• Journal of the Korean Ceramic Society
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• v.39 no.3
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• pp.315-320
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• 2002

Treatment effects of dilute hydrofluoric acid (6 wt% HF) on the surface properties of $Al(OH)_3$ were investigated at the molar ratio of F/Al(fluoride/aluminum)=0.15. Temperature and pH variations in the reaction system were recorded to analyze reaction mechanism between $Al(OH)_3$ and aqueous Hf. The reaction of HF to the surface of $Al(OH)_3$ accompanied with a quantity of heat evolution, resulting in increasing temperature of a reactionsystem. And also the reaction was proceeded as transitional state which metastable ${\alpha}-form\;AlF_3{\cdot}3H_2O$ was transferred to insoluble ${\beta}$-form. The resulting ${\beta}-form\;AlF_3{\cdot}3H_2O$ formed by a surface treatment was identified by FT-IR and X-ray diffractormetry. The formation of ${\beta}$-form aluminum fluoride hydrates with diameter less than $1{\mu}m$ on the surface of $Al(OH)_3$ could be visulaized by SEM imgae, making up a coating layer as precipitate-like. The surface whiteness of $Al(OH)_3$ treated with aqueous HF was furthermore increased approximately 6.6% due to the formation of surface hydrates.

• Journal of the Korean Electrochemical Society
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• v.11 no.1
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• pp.11-15
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• 2008

The composite membrane for direct methanol fuel cell (DMFC) was developed using $H_3O^+-{\beta}"-Al_2O_3$ powder and perfluorosulfonylfluroride copolymer (Nafion) resin. The perfluorosulfonylfluroride copolymer (Nafion) resin was mixed with $H_3O^+-{\beta}"-Al_2O_3$ powder and it was made to sheet form by hot pressing. The electrodes were prepared with 60 wt% PtRu/C and 60wt% Pt/C catalysts for anode and cathode, respectively. The morphology and the chemical composition of the composite membrane have been investigated by using SEM and EDXA, respectively. The composite membrane and $H_3O^+-{\beta}"-Al_2O_3$ were analyzed by using FT-IR and XRD. The methanol permeability of the composite membranes was also measured by gas chromatography (GC). The performance of the MEA containing the composite membrane (2wt% $H_3O^+-{\beta}"-Al_2O_3$) was higher than that of normal pure Nafion membrane at high operating temperature (e.g. $110^{\circ}C$), due to the homogenous distribution of $H_3O^+-{\beta}"-Al_2O_3$, which decreased the methanol permeability through the membrane and enhanced the water contents in the composite membrane.

• The Transactions of the Korean Institute of Electrical Engineers C
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• v.49 no.7
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• pp.394-399
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• 2000

The mechanical and electrical properties of pressed and annealed $\beta-SiC-TiB_2$ electroconductive ceramic composites were investigated as a function of the liquid forming additives of $Al_2O_3+Y_2O_3$. Phase analysis of composites by XRD revealed $\alpha$-SiC(6H), TiB2, and (Al5Y3O12). Reaction between Al2O3 and $Y_2O_3$ formed YAG but the relative density decreased with increasing $Al_2O_3+Y_2O_3$ contents. The Flexural strength showed the value of 458.9 MPa for composites added with 4 wt% $Al_2O_3+Y_2O_3$ additives at room temperatures. Owing to crack deflection and crack bridging, the fracture toughness showed 6.2, 6.0 and 6.6 MPa.m1/2 for composites added with 4, 8 and 12 wt% Al2O3+Y2O3 additives respectively at room temperature. The resistance temperature coefficient showed the value of $3.6\times10^{-3},\; 2.9\times10^{-3}\; and\; 3.0\times10^{-3} /^{\circ}C$$^{\circ}C$ for composite added with 4, 8 and 12 wt% $Al_2O_3+Y_2O_3$additives respectively at room temperature. The electrical resistivity of the composites was all positive temperature coefficient resistance(PTCR) in the temperature range of $25^{\circ}C\; to\; 700^{\circ}$.

