Thermal conductivity of mixed alkali silicate glasses at low temperature

Thermal conductivity of glass is one of the fundamental properties of it. However, that has not been studied enough. That of only less than 20 compositions has been measured below the room temperature. In this study, we measured the thermal conductivity of xNa₂O · (100 ? x)SiO₂ and (33 ? y)Na₂O · yCs₂O · 67SiO₂ glasses by a transient heating method in the temperature range from about 150 K to room temperature. The conductivity of xNa₂O · (100 ? x)SiO₂ is found to decrease with the increase in alkali content. The dominant factor of this behavior is the decrease in phonon mean free path, which is due to the increase of non-bridge oxygen. Thermal conductivity of (33 ? y)Cs₂O · yNa₂O · 67SiO₂ is decreased with the increase in Cs₂O/(Na₂O + Cs₂O) ratio. The dominant factor of this behavior is the decrease of sound velocity. However, composition dependence of the phonon mean free path also affects the thermal conductivity. Phonon mean free path of 33Cs₂O · 67SiO₂ glass is longer than that of 33Na₂O · 67SiO₂ glass, and should be related to the change in distribution of structural unit in glass. In addition, phonon mean free path of mixed alkali glasses are shorter than that of single alkali glasses.     

Activation volumes and site relaxation in mixed alkali glasses

Evidence is presented for site relaxations occurring in mixed alkali (cation) glasses based on activation volumes, V_A(σ) = −RT[d ln σ/dp]_T, which are determined for sodium aluminoborate glasses of varying Na₂O content, and for mixed alkali glasses where Na⁺ is partially replaced by Li⁺, K⁺ or Cs⁺ ions. In accordance with the ‘updated’ dynamic structure model, activation volumes are identified here with local expansions that accompany the opening up of C′ sites to admit incoming ions. ‘Anomalous’ increases in activation volume in mixed cation glasses correlate with the size of minority (guest) cations. This anomaly is interpreted in terms of a ‘leader follower’ mechanism that involves dynamic coupling between the faster (majority) and slower (minority) cations. Because of mismatch effects in mixed cation glasses this coupling requires the opening up of additional cation sites by the slower follower cations. The resulting disturbances in the glass network are responsible for many characteristic features of the mixed alkali effect, including the appearance of high temperature internal friction peaks and observed minima in glass transition temperatures and melt viscosities.     

Phosphate speciation in sodium borosilicate glasses studied by nuclear magnetic resonance

The structure of RNa₂O · B₂O₃ · KSiO₂ · xP₂O₅ (0.5 < R < 2; 0.86 < K < 3) borosilicate glasses has been studied by nuclear magnetic resonance (NMR). ³¹P magic angle spinning (MAS), double quantum-magic angle spinning (DQ-MAS) and ³¹P–¹¹B transfer of populations under double resonance magic angle spinning (TRAPDOR MAS) NMR were used to determine the phosphate speciation in the glasses and their connectivity with the borosilicate network. The structure of the glass network was characterized with ¹¹B, ²⁹Si and ²³Na MAS NMR. Ab initio calculations of the ³¹P chemical shielding were carried out in order to confirm the connectivity between phosphorus and the structural units of the borosilicate glass network. Na₃PO₄ (monophosphate), Na₄P₂O₇ (diphosphate) and P–O–B species (mono- and diphosphate groups with borate units as the next nearest neighbors) are found all along the compositional range studied. The proportion of the P–O–B groups increases as the glass optical basicity decreases, while the proportions of mono- and diphosphate species decrease. The change in the glass transition temperature of the phospho-borosilicate glasses with respect to that of the borosilicate ones is discussed in terms of the structural characterization. The formation of phosphate species gives rise to the increase in T_g, which is attributed to the re-polymerization of the silicate network, while the formation of P–O–B bonds weakens the glass network and produces a decrease in the glass transition temperature.     

