Imaging of surface atoms with simultaneous measurement of noncontact atomic force microscopy (NC-AFM) and scanning tunneling microscopy (STM) is performed. Si cantilever coated with PtIr provides stable AFM/STM operations to obtain high spatial resolution images. AFM/STM measurements on Si(111)-(7×7), Ge(111)-c(2×8), and TiO₂(110) surfaces are presented at room temperature. On the Ge(111)-c(2×8) surface, adatom and restatom sites which have the same atomic patterns are determined from dual bias AFM/STM measurements.

View full abstract

We investigated the microstructure of a hydrogen-storage (La_0.8Mg_0.2)Ni_3.5 alloy with block-stacking superstructures by electron diffraction and Z-contrast scanning transmission electron microscopy (STEM), particularly focusing on the type of stacking fault structures and possible occurrence of novel structural variants. It was found that two major phases coexist in the alloy, which are of 5:19-2H type and 5:19-3R type superstructures that are constructed by the common structural blocks but with different stacking sequences. A high density of stacking faults were often observed, most of which were of inter-block-layer type that does not change the intra-block structure but simply alters local stacking sequence of the blocks. As a minor phase in the alloy, we identified a novel polytype structure represented as 5:19-12R, whose long-period block-stacking sequence is described as ABCA′CABC′BCAB′.

View full abstract

An icosahedral quasicrystal (i-phase) and its 1/1 crystal approximant (1/1-phase) have been found to form in Al-Pd-Sc system. The formation areas of the i-phase and the 1/1-phase are limited to very small areas around the compositions of Al₅₄Pd₃₀Sc₁₆ and Al₅₆Pd₂₉Sc₁₅, respectively. The valence electrons per atom ratio (e/a ratio) is calculated to be 2.10 for the i-phase of Al₅₄Pd₃₀Sc₁₆. The 16 at% Sc concentration and e⁄a=2.10, as well as the intensity profile of the measured X-ray diffraction spectrum, indicate that this i-phase can be classified into the Tsai-type: the Tsai-type i-phases found so far commonly contain an element having relatively large atomic size with 15–16 at% and have e/a within the range 2.0–2.15. The formation of the i-phase has also been found in a series of quaternary Al-Pd-Cu-Sc alloys with different compositions keeping 16 at%Sc and e⁄a=2.10. This supports the fact that the two conditions are crucially important in the formation of this type of i-phase.

View full abstract

We investigated the local atomic and electronic structure around Ce in CeF₃ and Ce₂O₃ by the combination analysis of F- and O-K shell electron energy loss near edge structures (ELNES) and first-principles calculations. The energy width of the main edge structure depended on the interaction between Ce5d orbitals and the neighboring F/O atoms. Not only ELNES but also the reported emission and excitation spectra were qualitatively consistent with the electronic structures of density functional theory (DFT) calculations with Hubbard U for the Ce4f energy correction. Main factors determining the emission wavelength of the fluoride and Ce-doped oxides were discussed.

View full abstract

Microstructures of Sn-doped sintered indium oxide were investigated by transmission electron microscopy. It was found that Sn-rich nanosized precipitates were formed inside the ITO grains, in addition to the secondary phases of In₄Sn₃O₁₂ formed at grain boundaries. By nano-beam electron diffraction analysis, the crystal structure of the nanosized precipitates was determined to be the fluorite structure, which is different from the bixbyite structure of the ITO matrix. The structural change in the Sn-rich nanosized precipitates can be explained by the insertion of O²⁻ ions in the vacancy sites of the bixbyite structure during the substitution of In³⁺ sites by Sn⁴⁺.

View full abstract

The data cube of spectrum imaging (SI) by scanning TEM (STEM) and electron energy-loss spectroscopy (EELS) or energy-filtering TEM (EF-TEM) can be treated as the two-dimensional data array, each row corresponding to the EELS spectrum at a specific position. A multivariate curve resolution (MCR) technique can then apply to the dataset, which decomposes the set of spectra into the product of the constituent pure spectral components and their corresponding relative composition matrices without any reference spectra. This method allows us to provide a two dimensional spatial distribution map of different chemical states incorporated even when the multiply spectra overlap with one another. Several application examples are presented.

View full abstract

Phase-field method has recently been extended and utilized across many fields of materials science. Since this method can systematically incorporate, the effect of coherency strain induced by lattice mismatch and applied stress as well as external electrical and magnetic fields, it has been applied to many material processes including solidification, solid-state phase transformations, and various types of complex microstructure changes.
In this article, we focus on the ferroelectric domain microstructure changes followed by the structural phase transition from cubic to tetragonal phase in BaTiO₃, and its morphological developments are simulated on the basis of the phase-field method. The circuit structure of polarization moments of ferroelectric domains including twin defects is simulated, and the domain morphology is controlled by both the electric dipole-dipole interaction among polarization moments and the elastic interaction among domains with different tetragonal distortion. The ferroelectric domain exchange induced by external electric field is also simulated, then the dielectric property, i.e., the polarization hysteresis curve, is calculated by integrating all the x components of polarization moment vector over the microstructure. Furthermore, the simulation of the reversible ferroelectric domain switching, which is a new phenomenon recently discovered by Ren, is also simulated as an advanced application of the present simulation model.

