Effect of Phase Separation on Helium Migration in Sodium Silicate Glasses

Helium permeability, diffusion, and solubility were measured in sodium silicate glasses containing 0 to 40 mol% Na₂O. These properties appear to depend on the substructure of the glass, since definite inflections occur in the curves of diffusivity and solubility vs composition at the boundaries of the two-phase region. Similar inflections occur for the activation energy for diffusion and enthalpy of solution at the phase boundaries. These effects are explained by use of a two-component diffusion model within the miscibility gap and a one-component model in the single-phase region.     

Effect of Oxidation State of Iron on Phase Separation in Sodium Silicate Glasses

The effect of the oxidation state of iron on the phase separation of x Na₂O·(100 – x )SiO₂ glasses, x = 18.56 and 13, containing 0.5 mol% Fe₂O₃ was studied. The oxidation state of iron in the glasses was varied by changing the melting conditions, such as melting temperature and melting atmosphere. The oxidation states of the iron ion were determined using colorimetric and UV–VIS–NIR spectrophotometric methods, and a comparison was made between the results obtained using these two methods. Immiscibility temperatures of the glasses were determined using opalescence and clearing methods. The immiscibility temperature of the sodium silicate binary glasses decreased ∼25°C with the addition of 0.5 mol% Fe₂O₃. The immiscibility temperature of the doped glasses increased slightly with increased concentration of Fe²⁺ ion in the glass. The prediction of immiscibility tendency on the addition of a minor amount of third component was made using models proposed by Tomozawa and Obara and Nakagawa and Izumitani. The Tomozawa and Obara model showed good agreement with measured immiscibility values.     

Sodium Motion in Phase-Separated Sodium Silicate Glasses

The thermally stimulated polarization and depolarization current (TSPC/TSDC) technique was used to study Na^plus; ion motion in phase-separated silicate glasses containing between 10.3 and 20 mol% Na₂O. Generally, two TSDC peaks and a high-temperature background (HTB) current were observed. The two TSDC peaks were attributed to different types of Na^plus; motion, i.e. a short-range reorientation process and a longer-range translational motion. Changes in the microstructure affected only the size of the larger TSDC peak located at higher temperatures.     

Limiting Opalescence Temperature in Sodium Silicate Glasses

The calculation of the limiting opalescence temperature is proposed based on a model describing the mechanism of the formation of internal stresses in glass in cooling. The notion of the range of increased mobility of structural elements is introduced, which is identified with the glass softening range.     

Thermally Stimulated Currents in Sodium Silicate Glasses

Sodium-ion motion in three sodium silicate glasses and a sodium aluminosilicate glass was investigated by the thermally stimulated polarization (TSPC) and depolarization (TSDC) current techniques. The two TSDC peaks found in the sodium silicate glasses were attributed to localized sodium-ion movement around a nonbridging oxygen ion, a type of dipolar orientational polarization, and to a longer-range sodium motion leading to interfacial polarization at the immiscible phase boundaries. The high-temperature background (HTB) current corresponded to the sodium motion for dc conductivity and diffusion. The commonly observed dc absorption current was found to be related to the two TSDC peaks.     

Phase Separation and Structural Differences between Alkali Silicate Glasses Prepared by the Sol-Gel and Melt-Quench Methods

Glasses of composition x M₂O:(1- x )SiO₂ [M = Na, Li] with x = 0.15 and 0.2 have been prepared both by the melt-quench and the sol-gel methods. X-ray photoelectron spectroscopy shows that with increasing sintering temperature the structure of gels approaches that of the melt-quench glass but does not become identical even on heating above the glass transition temperature ( T _g). Phase separation occurs in gel much below T _g, and before it evolves into the structure of melt-quench glass. Thus phase separation is achieved more readily in sintered gels than in the corresponding melt-quench glass.     

Light Scattering and Spinodal Decomposition in Glasses

It is shown that the well-known anomalous Rayleigh scattering of phase-separated glasses, which can be accounted for by the assumption of nucleation of a new phase followed by diffusion-controlled growth, can also be qualitatively accounted for by the assumption that the phase separation is occurring within the spinodal region of the phase diagram; here there is no nucleation barrier to phase separation, only a diffusional barrier. A distinction between the two mechanisms in any actual system can be made only by the use of other experimental tools, such as electron microscopy. A quantitative theory of this process proposed by Cahn is examined; it is concluded that light scattering probably reveals only the later stages of the process, rather than the initial stages dealt with by Cahn's theory.     

Fluoride Analysis of Glasses and Silicate Materials by Pyrohydrolysis Separation

Fluoride in a wide variety of glasses and silicate materials can be separated by pyrohydrolysis and then determined by titration with thorium nitrate. Pyrohydrolysis is accomplished by passing steam over a mixture of sample and uranium oxide (U₃O₃) at temperatures of approximately 1000°C. The average reproducibility of results is 2 relative % and the results are in general slightly higher than those obtained by conventional methods of separation. A determination can be completed in about 1 hour.     

Peculiar Morphology in Some Phase-Separated Multicomponent Silicate Glasses

Several researchers have reported the formation of convoluted, silica-rich, dispersed-phase particles during amorphous phase separation in some glass-forming silicate systems. These appear to be analogous to crystalline growth instabilities such as dendritic, fingered, and cellular structures that can arise during diffusion-controlled solidification in undercooled systems.     

