Preparation and characterization of TiO2-BaO-ZnO-B2O3 glass systems for optical devices

We study the TiO₂-BaO-ZnO-B₂O₃ glass system, where the ZnO and B₂O₃ compositions were constant and the ratio TiO₂/BaO was varied from 0.87 to 1.76. A super kanthal resistance furnace was used to melt the compounds inside an alumina crucible, at 1200 °C, for 10 min. After melting, the glasses were poured out into steel moulds and rapidly cooled by quenching. The glasses obtained were homogeneous, bubble free and transparent. They were characterized by X-ray diffractometry, Fourier transform infrared spectroscopy (FTIR), UV-VIS spectroscopy, dilatometry, density and linear refractive index. An infrared cut off caused by the composition influence was found in both IR and UV-vis spectra. From dilatometry T _d and T _g were verified as being anomalous. The linear thermal expansion coefficient presented an anomalous behaviour in relation to TiO₂ concentrations. The density and linear refractive index increased with increasing TiO₂/BaO ratio arriving at their peak value of TiO₂/BaO=1.5 and then decreasing. The dependence of softening point T _d on the ratio TiO₂/BaO exhibited the same behaviour. It is suggested that Ti⁴⁺ plays a dual part in the glass system, assuming a predominantly tetrahedral coordination in the low titania region and a predominantly octahedral coordination in the high titania region. With a heat treatment of the glass around 600 °C, we observed a rapid change of refractive index with increasing temperature.