Bulent E. Yoldas

Effect of molecular separation on the hydrolytic polycondensation of Si(OC2H5)4

Abstract The course of network forming reactions during the hydrolytic polycondensation of alkoxysilanes and metal alkoxides is determined primarily by chemical encounter rates and the diffusion process. The host liquid and the molecular separation of the interacting species in the host liquid play a fundamental role in shaping the molecular structure and molecular size distribution in these cases. In this work, the molecular separation concept is discussed, and the effect of molecular separation during the hydrolytic polycondensation of Si(OC 2 H 5 ) 4 in ethanol on the molecular make-up of the resultant organosiloxane polymers is presented. The effect of the concentration of the reacting species, i.e., Si(OC 2 H 5 ) 4 and H 2 O in the solutions is distinguished from the effect of concentrating the solution. The concentration of water-rich siloxane solutions leads to significant molecular size expansion by oxide network formation. No similar polymer size growth occurs during the concentration of alcohol based solutions. This differences in the polymeric activities can be related to the difference in the terminal bonds under the two different conditions.

Technological significance of sol-gel process and process-induced variations in sol-gel materials and coatings

A significant aspect of sol-gel technology is the capability it provides to affect the substructure of materials by controlling the nature and the kinetics of chemical reactions. This capability allows us to produce novel materials, design unique molecular and pore morphologies, circumvent high-temperature reactions, and modify material properties. The modifications include strongly thermodynamic-dependent high-temperature properties such as sintering, crystallization, and viscosity in glass and ceramic materials. A particularly exciting area for investigation is the optical-electronic field, where a significant dependence of electro-optical properties and photosensitivity on process-induced molecular-structural variations occurs. Understanding the basis for the creation of structural variations in sol-gel processes should have significant impact on the technologies and systems that use these materials. In this article, some fundamental aspects of alkoxide-based, sol-gel processes and thermochemical bases for process-induced structural variates are discussed.

Thermochemically induced photoluminescence in sol-gel-derived oxide networks

Abstract Chemical and thermal processes can lead to the creation of photoluminescence centers in sol-gel-derived oxide systems. Photosensitivity commences upon heating the gel to ∼ 300–350°C, coinciding with the chemical bond cleavage temperatures, and diminishes with heat treatments about ∼ 650–700°C in air. Photoluminescence absorption peaks occur at ∼ 360 and ∼ 225 nm (3.4 and 5.5 eV) in binary Al 2 O 3 SiO 2 . The emission spectra cover the entire visible spectrum, peaking at ∼ 425 and ∼ 500 nm (2.9–2.5 eV). A strong phosphorescence is also associated with these centers. Luminescence lifetime measurements show relatively long decay time, and indicate at least two mechanisms for the phosphorescence. Electron paramagnetic resonance measurements indicate paramagnetic states which are attributed to paramagnetic states of oxygen, carbon and E′ centers. The initial observations are consistent with the model that photosensitivity is related to chemical bond cleavage and resultant carbon formation and/or non-stoichiometry. Coordination states appear to play a significant role in the alumina system.

Process-induced variations in sol-gel derived oxide coatings

Process parameters introduce a wide range of property and behavior variations in inorganic coatings deposited from metal-organic derived solutions. The modifications occur in the molecular, morphological, and the stoichiometric states and present some unique design opportunities for applications in the optical and electronic fields. The nature of the modifications and the processing parameters that introduce the modifications are discussed for the metal-organic derived oxide coatings.

Modifications of Molecular Size and Structure During the Hydrolytic Polycondensation of Metal Alkoxides

Inorganic materials, such as ceramics and glasses, are traditionally formed by thermal processes. Application of heat has been such an integral part of their formation that the scientific definition of the very materials often refers to this fact, e. g., product of fusion, etc. Recently, it has been demonstrated that inorganic networks of ceramics and glasses can also be formed by low temperature chemical polymerization. In the latter cases high temperature reactions which limit the glass formation can be avoided. More importantly, molecular-structural variations can be introduced into the polymeric structure, allowing property modifications without compositional alterations. These nonequilibrium conditions are found to be quite stable even in relatively high temperatures.