Liquid Immiscibility in the System K2O-B2O3-SiO2

The subliquidus miscibility gap in the system K₂O-B₂O₃-SiO₂ has been determined for compositions with molar ratios SiO₂/B₂O₃<2 and T≥550°C. The shape of the miscibility gap is an elongated dome similar in form to, but less extensive than those in the lithium and sodium borosilicate systems. The consolute composition (molar) and temperature are estimated to be 4 ± 1 K₂O -30±8 B₂O₃-66±8 SiO₂ and 629±5°C, respectively .     

Prediction of Immiscibility Boundaries of the Systems K2O-SiO2, K2O-Li20-SiO2, K20-Na2O-Si02, and K2O-BaO-SiO2

In earlier work, a prediction method of the immiscibility boundary of a ternary silicate glass system was developed involving two known binary immiscibility boundaries and a measured immiscibility temperature of one ternary glass composition. In the present work, the method is extended to the case where one of the two binary immiscibility boundaries is not known and is applied as an example to ternary silicate systems containing K₂O. First, the immiscibility boundary of the system K₂O-SiO₂ is estimated by measuring the immiscibility temperatures of three glasses in the system K₂O-Li₂O (or Na₂O)-SiO₂. Using this result the immiscibility boundaries of the systems K₂O-Li₂O-SiO₂, K₂O-Na₂O-SiO₂, and K₂O-BaO-SiO₂ are estimated. The results agree reasonably well with the experimentally determined immiscibility temperatures at selected compositions.     

Activities in Li2O-, Na2O, and K2O-SiO2 Solutions

Activities of alkali metal oxides and silica are calculated for regions of the Li₂O-SiO₂, Na₂O-SiO₂, and K₂O-SiO₂ phase diagrams where volatility, cell potential, and thermodynamic data are available. For the high-silica composition regions, activities are calculated from the stable phase diagrams and assumptions based on sub-regular solution behavior. These data permit estimates of the phase boundaries for the metastable two-liquid regions in the Li₂O and Na₂O systems which may be compared with experiment. An estimate is also made of the critical point for K₂O-SiO₂ to determine whether metastable phase separation in this system will be experimentally obtainable.     

Effect of Crystallization on the Auger Electron Signal Decay in an Li2O·2SiO2 Glass and Glass-Ceramic

Problems associated with Auger analysis of glasses are well recognized. When an electron beam impinges on the surface of a glass specimen, the Auger signal produced by the alkali species decreases with exposure time. The rate at which the signal decays depends on the glass composition, beam current density, accelerating potential of the primary beam, and substrate temperature, all of which must be carefully controlled to obtain a reliable analysis. A similar decay of the alkali Auger signal is observed for glass-ceramic systems. The rate of decay for the crystalline phase is different from that for the glass and depends on the extent of crystallization. This behavior may provide a basis for using Auger electron spectroscopy for microstructural as well as compositional analysis.     

Kinetics of Crystallization in Li2O-SiO2 Glasses

The kinetics of crystallization of Li₂Si₂O₅ from glasses in the Li₂O-SiO₂ system were studied using quantitative X-ray diffraction. Analysis of the data using the Johnson-Mehl-Avrami equation showed that crystallization occurred through the nucleation and growth of rods. The spherulitic nature of the crystals was substantiated by petrographic examinations of the partially crystalline glass. Analysis of the temperature dependence of the crystallization rate using a modification of the Johnson-Mehl-Avrami equation showed that the activation energy for nucleation was a function of composition and thermal treatment.     

Effect of Crystallization on the Mechanical Properties of Li2O-SiO2 Glass-Ceramics

Variations in the thermal expansion coefficients, elastic moduli, and fracture strengths of Li₂O-SiO₂ glass-ceramics were determined as a function of nucleation treatment and volume fraction of crystals present. Strength enhancement was attributed to a decrease in the mean free path between crystals as crystallization proceeds. It is postulated that the eventual reduction in strength in some glass-ceramics is caused by the development of localized cracks at the crystal-glass interface as a result of the volumetric changes which occur during crystallization.     

