Thermal analysis and infrared study of Nb₂O₅–TeO₂glasses

Tellurite glasses of the xNb2O5–(100–x) TeO2, (3 ≤ x ≤ 20 mol%) system have been prepared and studied by IR spectroscopy and differential thermal analysis to explore the role of Nb2O5 on their structure. IR analysis indicates that NbO6 transforms TeO4 units into tellurite structural TeO3 units, with a shift of lattice vibrations towards higher wavenumbers. The stretching force constant of the tellurite structural units increases with Nb2O5 content, a feature that is attributed to the higher bond strength and higher coordination number of Nb2O5 relative to TeO2. The crystallization kinetics has been studied under non-isothermal conditions using the formal theory of transformations for heterogeneous nucleation. The crystallization results are analyzed, and both the activation energy of the crystallization process and the crystallization mechanism are characterized. The thermal stability of these glasses are characterized in terms of characteristic temperatures, such as the glass-transition temperature, T g, the temperature of onset of crystallization, T in, the temperature corresponding to the maximum crystallization rate, T p, and two kinetic parameters, K(T g) and K(T p). The results reveal that thermal stability increases with increasing Nb2O5 content. XRD diffraction of the studied glasses indicates the presence of microcrystallites of α-tellurite, γ-telluride, Nb2Te4O13 and an amorphous matrix.