Property/Morphology Relations in Alkali Silicate Glasses

The effects of glass composition and thermal history on the glass-transformation range behavior, as indicated by thermal expansion curves, and on chemical durability, as indicated by the acid etch rate, were measured for binary lithium, sodium, and potassium silicate glasses and for the lithinm/sodium and lithiumlpotassium mixed-alkali silicate glasses. The results were used to determine the phase-separation boundaries and the limits of interconnectivity of each phase for all five systems. The results demonstrate the utility of such studies for the characterization of the morphology of glass-forming systems.     

Phase Separation in Borosilicate Glasses

Dielectric measurements and direct-transmission electron microscopy were used to analyze the phase separation and alteration in sodium borosilicate glasses. Metal impregnation of the glasses after leaching was particularly useful in observing the topology of separated phases whose characteristic dimensions are about 100 A. The results indicate that borosilicate glasses such as Vycor and Pyrex are fully phase separated when rapidly cooled from the melt and that subsequent heat treatment serves only to alter the structural characteristics of the separated phases. The topological characteristics of the initially segregated phases suggest that phase separation may occur by spinodal decomposition.     

Structural Relaxations in Alkali Silicate Systems by Brillouin Light Scattering

The Brillouin light scattering technique was used to investigate the viscoelastic relaxations in alkali silicate systems up to temperatures of 1500°C. The line shape analysis of the Brillouin spectra reveals information about the mechanical rigidity and the viscous dissipation in the scattering medium. Brillouin frequency shifts and the frequency-dependent viscosity coefficients, v ' were measured as a function of temperature. The shapes of the v ' vs temperature curves, and the location of their maxima on the temperature scale, reveal the activation energies and the resonance frequencies of the underlying mechanisms. In binary alkali silicates, small dissipation maxima were observed between the glass transition and the melting temperature, which are attributed to the increased cation mobility as a result of the accelerated disintegration of the silica networks. In mixed alkali systems, multiple dissipation maxima are observed. These are spread over a range of temperatures, which are distinctly higher than in binary systems.     

Influence of Platinum Nucleation on Constitution of and Phase Separation in Sodium Phosphate Glasses

Published work in the literature indicates that small amounts of platinum bring about phase separation in sodium phosphate melts which can be recognized in electron micrographs. The influence of 0.0038, 0.0076, 0.038, and 0.15% platinum on the constitution of and phase separation in sodium phosphate glasses with n, the number average chain length, varying from 3 to 9 has been investigated using both paper chromatographic and electron microscopic techniques. The results indicate that platinum nucleation apparently has no effect on the distribution of polyphosphates in the glasses studied, although electron micrographs show phase separation. The significance of the results in relation to immiscibility phenomena is discussed.     

Interpretation of Hydrated States of Sodium Silicate Glasses by Infrared and Raman Analysis

Infrared spectra of thin films (a few micrometers) of hydrated sodium silicate glasses after heating at temperatures from 100° to 400°C were measured in the spectral region from 400 to 4300 cm⁻¹. Also, Raman spectra of hydrated glasses were measured in the region from 100 to 1400 cm⁻¹. Water molecules in hydrated glasses were classified into three types by the bonding configuration: water molecules interacting with Si–OH groups, nonbridging oxygens, and bridging oxygens. On the basis of the fundamental bands observed here, we calculated the wave numbers of the combination-overtone bands of water molecules and SiOH groups in hydrated glasses. The calculated wave numbers satisfactorily agreed with observed wave numbers in the nearinfrared region.     

Secondary Phase Separation in Lead Borate Glasses

The effect of secondary phase separation on the apparent volume fraction of second phase was examined in the immiscible region of the PbO-B₂A₃ system. The volume fraction, V₂, of the PbO-rich phase agreed well with theoretical expressions relating V₂ to density and composition, except for 0.2< V₂ <0.5. The discrepancy in this region was explained in terms of the peculiar shape of the immiscible phase boundary and the resulting mutual supersaturation of the immiscible phases with respect to each other.     

Color and Light Scattering of Platinum in Some Lead Glasses

The coloration and light scattering of sodium lead silicate, borate, and phosphate glasses containing platinum, introduced as PtCl₄, have been studied. Under the normal atmosphere of a furnace heated by silicon carbide elements, the platinum is retained in the colloidal state in the sodium lead silicates and borates, producing gray glasses. In the phosphate glasses yellow to orange ionic colors as well as grays are produced. Lead oxide increases the stability of platinum ions in the phosphate glasses. Under strong oxidizing conditions, ionic platinum also can be obtained in the borate and silicate glasses. Tyndall scattering can be detected in what appear to be clear, colorless glasses. This light scattering has been observed in some glasses containing as little as 1 p.p.m. platinum.     

Absorption and Scattering Losses in Glasses and Fibers for Light Guidance

Fiber optics for telecommunications applications require very high purity glass. Light loss in glass results primarily from absorption and scattering. Transition metal ions and OH ions cause most of the absorption, whereas scattering is caused by microheterogeneity. Scattering losses vary from 1 to 4 dB/km at 850 nm; absorption losses are high near 630 nm, because of Cr³⁺ and Ni²⁺ impurities, but are lower near 850 nm, making this a preferred wavelength region. Flint glass with a total loss of 50 dB/km at 850 nm, prepared using pure material and special techniques, was used as the core of thin-clad optical fibers 70 μm in diameter; these fibers had an internal loss at 850 nm of ∼450 dB/km.     

Specifics of Phase Separation in Zirconium-Bearing Glazed Glasses

The results of studying phase separation in zirconium-bearing alumoboron silicate glazed glasses at the stages of melting and thermal treatment are described. The effect of glass composition on the specifics of its phase composition and structure is considered. The possibility of producing low-melting glaze coatings with preset properties is demonstrated.