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 < 470 °C, this reaction is kinetically frozen and 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).