Specific Features of Changes in the Properties of One- and Two-Alkali Borate Glasses Containing Water: I. Viscosity, Thermal Expansion, and Kinetics of Structural Relaxation in Binary Alkali Borate Glasses

Alkali borate glasses with different contents of residual water are prepared by varying the synthesis conditions. The temperature dependences of the viscosity and thermal expansion of glasses are obtained. The structural relaxation parameters are calculated from the hysteresis dilatometric curves measured. The water content is determined using the IR absorption spectra in the range of stretching vibrations of hydroxyl groups at room temperature. It is found that an increase in the water concentration in alkali borate glasses leads to a decrease in the viscosity. The character of variations in the viscosity logarithm with a change in the water content depends on the alkali cation concentration. The glass transition temperatures determined from the dilatometric curves for all the studied glasses decrease with an increase in the water content. As the water concentration increases, the thermal expansion coefficient (above and below the glass transition range) and the degree of fragility decrease for glasses containing 25 mol % Na₂O, increase for glasses with an alkali oxide content of 15 mol %, and remain virtually unchanged for glasses involving 5.5 mol % Na₂O. A change in the water content in the concentration range under investigation does not affect the structural relaxation parameters.     

Specific Features of Changes in the Properties of One- and Two-Alkali Borate Glasses Containing Water: II. Viscosity of Mixed Alkali Borate Glasses

The temperature dependences of the viscosity for mixed alkali borate glasses containing lithium, sodium, and potassium oxides with a total alkali oxide content of 15 mol % are measured by the central rod bending method in the temperature range corresponding to a change in the viscosity from 10¹⁰ to 10¹³ dPa s. Analysis of the results obtained and the data available in the literature demonstrates that the content of residual structural water affects the manifestation of the mixed alkali effect in the viscosity of borate glasses. No significant change in the degree of fragility is found upon replacement of one alkali oxide by another alkali oxide in the studied glasses.     

Thermal Expansion of Alkali Borate Glasses

The thermal expansion behavior of lithium, sodium, potassium, and rubidium borate glasses was measured. The results indicate that the "borate anomaly" in the thermal expansion coefficient occurs at ∼20 mol% alkali oxide. A maximum in the glass transformation and dilatometric softening temperatures also occurs at ∼27 mol% alkali oxide. No evidence for phase separation was observed in this study. These effects are related to the structural changes reported for these glasses by Bray and coworkers.     

Preparation and Properties of Silver Borate Glasses

Silver borate Basses containing 0 to 30 mol% Ag₂O were formed. Properties measured include density, thermal expansion coefficient, glass transformation and dilatometric softening temperatures, transformation-range viscosity, dc electrical conductivity, and helium permeability and diffusivity. Optical spectroscopy revealed that the color of these glasses results from absorption bands at ∼407, ∼310, and ∼250 nm. Each band increased in intensity as the silver oxide content of the glass was increased. The properties of these glasses were consistent with the structural model currently used to describe the structure of alkali borate glasses. No evidence for the existence of phase separation was found for the conditions of the present study.     

Viscosity and Thermal Expansion of Mixed-Alkali Sodium-Potassium Borate Glasses

The transformation-range viscosity and thermal expansion be-havior of sodium-potassium borate glasses were studied. The results indicate that negative deviations from additivity in the viscosity of these glasses are a function of the Na²O to K²O ratio and that the magnitude of these deviations increases with alkali oxide content. The thermal expansion coefficient exhibits a maximum deviation from additivity as a function of the Na²o to K²O ratio. The magnitude of this deviation increases with increasing total alkali oxide content. The results indicate that there is little, if any, interaction between the factors which lead to the mixed-alkali effect and the changes in the structure of the vitreous network which occur as the alkali oxide content of the glasses increases.     

Structural Interpretation of the Results of Investigations into the Influence of Residual Water on the Properties of One-Alkali Borate Glasses

The results obtained in earlier investigations into the influence of water on the viscosity, thermal expansion coefficients, structural relaxation parameters, and fragility parameters of one-alkali borate glasses are analyzed, and schemes of the interaction of water with the glass network are proposed. The variations observed in the properties of glasses are explained in terms of possible changes in the ratio between different borate groups in the structure of the glass with a change in its composition.     

Structure of Borate Glasses

Anomalous thermal expansion and other properties of borate glasses have previously been given a structural explanation by Abe. This explanation is here reinterpreted and revised in terms of the structon theory of Huggins. In agreement with Warren, it is postulated that, in glasses containing, in addition to boric oxide, small relative amounts of an alkali or alkaline-earth oxide, some of the boron atoms are surrounded by four oxygen atoms and all the oxygen atoms bridge between two boron atoms. At concentrations beyond that of the thermal expansion minimum, some oxygens are assumed to have only one boron neighbor. These ideas are formulated quantitatively and are used to interpret pertinent experimental data. From the concentrations where the property-composition curves show breaks, conclusions may be drawn as to the average number of oxygens surrounding each metal atom.     

