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Lithium aluminosilicate polymorphs α-LiAlSi2O6, β-LiAlSi2O6, and the LiAlSi2O6 glass have been studied comparatively using classical molecular dynamics ( MD) simulations with an aim to better understand the structural origin of the different thermomechanical behaviors and lithium ion diffusion properties. The melting behaviors and structural evolution were investigated for the three phases using MD simulations. The structural features of the three simulated samples were analyzed using coordination number, pair and bond angle distributions. The results showed that β-LiAlSi2O6 and the LiAlSi2O6 glass had similar melting behavior, had more random short-range atomic structures, and lower densities as compared to the α-LiAlSi2O6 phase, which has a more ordered and compact structure. The lithium ion diffusion behavior in α-LiAlSi2O6, β-LiAlSi2O6, and LiAlSi2O6 glass and their melts are determined and compared by calculating the mean square displacements. It was found that at high temperatures, the melts of α-LiAlSi2O6, β-LiAlSi2O6, and LiAlSi2O6 glass had similar diffusion properties. While at low temperatures, α-LiAlSi2O6 had the lowest diffusion coefficient and highest diffusion energy barrier due to its more close-packed structure and lacking of defects to facilitate lithium ion diffusion. Both the β-LiAlSi2O6 and glass show high ionic conductivity even at low temperatures. This originates from their lower density and thus relatively open structures, but slightly different diffusion mechanisms. Lithium ion diffusion in β-LiAlSi2O6 is through the large available interstitial sites while that in the glass is through vacancies due to high free volume. The glass phase had slightly lower lithium ion diffusion energy barrier and higher lithium ion diffusion coefficients as compared to the β-LiAlSi2O6 phase, indicating the glass phase can achieve high ionic diffusion and, in some cases, even higher than the crystalline phases with similar densities and short-range structures. [ABSTRACT FROM AUTHOR]

Low-Energy Ion-Scattering ( LEIS) spectroscopy is a technique with a unique sensitivity to the elemental composition of the top atomic layer of a solid surface. LEIS measurements of ternary silicate glasses modified with Na2O, Cs2O, CaO, and BaO show that the compositions of the as-cast (melt) surface and the in-vacuum fracture surface often differ. While the as-cast surface is usually depleted of alkali ions (Na+ or Cs+) compared to the nominal (batch) glass composition, there is often strong accumulation of the same mobile cations on the fresh fracture surface. Depth profiles obtained by sputter etching reveal elemental concentration gradients normal to the glass surface. The final concentrations often fail to reach the nominal glass composition, suggesting the likely presence of preferential sputtering effects and thereby the distortion of the measured concentration gradient. At present, the lack of reliable standards and preferential sputtering effects in the LEIS of multicomponent glasses limit somewhat the absolute chemical composition and structural information that can be obtained with this otherwise unique and powerful method of surface analysis. [ABSTRACT FROM AUTHOR]

This study presents a simple method for retrieving the effective thermal conductivity of semitransparent glassmelts from measured temperature profiles. Effective thermal conductivity of molten glass at high temperature is an important thermophysical property that affects the glassmelting and forming processes and thus the quality of the final glass products. In semitransparent glassmelts, heat is transferred by both conduction and radiation. In the limiting case of optically thick glassmelts, typically featuring high iron content, thermal radiation can be treated as a diffusion process. The total heat flux can be expressed as the sum of a phononic and a radiative heat fluxes based on Fourier's law. For weakly absorbing glassmelts, the temperature profile may be strongly nonlinear particularly neat container walls due to the contribution from emission and absorption. Steady-state measurement techniques, such as the linear heat flux method, have been developed to measure glassmelt effective thermal conductivity at high temperatures. However, they typically use only three temperatures measurements and assume linear temperature profile in the glassmelt. The new retrieval method addresses these drawbacks particularly for weakly absorbing glassmelts featuring nonlinear temperature profiles. It is demonstrated with experimental data collected for soda-lime silicate glasses with iron content ranging from 0.008 to 1.1 wt% and temperatures between 1100°C and 1550°C. [ABSTRACT FROM AUTHOR]

A series of alkaline-earth silicate glasses, with compositions ranging from the metasilicate to the ortho- and suborthosilicate, have been synthesized by aerodynamic levitation and CO2 laser melting. They have been studied by high-temperature oxide melt solution calorimetry with 2 PbO· B2 O3 as solvent. The enthalpies of formation from the oxides at room temperature ( [ABSTRACT FROM AUTHOR]

The structure and properties of polymer-derived Si-( B-) O- C glasses have been shown to be significantly influenced by the boron content and pyrolysis temperature. This work determined the impact of these two parameters on the thermodynamic stability of these glasses. High-temperature oxide melt solution calorimetry was performed on a series of amorphous samples, with varying boron contents (0-7.7 at.%), obtained by pyrolysis of precursors made by a sol-gel technique. Thermodynamic analysis of the calorimetric results demonstrated that at a constant pyrolysis temperature, adding boron makes the materials energetically less stable. While the B-containing glasses pyrolyzed at 1000°C were energetically less stable than the competitive crystalline components, increasing the pyrolysis temperature to 1200°C led to their enthalpic stability. 29 Si and 11B MAS nuclear magnetic resonance ( NMR) spectroscopy measurements on selected samples confirmed a decrease in the concentrations of mixed Si-centered SO i C4− i and B-centered BO j C3− j bonds at the expense of formation of SiO4 and B( OSi)3 species (indicating a tendency toward phase separation) when the boron content and pyrolysis temperature increased. In light of the findings from calorimetry and NMR spectroscopy, we propose a structure-energetic relationship in Si-( B-) O- C glasses. [ABSTRACT FROM AUTHOR]

