Microstructural Evolution on Firing Soda–Lime–Silica Glass Fluxed Whitewares

Density and microstructural evolution of porcelains containing 0–25 wt% soda–lime–silica (SLS) waste glass fired over a range of temperatures from 600° to 1400°C have been investigated. After firing 3 h at 1100°C, batches containing 6.25 wt% SLS glass and 18.75 wt% nepheline syenite flux system attained open-pore closure and a bulk density of 2.40 g/cm³, comparable to results from commercial porcelain after firing at 1200°C. SLS glass softens and melts, conferring early densification and overfiring on porcelains fired at normal commercial firing temperatures. The microstructural evolution examined using XRD, SEM/energy dispersive spectroscopy (EDS), and TEM/EDS revealed formation of a variable composition plagioclase, rounded wollastonite particles, sodium silicates, and tridymite in batches containing SLS glass, in addition to primary and secondary mullites, partially dissolved quartz, and a glassy matrix as found in the waste-free batch. Ca²⁺ and Na⁺ from the SLS glass migrate to regions containing the products of clay decomposition to form plagioclase, limiting the extent of mullite crystallization. The presence of a solution rim surrounding quartz and different glass compositions around wollastonite crystals indicate that the system is not in equilibrium, although phases predicted by the Na₂O–CaO–SiO₂ equilibrium diagram were formed.     

Oxygen Ion Activity and the Solubility of Sulfur Trioxide in Sodium Silicate Melts

The solubility of sulfur trioxide in sodium silicate melts was determined from 1150° to 1250°C by equilibrating melts in gas mixtures of known contents of sulfur dioxide and oxygen. Sulfate forms according to the reactions: O^2-+ SO₂+ 1/2O₂= O^2-+ SO₃= SO₄^2-. The data obtained at 1200°C were interpreted by the linear equation: log(SO₄^2-) = log( P so₂½ P o₂^1/2) + log Y in which Y is a function of the soda/silica ratio. A series of parallel lines was obtained. Relative free oxygen ion activities calculated for 1200°C were in good agreement with theoretical values calculated from the thermodynamic model of Toop and Samis.     

Surface-Energy Determinations of Tin Oxide-Coated Soda–Lime–Silica Glass

The dispersive and polar surface-energy components, as well as the total surface energy, of tin oxide coatings on soda–lime–silica glass were determined by the Owens–Wendt method. The total surface energy of tin oxide is greater than soda–lime–silica glass and, more importantly, exhibits significantly more-dispersive and less-polar character. These results indicate that tin oxide is significantly more covalent than soda–lime–silica glass. It is postulated that the more-covalent tin oxide coatings increase the bond strength of organic coatings to soda–lime–silica glass. These effects improve the friction-damage resistance of glass surfaces coated with metal oxides and organics, compared with glass surfaces coated with organics only.     

Foaming in Glass Melts Produced by Sodium Sulfate Decomposition under Ramp Heating Conditions

Ramp heating a glass batch with sulfate permits determination of the foaming temperature and maximum foam height as a function of sulfate addition. Experiments revealed that, in the soda–lime glass studied, foam was produced when the mass fraction of SO₃ in the glass was higher than 0.0027. With increasing sulfate content, the foaming temperature decreased and the maximum foam height increased until the mass fraction of SO₃ was 0.01. Further additions of sulfate had little effect on foaming temperature and decreased maximum foam height. The results are discussed in terms of sulfate solubility and nonequilibrium effects.     

Thermooptic Coefficients of Soda–Lime–Silica Glasses

The thermooptic coefficients, d n /d T , of two series of soda–lime–silica glasses, 25Na₂O· x CaO·(75 – x )SiO₂ and (25 – y )Na₂O· y CaO·75SiO₂, have been measured. When CaO is substituted for SiO₂, the magnitude of d n /d T passes through a maximum negative value. When CaO is substituted for Na₂O, there is a monotonic change from negative to positive d n /d T . These effects are accounted for by the changes in relative values of the coefficient of thermal expansion and the temperature coefficient of electronic polarizability, Θ. The effect of composition on Θ is explained by the changes in density of various nonbridging and bridging oxygens in the glass.     

