Non-isothermal crystallization kinetics of ZnO-BaO-B2O3-SiO2 glass

Glass of composition 40SiO₂-30BaO-20ZnO-10B₂O₃ (mol%) was made by conventional melting and casting process. Crystallization kinetics of above glass has been investigated under non-isothermal conditions, using the formal theory of transformations for heterogeneous nucleation. The procedure is applied to the experimental data obtained by differential thermal analysis (DTA), using continuous-heating techniques. The crystallization results are analyzed, and both the activation energy of crystallization process as well as the crystallization mechanism is characterized. Dilatometric measurement of this glass was also done and data obtained was used to calculate the viscosity of the formed glass. Development of crystalline phases on thermal treatments of the glass at various temperatures has been analyzed by X-ray diffraction. Microstructure of the crystalline phases was investigated by scanning electron microscopy (SEM) and the development of various structural features with variation in heat treatment cycle was observed. The nucleation and growth of these phases in the matrix of glass has been described and discussed.     

Structural characterization of phosphate glasses doped with silver

The present paper reports the influence of silver oxide addition on the local structure of 2P₂O₅ · CaO · 0.05ZnO glass matrix. The glass samples were investigated through several methods: X-ray powder diffraction (XRD), scanning electron microscopy (SEM), infrared absorption (FT-IR) and Raman scattering. The X-ray diffraction patterns confirm the vitreous character of these samples over the explored compositional range and the SEM pictures confirm this information. The phosphate structural units of the network former are assessed from FT-IR and Raman spectra as ultra-, meta-, pyro- and orthophosphate units. A slight structural depolymerization process of these phosphate-based glasses was evidenced for higher silver oxide content. In vitro behavior of the bulk glass sample with the highest silver oxide content was tested by immersion in simulated body fluid (SBF). X-ray diffraction and SEM measurements made on the SBF treated sample revealed growth of a crystalline phase on the surface sample.     

The ionic transition and the glass transition in ion-conducting glasses

In an ion-conducting glass, there may be the ionic transition which characterizes ‘melting’ of the modifying ions in a similar way as the glass transition characterizes ‘melting’ of the glass network former. The ionic transition can be observed, electrically, by the thermally stimulated depolarization current technique. It is demonstrated that the relaxation time for the ionic transition is the same (near 100 s) with that for the glass transition in so far as the heating/cooling rate lies within, say, 10⁻¹-10⁻⁵ K/s as adopted in our routine works.     

Crystallization and dielectric properties of low temperature dielectrics containing Li2O filler

Crystallization and dielectric properties of typical low temperature co-fired ceramics (LTCC) consisting of calcium zinc aluminoborosilicate glass and Al₂O₃ filler were investigated by substituting the Al₂O₃ filler partially with Li₂O at the levels of 2-10 wt%. Depending on the content of Li₂O, densification was found significantly affected by early crystallization that resulted from the formation of unexpected crystalline phases including LiAlSiO₄, Ca₂SiO₄, LiAlO₂, and LiAlSi₃O₈. The effect of hindering sintering via earlier crystallization became enormous regardless of firing temperature when >5 wt% Li₂O substitution occurred. It was observed that the substitution of 2 wt% Li₂O for Al₂O₃ was beneficial in producing promising performance at the low temperature of 750 °C, which can be highlighted with k ∼ 8.7 and tan δ ∼ 0.009 at 1 MHz.     

Chemical durability of lead silicate glass in HNO3, HCl and H2SO4 aqueous acid solutions

Acid dissolution of silicate glasses with different lead contents was rigorously investigated. Aqueous solutions containing 0.5, 1, and 2 N HNO₃, HCl and H₂SO₄ were used to measure the durability of the glass probes. Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), Inductively Coupled Plasma (ICP), X-ray Diffraction (XRD) and weight loss analyses were used to evaluate the morphological/compositional changes of the probes, the ash deposit, and the aqueous solutions produced due to the dissolution of the glass specimens. Empirical results showed that any increase in the lead content of the probes deteriorated the durability of the glasses by accelerating the hydrolysis of the silica network. ZrO₂ and TiO₂ additions had inverse effect and improved the chemical durability and the practical life-time of the lead glasses.     

