Specific Features of Changes in the Properties of One- and Two-Alkali Borate Glasses Containing Water: III. Thermal Expansion and the Structural and Mechanical Relaxation Parameters of Two-Alkali Borate Glasses

The thermal expansion and stress relaxation in mixed alkali borate glasses containing lithium, sodium, and potassium oxides with a total alkali oxide content of 15 mol % are measured on an inclined quartz dilatometer and a relaxometer. The experimental data obtained are used to determine the thermal expansion coefficients and the structural and mechanical relaxation parameters. No deviations from the additivity are found in the concentration dependences of the thermal expansion coefficient and the calculated parameters determining the width of the spectra of the structural and stress relaxation times. The IR absorption spectra of the studied glasses are recorded in the range of stretching vibrations of hydroxyl groups. Analysis of the IR spectra makes it possible to assume that the content of residual water in the structure of borate glasses affects the manifestation of the mixed alkali effect in the properties of these glasses.     

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.     

Synthesis and Properties of Glasses in the NaF–ZnSO4 System

The glass-forming ability is studied in the NaF–ZnSO₄ system. It is revealed that, when melts are cooled at a rate of 10³ K/s, the glass formation is observed in the concentration range 35–70 mol % ZnSO₄. The characteristic temperatures are determined by differential thermal analysis. According to these temperatures, fluorosulfate glasses can be assigned to low-melting glasses. The density of glasses is measured by hydrostatic weighing. The experimental data obtained are used to calculate the molar volume and the thermal stability parameters of fluorosulfate glasses.     

Towards highly transparent tungsten zinc sodium borate glasses for radiation shielding purposes

The impact of tungsten oxide (WO₃) additions on the structure, some physical and radiation shielding parameters of sodium zinc borate glasses have been scrutinized. These glasses were properly produced by the melt quenching method. The amorphous state was affirmed by X-ray diffraction (XRD) data. The internal structure within the short-range order of the glassy network was studied by the method of infrared spectroscopy (IR). The results of IR showed that the BO₄ units are transformed to BO₃ accompanied by the formation of nonbridging oxygens with further WO₃ doping. This transformation of BO₄ to BO₃ and nonbridging oxygens is employed to explain the increase in the molar volume values with changing WO₃/ZnO amount. Further, the optical transmittance was measured within the visible range to assure the transparency of the prepared glasses. The transmittance results confirm the absence of W⁵⁺, W⁴⁺, W³⁺ states; based on the absence of their absorption bands. Also, the transmittance results indicate that the only oxidation state in the present glasses is hexavalent tungsten (W⁶⁺). Additionally, the parameters of radiation protection of the manufactured glasses were investigated. It was found that, the addition of WO₃ improves not only the radiation protection parameters (such as the linear attenuation coefficient) but also the transparency of the prepared glasses. Finally, we concluded that, the addition of WO₃ to the glass samples leads to transparent glasses with an improved shielding ability at low energies but effects slightly at high energies. Due to the high transparency and the increased values of the linear attenuation coefficient of the prepared glasses, they are considered promising glasses in the field of nuclear radiation protection, especially at low energies.     

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.     

An Overview of the Structure and Properties of Silicon-Based Oxynitride Glasses

The silicon oxynitride glasses take advantage of nitrogen bonding to attain high elastic modulus, increased softening temperatures and viscosities, greater slow crack growth resistance, and modest gains in fracture resistance. Of the oxynitride glasses, the Si–Y–Al-based oxynitride glasses have been most extensively studied and a degree of success has been achieved in understanding how changes in glass composition affect structural parameters and their relationship with properties. More recent studies have focused on the Si–RE–Me oxynitride glasses, where Me is primarily Al or Mg and rare earth (RE) includes most of the lanthanide series elements. These glasses possess a range of elastic, thermal, mechanical, and optical properties, which can be correlated with the strength of the RE bond in terms of the cationic field strength. However, such correlations require knowledge of not only the RE valence state but also its coordination with the anions. Herein, the current state-of-the-art understanding of the properties and structural parameters of oxynitride glasses and their interrelationships are reviewed.     