Boson Peak and Medium-Range Order Structure of Alkali Borate Glasses

The boson peaks in the spectra of alkali borate glasses are investigated. The results obtained are interpreted within a model of Einstein oscillators localized at medium-range order clusters with hinged bonds. Inferences are drawn regarding the statistical characteristics and self-organization of medium-range order structures in glasses, their vibrational spectrum, and the relation to the parameters of alkali ions.     

Thermal Expansion of Binary Borate Glasses and Their Structure

The possible structural transformations in binary borate glasses are analyzed by comparing the phase diagrams and the data available in the literature on the thermal expansion of glasses.     

Viscosity and Thermal Expansion of Mixed-Alkali Borate Glasses

Transformation-range viscosity and thermal expansion measurements were made for five series (Li-Na, Li-K, Li-Cs, Na-Cs, and K-Rb) of mixed-alkali borate glasses containing 30 mol% total alkali oxide. In each case, negative deviations from additivity were observed in the isokom and glass transformation temperatures. Positive deviations from additivity were observed in the thermal expansion coefficients for the Li-Na, Li-K, and K-Rb glasses, while the Li-Cs and Na-Cs glasses exhibited negative deviations. Maximization of the deviation from additivity in the viscosity and glass transformation temperatures occurred when the radius ratio of the two alkali ions was approximately 1.7 to 1.8. Combination of the results of this study with those of earlier studies indicates that the deviations from additivity observed for these properties are independent of the identity of the glass former.     

Temperature-Induced Structural Modifications Between Alkali Borate Glasses and Melts

High-temperature neutron diffraction and Raman spectra have been obtained on M₂O–2B₂O₃ (M=Li, Na, K) glasses and melts. Both techniques indicate a coordination change of boron atoms: the tetrahedral boron sites present in the glasses are converted into triangular boron sites. These changes of the borate network yield modifications of the alkali environment, as assessed for Li using the isotopic substitution technique. We observe that Li atoms are in a charge-compensating position in the glass and in a modifying position in the liquid. These structural modifications have important implications toward understanding the physical properties of borate melts.     

Properties and Structure of Lanthanum Borate Glasses

Glass formation limits were determined for the lanthanum borate glasses. Stable immiscibility prevents the formation of clear glasses over the range 0 to 20 mol% La₂O₃, but excellent quality glasses could be formed between 20 and 28 mol% La₂O₃. Data are reported for the density, refractive index, thermal expansion coefficient, glass transformation and dilatometric softening temperatures, transformation range viscosity, helium permeability, and chemical durability of these glasses. A limited Raman and infrared spectroscopy study suggests that lanthanum plays a similar structural role in these glasses and in the related crystals.     

States of Sulfur in Alkali Borate Glasses

Simple binary alkali borate glasses of differing alkali content (alkali = Li, Na, or K) containing sulfur were prepared and their absorption spectra were measured in the range of 200 to 700 nm. The absorption bands obtained were used to determine the stable states of sulfur in these glasses by directly comparing them with published data. The following states of sulfur were identified and their ranges of stability were determined: the S₂ molecule in glasses containing at least 15 mol% alkali oxide, the S₃⁻ and S₂⁻ in glasses containing 20 to 30 mol% Na₂O or K₂O, and the polysulfide ion (S x ²⁻) in glasses of 30 to 35 mol% Na₂O or K₂O.     

Thermal Expansion and Glass Transition Temperature of Calcium Borate and Calcium Aluminoborate Glasses

The thermal expansion (the temperature coefficients of linear expansion for solid glasses and the structural temperature coefficients of linear expansion) and the glass transition temperatures are studied for glasses in the CaO–B₂O₃ and CaO–B₂O₃–Al₂O₃ systems. The results obtained are compared with the available data for barium borate and barium aluminoborate glasses. The revealed dependences are interpreted within the concepts of the borate glass structure.     

Viscosity and Thermal Expansion of Sodium and Potassium Galliosilicate Glasses

The transformation-range viscosity and thermal expansion behavior of several series of sodium and potassium galliosilicate glasses were studied. The trends observed in the results are similar to those found for alkali aluminosilicate glasses. This result suggests a similarity in the structural roles of aluminum and gallium ions. The present data are discussed in terms of this structural role. In addition, the influence of silica content, alkali content, and alkali identity on the thermal properties of these glasses is discussed.     

Optical Absorption Characteristics of Ni2+ in Mixed-Alkali Borate Glasses

Nickel-containing mixed-alkali borate glasses were prepared and their spectral absorption in the visible and near-ir were measured and used to determine the ligand field strength, Racah parameter, and state of coordination of Ni²⁺ ions. The change in these parameters with alkali content is discussed in terms of mixed-alkali effect. Nickel ions were found to occupy octahedral sites in the 20 mol% (Li₂O+Na₂O) glasses, whereas in the 30 mol% (Na₂O+K₂O) glasses Ni^Z+ ions are believed to occupy both octahedral and tetrahedral sites. In most cases slight deviations were observed which increased with increasing alkali content. The results obtained suggest that the substitution of one alkali for another in alkali borate glasses did not produce gross alterations of the glass network sufficient to induce deviations from linearity in property-composition relations.     