Effect of Na2O Additions in the Crystallization of Barium Ferrite from a BaO-B2O3-Fe2O3 Glass

A glass crystallization method was utilized to synthesize nanosized BaO-6Fe₂O₃ platelets from a 0.412BaO-0.258B₂O₃-0.330Fe₂O₃ batch composition. Quenched ribbons were inhomogeneous, showing microclustering and ∼1 μm hematite crystals. Na₂O substitutions for BaO greatly enhanced the glass-forming tendency of quenched ribbons, though quenched-in ∼0.5 μm barium ferrite crystals were infrequently present. The improved homogeneity with Na₂O substitution was attributed to lower vapor pressure of BaO during batch melting, which increased its retention in the as-quenched ribbons. Quantities of BaO equal to or in excess of Fe₂O₃ allowed iron ions to adopt stable network positions in the glass melt. With Na₂O substitution, devitrification of dispersed ∼40 nm barium ferrite particles from phase-separated regions occurred after secondary heat treatment. 5 mol% Na₂O batch substitution showed the lowest crystallinity in the as-quenched ribbons, and the highest crystallinity after secondary heat treatment. After optimum devitrification, the maximum values of saturation magnetization and coercivity were 21.22 emu/g and 2.82 kOe, respectively.     

Molybdenum Phosphate Glasses Containing Ag2O or K2O

Glasses in the systems MoO₃-P₂O₅-Ag₂O and MoO₃-P₂O₅-K₂O were investigated and general areas of composition for good glass formation were determined by X-ray diffraction methods. Temperature coefficients of electrical resistivity and linear expansion were determined for the glasses along with density, durability, and other thermal characteristics. Resistivity of the glasses was effectively reduced by the addition of Ag₂O.     

Effect of Na2O/K2O Ratio on Chemical Durability of Alkali-Lime-Silica Glasses

The chemical resistance of mixed-alkali glasses to water was measured by determining the sodium, potassium, calcium, and silicon ions extracted instead of evaluating it from the alkalinity of the mixed-alkali extracts. In glasses of the composition K₂O and Na₂O 18, CaO 10, and SiO₂ 72% the minimum extraction of alkali (maximum durability) occurred for a K₂O/-Na₂O ratio of 2.6/1.0 (by weight). Based on an ionic-composition analysis of the results, an empirical index is suggested for determining the relative degree of ion-exchange and network-breakdown modes of reactions that occur when such mixed-alkali glasses are attacked by water.     

Crystallization Behavior of CaO–P2O5 Glass with TiO2, SiO2, and Al2O3 Additions

TiO₂ above 4 mol% is an effective nucleating agent for CaO–P₂O₅ glass which also contains substantial SiO₂ and Al₂O₃ additions. Glass ceramics can be made from this glass using a single slow heating ramp with no need for a nucleating heat treatment step. Powder of this composition crystallizes rapidly to β-Ca₂P₂O₇, whereas bulk glass crystallizes from diphasic nuclei consisting of a central cubic Ca-P-Ti-Si-Al oxide phase surrounded by impure AlPO₄ dendrites. Metastable calcium phosphate grows on the AlPO₄ dendrites and later transforms to β-Ca₂P₂O₇.     

Internal Friction of Ion-Exchanged Li2O.Al2O3.2SiO2 Glass

Fibers of Li₂O.Al₂O₃.2SiO₂ glass were ion-exchanged for 1 to 300 min in an NaNO₃ bath at 366°C. The internal friction and the Li and Na concentration profiles were measured. As Na progressively replaced Li, the alkali internal friction peak became smaller while a new peak (mixed-alkali peak) appeared and increased in magnitude. These changes in internal friction are similar to those that occur when a second alkali is added to glasses prepared by conventional melting. The magnitudes of both internal friction peaks in the ion-exchanged glass depended on the overall composition of the glass; that of the alkali peak depended on the composition of the unexchanged glass core, whereas that of the mixed-alkali peak depended on the composition of the exchanged layer on the glass surface. When the exchanged surface layer was dissolved, the original alkali peak was restored, and the mixed-alkali peak disappeared. Changing the alkali distribution did not affect the mixed-alkali peak much; however, it caused the alkali peak to shift to higher temperatures and become smaller. The height of the alkali peak can be used to determine the maximum depth of penetration of the second alkali.     

