Takahiro Takei

Preparation of microporous silica from metakaolinite by selective leaching method

Microporous silica was prepared by selective leaching of metakaolinite, derived from the 1:1 layered clay mineral kaolinite (2SiO2Al2O32H2O) by firing at 600 °C for 24 h. Treatment with 20 mass% H2SO4 solution at 90 °C for 0.5–5 h with stirring, led to a decrease in the Al2O3 content and an increase in the specific surface area (SA) with increasing leaching time. The adsorption-desorption isotherms of N2 measured at 77 K show a type I curve, the average pore size, calculated by t-plot method, being about 0.6 nm and the maximum SA about 340 m2g−1. The 29Si MAS NMR spectrum of the 2 h leached sample showed peaks at −101 and − 110 ppm, which can be assigned to the layer structure unit (Q3) and the SiO4 tetrahedral framework structure unit (Q4), respectively. It. is suggested that the SiO4 tetrahedral structure of the microporous silica basically maintains the original kaolinite and is different from the general framework structure of silica gel.

Crystallization kinetics of mullite from Al2O3-SiO2 glasses under non-isothermal conditions

Abstract The kinetics of mullite crystallization from Al2O3–SiO2 glasses with chemical compositions ranging from 15 to 50 mol% Al2O3 were investigated under non-isothermal conditions using DTA. The glasses were prepared by ultra-quenching of molten droplets formed at the tip of a sintered aluminosilicate rod by an infra-red imaging furnace quenched by a twin-roller system. All the samples showed an exothermic peak at about 1000°C associated with mullitization. This crystallization temperature increased slightly with decreasing Al2O3 content. The exothermic peak split into two peaks at a glass composition of about 25 mol% Al2O3. This splitting is thought to be related to phase separation of the glass preceding mullitization. The activation energies for the nucleation-growth mechanism of mullitization, calculated by the Kissinger method, were 900–1300 kJ/mol, in good agreement with those for glass fibers obtained under isothermal conditions (Takei, T., Kameshima, Y., Yasumori, A. and Okada, K., Crystallization kinetics of mullite in alumina–silica glass fibers. J. Am. Ceram. Soc., 1999, 82, 2876–2880).

In situ zeolite Na–X coating on glass fibers by soft solution process

Abstract A soft-solution process for in situ coating of faujasite type zeolite crystals on glass fibers was investigated. Commercial SiO2–Al2O3 glass fibers (SA fiber) and SiO2 glass fibers (SO fiber) were reacted at 60–150°C for various times with NaOH solutions containing NaCl as a mineralizer. The surface of glass fibers was initially leached by the solution but after a certain time crystals of faujasite type zeolite precipitated on the fibers. Eventually the fiber surface was completely coated because of an increase in the concentrations of Si and Al in the solution due to dissolution of the fibers, especially the SO fibers. The important experimental parameters for formation of faujasite type zeolite were found to be the mixing ratio of SA fiber/SO fiber, the reaction time, NaOH concentration and the amount of added NaCl. The lattice constant of the resulting faujasite type zeolite identified it as zeolite Na–X with a Si/Al ratio of 1.28.

Preparation of CaSiO3 whiskers from alkali halide fluxes

Abstract CaSiO3 whiskers were prepared from fine CaSiO3 powder by use of alkali halide fluxes, NaCl, KCl, and their mixture. The CaSiO3 powder and the flux were mixed and heated at temperatures from 850 to 1000°C for 1 to 48 h. The average lengths, diameters and aspect ratios ranged from 3.24 to 10.5 μm, 0.33 to 1.12 μm and 7.1 to 21.7, respectively. The diameter of the whiskers increased with increasing heating temperature more rapidly than the length, so that the aspect ratio decreased with increasing temperature. The whiskers were considered to grow by an Ostwald ripening mechanism.

In-Situ Coating of Zeolite Na-A on Al2O3-SiO2 Glass Fibers

In-situ coating of zeolite Na-A crystals on Al2O3-SiO2 glass fibers was investigated by a low temperature chemical process. The glass fibers were reacted with various concentrations of NaOH solutions at 60–110°C for various times. The surface of glass fibers was first leached by the solution but crystals of zeolite Na-A precipitated on the glass fibers after a certain reaction time because the concentrations of Si and Al components in the solution increased by dissolution of the glass fibers. The precipitated zeolite was identified to be Na-A type but nosean-cancrinite-type phase and/or hydroxysodalite coexisted with zeolite Na-A at longer reaction times. With higher concentration of NaOH solution, the formation rate of zeolite Na-A was faster, the formation temperature was lower and the grain size was smaller ca. 2–3 μm. Dense zeolite Na-A coatings were produced when the glass fibers were reacted in 4 M NaOH solution at 60°C for 18 h. The amount of zeolite Na-A formed on the fibers was about 20 mass%.

SAXS analysis of textures formed by phase separation and crystallization of Al2O3–SiO2 glasses

Abstract Textures formed by phase separation and crystallization of Al2O3–SiO2 glasses with 15, 25 and 50 mol% Al2O3 compositions were investigated by small angle X-ray scattering (SAXS) and TEM. From the Porod analysis of the SAXS patterns, droplet structures were found in glasses with 15 and 50 mol% Al2O3 compositions, whereas an interconnected structure was formed by spinodal decomposition in the glass with 25 mol% Al2O3. In the initial-stage crystallization of mullite, nucleation was considered to occur in the Al2O3-rich regions. The crystallites of mullite were of similar size to the phase separated region. The relationship between the annealing time and the size of the interconnected texture formed by spinodal decomposition was analyzed using Cahns theory, giving good agreement with the result obtained by the Porod method. The distance distribution of the phase separated grains was obtained from the radial distribution function calculated by Fourier transformation of the SAXS patterns. The calculated intergrain distances increased with increasing annealing time and were in good agreement with those obtained from TEM observation.

Pore Structure and Thermal Stability of Mesoporous Mullite Fibers Prepared by Crystallization and Selective Leaching of Al2O3-SiO2 Glass Fibers

Porous mullite fibers were prepared by crystallization and selective leaching of Al2O3-SiO2 glass fibers using buffered HF-NH4F(BHF) aqueous solutions. The optimum concentration of BHF solution for selective leaching of the fibers was 0.9 mass% HF and 17.0 mass% NH4F. By firing at 1000–1300°C, the glass fibers changed into composite texture of mullite and glassy phase. Since the pores in the fibers were formed by selective leaching of glassy phase among mullite grains, they were tunable by changing the firing conditions of fibers. Pore size of the samples changed from around 4 nm in the 1100°C fired sample to 16 and 40 nm in the 1200 and 1300°C fired samples, respectively. The highest specific surface area obtained was around 30 m2/g, when the fibers were heat treated at 1200°C for 24 h and leached for 20 h in 0.9 mass% HF-17.0 mass% NH4F solution. From the thermal stability tests of the porous mullite fibers, its specific surface area was found to be maintained up to 1200–1300°C.