Papers by Author: Motozo Hayakawa

Abstract: Y-TZP presents excellent properties at room temperature but these properties decrease as the temperature increases. This paper studies the behavior 20vol%Si3N4-SiC when added in YTZP matrix and heated under no pressure system. Al2O3 and Y2O3 were used to maintain the stability of the matrix and as sintering aids. The addition of Si3N4 and SiC in a Y-TZP matrix leads to formation of silicon oxynitride and it increases the mechanical properties like toughness and hardness. The mixture was milled and molded by CIP. Samples were heated at 1500°C, 1600°C and 1700°C for 2h without pressure under atmospheric conditions in bed-powders of Si3N4. Samples were characterized by XRD. Density, hardness, toughness, bending strength were measured. The structure of the material was observed in SEM/EPMA to verify the distribution of the materials in the composite. The formation of Si2N2O was observed in the sintered material and it showed an increment of both hardness and toughness as temperature increases. The samples presented considerable resistance of oxidation at 1000°C.

Abstract: The effect of the processing conditions of dolomite powders on the antiviral activity was studied against H5N3 avian influenza virus. When dolomite was fired in the ambient atmosphere, it decomposed into MgO and CaO by two stages at different temperatures, namely into MgO+CaCO3 at ca.1010 K and MgO+CaO at ca. 1070 K. Strong antiviral activity was observed after the complete decomposition into CaO and MgO. However, when the firing temperature exceeded 1673 K, the antiviral activity was degraded. It was also found that the degraded antiviral activity was recovered by sever ball mill grinding.

Abstract: Small spherical specimens of ZrO2-1.5mol%Y2O3 with diameters ranging from 150 to 900μm were prepared and the burst temperature was measured on an individual specimen under optical microscope with cooling stage. The range of scatter of the burst temperature increased with decreasing specimen’s size. While the upper limit of the scatter remained nearly the same, the lower limit of the scatter markedly decreased with decreasing specimen’s size. The distribution of potential embryos was deduced by fitting the simulated distribution of burst temperatures to the observed result by adjusting the assumed distribution of potential embryos used for the simulation.

Abstract: Grain growth behavior in Al2O3 with a small amount of ZrO2 (< 5 vol%) was examined. Grain growth of Al2O3 was retarded by zirconia particles, despite the small amount of added zirconia. The fraction of the zirconia particles embedded within alumina grains (intragranular zirconia particles) increased with decreasing zirconia content and increasing sintering temperature. Grain growth inhibition of alumina in the Al2O3-ZrO2 showed good agreement with the prediction of modified Zener’s pinning effect by the zirconia particles on grain boundaries.

Abstract: Centrifugal casting method was used to fabricate composites with graded structures in alumina-zirconia system. The composition profiles of the graded composites showed three distinct regions, with a greater part of the specimen showing nearly linear variation of compositions. The composition profile can be varied by using zirconia powder of different size ranges. The top of the specimens showed nano-scaled microstructures. The composition profiles can also be varied by altering zirconia content in the starting suspension. It was found that the composites with graded structures showed higher bending strength than that of uniform composites. On the other hand, the specimens fractured with the lower zirconia content region being the crack initiate plane showed higher bending strength than that of fractured form higher zirconia content region.

Abstract: The effect of CoO addition into Y-TZP (Yttria doped Tetragonal Zirconia Polycrystals) was studied on the evolution of its sintering ability, grain size, grain boundary structure and mechanical properties. The doping of a small amount of CoO effectively reduced the sintering temperature. A small amount of CoO up to ~ 0.3 mol% was effective for the suppression of grain growth, but the addition of 1.0 mole % resulted in an enhanced grain growth. The hardness and toughness of the CoO doped TZP were about the same as those of undoped TZP. Furthermore, despite the grain refinement, CoO doped TZP did not exhibit improved mechanical properties. This may be suggesting that CoO dopant had weakened the grain boundary strength.