Carbon crucible technology for optical glass melting

Abstract

Fluoro crown and phosphate crown glasses are attractive candidates for application in the deep ultraviolet range. The calculated values for the ultraviolet resonance wavelenghts are comparable with those of CaF₂, BeF₂ and SiO₂. However, the experimental ultraviolet transmission of glass is frequently limited by extrinsic absorption due to trace impurities of metal ions, mainly iron, copper and lead. The influence of different technological parameters on the ultraviolet transmission and the glass quality was investigated, especially the effect of reducing melting conditions, by using a carbon crucible in a glove box. The ultraviolet absorption of the glass is strongly dependent on the redox conditions of the melt, the reducing temperature and time. Glasses melted under reducing conditions have a much higher transmission at 250 nm due to the shift of redox equilibria of polyvalent transition metal ions to the lower redox state and the fact that the molar absorption coefficients of the lower redox states are more than one order of magnitude lower than those of the higher redox state in the ultraviolet range. Undesirable side reactions that may occur under strong reducing melting conditions are described. Using carbon crucible technology for fluoro and phosphate crown glass melting it is possible to achieve high UV transmission values and a good inner glass quaUty.