This paper reviews the history of the author’s research on fine-particle processing conducted primarily at national research institute, and provides an overview of the conception of research themes, interdisciplinary collaboration, and the development of process technologies. Through research based on fine-particle synthesis and colloidal processing, the author has accumulated insights into the reactivity of powders, the control of agglomeration and dispersion of fine powders, and various sintering processes and the creation of nanocomposites. Furthermore, the author has systematized a diverse range of processing technologies—such as particle orientation, laminate structure, control of higher-order structures, and high-speed sintering techniques utilizing external fields such as strong magnetic fields, electric fields, and stress fields—and applied them to the creation of multifunctional ceramics. In addition, drawing on experience in fostering researchers, building academic communities, and conducting international collaborative research, the author emphasizes the indispensability of perspectives that transcend existing disciplinary boundaries, interdisciplinary integration, and the continued pursuit of basic and foundational research with a long-term vision.

Superplasticity is the ability of polycrystalline solids to exhibit exceptionally large tensile elongation at high temperatures, governed primarily by grain-boundary sliding and relative motion of fine grains. While widely exploited in metals for near-net-shape forming, ceramics have long been regarded as intrinsically brittle. This view was overturned by the first discovery of superplasticity in zirconia in 1985, followed by demonstrations in many fine-grained ceramics, including zirconia/alumina composites, silicon nitride, etc. Despite early expectations for improved ductility and forming rate, industrial adoption has remained limited because most ceramic components can be manufactured efficiently by conventional powder forming and sintering. In parallel, sintering theory has advanced by reinterpreting densification as high-temperature deformation driven by sintering stress, linking creep/superplasticity concepts with continuum mechanics. Recent progress in 3D synchrotron X-ray tomography, large-scale simulations, and emerging sintering technologies motivates a multiscale reconstruction of sintering theory. This article highlights selected topics closely related to the author’s research and a new graduate-level textbook on sintering fundamentals.

This review consolidates current understanding of particle surface–water interactions and their significance in powder processing and forming technologies. It examines the fundamental relationship between hydroxyl group density and chemical states on silica surfaces and the adsorption behavior of water molecules, emphasizing their critical role in particle adhesion phenomena. Building on these principles, the review discusses the development of non-fired solidification techniques based on mechanochemical surface activation and outlines future directions for surface engineering in powder-based systems.

When a green compact is heated while an electric field is applied, a phenomenon occurs wherein the power dissipated into the material increases abruptly at a specific threshold temperature dependent on the field strength. This power surge is called as the flash event, and the technique utilizing this phenomenon to accelerate densification is termed flash sintering. Pioneered in 2010 by Professor Raj’s group at the University of Colorado¹⁾, this method initially prompted research focused primarily on confirming its practical utility and achieving process optimization^2-5). As the research field expanded globally, investigators began reporting unique phenomena driven by non-thermal effects induced during the flash state. Consequently, current scientific interest has shifted toward the detailed exploration of these novel physical phenomena and the fundamental elucidation of their underlying mechanisms. To comprehensively understand these mechanisms, it is essential to transcend conventional macroscopic evaluations, such as densification rates and grain growth behavior. Instead, rigorous structural characterization and the precise evaluation of local electronic states at the nano/atomic scales have become indispensable. This review provides a comprehensive overview of our recent investigations into microstructural evolution and point defect formation under flash conditions, elucidated through advanced transmission electron microscopy.

We focused on the unique properties of microwave irradiation, namely rapid and selective heating, and applied them to the hydrothermal synthesis of ceramic particles, including zeolites from coal fly ash, spherical mesoporous silica and pudding-like ZnO microcrystals. Microwave irradiation enables rapid molecular-level heating of solutions, significantly reducing reaction times compared with conventional hydrothermal synthesis. Moreover, microwave-assisted hydrothermal processes are believed to enhance solute dissolution more effectively than conventional heating. These characteristics can lead to distinctive effects, such as enhanced crystal growth, changes in the resulting crystalline phases, and particle size reduction. Because microwave absorption depends on the dielectric loss coefficient of the solvent, selective heating can be achieved. Utilizing this feature, we developed spherical particles and unique ZnO crystals by selectively heating droplets in a water-in-oil (W/O) emulsion under microwave irradiation. This paper reports these results.

Particle breakage occurs frequently in powder processes; however, direct observation of instantaneous particle-scale phenomena remains difficult. This difficulty of observation leads to a lack of understanding of breakage mechanisms. Consequently, simulation-based analysis has gained attention as an effective approach.

Two modeling approaches exist: particle replacement models and bonded element models. While particle replacement models offer computational efficiency, they require prior knowledge of breakage patterns. Bonded element models represent particles as assemblies of fine elements connected by bonds, calculating the bond stresses and strains to simulate breakage. Although computationally demanding, they can analyze breakage mechanisms with fewer assumptions.

