Eric P. Bescher

STRUCTURES, PROPERTIES AND POTENTIAL APPLICATIONS OF ORMOSILS

Ormosils are organic-inorganic hybrid solids in which the organic component may be chemically bonded to a silica matrix. Somewhat similar to inorganic silicate glasses, the structure of the silica network can be modified by the presence of organic groups. The resulting properties of the Ormosils are then governed by the type and concentration of organics used. Examples are presented in which the mechanical, electrical and optical properties of selected Ormosils can be influenced by organic groups. For instance, small amounts of polydimethylsiloxane (PDMS) added to a solution of TEOS will give an Ormosil about ten times harder than the hardest organic polymer. Larger amounts of PDMS (20%) will now yield an Ormosil which is as rubbery as organic rubber. Ormosils in which the organic and inorganic constituents are covalently bound to each other are the focus of this critical review. The potential applications of such Ormosils are discussed.

Physical Properties of Sol-Gel Coatings

One of the most important applications of sol-gel technology is the fabrication of coatings. This is because of the possibility of applying oxide coatings with practically all types of chemical compositions at low ambient temperatures on many substrates of various shapes through the use of liquid solutions. Both oxides and different types of organic-inorganic hybrid coatings have been reported. Both oxides and hybrid coatings are usually amorphous at ambient temperatures but some oxides can be converted to the crystalline phase with heating. Regardless of the intended applications of the coatings their physical properties are always of importance. For instance, an anti-reflective coating for an automobile mirror is of little practical value unless it is fairly scratch-resistant. In this review which covers published information in the past fifteen years, some of the more important results of physical properties of sol-gel derived coatings are discussed firstly for oxides and then for organic-inorganic hybrids. It appears that properties such as the hardness of oxide coatings are inadequate unless the heat-treatment temperatures are in excess of about 400°C. The hybrid coatings, especially when they contain a dispersed phase of a hard solid like colloidal silica, can be processed at temperatures below about 150°C and can improve the performance of organic plastics such as the polycarbonates. There is insufficient scientific understanding of the relationship between physical properties and other interdependent variables such as processing conditions, chemistry and coating thickness. More research in this area will undoubtedly contribute to the availability of better and new coatings via the sol-gel approach.

Some factors governing the coating of organic polymers by sol-gel derived hybrid materials

Organic-inorganic hybrid materials can exhibit some properties of organic polymers, such as toughness and elasticity, and/or that of ceramics, such as chemical stability and hardness. In this review, we discuss the main factors that should be considered when coating a polymeric substrate with a sol-gel derived organic-inorganic hybrid material. The effects of the solution characteristics, the polymer substrate chemistry and preparation, the application process and materials characteristics are considered. Examples of commercial and published systems are discussed. We find that due to the wide diversity of the systems investigated, it is difficult to be specific in recommending guidelines applicable to all systems. However, some general considerations can be made that should be useful in the design of functional hybrid coatings aimed at improving the characteristics of polymeric surfaces.

Synthesis of PDMS-based porous materials for biomedical applications

Polydimethylsiloxane (PDMS) and tetraethoxysilane (TEOS)-based porous organically modified silicates (ORMOSILs) for biomedical applications were synthesized through a sol-gel process, using sucrose particles as templates. These materials were characterized by 29Si CP-MAS NMR spectroscopy, thin film X-ray diffraction, and scanning electron microscopy. Their bioactivity was evaluated using a simulated body fluid (SBF) of Kokubo recipe. These materials had a bimodal porous structure with pores of 300–500 μm and 10–50 μm in diameter. NMR showed that the silanol groups of the PDMS chain cross-linked to silica derived from the hydrolysis and condensation of TEOS. The samples containing Ca(II) exhibited apatite deposition on the pore walls within 3 days in SBF.

Sol-gel coatings for the protection of brass and bronze

The effectiveness of sol-gel Ormosil coatings as barriers coatings has already been demonstrated, and it is natural to assume that such coatings can play a unique role in art conservation, where object corrosion and decay are often a major issue. The main feature of ormosil coatings that would make them preferable to polymers is their potentially higher stability to ultra-violet radiation, controlled porosity and good adhesion to many different substrates. The permeability to various gases can also be tailored with changes in the chemical structure. In previous work, we have applied the sol-gel process as part of a multiplayer coating in the conservation of the Last Judgment mosaic in Prague. In the present work, we explore the use of sol-gel organic-inorganic hybrid coatings on various copper alloy substrates frequently encountered in art conservation.