View full abstract

The structural relaxations and the formation energies of intrinsic defects in cubic and orthorhombic CaTiO₃ were investigated by a first principles projector-augmented wave method. It was found that cations and oxygen vacancies in both phases cause extra levels near the valence band maximum and the conduction band minimum, respectively, and the Ti-vacancy induced level in orthorhombic CaTiO₃ is closer to the valence band maximum than that in cubic CaTiO₃. Among the neutral defect species, including neutral isolated vacancy, partial Schottky, and full Schottky, it was found that the V_Ca²⁻+V_O²⁺ and V_O⁰ are the most preferable defect species for orthorhombic CaTiO₃ under reduction and oxidization conditions, respectively, whereas the V_Ca²⁻+V_O²⁺ partial Schottky is always stable in any atmosphere in cubic CaTiO₃. As compared to cubic CaTiO₃, it was found that orthorhombic CaTiO₃ shows higher defect formation energies.

View full abstract

A thermodynamic analysis of the Nb–Ti–B ternary system was carried out by estimating the unknown thermodynamic properties of binary and ternary borides by using a first-principles method. The calculated value for the binary TiB₂ phase was in reasonable agreement with those found by experiment. Due to a lack of experimental values concerning thermodynamic properties of the ternary system, the formation enthalpies for the Nb₃B₂–Ti₃B₂, NbB–TiB (Cmcm), NbB–TiB (Pnma), Nb₅B₆–Ti₅B₆, Nb₃B₄–Ti₃B₄, and NbB₂–TiB₂ pseudo-binary sections were also determined by constructing various kinds of superstructures for binary borides. The thermodynamic functions determined using these theoretical values, as well as the available experimental information on the phase fields, successfully revealed the phase equilibria in the Nb–Ti–B ternary system across the entire composition and all temperature ranges.

View full abstract

Metamaterial with negative permittivity and permeability is studied by means of computer simulations. We analyze the electromagnetic response of nanostructured metamaterials to evanescent waves at optical frequency via the finite-difference time-domain simulatioins on parallel computer. Effects of the nanostructure on dielectric and magnetic properties are taken into account by introducing the Drude-Lorentz model in the materials dispersion. Size effect on the dispersion is examined by comparing the model with that of a noble metal particle. A re-focusing and an amplification of the evanescent waves propagating through a metamaterial, consisting of metal slabs/vacuum stacking, are demonstrated for the frequency range at 614–744 THz. By properly treating the materials dispersion, we show that the nanostructured metamaterial may behave as a left-handed material in the optical range.

View full abstract

Phonon states of Ag doped potassium halides, KX:Ag⁺ (X=Cl, Br and I), are computed by a first principles lattice dynamic method using 64-atoms supercells. Results are compared to experimental data in literature. Phonon density of states of host KCl and KI crystals satisfactorily agree to the experimental inelastic neutron scattering data. Experimental frequencies of the impurity-induced infra-red (IR) and Raman active modes in the low frequency region are reasonably well reproduced because the vibrations are localized within the first nearest neighbour anions of the Ag⁺-ion. On the other hand, limitations of present calculations to reproduce the high frequency impurity-induced modes are pointed out. They are less localized to the Ag⁺-ion.

View full abstract

Negative refraction of acoustic waves in a two-dimensional (2D) phononic crystal is studied by numerical simulation based on the finite-difference time-domain (FDTD) method. We calculate the phonon band structure of 2D phononic crystals, consisting of metal cylinders placed periodically in a liquid. By comparing several combinations of materials for metal cylinder and liquid, we analyze the dependence of the band structures on sound speed and density of liquid media. The negative refraction of the acoustic waves is observed at the interfaces between the phononic crystal slab and the liquid. We find that an acoustic “lens effect” with the slab appears due to the negative refractions. The relationship between the focal intensity in the lens effect and the band structure of the phononic crystal is quantitatively discussed.

View full abstract

Structure and configuration of boundary dislocations on various low angle tilt grain boundaries in alumina were considered based on the ideas that the boundary is composed of regularly arrayed edge dislocations and that the dislocations could dissociate into partial dislocations with maintaining the hcp-like oxygen sublattice. Moreover, the separation distance between the partial dislocations formed by the dissociation was evaluated by the calculations based on an elastic theory. The calculations indicated that the width of the stacking fault region between partial dislocations decreases with increasing tilt angles. As a consequence, the hypothesis and calculations used here would enable us to predict the structures of various low angle boundaries with dissociated boundary dislocations.

View full abstract

We perform molecular-dynamics simulations to investigate the atomic and electronic structures of a basal edge dislocation in α-Al₂O₃. The core structure consisting of two non-stoichiometric partial dislocations, which has been recently proposed by an experiment, is examined by an empirical interatomic-potential model and by a hybrid quantum/classical approach. The atomic rearrangements in the full and in the partial dislocation cores are analyzed. The local electronic structure in the full dislocation core is evaluated by the density-functional method applied for a quantum-cluster region in the hybrid simulations. Interaction potentials between partial dislocations are investigated by the classical model. Results preliminarily obtained show that the partials aligned normal to a basal plane ({0001}) has a short-ranged repulsive nature approximately within 8 Å.

