John D. Mackenzie

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

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.

Electrical properties of semiconducting oxide glasses

Abstract Studies of the electrical conductivity of semiconducting oxide glasses are reviewed in the framework of Motts theory for such materials. An examination of the conduction processes in semiconducting oxide glasses confirms the applicability of the polaronic hopping model of electrical transport. Some deviation from Motts theory are observed in phase-separated glasses. The thermal activation energy for conduction appears to be the dominating factor which controls conductivity, although in many cases the pre-exponential factor also has a great influence on conductivity. The diffusion-like conduction mechanism in a system of randomly distributed ions is probably not applicable to glasses that exhibit some kind of order in the structure, such as clustering.

Direct calculation of Young's moidulus of glass

Abstract An equation has been derived for the direct calculation of the Youngs modulus of oxide glasses from their chemical compositions. The method is based on a consideration of dissociation energy of the oxide constituents per unit volume and the packing density. For borate glasses, the ratio of four-coordinated to three-coordinated borons must be taken into consideration. Excellent agreement is obtained between calculated and measured values of Youngs modulus for over thirty different glasses.

Calculation of bulk modulus, shear modulus and Poisson's ratio of glass

Abstract Combining Gruneisens equation with our Youngs modulus equation of glass, new formulae were semi-empirically derived for the calculation of bulk modulus, shear modulus and Poissons ratio of glass. Considering packing density of atoms and bond energy in unit volume the elastic moduli of glass can be calculated. The agreements between calculated and observed values of the moduli of glass are satisfactory for more than 30 glasses.

Relation between apparent glass transition temperature and liquids temperature for inorganic glasses

Abstract The ratio of the glass transition temperature T g to the liquidus temperature T l (or T m ) was calculated for a wide variety of inorganic glass forming systems including the elements, oxides and sulfides and technologically important systems such as borates and silicates. It was found that a simple empirical rule T g /T l = 2 3 which was suggested for organic polymers holds suprisingly well for inorganic systems.

Transparent silica gel–PMMA composites

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).

Glasses from melts and glasses from gels, a comparison

Abstract In recent years, an increasing number of glasses have been prepared by the sol-gel method. Many of these glasses have identical chemical compositions to glasses prepared by the conventional method of cooling the melts. The sol-gel glasses do not involve the melting of crystalline mixtures. It is of both scientific and technical interest to make comparisons between the two different processes of preparing glasses which have the same chemical compositions. It is important to inquire in the structure and microstructure of these glasses are similar, and if not, whether some important properties for certain applications are then different. In this paper, published data on one-component and polycomponent sol-gel glasses are utilized for comparison. The advantages and disadvantages of the sol-gel method as compared with the conventional melting method are discussed.

Applications of the sol-gel process

Abstract The advantages and disadvantages of the sol-gel process are reviewed. Successful applications are dependent on sufficient scientific understanding of the various stages of the process. The many factors governing the development of structure and microstructure during gelation and subsequent heating are mentioned. Examples are described for applications in the areas of composites, porous solids and coatings. Opportunities for many new applications are presently available for exploitations.

Ferroelectric thin films prepared by sol-gel processing

Abstract Ferroelectric thin films are playing a growing role as key elements in variety of devices. For various techniques of ferroelectric thin film preparation, the sol-gel processing is one of the most promising. In comparison with different deposition techniques, we survey the intrinsic advantages and the recent improvements of the sol-gel processing. In this review paper, several interesting topics, including epitaxial growth and grain-orientation, symmetric and asymmetric P-E hysteresis loops, heterojunction effect of the interface between film and substrate, to attest pyroelectricity of the films, electrooptic coefficients in poled and unpoled films, possibility of amorphous ferroelectrics, etc., are introduced.

Structural evolutions from polycarbosilane to SiC ceramic

The pyrolysis process of a polycarbosilane into a microcrystalline silicon carbide ceramic has been followed up to 1700 ° C mainly by means of solid state29Si and13C nuclear magnetic resonance, transmission electron microscopy and X-ray diffraction analysis. A structural model has been proposed for the amorphous silicon carbide phase that is formed during the pyrolysis process. The ceramic obtained at high temperature is formed by a mixture of β-SiC and α-SiC; however, some difficulties in the identification of the crystalline phases have been pointed out.