New Lutetium Silicate Scintillators

Cerium-doped lutecium orthosilicate (LSO) is the most promising scintillator discovered in almost five decades. It exhibits a unique combination of important properties for x and gamma-ray spectroscopy: high density, fast decay, and large light yield. However, the practical use of LSO is hindered by difficulties related to its fabrication as a single crystal by the Czochralski method. We report on the usefulness of the sol-gel process in obtaining lutecium silicate scintillators. Upon appropriate drying and firing, lutetium silicate crystals can be grown in a silica matrix. The bulk, polycrystalline transparent scintillators are characterized by XRD, optical absorption, light decay measurement and gamma-ray spectral response. Their properties are comparable to that of traditional LSO single crystals.

Long-Term Protection of the Last Judgment Mosaic in Prague

A sol-gel coating was used in the long-term protection of the 14th Century mosaic situated above the gates of St. Vitus cathedral, in the center of Prague Castle. The choice of a sol-gel coating is the result of a research effort involving both on-site testing in Prague and accelerated ageing of various polymers and sol-gel materials in the laboratory. The coating selected for treatment of the entire 13 m × 10 m mosaic is a multi-layer system in which an organic-inorganic sol-gel layer is placed between the glass substrate and a fluoropolymer coating. This coating combination performed very well in an accelerated weathering chamber as well as during onsite testing. Since the top layer is scheduled to be removed and re-applied periodically, it is hoped that this concept will allow long term protection to the largest outdoors mosaic north of the Alps. On-site application of the coatings on the entire mosaic started in 1998 and will be completed in 2000.

New low temperature multiphase ferroelectric films

This article describes the low-temperature synthesis of new multiphase ferroelectrics containing an inorganic ferroelectric phase entrapped in amorphous silica or in an organically modified silicate (ormosil). Sol gel derived LiNbO3 and BaTiO3 crystals were grown in SiO2 and in RSiO1.5 glass where R contains a chromophore (TDP) insensitive to hydrolysis and condensation reactions. The LiNbO3–SiO2 and BaTiO3–SiO2 compositions as well as the TDP–LiNbO3–SiO2 and TDP–BaTiO3–SiO2 ormosils exhibit ferroelectric-like properties. This unusual characteristic is due to the presence of small, partially ordered crystallites of the ferroelectric, dispersed in the amorphous matrix. In addition to their ferroelectric properties, the ormosils also exhibit interesting optical characteristics: the TDP–BaTiO3–SiO2 materials are red, whereas the TDP–LiNbO3–SiO2 are yellow. The materials described in this article are representative of two new classes of weak ferroelectrics. In the first class, a ferroelectric is dispersed in ...

A new technique for the measurement of the impact resistance of wall coatings

A new technique for the quantitative assessment of the impact resistance of thick plaster or cement coatings on walls is presented. The technique uses a new device that measures the energy absorbed by the coating during the impact of a steel ball striking its surface. The energy absorbed is proportional to the height of rebound of the steel ball. Not only is it possible to measure the damage caused to the coating, but the technique also allows a quantitative evaluation of the extent of damage per unit of energy absorbed. These measurements can help in predicting the operational lifetime of wall coatings. Some results are presented for various types of wall coatings.

Rare-Earth Doped, Low Hydroxyl Organically Modified Silicates

Low-hydroxyl organically modified silicates (ORMOSILS) were fabricated by a nonhydrolytic reaction between methyl-modified silicon halides and tertiary butyl alcohol under controlled atmosphere. The gels were characterized by DTA and TGA., FTIR, {sup 1}H NMR and {sup 29}Si NMR. The solutions were doped with anhydrous ErI{sub 3}, which was dissolved in the sol prior to gelation. Er{sup 3+} luminescence in the gels was observed at 570 nm under excitation at 488 nm.