Chihiro Kawai

Crystal growth of silicon nitride whiskers through a VLS mechanism using SiO2Al2O3Y2O3 oxides as liquid phase

Abstract Si 3 N 4 whiskers were coated on Si 3 N 4 whisker substrates through a VLS mechanism using SiO 2 Al 2 O 3 Y 2 O 3 oxides as liquid phases. Alpha-Si 3 N 4 whiskers having diameters of 0.4 to 0.7 μm were used as seeds. After the seed whiskers were coated with SiO 2 Al 2 O 3 Y 2 O 3 oxides, the oxide-coated whiskers were heated at 1490 °C N 2 including gas species generated by reacting amorphous Si 3 N 4 and TiO 2 powders. Resultantly, whisker-like products were newly formed on the seeds. The morphologies of the resulting whisker growth on the seeds resembled rose-twigs and centipedes with long legs. X-ray diffraction analysis indicated that the newly formed whiskers were β-Si 3 N 4 . Droplets were observed on the tips of some whiskers. This suggested that the whisker growth proceeded through a vapour-solid-liquid (VLS) mechanism.

Separation-permeation performance of porous Si3N4 ceramics composed of columnar β-Si3N4 grains as membrane filters for microfiltration

The separation–permeation performance of porous silicon nitride (Si3N4) ceramics (consisting of columnar grains connected at random in three dimensions) as membrane filters was evaluated, and compared with commercial Al2O3 membranes having a three-layer structure. Si3N4 membranes separate particles with diameters much less than their pore diameters. The permeability of Si3N4 membranes with separability values the same as those of the Al2O3 membranes was about 1.3–2.4 times as large as the Al2O3 membranes. Dead-end filtration examination, using Al2O3 particles with a particle size distribution, indicated that the Si3N4 membrane filtration mechanism obeyed the cake filtration mechanism although the particle size was smaller than the pore size of the Si3N4 membranes.

Effect of grain size distribution on the strength of porous Si3N4 ceramics composed of elongated β-Si3N4 grains

The grain size distributions (diameter and aspect ratio) of porous Si3N4 ceramics composed of elongated β-Si3N4 grains were evaluated statistically, and their effect on the pore size distribution and the flexural strength of the porous Si3N4 was investigated. Porous Si3N4 ceramics having porosities of 27 to 43% and median pore diameters of 0.56 to 0.96 μm were used as specimens. The grain diameter distribution was well correlated to the pore size distribution of the porous Si3N4 ceramics. We concluded that the strength of the porous Si3N4 ceramics increased with increasing grain length of β-Si3N4 as well as with decreasing porosity.

Fabrication of C/SiC composites by an electrodeposition/sintering method and the control of the properties

Carbon fibre reinforced SiC matrix composites (C/SiC composites) were fabricated using an electrodeposition/sintering method and the control of properties such as flexural strength. Youngs modulus and thermal expansion coefficient was investigated in order to fabricate C/SiC-based functionally gradient materials. By means of choosing the condition of electrodeposition and sintering, C/SiC composites with volume fraction of fibre (Vf) ranging from 45 to 78% were fabricated. Maximum flexural strength and Youngs modulus were 185 MPa and 47.5 GPa with Vf of 75%, but both properties decreased with the decrease in Vf. Conversely, the thermal expansion coefficient increased with the decrease in Vf; the value varied from 0.2 to 2.75 × 10−6K−1.

Effect of shock compression on wurtzite-type ZnMgS crystals

It is hard to generate twin defects in wurtzite-type ZnMS (M: metal element) crystals by static mechanical method, while it is important for the luminescence. It was found that many twin defects were formed in wurtzite-type ZnMgS crystals by shock compression of >15 GPa in stress, and the absorption and luminescence spectra changed.

High-rate deposition of titanium silicides under high gas flow rate by chemical vapour deposition

Titanium silicides are well known as intermetallic compounds having excellent heat resistance and high hardness. However, it is difficult to fabricate the compounds using the powder sintering method to produce the strong bonding force between atoms characteristic of the compounds. Chemical vapour deposition (CVD) has been a useful method of fabricating a series of intermetallic compounds. In many CVD processes, thermal CVD without plasma is known as a thermally equilibrated process. Therefore, it is possible to predict the deposited phases by means of previous thermodynamic calculations. Many studies have been reported on titanium silicides. Wahl et al. reported the deposition of Ti-Si compounds from the Ti -S i -H-C1 system [1]. Other studies have considered the T i S i C H C 1 system. Most of these have reported both the synthesis and the thermodynamic calculations for a variety of composites consisting of Ti-Si-C, and compared the experimental results with the calculations [2-8]. These reports mention the rare case in which non-equilibrium phases deposit, even in the thermal CVD process. The fabrication of TiC-SiC composites performed by Stinton et al. [2] included the deposition of TiSi2 phase, which was not predicted by the thermodynamic calculation. Similarly, Goto and Hirai [3, 4] suggested that the difference in the deposited phase between the experiments and the calculations might be caused by kinetic restriction. However, detailed kinetic studies, such as deposition rate, have not been reported. In the research reported here, we found a high rate deposition of titanium silicides, which were non-equilibrium phases in a CVD system, under high gas flow rate. Fig. 1 shows the CVD apparatus. TiCI4, SiC14, CH 4 and H2 were used as source materials. The L

Residual Stress of TiC/SiC Composite Film Coated by Chemical Vapor Deposition.

X-Ray stress measurement was applied to TiC and SiC monolithic films and TiC/SiC composite films coated by CVD process on graphite substrates. Textures of films were examined by the X-ray diffraction method. TiC coating had preferential growth of the (2 2 0) plane. SiC coating had preferential growth of the (1 1 1) plane. TiC/SiC composite coating showed a strong texture of the (2 2 0) plane perpendicular to the substrate surface. The diffractions from TiC (3 3 1) and SiC (3 3 1) planes by Fe-Kα characteristic X-rays are suited for X-ray residual stress measurement in thin films. In TiC/SiC composite coating, X-ray diffraction profiles from the TiC phase and SiC phase overlap. These line profiles were separated into TiC and SiC line profiles by the modified DFP method. Phase stresses in the TiC phase and SiC phase in TiC/SiC composite coating showed a triaxial stress state. The measured residual stress of the TiC phase is tensile and that of the SiC phase is compressive. Although the surface residual stress in SiC monolithic coating with thicknesses below 30μm was compressive, the residual stress was tensile for thick coating, and increased with increasing film thickness.