Densities of SiO2-Al2O3 Melts

The densities of binary aluminosilicate melts were measured X-radiographically as a function of Al₂O₃, concentration between 1800° and 2000°C. Within this temperature range, the density curves vary linearly and are parallel from fused SiO₂ to ≊30 to 45 mol% Al₂O₃, depending on the temperature. At higher Al₂O₃ contents, negative deviation from linearity increases with increasing temperature. Recent supplementary research efforts on various aspects of the system SiO₂-Al₂O₃ indicate that the changing coordination and structural role of the aluminum ion may be a primary factor in determining the shapes of the density curves.     

Mullite Crystallization from SiO2-Al2O3 Melts

SiO₂-Al₂O₃ melts containing 42 and 60 wt% A1₂O₃ were homogenized at 2090°C (∼10°) and crystallized by various heat treatment schedules in sealed molybdenum crucibles. Mullite containing ∼78 wt% A1₂O₃ precipitated from the 60 wt% A1₂O₃ melts at ∼1325°± 20°C, which is the boundary of a previously calculated liquid miscibility gap. When the homogenized melts were heat-treated within this gap, the A1₂O₃ in the mullite decreased with a corresponding increase in the Al₂O₃ content of the glass. A similar decrease of Al₂O₃ in mullite was observed when crystallized melts were reheated at 1725°± 10°C; the lowest A1₂O₃ content (∼73.5 wt%) was in melts that were reheated for 110 h. All melts indicated that the composition of the precipitating mullite was sensitive to the heat treatment of the melts.     

Optical and NMR Studies on Interdiffusion of Silver in SiO2-B2O3-AI2O3-R2O Glasses

Interdiffusion of silver ions in SiO₂-B₂O₃-AI₂O₃-R₂O glasses where R=Na or K was investigated, using optical transmission, ESR, and wet chemical methods to determine concentration and the chemical state of silver, and NMR spectra as a probe of the glass structure. The concentration of silver introduced by ion exchange increased monotonically, as the line widths of²⁷AI NMR spectra decreased. The sharp and narrow features of ²⁷Al line shapes were broadened and the amount of colloidal silver produced by ion exchange decreased, as R₂O/B₂O₃ approached unity with fixed AI₂O₃. The BO₄ to BO₃ ratio approached unity and the quadrupole coupling constant of BO₃ units varied from 2.70 to 2.96 MH_Z, as R₂O/AI₂O₃, approached unity for fixed B₂O₃. These diverse data suggest a relation between silver diffusion and glass structure, although the phenomena of phase separation and the mixed-alkali effect could also influence silver-colloid formation in the glasses studied.     

Oxidation-Reduction Equilibria in Molten Na2O.2SiO2 Glass

The oxidation-reduction equilibria in molten glasses having a soda/silica ratio of 1/2 and containing small amounts of variable-valence ions of the transition elements titanium, vanadium, iron, cobalt, and nickel, the post-transition elements tin and antimony, and the rare-earth element cerium were obtained by equilibrating the melts with various atmospheres. The simple mass expression
was always applicable. In the expression, n is the number of electrons involved in the valence change of the metal M. The value of n is 1 for all systems except for the ions of antimony and tin where n is 2. The ions found are those generally accepted as existing in glass melts which are Ti³⁺, Ti⁴⁺; Fez+, Fe³⁺; Ce³⁺, Ce⁴⁺; Mn²⁺, Mn³; Co²⁺, Co³⁺; Ni²⁺, Ni³; Sb³⁺, Sb⁵⁺; and Sn²⁺, Sn⁴⁺. Two mass action expressions were needed for vanadium-containing glasses to describe the equilibria between 5+, 4+, and 3 f species.     

Subsolidus Relations in the System SiO2— Al2O3— P2O5

Phase relations in that part of the system SiO₂-Al₂O₅-P₂O6 below 50 mole % P₂O₃ were investigated at subsolidus temperatures. The only ternary compositions which crystallized readily were those lying close to binary compounds and no evidence for ternary compound formation was found. Solid solution between silica and aluminum phosphate, expected on the basis of closely similar lattice dimensions and physical properties, was not indicated by X-ray diffraction, refractive index, or electron-beam microprobe results. Above 80% silica and at a temperature of 1400°C, the only crystalline species was cristobalite whereas below 60% silica, only the cristobalite form of AlPO₄ was present. Coexistence of these two constituents seemed to be inhibited, possibly because of the strong glass-forming properties of the component oxides.     

