Papers by Author: Satoshi Kitaoka

Abstract: The oxygen permeabilities of polycrystalline Yb₂Si₂O₇ wafers cut from sintered bodies to serve as models of environmental barrier coatings were determined at temperatures up to 1673 K under various oxygen potential gradients (dµ_O), produced by exposing the upper and lower surfaces of the wafer to atmospheres with different oxygen partial pressures (P_O2). Oxygen permeation proceeded via grain boundary (GB) diffusion of oxygen from the higher P_O2 surface to the lower P_O2 surface, concurrently with GB diffusion of ytterbium in the opposite direction. Mass transfer analysis allowed for a design guideline for selectively enhancing the structural stability of the ytterbium silicate layer with applying a dµ_O.

Abstract: MC3T3-E1 cell differentiation and related surface potentials of rutile-type TiO₂ scales formed on Ti are controlled by varying the Ti heat treatment conditions in a N₂ atmosphere containing a trace amount of O₂. The zeta potentials of the samples heated at 873 and 973 K for 1 h show large negative and positive values, respectively, while cell differentiation on the surface is enhanced in both cases (14 days incubation). In the case of untreated Ti, the cell differentiation diminishes and the zeta potential becomes more neutral. Protein detection by an immunogold-labeling technique and Ca and P detection by time-of-flight secondary ion mass spectrometry reveal that Ca and P, rather than an adhesive protein such as fibronectin, predominantly adsorbed on the scales formed in 1 h at 873 and 973 K, respectively. In the case of untreated Ti, both fibronectin and a non-adhesive protein such as albumin adsorbed, but no Ca and P were detected. The present findings illuminate the relationship between charged surfaces and MC3T3-E1 cellular response.

Abstract: The surface potential of the TiO₂ scale formed on Ti was controlled by varying the Ti heat treatment conditions in a N₂ atmosphere containing a trace amount of O₂. The surface potential was attributed to the effective charge of nitrogen-related defects in the TiO₂, where the positive and negative surface charges were associated with (N₂)₀+2 and (NO)₀^-1, respectively. The latter defects were formed only during the early stages of the heat treatment, and with increasing treatment time, this was followed by the formation of voids containing N₂ in the scale rather than the disappearance of the defects in the TiO₂ crystal lattice, resulting in zero surface charge. Hydroxyapatite (HAp) formation and osteoconductivity were enhanced on nitrogen-doped TiO₂ scale with either a positive or negative surface potential. In contrast, for the unchanged TiO₂ scale, no HAp formation was observed and the osteoconductivity was low.

Abstract: The oxygen permeability of polycrystalline α-alumina wafers, which served as model alumina scales formed on heat-resistant alloys, was evaluated at a temperature of 1873 K. Mass transfer along grain boundaries (GBs) in an alumina wafer exposed to a large oxygen potential gradient (dμ_O), where both oxygen and aluminum mutually diffuse along GBs, was analyzed using ¹⁸O₂ and SIMS. ¹⁸O was concentrated at GB ridges on the high oxygen partial pressure (P_O2(hi)) surface and along the GBs near the P_O2(hi) surface. ¹⁸O adsorbed on the surface diffused almost immediately to surface GBs, resulting in the formation of new alumina by reaction with aluminum diffusing outward along the GBs. Oxygen GB diffusion coefficients in the vicinity of the P_O2(hi) surface were determined from the ¹⁸O depth profile along each GB for the ¹⁸O map of the cross section of the exposed alumina wafer. The oxygen GB diffusion coefficients were comparable to the values calculated from the oxygen permeability constants assuming an electronic conductivity and were obviously lower than those of oxygen GB self-diffusion without an oxygen potential gradient.

Abstract: The oxygen permeability of polycrystalline α-alumina wafers, which served as model alumina layers, under an oxygen potential gradient ΔP_O2 was evaluated at a temperature of 1873 K. When mutual grain boundary (GB) diffusion of oxygen and aluminum occurred in wafers subjected to a steep ΔP_O2, the oxygen and aluminum fluxes at the inflow side of the wafer were significantly smaller than those at the outflow side. It was noteworthy that Lu and Hf segregation at the GBs selectively reduced the mobility of oxygen and aluminum, respectively. It was found that a wafer with a bilayer structure, in which a Lu-doped layer was exposed to a low partial oxygen pressure (P_O2) and a Hf-doped layer was exposed to a high P_O2, exhibited excellent oxygen shielding properties at high temperatures.

Abstract: The effect of dopants such as zirconium and nitrogen on the releasability of Y₂O₃-based ceramics from molds was investigated for integrated circuit packaging using epoxy molding compounds (EMCs). Co-doping of these elements was carried out by annealing the surfaces of 5mol% ZrO₂-Y₂O₃ samples under a N₂ flow at 1100-1300 °C, resulting in concentration of nitrogen near the surfaces of the samples. The adhesion strength was minimized by exposure at about 1200-1250 °C, which was less than half the value for the undoped Y₂O₃. The co-doping remarkably decreased the polar part of the surface energy and consequently hydrophobicity of the ceramic surfaces increased. The excellent releasability characteristics were likely related to the depression of dissociative adsorption of water molecules, which are considered to act as active sites for the adhesion of EMCs.

Abstract: The transformation from metastable polymorphs to stable alpha-Al2O3 in the scale formed on a CoNiCrAlY alloy is accelerated under lower oxygen partial pressure (PO2), where both Al and Cr in the alloy are simultaneously oxidized, resulting in the formation of a dense and monolithic alpha-Al2O3 scale. Under higher PO2, where all components of the alloy are oxidized, the transformation is retarded and (Co,Ni)(Al,Cr)2O4 is also produced. The oxygen permeability in polycrystalline alpha-Al2O3 wafers exposed to steep oxygen potential gradients is evaluated at high temperatures to investigate the complicated mass-transfer phenomena through the scale formed on the alloy. The diffusion of Al and O species, which are responsible for the oxygen permeation along the grain boundaries of Al2O3, is dependent on the formation of an oxygen potential gradients. For Lu-doped Al2O3 polycrystals, it was found that Lu depressed the mobility of oxygen, but did not directly influence the migration of Al.

Abstract: The effect of carbon nano-fiber (CNF) dispersion on the tribological behavior of Si3N4 based composites against bearing steel were investigated in kerosene. The friction coefficients were not affected by the CNF addition. On the other hand, the wear rates of Si3N4/CNF composites showed a minimum value on 3 mass% CNF additions. The primary wear mechanism of Si3N4 is considered to be controlled by oxidation wear of Si3N4 due to a trace amount of water as impurity in the kerosene.

Abstract: The coating of molten silicate glass on a porous carbon substrate was developed, without the formation of cristobalite at the carbon-glass layer interface, in order to improve the steam oxidation and thermal shock resistance. Initially, suitable conditions for coating were assumed from thermodynamic analysis. Based on these calculations, the wettability of the carbon to molten glass was modified by infiltration and pyrolysis of a Si-N precursor, and the coating with glass was carried out under higher N2 partial pressures. As a result, carbon substrates were completely sealed with glass, without the production of cristobalite at the interface, and the glass was infiltrated into the substrate. In contrast, coating with glass at lower N2 partial pressures, such as in Ar, were followed by the formation of cristobalite along with many pores at the interface. The structural changes occurring as a result of variation of the N2 partial pressure during sealing with glass are in good agreement with the thermodynamic analysis. The glass-coated carbon materials, which were fabricated at higher N2 partial pressure, possessed excellent steam oxidation resistance and thermal shock resistance.