Kiyoshi Hirao

Measuring the anisotropic thermal diffusivity of silicon nitride grains by thermoreflectance microscopy

Abstract High-resolution thermoreflectance microscopy measurements were performed at five frequencies on rod-shaped Si 3 N 4 grains in a ceramic. Our heat diffusion model takes account of the coating and of a coating/substrate resistance. The parameters are adjusted to fit the measurements at the five frequencies simultaneously. The principal diffusivities obtained in individual grains are 0·32xa0cm 2 xa0s −1 along the a -axis, and 0·84xa0cm 2 xa0s −1 along the c -axis (corresponding conductivities: 69 and 180xa0Wxa0m −1 xa0K −1 ). The thermal anisotropy inside individual Si 3 N 4 grains is found to be intrinsic, without direct connection with their elongated shape. ‘Macroscopic’ diffusivities, obtained by mirage effect, are different from the values measured inside individual grains, as a consequence of the dispersion of the grains orientations in the ceramic and of a second-phase effect.

Hot Isostatic Pressing to Increase Thermal Conductivity of Si 3 N 4 Ceramics

Highly anisotropic Si 3 N 4 ceramics were successfully fabricated by tape-casting of raw α–Si 3 N 4 powders with β–Si 3 N 4 single-crystal particles as seed particles and Y 2 O 3 as an effective sintering aid, followed by hot isostatic pressing at a temperature of 2773 K for 2 h under a nitrogen gas pressure of 200 MPa. The microstructure consists of very large elongated grains (diameter ~10 μm; length of ~200 μm), highly oriented in the tape-casting direction. The thermal conductivity along this direction reaches 155 W m -1 K -1 at room temperature, but varies significantly between room temperature and 1273 K. This thermal conductivity is closely related to (1) formation of extremely large elongated β–Si 3 N 4 grains with a reduced amount of crystal defects due to the high-temperature firing and to (2) orientation of β–Si 3 N 4 grains due to addition of seed particles and to tape-casting.

Modeling and simulation of grain growth in Si3N4—I. Anisotropic Ostwald ripening

Abstract The anisotropic Ostwald ripening model has been developed for completely faceted crystals. This model has been applied to the simulation of grain growth in β -Si 3 N 4 with a highly anisotropic rod-like grain shape developed in the liquid phase. The reduction of aspect ratio after the phase transformation observed by previous studies is proved to be a consequence of the anisotropic Ostwald ripening. This model predicts a growth exponent n =3 for totally interfacial reaction controlled kinetics, and higher values when the diffusion constant approaches the interfacial reaction constants. This would explain the puzzling results reported by previous works that growth exponents n =3 or higher have been observed in the grain growth of faceted crystals. While the length distribution becomes wider with time, the reduced radius distribution approaches the shape that is known as the asymptotic distribution function derived from the LSW theory.

Microstructure designing of silicon nitride

A new concept of materials design that allows simultaneous control of the morphologies and distribution of the structural elements at plural scale levels to create a new family of advanced ceramics was proposed. The validity of the concept was experimentally demonstrated using silicon nitride ceramics as model materials. Controlling anisotropic grain growth by seeding of small amounts of morphologically regulated, β-silicon nitride single crystals (micro-scale level control), combined with alignment of the seed particles by tape casting followed by stacking of laminates (macro-scale level control) allows compatibility of high strength and high fracture toughness in this material, with a high degree of reliability for the mechanical strength.

Modeling and simulation of grain growth in Si3N4—II. The α–β transformation

Abstract A model for the α – β transformation has been developed for completely faceted crystals as an extension of the anisotropic Ostwald ripening model developed in the companion paper. Si 3 N 4 grain growth simulations have been performed using various relationships between diffusion and interfacial reaction constants. It has been found that length growth is dominant, and that its growth rate is independent of width and does not change with time in the totally interfacial reaction controlled case during the α – β transformation. Simulation predicts that a time–length relationship deviates from a straight line as the growth kinetics in the length direction shifts from interfacial reaction controlled to diffusion controlled. It has been confirmed that the ratio of the interfacial reaction constants of the (100) and (001) interfaces and the α – β ratio are the key factors for determining the aspect ratio of β -Si 3 N 4 grains.

