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Kinetics of phase separation in a 6.5 Na2O ∙ 33.5 B2O3 ∙ 60 SiO2 glass
Industrially melted glasses with the composition (in mol%) 6.5 Na2O ∙ 33.5 B2O3 ∙ 60 SiO2 were thermally treated at temperatures in the range of 660 to 750 °C. This resulted in phase separation, i.e. in the formation of a silica- and a sodium borate-rich phase with an interconnected microstructure. Both, the volume content of the borate-rich phase and the mean structure thickness (the correlation length) increased with time as well as with temperature. The volume content approached to a limiting value at constant temperature. The correlation length increased with time according to a power law (~ t^1/n). By contrast to previous studies, n was in the range of 1 to 1.2 within the temperature range and time scale studied. The correlation lengths were much larger (up to 12 µm) and the viscosities much lower than in most previous studies. The kinetic law was explained as controlled by visous flow.
Effect of TiO2 , ZrO2 and ZnO addition on the hydrolytic durability, viscosity and crystallization behaviour of fibre glasses
Glasses of the System Na2O-K2O-MgO-CaO-BaO-AI2O3-SiO2-B2O3 with additions of TiO2, ZrO2 and ZnO were melted from the raw materials. The glasses were characterized with respect to their hydrolytic durabilities, the working ranges (difference of temperatures attributed to viscosities of 10^2 and 10^4 dPa s), the temperatures attributed to the working point (η = 10^3.5 dPa s), and the crystal growth velocities. In the most cases, the additions led to an increase in viscosity and to a slight decrease in the workability range. The crystal growth velocities decreased with the addition of Ti02 and/or Zr02, while they increased if adding ZnO. The addition of ZnO or of TiO2 and ZrO2 resulted in better chemical durability.
Glasses for the preparation of gradient index lenses in the Na2O-Al2O3-B2O3-SiO2 system - hydrolytic durability, thermal and optical properties
Glasses in the system Na2O-Al2O3-B2O3-SiO2 were melted from the raw materials and studied with respect to their chemical durability, their crystal growth velocities, their refractive index and dispersion, their glass transition temperature and their thermal expansion coeffieient. With inereasing Na2O concentration, the crystal growth velocity increases and the chemical durability decreases. Equimolar subtitution of B2O3 for SiO2 results in a decrease in both the chemical durability and in crystal growth velocities. Equimolar subtitution of AI2O3 for SiO2 leads to increasing chemical durability and decreasing crystal growth velocities. Optimum glass compositions to produce gradient index lenses should possess B2O3 concentrations of around 12.5 to 15 mor% and molar AI2O3 concentrations some percent larger than the Na2O concentration. In these glasses, nonbridging oxygen does not occur.
Structurization of a partially crystalline melt by a stationary ultrasonic wave
At a partially crystalline melt composed by a lead borate glass and crystalline SnO2, a stationary ultrasonic wave was attached at a temperature of 600 °C. This led to an accumulation of the SnO2 particles at the pressure nodes. After cooling the melt, a layered structure was obtained. The effect of the particle size upon the particle distribution is shown. The movement of the particles in the stationary ultrasonic wave is described by a differential equation which is numerically solved. Results of the numeric simulation are compared with experimental results and discussed.
Studies on viscosity of dried sodium water glasses by extrusion
Α study on viscosity and flow behavior of dried water glasses using an extrusion method is first reported. The rheological properties of water glass samples with water concentrations in the range from 22.2 to 36.4 wt% and also some glycerin-containing samples were studied. The extrusion method used enables the determination of viscosities in the range from 10^7 to 10^10 dPa s. By comparison to glass melts, relatively low temperatures in the range from 50 to 85 °C are used. During extrusion of dried water glasses, the viscosity decreases significantly with increasing shear rate. Lower water concentrations lead to a more pronounced deviation from Newtonian flow behavior. This is attributed to nonlinear flow behavior rather than to localized viscous heating effects by mechanical deformation work. Furthermore, the effect of composition on the viscosity-temperature dependence is studied. The higher the water concentration, the lower is the viscosity. Glycerin additions intensify this effect. Generally, the viscosity decreases with increasing temperature. Vogel-Fulcher-Tammann parameters are obtained by ftting the data of extrusion experiments and the corresponding Tg values.