Effect of alkali and alkaline-earth cations on the neodymium environment in a rare-earth rich aluminoborosilicate glass

The local structure around neodymium in an aluminoborosilicate glass bearing 3.6 mol% Nd₂O₃ is studied by optical absorption spectroscopy and EXAFS at the Nd L_III- and K-edges. The influence of the nature of alkalis (M⁺ = Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺) and alkaline-earths (M²⁺ = Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺) on the coordination sphere of Nd³⁺ ions in the glass is particularly investigated. The Nd³⁺ sites are well-defined with NdO mean distances of 2.46 ± 0.03 Å, whatever the alkali and alkaline-earth ion type except Li⁺ and Mg²⁺, for which glasses exhibit slightly more disordered Nd sites and longer NdO distances (2.49 ± 0.03 Å). Using bond valence considerations, a model is proposed for the Nd site, and consists in 7–8 non-bridging oxygens (NBO), every NBO being charge compensated by 2–3 alkalis and alkaline-earths. The NdO mean distance is adjusted according to the mean field strength of these cations, to avoid overbonding of the NBO’s. A glass series with varying Ca²⁺/Na⁺ concentration ratio shows that Nd³⁺ cations are able to maintain this average coordination site even at high alkaline-earth content.     

Er3+-doped germanate glasses for active waveguides prepared by Ag+/K+ ↔ Na+ ion-exchange

Planar waveguides were prepared by Ag⁺/K⁺ ? Na⁺ ion-exchange on Er⁺³-doped GeO₂–ZnO–Na₂O–Li₂O glasses obtained by a melting–casting method. Optical parameters of the waveguides were measured at 543.5, 632.8, and 1550 nm by m-line technique as a function of the Ag⁺ ion-exchange time. The optimized planar waveguides show an effective diffusion depth (d) of 2.95 μm and well confined propagating TE₀ and TM₀ modes at 1550 nm. Spectroscopic properties as photoluminescence emission and emission decay time were evaluated for the erbium-doped planar waveguide, indicating that Ag⁺ ? Na⁺ ion-exchange enhance the photoluminescence emissions in the green and infrared regions from erbium ions. The glass system studied is promising to be applied as optical amplifiers in the C-telecom band. Green emission sensitized by Ag⁺ was also observed.     

The glass transition temperature of mixed glass former 0.35Na2O + 0.65[xB2O3 + (1 − x)P2O5] glasses

The mixed glass former effect (MGFE) is defined as the non-linear and non-additive change in the ionic conductivity with changing glass former fraction at constant modifier composition between two binary glass former compositions. In this study, sodium borophosphate glasses, 0.35Na₂O + 0.65[xB₂O₃ + (1 ? x)P₂O₅] with 0 ≤ x ≤ 1, have been prepared and their glass transition temperatures (T_g) have been examined as an alternative indicator of the MGFE and as an indicator of changes in the short range order (SRO) structural network units that could cause or contribute to the MGFE. The changes in T_g show a positive non-additive and non-linear trend over the changing glass former fraction, x. The increase in T_g is related to the increasing number of bridging oxygens (BO) in the glass samples, which is caused by the increase in the number of tetrahedral boron, B⁴, units in the SRO structure.     

Mixed transition-ion effect in the glass system: Fe2O3-MnO-TeO2

In earlier studies on phosphate and tellurite glasses containing vanadium and iron oxides, non-linear variation of physical properties as functions of the ratios of the transition ions (V/V + Fe) were observed. The most striking effect was observed with electrical conductivity, where a 3 orders of magnitude reduction in conductivity was observed at a V/V + Fe ratio of ~ 0.4. The effect was termed Mixed Transition-ion Effect or MTE. In phosphate glasses, however, MTE was not observed when one of the transition ions was manganese. It was concluded that Mn does not contribute to conduction in these glasses. In the present study, we demonstrate a mixed transition ion effect in tellurite glasses containing MnO and Fe₂O₃ (xFe₂O₃(0.2 ? x) MnO0.8TeO₂ with x varying from 0 to 0.2). A maximum in the property at an intermediate composition (x = 8.5 mol%), was observed in DC resistivity, activation energy, molar volume etc. Mossbauer and optical absorption (UV–VIS–NIR) measurements were performed on these glasses and the transport mechanism has been identified to be hopping of small polarons between Fe^3 + (Mn^3 +) and Fe^2 + (Mn^2 +) sites.     