View full abstract

Defect energetics in Σ13 pyramidal twin grain boundary (GB) of Al₂O₃ was investigated by a first principles projector-augmented wave method. It was found that the vacancy formation energy depends on the atomic site and the defect energetics at the GB is similar to that in the bulk Al₂O₃, namely the oxygen vacancy shows much higher formation energy than the aluminum vacancy and the Schottky defect is the most preferable species in a wide range of atmospheres. By analyzing the atomic structures of the GB in detail, it was found that the defect energetics at the GB is closely related to the structural distortions, such as strains and dangling-bonds in the vicinity of the GB.

View full abstract

The oxygen permeability of an undoped polycrystalline α-Al₂O₃ wafer that was exposed to oxygen potential gradients was evaluated at temperatures up to 1973 K. Oxygen preferentially permeated through the grain boundaries of α-Al₂O₃. The main diffusion species, which were attributed to oxygen permeation, depended on oxygen partial pressures (P_O2), forming oxygen potential gradients. Under oxygen potential gradients generated by P_O2 below about 1 Pa, oxygen permeation occurred by oxygen diffusing from regions of higher P_O2 to regions of lower P_O2. By contrast, under oxygen potential gradients generated by P_O2 above about 1 Pa, oxygen permeation proceeded by aluminum diffusing from regions of lower P_O2 to regions of higher P_O2. In other words, O₂ molecules were adsorbed onto a surface at higher P_O2 and subsequently dissociated into oxygen ions (forming Al₂O₃), while oxygen ions on the opposite surface at lower P_O2 were desorbed by association into O₂ molecules (decomposition of Al₂O₃). The grain-boundary diffusion coefficients of oxygen and aluminum as a function of P_O2 were determined from the oxygen permeation constants.

View full abstract

The densification behavior during sintering of 0.1 mol% TiO₂-doped Al₂O₃ was measured in either an air or N₂+5%H₂ gas atmosphere at the sintering temperature of 1573–1673 K. The grain boundary diffusivity was evaluated from the densification rate. High-resolution transmission electron microscopy (HRTEM) and nano-probe energy-dispersive X-ray spectroscopy (EDS) analyses revealed that the doped Ti cations segregate in the vicinity of the grain boundaries in the Al₂O₃. An electron energy loss spectroscopy (EELS) investigation indicated that the valence state of Ti in the Al₂O₃ sintered in the reducing atmosphere was close to +3. The grain boundary diffusivity in undoped Al₂O₃ was insensitive to the atmosphere, but was enhanced by the grain boundary segregation of Ti⁴⁺. The grain boundary diffusivity of alumina in the reducing atmosphere was, however, retarded by the Ti³⁺-doping. The retarded diffusivity by Ti³⁺-doping must be related to the lack of aluminum vacancies and the large ionicity of Ti-O compared to Al-O.

View full abstract

In this paper, we characterized aluminum/sapphire interface structure by using high-resolution transmission electron microscopy. It was found that step structures of sapphire formed at the aluminum/sapphire interfaces during the liquid-phase bonding. It was discovered that the addition of silicon in aluminum significantly reduces the step growth at the interface. Silicon was found to segregate and precipitate at the interface. These results suggest that the strong preference of silicon at the interface may inhibit the step growth reactions during liquid-phase bonding.

View full abstract

First-principles calculations are performed to investigate atomic and electronic structures of Na⁺ and K⁺ ions substituting for Ca²⁺ in hydroxyapatite (HAp). Formation energies of the substitutional defects are obtained from total energies of defective HAp supercells and chemical potentials determined by assuming chemical equilibrium between HAp and HAp-saturated aqueous solution containing Na⁺ or K⁺. It is found that substitutional Na⁺ with a charge-compensating interstitial proton is more stably formed, as compared to substitutional K⁺. This may be related to the fact that Na⁺ is generally more abundantly involved in bones and tooth enamels than K⁺.

View full abstract

Silicon was doped to hydroxyapatite by hydrothermal techniques for higher biocompatibility. Products contained tetra-ethyl-orthosilicate (TEOS) as a silicon source in the range of 0 to 15 mass%. In order to evaluate bioactivity of silicon-doped hydroxyapatite, the samples were soaked in simulated body fluid (SBF). Silicon doped samples showed faster apatite forming ability than the undoped samples. The samples were examined by transmission electron microscopy (TEM), X-ray diffraction patterns (XRD), Fourier transform infrared spectroscopy (FTIR) and X-ray absorption fine structure (XAFS). There results indicated that SiO₄⁴⁻ ion substituted PO₄³⁻ ion site in apatite structures. And it was found that appropriate TEOS doping ratio was 10 mass% for superior biocompatibility due to amorphous SiO₂ segregation in the 15 mass% TEOS doped samples.

View full abstract

Zeolites possess excellent ion exchange capabilities, so that it is easy to synthesize Ag-zeolite (Ag_xAl_xSi_1−xO₂) using an AgNO₃ aqueous solution. When this Ag type zeolite is irradiated with a high energy electron beam of several hundred keV in a TEM, the chemical bonds in the zeolite crystal are broken and the crystal eventually becomes amorphous. (L. A. Bursill, E. A. Lodge and J. M. Thomas: Nature 286 (1980) 111–113.) We have found that ordered clusters of Ag atoms are formed in the amorphous region during this process. It is anticipated that quantum effects will appear in this material. (Y. Nozue, T. Kodaira and T. Goto: Phys. Rev. Lett. 68 (1992) 3789.)

