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.