A porous mixed conductive phase consisting of (Ba,Sr)(Co,Fe)O_3-δ (BSCF) was fabricated on a stainless steel-zirconia dual-phase oxygen separation membrane by electrophoretic deposition. To suppress the reaction between stainless steel, zirconia, and BSCF, a gadolinium-doped ceria (GDC) buffer layer was inserted. The GDC buffer layer suppressed the formation of zirconate, a reaction product of BSCF and YSZ, and formed a uniform BSCF layer with a thickness of 10 micrometers. This layer showed good correlated adhesion.

Vanadium nitride (VN_x) powder was synthesized by heating a compact of V₂O₅ and BN powders with Na metal at 600–1100°C under an Ar atmosphere. The process proceeded via a metathesis reaction using BN as the nitrogen source, accompanied by the formation of Na₃BO₃ as a by-product (3V₂O₅ + 5BN + 15Na → 6VN_0.83 + 5Na₃BO₃). For the samples heated at or above 900°C, nearly single-phase VN_x powder was obtained after washing with water to remove the by-product. Rietveld analysis and elemental analysis revealed that the product was nitrogen-deficient VN_x with x = 0.79–0.88, and the particle size increased from 0.5–1 µm (900°C) to 2–5 µm (1100°C) with increasing heating temperature. The molten Na₃BO₃ by-product acted as a flux, promoting both the formation and grain growth of VN_x. Furthermore, reacting a V₂O₅ compact with a BN crucible at the contact interface produced coarse, octahedral euhedral crystals of VN_x (10–50 µm). This method allows for the synthesis of VN_x powders and crystals at lower temperatures than conventional methods and is effective for controlling both nitrogen composition and particle size.

Binary phase diagrams of Group 4 (Ti, Zr, Hf), Group 5 (V, Nb, Ta), and Group 6 (Cr, Mo, W) transition metal carbides were calculated using the CALPHAD method with FactSage and Thermo-Calc software. The calculated results were compared with classical experimental phase diagrams compiled by Suzuki, referencing data from Storms and Rudy. The calculated diagrams showed good agreement with experimental data regarding liquidus shapes and invariant reaction temperatures, such as eutectic and peritectic points, across all systems. However, discrepancies were observed in the homogeneity ranges of MC-type carbides (Group 4 and 5); the calculated single-phase regions tended to be narrower and shifted toward stoichiometric compositions compared to experimental reports. Additionally, differences were noted in the stability of specific phases, such as ζ-phases in V-C and Nb-C systems, which are often stabilized by impurities in experiments, and the treatment of certain carbides as line compounds in databases. These results suggest that while thermodynamic databases are powerful tools for predicting phase equilibria, careful consideration of solid solution ranges and phase modeling assumptions is necessary for material design. Furthermore, future challenges, including improving homogeneity range accuracy, evaluating impurity effects, and refining phase modeling for order-disorder transformations, are identified to enhance database reliability.

In this study, Ti₃SiC₂, a nanolayered ternary carbide, which shows superior oxidation resistance than carbon and hexagonal-boron nitride (h-BN), was selected as the interphase for SiC_f/SiC composites, and Ti₃SiC₂ coating was formed on SiC fibers by the electrophoretic deposition (EPD) method. The SiC_f/SiC composites with the Ti₃SiC₂ interphase with the different thickness were prepared, and the effect of the Ti₃SiC₂ interphase on their mechanical properties was discussed. Whereas the SiC_f/SiC composites using uncoated SiC fibers showed brittle fracture behavior without fiber pullout and interfacial debonding, the SiC_f/SiC composites with Ti₃SiC₂ interphase exhibited pseudo-ductile fracture behavior with the large amount of fiber pullout and interfacial debonding. Bending strength and fracture energy of the SiC_f/SiC composites with Ti₃SiC₂ interphase were much higher than those of the SiC_f/SiC composites without Ti₃SiC₂ interphase. From the results of mechanical properties of the SiC_f/SiC composites with Ti₃SiC₂ interphase varying the thickness, it was suggested that the Ti₃SiC₂ interphase thickness affected their bending strength and fracture energy. It was demonstrated that Ti₃SiC₂ interphase was successfully formed on SiC fibers by EPD method, the SiC_f/SiC composites with excellent mechanical properties were achieved by controlling the Ti₃SiC₂ interphase thickness, and Ti₃SiC₂ was effective as the interphase for SiC_f/SiC composites.

