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Redox equilibria of polyvalent elements in binary Na2O · xSiO2 melts
Glass melts with the basic compositions (in mol%) of 15 Na2O ∙ 85 SiO2 and Na2O ∙ 2 SiO2 were doped with oxides of various polyvalent elements. At temperatures in the range of 800 to 1550°C, square-wave voltammograms were recorded, which exhibit distinct maxima attributed to respective reduction processes. The following redox steps were observed: Sb5+/Sb3+, Sb3+/Sb0, As5+/As3+, As3+/As0, Cu+/Cu0, V5+/V4+, V4+/V3+, Cr6+/Cr3+, Cr3+/Cr2+, Fe3+/Fe2+ and Ti4+/Ti3+. Peak potentials measured in the Na2O ∙ 2 SiO2 glass depended linearly on temperature, while those of the 15 Na2O ∙ 85 SiO2 melt were affected by the crystallization of cristobalit at temperatures < 1300°C. Redox equilibria in the alkali-rich Na2O ∙ 2 SiO2 melt were generally shifted to the oxidized state by comparison to those measured in the 15 Na2O ∙ 85 SiO2 melt.
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.
Voltammetric investigations of the redox behaviour of Fe, Ni, Co and Sn doped glass melts of AR® and BK7® type
The redox behaviour of iron, nickel, cobah, and tin ions was studied in glass melts with compositions near AR® and BK7® type by means of square-wave voltammetry (SWV). From the linear dependence of the peak potentials on the temperature, the standard enthalpies, ΔH°, and the standard entropies, ΔS° of the transitions Fe3+/Fe2+, Co2+/Co0, Ni2+/Ni0, Sn4+/Sn2+ and Sn2+/Sn0 were calculated. The reduced states are formed in the AR glass melt already at less reducing conditions than in the BK7 melt. The dependence of the redox ratios upon temperature is larger in the AR melt. As colouring ion Co2+ is more advantageous than Ni2+ because of its higher stability against reduction. It is possible to reduce the Fe3+/Fe2+ ratio without formation of Co0 in a disturbing concentration.
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.
High-temperature UV-VIS-NIR spectroscopy of chromium-doped glasses
Chromium-doped glasses with the basic composition (in mol%) 16 Na2O · 10 CaO · 74 SiO2 were melted under different redox conditions. From these glasses, UV-VIS-NIR absorption spectra were recorded at temperatures up to 1200 °C. While the intensity of the peak attributed to Cr6+ decreases, some of the peaks caused by Cr3+ increase in intensity at higher temperature. All peaks are slightly shifted to larger wavelengths and get broader with increasing temperature. Glasses melted under oxidizing conditions were slowly cooled as well as quenched. Using EPR spectroscopy, in the quenched sample, Cr5+ was detected in a larger concentration than in the slowly cooled sample. Otherwise, the Cr6+ concentration was larger in the slowly cooled sample. This is explained by a redox reaction, i.e. a disproportionation of Cr5+ to Cr6+ and Cr3+ during cooling. As shown by high-temperature spectroscopy of the quenched sample, this redox reaction is frozen in below 550 °C.
High-temperature UV-VIS-NIR absorption and emission spectroscopy of soda-lime-silica glasses doped with Nd2O3
Absorption spectra were recorded from a glass with the basic composition 16Na2O ∙ 10CaO ∙ 74SiO2 doped with 4 wt% Nd2O3 at temperatures in the range from 25 to 1400°C. The effective width of the observed absorption peaks increased with increasing temperature, while the peak positions remained nearly constant. Some absorption coefficients decreased with temperature while that at a wavelength of 657 nm increased notably. Emission spectra were recorded from glass melts exhibiting a temperature gradient at the surface (cold surface). The spectra showed distinguished minima at those wavelengths where the absorption maxima occurred. Numerical simulation of the emission spectra assuming a constant temperature gradient at the surface is in agreement with the experimental spectra.
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.
High-temperature spectroscopic study of redox reactions in iron-and arsenic-doped melts
Glasses with the basic composition 16Na2O - 10CaO - 74SiO2 doped with iron or with both iron and arsenic were studied by means of high-temperature UV-VIS-NIR spectroscopy. Increasing temperatures led to a shift of the UV absorption edge caused by Fe3+ -charge transfer bands to larger wavelengths. All other bands, especially the Fe2+ absorption band at around 1100 nm, decreased in intensity at higher temperatures. For glasses, solely doped with iron, the temperature dependency of the extinction coefficient was quantitatively determined. Glasses doped with both arsenic and iron showed a different behaviour: the intensity of the bands decreased up to a temperature of 600 to 650 °C and then increased again. This can be explained by the temperature-dependent redox reaction 2Fe3+ + As3+ ⇌ 2Fe2+ + As5+. Increasing temperatures lead to a shift of the reaction to the right. This reaction is in equilibrium at temperatures > 650 °C and gets frozen in at smaller temperatures, depending on the respective iron and arsenic concentrations. The latter is explained by a numerical simulation assuming the redox reactions to be controlled by diffusion.
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.
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.