Mikito Kitayama

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.

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.

Surface and interface properties of alumina via model studies of microdesigned interfaces

The ability to produce controlled-geometry, controlled-crystallography internal voids in ceramics has made possible several new model experiments for studying the high-temperature properties of surfaces and interfaces in ceramics. Recent advances have enabled the production of more complex microdesigned internal defect structures, and have exploited new means of examining them, thus, broadening the range of problems that can be addressed. A particular topic of concern is the effect of surface energy anisotropy on both the driving force for and the mechanism of shape changes. This paper reviews and previews recent research focussing on improving our understanding of surface diffusion in ceramics. Rayleigh instabilities provide one means of examining morphological evolution. The modelling of Rayleigh instabilities in materials with surface energy anisotropy is reviewed, and the results of experiments utilizing microdesigned pore arrays in sapphire are summarized. In a material with anisotropic surface energy and a facetted Wulff shape, the driving force for shape changes hinges on both the absolute and relative surface energies. Microdesigned pore structures have been used to determine the stable surfaces in both undoped and doped sapphire and to provide the relative values of the energies of these stable surfaces. Nonequilibrium shape, controlled-crystallography cavities have been introduced into undoped sapphire, and the effect of crystallographic orientation on their morphological evolution has been studied. Comparisons of the results with predictions of models of surface-diffusion-controlled evolution indicate that surface-attachment-limited kinetics (SALK) play an important role.

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.

The Kinetics of Pore Shape Evolution in Alumina

The kinetics of shape evolution of a completely faceted crystal/internal void by surface diffusion was modeled. Arrays of micron-sized cavities were generated in sapphire substrates with known surface orientations using microlithography and ion beam etching and converted to internal intragranular pores of nonequilibrium shape by diffusion bonding of the etched substrate to an identical-orientation unetched sapphire substrate. Pore shape evolution rates during high-temperature anneals were monitored and found to be highly sensitive to the orientation of the substrate surface. The observed evolution rates were compared with the predictions of the kinetic model using diffusivity values for alumina that span the range from the highest to the lowest diffusion constants reported in the literature. The comparison suggests that surface-attachment-limited kinetics (SALK) play a major role in surface mass transport on stable low-index planes of alumina.

Catalytic activities of zeolite compounds for decomposing aqueous ozone

The advanced oxidation process (AOP), chemical oxidation using aqueous ozone in the presence of appropriate catalysts to generate highly reactive oxygen species, offers an attractive option for removing poorly biodegradable pollutants. Using the commercial zeolite powders with various Si/Al ratios and crystal structures, their catalytic activities for decomposing aqueous ozone were evaluated by continuously flowing ozone to water containing the zeolite powders. The hydrophilic zeolites (low Si/Al ratio) with alkali cations in the crystal structures were found to possess high catalytic activity for decomposing aqueous ozone. The hydrophobic zeolite compounds (high Si/Al ratio) were found to absorb ozone very well, but to have no catalytic activity for decomposing aqueous ozone. Their catalytic activities were also evaluated by using the fixed bed column method. When alkali cations were removed by acid rinsing or substituted by alkali-earth cations, the catalytic activities was significantly deteriorated. These results suggest that the metal cations on the crystal surface of the hydrophilic zeolite would play a key role for catalytic activity for decomposing aqueous ozone.

Microdesigned Interfaces: New Opportunities for Studies of Surfaces and Grain Boundaries

The development of methods for producing highly controlled internal voids in ceramics provided a new class of experimental methods for studying the properties of surfaces and interfaces in ceramics. Recent work, based on refined capabilities for producing microdesigned internal defect structures has broadened the range of problems that can be addressed. This paper reviews and previews recent research focussing on improving our understanding of surface diffusion in ceramics, providing experimentally determined values of surface energies in doped and undoped sapphire, and on developing new approaches to generating graded microstructures and single crystals by solid-state routes.

Development of Silicon Nitride Porous Bodies with Micro-Macro Complex Pore Structure for the Application of the ‘Bio-Filter’

In order to develop the ‘Bio-filter’, Si3N4 porous bodies with micro-macro complex pore structure were fabricated. The micro-pores were successfully introduced by inserting PE meshes between the green tapes. Micro-pore size was controlled by adding a small amount of the β-Si3N4 ‘seed’ particles to the α-Si3N4 raw powder. It is expected that the aerobic microbes colonize along the micro-pores consuming dissolved oxygen, which makes the anaerobic ones colonized within the micro-pores to develop the microbe consortium. In this study, this concept was verified by using the S. cerevisiae and B. bifidum as the aerobic and anaerobic microbes, respectively.

Effects of Lanthanide Substitution on the Sintering Behavior of Bismuth Titanate Ceramics Prepared by the Chemical Coprecipitation Method

Bi4-xLnxTi3O12 (Ln = La, Nd or Gd; x = 0, 0.25, 0.5, 0.75 or 1.0) fine powders were prepared by the chemical coprecipitation method. Suitable calcination temperature for each powder was determined. The effects of Ln-substitution on the sintering behaviors, crystal structures, lattice constants and microstructures of sintered bodies were investigated. The crystallization temperature was significantly influenced by lanthanide elements and their amounts, and thus, a suitable temperature for calcination was determined for each substitution. Each calcined powder was milled and fired at 950, 1000, 1050 or 1100°C for 2 hr. It was found that the Ln-substitution drastically inhibited both sintering and grain growth, although the effect was not consistent with the order of Ln ionic radii.

Anisotropic Ostwald Ripening in Silicon Nitride: On the Reaction-Controlled Kinetics

A model for anisotropic Ostwald ripening was developed using a chemical potential (weighted mean curvature) difference as a driving force for mass-transport. Based on this model, grain growth simulations of silicon nitride during the phase transformation and Ostwald ripening were performed. Comparison with experimental results during the phase transformation suggests that grain growth be controlled by interfacial reaction. Simulations of Ostwald ripening predict that the growth exponent be 3 for the reaction-controlled case, and increases up to 5 as the growth kinetics shifts from reaction-controlled to diffusion-controlled. It was reported that the mean aspect ratio of silicon nitride crystals increased during the phase transformation, and decreased during Ostwald ripening. These behaviors were successfully simulated by this model. The concave depression at the tip of silicon nitride crystal that was experimentally observed. Simulations by the Ostwald ripening model demonstrated that it could be developed when the liquid phase was super-saturated, and further that the tip shape was a function of the liquid concentration.