Medium-range order in phospho-silicate bioactive glasses: Insights from MAS-NMR spectra,chemical durability experiments and molecular dynamics simulations

The medium-range order of phospho-silicate bioactive glasses (with compositions (2 ? p)SiO₂ · 1Na₂O · 1.1CaO · pP₂O₅, in which p = 0.10, 0.20, 0.26) has been studied by means of a combined-experimental (MAS-NMR, chemical durability measurements) and computational (classical molecular dynamics (MD)) approach. The structural model obtained by MD is showed to be helpful in the interpretation of the NMR spectra. A small amount of Si–O–P link units has been detected in glasses with low P₂O₅-content, but at high P₂O₅ concentration the percentage of Si–O–P bridges becomes important. However, Qⁿ distributions show that the HP5 (p = 0.20) glass structure is less polymerized with respect to the H (p = 0.10) and HP6.5 (p = 0.26) glasses. These results provide useful explanation of the behavior of these glasses in water and highlight the influence of the medium-range order on a very important property of potential bioactive glasses such as the chemical durability.     

Monte Carlo simulations of the dissolution of borosilicate glasses in near-equilibrium conditions

Monte Carlo simulations were performed to investigate the mechanisms of glass dissolution as equilibrium conditions are approached in both static and flow-through conditions. The glasses studied are borosilicate glasses in the compositional range (80 ? x)% SiO₂ (10 + x / 2)% B₂O₃ (10 + x / 2)% Na₂O, where 5 < x < 30%. In static conditions, dissolution/condensation reactions lead to the formation, for all compositions studied, of a blocking layer composed of polymerized Si sites with principally 4 connections to nearest Si sites. This layer forms atop the altered glass layer and shows similar composition and density for all glass compositions considered. In flow-through conditions, three main dissolution regimes are observed: at high flow rates, the dissolving glass exhibits a thin alteration layer and congruent dissolution; at low flow rates, a blocking layer is formed as in static conditions but the simulations show that water can occasionally break through the blocking layer causing the corrosion process to resume; and, at intermediate flow rates, the glasses dissolve incongruently with an increasingly deepening altered layer. The simulation results suggest that, in geological disposal environments, small perturbations or slow flows could be enough to prevent the formation of a permanent blocking layer. Finally, a comparison between predictions of the linear rate law and the Monte Carlo simulation results indicates that, in flow-through conditions, the linear rate law is applicable at high flow rates and deviations from the linear rate law occur under low flow rates (e.g., at near-saturated conditions with respect to amorphous silica). This effect is associated with the complex dynamics of Si dissolution/condensation processes at the glass–water interface.     

Structure and properties of the 70Li2S · (30 − x)P2S5 · xP2O5 oxysulfide glasses and glass–ceramics

The 70Li₂S · (30 ? x)P₂S₅ · xP₂O₅ (mol%) oxysulfide glasses were prepared by the melt quenching method. The glasses were prepared in the composition range 0 ≦ x ≦ 10. The glass–ceramics were prepared by heating the glasses over crystallization temperatures. The PO_nS_3?n (n = 1–3) oxysulfide units were produced in the glasses and glass–ceramics by partial substituting P₂O₅ for P₂S₅. In particular, the P₂OS₆^4? unit would be produced by substituting a small amount of P₂O₅ for P₂S₅. The oxygen atoms were incorporated into the Li₇P₃S₁₁ crystal structure because the diffraction peaks of the oxysulfide glass–ceramic shifted to the higher angle side. The glass–ceramic with 3 mol% of P₂O₅ exhibited the highest conductivity of 3.0 × 10^?3 S cm^?1 and the lowest activation energy for conduction of 16 kJ mol^?1. The P₂OS₆^4? dimer units in the oxygen-incorporated Li₇P₃S₁₁ crystal would improve conductive behavior of the Li₂S–P₂S₅ glass–ceramics.     