Structure and disorder in MgSiO3 glasses above megabar pressures via nuclear magnetic resonance: DFT calculations

The structural modifications in oxide glasses under extreme compression may account for the pressure-induced increase in their mechanical toughness and rigidity, rendering potential for technological applications of the compressed glasses. High-resolution solid-state nuclear magnetic resonance has provided a structural information regarding glasses by identifying how nuclear spins behave and interact with nearby elements. However, knowledge of nuclear spins resonance in oxide glasses under extreme pressure above 1 million atmospheres has not been available, making the origins of glass densification illusive. In this article, ab initio calculations of prototypical magnesium silicate glasses quantify how structural changes in glasses affect the nature of nuclear spin interactions at high pressure beyond megabars. The calculated results establish novel correlations between pressure-induced evolution of atomic structures, such as oxygen and cation coordination numbers, bond angle and lengths, and structurally relevant nuclear magnetic resonance parameters for Mg, Si, and O in compressed oxide glasses above megabar pressures. The established correlations highlight that the nuclear spins in glasses can serve as a new indicator to the extreme densification paths. Pressure-induced dispersion in nuclear spin parameters also reveals an overall increase in the topological entropy. This entropy gain may weaken glasses at an elevated pressure conditions, accounting for potential softening of the compressed glasses. The proposed relationships open a new window to the evolution of diverse complex glasses under extreme stress and compression with high-resolution solid-state nuclear magnetic resonance.     

Myuller''s Structural Models in Statistical Thermodynamics of Glasses1

The impact of Myuller's chemical–structural method on the development of the modern concepts concerning the structure and physicochemical properties of glasses is considered. Making allowance for the switching of homo- and heterobonds, it is demonstrated that this method permits one to investigate the ranges of variations in glass parameters (adaptivity) and to estimate the energy accumulated during structural transformations. The partition function and the main thermodynamic functions of glasses are calculated by choosing a set of structural states that can be realized in glasses. It is proved that the adaptivity leads to multivaluedness of thermodynamic functions even for a particular glass composition.     

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.     

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.     

Structural transformations in thermally modified porous glasses

The changes in the structural parameters of porous glasses (pore radius, pore volume, specific surface of pores, structural resistance coefficients) upon heating are investigated as a function of the composition of initial two-phase alkali borosilicate glasses with the use of a number of independent methods, such as the adsorption techniques (water vapor adsorption, mercury porosimetry, thermal desorption of nitrogen), transmission electron microscopy, small-angle X-ray scattering, and membrane conductivity measurements. It is demonstrated that the structural transformations in thermally modified porous glasses are associated with the processes of overcondensation of pores and viscous flow in the silica network.     

Porous glass desalination membranes: phenomenological transport coefficients and fixed charge salt exclusion

The independent phenomenological transport coefficients for two porous glasses of different average pore diameter and surface charge densities were measured and compared with similar parameters of the asymmetric cellulose acetate membrane.The bound water model is used to explain the variation of the specific hydraulic permeability with pressure. Viscous and diffusive flow are discussed and the fixed charge model tested for one of the porous glasses.The results indicate that these porous glasses have inherently good potential practical desalination properties, their main limitation being imperfections or large cavities which are responsible for viscous flow, membrane degradation and solute leakage.     

Application of the Free Volume Concept to Glasses in the Ge–As–S System

The energy of hole formation E _h, the hole volume V _h, and the fraction f _g of fluctuation free volume for glasses in the Ge–As–S system are calculated from the data on the elastic moduli, the microhardness H, and the glass transition temperature T _g. It is shown that the fraction of fluctuation free volume in ternary glasses is considerably larger than that in arsenic chalcogenide glasses. For the glasses studied, the E _h energies fall in the range 13–18 kJ/mol and the hole volumes are equal to (7–84) × 10^–6 m³/mol. The concentration dependences of the elastic moduli and the parameters of the free volume theory for glasses in the As₂S₃–GeS₂ system are less pronounced than those for glasses in the As–S system with small coordination numbers of glass-former ions. This is explained by the topological transition observed in glasses of this system at a high germanium content. It is demonstrated that the parameters of the fluctuation free volume theory are related to the characteristics of the boson peak in the Raman spectra.     

Judd-Ofelt Parameters, Hypersensitivity, and Emission Characteristics of Ln3+ (Nd3+, Ho3+, and Er3+) Ions Doped in PbO-PbF2 Glasses

The Judd–Ofelt parameters, Ω₂ and ΣΩ_λ (λ= 2, 4, 6), for Nd³⁺, Ho³⁺, and Er³⁺ doped in the oxyfluoride glass 30PbO70PbF₂ lie intermediate between fluoride glasses and oxide glasses such as borate and phosphate glasses, providing evidence for the sensitivity of these parameters to the bonding environment. The variation of Ω₂ unlike Ω₄ and Ω₆ for the lanthanide series is qualitatively different for glass matrices compared to crystalline matrices. Plots of oscillator strengths of hypersensitive transitions for these ions against ΣΩ_λ (λ= 2, 4, 6) are found useful in discerning the degree of hypersensitivity of these transitions due to change in the host matrix. The ⁵F₂³K₈⁵G₆←⁵I₈ transition of Ho³⁺ is found to be the most hypersensitive. The radiative parameters for the oxyfluoride glasses are close to fluoride glasses and the branching ratio of the important lasing transition, viz., ⁴F_3/2→⁴I_11/2, of Nd³⁺ is higher for the present case compared to fluoride glass. The results suggest that the oxyfluoride glasses may be used as hosts in the place of fluoride glasses wherever suitable as they are more stable and easy to prepare and have similar radiative properties.     

