Papers by Author: Hidehiko Tanaka

Abstract: Synthesis of titanium carbide ceramic powders and control of particle size were tried by carbothermic reduction of a precursor, comparing synthesis of silicon carbide from a precursor. Metal alkoxides were used as a metal source and phenolic resin was done as a carbon source. Homogeneous sol-like mixture was obtained by mixing ethylsilicate and phenolic resin with alcohol. Gel-like precursor was formed by drying the sol-like mixture. Precipitation was formed by mixing titanium(IV) tetraisopropoxide and phenolic resin. The precursor was amorphous and ceramization occurred by heat treatment in argon flow. Growth of particles and phase formation were investigated.

Abstract: Nano sized SiC powder was successfully synthesized by the carbothermal reduction in SiO2. Precursor for SiC was prepared by using phenolic resin as a carbon source and ethylsilicate as a silicon source. After mixing, hydrolysis, drying and pyrolysis at 1000°C, SiC precursor consisted of C and SiO2 was obtained. The precursor was heat treated at 1500-1800°C in Ar to synthesize SiC by the carbothermal reduction. The carbothermal redction reaction was almost completed at 1700°C and then SiC particle with suitable size was obtained at this temperature. Nano-sized SiC particles could be achieved at 1600 °C, and unreacted SiO2 and C remained in the sample. Pure SiC particles were obtained by oxidation and acid treatment. Nano-sized SiC powder had the diameter of 10-20 nm and BET surface area of 156 m²/g.

Abstract: - and -SiC powder mixtures with 0.3 mass% of AlB2 were sintered at 2200oC. Porous materials having 43-45% porosity were obtained. Grains in the powder mixtures grew into plate-shaped grains of 6-8 m size, and -SiC transformed into -SiC. Free energy due to the polytype transformation enhanced material transport very much. The new rate theory proposed here explained very well the behavior of the grain growth.

Abstract: Single-phase and hexagonal plate-like Al4SiC4 powder was successfully synthesized using a mixture of Al(OH)3, SiO2 and carbon via a carbothermal reduction process. Two-step reaction was compartmentally observed during the calcination; the carbothermal reduction of SiO2 between 1300 and 1600oC and that of Al2O3 above 1500oC. The mechanisms which were responsible for the synthesis of the ternary carbide (Al4SiC4) powder were discussed.

Abstract: A commercial silicon nitride powder with sintering additives was ground by high-energy milling to reduce particle size. Nanometer sized powder was obtained. The powder was densified for short time by spark plasma sintering to prevent grain growth. Nanometer-grained Si3N4 ceramics were obtained. Plastic deformation of the Si3N4 nano-ceramics has been studied in compression over a wide range of strain rates and temperatures. The experimental results revealed that a transition in stress exponent, n, at each temperature. The n value decreased from ~ 2 to ~ 1 with increasing applied stress. Activation energy was also different for the two regions, decreasing from 858.2 kJ/mol in the n ~ 2 region to 571.8 kJ/mol in the n ~ 1 region. Effect of sintering additives on plastic deformation was also discussed.

Abstract: Diffusion phenomena in solid particles were analyzed with the new material transport concept. It was assumed that total excess free energy in a system acted as a driving force for material transport so that the system changed to an equilibrium state. The new rate equation was adopted to analyze shape change, sintering and growth of grains. It was found that surface energy or ratio of grain boundary energy to surface energy was key factor for shape changes in these processes.

Abstract: We report sintering additive systems to decrease the densification temperature of the corrosion resistant AlN-SiC-TiB2 system. Since oxide additives degrade the high temperature properties of the system, and other kinds of metallic additives may affect the formation of protective mullite during oxidation, only the constituent elements were applied as additives. Dense samples ( > 98 % relative density) could be fabricated at 1850 oC and 1900 oC by spark plasma sintering (SPS) and hot pressing method, respectively.

Abstract: In previous report, we succeeded in preparing dense nano-sized ceramics with the composition of Y- α-sialon (m=1.35, n=0.675) by high-energy mechanical milling followed by spark plasma sintering. The superplastic deformation of the obtained nano-ceramics was studied in this report. A good ductility of the nano-sized ceramics has been confirmed, which arised from the nano-sized grains and large amount of transient liquid phase. The effects of deformation on the phase and microstructural evolution were also studied. The nano-sized grains promoted the formation of elongated α-sialon grains during post-annealing at 1850oC for 3 h, which would strengthen and toughen the deformed ceramics.

Abstract: Y- α-sialon (m=1.35, n=0.675) ceramics were prepared by high-energy mechanical milling followed by spark plasma sintering. The milling promoted not only liquid-phase sintering, but also phase transformation from β-Si3N4 to α-sialon. Under the same holding time of 5 min, milled powder could be completely densified at 1500oC, which is about 250oC lower than that required for as-received powder. The temperature where the phase transformation finished was 1600oC and 1750oC for milled and as-received powder, respectively. The grain size of obtained dense ceramics from milled powder was significantly decreased. Nano-sized dense ceramics have been obtained by sintering the milled powder at 1500oC for 5 min. Although 100 % α-sialon has not been achieved, the nano-sized ceramics can be used for superplastic deformation, taking advantage of small grain size and large amount of transient liquid phase.

Abstract: We sintered α (6H)- and β(3C)-SiC powders using an Al-B-C additive. SiC powders were densified to > 98% of the theoretical density from 1950 to 2150oC with 0.67-2.7 mass % AlB2 and 2.0 mass % C. Sintering temperatures are 150-200 oC lower than the conventional. During sintering, 6H polytype in α-SiC powder was partly transformed to 4H. α-SiC powder grew moderately into plate-shaped grains. β-/SiC powder was completely transformed to 6H and subsequently 4H with large grain growth. Low-temperature sintering enabled us to use hot isostatic pressing resulting in pore-free SiC materials.