CARBON CRUCIBLE TECHNOLOGY FOR OPTICAL-GLASS MELTING

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 CaF 2, BeF2 and SiO2. However, the experimental ultraviolet transmission o f glass is frequently limited by extrinsic absorption due to trace imp urities 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 ul traviolet absorption of the glass is strongly dependent on the redox c onditions of the melt, the reducing temperature and time. Glasses melt ed under reducing conditions have a much higher transmission at 250 nm due to the shift of redox equilibria of polyvalent transition metal i ons to the lower redox state and the fact that the molar absorption co efficients of the lower redox states are more than one order of magnit ude lower than those of the higher redox state in the ultraviolet rang e. Undesirable side reactions that may occur under strong reducing mel ting conditions are described. Using carbon crucible technology for fl uoro and phosphate crown glass melting it is possible to achieve high UV transmission values and a good inner glass quality.