Voltammetry in a sulfur and iron-containing soda-lime-silica glass melt
With the aid of square-wave voltammetry soda-lime-silica melts with sulfate fining were investigated in order to enable a quantitative in-situ determination of sulfur and iron. In this study, glass melts with low iron and high sulfate contents, typical for technical white glasses, were examined. The current-potential curves are predominantly influenced by sulfur and not solely controlled by diffusion. This behavior is supposedly caused by deposition of a sulfur layer on the surface of the working electrode. However, a simultaneous quantitative in-situ determination of iron and sulfur in melts of white glasses is possible.
Crystallization and vicosity of phosphate melts in the system Na2O-CaO-MgO-P2O5-Al2O3-TiO2
Glasses in the system Na2O-CaO-MgO-P2O5-Al2O3-TiO2 were studied with respeet to their crystallization and viscosity. Glasses in the metaphosphate range are composed of phosphate chains, and possess a comparably small tendeney to crystallization. Here, AlPO4 in a cristobalite-like modification and Ca2P2O7 are formed. Glasses in the intermediate range between metaphosphate and invert glass structures are composed of smaller phosphate units and show higher erystal growth velocities as well as higher viscosities. Additions of MgO, AI2O3 and TiO2 result in a strengthening of the glass network, and hence, inereasing viscosities and a notably smaller tendeney to crystallization.
Alumina and zirconia as inhibitors of crystallization during sintering of borosilicate glasses
The glass composition (in mol%) 16 Na2O ∙ 24 B2O3 ∙ 60 SiO2 has suitable viscosity and dilatometric properties to be used as sealing material for the gas manifolds of molten carbonate fuel cells (MCFC). Nevertheless, quartz crystallization takes place during sintering of the borosilicate glass powder at temperatures between 600 and 700 °C. The quartz crystallization leads to the formation of pores and a variation in the thermal expansion coefficient; both giving rise to defects and tensions in the seal. In this work, the possibility of inhibition of quartz crystallization by substituting AI2O3 or ZrO2 for 2 or 5 mol% of SiO2 is described. The thermal properties and the viscosity-temperature curve of the new compositions, especially those containing alumina do not differ much from the original glass.
Influence of process parameters on the formation of protective MoSiₓ layers on molybdenum electrodes during electric melting of glass
With the aid of laboratory experiments, the cathodic passivation of molybdenum electrodes during electrical melting of glass was studied. The formation of molybdenum silicide layers does not only depend on the cathodic passivation currents, but also on time and the AC heating current density If the melt has freshly been prepared from raw materials, the molybdenum silicide layer is thinner than under otherwise same conditions. While at an AC heating current density of 1 A/cm², a cathodic current density of 3.75 mA/cm² led to Optimum layer formation, at a heating current density of 2 A/cm², higher cathodic current densities are required.
Chemical solubility of phosphate glasses in the system Na2O-CaO-MgO-P2O5-Al2O3-TiO2 in aqueous solutions of different pH values
Phosphate glasses of different composition ranges show very different solubility behaviour in aqueous solutions of pH 4 and 5.8. With inereasing P2O5 concentrations between 38 and 52 mol% , in the system Na2O-CaO-MgO-P2O5-Al2O3-TiO2 , the dissolution rate generally increases, however, the most notable increase is observed at P2O5 concentrations larger than 48 mol%. Additives of TiO2 and AI2O3 increase the stability to corrosion. Glasses with compositions in the metaphosphate range show larger dissolution rates in slightly acidic medium than glasses near the invert glass range. Glasses in the invert glass range show a stronger increase in the dissolution rates with decreasing pH value and also deceleration of the dissolution with time.