Direct Measurement of Relative Partial Molar Enthalpy of SiO2 in SiO2–M2O (M=Li, Na, K, Cs) Binary and SiO2–CaO–Al2O3 Ternary Melts

The relative partial molar enthalpies, Δ _SiO2, of SiO₂ in SiO₂–M₂O (M = Li, Na, K and Cs) binary and SiO₂–CaO–Al₂O₃ternary melts were directly measured by drop-solution calorimetry at 1465 K and 1663 K. Δ _SiO2 changes from exothermic to endothermic as silica content increases, confirming the tendency toward immisciblity seen from activity measurements. It is concluded that Δ _SiO2 is negative due to acid-base reactions and charge-coupled substitutions when the melt is composed of fewer Q ⁴ and more Q ³ and Q ² species, but positive due to structural strain when the melt is composed of mostly Q ⁴ species. The Δ _SiO2 obtained by calorimetry is a useful measure of basicity, when comparing different alkali and alkaline earth oxides.     

Kinetics of Crystallization of Stoichiometric SiO2 Glass in H2O Atmospheres

Crystallization rates were measured in vacuum, dry nitrogen, and water-saturated nitrogen atmospheres from 1300° to 1540°C. In all cases the observed rates were linear. Three reactions appeared to contribute to crystallization: the intrinsic crystallization, the impurity effect of H₂O vapor, and furnace contamination. Enhancement of crystallization by both water vapor and furnace contamination is attributed to the breaking of silicon—oxygen bonds of the glass structure. Competitive adsorption mechanisms were proposed to characterize the adsorption of water and impurity species. The activation energy for apparent intrinsic crystallization was 134 kcal/mole; the activation energy for crystallization in H₂O vapor was 77 kcal/mole.     

The Glass System SiO2-B2O3-Bap-Na2O for High-Numerical-Aperture Optical Fibers

The glass system SiO₂-B₂O₃-BaO-Na₂O was investigated to obtain high-refractive-index glasses for high-numerical-aperture (N.A.) optical fibers. Compositions having a mole ratio [B₂O₃]/[BaO+Na₂O]=2 have unique properties. They are both highly viscous and chemically durable despite the fact that they contain highly alkaline metals and alkaline-earth metals. Glass properties at this mole ratio are favorable for optical fibers. It is proposed that the composition for core glass of high-N. A. optical fibers be based on this mole ratio.     

Electrical Conductivity of HF-H2O- Impregnated Na2O.3SiO2 Glass

Up to 5.75 WHO of concentrated (49 wt%) HF solution was placed in Nazo.3SiO₂ glass under hydrothermal conditions. Electrical conductivity of the resulting glasses was primarily determined by the water content.     

Free-Energy-Based Model of Chemical Equilibria in the CaO–SiO2-H2O System

The chemical equilibrium between the C-S-H gel phase and the aqueous phase in the CaO-SiO₂-H₂O system was modeled using the free-energy-based Thermo-Calc commercial software system. Unlike programs that are based on solubility products, the minimization of free energy allows the composition and solubility of the C-S-H phase to vary, as in real cementitious systems. The C-S-H gel is modeled as a defect tobermorite structure, based on recent compositional data gathered using ²⁹Si NMR. The composition is varied by removal of some of the bridging silica tetrahedra in the SiO layers and by incorporating different amounts of Ca in the interlayer spaces.     

Dehydration Series in the System CaSiO3— SiO2— H2O

Hydrothermal studies show a dehydration series involving CSH(I), tobermorite, xonotlite gyrolite, truscottite (reyerite), silica, and wollastonite. The synthetic equivalents of gyrolite and Qruscottite (reyerite) have ideal compositions very close to 2CaO.3SiO₂.2H₂O and 3CaO.5SiO₂.1SH₂O respectively. With rising temperature the following reactions take place: (1) CSH(1) or tobermorite + silica = gyrolite (+H₂O); (2) gyrolite + silica = truscottite + H₂O; (3) gyrolite = tobermorite + truscottite + H₂O (high pressure); (4) gyrolite = xonotlite + truscottite + H₂0 (low pressure); (5) tobermorite = xonotlite + H₂0; (6) truscottite = xonotlite + quartz + H₂O; (7) xonotlite = wollastonite + H₂O. Data on reactions (l), (5), and (7) confirm results of previous workers. The water pressure-temperature curves or bands, except the one involving the breakdown of tobermorite, are steep. This curve crosses the curve representing the breakdown of gyrolite at a quintuple point. None of the reactions was reversed, and the temperatures indicated are approximate and are based on runs of about 4-weeks duration. For much shorter times somewhat higher temperatures are needed to cause the reactions, and conversely in much longer runs slightly lower temperatures might suffice. The silica was usually amorphous, cristobalite, or quartz with a mineralizer and therefore of higher activity than quartz alone. Up to 2 % alkali fluorides facilitated the formation of gyrolite; this is probably related to the fact that apophyllite in many cases accompanies gyrolite in amygdules. Truscottite (reyerite) forms very easily, and the rarity of reported occurrences, compared with those of xonotlite, gyrolite, and opaline silica, suggests that some workers may have overlooked it.     