This study investigated the effects of alumina particle properties, including particle size, aspect ratio, and circularity, on the packing structure, relative density, and high-temperature electrical resistivity of filler materials for immersion heaters used in aluminum melting. Natural and vibration filling tests showed that the relative density of single coarse particles strongly depended on particle properties, and particles with high circularity exhibited relative densities close to the random packing limit. Porous compacts prepared by filling the interstices between coarse particles with fine particles showed relative densities as high as 80%. LN2D analysis revealed that the dispersion state of the coarse particles governed the packing structure, and the relative density showed a parabolic relationship with LN2D variance. High-temperature volume resistivity depended more strongly on particle dispersion and cluster formation than on relative density itself. Particles with large variations in size, aspect ratio, and circularity produced tortuous conduction paths and improved electrical insulation performance. These results indicate that particle-property design is important for optimizing the high-temperature insulation performance of immersion-heater fillers.

A liquid-phase carbon coating for inorganic particle functionalization has been developed using a novel soluble carbon material (SCM). Conventional carbon materials suffer from poor solubility and strong aggregation, limiting their use in thin and uniform coatings on fine particles. SCM, designed via bottom-up carbonization of phloroglucinol as precursor which was high reactive oxygen-containing aromatic compound, exhibits high solvent solubility and a two-dimensional nanocarbon structure. In the solvents, SCM spontaneously adsorbs onto inorganic particle surfaces through electrostatic and coordination interactions, forming an ultrathin, uniform, and mechanically stable carbon layer, typically thinner than 1 nm. The coating process is self-limiting at nearly a monolayer level, enabling precise thickness control. Post-heat treatment allows tunable carbonization, providing continuous control over surface polarity and electrical conductivity. This method is applicable to a wide range of inorganic materials, including metals, oxides, nitrides, and carbides, across nano- to micrometer-sized particles. Representative applications demonstrate improved dispersibility, lubricity, powder flowability, packing density, and sintering behavior, highlighting the broad potential of this technology in advanced materials processing.

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.

Densification enhancement and property improvement must be achieved in tandem, and controlling the dispersion state of sintering aids is crucial for this purpose. In this study, we propose a new addition method in which a sintering-aid component is uniformly coated onto the target particle surfaces at the nanoscale to suppress segregation within the sintered body. LiTa₂PO₈ (LTPO) was synthesized by a solid-state reaction, and an MgO coating precursor solution was prepared via a metal alkoxide process. LTPO–MgO core–shell particles ([MgO]/[LTPO] = 0.05 and 0.1) were obtained by wet coating process. These powders were sintered by spark plasma sintering (SPS), and after a decarbonization treatment, the microstructure, relative density, and electrochemical properties were evaluated by SEM/elemental mapping, and electrochemical impedance spectroscopy (EIS). The coating-based addition suppressed local Mg segregation and produced a denser microstructure with fewer pores than that obtained by conventional powder mixing, resulting in a marked increase in densification. A maximum relative density of 99.2% was achieved, while the electrochemical performance was optimized at [MgO]/[LTPO] = 0.05, where the grain-boundary resistance was most significantly reduced and the total lithium-ion conductivity reached 4.6 × 10^-5 S cm^-1. These results demonstrate that combining nano-coating for sintering-aid dispersion control with SPS is an effective approach to simultaneously realize densification and improved grain-boundary ionic transport.

