Nucleação e crescimento de cristais, difusão e relaxação estrutural em vidros 2Na2O.CaO.3SiO2 e 5BaO.8SiO2

Abstract

Despite intensive research regarding glass crystallization, there are still flaws in the theories and especially in their application. Aiming to verify the applicability of the classical nucleation theory (CNT) and the relationship between the diffusional process related to crystallization and viscous flow, two silicate glasses, 2Na2O∙CaO∙3SiO2 and 5BaO∙8SiO2 were used. Due to the numerous difficulties in the direct measurement of the “structural units” mobility in the crystal-liquid interface, the effective diffusivity for nucleation (DI) has been estimated using three parameters – nucleation time-lag (Dτ), viscosity (Dη) and crystal growth velocity (DU). Additionally, the possible effect of structural relaxation in the nucleation kinetics at low temperatures was evaluated, which is often neglected in the application of the CNT. It was observed that Dτ and Dη yields similar values of DI, leading to similar conclusions about the CNT consistence. Only nucleation data of 2Na2O∙CaO∙3SiO2 glass at very low temperatures showed some evidence of a failure of the CNT, which was assigned to the experimental limitation of nucleation time for this composition. Generally, the temperature dependence of Ist was best described by assuming DI = Dτ or DI = Dη, when compared to DI = DU. However, the inclusion of the effect of structural relaxation suggested a progressive increase in the nucleation rate until reaching the definitive value corresponding to the fully relaxed glass, which agrees with the theoretical predictions considering DI = DU. Therefore, it was shown that the alleged “CNT breakdown” is related to the long time required for the actual steady-state be reached at the lowest temperatures, which is a result of transient nucleation and structural relaxation. We also verified that with increasing nucleation time, the temperature of maximum Ist shifts to lower values.