Redox behavior of glasses doped with copper and arsenic, antimony or tin

Soda-lime-silicate glasses (16Na2O-10CaO-74SiO2, in mol %) doped with copper and arsenic were studied by high temperature UV-vis spectroscopy. The base glass is a model for sheet glasses. Arsenic was one of the classical fining agents and is still used as an oxidizing additive in heavy metal oxide glasses. During heating, the Cu2+ absorptivity slightly decreased up to a temperature of 470 °C. At further increasing temperatures, the absorptivity decreased more notably and after reaching a minimum at temperatures >600 °C strongly increased again. At smaller heating rates, the minimum was more pronounced and was shifted to lower temperatures. This was explained by the redox reaction: 2Cu2+ + As3+ ↔ 2Cu+ + As5+ This reaction is shifted to the left during heating. At temperatures 600 °C, it is in equilibrium. In-between, the kinetics play an important part. Rate constants of the redox reaction were determined from relaxation times. The rate constants showed Arrhenius behavior and were inserted into a kinetic differential equation. Numerical solutions of this differential equation were in good agreement with the results from high temperature spectroscopy. The activation energy is 210 kJ∙mol-1. This value is much smaller than the activation energy of viscous flow and hence the rate determining step is assumed to be the diffusion of Cu2+. Sodium borasilicate glasses (25 Na2O-15 B2O3-60 SiO2, in mol%) doped with copper and arsenic, antimony, or tin as redox agents were studied by optical absorption spectroscopy in the temperature range from 25 to 620 °C. In general, the redox agents decrease the Cu2+-concentration in the glasses. Increasing the temperature to 620 °C resulted in a shift of the Cu2+ absorption band from 12,600 to 11,800 cm-1. In glasses solely doped with copper or with copper and tin, the absorptivity decreased by about 5 % in that temperature range. In principle, glasses doped with both copper and arsenic or antimony showed the same behaviour up to a temperature of 420 °C. For these glasses, heating to higher temperatures resulted in a minimum absorptivity at around 540 to 500 °C and a subsequent strong re-increase in absorptivity. The rate constants showed Arrhenius behavior and were inserted into a kinetic differential equation. The activation energy are 270 kJ.mol-1 (Cu/As) and 265 kJ.mol-1 (Cu/Sb). During cooling from 620 °C, a steep decrease in absorptivity down to a temperature of 520 °C and after passing through a minimum a slight re-increase was observed. The Cu2+ concentration, and hence the absorptivity after cooling depends on cooling rate (10 to 0.5 K·min-1)

Modification of silicophosphate glass composition, structure, and properties via crucible material and melting conditions

Abstract Ceramic crucibles are known to corrode in contact with glass melts. Here, we investigate the effect of alumina and fused silica crucibles on the composition, structure, and properties of silicophosphate glasses. Glasses in the system 0.3 Na2O‐0.6 P2O5‐0.1 SiO2 were melted in platinum, alumina, or fused silica crucibles at 900°C or 1200°C for 0.5‐12 hours. Al2O3 and SiO2 were found to leach from the crucibles into the glass melt and alter the glass composition: Al2O3 content increased with melting temperature and time, resulting in up to 10 mol% Al2O3; SiO2 from fused silica crucibles was also introduced into the glass, resulting in a 25% higher SiO2 content compared to the nominal composition. Glass density, transition temperature, thermal expansion, and mechanical properties were strongly affected by these compositional changes. Based on vibrational spectroscopy, this is explained by increasing numbers of P–O–Al or P–O–Si bonds, resulting in a depolymerization of the phosphate network, and ionic cross‐linking by high field strength aluminum or silicon ions. With increasing alumina content, P–O–Si bonds were replaced by P–O–Al bonds. 31P and 27Al MAS NMR spectra revealed that aluminum is present in sixfold coordination exclusively and fully bonded to phosphate species, connecting phosphate groups by P–O–Al–O–P bonds