Structure Transition in Alkali Silicate Glasses

The logarithm of measured high-temperature fluidities is related to the square of the calculated probability of finding a nonbridging oxygen in a randomly chosen SO4 group. This is interpreted in terms of a flow mechanism (gliding) dependent on juxtaposition of two such oxygens. A theory of the glass transformation is developed. On cooling, the coordination number of nonbridging oxygens increases until, ideally, at T , each touches three oxygens of an external Si04 group (stabilization). Below this temperature, thermal expansion/contraction contains no coordination change component. These changes constitute a second-order transition. Observed volume data are in agreement with hypothesis. On this view, stabilization creates vacant tetrahedral environments adjacent to existing ones. Migration of glass-forming ions to these empty sites can unlock the structure and permit the operation of a new set of flow processes. The coordination requirements for stabilization severely restrict the types of local arrangement possible at high silica contents, and therefore reduce the configurational entropy and induce unmixing. The overall transformation is composite in character. Rather extensive bond rearrangement and unmixing (when present) are features of first-order kind.     

Impurity Alkali Diffusion in Sodium-Cesium Silicate Glasses

The tracer diffusion coefficient of ⁸⁶Rh was measured as a function of Cs/Na ratio in (Na, CS)₂O:3SiO₂ glasses. A comparison of the compositional dependence of this impurity alkali diffusion with the self-diffusion coefficients of ²²Na and ¹³⁷Cs suggests that the alkali-ion pair concept in the weak-electrolyte model is not sufficient to explain the mixed-alkali effect.     

Heat of Annealing in Simple Alkali Silicate Glasses

The enthalpy changes associated with annealing of glass were studied in simple and mixed alkali silicate glasses. The data indicate that during prolonged annealing the glass comes to a metastable equilibrium state and has a unique heat of solution which depends on its fictive temperature. The heats of solution of these glasses show a linear dependence on fictive temperature, and the magnitude of this dependence is related to the molar volume of the glass. The significance of these heat effects is discussed. The maximum heat effects which can occur on annealing sodium silicate glasses were measured and were approximately half as large as the enthalpy changes associated with the structural arrangements that occur during crystallization of these glasses.     

Electrode Properties of Fluorine-Containing Alkali Silicate Glasses

An attempt is made to change the electrode properties of alkali silicate glasses containing various glass-formers and modifiers by introducing fluorine components into their compositions. Model systems with the initial hydrogen and metallic electrode functions are investigated. The study is based on the assumption that new ionogenic structural units with a mixed-anion constituent are formed in the glass structure. Lithium, sodium, and potassium silicate glasses with different fluorine contents are synthesized. Their potentiometric properties are studied. It is shown that the introduction of fluorine exerts the strongest effect on the electrode properties of alkali aluminosilicate glasses. An indirect corroboration is obtained for the assumption that strongly acid groupings containing fluorine can be formed in the glass network. It is found that the technological properties of the electrode glasses are significantly improved and the chemical durability in the potassium-containing system also becomes much higher. From the results obtained, the inference is drawn about the prospects of fluorine introduction into electrode glasses.     

Ionic Conduction in Alkali and Thallium Silicate Glasses

Electrical conductivities of binary alkali and thallous silicate glasses have been measured as a function of composition, temperature, and frequency. The best approximation to the activation energy for dc conduction can probably be obtained by extrapolating its frequency dependence to zero frequency, although values obtained at frequencies below 2500 hz do not differ greatly from the dc values. The plot of activation energy for conduction against modifier content consists of two straight lines of different slope. For all the alkali silicate glasses the break in slope occurs near 25 mole % alkali oxide and at an activation energy near 14.7 kcal/mole. The slope of the line below 25 mole % alkali oxide is proportional to the alkali ion radius. The behavior of thallous silicate glass is similar but not identical to that of the alkali silicates. It is concluded that (1) the reported conduction behavior does not result primarily from phase separation, (2) the principal contribution to the activation energy is probably the work required for a mobile ion to pass through the glass network rather than to leave its initial position, and (3) a structural change independent of the nature of the modifier cation occurs near a modifier content of 25 mole %.     

Stannous-Stannic Equilibrium in Molten Binary Alkali Silicate and Ternary Silicate Glasses

The stannous-stannic equilibrium in binary alkali silicate and ternary silicate glasses was studied by equilibrating glassmelts with air at 1400°C. The Sn²⁺-Sn⁴⁺ equilibrium shifts more toward the oxidized state with increasing ionic radii of the alkali ions or with increasing concentration of the alkali ions in the same series of glasses. The slope of the straight lines obtained on plotting log (Sn⁴⁺)/(Sn²⁺)( pO₂ )^n/2 vs mol% R₂O increased in the order Li→Na→K. In ternary silicate glasses having the base glass composition 20Na₂O·10RO·70SiO₂, the Sn²⁺-Sn⁴⁺ equilibrium shifts more toward the reduced state, with increasing bond strength between the divalent cations and the nonbridging oxygens. With increasing temperature, the equilibrium shifts more toward the reduced state.     

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.     