The morphological and structural properties of sodium silicate ( Na2 O- SiO2) glasses were analyzed using atomic force microscopy ( AFM) and light scattering following thermal treatments. AFM observations indicated that the glass surface microstructure evolves during the phase separation mechanisms from continuous interpenetrating phases in the spinodal decomposition process to separated droplets embedded in a continuous matrix for the nucleation/growth one. Raman mapping gave evidence of a phase separation through the nucleation/growth process with formation of silica-rich clusters characterized by higher polymerization degree as separate droplets. The variations in inhomogeneities versus temperature investigated by Brillouin are exponential for spinodal decomposition and linear in the case of nucleation/growth mechanism. Nuclear magnetic resonance spectroscopy was used to investigate the spatial distribution of the various Qn species present in thermally treated glasses and allows determining fractal dimension between two and three. [ABSTRACT FROM AUTHOR]

The dynamic structure factor of a series of sodium aluminophosphate melts has been measured using photon correlation spectroscopy. These melts consist of oxide networks that undergo a uniform increase in the lattice connectivity (per structural unit) from 〈 n〉 = 2 to 〈 n〉 ≈ 2.8 with the addition of alumina. Characteristics of the main viscoelastic relaxation (its shape parameter β and fragility m) were evaluated and the fragility is seen to decrease in a manner identical with that reported previously for both chalcogenide and sodium phosphate melts. To explain this common dependence on connectivity, we turn to the bond lattice model and simulations that indicate a direct proportionality between fragility and the excess entropy gained in the removal of a lattice constraint. We propose that the variation in the fragility with lattice connectivity might be decomposed into two regimes. In the first regime at 〈 n〉 < 2.4, the linear decrease in m with 〈 n〉 is argued to reflect a decreasing configurational entropy associated with the formation of covalent crosslinks between polymer-like chains that vanishes in the vicinity of the rigidity threshold. In the second regime at 〈 n〉 > 2.4, we speculate that the variation in m with 〈 n〉 results from a weaker, vibrational contribution to the entropy. [ABSTRACT FROM AUTHOR]

Porous glasses with an aligned pore system were prepared by using a stretch-forming device. For the experiments a sodium borosilicate glass with the composition 62.5 wt% SiO2, 30.5 wt% B2O3 and 7 wt% Na2O was prepared. The characterization of this glass includes X-ray fluorescence analysis and dilatometry. The applied stretch-forming apparatus allowed the simultaneous phase separation and elongation of the glass samples. The obtained glass templates were chemically treated with hydrochloric acid and sodium hydroxide solution to extract the soluble sodium rich borate phase. The resulting porous glass samples were characterized by environmental scanning electron microscopy and mercury intrusion. Different degrees of pore alignment were realized by varying the parameters of the stretch-forming process. [ABSTRACT FROM AUTHOR]

A series of alkaline-earth silicate glasses, with compositions ranging from the metasilicate to the ortho- and suborthosilicate, have been synthesized by aerodynamic levitation and CO2 laser melting. They have been studied by high-temperature oxide melt solution calorimetry with 2 PbO· B2 O3 as solvent. The enthalpies of formation from the oxides at room temperature ( [ABSTRACT FROM AUTHOR]

In order to explore the structural roles of sodium in boroaluminosilicate glasses, we have designed ten Na²0-B²0³-Al²0³-Si0² glasses with varied [Al²0³]/[Si0²] ratio to access different regimes of sodium behavior. Multinuclear nuclear magnetic resonance (NMR) experiments on 11B, 27Al, 29Si, and 23Na were performed to determine the complicated network former speciation and modifier environments as a function of glass composition. The different roles of sodium in relation with the network-forming cations (Si, B, and Al) have been clarified and quantified. When [Na²0] < [Al²0³], all available sodium is used to charge compensate fourfold coordinated aluminum (A1iv), and deficiency in sodium concentration leads to fivefold coordinated aluminum (Aliv) groups. When [Na²0] < [AI²O³], sodium first charge compensates Aliv, and thus all aluminum is fourfold coordinated and unaffected by other compositional changes. Hence, the preference in the formation of Aliv over that of fourfold coordinated boron (Biv) is confirmed. Excess sodium can be used to convert threefold coordinated boron (Biii) to Biv orto create nonbridging oxygen (NBO) on Si and B, with a thermodynamic competition among these mechanisms. The NBOs on Si are quantified using 29Si wide-line and magic angle spinning NMR. The fraction of silicon atoms associated with NBOs is calculated using a random model and compared with the NMR results. Finally, we have found that our previously proposed two-state statistical mechanical model of boron speciation accurately predicts the fraction of tetrahedrally coordinated boron atoms (N4) in these mixed network former glasses. [ABSTRACT FROM AUTHOR]