Low-Melting Glasses Based on Borate Systems

The results of analyzing systems of low-melting glasses and their application areas based on published data are used to select an optimum system. The melting properties of low-melting glasses based on the borate system ZnO – SrO – B₂O₃ are studied. The dependences of the main properties of glasses (TCLE, softening temperature, microhardness, chemical resistance to water) on their compositions are determined. The optimum low-melting glass compositions with required physicochemical and technological parameters are identified.     

Effect of CuO-addition on the Dielectric Parameters of Sodium Zinc Phosphate Glasses

The dielectric properties of sodium zinc phosphate glasses doped with copper oxide have been investigated over a frequency range of 1.05–100 kHz in a vacuum medium (10^?5 torr). Some dielectric parameters, such as dielectric constant ε′, dielectric loss ε′′, dielectric loss tangent (tan δ) and AC conductivity, have been estimated. It is observed that the dielectric constant is strongly dependent on the CuO concentration in the investigated phosphate glasses. The AC conductivity measurements show that these phosphate glasses are good electronic conductors. With the aid of transmission spectra of these glasses, the absorption coefficient and the optical direct bandgap are estimated. The optical bandgap is found to decrease with increasing CuO content indicating the formation of non-bridging oxygens.     

The glass transition and the glassy state in isotropic and liquid crystalline polymers

PVT-measurements on atactic polystyrene shows that the Ehrenfest equations, which describe a thermodynamic transformation of the second order, are not applicable for the glass transition. The introduction of one internal ordering parameter ζ in addition to the conventional variables T and P, is sufficient to describe the behaviour in case of atactic PS in the isotropic glassy state. The usually observed way dependences in the glassy state can be explained by the concept of ordering parameters. In recent years liquid crystalline side chain polymers were developed. The liquid crystalline phases of these polymers can be supercooled and frozen-in like isotropic liquids. As the nematic or smectic liquid crystalline structure freezes-in, and can be conserved by this process, glasses with anisotropic properties are obtained. These glasses show e.g. a high optical birefringence. The glass transition of the liquid crystalline polymers can only be described by the assumption of at least two parameters ζ_i. Therefore the Ehrenfest equations are not applicable for the glass transition of isotropic as well as anisotropic glasses.     

The Mechanical Properties of Polymer Glasses

Polymer glasses are widely used in the container, appliance, construction, aerospace, and automobile industries in the form of compression or injection molded parts, solvent cast films, extruded films, coatings, adhesives, and composites. The increasing use of polymeric glasses in extreme service environments, particularly in the aerospace industry, requires a knowledge of their lifetime in such environments. To predict the lifetime of polymeric glasses in a service environment requires knowledge of: (1) details of the particular environment, (2) the nature of the failure processes, (3) the effect of the failure processes on the mechanical properties, (4) the structural parameters affecting the failure processes, and (5) how these structural parameters are modified by fabrication procedures and the service environment.     

Investigation into the Influence of Chloride Additives on the Properties of Alkali Silicate Glasses

High-alkali silicate glasses M-27 containing chloride additives (MCl, where M = Na and K) introduced in the course of synthesis (0, 1, 5, 10, 12.5 wt % Cl) are investigated. The glasses synthesized are analyzed for chlorine content, and their thermal and electrical characteristics are measured. The upper limits of the chlorine content in the glasses are determined. The nonlinear dependence of the as-analyzed chlorine content in a binary glass on the as-batched chlorine content introduced into a batch is explained by structural transformations during which chlorine in the glass interacts with polar groupings that are bonded to at least two polar tetrahedra. Only large-sized associates are able to fix chlorine. The proposed concept of structural transformations is used to interpret the changes observed in the bulk (thermal, electrical) parameters of glasses. It is shown that these changes are governed by two competitive processes, namely, the incorporation of chlorine into the glass structure and the change in the total glass composition due to volatilization of the components in the course of the synthesis. It is noted that the effect of chloride additives on the high-temperature viscosity of glass-forming melts is weaker than that of fluoride additives.