E.M. Rabinovich

Multicomponent glasses from particulate gels

Abstract The principles of making multicomponent glasses from particulate gels are discussed and the applications of these principles in the synthesis of bulk samples of low-alkali high-silica glasses containing up to 6 mol% Li 2 O, K 2 O, Rb 2 O or Cs 2 O and up to 3% Al 2 O 3 are described. Several samples were also doped with Er 3+ and Nd 3+ . A feasibility study directed at the preparation of entire-gel optical fiber performs was conducted. It is shown that the impregnation of the inner surface of a low-alkali gel tube with Al 3+ - and Er 3+ -containing solutions can yield, after sintering, a glass tube with sharp compositional and refractive index profiles. The results of this study indicate that bulk samples of previously unavailable multicomponent glasses can be fabricated from particulate gels. The boron-free glasses prepared in this study have low thermal expansion (between silica and Pyrex®), relatively low T g (750–800°C) and softening temperatures and may be considered for applications in planar waveguide technology based on silicon.

Raman study of the structure of sodium aluminosilicate gels

Abstract The structure of sodium aluminosilicate gels of various composition, prepared by sol-gel methods using TEOS and nitrates, was studied with Raman spectroscopy. Some of these gels, 12.5Na2O12.5Al2O375SiO2 (albite) and 7.5Na2O7.5Al2O385SiO2, can be easily densified into glasses by heat treating them at 1100°C. The similarity between Raman spectra of the gel-derived and melt-derived glasses, in particular the absence of the non-bridging-oxygen peak at 1095 cm−1, shows that for the albite composition the structure consists of a 3D-network of interconnected AlO4/SiO4 tetrahedra with the negative charge of the AlO4 groups compensated by the sodium ions. The network structure of the glass phase is already present in the gel phase at temperatures as low as 500°C. For other compositions, such as 15.5Na2O15.5Al2O369SiO2 , densification of the gel into a glass is not possible because of crystallization during the heat treatment.

Raman study of the gel-to-glass conversion of fluorine-doped silica

Abstract Raman spectroscopy was used to investigate the structural and chemical changes of F-doped silica gels as a function of sintering temperature and atmosphere. The results show that, regardless of the initial gelation conditions, most SiF groups, that are initially present in the dried gel, are lost from the gel after heating to 1000°C. Since SiF groups are eliminated by reaction with SiOH groups, higher concentrations of fluorine can be retained when SiOH groups are removed by adding Cl2 to the sintering atmosphere. For the glasses, that are formed after heating at 1400°C, the highest F concentrations are found when SiF4 is present during densification.

Phase Transformations in ZrO 2 -SiO 2 Gels

Transparent sols and gels in the ZrO 2 -SiO 2 system containing 25, 50 and 75 mole % ZrO 2 were prepared using tetraethyl orthosilicate (TEOS) and Zr n-propoxide solution in the presence of acids. Heating the gels to 600°C yielded an amorphous product which at higher temperature crystallized with formation of tetragonal and monoclinic ZrO 2 . Zircon crystallized slowly at 1600°C. The results are explained by metastable liquid-liquid phase separation in the studied system.

Energetics, Composition, and Structure of Alkoxide Derived Silica Gels

Acid and base catalyzed alkoxide silica gels are studied by simultaneous DSC/DTGA under oxidizing and oxygen-free conditions and by intrared spectroscopy. The results are utilized to determine the composition of the dried gels and to estimate the extent of the net hydrolysis reaction. Our results suggest that, with sufficient water, condensation of partially hydrolyzed TEOS is the dominant reaction at high pH whereas, at low pH hydrolysis is rapid, relatively complete and precedes condensation. Thus the dry gels formed at high pH, with sufficient water, contain more alkoxy groups and fewer hydroxyl groups bound to Si than the dry gels formed at low pH.