Existing bonded element models face a challenge: they require many parameters (typically 7–8) that are difficult to determine. To address this, the authors developed the Cross Bond Discrete Element Method (XB-DEM). XB-DEM simplifies the bonding model by focusing on the roles of bonds, reducing parameters to only two: spring constant and maximum elongation. It was confirmed that these parameters can be determined through a compression test. Furthermore, XB-DEM was validated by comparing particle breakage behavior under different conditions with corresponding experimental results. Therefore, it is expected that such developments in particle breakage modeling will contribute to the advancement of powder process analysis.

Ceramic materials are indispensable for high-temperature structural applications because of their high melting points and excellent thermal stability. Owing to the difficulty of melt-processing, most ceramics are fabricated via powder metallurgy. Although the increasing availability of nanosized powders has enabled reduced sintering temperatures and enhanced properties through nanocomposite design, powder refinement also introduces critical challenges, such as poor handling during forming and difficulty in achieving uniform mixing of dissimilar particles. In addition, conventional mechanical mixing generally produces only uniformly dispersed microstructures, restricting the development of anisotropic or hierarchical architectures. This review summarizes recent progress in electrostatic assembly of ceramic particles as a wet-processing strategy. By controlling particle surface charge via polyelectrolyte adsorption and Layer-by-Layer modification, oppositely charged particles can be assembled into composite particles or spherical, monodisperse composite granules. These granules exhibit high flowability and packing ability, significantly improving processability while eliminating the need for mechanical mixing. Furthermore, the use of designed composite and core-shell granules enables microstructure control from the nanoscale to the macroscale, including the formation of three-dimensionally interconnected functional layers. Electrostatic assembly thus provides a versatile platform for advanced ceramic composite design beyond the capabilities of conventional powder metallurgy routes.

In wet forming processes such as slip casting and tape casting, slurry properties strongly influence green body density and final product performance. Apparent viscosity is commonly used as an indicator of particle dispersion and is generally assumed to correlate with green body density. However, cases have been reported in which the slurry with the lowest apparent viscosity does not produce the highest density. This study investigates the relationship between slurry flowability (apparent viscosity) and packing characteristics, including the packing density of sedimentation layers and filter cakes, to clarify their correlation with green body density.

In slip casting, the packing density of filter cakes obtained by constant-pressure filtration showed a stronger correlation with green body density than apparent viscosity. This is attributed to changes in medium conditions during casting, where unadsorbed dispersants and binders are partially removed into the mold, altering interparticle interactions. In tape casting, the packing density of sedimentation layers more reliably predicted green sheet density because drying-induced concentration changes modify particle interactions.

These findings demonstrate that flowability and packing characteristics are not interchangeable and should be evaluated separately when optimizing slurries for specific wet forming processes.

The fabrication of eutectic ceramics has been recently explored as one application of flash sintering technology. The present paper provides a brief overview of the fabrication of eutectic ceramics by flash sintering. Our group has fabricated two types of Al₂O₃-based eutectic ceramics—Al₂O₃-GdAlO₃ (GAP) and Al₂O₃-8 mol% Y₂O₃-stabilized ZrO₂ (8YSZ)—using flash sintering technology. Using flash sintering technology for melting and rapid solidification, Al₂O₃-GAP eutectic material was fabricated at a furnace temperature more than 300°C below the eutectic temperature, in which rod-like GAP phases grew unidirectionally within Al₂O₃ single crystals with an interphase spacing of 170 nm. In addition, an Al₂O₃-8YSZ eutectic material with a fine lamellar structure and a phase spacing of 135 nm was obtained at a low furnace temperature of 600°C. The phase spacing in the Al₂O₃-8YSZ eutectic material was determined by the cooling rate and followed the relationship between phase spacing and cooling rate observed in conventional eutectic solidification microstructures. The flash sintering technology established through these studies has the potential to become a new method for obtaining fine eutectic microstructures under furnace temperature conditions well below the eutectic temperature and in a short processing time.

Ceramic materials are very useful functional and structural materials that can be utilized even in harsh environments, but their application field is relatively limited due to its brittle nature and resultant lower reliability. One of the main reasons is that microscopic cracks and damages, which were introduced during machining processing or usage, act as fracture sources, leading to a rapid degradation of their mechanical properties. If those damages causing the degradation could be healed, it would enable to provide not only highly reliable ceramic components but also new material recycling processes aiming re-use of limited material resources. This article reports on a crack healing method of ceramics by employing a strong electric field/current. Using the electric field/current technique, we successfully demonstrated not only the crack healing in the material from microstructural point of view but also recovery of the mechanical properties more than its initial state.