View full abstract

Silica-containing TiO₂-derived titanate nanotubes were prepared by the addition of a small amount of tetraethyl orthosilicate (TEOS) to TiO₂-derived titanate nanotubes prepared by the hydrothermal process and a subsequent heat-treatment at 473 K in air. The microstructure and thermal behavior of synthesized silica containing TiO₂-derived titanate nanotubes were investigated by various methods such as X-ray diffraction (XRD), X-ray absorption fine structure (XAF), and X-ray photoelectron spectroscopy (XPS). As a result, the addition of a small amount of TEOS leaded to the improvement of the thermal stability for TiO₂-derived titanate nanotubes. XPS results revealed that Si was combined onto the surface of TiO₂-derived titanate nanotubes, forming partial Si-O-Ti chemical bonds. Therefore, it was inferred that the thermal stability could be modified by forming partial Si-O-Ti chemical bonds at interface of silica and TiO₂-derived titanate nanotubes.

View full abstract

As part of an effort to develop low-voltage surge filters, zinc oxide (ZnO) bicrystals were investigated to clarify the electronic state of grain boundaries in ZnO varistors. The varistors had a ZnO/glass/ZnO-type sandwich structure, and the interfacial glass layer was composed of oxide glass of the Bi-B-O system. A highly nonlinear current-voltage relationship was obtained for a sandwich structure composed of Co-doped single crystal ZnO, and the current through the junction was proportional to roughly the 30th power of the bias voltage at the breakdown stage. In contrast, the junction using undoped single crystal ZnO showed insignificant nonlinearity. Dielectric measurements revealed that a double-sided depletion layer was formed at the interface and the high nonlinearity was ascribed to the corresponding potential barrier formed at the interface.

View full abstract

Solid solutions of Zn_1−xCd_xO and Zn_1−xMg_xO (0≤x≤1) with wurtzite structures are systematically investigated by first principles calculations with special interests on the dependence of energetics and electronic structures on the alloy structure. Alloying ZnO with CdO shows a linear increase in the lattice parameters when we choose energetically favorable alloy structures. On the other hand, they do not show such a linear trend when alloyed with MgO. Formation energy with reference to the end-member oxides of wurtzite structures is positive for Zn_1−xCd_xO alloys, whereas it is slightly negative for Zn_1−xMg_xO alloy. They are consistent with the experimental fact that highly concentrated solid solutions are easier to be formed in Zn_1−xMg_xO alloys. The band gap shows a monotonous decrease in Zn_1−xCd_xO and increase in Zn_1−xMg_xO with the solute concentration. In Zn_1−xCd_xO, Cd has an orbital component as large as that of Zn at the bottom of the conduction band. On the other hand, the contribution of Mg is much smaller in Zn_1−xMg_xO. They are consistent with the changes in the band gap with the composition.

View full abstract

Growth and microstructure of ternary Ti₃SiC₂ compound layers on 4H-SiC, which play am important role in formation of TiAl-based ohmic contacts to p-type SiC, were investigated in this study. The Ti₃SiC₂ layer was fabricated by deposition of Ti/Al contacts (where a slash “/” indicates the deposition sequence) on the 4H-SiC(0001) substrate and subsequent rapid thermal anneal at 1000°C in ultra high vacuum. After annealing, reaction products and microstructure of the Ti₃SiC₂ layer were investigated by X ray diffraction analysis and transmission electron microscopy observations in order to understand the growth processes of the Ti₃SiC₂ layer and determination of the Ti₃SiC₂/4H-SiC interface structure. The Ti₃SiC₂ layers with hexagonal plate shape were observed to grow epitaxially on the SiC(0001) surface by anisotropic lateral growth process. The interface was found to have a hetero-epitaxial orientation relationship of (0001)_TSC||(0001)_S and [0\\bar110]_TSC||[0\\bar110]_S where TSC and S represent Ti₃SiC₂ and 4H-SiC, respectively, and have well-defined ledge-terrace structures with low density of misfit dislocations due to an extremely low lattice mismatch of 0.4% between Ti₃SiC₂ and 4H-SiC.

View full abstract

Spinel-type 0.4Fe₃O₄·0.6Fe₂TiO₄ (molar ratio) solid solution thin films have been deposited on c-sapphire substrates by a pulsed laser deposition technique. A single phase of (111)-oriented solid solution can be obtained by adjusting the oxygen partial pressure and substrate temperature. The epitaxial solid solution thin film exhibits ferrimagnetism with Curie temperature above room temperature and is a semiconductor with n-type conduction carriers. Anomalous Hall effect is observed at room temperature for the solid solution thin films, implying the presence of spin-polarized charge carriers.

View full abstract

Epitaxial La_0.6Sr_0.4MnO₃ (LSMO) thin films were grown by pulsed laser deposition. Relationships between magnetic properties of LSMO epitaxial thin films and ablation conditions such as ablated spot area and total incident laser energy in a pulsed laser deposition technique were studied. Ablated spot area was controlled by changing the focus lens position and total laser energy. Epitaxial growth mode and magnetic properties of LSMO thin films were strongly dependent on the ablated spot area and total laser energy. Under the optimal laser ablation condition, epitaxial LSMO films with Curie temperature of about 340 K and atomically flat surfaces were obtained.