ZrO₂, TiO₂, and TiO₂-ZrO₂ composite oxide gels prepared by the sol-gel method underwent mechanical milling treatment, and their crystallization behaviors and structures were analyzed in comparison to heat treatment. Oxide gels in the TiO₂-ZrO₂ system exhibited the formation of high-temperature, high-pressure phase ZrO₂ and Ti_1+xZrO_2(2+x) crystals through milling treatment, which were not formed by heat treatment. These results suggest that mechanical milling of sol-gel-derived gels is a method of high potentiality to obtain novel or high-performance functional oxides at ambient conditions.

Synchrotron X-ray computed tomography (CT) in SPring-8 was utilized to analyze the size and shape distribution of internal defects in Laser Powder Bed Fusion (L-PBF) manufactured Inconel 738LC. This technique enabled the detection of defects smaller than 10 μm, which are considered difficult to detect with laboratory high tube voltage X-ray CT. This was achieved due to the high signal-to-noise ratio of synchrotron X-ray CT. Based on CT image, the defects were classified according to Feret diameter and sphericity. Defects consisted only of lack of fusion (LOF) when the laser energy density was less than 40 J/mm³. These LOF defects were characterized by a Feret diameter between 3 mm and 0.2 mm, and a low sphericity less than 0.3. LOF disappeared and defects consisted of cracks and gas pores when the energy density is higher than 50 J/mm³. Cracks were characterized by a Feret diameter between 0.3 mm and 10 μm, and a sphericity of 0.3-0.7. Gas pores were characterized by a Feret diameter between 30 μm and 5 μm and a higher sphericity of 0.6-0.8. Appropriate laser energy density was 56 J/mm³ to obtain an additively manufactured body with the highest relative density (99.6%) without LOF.

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.

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.

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.

Aerosol (Gas) Deposition (AD or AGD) has evolved as a film deposition technique capable of forming dense films at room temperature without heating and melting ceramic powders. This paper reviews the current understanding of the film formation mechanism in AGD (including AD), focusing particularly on alumina film formation. It outlines the effects of raw powder characteristics, deposition conditions, and advancements in deposition equipment on film structure and electrical properties.

The room-temperature film deposition mechanism has been explained by particle fragmentation and plastic deformation. However, phenomena such as plasma emission, the formation of atomic-scale bonding interfaces, and the generation of zirconia’s high-temperature phase suggest the involvement of electrostatic induction plasma and sputtering processes arising from particle charging and discharge. Building on these findings, a target-based AGD (T-AGD) method was developed, where particles collide with a target to generate reactive species, successfully forming highly insulating alumina films. Furthermore, combining this with a masking technique suppressed large particle contamination, demonstrating the feasibility of forming large-area, highly reliable insulating films. As a room-temperature film deposition technique integrating powder engineering and thin-film technology, the AGD method holds promise for further mechanism elucidation and application development.

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.

The ultra-high-temperature sintering (UHS) method, developed by Wang et al. in 2020, utilizes carbon felt as a resistive heating element to achieve temperatures up to 3000°C within seconds. Since its introduction, UHS has been widely adopted as a superior sintering technique capable of achieving rapid densification. However, new applications leveraging its unique utility as a general heat treatment method are increasingly being reported. This paper reviews three topics conducted by our research group. WC-Co cemented carbides were fully densified in just 65 seconds at 1400°C. This rapid process successfully suppressed cobalt volatilization via insulation between carbon felts and green compacts. Microcracks on yttria-stabilized zirconia were completely healed within 20 seconds, facilitated by accelerated neck formation driven by rapid heating. A conductive TiO layer was formed on (001) surface of SrTiO₃ single crystals via UHS. The strong reducing atmosphere generated by the carbon felt induced selective strontium volatilization, creating a metallic TiO surface phase. These findings demonstrate that the UHS method is a powerful tool not only for rapid densification but also for advanced materials processing.

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.

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.