Subliquidus Immiscibility in Binary Alkali Borates

The phase boundaries of subliquidus immiscibility in the lithium, sodium, potassium, rubidium, and cesium borate systems were determined. Two-phase structures were developed in thermal gradients and examined by direct-transmission electron microscopy. No opalescence was observed. Microstructures were of the order of a few hundred angstroms. The extent of immiscibility in each system was determined by the approximate intersection of the immiscible phase boundary with the glass transition region.     

Development of Two-Phase Structure in Glasses, with Special Reference to the System BaO-SiO2

Vapor deposition was used to obtain homogeneous glass films of several BaO-SiO₂ compositions in the region of the metastable miscibility gap. The films were electron-beam heated in the electron microscope, and the phase separation process was observed. At low BaO contents, separation was characterized by the appearance of discrete, amorphous, second-phase particles. For compositions near the center of the miscibility gap, separation resulted in two continuously interconnected phases similar to those in the corresponding bulk glasses. In the thinnest of these films, the process proceeded by the coalescence of isolated particles into the interconnected second-phase submicrostructure. At high BaO contents, rapid crystallization of the films prevented study of the separation process. Two processes which could lead to coalescence of discrete second-phase particles into an interconnected submicrostructure are discussed. It is suggested that coalescence may occur when either of these processes results in the particles approaching within some small distance of each other. It is indicated that under appropriate conditions, this requirement may be satisfied by either process. These conclusions, together with the present observations of BaO-SO₂ thin films and those previously reported for bulk samples, indicate that observations of final phase-separated morphologies are inadequate for specifying the processes by which those morphologies arose.     

Behavior of Bubbles in Glassmelts: III, Dissolution and Growth of a Rising Bubble Containing a Single Gas

Finite difference solutions of the mass transport equations governing the dissolution (growth) of a rising gas bubble, containing a single gas, in a glassmelt were obtained. These solutions were compared with those obtained from an approximate procedure for a range of the controlling parameters. Applications were made to describe various aspects of O₂ and CO₂ gas-bubble behavior in a soda-lime-silicate melt.     

Phase Separation in the System BaO-SiO2

A simple, rapid-quenching device was used to investigate the extent of liquid-liquid immiscibility in the system BaO-SiO₂. A metastable miscibility gap was found with an upper consolute point at about 10 mole % BaO and 146OOC. Bulk glass samples of different compositions and heat treatments were examined by direct-transmission electron microscopy to determine the phase separation morphology. At low BaO contents the separation was characterized by spherical barium-rich particles in a silica-rich matrix. For compositions near the center of the gap, the structure consisted of two phases continuously interconnected throughout the bulk of the glass. At higher BaO contents, spherical silica-rich particles were found in a barium-rich matrix. The dependence of the morphology on quench rate and heat treatment was investigated. Predictions of other authors concerning the occurrence of BaO-SiO₂ immiscibility are compared with the present results.     

Consequences for sun protection factors when solar simulator spectra deviate from the spectrum of the sun

The sun protection factor (SPF) of two products, one with an expected SPF of 4 and another with an expected SPF of 15 were determined, using two solar simulators: Multiport Solar UV Simulator (xenon, Solar Light, Philadelphia, PA, USA), and Supersun 5000 (metal halide, Mutzhas, Munich, Germany). The mean SPFs using the Multiport were: 4.8 for the low SPF product and 19.4 for the high SPF one. The results using the Supersun were lower: 2.6 for the low SPF product and 7.2 for the high SPF one. Relative emission spectra of the two sources were recorded using a fluorescence spectrophotometer in bioluminescence mode. Efficacy spectra were calculated and compared with the corresponding spectrum of natural sunlight. It was evident that the spectral power of the xenon source is too high in the UVB, leading to overestimation of SPFs, whereas the Supersun irradiator emits too much in the UVA, resulting in too low SPFs. Heat effects and photodegradation of UV filters are discussed as further possible reasons for the discrepancies between the experimentally determined SPFs. Our results confirm a recent publication about theoretical SPFs, calculated with emission spectra of a xenon source and spectra of the sun at different elevation angles, where the authors provide evidence that in natural sunlight the contribution of UVA to total UV radiation is twice as high as in a xenon source. This may contribute to an understanding of why sunscreens tested according to the FDA method (xenon sources) often yield higher SPFs than those obtained from European testing procedures.     

Gelation of Aluminum Hydroxide Sols

The gelation of aluminum hydroxide sols obtained from aluminum sec -butoxide has been studied as a function of the conditions of preparation, including pH, temperature, and time of aging. The preceding studies of Yoldas have been extended in more acidic conditions, up to a molar proportion of 1.12 HNO₃ per Al(OC₄H₉)₃. It has been found that stable sols can easily be obtained by processing at any temperature between 20° and 90°C beyond a molar proportion of 0.28 HNO₃. The corresponding gels are suggested to have a boehmite-like structure in which there is a metastable folding of the (020) layers at 20°C, and a stable unfolding at 80°C, the two structures coexisting at intermediate processing temperatures.     

DOMAIN DECOMPOSITION WITH BEM AND FEM

The Finite Element Method and the Boundary Element Method are two different structure analysis methods with a totally different numerical character. Therefore, it makes no sense to couple these two methods pointwise at the interface. In contrast to a lot of coupling strategies in the past, in this paper a method is constructed where we have coupling of the two different methods in a weak form. As a result we can analyse the given structure with two different grids independent of each other. On this account, we see that the big advantage of the proposed method is in its ablity to couple BEM and FEM. The construction of a robust and reliable numerical algorithm depends on the adaptive control of symmetry and definiteness of the coupling matrix. Therefore, we use an iterative method for solving the boundary integral equation by expanding the Calderon projector in a Neumann series. Numerical results show the preciseness and efficiency of the method. © 1997 John Willey & Sons Ltd.     

Transmission Electron Microscopy of Thin Glass Samples

Mechanical thinning and fracturing techniques for preparing thin glass samples for direct-transmission electron microscopy are discussed. A modification of the Doherty and Leombruno procedure for mechanically thinning ceramic materials is described. These techniques make possible more reliable electron microscope studies of fine-scale submicrostructure in glass systems. Electron microscope observations on fused silica, an alkali borosilicate glass, and some binary silicate glasses are reported and discussed in terms of present understanding of glass structure.     

Glass Formation, a Contemporary View

The process of glass formation is discussed from several perspectives. Particular attention is directed to kinetic treatments of glass formation and to the question of how fast a given liquid must be cooled in order to form a glass. Specific consideration is paid to the calculation of critical cooling rates for glass formation, to the effects of nucleating heterogeneities and transients in nucleation on the critical cooling rates, to crystallization on reheating a glass, to the experimental determination of nucleation rates and barriers to crystal nucleation, and to the characteristics of materials which are most conducive to glass formation.