Investigation of the chemical solubility of mixed-alkali fluorcanasite forming glasses

There is currently significant interest in all-ceramic dental restorations due to the demand to replace metals as the primary load-bearing tooth restorative material. A promising non-metallic material for such restorations is fluorcanasite, a chain-silicate glass-ceramic that is castable using conventional dental metal-casting techniques and which exhibits enhanced fracture toughness and flexural strength compared with currently available resin-bonded ceramics. Unfortunately, because of its relatively low silica content, it exhibits poor chemical durability. The aim of this study was to assess the influence of compositional changes on the formation and chemical solubility of the fluorcanasite forming glass, crystalline phase and residual glass. To this end, mixed-alkali compositions have been investigated and it has been shown that the solubility of the glass is a function of the alkali species present in the glass. By changing the alkali ratio of the fluorcanasite forming glass from 0.33 ([K]/[K + Na]) of the base composition derived from stoichiometry to 0.47, it was possible to reduce the chemical solubility of the fluorcanasite material significantly. The addition of extra CaF₂ to refine the grain structure resulted in a decrease in the durability of the material, making it currently unacceptable for dental applications. The glass-ceramic exhibits a minimum chemical solubility at the composition K⁷/Na⁸. The residual glass may have a slightly elevated K content compared to the original glass. The addition of extra CaF₂ to refine the grain structure resulted in an unacceptable decrease in the durability of the material.     

Elastic properties and structural studies on lead-boro-vanadate glasses

This paper describes elastic properties and spectroscopic studies on the xPbO-50B₂O₃-(50 - x)V₂O₅ (where x = 20, 25, 30, 35, 40, 45 and 49 mol%) glass system. Elastic moduli and spectroscopic parameters exhibit compositional dependent trends and the existence of characteristic borovanadate groups in these glasses. The bulk modulus and shear modulus increase with the concentration of [BO_4/2]⁻ and [B₂V₂O₉]²⁻ groups, which increases the dimensionality of the network. The scheme of modification of borate and vanadate groups has been explained by considering the Sanderson’s electronegativity principle. Analysis of infrared(IR) and magic angle spinning-nuclear magnetic resonance (MAS-NMR) spectra suggests the presence of characteristic diborovanadate groups also in these glasses. The results are examined in view of the structural groups formed due to the presence of PbO as a modifier.     

Various transmission electron microscopic techniques to characterize phase separation - illustrated using a LaF3 containing aluminosilicate glass

Various transmission electron microscopy techniques which are useful for the characterization of phase-separation phenomena in glass materials are exemplarily demonstrated at a 40 SiO₂-30 Al₂O₃-18 Na₂O-12 LaF₃ glass. It is shown that direct imaging of phase-separated regions possesses manifold advantages over the traditional replica technique, particular if the feature size approaches the nano-scale. Although surface replica can be accomplished effortless and made a great leap in glass structure research possible, the method is indirect, i.e. requires a deep understanding of the chemical attack leading to the surface topography eventually imaged. Moreover, surface replicas reach their limits for nanosized features due to the inherent structure of the deposited replica film.     

High resolution O MAS and triple-quantum MAS NMR studies of gallosilicate glasses

Gallosilicate (or ‘galliosilicate’) glasses have been widely studied as analogs of aluminosilicates, and the variations with composition in properties are known to be similar in both systems. We have applied ¹⁷O MAS and triple-quantum MAS (3QMAS) NMR spectroscopy to investigate the oxygen local environments in Na-, Li-, Ca- and Y-gallosilicate glasses. Signals due to several different oxygen species can be resolved and their concentrations quantified and NMR parameters determined. The NMR spectra generally resemble those of aluminosilicate glasses, indicating that the current model of gallium ions occupying the same types of sites as aluminum ions is a good first approximation. Quadrupolar coupling constants for the various oxygen sites tend to be larger than those for aluminosilicates, however, and vary less among different types of sites. Broader ¹⁷O spectra at low to medium external magnetic fields result. In detail, several types of differences between the observed oxygen species populations for gallosilicate and aluminosilicate are consistent with the larger radius of Ga³⁺ in comparison to Al³⁺, and possibly with a somewhat greater tendency for the former to form groups with oxygen coordination numbers greater than four. These include more Si-Ga disorder than Si-Al disorder in corresponding sodium gallo- vs. aluminosilicates, and more ‘non-stoichiometric’ non-bridging oxygen in a calcium gallosilicate than in a calcium aluminosilicate glass.     

Molecular dynamics simulations of calcium aluminate glasses

Molecular dynamics simulations of a series of calcium aluminate glasses have been carried out using empirical potentials with a covalent term. The simulations closely reproduce the total neutron correlation function of glasses with 30 and 38 mol% Al₂O₃ and physical properties such as elastic constants. For compositions close to the eutectic 37 mol% Al₂O₃, aluminum is tetrahedrally coordinated by oxygen, but the proportion of five-fold and six-fold coordination and also edge-sharing tetrahedra gradually increases with increasing Al₂O₃ content. The coordination number for oxygen around calcium atoms is close to 6 and this involves a larger coordination at a shorter, well-defined distance, followed by a smaller coordination with a broader range of distances. When the Al₂O₃ content decreases, the calcium aluminate structure becomes depolymerised and the average ring size increases. Also reported is an X-ray photoelectron spectroscopy measurement on a glass sample with 38 mol% Al₂O₃, which shows that 38.6 ± 0.9% of the oxygens are non-bridging, in excellent agreement with the simulations.     

