Akira Yamakawa

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.

High temperature deformation of silicon nitride ceramics with different microstructures

Abstract The deformation of some silicon nitride ceramics which consist of elongated β-Si 3 N 4 grains in a fine-grained matrix containing an amorphous grain boundary phase was studied by tensile testing at elevated temperature. Superplastic elongations larger than 250% could be observed, and characteristic microstructural development during deformation was studied.

Thin film magnetic heads and substrates therefore

A novel thin film magnetic head is provided characterized by the use of a new substrate material excellent in mechanical property as well as machinability. This substrate is composed of a ceramic compact comprising 4 to 45% by volume of Component A, 55 to 96% by volume of Component B and at most 3% by volume of unavoidable impurities: Component A: at least one member selected from the group consisting of carbides, nitrides, carbonitrides, carboxides, oxynitrides and carboxynitrides of Group IVa, Va and VIa elements of Periodic Table and mixtures or solid solutions thereof. Component B: ZrO2 consisting of at least 70% by weight of tetragonal and/or cubic system and the balance of monoclinic system, in which at least one member selected from the group consisting of oxides of Group IIIa elements of Periodic Table, CaO and MgO, and mixtures thereof is dissolved to form a solid solution.

One role of titanium compound particles in aluminium nitride sintered body

Microstructure of titanium compound particle in polycrystalline aluminium nitride (AlN) has been investigated using micro-auger electron spectroscopy (μ-AES). AlN-0.5 wt% TiO2-1.5 wt% Y2O3-0.4 wt% CaO system was sintered at 1850°C in nitrogen atmosphere using a graphite furnace. The AES studies show that the composition of the titanium compound particle is titanium, aluminium, carbon, oxygen, nitrogen and calcium. On the other hand, no calcium is observed by AES in the AlN grains and grain boundary. It is found that one role of the titanium compound particle is to trap calcium included in polycrystalline AlN.

Indentation cracks in superplastically deformed silicon nitride consisting of strongly aligned rod-shaped grains

Vickers indentation in silicon nitride, which was produced by superplastic tensile deformation of 150%, was conducted. The specimen had a highly anisotropic microstructure, where rod-like grains aligned along the tensile direction, and showed unusual crack systems. In the case of the indentation whose diagonals point to 0°/90° (0° direction is the tensile direction), two V-shaped surface cracks were propagated from each 90° edge and one crack from each 0° edge. In the case of the 45°/45° indentation, surface cracks emanated from the edges of the indent and deflected towards the tensile direction. Subsurface crack systems also showed unique shapes, differing from a cross-shaped crack system in isotropic brittle materials. Three-dimensional crack systems were estimated on the basis of the observation, and the crack evolution process was considered. Furthermore, the mechanical properties of the anisotropic silicon nitride were measured, and showed superior bending strength and fracture toughness in specific directions, compared to the undeformed material.

Room temperature strength and microstructure of Si3N4-Y2O3-ZrO2-Al2O3 ceramics☆

Si3N4-(0–24)mol.% Y2O3- (0–27)mol.%ZrO2-(4.0–4.6)mol.%Al2O3 compacts were sintered in nitrogen gas at 0.5 MPa and 2073 K for 7.2 ks. The room temperature mechanical properties, and in particular the transverse-rupture strength (TRS), were investigated in relation to the Y2O3 and ZrO2 contents. It was found that the compact showed a high strength of about 0.7 MPa in the composition range of low Y2O3, low to high ZrO2 or low to medium Y2O3+ZrO2 contents, and it showed a low strength of about 0.45–0.55 GPa in the other composition ranges. The dependences of both fracture toughness and hardness on the composition were nearly the same as that of the TRS. The low values of TRS in the above composition range were discussed in relation to the microstructure.

Determination of the range of lattice distortion in AIN sintered body by higher order laue zone pattern

To analyse the magnitude and range of lattice distortion which is responsible for the low thermal conductivity in aluminium nitride (AIN) crystal grains, the higher order laue zone (HOLZ) pattern of transmission electron microscopy was used. The HOLZ patterns obtained from various positions in the AIN crystal grain show that the AIN crystal lattice is distorted in the vicinity of the grain-boundary phase, and the magnitude of lattice distortion becomes large as it approaches the grain-boundary phase. Also, the range of distortion extends to approximately 300 nm from the grain-boundary phase.

Fabrication, Microstructure, and Some Properties of Si3N4-Based Nanocomposites

The fabrication process and microstructure of Si3N4-based nanocomposites were investigated for the improvement of fracture toughness. The fine Si3N4 powders containing metal nitride particles such as VN, NbN, TaN and TiN were synthesized by heating the mixtures of Si3N4 and corresponding oxide powders in a N2 atmosphere. The obtained VN/Si3N4 and TiN/Si3N4 powders were confirmed to be very fine (20 to 30 nm). These composite powders were sintered with sintering aids. No improvement in fracture toughness was observed for the VN/Si3N4 composites. For the TiN/Si3N4 composites, in which nano-sized TiN grains were dispersed within the Si3N4 matrix grains, the fracture toughness was increased. This was probably due to crack propagation through intragrains of the Si3N4 matrix.

Room Temperature Strength and Microstructure of Si3N4-Y2O3-ZrO2-Al2O3 Pressure-Sintered Compact

The Si3N4-(0-24) mol%Y2O3-(0-27) mol%ZrO2-(4.0-4.6) mol%Al2O3 compacts were sintered under 0.5 MPa of nitrogen gas at 2073 K for 7.2 ks. The room temperature mechanical properties such as transverse-rupture strength (σm), fracture toughness (KIC) and hardness (HV) and the microstructure of the compacts were investigated.It was found that the compact showed high strength of about 0.7 MPa in the composition range of low Y2O3, low-high ZrO2 or low-medium Y2O3+ZrO2 contents, and it showed low strength of about 0.3-0.5 GPa in the other composition range. The dependences of both KIC and HV on the composition were nearly the same as am. The low values of the mechanical properties in the above composition range could mainly be attributed to the appearance of the soft grain boundary phases such as Y2SiO5, Y20N4Si12O48, etc., and also to the largeness of the domain of those grain boundary phases.