View full abstract

In order to examine the effect of threading dislocations on the structure of InGaN/GaN multiple quantum wells (MQWs) grown by metal-organic vapor-phase epitaxy (MOVPE), epitaxial lateral overgrowth (ELO) on a patterned sapphire substrate was employed and the MQWs were characterized as a function of the lateral position in terms of cathode luminescence (CL), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). The intensity of a blue luminescence peak (426 nm) was larger for the epitaxial layer overriding on the masks than that for the layer on the unmasked area, while the peak wavelength was independent of the position. Threading dislocations, which were generated at the GaN/sapphire interface, did not propagate to the surface of GaN layer on the masks. The thicknesses of both the InGaN well and the GaN barrier, on the other hand, were the same for the MQWs both on the unmasked surface and on the masks, which is consistent with the invariable peak wavelength of the blue luminescence. For an InGaN well with the indium content of around 10%, it seems that the existence of threading dislocations does not affect the structure of the MQWs but just reduces the luminescence intensity through a recombination via mid-gap states.

View full abstract

The effect of small amount of insoluble dopant on tetragonal to monoclinic (t-m) phase transformation of 3 mol%Y₂O₃ stabilized tetragonal zirconia polycrystal (3Y-TZP) is examined by ageing in hot water. The materials used are 3Y-TZP and 0.1 mol%SiO₂-doped 3Y-TZP with grain size of 0.55 μm. The t-m phase transformation of 3Y-TZP is retarded by 0.1 mol%SiO₂ doping. Since the doped silicon ion segregates along the grain boundaries, the change in phase transformation behavior must be originated from the change in grain boundary diffusivity of hydroxyl ion. Analysis of the transformation kinetics by the Mehl-Avrami-Johnson equation reveals that the activation energy does not change in the two materials but the pre-exponential term significantly changes. Grain boundary diffusion of hydroxyl ion must be blocked by the presence of silicon ion which reduces the effective area of grain boundary diffusion.

View full abstract

Microstructures in ZrC-doped WC-Co alloys, which were prepared by liquid-phase and solid-phase sintering, were investigated mainly by high-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray spectrometry (EDS). HRTEM study has revealed that multi facet structures, which are often observed in VC-doped alloys, were not formed in ZrC-doped alloys. In addition, the segregation of Zr at WC/Co interfaces could not be detected by EDS analysis even with nano size electron probe. Doped ZrC was found to exist as other carbide grains without dissolving into γ-phase, i.e., Co phase by EDS-mapping performed for liquid-phase sintered alloys. In the case of liquid-phase sintering, the inhibition effect for the grain growth caused by ZrC-doping was confirmed to be very small as previously reported. In contrast, it was found that ZrC-doping largely retards WC grain growth in the case of solid-phase sintering. The retardation of WC grain growth observed in solid-phase sintered alloys was considered to be closely related to a low sinterbility due to a low wettability between Co and ZrC.

View full abstract

Various pore sizes of anodic aluminum oxide (AAO) were fabricated using a two-step anodization process. AAO pores with diameters ranging from 5 to 500 nm were formed in diluted electrolyte solutions of sulfuric acid, oxalic acid, and phosphoric acid. Irregularly shaped tin particles in nano sizes were formed on the AAO using a thermo-vaporizing process. Tin spheres with diameters ranging from 3 to 500 nm were then obtained by reflowing vaporized AAO in silicate oil. A transmission electron microscope (TEM JEOL 2000) was employed to characterize the crystalline structure of a tin sphere with 18 nm diameter.

View full abstract

The effects of dynamic electropulsing on phase transformations of the ZA22 alloy were studied during tensile deformation, by using scanning electron microscopy, X-ray diffraction and transmission electron microscopy. It was found that electopulsing accelerated tremendously phase transformation in two stages, sequentially: (a) quenching from supersaturated state approaching the final stable state, and (b) up-quenching from the final stable state to a higher temperature state. The relationship between plasticity and the dynamic electropulsing is discussed from the point of view of Gibbs free energy, phase transformation and microstructual changes.

View full abstract

A self-consistent thermodynamic model of the Mg-Mn, Al-Mn and Mg-Al-Mn systems has been developed. The major difference between this work and the already existing assessments of these systems is the application of the modified quasichemical model for the liquid phase in each system while most of the existing descriptions use the random mixing model. In the absence of key data for the Mg-Mn system, the calculated thermodynamic properties from the model have been found comparable to other similar systems and the estimated critical temperature of the Mg-Mn liquid miscibility gap using the available empirical equation has been found to be in acceptable agreement with the calculated value. A comparison between the current work and the most recent work on the Al-Mn system that uses the same model for the liquid phase reveals that better agreement with the experimental data with less number of model parameters has been achieved in the current work. Kohler symmetric extrapolation model with only one ternary interaction parameter has been used to calculate the ternary Mg-Al-Mn system. The thermodynamic description of the Mg-Al-Mn system has been verified by extensive comparison with the available experimental data from numerous independent experiments. The model can satisfactorily reproduce all the invariant points and the key phase diagram and thermodynamic features of the ternary as well as the constituent binary systems.