Sodium Coordination Environments in Silica Films

The local structure centered on sodium after diffusion in silica (Na-SiO₂ samples) has been determined by means of extended X-ray absorption fine structure (EXAFS) studies. The Na-SiO₂ samples are of particular interest because (i) their sodium content can be varied over a wide range of concentration and (ii) their local structure is representative of that of soda–silica glass. EXAFS analyses reveal the existence of a well-defined local structure involving oxygen, sodium, and silicon neighbors. The Na-O, Na-Na, and Na-Si bonds lengths, which amount to 0.23, 0.30, and 0.38 nm, respectively, do not depend on sodium concentration. This environment closely resembles that found in soda–silica glass. Moreover, it is compatible with the "target site" and "the site memory effect" suggested by recent theories of the ionic conductivity in oxide glasses.     

Measurement of Crack Tip Displacement Fields in Mixed Mode I–Mode III Fracture

A new method has been devised to propagate stable subcritical mixed mode I–mode III cracks in soda–lime–silicate and borosilicate glasses. The mode I content was measured using an original optical tunneling technique and mode III content via multiple-beam interferometry. Measurement of the mode III displacement field allows values of mode III fracture toughness, K_III, to be derived. K_III vs crack velocity (v) diagrams have been constructed for both glasses, and, in the case of the soda–lime–silicate glass, v vs K_III relationships have been determined for a variety of humidities. Comparison has been made with published K_I vs v data for this glass.     

Hot Corrosion of Silica

The high-temperature corrosion of bulk silica glass was studied in pure oxygen and in SO₃-containing oxygen atmospheres in the presence of liquid sulfate deposits at temperatures of 700° and 1000°C. No reaction and devitrification were observed without Na₂SO₄ on the surface. The wetting of the silica by the sulfate, the tendency toward basic fluxing, and the crystallization of the silica incrased with the activity of Na₂O. The most extensive degradation of vitreous silica occurred by crystallization, and the resulting spalling under basic conditions and thermal cycling at basic conditions were parabolic. This behavior is explained by a model in which the crystallization is controlled by sodium at the glass-crystal interface and its diffusion into the glass. This sodium diffuses into the glass before crystallization and is swept ahead of the crystallization front.     

Dispersion of Thermooptic Coefficients of Soda–Lime–Silica Glasses

The thermooptic coefficients, i.e., the variation of refractive index with temperature (d n /d T ), are analyzed in a physically meaningful model for two series of soda–lime–silica glasses, 25Na₂O· x CaO·(75 – x )SiO₂ and (25 – x )Na₂O· x CaO·75SiO₂. This model is based on three physical parameters—the thermal expansion coefficient and excitonic and isentropic optical bands that are in the vacuum ultraviolet region—instead of on consideration of the temperature coefficient of electronic polarizability, as suggested in 1960. This model is capable of predicting and analyzing the thermooptic coefficients throughout the transmission region of the optical glasses at any operating temperature.     

Thermal Analysis of Reactions in Soda–Lime Silicate Glass Batches Containing Melting Accelerants: I, One- and Two-Component Systems

To identify each glass melting reaction in a multicomponent system, one-component and two-component reaction processes were studied using DTA, TGA, and XRD. Two-component mixtures were prepared by choosing pairs in the same ratio as in a commercial container glass batch composition (sand-soda ash-calcite-dolomite-feldspar). The presence of silica in the silica-calcite system decreased the termination temperature of the decomposition of calcite, but did not alter the onset of decomposition. Similar behavior was found in the dolomite-silica system. A double carbonate (Na₂Ca(CO₃)₂) formed via solid-state reaction in the calcite-soda ash system, and metasilicate/disilicate phases were detected during the fusion process in the soda ash-silica system. The effects of reaction accelerant additions in the soda ash-silica system were investigated using 1 wt% additions of sodium sulfate, sodium nitrate, and sodium chloride. Sodium chloride was the most effective melting accelerant, lowering the termination temperature of CO₂ release by ∼80°C compared with the soda ash-silica system with no additives. NaCl additions caused complete reaction and/or fusion of Na₂CO₃ prior to its melting temperature. Sodium sulfate additions acted to suppress metasilicate/ disilicate formation by coating quartz grains and shifted consequent CO₂ release to higher temperature.     

