Gaku Okuma 교수

Sintering is a common process during which nanoparticles and microparticles are bonded, leading to the shrinkage of interstitial pore space. Understanding morphological evolution during sintering is a challenge, because pore structures are elusive and very complex. A topological model of sintering is presented here, providing insight for understanding 3-D microstructures observed by X-ray microtomography. We find that the topological evolution is described by Euler characteristics as a function

Synchrotron X-ray nano computed tomography was used to investigate the microstructural evolution during co-sintering of multi-layer ceramic capacitors (MLCC) consisting of Ni electrodes and BaTiO3 dielectric layers stacked alternately. As the electrode thickness reduced to submicron at the scale of a few particle diameters, the process produced the defect of inner electrode leading to capacitance loss. The discontinuous electrode region contained round holes and irregularly-shaped channels. The

The characterization of the processing-induced defects is an essential step for developing defect-free processing, which is important to the assurance of structural reliability of brittle ceramics. The multiscale X-ray computed tomography, consisting of micro-CT as a wide-field and low-resolution system and nano-CT as a narrow-field and high-resolution system, is suitable for observing crack-like defects with small length and with very small crack opening displacement. Here we applied this power

The characterization of subsurface cracks induced by indentation is a challenge for understanding contact damage, impact, wear, erosion, and abrasion of brittle materials, because the crack pattern observable on the surface is only a part of the total crack system. Here we applied synchrotron X-ray multiscale tomography to observe the morphology of subsurface cracks produced by Vickers indentation in a novel CaO-Al₂O₃-SiO₂ glass-ceramic with plate-like crystals f

The representative volume element (RVE) is a basic concept in the continuum mechanics of sintering of random heterogeneous porous materials. A quantitative determination of its size was performed by using synchrotron X-ray microtomography data of constrained sintering of thin glass film on a rigid substrate. A RVE size is associated with a property of interest; we determined it for relative density, specific surface area, and hydrostatic component of sintering stress. The RVE size was estimated

Abstract Sintering stress and bulk viscosity were derived as functions of relative density from microtomographic images in viscous sintering of glass particles. Three methods were proposed to estimate the sintering stress from relative density, specific surface area, and average of curvature on pore surface, which were directly measured by X‐ray microtomography. The surface energy method gave valid value in the final stage of sintering, while the mixed method gave better estimation in the interm

The uniaxial die compaction and sintering is an important industrial production method for metal and ceramic components. While submicron- and nano-sized powders are finding increasing use for making dense materials with finest grain sizes, the uniformity of microstructure, and then, the reliability of the products depend on the heterogeneity in powder packing structure. Here we applied the synchrotron X-ray multiscale-CT to characterize the complex domain structures, i.e., agglomerates, in powde

Abstract Synchrotron X‐ray multiscale tomography is a three‐dimensional (3D) imaging method that combines microtomography and nanotomography to perform a high spatial resolution measurement with a wide field of view. We show how this method can be used to track the 3D microstructural evolution at multiple length scales in powder processing, sintering, and microfracture of ceramics. Heterogeneity, complexity, diversity, and hierarchical structure of powder compact lead to various types of defects

The mechanical reliability of products must be assured for scaling up and production of complex‐shaped components by spark plasma sintering (SPS) of spray‐dried granules. The evolution of morphologies of pores and defects, which control the mechanical strength, is investigated by using synchrotron X‐ray multiscale tomography during SPS of alumina granules at 1300 °C. While large defects arising from the hierarchical granule packing structure cannot be removed by pressureless sintering, crack‐lik

Sintering is a crucial process in the production of ceramics, where particles are bonded and interstitial pore space is reduced. The morphological evolution during sintering was studied by X-ray microtomography, which reveals a topological model described by Euler characteristics as a function of relative density. Domain coarsening was observed, where the characteristic length increased with densification. Sintering stress and bulk viscosity were derived from microtomographic images and compared