Tatsuki Ohji

Formation and Photocatalytic Application of ZnO Nanotubes Using Aqueous Solution

Vertically aligned ZnO nanotubes were prepared by etching ZnO rod arrays in aqueous solution, which were previously developed by chemical bath deposition method. The morphological, structural, photoluminescence, as well as photocatalytic properties of the ZnO nanotubes were examined with respect to the pH values of chemical bath solution. The morphology of the products was found to be sensitive to the pH values and chemical bath temperatures. The nanotubes synthesized at a low pH value (5.82) exhibited a strong UV emission and a weak defect-related visible emission. The highest photocatalytic efficiency was also observed at pH = 5.82. The possible mechanism for the difference of photocatalytic efficiency was discussed.

Pore structure of porous ceramics synthesized from water-based slurry by freeze-dry process

A unique porous ceramic with complex pore structure was synthesized by the freeze-dry process. A water-based ceramic slurry was frozen while controlling the growth direction of ice, and sublimation of the ice were generated by drying it at a reduced pressure. By sintering this green body, a porous ceramic with complex pore structure was obtained, where macroscopically aligned open pores exceeding 10 μm in size contained minute pores of about 0.1 μm in their internal walls. Wide control of the porosity was possible by changing the concentration of the starting slurry. The pore size distribution as well as the microstructure were substantially affected by the freezing and sintering temperatures. Optimization of the synthesis conditions was investigated in order to obtain the desired pore structure.

Fabrication and characterisation of porous silicon nitride ceramics using Yb2O3 as sintering additive

Abstract Porous Si 3 N 4 ceramics were fabricated by liquid-phase sintering with a Yb 2 O 3 sintering additive, and the microstructure and mechanical properties of the ceramics were investigated, as a function of porosity. Low densification was achieved using a lower Yb 2 O 3 additive content. Fibrous β-Si 3 N 4 grains developed in the porous microstructure, and the grain morphology and size were affected by different sintering conditions. A high porosity, ∼40–60%, with β-Si 3 N 4 grain development, was obtained by adjusting the additive content. Superior mechanical properties, as well as strain tolerance, were obtained for porous ceramics with a microstructure of fine, fibrous grains of a bimodal size distribution.

Oxidation bonding of porous silicon carbide ceramics

A oxidation-bonding technique was successfully developed to fabricate porous SiC ceramics using the powder mixtures of SiC, Al2O3 and C. The oxidation-bonding behavior, mechanical strength, open porosity and pore-size distribution were investigated as a function of Al2O3 content as well as graphite particle size and volume fraction. The pore size and porosity were observed to be strongly dependent on graphite particle size and volume fraction. In contrast, the degree of SiC oxidation was not significantly affected by graphite particle size and volume fraction. In addition, it was found that the fracture strength of oxidation-bonded SiC ceramics at a given porosity decreases with the pore size but increases with the neck size. Due to the enhancement of neck growth by the additions of Al2O3, a high strength of 39.6 MPa was achieved at a porosity of 36.4%. Moreover, such a porous ceramic exhibited an excellent oxidation resistance and a high Weibull modulus.

Boron carbide and nitride as reactants for in situ synthesis of boride-containing ceramic composites

Abstract Using boron carbide (B 4 C) and boron nitride (here refers to hexagonal BN) as reactants, a series of boride-containing ceramic composites, mainly in the present work zirconium diboride-containing composites including zirconium diboride–zirconium carbide (ZrB 2 –ZrC), zirconium diboride–zirconium nitride (ZrB 2 –ZrN), zirconium diboride–silicon carbide (ZrB 2 –SiC) and zirconium diboride–aluminum nitride (ZrB 2 –AlN) were prepared by in situ reactive hot pressing. The features and the development mechanisms of the composite microstructures were characterized and modeled. The obtained zirconium diboride-containing composites demonstrated high bending strength. In addition, some general problems such as transformation between B 4 C and BN and thermodynamics of using B 4 C and BN as reactants were also briefly discussed.