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.     

Properties of CeO2–Al2O3–SiO2 Glasses Prepared in Air

Properties of 15–30 mol% CeO₂/23 mol% Al₂O₃/62–47 mol% SiO₂ glasses prepared in air have been investigated in this study. Experimental results show that the glass transformation temperature, the dilatometric softening temperature, and the transmittance in the visible region decrease with increasing ceria content; but the thermal expansion coefficient, the bulk density, and the microhardness increase with increasing ceria content.     

Change of Molar Volume and Interdiffusion Coefficients in the System MgO-Cr2O3

The interdiffusion coefficients in the system MgO-Cr₂O₃ were measured in the MgO phase. They are concentration-dependent and increase linearly, below 1600°C, with increasing mole fractions of cation vacancies. Interdiffusion coefficients calculated by the Boltzmann-Matano and Wagner treatments for the system MgO-Cr₂O₃ are compared; some of those obtained by the Boltzmann-Matano treatment are about 70% greater than those obtained by Wagner's analysis. Wagner's treatment gives correct values for the interdiffusion coefficients. The results of the Boltzmann-Matano treatment show the effect of neglecting the molar volume change in the system.     

Dielectric Characteristics of Na2O · 3SiO2 Glasses with High Water Contents

Dielectric characteristics of Na₂O·3SiO₂ glasses with water contents up to ∼12 wt% were found to be drastically affected by incorporated water. The high-frequency dielectric constant increased with water content, while both the static dielectric constant and the low-frequency dielectric relaxation strength showed a pronounced minimum at a water content of ∼3 wt%.     

Oxidation-Reduction Equilibria in Molten Na2O.2SiO2 Glass in Contact with Metallic Copper and Silver

The oxidation-reduction equilibrium in Na₂O-2SiO₂ glass melts containing either copper or silver was studied by systematically varying the oxygen partial pressure above the melt. For copper the two equilibria are
For silver the equilibrium is analogous to the second of the two copper reactions.     

Effect of Water Content on the Chemical Durability of Na2O 3SiO2 Glass

Na₂O 3SiO₂ glasses with high water content were prepared under high-pressure, hydrothermal conditions and their chemical durabilities in water were measured at atmospheric pressure and 30°C. Chemical durability decreases with increasing water content, which is probably due to a positive dependence of H⁺ (or H₃O⁺) diffusivity on water content.     

Indentation Creep of Na2O · 3SiO2 Glasses with Various Water Contents

Time-dependent deformation behavior induced by an indentation, called indentation creep, was investigated for Na₂O · 3SiO₂ glasses with various water contents by measuring the indentation depth as a function of time using a conical indenter. It was found that the indentation creep behavior of the glasses could be best explained as steady-state inhomogeneous (non-Newtonian viscous) flow. Water in the glasses appeared to reduce the stress (or yield stress) needed to cause this flow. Crack initiation of the glasses was promoted by water through this inhomogeneous flow.     

Interdiffusion in the System SrF2-BaF2

A microprobe study of interdiffusion between 1033° and 1238°C in the system SrF₂-BaF₂ was conducted using single crystals. The interdiffusion coefficient, D , generally decreased with increasing SrF₂ concentration, but the composition dependence of D decreased with decreasing temperature. Extrapolation of D values to infinite dilution allowed the impurity cation self- diffusion coefficients:

The values for D_Sr in BaF₂ are in excellent agreement with tracer results. An analysis of the activation energies for cations diffusing in alkaline-earth fluorides shows a direct correlation between constriction ratio and migration energy for a single ion diffusing in different fluorides. An apparent anomaly arises when attempting to correlate migration energy with structural information for different cations diffusing in the same fluoride.