Molar volume,glass-transition temperature,and ionic conductivity of Na- and Rb-borate glasses in comparison with mixed Na–Rb borate glasses

Mass densities, molar volumes, glass-transition temperatures, and ionic conductivities are measured in series of YNa₂O · (1 − Y)B₂O₃ glasses, with Y = 0.00, 0.04, 0.08, 0.12, 0.16, 0.20, 0.25, 0.30 and YRb₂O · (1 − Y)B₂O₃ glasses, with Y = 0.00, 0.12, 0.16, 0.20, 0.25, 0.30. Measurements of the molar volumes indicate that the incorporation of rubidium ions leads to a considerable expansion of the network, which is not observed for sodium ions. The glass-transition temperature increases with increasing alkali content and reaches a maximum near Y = 0.25 for both glass systems. These trends are attributed to changes in the glass network. For each glass composition an Arrhenius-activated increase of the product of dc conductivity and temperature is observed. The activation enthalpy decreases with increasing number density of ions. A comparison between the binary sodium- and rubidium-borate glasses from this work, with the ternary sodium–rubidium borate glasses studied earlier in our laboratory, provides interesting insights in the influence of the glass structure on ionic transport processes and the mixed-alkali effect.     

Sodium tracer diffusion in sodium boroaluminosilicate glasses

Sodium tracer diffusion coefficients, D*_Na, have been measured using the radioactive isotope Na-22 in sodium boroaluminosilicate (NBAS) glasses containing either a small amount of As₂O₃ or Fe₂O₃. The chemical compositions of the first type of glasses are given by the formula [(Na₂O)_0.71(Fe₂O₃)_0.05(B₂O₃)_0.24]_0.2[(SiO₂)_x(Al₂O₃)_1 ? x]_0.8 and those of the second type of glasses correspond to the formula [(Na₂O)_0.73(B₂O₃)_0.24(As₂O₃)_0.03]_0.18[(SiO₂)_x(Al₂O₃)_1 ? x]_0.82. Tracer diffusion measurements were performed at different temperatures between 198 and 350 °C. Pre-annealing of the glass samples at their glass transition temperatures in common air was found to lead to changes in the values of sodium tracer diffusion coefficients. For the NBAS glasses containing Fe₂O₃, after pre-annealing for 5 h, the activation enthalpy derived for the sodium tracer diffusion increases almost linearly from 57.5 to 71.3 kJ/mol with a decrease in the alumina content while the pre-exponential factor of the sodium tracer diffusion coefficient increases from 2.1 · 10^? 4 to 5.3 · 10^? 4 cm²/s. For the iron-free NBAS glasses pre-annealed for 5 h, the activation enthalpy varies between 63.9 and 71.4 kJ/mol while the pre-exponential factor varies between 1.5 · 10^? 4 and 1.2 · 10^? 3 cm²/s. In addition, it was observed that the considered glasses take up water when annealed at 300 °C in wet air with P_H2O = 474 mbar.     

Properties of sodium-based ternary phosphate glasses produced from readily available phosphate salts

Three series of phosphate glasses were produced by melting together sodium phosphate salt (NaH₂PO₄) and the phosphate salts of either calcium (CaHPO₄), magnesium (MgHPO₄ · 3H₂O) or iron (FePO₄ · 2H₂O) in a 5% gold/95% platinum crucible at 1200 °C. The glass compositions were confirmed by EDX and XRD analysis. Glass transition temperature (T_g), density and durability in water were determined for all the compositions. Maximum metal oxide contents before devitrification were between 55% and 59% for CaO + Na₂O and 59% and 62% for MgO + Na₂O. The normalized equivalent for Fe₂O₃ + Na₂O was between 55% and 61%. Density values for the glasses lay between 2.49 and 2.75 g cm⁻³. T_gs lay between 295 °C and 470 °C. Degradation rates in deionized water at 37 °C lay between 0.03 g cm⁻² h⁻¹ for Na phosphate glasses and 9 × 10⁻⁶ g cm⁻² h⁻¹ for Ca phosphate glasses, 3 × 10⁻⁶ g cm⁻² h⁻¹ for Mg phosphate glasses and <3 × 10⁻⁶ g cm⁻² h⁻¹ for Fe phosphate glasses. The effect of metal addition on properties goes as Fe > Mg > Ca for degradation rates and T_g and Fe > Mg ≈ Ca for density. The change in properties with metal addition was seen to be linear for Fe and Ca additions but not with Mg addition. This is in agreement with the anomalous behavior of magnesium phosphate glasses.     