A multi-peak transformation kinetics model and its application to the isothermal crystallization of Mg65Cu25Y10 amorphous alloy

Transformation kinetic analytical model plays an important role in the prediction of the microstructural evolution. In this paper, a simple formula has been developed for isothermal mixed nucleation transformation as the kinetic parameters of its JMAK-form formula vary upon time. The explored multi-peak transformation kinetics shows that each peak can be treated as a JMAK case, which is consistent with the classical JMAK model in only one peak case. Thereafter, a method has been developed to deal with the isothermal DSC data of multi-peak overlapping transformation. The isothermal crystallization process of Mg₆₅Cu₂₅Y₁₀ amorphous alloy has been explored and fitted well with the multi-peak kinetics model, which indicates a continuous nucleation, three dimensional interface-controlled growth mechanism with three crystallization peaks overlapping each other.     

Effect of antimony addition on the thermal and electrical-switching behavior of bulk Se-Te glasses

The thermal properties and electrical-switching behavior of semiconducting chalcogenide Sb_xSe_55−xTe₄₅ (2 ? x ? 9) glasses have been investigated by alternating differential scanning calorimetry and electrical-switching experiments, respectively. The addition of Sb is found to enhance the glass forming tendency and stability as revealed by the decrease in non-reversing enthalpy ΔH_nr, and an increase in the glass-transition width ΔT_g. Further, the glass-transition temperature of Sb_xSe_55−xTe₄₅ glasses, which is a measure of network connectivity, exhibits a subtle increase, suggesting a meager network growth with the addition of Sb. The crystallization temperature is also observed to increase with Sb content. The Sb_xSe_55−xTe₄₅ glasses (2 ? x ? 9) are found to exhibit memory type of electrical switching, which can be attributed to the polymeric nature of network and high devitrifying ability. The metallicity factor has been found to dominate over the network connectivity and rigidity in the compositional dependence of switching voltage, which shows a profound decrease with the addition of Sb.     

Thermal properties and crystallization of iron phosphate glasses containing up to 25 wt% additions of Si-, Al-, Na- and U-oxides

The thermal properties (expansion, T_g and T_SOFT.) of glasses, having 56-66% P₂O₅, 14.8-34.2% Fe₂O₃ and 2-25 wt% additions of SiO₂, Al₂O₃, Na₂O and UO₂, were comparatively estimated from dilatometric measurements in similar conditions. The T_g reversibility was clearly verified by varying the heating rates between 1 and 5 °C min⁻¹. From linear equations fits of the various glass properties as functions of the six components it is suggested the iron, sodium and uranium oxides decrease the thermal expansion (for 50 < T ? 300 °C), T_g and T_SOFT. From DTA/XRD analysis of three glasses it was confirmed the crystallization tendency decreased with increasing the UO₂ level in the glasses. Leaching test data for two compositions containing Na₂O suggest addition of UO₂ increases the chemical durability of the related glass. The roles of UO₂, Na₂O and Fe-oxide species as structural components of the glass network are discussed.     

Simulation and scaling of ac conductivities in ion-conducting glasses

Simulation is made about ac conductivity, σ(ω), in ion-conducting glass by recurring to one of the Maxwell’s equations. It is concluded that (1) σ(ω) is made of conduction and conduction-related polarization of the mobile ions, both governed by the same relaxation time, τ, and that (2) high frequency dispersion in σ(ω), high frequency dispersion in the electric modulus peak, and distortion of a semicircle in the complex impedance plane, all arise from a distribution in τ, caused by the disordered structure of glass. Finally, a procedure of scaling ac data measured at various temperatures into a single master curve is given.     

Effect of P2O5 content in two series of soda lime phosphosilicate glasses on structure and properties - Part I: NMR

