High-Temperature Energy Relations in the Alkali Borates: Binary Alkali Borate Compounds and Their Glasses

The energy relations existing between the congruently melting compounds Li₂O -2B₂O₃, Na₂O–2B₂O₃, Na₂O-4B₂O₃, and K₂O-4B₂O₃ and their glasses have been determined for the temperature range 25° to 1100°C. High-temperature heat-content, entropy, and heat of solution data are given for both the glasses and the corresponding devitrified materials. A comparison of the heats of fusion of the alkali borates on a gram atom of oxygen basis shows that they follow the order Li > Na > K. The entropy differences between the glass and the corresponding crystalline material have been determined at 25°C. The free-energy change at 25°C. for the reaction crystal → glass has been calculated for the four compounds.     

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.     

Infrared Absorption of Some Alkali Borate and Alkali Silicate Glasses Containing Nickel & Iron Oxides

This work has been carried out to investigate the infrared absorption spectra, of a number of alkali borate and alkali silicate glasses containing nickel oxide and/or iron oxide to investigate the structure of theses glasses. All glasses studied were melted in platinum 2 % rhodium crucibles in an electrically heated furnace at different temperature (1000 – 1400) ⁰C for two hours. The results are explained on the basis that the nickel and ferric ions exist in glass as divalent nickel and trivalent ferric ions respectively in two different states, i.e., octahedral and tetrahedral coordinations.     

Manganous-Manganic Equilibrium in Alkali Borate Glasses

The manganous-manganic equilibrium was studied in potassium, sodium, and lithium borate glasses. The glasses were analyzed chemically for total and trivalent manganese. The manganese equilibrium shifts more toward the oxidized state when the alkali concentration in the glass is increased and/or when the molar equivalent of lithia is replaced by soda and soda is replaced by potash. A linear relation between log (Mn³⁺/Mn²⁺) and mole percent alkali oxide was observed. Straight lines were obtained for lithium, sodium, and potassium borates. The disadvantages of using a concentration equilibrium constant where the glass composition is varied are discussed.     

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.     

Solubility of Water Vapor in Alkali Borate Melts

The solubility of water vapor at 750° to 1050°C was determined for alkali borate melts containing 0 to 40 mole % Li₂O, Na₂O, or K₂O. In all cases the solubility in these melts is linearly proportional to the square root of the H₂O partial pressure. At p H₂O = 1 atm and T = 900°C, o.5 to 2.2 mole % H₂O are dissolved in the melts in equilibrium. In the potassium borate melts a minimum of solubility was observed at about 25 mole % K₂O; in the sodium borate melts the minimum was at 35 to 40 mole % Na₂O. In the lithium borate melts a minimum of solubility was not reached in the range of compositions investigated. These results are discussed in terms of a concept for the acid-base properties of melts and glasses in which the position of the solubility minimum corresponds to the "neutral point" of an acidity-basicity scale. Some corroborating qualitative observations concerning the evaporation of components from the glass melts and the chemical resistivity of the corresponding solid glasses are discussed.     

Subliquidus Immiscibility in Binary Alkali Borates

The phase boundaries of subliquidus immiscibility in the lithium, sodium, potassium, rubidium, and cesium borate systems were determined. Two-phase structures were developed in thermal gradients and examined by direct-transmission electron microscopy. No opalescence was observed. Microstructures were of the order of a few hundred angstroms. The extent of immiscibility in each system was determined by the approximate intersection of the immiscible phase boundary with the glass transition region.     

Energy Relations in Binary Alkali Borates

The heats of solution of glasses and devitrified alkali borates in the systems Li₂O–B₂O₃, Na₂O—B₂O₃, and K₂O–B₂O₃ were measured in 2 N nitric acid with a simple vacuum-bottle calorimeter. The standard deviation was 0.35 cal. per gm. for approximately 130 determinations. From these data, together with available thermodynamic data, heats of formation were calculated for alkali borate glasses and devitrified alkali borates from two combinations of possible reactants. All the curves for the heats of solution of glasses showed minima at 20 mole % alkali oxide, but similar curves for devitrified alkali borates showed minima at 20 mole % for potassium and sodium borates only. The curve for devitrified lithium borates showed a minimum at 25 mole % alkali. The conclusion was drawn that there is probably no congruently melting Li₂O.4B₂O₃ compound.     

Compressibility of Binary Alkaline-Earth Borate Glasses at High Pressures

Compression data to 10,000 atm. were obtained for a series of barium, strontium, and calcium borate glasses at 21°C. The compressibility was found to be in the order Ba > Sr > Ca at corresponding molar concentrations and was not appreciably affected by pressure in the experimental pressure range. Comparison of data on alkali and alkaline-earth borate glasses shows that the compressibility at corresponding molar concentrations is smaller in the alkaline-earth glasses. Numerical comparisons at appropriate compositions show that a large part of the decreased compressibility produced by the alkaline-earth oxides arises from electrovalent sources.     

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.     

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.     

Volatility of Lead Silicate and Lead Borate Binary Melts

The volatility of binary lead silicate and lead borate melts has been determined at reduced pressures. These systems contain more than 71%, lead oxide. At comparable temperatures, the volatility of the binary lead silicate melt is higher than that of the lead borate of corresponding PbO to boron ratio. The volatilization of PbO is diffusion controlled. The approximate vapor pressure of PbO at 900°C. is 0.2 mm. Hg.     