Reactions between sol–gel coatings and some technical glass substrates during consolidation

SiO₂ and Na₂O–SiO₂ coatings have been applied on float glass and other technical glass substrates by a sol–gel dip-coating process. After drying and baking these films at temperatures up to 500 °C and for times up to 1020 min, the in-depth profiles of the different constituents were measured by secondary neutral mass spectrometry (SNMS). Sn, Al, and Si turned out to be immobile, whereas a diffusion coefficient of ≈10⁻¹⁷ cm²/s could be evaluated for Mg at 500 °C for the transport from float glass into the films. Ca diffused a little faster, however, especially for the Na₂O–SiO₂ films a saddle point and finally a peak occurred in the interface region. This interface peak was even stronger for Na, showing quite anomalous profiles. The mechanism of this peak formation is explained mainly as an up-hill diffusion process. According to this model at the interface non-bridging –O⁻ ions are formed, whose electroneutrality has to be maintained by mobile cations like Na⁺ and Ca²⁺, even diffusing against their own concentration gradient. The other glass substrates, two borofloat glasses and an alkali-poor display glass showed similar but less pronounced effects.     

Crystallization kinetics of 1Na2O · 2CaO · 3SiO2 · glass monitored by electrical conductivity measurements

We investigated the kinetics of crystal nucleation, growth, and overall crystallization of a glass with composition close to the stoichiometric 1Na₂O · 2CaO · 3SiO₂. The nucleation and subsequent growth of sodium-rich crystals in this glass decreases the sodium content in the glassy matrix, drastically hindering further nucleation and growth. Compositional changes of the crystals and glassy matrix at different stages of the crystallization process were determined by EDS. These compositional variations were also monitored by electrical conductivity measurements, carried out by impedance spectroscopy, in glassy, partially, and fully crystallized samples. The electrical conductivity of both crystalline and glassy phases decreases with the increase of the crystallized volume fraction. Starting at a crystallized volume fraction of about 0.5, the crystalline phase dominates the electrical conductivity of the sample. This behavior was corroborated by an analysis of the activation energy for conduction. We show that electrical conductivity is highly sensitive and can indicate compositional shifts, changes in the spatial distribution of mobile ions in the glassy matrix. Conductivity measurements are thus a powerful tool for the investigation of complex heterogeneous systems, such as partially crystallized glasses and glass-ceramics.     

Thermal and spectroscopic properties of Tm3 + doped TZPPN transparent glass laser material

In this work a transparent bulk glass with the mol% composition 76TeO₂·10ZnO·9.0PbO·1.0PbF₂·3.0Na₂O doped with Tm^3 + has been synthesized. Results of differential thermal analysis (DTA) indicate a high thermal stability and low tendency to crystallization of this glass. The refractive indices at different wavelengths, the Urbach energy, the optical energy gap, the Sellmeier gap energy and the dispersion energy have been estimated. Spectroscopic quality factor of Tm^3 + was evaluated from optical absorption spectra. Electric and magnetic dipole transition probabilities, branching ratios, and radiative lifetimes of several excited states of Tm^3 + have been predicted using calculated intensity Judd–Ofelt parameters. The classical McCumber theory has been applied to evaluate the emission cross-sections for ³F₄ → ³H₆ transition around 1.8 μm. This study shows that TZPPN glass doped with Tm^3 + ions is a promising candidate for laser applications.     

