Phase Separated Glasses

Phase separation has almost no effect on the thermal expansion coefficient of glasses. The measured thermal expansion coefficient is a volume average of the thermal expansion coefficients of the two phases present, in much the same way as the density. 

On the other hand, neither Tg nor Td is an averaged property. Each glass transformation will affect the curve independently of the existence of the other, so that each Tg may be observed. Softening of the sample, which determines the value of Td, will be controlled by the more viscous phase if that phase is continuous, le., if the microstructure consists of either a matrix of the more viscous phase with spheres of the lower viscosity phase, or if both phases are interconnected. 

Regardless of the overall shape of the thermal expansion curve, the Tg observed at the lower temperature, or if only one occurs, the lone Tg, will be that of the less viscous phase. It follows that, if two transitions are detected, the lower temperature will represent the Tg of the less viscous phase, while the higher temperature will indicate the Tg of the more viscous phase. 

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Glass-ceramics and phase-separated glasses. Properties,... 

At lower frequencies, orientational polarization may occur if the glass contains permanent ionic or molecular dipoles, such as H2O or an Si—OH group, that can rotate or oscillate in the presence of an appHed electric field. Another source of orientational polarization at even lower frequencies is the oscillatory movement of mobile ions such as Na". The higher the amount of alkaH oxide in the glass, the higher the dielectric constant. When the movement of mobile charge carriers is obstmcted by a barrier, the accumulation of carriers at the interface leads to interfacial polarization. Interfacial polarization can occur in phase-separated glasses if the phases have different dielectric constants. 

Ellison Warrens (1987) have reported NMR results on an atypical phase-separated glass of extreme composition G-309 (Table 5.8) finding... 

Phaseolotoxin, 13 300 Phase-separated glass, 12 578-579 Phase-separated systems, 24 701 Phase separation, 10 765... 

Fig. 5. Plot of logm B and Na normalized release rates vs. the activity of silicic acid for both phase-separated and physically homogeneous glass specimens. All rates were plotted at steady-state conditions. MAGNOX and HLP-31 represent phase-separated whereas LAWA33 and HLP-9 represent homogeneous glass specimens. The behaviour of homogeneous glass includes an inverse relationship between rates and silicic acid activity and a difference between B and Na rates as silicic acid activity increase. The magnitude of the difference between B and Na rates is related the amount of excess Na (see Fig. 3). Relatively faster element release rates ( lOx), identical release rales of Na and B, and independence from activity of silicic acid appears to characterize phase-separated glass.

A nanomaterial can be loosely defined to be any material containing heterogeneity at the nanoscale in one or more dimensions. In the broadest sense, then, the following are nanomaterials phase-separated glasses or crystals with domains in the nanoregime, zeolites and mesoporous materials with pores of nanometer dimensions, clays with nanometer sized alternations of aluminosilicate layers and interlayer hydrated cations, and nanoscale leach layers at the mineral-water interface. 

L. R. Pinckney, Phase-separated glasses and glass ceramics, in Engineered Materials Handbook, Vol. 4, ASM International, Metals Park, OH, 1991, p. 431. 

Figrue 4.3 Micrograph of a phase separated glass with a sphere in a matrix morphology... 

Figrue 4.7 Effect of heat treatment temperature on the scattering of light from a phase separated glass (Data supplied by P. B. McGinnis)... 

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