Redox Relaxation in Glass Melts Doped with Copper and Arsenic

Abstract

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 an 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. 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+.