High performance porous silicon nitrides

Abstract In structural materials, pores are generally believed to deteriorate the mechanical reliability. This study, however, demonstrates pores can cause improved or unique performance when the porous microstructure is carefully controlled. The first example is a silicon nitride of 14% porosity fabricated by tape-castng whiskers. This material, where the characteristic fibrous grains were aligned uniaxially, shows a high fracture strength in excess of 1 GPa as well as high damage tolerance. The fracture energy obtained by a chevron-noteched beam technique was about seven times larger than that of dense silicon nitride, which was primarily attributable to grain “pull-out” mechanism enhanced by the pores. The other example was a silicon nitride of 24% porosity, fabricated by sinter forging technique which exhibited excellent strain tolerance.

Reinforcement by crack-tip blunting in porous ceramics

A crack-tip blunting mechanism in porous ceramics was identified in this study. The reinforcement by crack tip blunting in porous ceramics was analyzed quantitatively, based on a grain fracture model. The investigation revealed that the crack tip blunting increases the fracture toughness of porous ceramics, depending on pore morphology and grain arrangement. The theoretical predictions were in good agreement with the experimental results.

Nonoxide-boron nitride composites: in situ synthesis, microstructure and properties

Hexagonal graphitic boron nitride (h-BN) composites show excellent corrosion and thermal shock resistance, good mechanical tolerance and machinability, especially Si3N4–BN and Sialon–BN composites; they have already been used as break rings for horizontal continuous casting of steel. However, the strength of the conventionally processed BN composites were remarkably degraded by the addition of BN due to the poor densification behavior and the existence of large BN flakes or agglomerates of BN flakes that acted as fracture flaws. This means that BN dispersoids with fine particle size and homogeneous distribution are the key factors to obtain high strength composites. By in situ process, such microstructural features can be realized. In this work, by using the proposed in situ reactions, synthesis, microstructures and properties of various in situ nonoxide-boron nitride (Nobn) composites including SiC–BN, Si3N4–BN, AlN–BN, Sialon–BN and Alon–BN composites were investigated. For some Nobn composite systems, due to the large volume expansion during the reaction processes, near-net shape sintering can be realized. For example, the sintering shrinkage of AlN-30 vol.% BN was 3.1% and that of Alon-21 vol.% BN was 4.2%. This will be an advantage for the fabrication of large and complicated products. # 2002 Elsevier Science Ltd. All rights reserved.

Porosity and microstructure control of porous ceramics by partial hot pressing

Precise control of porosity and microstructure of porous ceramics is essential. This paper describes a method for the preparation of porous ceramics with porosity from 0.0 to 0.4. The ceramics were prepared via hot pressing a powder mixture to a definite dimension, and the porosity was easily adjusted by the powder amount. As examples, porous Si 3 N 4 or SiC ceramics were produced by powder mixtures that contained Si 3 N 4 or SiC and a sintering additive of oxide. It was demonstrated that the present method is advantageous for producing ceramics with controllable porosity and microstructure.

Reaction mechanism and microstructure development of strain tolerant in situ SiC-BN composites

Abstract The reaction mechanism and microstructure development of strain tolerant in situ SiC–BN composites fabricated from the in situ reaction of Si 3 N 4 , B 4 C and C were investigated. This exothermic reaction took place at about 1400°C in an argon atmosphere according to the results of X-ray diffraction analysis and differential thermal analysis. The reaction finished after hot pressing at 1700°C for 60 min, and densification occurred mainly in the temperature range of 1700°C to 2000°C. In spite of poor sinterability of BN, composites with rather high density were obtained. Chemical composition analysis of the composites obtained showed that there was no obvious change in the composition after hot pressing. The in situ formed SiC was of the β-type with a quasi-spherical shape, whereas the in situ BN was graphitic hexagonal with a flake shape, and was located at the grain boundaries of SiC. The composite obtained showed a very fine and homogeneous microstructure. The bending strength of the composite was high, while the elastic modulus decreased substantially.