Differences in Surface Behavior of Alkali Ions in Li2O ·3SiO2 and Na2O · 3SiO2 Glasses

The molecular dynamics computer simulation technique was used to determine the short-time dynamics behavior and resultant structures of ions at the surface of Li₂0·3SiO₂ and Na₂O-3SiO₂ glasses. Room temperature and elevated temperatures were used. Results are compared with similar studies of the K₂O·3SiO₂ glass surface and with recent experimental ion-scattering-spectroscopy data. The simulations indicate that a localized surface rearrangement occurs within picoseconds after formation of the free surface, creating a surface excess of alkali in the Na (and K) case, but not in the Li case. Elevated temperatures, even for brief times, enhance the observed surface excess of Na and K. The results correspond to those obtained from the ion-scattering-spectroscopy studies.     

Direct Observation of the Crystallization of Li2O-Al2O3-SiO2 Glasses Containing TiO2

The nucleation and crystallization of Li₂O-Al₂O₃-SiO₂ glasses containing TiO₂ were investigated using transmission electron microscopy of thin sections produced from bulk samples. Phase separation occurs during cooling from the melt, and on heating, a large number of titanium-aluminum crystals approximately 50 A in diameter are formed. These crystals are the heterogeneous nuclei for the crystallization of the remaining glass. Photomicrographs of various stages of crystallization show the development of the fine-grained glass-ceramic.     

Behavior of Atoms at the Surface of a K2O · 3siO2 Glass—A Molecular Dynamics Simulation

Recent ion scattering spectroscopy (ISS) studies indicate that an excess of K ions occurs at the surface of a K₂O · 3SiO₂ glass. Molecular dynamics (MD) computer simulations were used to evaluate the short-time dynamic behavior of atoms at the surface of such a glass in order to determine a mechanism for the K ion enrichment. In the simulations, a bulk glass of several hundred atoms was melted using three-dimensional periodic boundary conditions, and subsequently quenched to lower temperatures. Periodic boundary conditions were removed in one dimension near room temperature so as to create free surfaces. The distribution of species perpendicular to the free surface was determined. The MD simulations show that K ions can build up at the outermost surface of the glass within several picoseconds after formation of the surface.     

Crystallization of Li2O · Al2O3· 6SiO2 Glasses Containing Niobium Pentoxide as Nucleating Dopant

Niobium pentoxide (T form, orthorhombic system) was utilized to promote devitrification in Li₂O · Al₂O₃· 6SiO₂ glasses. Two or more mole percentage of this nucleating dopant enhanced crystallization in these glasses. Glasses containing 4.0 and 8.0 mol% T-Nb₂O₅ exhibited a high tendency to form dispersed TT-Nb₂O₅ (monoclinic system) precipitates during the glass quenching process. The crystallization process in glasses containing 2.0 or 4.0 mol% T-Nb₂O₅ occurred as microphase separation, followed by the formation of dispersed TT-Nb₂O₅ crystalline precipitates (760°C), followed by β-quartz solid-solution ( ss ) formation (850° to 900°C) heterogeneously nucleated from the precipitates. β-quartz( ss ) transformed to β-spodumene( ss ), along with a polymorphic transition from the TT-Nb₂O₅ to M-Nb₂O₅ (tetragonal system) crystalline phase.     

Effects of Fast Neutron Irradiation on Thermal Conductivity of Li2O and LiAlO2

Li₂O and LiAlO₂ are two candidates for solid breeder materials in the United States' Fusion Power Program. Critical to breeder design efforts are thermophysical data, the bulk of which have only recently become available, for un-irradiated lithium ceramics. This paper expands the current limited data base by presenting thermal conductivity data between 373 and 1173 K for both materials following fast neutron irradiation. Samples were irradiated at 773 to 1173 K to lithium burnups ≤11.5 × 10²⁰ captures/cm³. Comparisons are made between these data and those from unirradiated archive samples of these same materials.