Papers by Author: Tetsuo Uchikoshi

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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.
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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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Authors: Tetsuo Uchikoshi, Tohru Suzuki, Hideo Okuyama, Yoshio Sakka
Abstract: Highly crystalline-textured alumina ceramics were fabricated by electrophoretic deposition (EPD) in a strong magnetic field of 12 T. Preferred orientation of the bulk was controlled by changing the direction of the applied electric field E relative to the magnetic field B during the EPD. Average orientation angle of the prepared monoliths as a function of the angle between the vectors E and B, ϕ B-E was estimated from the X-ray diffraction analysis. Alumina/alumina laminar composites with crystalline- oriented layers were also fabricated by alternately changing the ϕ B-E layer by layer during EPD in a magnetic field of 12 T.
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Authors: Tetsuo Uchikoshi, Seiichi Furumi, Tohru Suzuki, Yoshio Sakka
Abstract: Cathodic electrophoretic deposition of bimodal alumina suspension was performed using conductive polypyrrole (Ppy) film as an electrode. The coating of the Ppy on nonconductive ceramic substrates was performed by polymerization of pyrrole (Py) in an aqueous solution. The relative green density of the deposits measured by Archimedes’ method was 68 %. Alumina ceramics were obtained by sintering the deposits together with the Ppy coated ceramic substrates in air.
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Authors: Sanjukta Dey, Sarama Bhattacharjee, Tapan K. Rout, Dilip K. Sengupta, Tetsuo Uchikoshi, Laxmidhar Besra
Abstract: Non-aqueous solvents have been popularly used as suspending medium in Electrophoretic deposition (EPD) for several applications. The drawbacks of these solvents are that they cause environmental concerns and are expensive. Therefore, there is a great interest for the development aqueous EPD, which is low-cost and environmental friendly. There are several problems when using water as the suspending medium in EPD. First, there is gas evolution at the electrodes on application of the electric field. Hydrogen is generated at cathode and oxygen at anode. This results in incorporation of bubbles in deposits and the quality of deposit suffers. Several approaches, such as the use of pulse DC, asymmetric AC, and palladium electrode have been reported in the literature to overcome this problem.However, the biggest problem concerning aqueous EPD is reproducibility of deposition. As an example, in batch EPD of Al2O3 from aqueous suspension, we observed the best quality of deposition from the first deposit. Invariably, the amount and quality of deposit decreased progressively with increased number of deposition from the same suspension. For prolonged first deposition, the deterioration became severe and no deposition occurred from the second deposition onwards. This occurred even for cases when only a small fraction of the powder suspension is depleted in the first deposit. A closer look showed destabilized suspension and significant change in bulk pH of the suspension. Here, we measured the change in bulk pH as a function of time, and starting pH during water electrolysis as well as during aqueous EPD of Al2O3. The bulk pH of suspension increased with increasing time of deposition and with increasing applied voltages, resulting in progressive destabilization of suspension and decrease in deposit yield and quality. Use of suitable dosages of cationic dispersant polyethyleneimine (PEI) in the suspension decreased the extent of bulk pH variation during aqueous EPD.
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Authors: Yoshio Sakka, Tohru Suzuki, Tetsuo Uchikoshi
Abstract: Recently to improve properties, highly microstructure controlled ceramics such as fine-grained, textured and laminated structures are required. We have demonstrated a new processing of textured ceramics with a feeble magnetic susceptibility by colloidal processing in a high magnetic field and subsequent heating. As colloidal processing, slip casting and electrophoretic deposition (EPD) have been conducted successfully. Colloidal processing is known to be a powerful method for consolidating fine particles with a high density and homogeneous microstructure. The degree of orientation strongly depends on the particle dispersion and some processing factors, such as particle size, applied magnetic field, concentration of the suspension, sintering temperature, etc. Crystalline-textured controlled laminated composites can be fabricated using EPD by varying the angle between the vectors of electric field and magnetic field. Also textured ceramics with complicated structure can be fabricated by reaction sintering. The colloidal processing in a high magnetic field confers several advantages and it is possible for this type of processing to be applied to non-cubic ceramics, such as alumina, titania, zinc oxide, tin oxide, hydroxy apatite, aluminium nitride, silicon carbide, silicon nitride, etc. The textured ceramics showed anisotropic properties depending on the crystal plane.
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Authors: Chika Matsunaga, Tetsuo Uchikoshi, Tohru S. Suzuki, Yoshio Sakka, Motohide Matsuda
Abstract: Orientation technique based on the magneto-scientific crystal alignment phenomenon combined with electrophoretic deposition (EPD) technique was applied for the seeding process of zeolite L particles. Well-dispersed, ethanol-based zeolite L suspension was prepared and then consolidated on porous zirconia substrates by EPD. Conducting polypyrrole film synthesized on the zirconia substrate was used as an anodic substrate for the EPD process. The EPD was performed in a superconducting magnet with applying 12 T strong magnetic field to the suspension. The degree of orientation was characterized by XRD and compared with that of the zeolite L layer prepared by slip casting in a 12 T strong magnetic field using the same suspension.
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Authors: Yoshio Sakka, Tetsuo Uchikoshi, Tohru Suzuki
Abstract: Highly structure controlled ceramics were required for improving their properties. Here, we demonstrate such ceramics fabricated by novel colloidal processing; (i) dense nano-sized zirconia without pressure sintering, (ii) porous ceramics with controlled pore size by hetero-coagulated suspension of ceramics and polymer, and (iii) textured ceramics with feeble magnetic susceptibility by electrophoretic deposition in a high magnetic field.
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Authors: Shigeru Horii, Masayuki Sakurai, Tetsuo Uchikoshi, Ryoji Funahashi, Tohru Suzuki, Yoshio Sakka, Hiraku Ogino, Jun Ichi Shimoyama, Kohji Kishio
Abstract: We report the fabrication of p- and n-type thermoelectric oxide thick films laminated by insulating alumina using electrophoretic deposition and their thermoelectric performance. From the experimental studies performed for optimization of the thermoelectric performance in the p- and n-type mono-layers, the control of sintering temperature for densification and the usage of fine powder were effective for reducing the electrical resistivity of thermoelectric layers. These findings could be applicable also to the triple-layered thick films. When one assumes that two triple-layered films of p- and n-type thermoelectric materials are combined as unicouple of thermoelectric module, an estimated maximum output power was 20 times higher than a measured maximum output power of a previously reported multi-layered thermoelectric module. It was found that precise control of the microstructure in the thermoelectric layers is indispensable for development of the thermoelectric modules based on the electrophoretic deposition.
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Authors: Mamiko Kawakita, Tetsuo Uchikoshi, Laxmidhar Besra, Tohru Suzuki, Jin Kawakita, Yoshio Sakka
Abstract: Crystal-oriented and crack-free thin TiO2 films with a good interfacial adhesion on indium-tin oxide (ITO) glass substrates for photoelectrodes were fabricated by the electrophoretic deposition (EPD) method in a 12 T strong magnetic field. A binder-free suspension for the EPD was prepared by dispersing TiO2 in the mixture of 2-propanol and 2,4-pentanedione. The electrophoretic mobility and the sedimentation rate were measured at various ratios of the mixed solution. The optimized state of the suspension exhibiting the highest surface charge potential and producing deposits with the highest green density was obtained at the 50:50 mixing ratio. The TiO2 films were characterized by XRD and SEM analyses. The photo-current measurement was also conducted to investigate the relation between the photo-anode characteristics of a dye-sensitized solar cell and the plane orientation of the TiO2 films
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