• Korean Journal of Materials Research
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• v.3 no.3
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• pp.207-214
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• 1993

Glass-ceramic monoliths with an ultra-low thermal expansion coefficient have been synthesized by the sol-gel technique using metal alkoxides as starting materials and dimethyl formamide as a drying control chemical additive. The ternary gels: $Li_2O\cdot Al_2O_3\cdot 2, 4 or $6SiO_2$ were obtained by hydrolysis and polycondensation reactions of metal alkoxides of silicon, aluminum and lithium. To produce cylindrical crack-free gel monoliths, excess water was used to the starting solutions and drying rates were controlled precisely to prevent cracking. In conversion process ,${\beta}$-eucryptite, $Li_2O\cdot Al_2O_3\cdot 3SiO_2$ and P-spodumene with ,${\beta}$-quartz solid solution phase were obtained by heating at the range of 750 ~$1000^{\circ}C$. Above $800^{\circ}C$, the ,${\beta}$-spodumene phase increased while ,${\beta}$-eucryptite phase decreased. The thermal expansion coefficient of the crystallized specimens were -15~ $+5{\times}{10^{-7}}/{\circ}C$ over the temperature range from room temperature to $600^{\circ}C$.

• Journal of the Korean Ceramic Society
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• v.32 no.9
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• pp.1076-1084
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• 1995

A mixture of aluminum isopropoxide (Al(OC3H7i)3) solution and sodium hydroxide (NaOH) solution was hydrolyzed in the range between pH 1~14. the powder obtained from sol-gel process was calcined at several temperatures and crystallization behaviors of various samples were investigated. The hydrolyzed sols of pH 1~6 wre clear, or near clear. On the other hand, powder precipitated from sols of pH 7~14. The sample obtained from pH 3 solution crystallized via complicated route, and $\beta$-Al2O3 and $\beta$"-Al2O3 phases appeared at lower temperature than samples from other pH conditions. And the quantity of remained $\beta$"-Al2O3 phase after heat treatment at 150$0^{\circ}C$ was more than samples from other pH conditions. After sintering, ionic conductivities were 1.3$\times$10-4S.cm-1 to 0.76$\times$10-4S.cm-1.0-4S.cm-1.

• Proceedings of the KIEE Conference
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• 1999.07d
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• pp.1913-1915
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• 1999

The ${\beta}-SiC+ZrB_2$ ceramic composites were hot-press sintered and annealed by adding 1, 2, 3wt% $Al_2O_3+Y_2O_3$(6 : 4wt%) powder as a liquid forming additives at $1950^{\circ}C$ for 4h. In this microstructures, no reactions were observed between $\beta$-SiC and $ZrB_2$, and the relative density is over 90.79% of the theoretical density and the porosity decreased with increasing $Al_2O_3+Y_2O_3$ contents. Phase analysis of the composites by XRD revealed of $\alpha$-SiC(6H, 4H), $ZrB_2$, $Al_2O_3$ and $\beta$-SiC(15R). Flexural strength showed the highest of 315.46MPa for composites added with 3wt% $Al_2O_3+Y_2O_3$ additives at room temperature. Owing to crack deflection and crack bridging of fracture toughness mechanism, the fracture toughness showed the highest of $5.5328MPa{\cdot}m^{1/2}$ for composites added with 2wt% $Al_2O_3+Y_2O_3$ additives at room temperature.

• Journal of the Korean Ceramic Society
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• v.24 no.1
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• pp.23-32
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• 1987

Synthesis of high purity ultrafine Si3N4 and ${\beta}$-Sialon powders was investigated via the simultaneous reduction and nitriding of amorphous SiO2, SiO2-Al2O3 system prepaerd by hydrolysis of alkoxides, using carbonablack as a reducing agent. In Si(OC2H5)4-C2H5 OH-H2 O-NH4OH system, hydrolysis rate increased with increasing reaction temperature and pH. Pure ${\alpha}$-Si3N4 was formed at 1350$^{\circ}C$ for 5 hrs in N2 atmosphere. In Si(OC2H5)4-Al(OC3H7)3-C6H6-H2 O-NH4OH system, weight loss increased as Si/Al ratio decreased. Single phase ${\beta}$-Sialon consisted of Si/Al=2 was formed at 1350$^{\circ}C$ in N2 and minor phases of ${\alpha}$-Si3N4, AIN, and X-phase were existed besides theSialon phase at other Si/Al ratios. The Si3N4 and Sialon powders synthesized from alkoxides consisted of uniform find particles of 0.05-0.2$\mu\textrm{m}$ in diameter, respectively.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.7 no.4
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• pp.658-664
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• 1997