Nano-composite soda lime silicate glass prepared using silver ion exchange

Commercial soda lime silicate glasses have been subjected to ion exchange at different temperatures ranging from 320 to 500 °C in a molten mixture of AgNO₃ and NaNO₃ with molar ratio of 10:90, 02:98 and 50:50 for different time periods ranging from 40 to 180 min. Optical and structural properties of the ion exchanged glass are measured using UV–Vis–NIR absorption spectroscopy, Fourier transform infrared (FTIR) spectroscopy, fluorescence spectroscopy and transmission electron microscopy (TEM). Signature of silver nanoparticle formation is obtained from the UV–Vis–NIR spectra, which shows a peak at 425 nm due to surface plasmon resonance (SPR). Replacement of Na⁺ ions by Ag⁺ ions is inferred from FTIR spectra. Fluorescence spectra reveal the formation of Ag⁰ atoms from Ag⁺ ions at higher temperatures. TEM image shows the silver nanoparticles of average size 3.75 nm. At exchange temperature of 500 °C Ag nanoparticles are formed without post-exchange annealing treatment.     

Germanosilicate and alkali germanosilicate glass structure: New insights from high-resolution oxygen-17 NMR

We present here triple-quantum, magic-angle spinning (3QMAS) NMR spectra for ¹⁷O in a SiO₂–GeO₂ binary glass, and for two sodium germanosilicate glasses, all with Si/Ge ratios of 1. In the binary germanosilicate, three NMR peaks are partially resolved, and correspond to the three types of bridging oxygens, Si–O–Si, Si–O–Ge, and Ge–O–Ge. Peak areas indicate that the relative abundances of these species are close to those expected for random mixing of the Si and Ge in the network. In a sodium germanosilicate glass with a relatively low Na content (Na₂O ≈ 8 mol%), the spectra demonstrate the formation of significant fractions of both nonbridging oxygens bonded to Si, and of oxygens bonded to Ge in five- or six-coordination. At higher Na content (Na₂O ≈ 31%), most or all Ge is four-coordinated and network modification is dominated by the formation of NBO on Si and on Ge. Models of physical properties of alkali germanosilicates, in which modifier oxides are distributed between the Si and Ge components of the network in proportion to the Si/Ge ratio, are thus supported, as is extensive mixing of Si and Ge.     

Microstructural analysis of Ga2S3–2MCl (M = K, Rb, Cs) glasses using Raman scattering

Raman scattering spectra of Ga₂S₃–2MCl (M = K, Rb, Cs) glasses have been conducted at room temperature. Based on the analysis of the local co-ordination surroundings of Cs⁺ ions, the similarities and differences of Raman spectra for the glass Ga₂S₃–2CsCl and the bridged molecular GaCl₃ were explained successfully. Through considering the effect of M⁺ ions on mixed anion units [GaS_4?xCl_x] and bridged units [Ga₂S_6?xCl_x] and the corresponding microstructural model, the Raman spectral evolution of the Ga₂S₃–2MCl (M = K, Rb, Cs) glasses was reasonably elucidated.     

Na2CaSi2O6–P2O5 based bioactive glasses. Part 1: Elasticity and structure

The glass structure and elastic properties of two bioglasses having bulk compositions near Na₂CaSi₂O₆ (45S5.2) and Na₂CaSi₃O₈ (55S4.1) were studied using both Raman and Brillouin scattering techniques. The annealed 45S5.2 glass has more Q² and Q⁰ but less Q³ species than 55S4.1 glass due to lower (Si⁴⁺ + P⁵⁺)/(Na⁺ + Ca²⁺) ratio. Brillouin scattering measurements of the as-annealed glasses indicated that 45S5.2 glass is ca. 2% and 9% higher in Young’s and bulk moduli than 55S4.1 glass due to more modifiers in the 45S5.2 glass. Nearly full crystallization of 45S5.2 glass was observed after treating it at 715 °C for ca. 30 min. Devitrification of the 45S5.2 glass caused an increase in the elastic moduli up to ca. 30% (fully crystallized) but a negligible change in density. This 45S5.2-derived crystalline phase displayed at least 17 Raman bands, and has the average elastic moduli of 72.4 (bulk), 41.6 (shear), and 104.7 (Young’s) GPa. The comparable elastic moduli with hydroxyapatite and the ability for developing a HCA layer in simulated body fluid indicate that the 45S5.2-derived phase may be better for using as a substitute of bone than its parent glass.