Lanthanide-based luminescent hybrid materials.

Sol-gel chemistry of transition metal oxides

The sol-gel process, with its associated mild conditions, offers a new approach to the synthesis of composite materials with domain sizes approaching the molecular level. Transparent organic-inorganic composites can be prepared by dissolving preformed polymers into sol-gel precursor solutions, and then allowing the tetraalkyi orthosilicates to hydrolyze and condense to form glassy SiO, phases of different morphological structures. Alternatively, both the organic and inorganic phases can be simultaneously formed through the synchronous polymerization of the organic monomer and the sol-gel precursors. Depending upon such factors as the structures of the organic and inorganic components, the phase morphology, the degree of interpenetration, and the presence of covalent bonds between the phases, the properties of these composites can vary greatly and range from elastomeric rubbers to high-modulus materials.

Hybrid nanocomposite materials―between inorganic glasses and organic polymers

Concepts for the incorporation of inorganic building blocks into organic polymers on a nanoscale

The hydrolysis and condensation reactions of monomeric alkoxysilanes and organylalkoxysilanes utilized in sol-gel processing are reviewed. Both reactions occur by acid or base-catalyzed bimolecular displacement reactions. The acid-catalyzed mechanisms are preceded by protonation of OH or OR substituents attached to Si, whereas under basic conditions hydroxyl or silanolate anions attack Si directly. Many of the observed structural trends are understood on the basis of the pH and [H2O] dependence of the hydrolysis, condensation, and dissolution reactions.

Hydrolysis and condensation of silicates: Effects on structure

In the sol-gel synthesis of silica, a silicon-containing raw material, a solvent, water, and a catalyst are utilized. By varying the catalyst, dramatic effects on gelation time, porosity, bulk and apparent density, and volume shrinkage on drying were observed. For example, porosities ranging from two to sixty-eight per cent can be obtained for dried and fired gels. Mechanisms for catalysis have been proposed for the catalysts considered.

Sol-gel processing of silica. II: The role of the catalyst

Transparent silica gel–polymer composites have been prepared by the impregnation of porous gels with organic monomer and polymerization in situ. The relative amount of each phase was adjusted by varying the porosity of the silica gel prior to impregnation. These materials constitute a new class of transparent composites. Properties, such as density, refractive index, modulus of rupture, compressive strength, abrasion rate, and Vickers hardness, have been measured over the compositional range of 100% silica to 100% polymethyl methacrylate (PMMA).

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S088429140000491X

Transparent silica gel–PMMA composites

Silica glass containing neodymium is a potentially important laser glass because of its low thermal expansion coefficient and high temperature stability. The sol-gel method is highly advantageous in that it permits the uniform distribution of relatively large concentrations of neodymium in a silica host with respect to conventional processing techniques. In this paper, the evolution of the Nd-doped silica gel as a function of heat treatment temperature, from the liquid state to the glassy state, is discussed. The fluorescence spectra for sol-gel derived neodymia-silica glass is presented. A tentative structural model of the local environment around the neodymium ions in silica glass is presented.

Nd-doped silica glass I: Structural evolution in the sol-gel state

In the preparation of oxides by the sol-gel route, it has been observed that the viscosity increases with time. Throughout this process, the molecular weight of species is also increasing. Theoretical models are proposed to elucidate the structural evolution of gels utilizing measurements of viscosity vs. time dependencies. Two models, one for nearly linear growth and another for fractal growth, are presented.

Theoretical modelling of the structural evolution of gels

Neodymia-doped glasses are currently commercially available for use in laser systems. Typically, these glasses are low-softening-temperature borates, phosphates, silicates, and fluorides. With its high softening temperature, high thermal shock resistance, and low nonlinear index of refraction, silica may be an ideal candidate for a neodymia bost glass for high-power laser applications. Conventional glass processing methods, however, have thus far succeeded in incorporating only low levels of neodymia in silica. In this paper, the fabrication of neodymia–silica glasses with high neodymia content by the sol-gel route is described. A possible structure for neodymia in silica is discussed.

Edward J. A. Pope

Papers

Sol-gel processing of silica. II: The role of the catalyst

Transparent silica gel–PMMA composites

Nd-doped silica glass I: Structural evolution in the sol-gel state

Theoretical modelling of the structural evolution of gels

Sol-gel processing of neodymia-silica glass