Crystallization kinetics of a barium-zinc borosilicate glass by a non-isothermal method

The crystallization kinetics of a glass with a molar composition 40BaO-20ZnO-30B2O3-10SiO2 was investigated.The kinetic parameters, activation energy for crystallization (Ec) and Avrami exponent (n), wereevaluated under non-isothermal conditions using the results obtained by differential thermal analysis(DTA) performed at different heating rates. DTA curves exhibited two overlapping exothermic peaks associatedwith the crystallization of the glass. Barium borate (BaB4O7) was the first crystalline phase to beformed and it was followed by the formation of barium zinc silicate (BaZnSiO4), as identified by XRD.For the first exothermic peak, when the fraction of crystallization (v) increased from 0.1 to 0.9, the localactivation energy (Ec(v)) decreased from 700 to 500 kJ/mol, while for the second exothermic peak, Ec(v)slightly increased from 490 to 570 kJ/mol. For the range of 0.1 < v < 0.9, the local Avrami exponent (n(v))increased from 1 to 1.4 for the first exothermic peak and it decreased from 1.7 to 1.4 for the secondexothermic peak. Observation by SEM of the microstructure of sintered glass samples revealed that crystallizationstarted at the surface of glass particles, with growth of lamellar crystallites, that together withsome quasi-spherical nano-sized crystallites progressed towards the inside of the glass at the highestsintering temperatures. The change of the local activation energy with the fraction of crystallizationsuggested that a multi-step kinetic reaction took place during sintering and crystallization of the glass.