Papers by Author: Yoshio Sakka

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Authors: Sea Hoon Lee, Yoshio Sakka, Yutaka Kagawa
Abstract: Hydrolysis and dispersing behavior of aqueous ZrB2 slurry were investigated. The isoelectric point (IEP) of ZrB2 slurry was measured to be pH 6.3. The acid and base slurry underwent rather rapid change of pH after fabrication for one and three hours, respectively. The oxygen content of the boride powder increased slowly with static ageing time in acid and neutral slurries, while the value decreased in base slurry. Application of poly(ethyleneimine) changed IEP of the boride slurry to pH 10.4. Sedimentation test informed that stability of the slurry was improved when increasing the molecular weight of poly(ethyleneimine) to 70,000 and in acid slurry (pH 3).
Authors: Xin Wen Zhu, Tetsuo Uchikoshi, Yoshio Sakka
Abstract: Recently, textured microstructure has received particular interest in the processing of advanced Si3N4 ceramics because of significant improvement in the mechanical properties and thermal conductivity. This work will report a highly textured β-Si3N4 ceramic by aqueous slip casting in a magnetic field and subsequent pressureless sintering, using commercial α-Si3N4 raw powder and a mixture of Y2O3 and Al2O3 as sintering aids. To obtain the well-dispersed Si3N4- Y2O3-Al2O3 slurries, polyethylenimine (PEI) was chosen as a dispersant. Effects of the sintering aids, PEI amount, pH and stirring time on the stability of the Si3N4 slurries were studied. It is shown that PEI is an effective dispersant for stabilizing the Si3N4-Y2O3-Al2O3 slurries that does not show a time-dependent behavior at an optimum pH ≈ 10, compared to the case in the absence of PEI. Using the 30 vol% Si3N4-Y2O3-Al2O3 slurries stabilized with 1.5 dwb% PEI at pH 10, the highly textured β-Si3N4 with 97 % relative density could be obtained by slip casting in a magnetic field of 12 T and subsequent sintering at 1800 oC for 1 h. The textured microstructure is featured by the alignment of c-axis of β-Si3N4 crystals perpendicular to the magnetic field, and the Lotgering orientation factor, f, is determined to be 0.8.
Authors: Xin Wen Zhu, Yoshio Sakka, Shou Hong Tan, Dong Liang Jiang
Abstract: This work will report the deformation behavior of silicon carbide reticulated porous ceramics (SiC RPCs) under three-point bend test. SiC RPCs were fabricated by replication processing using an open-cell polyurethane sponge with cell size of ~ 13 pores per inch (ppi). It is shown that the deformation behavior strongly depends on the loading uniformity and the macrostructure. Using a compliant layer (0.5 mm paper pad), the uniform loading leads to a significant transition in the load-displacement curve of RPCs from the complex saw-tooth shape to the one similar to dense ceramics, despite the presence of some macrostructural flaws and partial clogged cells. However, this dependence of loading uniformity is alleviated by developing highly uniform macrostructure with fewer flaws and clogged pores. Even, this dependence becomes less important as the struts become thicker and stronger, leading to an increase in relative density, accordingly. The bend result of RPCs with highly uniform macrostructure is in excellent agreement with the GA (Gibson and Ashby) model, but the one with un-uniform macrostructure deviates from the GA model. This work shows that the macrostructure plays an important role in deformation behavior of RPCs under bend.
Authors: Keijiro Hiraga, Yoshio Sakka, Tohru Suzuki, K. Nakano
Authors: Keijiro Hiraga, K. Nakano, Tohru Suzuki, Yoshio Sakka
Authors: Keijiro Hiraga, K. Nakano, Tohru Suzuki, Yoshio Sakka
Authors: Yoshio Sakka, Tohru Suzuki, Koji Morita, Keijiro Hiraga
Authors: K. Inoue, Kensuke Sassa, Yoshiyuki Yokogawa, Yoshio Sakka, Masazumi Okido, Shigeo Asai
Authors: Tohru Suzuki, Tetsuo Uchikoshi, Yoshio Sakka
Abstract: The mechanical properties of ceramics materials can be tailored by designing their microstructures. Residual stress is one of the important factors for controlling the crack propagation and consequently improving the mechanical properties. On the other hand, development of the crystallographic orientation even in a diamagnetic ceramic can be controlled by colloidal processing in a strong magnetic field. In this study, alumina/alumina laminar composites with different crystalline-oriented layers were fabricated by EPD in a strong magnetic field in order to control the residual stress using the difference in the thermal expansion of each layer.
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