Microstructural characterization and crystallization behaviour of (1 − x)TeO2–xWO3 (x = 0.15, 0.25, 0.3 mol) glasses

DTA, XRD and SEM investigations were conducted on the (1 − x)TeO2–xWO3 glasses (where x = 0.15, 0.25 and 0.3). Whereas the 0.75TeO2–0.25WO3 and 0.7TeO2–0.3WO3 glasses show no exothermic peaks, an indication of no crystallization in their glassy matrices, two crystallization peaks were observed on the DTA plot of the 0.85TeO2–0.15WO3 glass. On the basis of the XRD measurements of the 0.85TeO2–0.15WO3 glass samples heated to 510 °C and 550 °C (above the peak crystallization temperatures), α-TeO2 (paratellurite), γ-TeO2 and WO3 phases were detected in the sample heated to 510 °C and the α-TeO2 and WO3 phases were present in the sample heated to 550 °C. SEM micrographs taken from the 0.85TeO2–0.15WO3 glass heated to 510 °C showed that centrosymmetrical crystals were formed as a result of surface crystallization and were between 3 μm and 15 μm in width and 12 μm and 30 μm in length. On the other hand, SEM investigations of the 0.85TeO2–0.15WO3 glass heated to 550 °C revealed the evidence of bulk massive crystallization resulting in lamellar crystals between 1 μm and 3 μm in width and 5 μm and 30 μm in length. DTA analyses were carried out at different heating rates and the Avrami constants for the 0.85TeO2–0.15WO3 glass heated to 510 °C and 550 °C were calculated as 1.2 and 3.9, respectively. Using the modified Kissinger equation, activation energies for crystallization were determined as 265.5 kJ/mol and 258.6 kJ/mol for the 0.85TeO2–0.15WO3 glass heated to 510 °C and 550 °C, respectively.