Papers by Author: Wakako Araki

Abstract: Ionic devices consisted of ion-conductive ceramics such as solid oxide fuel cells (SOFC) and oxygen separation membranes have been developed in the last decades. La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF), which is a mixed ionic-electronic conductive ceramics, is especially expected to play an important role in those ionic devices and so its electrochemical properties have been intensively studied.

Abstract: The oxygen diffusion in 4, 8, and 14 mol%-yttria-stabilized zirconia subjected to uniaxial stresses in the [100], [110], and [111] directions is investigated. In the case of uniaxial stress in the [100] direction, the oxygen diffusion in 4YSZ and 8YSZ is facilitated in the tensile direction and deteriorated in the compressive direction without changing the total diffusion property, which could be caused by the elastic recovery force. For 14YSZ, however, the oxygen diffusion remains unchanged regardless of the stress, probably due to high yttria concentration. In the case of tensile stress in the [110] and [111] directions, the oxygen diffusion is also increased in the tensile direction, which could be attributed to the improvement of the oxygen diffusion in the <100> direction caused by the stress component in the <100> direction.

Abstract: An asymptotic analysis for singular stress fields around an interface-edge of dissimilar power-law hardening materials joint has been presented under plane strain condition and J2 deformation plasticity theory. Both the balance of force and the continuity of displacement are satisfied on the interface. In the higher order approximation, the nonlinear effective stress term was expanded by Taylor series. An iteration method is proposed for the determination of singular fields around interface edge. Multiple stress singular terms exist for in the higher order approximation. The order of stress singularity has a dependency with the combination of hardening exponents, .

Abstract: Zirconium oxide doped with scandium oxide is one of the most promising materials for electrolytes of solid oxide fuel cells because of its high ionic conductivity. In this study, temperature dependence of mechanical property, especially strength, of 9 mol% scandia doped zirconia polycrystalline samples with 0 to 30 wt% of alumina additions was examined at temperatures from 293 to 1273 K. Ionic conductivity was also measured by AC impedance analysis. From both mechanical and electrical standpoints, the experimental results were discussed in terms of alumina addition.

Abstract: Two different processes were used to manufacture epoxy/silica hybrid materials. Observations by AFM, TEM, and Raman spectroscopy demonstrated different silica structures, depending on the hybrid process. DMTA results indicated that the “sequentially” hybridised sample had a new glass transition at higher temperatures, which could be attributed to the physical interaction between the silica chain and the epoxy resin. The dynamic modulus was significantly improved and depended only on the silica content, not on the silica structure nor the aging.

Abstract: The effect of adding a high molecular weight epoxy monomer (epikote 1001) to a low molecular weight one (epikote 828) on fracture toughness properties was investigated according to the crosslinking degree and density heterogeneity. To characterize the crosslinking degree and density heterogeneity, the glass transition temperature, Tg, and fragility, m, were deduced from thermo-viscoelastic properties. The characterization of Tg and m revealed that blends can be divided into two groups: one group with (φ < 10 wt%) and another one with (φ > 10 wt%), where φ is the weight ratio of epikote 1001 to epikote 828. The first group had the same average crosslinking degree (the same Tg) but different density heterogeneities (m decreased). The other group had a lower crosslinking degree (Tg decreased) and even more density heterogeneity (m decreased). The fracture toughness results showed that KIC of blends of the first group was approximately constant because the increase in density heterogeneity was still too weak (ineffective m), whereas KIC of blends of the second group was higher due to the simultaneous decrease in average crosslinking degree and increase in density heterogeneity. Therefore, the lower crosslinking degree (lower Tg) is and the more heterogeneous the blend (lower m) is due to the addition of high molecular weight monomer, the higher KIC becomes.

Abstract: Mechanical properties of nano/micro-silica particles bidispersed epoxy composites were investigated based on experimental results. The composite specimens varied with different compositions of nano and micro-silica particles (240 nm and 1.56$m) were prepared with the constant volume fraction, 0.30. The thermo-viscoelastic properties for the composites and the neat epoxy measured in the temperature ranges from 123 K to 523 K and compared to theoretical results according to Lewis and Nielsen’s law with the maximum particle packing given by Ouchiyama and Tanaka’s model. In addition, fragility derived from the thermo-viscoelasticity measurements was used to characterize the strength and fracture toughness of the composites. From results, we found that the thermo-viscoelasticity of the composite was dependent on nano and micro-particles packing, and its strength and fracture toughness were effectively evaluated by fragility.

Abstract: The time-temperature dependence of the compressive behavior of polypropylene (PP) foam was investigated to make predictions about what sort of behavior for wide ranges of temperature and strain rate. Compressive stress relaxation tests were conducted at 213 K and 373 K. Compression tests were also conducted. The strain rate was 2×10-3 1/s at 213 K and 373 K. The compressive stress-strain curves were roughly linear and dependent on temperature until the maximum stress was reached. The maximum stress occurred at 5% strain regardless of temperature. The plateau stresses decreased as temperature increased. By plotting compressive behavior of the PP foam at the master curve of the stress relaxation modulus, its temperature dependence could be explained by the thermo-viscoelastic properties. Therefore, the behavior of PP foam at different strain rates could be approximately predicted from the stress relaxation modulus with the timetemperature equivalence principle.

Abstract: We investigated the particle size effects on the fracture toughness of epoxy resin composites reinforced with spherical-silica particles. The silica particles had different mean particle diameters of between 1.56 and 0.24µm and were filled with bisphenol A-type epoxy resin under different mixture ratios of small and large particles and a constant volume fraction for all particles of 0.30. As the content with the added smaller particle increased, the viscosity of each composite before curing remarkably increased. We conducted the single edge notched bending test (SENB) to measure the mode I fracture toughness of each composite. The fracture surface with the small particle content exhibited more rough areas than the surface with larger particles. The fracture toughness increased below the small particle content of 0.8 and saturated above it. Therefore, near the small particle content of 0.8, the composite had a relatively low viscosity and a high fracture toughness.

Abstract: We experimentally investigated compressive behaviors of a paper and a paper coated by polyethylene resin. The initial thickness of sheets of paper measured under a compressive stress of 4.5kPa was widely distributed in comparison with the basis weight, although the results based on the ISO 534 standard (under compressive stress of 100kPa) had relatively little dispersion. For compressive deformation, the thickness immediately decreased under low compressive stress, because the gap between fibers in the paper collapsed. After that, the variation of the thickness under higher compression was small due to the compression of fibers. We found that although the resin-coat layers did not have an insignificant effect on compressive compliance above 250kPa, the compliance of the coated paper was larger than that of uncoated paper.