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Nanostructure Ceramics of Silicon Nitride Produced by Spark Plasma Sintering
Hojo Junichi,Hotta Mikinori 한국분말야금학회 2006 한국분말야금학회 학술대회논문집 Vol.2006 No.1
The nanostructure control of ceramics can be achieved by using fine starting powder and retardation of grain growth. The spark plasma sintering technique is useful to retard the grain growth by rapid heating. In the present work, the change of microstructure was investigated with emphasis on the particle size of starting powder, the amount of sintering additive and the heating schedule. The rapid heating by spark plasma sintering gave the fine microstructure consisting of equiaxed grains with the same size as starting particles. The spark plasma sintering of fine powder was effective to control the microstrucutre on nano-meter level.
Electrical and mechanical properties of cubic ZrO2 stabilized with Sc2O3 and Y2O3
Junichi Hojo,Masato Uehara,Naoya Enomoto,Hiromichi Yasuda 한양대학교 세라믹연구소 2004 Journal of Ceramic Processing Research Vol.5 No.1
It has been reported that Sc2O3- doped ZrO2 has the highest ionic conductivity among zirconia ceramics. The xmol%Sc2O3 ñadded 3YSZ (abbr. xSc3Y; x = 0 to 8) powders were prepared by homogeneous precipitation reaction using urea. A mixture of cubic and tetragonal phases was observed in every xSc3Y sample by XRD, and the amount of cubic phase increased with increasing Sc2O3 content. The ionic conductivity of xSc3Y exhibited a maximum, 0.244 S/cm at x = 6. Although the fracture toughness of xSc3Y decreased with an increase in x, xSc3Y samples were tougher than 8YSZ. The improvement in fracture toughness is due to the fine microstructure compared to 8YSZ and the difference in fracture mode. Sc2O3 -added 3YSZ should be applicable as an electrolyte for high performance solid oxide fuel cells (SOFC) replacing 8YSZ.
Previous aging as a parameter for a liquid phase synthesis of ceramic nanoparticles
Naoya Enomoto,Akihiro Kumagai,Junichi Hojo 한양대학교 세라믹연구소 2004 Journal of Ceramic Processing Research Vol.5 No.4
Particular attention has been paid to the “age” of the starting solution for a liquid phase synthesis of ceramic nanoparticles. That is, starting solutions were prepared and kept in a container for several days and then were used to make a precipitation. Metal oxalates (hydrate) were precipitated from ethanolic solutions of oxalic acid and metal nitrate (hydrate). Aging of the starting solutions resulted in a marked delay of the precipitation. Silica spheres were prepared from ethanolic solutions of tetraethylorthosilicate (TEOS) and ammonia water. Aging of the starting solutions enhanced the size and monodispersibility of the spheres. We infer that the delay in precipitation is due to microscopic homogenization in the solution structure among ethanol, water, and solute species. That is, even when the prepared starting solutions look clear, transparent, and homogeneous, they may involve less-dispersed aggregates of solute/solvent, which would enhance heterogeneous nucleation to reduce the delay.
Effect of soft sonication on starting solution for spherical silica synthesis
Naoya Enomoto,Akihiro Kumagai,Junichi Hojo 한양대학교 세라믹연구소 2005 Journal of Ceramic Processing Research Vol.6 No.4
Monodispersed spheres of silica were prepared with particular attention to the age of starting solutions, and a subtle irradiation (~mW power) of ultrasonic vibration, which we termed soft sonication, was applied to enhance the aging process. Aging effects on the precipitation behavior are as follows, (1) increase of the incubation period for precipitation, (2) increase of the final particle size, (3) improvement of the monodispersibility. The increment of the final particle size as a function of aging time is twice larger in the case of soft sonication than in the case of normal aging. We assume from these results that a starting solution, especially that including an organic solvent with a low dielectric constant, may be inhomogeneous at the microscopic level just after the preparation. In other words, the as-prepared solution may involve some clusters, which may cause unfavorable nucleation to degrade the monodispersibility. It is inferred that these “clusters” can be broken or dispersed by aging for days or weeks. Reduction of the aging period by soft sonication would be due to high frequency (~tens of kHz) and small amplitude (~nm) of the vibration.