Takashi Maekawa

Nuclear magnetic resonance studies of alkaline earth phosphosilicate and aluminoborosilicate glasses

The 11B, 27Al, 29Si and 31P magic angle spinning (MAS) NMR spectra of MO–P2O5, MO–SiO2–P2O5 and MO(M′2O)–SiO2–Al2O3–B2O3 (M=Mg, Ca, Sr and Ba, M′=Na) glasses were examined. In binary MO–P2O5 (M=Ca and Mg) glasses, the distributions of the phosphate sites, P(Qn), can be expressed by a theoretical prediction that P2O5 reacts quantitatively with MO. In the ternary 0.30MO–0.05SiO2–0.65P2O5 glasses, the 6-coordinated silicon sites were detected, whose population increases in the order of MgO<CaO<SrO≒BaO. In xCaO–0.05SiO2–(0.95−x)P2O5 glasses, its population increases with an increase in f (=([P2O5]−[MO]−[B2O3]−[Na2O])/[SiO2]) and has maximum at f=9. The signal due to the 5-coordinated silicon atoms is also observed when x is smaller than 0.45. When three network-forming oxides such as SiO2, Al2O3 and B2O3 coexist, Al2O3 reacts preferably with MO. The populations of 4-coordinated boron atoms, N4, are expressed well with r/(1−r), where r=([Na2O]−[Al2O3])/([Na2O]−[Al2O3]+[B2O3]). The correlation of the Raman signal at 1210 and 1350 cm−1 with the NMR signal of Si(Q6) at −215 ppm is also seen.

Structural studies of 30Na2O–5SiO2–65[(1 − x)P2O5–xB2O3] glasses by nuclear magnetic resonance, Raman and infrared spectroscopy

Abstract The 29 Si, 31 P and 11 B magic angle spinning (MAS) NMR spectra of 30Na 2 O–5SiO 2 –65[(1xa0−xa0 x )P 2 O 5 – x B 2 O 3 ] glasses were examined together with Raman and IR data. A correlation of the Raman signal at 1200 cm −1 or IR signal at 660 cm −1 with the 29 Si NMR around −215 ppm is seen. The 6-coordinated silicon atoms are observed in glasses when x is smaller than 0.250. The 3-coordinated boron atoms begin to appear in glasses with x xa0>xa00.500. It is assumed that the phosphate species were made up of six types: O=P– O 3/2 (P), O=P–(ONa) O 2/2 (P), (NaO)–P– O 3/2 (T), (NaO) 2 –P– O 2/2 (T), (NaO) 3 –P– O 1/2 (T) and P– O 4/2 (T), where O is bridging oxygen and T is either B, P or Si. The distribution curves derived from the deconvolution of the spectra by component signal of each phosphate and borate species agree with those calculated using species with different [Na 2 O]/[P 2 O 5 ] ratios when P 2 O 5 reacts with Na 2 O and B 2 O 3 preferably.

Influence of solvents on the structure of SiO2 gel from hydrolysis of tetramethylorthosilicate

Abstract The effects of solvents on bulk density, specific surface area and pore size distribution of alkoxy-derived silica dry gel were investigated. Bulk densities of dry gel prepared using solvents of high dipole moment such as N,N-dimethylformamide (DMF), ethylene glycol (EG), formamide (FA) and ethyleneglycolmonomethylether (EGM) decreased with increasing amount of solvent. The specific surface area of the gels using EGM and EG increased monotonically with an increase of their content, whereas it decreased drastically by small addition and then increased when DMF or FA were used. These results were rationalized in terms of competition between the promotion of polymerization and the inhibition of coalescence among primary particles in the presence of organic solvent.

Oxidation of aromatic hydrocarbons with H2O2 catalyzed by a nano-scale tubular aluminosilicate, Fe-containing imogolite

Imogolite is a nano-scale tubular clay mineral with the typical chemical composition (OH)3Al2O3SiOH. The Fe-containing imogolite was synthesized from FeCl3, AlCl3 and Na4SiO4 aqueous solutions. The fibrous morphology observed by AFM in this sample is similar to that of synthetic imogolite. In the catalytic oxidation of aromatic hydrocarbon compounds such as benzene, phenol and chrolobenzene, the aromatic ring was oxidized. When the side-chain is a hydrocarbon group such as methyl, both benzene ring and the side-chain group were oxidized. It was found that the side-chain group was preferentially oxidized.

Electrochemical studies of the redox behavior of antimony ions in sodium borate and silicate melts

Abstract An electrochemical study of antimony ions doped in soduim borate and silicate glass melts was investigated by differential pulse voltammetry at various temperatures, by which Sb 5+ ue5f8Sb 3+ redox reactions were observed. The half wave potentials for reduction of Sb 5+ to Sb 3+ shifted toward negative values with increasing Na 2 O content of the glass-forming liquids and with decreasing temperature. Substitution of Na 2 O by CaO in sodium silicate liquids shifted the half wave potential to the positive side. These results correspond to an increase of the content of pentavalent antimony ions with an increase of the basicity of the glass-forming liquids.

Electrochemical study of Fe ions in alkali borate melts

Abstract Half wave potentials of reductions of Fe3+ in alkali borate glass liquids were determined by a differential pulse voltammetry. The experimental i-E curves were fitted by a theoretical curve calculated by a one-electron reaction on a working electrode. The half wave potentials become negative with decrease of temperature and increase of alkali oxide when they exceed 20 mol%. The half wave potentials of 30A2O · 70B2O3 samples (A = Li, Na, K, Rb) become positive with decrease of the ionic radius of the alkali ions. When alkali oxide is less than about 20 mol%, the data can be fitted by two reduction peaks, i.e. the Fe3+-containing complex and free Fe3+ to Fe2+.

Preparation and optical properties of ZnS-microcrystals deposited in silica gels

Abstract ZnO doped silica gels were synthesized by the hydrolysis and condensation of complex solution tetraethyl orthosilicate (TEOS) and zinc acetate (Zn(Ac) 2 ) and heating at 500°C in the air. For conversion to ZnS doped gels, they were exposed to H 2 S gas at appropriate temperatures. The crystal size of ZnS determined by the width of X-ray diffraction pattern and direct transmission electron micrograph (TEM) grows from 1 to 4 nm with an increase of reaction temperature with H 2 S gas. The edge energies of optical absorption shifted to higher energy side compared with that of bulk crystal and reciprocally increased in proportional to the square of the crystal size. The peak energy of fluorescence spectra also shifted to lower energy side with an increase of reaction temperature exposing to the H 2 S gas. Thus, the quantum size effect could be found for ZnS microcrystals in the gels.