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, T_p. SEM/EDS investigations of (1 - x)TeO₂-xK₂O (x = 0.05, 0.10, 0.15, and 0.20 in molar ratio) glasses heated above T_p 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.