View full abstract

Samples of Ni₃Al intermetallic compound were subjected to deformation by high-pressure torsion (HPT). The plastically-deformed structure revealed a bimodal character: coarse grains, retaining a degree of long-range order, surrounded by regions of nanocrystalline, disordered grains. It was inferred that the grain refinement proceeds in an inhomogeneous manner throughout the sample. Grains as large as 100 nm in size were shown to contain only a low density of perfect dislocations, but a large density of nanotwins and stacking faults. These planar defects appeared to originate from the grain boundaries, suggesting that grain boundaries are active sources for Shockley-partial dislocations. Their formation is accompanied by a deviation of the microhardness dependence on grain size from the Hall-Petch behavior, potentially suggesting the activation of a deformation mechanism different from the one acting in coarse structures. The hardness saturates at a significantly larger grain size than in the case of nanostructured pure Ni.

View full abstract

The reduced pressure frozen mold casting process has been known as a recycling-based casting method with several advantages, such as improvement of the work environment, reduction of industrial waste and significant improvement of product yield. In this method, only water and silica sand were used to make mold, which was rapidly frozen at −40°C, then molten metal was poured into it. In the present investigation, samples were made by the reduced pressure frozen mold casting process and previous processes, and comparisons of their mechanical properties, especially the fatigue strength, were reported.
As a result, it was clarified that cast iron made by the reduced pressure frozen mold casting process has a sufficient strength; therefore the reduced pressure frozen mold casting process was expected to be applicable to other castings that have made by previous casting processes.

View full abstract

Wettability of low silver (Ag) content Sn-xAg-0.7Cu-0.5In lead-free solder (x=0.1,0.3,0.5) on copper substrate was investigated. Wetting balance test results indicated that varying small Ag content in the alloy affects the wettability of solders with higher Ag content showing better wettability. The contact angles of the solder alloy showed dependence on Ag content in solder and temperature. Solder alloy with no Ag content and 0.5 mass% Ag content displayed similar contact angle and qualitatively similar surface tension; however, the maximum wetting force is significantly difference, which indicates difference in the density of solder alloy. Effects of different fluxes on wettability were also studied.

View full abstract

The behavior of ethanol oxidation on electro-deposited Ni-Pt electrodes was investigated using cyclic voltammetry and polarization curves in 0.4 M KOH solutions. The ethanol concentration was in the ranges of 0∼0.25 M and 0∼1000 ppm. The rate constants of Ni(OH)₂/NiOOH and the ethanol oxidizing reaction on the Ni-Pt electrodes were calculated and the results were compared with those for Ni electrodes. The anodic current of Ni-Pt electrodes increased with the concentration of ethanol in the forward sweep in both ranges of ethanol concentration. The anodic and cathodic currents on Ni-Pt electrodes increased with scan rate at 0.1 M and 200 ppm concentrations of ethanol. No intersection of anodic current was observed on Ni-Pt electrodes. The kinetic parameters of Ni(OH)₂/NiOOH and ethanol oxidation on Ni-Pt electrodes were determined by comparing the experimental results with kinetic equations. The anodic transfer coefficient of the Ni(OH)₂/NiOOH reaction on Ni-Pt electrodes was larger than that on Ni electrodes while the rate constant, k_c1, of Ni-Pt electrodes was smaller than that of Ni electrodes.

View full abstract

The distribution ratio of Se between Ag-Pb alloy and PbO phases was investigated at 1273 K. A chemical equilibrium technique was used for the measurement. The oxygen partial pressure was in equilibrium with both the phases, and it was measured by an EMF method. The distribution ratio, defined as the mole fraction of Se in PbO to the mole fraction of Se in metal, was plotted against the oxygen partial pressure. The distribution ratio decreased with an increase in the oxygen partial pressure. The slope of the plot indicates that Se dissolves in the PbO phase as oxide, which is unreasonable. The activity coefficient of Se in the Ag-Pb alloy was also measured, and it was found to decrease with an increase in the concentration of Ag. Se dissolved in the PbO phase in the non-oxide form. The activity coefficient of Se in Ag was estimated as 0.0009 at 1273 K.

View full abstract

The solubility of carbon in liquid silicon equilibrated with silicon carbide has been studied in the temperature region 1414–1559°C. High purity silicon was melted in graphite crucibles under Ar atmosphere with various boron additions. The equilibrium was observed to be established within minutes, after which no evolution with time could be observed. The addition of boron to the system was found to increase the carbon solubility, and an equation was derived describing the solubility as a function of both temperature and boron content. The solubility of carbon in pure liquid silicon was determined to be 65 ppm mass at the melting point of silicon. Expressions for the dissolution energy of carbon and the B-C interaction coefficient were also derived.

View full abstract

Molecular dynamics (MD) simulation is employed to simulate the imbibition of a designed nanopore by a fluid. The fluid is considered as a simple Lennard-Jones (LJ) fluid. For this system (i.e., LJ fluid and nanopore), the length of imbibition as a function of time for various interactions between the fluid and the pore wall is recorded. In almost all cases, the kinetics of imbibition is successfully described with the Lucas-Washburn (LW) equation. However, the deviation from the LW equation is observed in some cases. This nonconformity is contributed to the neglecting of the dynamic contact angle (DCA) in the LW equation. A hydrodynamic model (i.e., the Cox equation) is taken into consideration to calculate the DCA. It is demonstrated that the LW equation together with the Cox equation is able to justify the simulation results for those cases, which are not in good agreement with the simple LW equation. Further investigation on the MD simulation data reveals that the Cox equation is only appropriate to derive the DCA at small capillary numbers. This finding is in consonance with the theoretical background of this equation as well as experimental work.

