Microstructural characterization and crystallization kinetics of (1 − x)TeO2–xK2O (x = 0.05, 0.10, 0.15, 0.20 mol) glasses

Microstructural characterization and crystallization kinetics of (1 − x)TeO2–xK2O (x = 0.05, 0.10, 0.15, and 0.20 in molar ratio) glasses were investigated using DTA, XRD, Raman spectroscopy, optical microscopy and SEM techniques. Whereas only one exothermic peak was observed for the 0.95TeO2–0.05K2O and 0.90TeO2–0.10K2O glasses, two crystallization peaks were present on the DTA plots of the 0.85TeO2–0.15K2O and 0.80TeO2–0.20K2O glasses. On the basis of the XRD and Raman spectrophometry investigations, α-TeO2, γ-TeO2 and K2Te4O9 crystal phases were present in the (1 − x)TeO2–xK2O (x = 0.05, 0.10, 0.15, and 0.20 in molar ratio) glass samples heated above the peak crystallization temperatures, Tp. SEM/EDS investigations of (1 − x)TeO2–xK2O (x = 0.05, 0.10, 0.15, and 0.20 in molar ratio) glasses heated above Tp revealed the presence of distinct TeO2-rich and K2Te4O9 in the 0.95TeO2–0.05K2O and triangular wedge-shaped crystalline regions in the 0.90TeO2–0.10K2O, 0.85TeO2–0.15K2O and 0.80TeO2–0.20K2O glasses. DTA analyses were carried out at different heating rates and the Avrami constant for the 0.95TeO2–0.05K2O glass was calculated as 0.94, an indication of surface crystallization also confirming SEM results. On the other hand, the n values were between 1.7 and 1.87 for the exothermic peaks of the 0.80TeO2–0.10K2O, 0.85TeO2–0.15K2O and 0.80TeO2–0.20K2O glasses, indicating one-dimensional crystalline growth mechanisms for these glasses. Activation energies for one-dimensional crystal growth mechanisms in these crystals determined from the modified Kissinger plots were found to vary between 550 and 650 kJ/mol.