The effect of metal oxides($K_2O, MgO, CaO,Al_2O_3$, and $TiO_2$) on the kinds of pure $SiO_2$ phase, and $SiO_2$ phases in the composition of vitreous porcelain body was investigated. Also, the effect of the ratio $SiO_2$ to $Al_2O_3$ in the composition of porcelain body with stabilized of cristobalite phase was investigated. In the case of the addition of $K_2O, MgO, CaO, Al_2O_3$, and $TiO_2$ to pure $SiO_2$, the major phase was $\alpha$-cristobalite, $\alpha$-cristobalite, $\alpha$-quartz, $\alpha$-quartz and amorphous, respectively. As the ratio of $SiO_2$ to $Al_2O_3$ in the composition of porcelain body was decreased, the stabilization of cristobalite phase was promoted and only the critical value of $SiO_2/Al_2O_3$ ratio that stabilizing the cristobalite phase in it was 68.10/22.75. The addition of $K_2$O, MgO, CaO,Al_2O_3$, and $TiO_2$ to the composition of porcelain body stabilized already did not affect on the formation of $\alpha$-cristobalite phase which degraded the thermal properties of porcelain body, and suppressed the formation of a, $\beta$-cristobalite.

• Proceedings of the KIEE Conference
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• 1998.11c
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• pp.847-849
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• 1998

The effect of $Al_{2}O_{3}$ additives on the microstructure, mechanical and electrical properties of ${\beta}$-SiC+39vol.%$ZrB_2$ electroconductive ceramic composites by pressureless sintering were investigated. The ${\beta}$-SiC+39vol.%$ZrB_2$ ceramic composites were pressureless sintered by adding 4, 8, 12wt.% $Al_{2}O_{3}$ powder as a liquid forming additives at $1950^{\circ}C$ for 1h. Phase analysis of composites by XRD revealed mostly of $\alpha$-SiC(6H), $ZrB_2$ and weakly $\alpha$-SiC(4H), $\beta$-SiC(15R) phase. The relative density of composites was lowered by gaseous products of the result of reaction between $\beta$-SiC and $Al_{2}O_{3}$ therefore, porosity was increased with increased $Al_{2}O_{3}$ contents. The fracture toughness of composites was decreased with increased $Al_{2}O_{3}$ contents, and showed the maximum value of $1.4197MPa{\cdot}m^{1/2}$ for composite added with 4wt.% $Al_{2}O_{3}$ additives. The electrical resistivity of ${\beta}$-SiC+39vol.%$ZrB_2$ electroconductive ceramic composite was increased with increased $Al_{2}O_{3}$ contents, and showed positive temperature coefficient resistance (PTCR) in the temperature from $25^{\circ}C$ to $700^{\circ}C$.

• The Transactions of the Korean Institute of Electrical Engineers C
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• v.48 no.2
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• pp.92-97
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• 1999

The mechanical and electrical properties of the hot-pressed and annealed $\beta-Sic$+39vol.%$ZrB_2$ electroconductive ceramic composites were investigated as a function of the liquid forming additives of $Al_2O_3+Y_2O_3(6:4wt%)$. In this microstructures, no reactions and elongated $\alpha$-SiC grains with equiaxed $ZrB_2$, gains were observed between $\beta-SiC$ and $ZrB_2$, and the relative density was over 97.6% of the theoretical density. Phase analysis of the composites by XRD revealedmostly of $\alpha$-SiC(6H, 4H), $ZrB_2$, and weakly $\beta-SiC$(15R) phase. The fracture toughness decreased with increasing $Al_2O_3+Y_2O_3$ contents and showed the highest of $6.37MPa.m^{\fraction ane-half}$ for composite added with 4wt% $Al_2O_3+Y_2O_3$ additives at room temperature. The electrical resistivity increased with increasing $Al_2O_3+Y_2O_3$contents and showed the lowest of $1.51\times10^{-4}\Omega.cm$ for composite added with $Al_2O_3+Y_2O_3$ additives at $25^{\circ}C$. This reason is the increasing tendency of pore formation according to amount of liquid forming additives $Al_2O_3+Y_2O_3$. The electrical resistivity of the composites was all positive temperature coefficient resistance(PTCR) against temperature up to $700^{\circ}C$.