View full abstract

The solidification microstructure, the thermal property, and the hardness were investigated on AZ91D magnesium alloy and on AZ91D magnesium alloy with 20 mass% gadolinium addition. AZ91D and AZ91D + 20 mass% Gd were solidified by the furnace cooling technique starting from 700°C in an Ar flow atmosphere. The microstructure of AZ91D was composed of main αMg grains and web-like grain boundary phases of eutectic αMg + Mg₁₇Al₁₂, while that of AZ91D + 20 mass% Gd changes into an αMg matrix with dispersed Al₂Gd particles. SEM-EDS analyses showed that the Al content in an αMg matrix of this alloy was very low compared to AZ91D, because Al is consumed in the Al₂Gd particles. Differential thermal analysis and quenching experiments were performed in order to clarify this microstructure formation. The thermal conductivity of this alloy, as measured by the laser-flash method, was 129.2 W/mK at room temperature. This alloy exhibited a substantial variance from that of AZ91D at 45.1 W/mK. A higher Vickers hardness H_V=96.6 was yielded compared to AZ91D at H_V=63.7. These properties were well correlated with the results of microstructure and quantitative analysis.

View full abstract

Low carbon steel S15CK was nitrided by active screen plasma nitriding (ASPN) using various stainless steel cages to investigate the effect of the cage on the nitriding properties. Three types of austenitic stainless steel cages, such as pipe, foil, and wire mesh, were used. The sample was treated for 18 ks at 773 K under 630 Pa in 50% N₂ + 50% H₂ gases. The nitrided samples were characterized by surface roughness tests, optical microscopy, scanning electron microscopy, X-ray diffraction, and microhardness testing. In all samples nitrided by the ASPN process, the ‘edging effect’ was completely eliminated whereas hardness and thickness of the surface layer were comparable with those obtained from the DC plasma nitriding. Moreover, a comparison of the screens used in the ASPN process revealed that the screen hole size had a slight influence on surface properties such as microstructure and hardness.

View full abstract

Microwave (MW) heating is applied to fabrication of composite materials using a reaction between soda-lime (S-L) glass and liquid Al. Because of insufficient heating capability of Al powder in the present conditions of MW power, Fe powder was mixed for the better heating to melt Al. In fabrication of the composites, mixture of S-L glass beads and Al-Fe powder was MW-heated at 700°C for 10 min.
It was observed that not only the liquid Al reacted with S-L glass, but also melting of S-L glass beads and their connection occurred. It was confirmed that Fe particles were not reacted with the glasses and that Al-Fe intermetallic compound was not virtually formed. Namely, it was shown that Fe particles took a major role of the heating agent. There were reaction layer formed between S-L glass and Al having thickness of about 50 microns. The heated mixture was consolidated as a composite body.

View full abstract

In our previous papers, we have reported the hydrogenation of MgNi₂ with C36-type structure under GPa-order hydrogen pressure. In this paper, the effect of Cu substitution in MgNi₂ was studied on the crystal structures, thermal stabilities and hydrogen contents of Mg(Ni_1−xCu_x)₂ hydrides (x=0–0.2). The hydrides were obtained by high pressure synthesis using cubic-anvil-type apparatus at 973 K for 8 h under 5 GPa. The hydrides were found to have primitive orthorhombic (Pmmm, x=0.0–0.1) and body-centered tetragonal structures (I4⁄mmm, x=0.15–0.2). Their hydrogen contents were estimated to be 2.23–2.32 mass%. Dehydrogenation temperature decreased from 460 K (x=0.0) to 429 K (x=0.20) with increasing amount of Cu substitution. After dehydrogenation, the C36-type phase was observed by X-ray diffraction (XRD). Consequently, it is noteworthy that Mg(Ni_1−xCu_x)₂ could be hydrogenated reversibly without disproportionation under high pressure of the order of gigapascals.

View full abstract

The relationship between hydrogen absorption and the corrosion behavior of Ti-6Al-4V alloy has been examined by immersing the alloy in acidulated phosphate fluoride (APF) solution with various concentrations. Upon immersion in 0.1 mass% APF solution, no hydrogen absorption is observed despite the occurrence of general corrosion. In 0.2% APF solution, slight hydrogen absorption occurs soon after immersion, and then the surface of the specimen becomes uniformly covered with a crystalline corrosion product (Na₃AlF₆), thereby inhibiting further hydrogen absorption. In APF solutions with concentrations higher than 0.4%, the amount of absorbed hydrogen increases with increasing concentration of APF solution and immersion time. The type and morphology of corrosion products sensitively depend on the concentration of APF solution. Upon immersion in 0.4% APF solution, crystalline and massive Na₃AlF₆ are observed on the surface of the specimen. In 1.0% and 1.5% APF solutions, Na₃AlF₆ with a foldlike shape and granular Na₃AlF₆ and Na₃TiF₆ are observed. In 2.0% APF solution, only granular Na₃TiF₆ is observed. The corrosion potentials and anodic polarization curves are analogous in all APF solutions except the 0.2% and 0.4% APF solutions. In contrast, the current density of cathodic polarization curves increases with the concentration of APF solution. The results of the present study suggest that the corrosion products play an important role in the hydrogen absorption behavior of Ti-6Al-4V alloy.