The effect of the variation in phosphate (P₂O₅) content on the structure of two series of bioactive glasses in the quaternary system SiO₂-Na₂O-CaO-P₂O₅ was studied. The first series (I) was a simple substitution of P₂O₅ for SiO₂ keeping the Na₂O:CaO ratio fixed (1.00:0.87). The second series was designed to ensure charge neutrality in the orthophosphate , therefore as P₂O₅ was added the Na₂O and CaO content was varied to provide sufficient Na⁺ and Ca²⁺ cations to charge balance the orthophosphate present. The glass network connectivity (NC) was calculated for each glass and a modification for the presence of a separate P₂O₅ phase was included (NC′). ³¹P and ²⁹Si magic-angle-spinning nuclear magnetic resonance (MAS-NMR) spectroscopy was performed on glass series I and II to determine the structural units present and their relation to glass properties. ³¹P MAS-NMR spectra of series I resulted in a broad resonance around 9 ppm corresponding to orthophosphate in an amorphous environment. The 9.25 mol% P₂O₅ glass shown to be partially crystalline by X-ray diffraction was heat treated, and the ³¹P MAS-NMR spectrum showed a sharp peak around 3 ppm corresponding to calcium orthophosphate or sodium pyrophosphate and overlapping broader peaks at 8.5, 10.5 and 14 ppm possibly corresponding to two mixed calcium-sodium orthophosphate phases and amorphous sodium orthophosphate respectively. ³¹P MAS-NMR spectra of series II resulted in a broad resonance around 10.5 ppm corresponding to orthophosphate in an amorphous environment. ²⁹Si MAS-NMR spectra of glasses from series I showed a shift in the resonance peak from around −78 to −86 ppm indicating an increase in Q³ species in the glass and a reduction in Q² with phosphate addition confirming the presence of orthophosphate. The heat treated sample showed a sharp ²⁹Si-NMR resonance at −88 ppm, indicating a crystalline Q² six-membered combeite (Na₂O · 2CaO · 3SiO₂) silicate-type phase, which was confirmed by powder X-ray diffraction. ²⁹Si MAS-NMR spectra of glasses from series II showed no shift in the resonance at around −78 ppm across the series, confirming an orthophosphate environment.     

Kinetics of relaxation and crystallization of sodium metaphosphate glass

Kinetics of structural relaxation and crystallization of NaPO₃ glassforming melt is studied by means of thermal analyses. It is demonstrated that activation energy depends strongly on fictive temperature T_f. The dependence of the onset temperature T_o of the glass transition interval on the heating rate q⁺ is investigated for samples that were previously cooled down at a rate q⁻ of about 850 K/min. The dimensionless fragility F is a measure of the dependence of the activation energy for spatial rearrangement on the changes of structure. According to the present results F is large for NaPO₃ (i.e. phosphates are ‘fragile’ substances).     

Deep-UV Raman spectroscopic analysis of structure and dissolution rates of silica-rich sodium borosilicate glasses

As part of ongoing studies to evaluate relationships between structure and rates of dissolution of silicate glasses in aqueous media, sodium borosilicate glasses of composition Na₂O·xB₂O₃·(3 − x)SiO₂, with x ≤ 1 (Na₂O/B₂O₃ ratio ≥ 1), were analyzed using deep-UV Raman spectroscopy. Results were quantified in terms of the fraction of SiO₄ tetrahedra with one non-bridging oxygen (Q³) and then correlated with Na₂O and B₂O₃ content. The Q³ fraction was found to increase with increasing Na₂O content, in agreement with studies on related glasses, and, as long as the value of x was not too high, this contributed to higher rates of dissolution in single pass flow-through testing. In contrast, dissolution rates were less strongly determined by the Q³ fraction when the value of x was near unity, and appeared to grow larger upon further reduction of the Q³ fraction. Results were interpreted to indicate the increasingly important role of network hydrolysis in the glass dissolution mechanism as the BO₄ tetrahedron replaces the Q³ unit as the charge-compensating structure for Na⁺ ions. Finally, the use of deep-UV Raman spectroscopy was found to be advantageous in studying finely powdered glasses in cases where visible Raman spectroscopy suffered from weak Raman scattering and fluorescence interference.     

Sub-critical crack growth rate of soda-lime-silicate glass and less brittle glass as a function of fictive temperature

Sub-critical crack growth rates of soda-lime-silicate glass and less brittle glass with different fictive temperatures were compared using the DCDC method under both dry and humid atmospheres in order to investigate the origin of the unique mechanical features of the less brittle glass developed by Ito and his collaborators. In both dry and humid atmospheres, the crack velocity of the soda-lime-silicate glass was slower than that of the less brittle glass. For both glasses, the glass sample with higher fictive temperature showed a slower crack growth rate under both dry and humid atmospheres. These observations can be explained by the tendency for the plastic flow at the crack tip; the soda-lime-silicate glass is expected to show easier plastic flow under tension than the less brittle glass, and also the samples with higher fictive temperatures are expected to show easier plastic flow, leading to greater fracture toughness, K_IC, and slower crack growth rate.     

Effect of Al2O3 on phase separation of SiO2-Nd2O3 glasses

The immiscibility boundary and the critical point of SiO₂-Nd₂O₃ system glass were determined as a function of Al₂O₃ addition. The critical temperature of the immiscibility boundary was observed to decrease with the addition of Al₂O₃. Using the regular solution model, the observed decrease of the immiscibility boundary was directly related to the decrease of the concentration fluctuation of Nd₂O₃ in SiO₂. It is concluded that the Al₂O₃ addition to Nd₂O₃ containing silica glass is beneficial in decreasing the concentration quenching effect, deterioration of the optical efficiency due to clustering of rare earth element, because Al₂O₃ addition diminishes the concentration fluctuation of Nd₂O₃ in silica glass.