Relative Activity of Oxygen Ions in Molten Sodium Phosphate as Determined from the Solubility of Sulfur Trioxide

The solubility of sulfur trioxide in sodium phosphate melts with Na₂O/P₂O₅ ratios from 1.0 to 1.4 was investigated at 653° to 1021°C. The oxygen-ion activity estimated from the solubility of SO₃ increased exponentially as the Na₂O/P₂O₅ ratio increased. This compositional dependence of the activity is discussed on the basis of a polymer model.     

Effect of Lead Compounds on the Properties of Stannous Fluorophosphate Glasses

The effects of PbO or PbF₂ additions on the density, thermal-expansion behavior, and chemical durability of stannous fluorophosphate glasses have been determined. Lead compounds dramatically improve the chemical durability of these glasses in water, resulting in low-melting-temperature glasses (T_g<150°C) which have durabilities approaching those of commercial soda–lime–silica glasses. Possible causes of these effects are discussed.     

Effect of Enameling on the Strength and Dynamic Fatigue of Soda–Lime–Silica Float Glass

The effect of a glass enamel coating on the strength and fatigue behavior of float glass was investigated. Commercially available enamel that was comprised of Cu₂Cr₂O₄ pigment particles in a bismuth-zinc borosilicate glass matrix was applied to a soda–lime–silica float glass via screen printing, followed by fusion at elevated temperature. Strengths of the enameled specimens were evaluated in biaxial flexure using a ring-on-ring (ROR) test geometry, and the data were analyzed using a conventional two-parameter Weibull distribution. Enameling was found to significantly degrade the strength of the float glass. There was no statistical difference in the characteristic strengths of samples enameled on the air side (66 MPa) compared with samples enameled on the tin side (61 MPa) of the float glass. Fractographic analysis revealed that the failures in the enameled float glass samples initiated at pores and pigment aggregates in the enamel, whereas failures in float glass samples initiated solely from surface flaws. Dynamic fatigue tests were performed on enameled float glass and indented float glass samples to determine the effect of the enamel on the stress corrosion behavior of the enameled components. There was no statistically significant difference between the stress corrosion exponents for the float glass and enameled float glass specimens.     

Hot Corrosion of Sintered α-Sic at 1000°C

The hot corrosion of sintered α-Sic by thin films of Na₂SO₄ and Na₂CO₃ was studied at 1000°C in controlled gas atmospheres. Under all conditions corrosion led to 10 to 20 times the amount of SiO₂ formed in pure oxidation after a 48-h exposure. In addition, small amounts of sodium silicate formed. Melts of Na₂SO₄/SO₃ caused uniform pitting of the Sic substrate; Na₂CO₃/CO₂ melts caused localized pitting and grain-boundary attack. In all cases the protective SiO₂ layer dissolved to form silicate, leading to corrosion. In the sulfate case, free carbon in the Sic promotes this process. In all cases the presence of liquid films is responsible for rapid transport rates and the subsequent rapid reaction.     

Solid-State SO2 Sensor Using a Sodium-Ionic Conductor and a Metal–Sulfide Electrode

All solid-state sulfur oxides (SO _x ) sensor devices combined with a sodium ionic conductor (Na₅DySi₄O₁₂) disk and metal sulfide-sensing electrodes synthesized via solution routes have been systematically investigated for the detection of SO₂ in the range of 20–200 ppm at 150–400°C. Among the various sulfide-sensing electrodes tested, the metal monosulfide-based electrodes gave good SO₂ sensitivity at 400°C. The Pb_{1− x} Cd _x S ( x =0.1, 0.2)-based solid electrolyte sensor element showed the best sensing characteristics, i.e., the EMF response was almost linear to the logarithm of SO₂ concentration in the range between 40 and 400 ppm, with a 90% response time to 100 ppm SO₂ of about 3–15 min, and also showed high selectivity to SO₂ at 400°C.     