Silicon carbide ceramics prepared by pulse electric current sintering of β–SiC and α–SiC powders with oxide and nonoxide additives

Using a pulse electric current sintering (PECS) method, β–SiC and α–SiC powders doped with a few weight percent of Al 2 O 3 –Y 2 O 3 oxide or Al 4 C 3 –B 4 C–C nonoxide additives were rapidly densified to high densities (95.2–99.7%) within less than 30 min of total processing time. When Al 2 O 3 –Y 2 O 3 additive was used, both ceramics resulting from β–SiC and α–SiC had fine, equiaxed microstructures. In contrast, when Al 4 C 3 –B 4 C–C additive was used, the ceramic resulting from α–SiC had a coarse, equiaxed microstructure, whereas the ceramic resulting from β–SiC was composed of large elongated grains whose formation was accompanied by the β →?α phase transformation of SiC. Compared with the Al 2 O 3 –Y 2 O 3 -doped SiC ceramics, the Al 4 C 3 –B 4 C–C-doped SiC ceramics had higher densities, lower fracture toughness, and higher hardness. The fracture mode of the oxide-doped SiC was mainly intergranular, whereas the nonoxide-doped SiC exhibited almost complete intragranular fracture that was attributed to the higher interfacial bonding strength.

Factors affecting mechanical properties of silicon oxynitride ceramics

Abstract The effect of additives and impurities on the mechanical properties of silicon oxynitride ceramics was investigated. The toughening of the ceramics was affected by three factors: (1) the thermal tensile stress in the intergranular glassy phase and the compressive stress in Si2N2O grains, developed by a thermal expansion mismatch between Si2N2O grains and the intergranular glassy phase; (2) the large grain size of Si2N2O; and (3) the concentration of impurities in the intergranular glassy phase. The degradation of high-temperature strength was primarily dependent on the basic chemical composition (the kind of additives (MeOx) and Me Si ratio) of the intergranular phase and impurities.

Modeling and simulation of grain growth in Si3N4. III. Tip shape evolution

Abstract Simulations based on the anisotropic Ostwald ripening model have been performed for the tip shape evolution of the β-Si 3 N 4 crystal in the liquid phase. It was found that tip shape is a function of the liquid concentration and the relationship between the interfacial reaction and diffusion constants. Depending on the concentration of the liquid phase, a convex, almost flat, or concave surface appeared at the tip surface. The following two factors are crucial for understanding the development of concavity at the tip surface of the β-Si 3 N 4 crystal: (1) strong growth anisotropy of the β-Si 3 N 4 crystal that is the interfacial reaction and diffusion controlled kinetics of the (100) and (001) surfaces, respectively, and (2) supersaturation of the liquid phase.

Grain bridging of highly anisotropic silicon nitride

Abstract This study assessed the contribution of grain bridging by elastic deformation to the total toughening, in a highly anisotropic silicon nitride where the large rod-like grains were uniaxially aligned, when a crack propagated perpendicularly to the grain alignment. The length of debonded part of the grains was estimated to be about 1 μm, from distribution of grain fracture sites in the crack wake. It was predicted that the grain bridging operated in a very short crack extension like 10 μm, and caused a large toughness increase of ∼4 MPa m 1/2 . This accounted for a substantial part of the total toughening of this material.

Grain Growth Behavior During Microwave Annealing of Silicon Nitride

A comparative study of grain growth behavior in silicon nitride under conventional and microwave annealing is presented. Microwave annealed specimens showed a faster growth rate as indicated by the quantitative microstructural analysis. The phenomenon was used in combination with seeding techniques to develop a silicon nitride exhibiting a bi-modal microstructure. Microwave annealing was carried out using a microwave radiation frequency of 28 GHz.