View full abstract

We have studied the formation and carried out an in vivo evaluation of hydroxyapatite (HAp)/collagen and HAp/denatured collagen (gelatin) composite coatings on titanium substrates using a thermal substrate method. The coatings were formed on commercial pure titanium rods (diameter = 2 mm, length = 5 mm) and plates (thickness = 0.3 mm) using a thermal substrate method in aqueous solutions that contained 0.3 mM Ca(H₂PO₄)₂, 0.7 mM CaCl₂, and a concentration of acid-soluble collagen (Type I) of ∼432 mg dm⁻³. The coating experiments were conducted at 40–140°C and pH=8 for periods of 15 or 30 min. The coating temperature and collagen content in the solution influenced the surface morphology and collagen (or gelatin) content in the films. A coated rod was implanted in a 10-week-old male rat’s tibia with a non-coated titanium rod being used as a control. The constructs were retrieved after a period of 14 d postimplantation and examined for new bone formation and for tissue response in the cancellous and cortical bone parts, respectively. HAp/gelatin composite films coated at >60°C showed a slight improvement in osteoconductivity in the cortical bone. In contrast, there was no improvement in the cancellous bone, compared with HAp, which had no gelatin. However, the HAp/collagen composites showed a high osteoconductivity in the cortical bone region, and this increased with increasing collagen content in the films. There was the same tendency in the cancellous bone part. However, too higher a collagen content (40 mass%) in the films gave rise to an obvious negative osteoconductive effect.

View full abstract

A novel method for synthesizing large scorodite (FeAsO₄·2H₂O) particles was recently developed for the fixation of arsenic. This method involves the coprecipitation of scorodite particles from an Fe(II) and As(V) aqueous solution at approximately 95°C by oxygen injection. In order to understand the process of coprecipitation of scorodite particles by this method, X-ray diffractometry (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) were used for characterizing reaction products extracted from the suspension during the reaction. The SEM observation showed that the formation of large scorodite particles was almost completed after a reaction time of 3 h, and then, fine particles precipitated on the large particles by further reactions. The XRD results indicated that scorodite particles with specific lattice parameters were formed in the reaction. The XPS results indicated that the arsenic composition on the surface of the scorodite particles decreased until 3 h from the start of precipitation reaction and increased thereafter. These results correspond to the results on the morphology of the scorodite particles obtained by SEM. Furthermore, X-ray absorption spectra (XAS) in the range of X-ray absorption near edge structure (XANES) were measured for gel-like reaction products formed in the initial stages of the reaction. The spectra revealed that the gel-like reaction products were composed of Fe(II) and Fe(III). The coprecipitation of scorodite particles synthesized by the novel method is discussed on the basis of these results together with previous results on the analyses of iron and arsenic concentrations in solution.

View full abstract

Thermodynamic calculation of phase equilibria in As-Fe-In ternary system is performed based on Calphad approach, directing a special attention to fabrication process of Fe/InAs hybrid structure for spin injection device. For this, the thermodynamic assessment of Fe-In binary system is first carried out utilizing reported experimental data. Then, the liquidus surface of the ternary system and invariant reactions are calculated. The isothermal sections in low temperature region are presented and discussed in the light of the optimization of the growth temperature of Fe film on InAs substrate during the fabrication process.

View full abstract

X-ray diffraction and electron microscopic studies on co-sputtered Zn_1−xFe_xO single crystalline films showed the solubility limit to be about x=0.50 at 673 K. An increase of Fe greater than 0.50 resulted in the existence of Fe metal and (ZnFe)Fe₂O₄ spinel solid solution in a matrix of (ZnFe)O wurtzite solid solution below a substrate temperature of 573 K. Above a substrate temperature of 673 K, a ZnFe₂O₄ spinel phase rather than a Fe metal phase was seen. Annealing of as-sputtered films with a single phase of Zn_1−xFe_xO in air led to decomposition to the two-phase structure of Zn_1−xFe_xO wurtzite and ZnFe₂O₄ spinel and the decrease of the solubility limit of Fe in ZnO to about x=0.11 at 873 K.

View full abstract

Selective dissolution of hyper duplex stainless steel was studied by potentiodynamic and potentiostatic test in various concentrations of H₂SO₄/HCl solutions at various temperatures. There were two peaks in the active-to-passive transition region in potentiodynamic test in 2 M H₂SO₄ + 0.5 M HCl solution at 60°C. In potentiostatic tests, the curve at −340 mV showed stable current density. As the potential increased, the current density increased and at above −310 mV potential, there was a much longer initial period of nonsteady current value. As the potential reached at −280 mV, the current density started to be stabilized and the current density was completely stabilized at −250 mV. It was found that a preferential dissolution of ferrite phase occurred at −330 mV and with the increase of potential, austenite phase was corroded at a high rate. On the other hand, both two phases were passivated at the potential above −270 mV, so that selective dissolution was absent.

View full abstract