Structure and Properties of Silver Borate Glasses

Clear glasses form in the system Ag₂O-B₂O₃ up to about 35 mol% (65 wt%) Ag₂O. Infrared absorption, thermal expansion, and density data indicated an analogy to the Na₂O-B₂O₃ system. Pentaborate-triborate group pairs appear to be formed on addition of Ag₂O to B₂O₃ up to 20 mol% Ag₂O and diborate groups from 20 to 33 mol% Ag₂O. This interpretation is supported by the comparison of the infrared absorption spectra of quenched and crystallized glasses. One crystallization product, Ag₂O-4B₂O₃, was identified previously. A new compound starts to appear at 28 mol% Ag₂O. The theory that silver is generally present as a network modifier like sodium was substantiated by the comparison of the molar volume of sodium and silver borate glasses. Above 27 mol% Ag₂O some atomic silver is assumed to be present; below 15 mol%, exploratory studies indicate a two-phase structure within an immiscibility gap. A low-temperature internal friction peak in the glasses up to 28 mol% Ag₂O corresponds to the alkali peak in other glasses; a high temperature peak appearing in the 34 mol% Ag₂O glass is associated with the appearance of nonbridging oxygen in the system.     

Infrared Spectroscopy of Wide Composition Range xNa2S + (1 –x)B2S3 Glasses

Wide composition range studies of the IR spectra of x Na₂S + (1 – x )B₂S₃ glasses are reported for the first time. Glasses can be prepared in two composition regions: 0 x 0.33 in a low-alkali region and 0.55 x 0.80 in a highalkali region. The structures of glasses in the former region are dominated by the creation of tetrahedral boron units similar to those observed in the alkali borate glasses. For pure B₂S₃, a large fraction of six-membered rings is found, as in glassy B₂O₃. As alkali is added, the vibrational frequency of the six-membered ring mode decreases, but not the intensity, and a new band grows in which is used to propose, by analogy with the alkali borate glasses, that the fraction of tetrahedral borons increases with alkali sulfide content. These observations suggest that isolated six-membered rings persist even in the presence of tetrahedral borons. For the high-alkali glasses, both the six-membered rings and tetrahedral boron structures are destroyed in preference to the formation of isolated orthothioborate groups, Na₃BS₃. The IR spectra of the high-alkali glasses show a monotonic increase in symmetry and simplicity, indicating an increase in structural simplicity as the orthothioborate composition is approached. For the orthothioborate composition, both glass and polycrystal can be prepared. The IR spectra of the two phases are very similar, with the glass exhibiting a broadened absorption envelope.     

Titanium Centers Induced in Borate Glasses by Irradiation

The electron paramagnetic resonance of alkali borate glasses containing Ti was observed at 9.1 GHz before and after room-temperature X irradiation. The irradiation-induced spectra can be separated into (1) the well-known complex borate resonance with an average g value greater than that of the free electron (2.0028) and (2) a resonance with a lower g value. The latter resonance consists of two asymmetric ESR lines, one broad and the other narrow, whose characteristics are reported. These lines were not observed in the corresponding Ti-free base glasses; they arise from induced Ti centers. The broad asymmetric line ( T ₁) corresponds to a Ti^3.1 ion in sixfold coordination, whereas the narrow asymmetric line ( T ₂) corresponds to a Ti³⁺ ion in a different structural configuration. Lines T ₁ and T ₂ are stable at room temperature, not easily saturated with microwave power, and exhibit behavior independent of the complex borate resonance. The total relative intensity of these lines depends on the total Ti concentration in the glass, the Ti⁴⁺/Ti³⁺ ratio in the glass before irradiation, and the structure of the glass.     

Visible Spectroscopy of Irradiated High-Alkali Borate and Mixed-Alkali Phosphate Glasses

Expectations of increased stability for trapped electrons in high-alkali glasses, based on extrapolations from observations on low-alkali borate glasses, are not borne out. In 69Na₂O-31B₂O₃ glass, electron centers have approximately the same thermal stability as in Na₂O·2B₂O₃ glass. In Na₂Oplus;P₂O₅ glasses the lifetimes, 3 · 0.5 μS, of transiently trapped electrons as well as their absorption spectra prove to be independent of increase of Na₂O content from 50 to 60 mol%. The same composition change destabilizes "permanent" hole centers. Exchange of Na₂O with K₂O in the metaphosphate glass also has no effect on the trapped electron lifetime. Small linear shifts in the trapped-hole absorption peak wavelengths are observed in the latter case. The most important positive finding in the phosphate glasses is a pronounced mixed-alkali effect on the yield of transiently trapped electrons and holes and of permanently trapped holes. The yield is a minimum at Na:K=1:1, due either to the elimination of trap sites or to the reduction of alkali ion mobilities which play a role in trap formation.