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.     

Cation Field Strength Effects on Boron Coordination in Binary Borate Glasses

The field strength of modifier cations in boron‐containing oxide glasses has important but complex effects on boron coordination, and has long been known to have major effects on glass and liquid properties. With well‐constrained compositional and fictive temperature information in three binary borate glass series, we report how different modifier cations (Na⁺, Ba²⁺, Ca²⁺) affect boron coordination (¹¹B MAS NMR), as well as glass transition temperatures and configurational heat capacities (DSC). Using estimated reaction enthalpies for converting a ^[4]B to a ^[3]B with an NBO from previous studies, we compare boron coordinations in glasses with different modifier cations on an isothermal basis. Temperature and modifier cation effects can thus be isolated. At low modifier contents [R = (Na₂,Ca,Ba)O/B₂O₃<0.45], N₄ is systematically higher in the order Na>Ba>Ca, suggesting the enhanced stabilization of NBO for the divalent cations, especially for the smaller Ca²⁺. At higher R values, N₄ for Na borates drops below values for Ca and Ba borates. The trend in N₄ with modifier field strength reverses at high R values (~ R > 0.7), with Ca > Ba > Na. The transition may be related to the enhanced stabilization of ^[4]B‐O‐^[4]B groups by higher field strength cations in NBO‐rich glasses in which boron is the primary network component.     

Thermodynamic Modeling of Alkali Metal Oxide-Silica Binary Melts

The evaluation of the thermodynamic properties as well as the phase diagrams for the binary Na₂O–SiO₂, K₂O–SiO₂, and Li₂O–SiO₂ systems are carried out with a structural model for silicate melts and glasses. This thermodynamic model is based on the assumption that each metallic oxide produces the depolymerization reaction of silica network with a characteristic free-energy change. A least squares optimization program permits all available thermodynamic and phase diagram data to be optimized simultaneously. In this manner, data for these binary systems have been analyzed and represented with a small number of parameters. The resulting equations represent the thermodynamic and phase diagram data for these alkali metal oxide–silica systems within experimental error limits. In particular, the measured limiting liquidus slope at     is well reproduced.     

A Gamma-Ray Induced Absorption Band in Some Lead Borate Glasses

Observations made on some lead borate, lead aluminoborate, and lead boroaluminosilicate glasses have shown that an absorption band is induced at 1.5 e.v. (825 mμ) by gamma irradiation. Experimental evidence indicates that this band is associated with Pb²⁺ ions and boron in the structure of these glasses. On replacing PbO by Tl₂O in a borate glass, two strong bands were observed; one band corresponds to the 1.5 e.v. band induced in lead borate glasses and a completely new band at 1.0 e.v. (1235 mμ). A postulate for the center responsible for the absorption band at 1.5 e.v. is proposed.     

Ultraviolet Absorption Spectrum of Toluene-D8

The near ultraviolet absorption spectrum of toluene-d₈ has been recorded with the help of a E₁ large quartz spectrograph. The (O,O) band has been identified at 37672 cm^?1. The spectrum has been analysed with the help of the excited state fundamentals 282, 378, 431, 454, 508, 700, 926, 1150 and 1432 cm^?1. These frequencies have been correlated with the corresponding frequencies of the normal molecule. The ration R = ν(C₆H₅CH₃)/ν(C₆D₅CD₃) has been calculated for each excited state mode of vibration and compared with the corresponding value of R found for the ground state vibrations.     

Absorption Spectra of Some Alkali Borate Glasses Containing NiO or Fe2O3 or Mixed NiO + Fe2O3

The absorption spectra of a numbers of alkali borate glasses containing nickel oxide, ferric oxide or both nickel and ferric oxides together were studied to obtain information on their structure. The experimental results were obtained analyzed and explained by assuming that nickel exists in glass as divalent nickel ions in two different states of coordination octahedral and tetrahedral while iron exists in glass as ferric and ferrous states in two different forms, octahedral and tetrahedral according to the amount of the alkali oxide content. It was found that the change in the values of position and intensity of absorption bands may change as the result to the increased d-p orbital mixing which is related presumably to the lower ligand field strength at highest alkali oxide content.     

Gamma-Induced Absorption and Structural Studies of Arsenic Borate Glasses

Gamma-induced coloration in borate glasses containing arsenic was studied to secure information on: ( a ) the effect of addition of As₂O₃ on the induced coloration of the alkali borate base glass, and ( b ) the possibility of using radiation-induced coloration in studying structural changes. At concentrations of less than 10 mole % As₂O₃, arsenic takes network-modifying positions and at concentrations greater than 10.0 mole % As₂O₃, it takes network-forming positions. Between 15.0 and 25.0 mole % As₂O₃, an additional change in the structure was detected. This change may be confined to the mode of arrangement of the structural units associated with the formation of groups or compounds. Additional studies based on infrared absorption, ultraviolet absorption edge, and density measurements, as well as the chemical analysis of the glasses, confirmed the structural changes suggested from irradiation studies.