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.     

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.     

Interaction Between Structural Elements in Binary Glasses

Expressions for the interaction parameter as a function of macroscopic glass parameters are derived within the framework of a model taking into account the interaction between the structural elements in binary glasses. Analysis of published data established that the interaction index in alkali-silicate glasses within the temperature interval in which the temperature coefficients of expansions are constant does not depend on temperature.     

Acid-Base Reactions Between Components in Sodium-Aluminoborosilicate Glasses

The effect of coordination groups of boron, aluminum, and silicon on acid-base properties and specifics of the variation in the redox state of copper in boron-containing melts are investigated. An approximate evaluation of acid-base properties of industrial glasses synthesized in an alkaline-aluminoborosilicate system is implemented.     

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.     

Compressibility of Binary Alkali Borate and Silicate Glasses at High Pressures

Isothermal compressibility data at 21°C. at pressures between 1000 and 10,000 atmospheres are reported for the binary alkali borate and silicate glass systems. In general, a decreasing compressibility is noted with increasing alkali content in both silicates and borates, the effect being much greater in the borates in the range of compositions studied. A maximum compressibility appears to be indicated for the potassium silicates at low potassium concentrations. In silicates and at high alkali contents in the borates, the order of compressibility is K > Na > Li. At low alkali contents in the borates, this order appears to be reversed. Possible interpretations of these data are discussed in terms of the concepts of the structura of glasses.     

Substructures in Silicate Glasses

Glass compositions are correlated with published data for refractive index, specific volume, and fluidity, which indicate the presence of different structural types in silicate glasses. These structural types are shown to be uniquely related to the composition areas of primary crystallization phase fields. It is suggested that these different structural types are to be understood as silicate framework structures or "substructures" in which other appropriate cations may be accommodated. The described data relations definitely establish the presence of different recognizable substructures in silicate glasses but do not permit judgments on their detailed characteristics.     

Structure of Thin Films of Binary and Multi-Component Lead Silicate Glasses on Metals

The topography and composition of surface layers in thin films of lead silicate glasses obtained by thermodiffusion homogenization on metal substrates is investigated. The composition of the film – substrate transitional layer is analyzed.     

Photoelastic Effects in Silicate Glasses

The photoelasticity of R_nO_mSiO₂(R_nO_m=Li₂O, Na₂O, K₂O, and ZnO), Li₂O-Al₂O₃-SiO₂, Na₂O-Al₂O₃-SiO₂, and Na₂O-ZnO-SiO₂ glasses was analyzed on the basis of the following equation relating the stress-optical coefficient and the strain-optical coefficient:
where C is the stress-optical coefficient, n is the refractive index, G is the shear modulus, pI2and pI1are the Pockels strain-optical coefticients, and p and q are the Neumann strain-optical coefficients. No simple relation was found between the stress-optical coefficient and the shear modulus, and the factor (p−q) was found to be an essential factor deter- mining the stress-optical coefficient of silicate glasses. It was found that (p−q) decreases markedly with increasing content of modifier oxide, and that the change of (p − q) with composition is governed by the change of the atomic effect, while the lattice effect does not change as much. It is argued that the bridging oxygen with covalent bonds increases the atomic effect and the nonbridging oxygen with ionic bonds reduces the atomic effect.     

Na-K Exchange in Silicate Glasses

Electrolytic Na-K displacement in commercial sheet glass was studied. Concentration profiles and compressive surface stresses were determined.     

Microstructure in Hydrated Silicate Glasses

A microstructure ranging from 5 to 20 nm in size was detected by small-angle X-ray scattering in Na₂O·3SiO₂ and 4EaO·28Na₂O·68SiO₂ glasses hydrated at ∼100° and 80°C, respectively. This observation suggests the presence of silica gel-solvent (water) phase separation.     

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.     

Ionic Conductivity in Calcium Silicate Glasses

Glasses in the system CaO–SiO₂ were prepared with the composition varying from 40 to 55 mole % CaO. The Na₂O impurity content ranged from 0.01 to 1.30 mole %. The electrical resistivity was insensitive to the Na₂O content but decreased with increasing CaO concentration. The activation energy for conduction ranged from 33.54 to 31.23 kcal/mole as the CaO concentration increased. The conduction mechanism was considered in terms of three arbitrary parameters: (1) equivalent oxygen packing density, (2) ionic radius, and (3) ion-oxygen attraction.     

Undulating Structure in Complex Silicate Glasses

Complex silicate glasses (e.g. commercial plate, sheet, and ophthalmic compositions), mica, and fused SiO₂ were carefully examined in the electron microscope using Pt-C replicating techniques. The complex glasses show an undulating structure at 300 to 500 Å, whereas the mica and fused SiO₂ do not. The low contrast observed between peak and valley is consistent with a small difference in chemical composition. The observed structure is explained in terms of an extension of the anionic model for molten silicates whereby the peaks and valleys result from clustering of silicate anions accompanied by partial expulsion of cations to the periphery.     

Viscoelastic Indentation of Silicate Glasses

The time-dependent viscoelastic deformation and flow of various types of silicate glasses are examined by the use of a pyramidal Berkovich indenter. It is demonstrated that a pyramidal indenter is an efficient microprobe for viscoelastic studies of glass-forming materials at temperatures near the glass transition point. Some important rheological functions of silicate glasses are determined as functions of time on the basis of a linear viscoelastic constitutive equation for pyramidal indentation. The test results are theoretically related to the dimension of flow units of these glasses, suggesting that a model of thermally fragmented silicate clusters is appropriate for consistently understanding the present rheological test results. However, the microstructural details (crystallite-like or network-like microstructures) of fragmented clusters are not inferred from the rheological information.     

Substructure Classification of Silicate Glasses

It is proposed that silicate glasses be classified in terms of substructures uniquely identified with primary crystallization phases. All glasses having compositions within the ranges or areas covered by a given primary crystallization phase field should constitute a separate glass type or class. This classification is definitive for properties determined principally by the silica frameworks or substructures. Such composition-property relations can be reproduced by independent investigators if the properties are measured under temperature equilibrium conditions.