Resistance of Tin Dioxide to Chemical Reaction with Vanadate—Sulfate Melts

A thermogravimetric analysis/SO₃ equilibrium technique has been used to show that tin dioxide (SnO₂) is inert to chemical reaction with NaVO₃ at 700° and 800°C under SO₃ partial pressures as high as 5 × 10⁻² bar (5 kPa). The results suggest that SnO₂ may be potentially useful as a material for protection against molten vanadate–sulfate hot corrosion at moderate temperatures (<800°C).     

Phase Equilibria at Liquidus Temperatures in the System Cobalt Oxide—Iron Oxide—Silica in Air

The quenching method was used to determine phase relations at liquidus temperatures in the system cobalt oxide-iron oxide-silica in air. The crystalline phases coexisting in equilibrium with liquids in the system are silica (SO₂), olivine (Co₂SiO₄), a spinel solid solution ((Co,Fe)₃O₄), and an oxide solid solution ((Co,Fe)O) with peri-clase-type structure. The lowest liquidus temperature is 1313°C. Composition relations among coexisting liquid and solid phases were established in the areas where solid solution crystals are involved in the equilibria.     

Strengthening of Glass Surfaces by Sulfur Trioxide Treatment

Removal of soda from a glass surface at high temperature can, on cooling, give appreciable strengthening due to lowering of the expansion coefficient at the surface. Soda depletion has been accomplished previously by treating a high-expansion soda-lime glass with SO₂ in the presence of oxygen and water. The depletion rate can be doubled by using dispersed platinum to catalyze the oxidation of SO₂ to SO₃ before the gas reacts with soda from the glass. The data are consistent with the soda depletion being Fickian both with and without the catalyst. Alumina in the glass composition markedly increases the strength which can be attained by sulfur oxide treatment. Although the percentage of soda retained in the leached layer increases, the total amount of soda removed from the glass also increases as alumina increases. It must follow that the depth of the layer is increased.     

Leaching Behavior of Sodium from Fine Particles of Soda–Lime–Silicate Glass in Acid Solution

The leaching behavior of Na from soda–lime–silicate glass was investigated by preparing glass powders with average particle diameters of 53 and 19 μm, and leaching in HNO₃ at 90°−140°C. A new theoretical equation for Na leaching from a spherical particle is proposed based on the assumption that a rate-determining process is the three-dimensional self-diffusion of Na in glass. The diffusion constant ( D ) of Na in glass was obtained by fitting the experimental data to a theoretical equation. The values of D and activation energy obtained are comparable to those obtained in other studies on larger particles.     

Activity–Composition Relations of Chromium Oxide in Silicate Melts at 1500°C under Strongly Reducing Conditions

The solubility and activity–composition relations of chromium oxide in melts of the systems CaO–CrO _x –SiO₂ and CaO–Al₂O₃–CrO _x –SiO₂ have been determined at 1500°C by equilibrating melts with Pt–Cr alloys at known oxygen pressures. It is shown that the increase in the concentration of divalent chromium ions, as the oxygen pressure and the basicity of melt decrease, results in a dramatic increase in the solubility of chromium oxide in the liquid phase. An increase in the Al₂O₃ content of the melt leads to a decrease in the solubility of chromium oxide over the whole composition range studied. The activity coefficient of CrO has been found to increase with increasing melt basicity and decreasing oxygen pressure whereas the activity coefficient of CrO_1.5 decreases sharply with increasing melt basicity for siliceous melts but levels off at a basicity ratio (wt% CaO/wt% SiO₂) of about 0.7. An increase in the Al₂O₃ content of the melts results in an increase in the activity coefficient of CrO.