Lead-Ion Stability in Soda–Lime Lead Silicate Glasses

Sodium-calcium-lead silicate glass compositions were prepared over a wide compositional range by conventional glass-melting methods. The acid chemical stability of the glass structure was studied by corroding samples of glass in 4% acetic acid solution at 22°C for periods up to 24 h. Lead corrosion stability was evaluated by measuring lead concentrations in the corrosion solution. At short times, parabolic time dependence was observed and the parabolic time coefficients were regressed against composition, yielding a simple additive relationship. A similar model was fitted to 24-h release data, which showed compositional effects similar to the 2-h data. Of the oxides studied, sodium was the most detrimental to durability, and the coefficients of the oxides decreased in the series: Na₂O, PbO, CaO, SiO₂. The effects of the oxides could be partially explained by the number of nonbridging oxygens expected in the glass structure, and the residual effect was attributed to cation characteristics. Small phosphate additions to the glass greatly increased the lead-ion stability under nearly all experimental conditions examined.     

Structural and Thermal Expansions in Alkali Silicate Binary Glasses

Two volume-changing processes that should be considered in detail are (1) distention of networks by added oxides which may swell glass volumes in proportion to the added oxide volumes and (2) angular change in intertetrahedral bonding which may take place at and above annealing temperatures in proportion to the number of metal ions introduced by the modifications. Accordingly, the partial molar volumes of modifiers, ν _R , are assumed as those of their crystal or liquid states and the partial molar volumes of silica are computed from (1/ν_S) = 2.20 + 0.00057 ( r_m – 0.12) (1723 – t ). Tentative glass volumes are then computed as V_g= S_mν_S+ r_mν_R by use of the mole fractions S_m and r_m for silica and nonsilica, respectively. The excess of computed glass volumes, V₀ , over V as derived from observed densities is taken as a measure of the penetration of modifiers into the network cavities without corresponding distention of the network. By using the computed density of 1.95 for vitron, the value 2.20 for silica glass, and 2.30 as reported for irradiated silica, one can estimate that silica glass consists of 28% vitron and 72% disordered silica in the intervitron tissues. The values of V_g , – V mentioned are found to be in fair accord with expectations computable from the vitron theory.     

Indentation Creep of Hydrated Soda–Lime Silicate Glass Determined by Nanoindentation

Time-dependent deformation behavior was investigated for soda–lime silicate glass with various water contents, using a nanoindentation technique. The complete indentation curve, loading and unloading part, is analyzed. It is shown that this deformation behavior may be represented in terms of a simple mechanical model analogous to a viscoelastic system. Values for Young's modulus were derived, a retardation spectrum was deduced, and apparent viscosity values were calculated. Structural rearrangements of the glass appear to be responsible for the observed changes of the viscoelastic properties. Water in the glass reduces Young's modulus and yield stress and thus promotes viscous flow.     

Effect of Cerium on the Ion-Exchange Parameters in Alkali Silicate Glasses

Alkali gallosilicate glasses containing 1–8 mol % CeO₂ after ion-exchange treatment in a NaNO₃ melt at different temperatures are studied. The refractive index distributions in glasses with different cerium oxide contents are obtained, and the profiles of the Na₂O concentration distribution in the samples containing 2 mol % CeO₂ after ion-exchange treatment at temperatures T = 500–550°C are determined. It is revealed that, as the cerium oxide content increases, the interdiffusion coefficient decreases, whereas the refractive index increment increases. The results obtained are analyzed within the framework of the model of dopant segregation, according to which cerium undergoes selective incorporation and concentration displacement into polar microregions of the glass.     

Relation of Alkali Mobility and Mechanical Relaxation in Mixed-Alkali Silicate Glasses

Internal friction and Na and K self-diffusion coefficients for (I–X)Na₂OXK₂O-3SiO₂ glasses were measured. Alkali diffusion was measured between 300° and 500°C using radioactive isotopes and a thin-sectioning technique; internal friction was determined at room temperature to 500°C at 0.1 to 2000 Hz. Comparison of the data for alkali diffusion and internal friction in Na-K glasses with those for other mixed-alkali glasses shows that the mixed-alkali peak is related to alkali mobility. It is concluded that the mechanism of mixed-alkali internal friction is cooperative rearrangement of dissimilar alkali ions, with the slower-moving ion controlling the rate of reorientation.     

Nature of Incorporated Water in Hydrated Silicate Glasses

Silicate glasses show a wide range of responses to exposure to water or steam at high temperatures and pressure. The response patterns and the factors determining these patterns are discussed. The manner in which water is incorporated in the silicate glasses is elucidated by a study of the near infrared spectra and differential thermogravimetric analysis of hydrated glasses. Factors affecting relative amounts of tightly bonded and loosely bonded water are discussed.     

Physical Properties and Structure of Silicate Glasses: I, Additive Relations in Alkali Binary Glasses

Correlations between certain physical properties, P , and all eighteen compositional ratios between the five ionic parameters of the system were studied with a computer for the five alkali silicates. Linear relations were found between P and the alkali/silica, alkali/oxygen, and silica/alkali ratios for certain regions of the glass field when the P's are expressed in proper molar terms. Specific volume and heat of formation data indicate three linear regions with two sharp break points between. The log electrical resistivity and the volatilization loss show two such regions and only the second break point. The break points, while structural in origin, must therefore occur for different reasons. The break points are the same for all P's and coincide with the eutectics. The constants of the linear relations are the partial molar quantities associated with P. Break points are not found in the molar refractivity data, so that no change in ligancy for these systems is indicated.