁄⁄(0001)Al2O3, cu⁄⁄Al2O3 in the Cu/Al2O3(0001) interface and (001)Cu⁄⁄Al2O3, Cu⁄⁄[0001]Al2O3 in the Cu/Al2O3 interface. Geometrical coherency of the Cu/Al2O3 system has been evaluated by the coincidence of reciprocal lattice points method, and the result showed that the most coherent ORs were (111)Cu⁄⁄(0001)Al2O3, Cu⁄⁄[11Ī00]Al2O3 and Cu⁄⁄Al2O3, [111]Cu⁄⁄[0001]Al2O3, which are equivalent to each other. These ORs were not consistent with the experimentally observed ORs, and it was possible that crucial factors to determine the ORs between Cu and Al2O3 were not only geometrical coherency, but also other factors such as chemical bonding states. Therefore, to understand the nature of the interface atomic structures, the electronic structures of the Cu/Al2O3 interfaces have been investigated by electron energy-loss spectroscopy. It was found that the pre-edge at the lower energy part of the main peak appeared in the O-K edge spectra at the interface region in both the Cu/Al2O3(0001) and Cu/Al2O3 systems. This indicates the existence of Cu—O interactions at the interface. In fact, HRTEM simulation images based on O-terminated interface models agreed well with the experimental images, indicating that O-terminated interfaces were formed in both systems. Since the overlapped Cu atomic density in the experimental ORs were larger than that in the most coherent OR, it is considered that the on-top Cu—O bonds stabilize the O-terminated Cu/Al2O3 interfaces.

Direct Imaging of Pt Single Atoms Adsorbed on TiO2 (110) Surfaces

Noble metal nanoparticles (e.g., gold and platinum) supported on TiO2 surfaces are utilized in many technological applications such as heterogeneous catalysts. To fully understand their enhanced catalytic activity, it is essential to unravel the interfacial interaction between the metal atoms and TiO2 surfaces at the level of atomic dimensions. However, it has been extremely difficult to directly characterize the atomic-scale structures that result when individual metal atoms are adsorbed on the TiO2 surfaces. Here, we show direct atomic-resolution images of individual Pt atoms adsorbed on TiO2 (110) surfaces using aberration-corrected scanning transmission electron microscopy. Subangstrom spatial resolution enables us to identify five different Pt atom adsorption sites on the TiO2 (110) surface. Combining this with systematic density functional theory calculations reveals that the most favorable Pt adsorption sites are on vacancy sites of basal oxygen atoms that are located in subsurface positions relative to the top surface bridging oxygen atoms.

Strontium Substitution in Bioactive Calcium Phosphates: A First-Principles Study

First-principles calculations are performed to investigate atomic and electronic structures of Sr(2+) ions substituting for Ca(2+) in octacalcium phosphate (OCP). The defect formation energies are evaluated from total energies of supercells and ionic chemical potentials of Sr(2+) and Ca(2+) determined under the chemical equilibrium with aqueous solution saturated with hydroxyapatite (HAp). The defect formation energy depends on the solution pH and the substitutional Ca sites in OCP, and the estimated equilibrium concentrations of Sr(2+) in OCP and HAp are in reasonable agreement with previous experimental results obtained in physiological conditions. It is also found that Sr(2+) ions can be more favorably substituted in OCP than in HAp. It is thought, therefore, that Sr(2+) plays its role to promote bone formation by being incorporated into the metastable OCP phase occurring during HAp nucleation.

Sr vacancy segregation by heat treatment at SrTiO3 grain boundary

Electron energy loss near-edge structures (ELNES) and first-principles band structure calculations were combined in order to identify native defects which are segregated by heat treatments at the vicinity of SrTiO3 grain boundary (GB). Spectral differences between the bulk and the vicinity of GB mainly appear around the second peak of OK ELNES. The spectral differences can be reproduced by calculating the theoretical OK ELNES from the first-nearest-neighbor oxygen to a Sr vacancy. It is therefore concluded that the Sr vacancies are concentrated in the vicinity of GB by the heat treatment.