Thermal studies of ZnO-B2O3-P2O5-TeO2 glasses

The effect of TeO2 additions on the thermal behaviour of zinc borophosphate glasses were studied in the compositional series (100 - x)[0.5ZnO-0.1B(2)O(3)-0.4P(2)O(5)]-xTeO(2) by differential scanning calorimetry, thermodilatometry and heating microscopy thermal analysis. The addition of TeO2 to the starting borophosphate glass resulted in a linear increase of glass transition temperature and dilatometric softening temperature, whereas the thermal expansion coefficient decreased. Most of glasses crystallize under heating within the temperature range of 440-640 degrees C. The crystallization temperature steeply decreases with increasing TeO2 content. The lowest tendency towards crystallization was observed for the glasses containing 50 and 60 mol% TeO2. X-ray diffraction analysis showed that major compounds formed by annealing of the glasses were Zn2P2O7, BPO4 and alpha-TeO2. Annealing of the powdered 50ZnO-10B(2)O(3)-40P(2)O(5) glass leads at first to the formation of an unknown crystalline phase, which is gradually transformed to Zn2P2O7 and BPO4 during subsequent heating.