Diffusion zone evolution around growing crystals in combeite glass

Abstract

AbstractThe precipitation of nonstoichiometric crystals (of different composition from the parent glass) motivated us to study the stoichiometric Na2O·2CaO·3SiO2 ( N2CS3) glass‐forming system comprehensively. The concentration profiles of Na and Ca in the diffusion zone around Na4+2xCa4−x[Si6O18], , crystals in an N2C3S glass were characterized by energy‐dispersive spectroscopy and were then theoretically described. We show that the diffusion zone is formed because of the rejection of part of calcium by the growing crystal. The radial variation of the Ca concentrations and the crystal nucleation rate through the diffusion zone were analyzed simultaneously. We found that the nucleation rate in the diffusion zone near the crystal–glass interface drastically decreases due to the increase in the work of critical cluster formation resulting from the composition change. In contrast, the diffusion activation barrier remains practically constant. Moreover, the Ca diffusion coefficients, DCa, in Na2O·2CaO·3SiO2 and CaO·SiO2 glasses show the same Arrhenius dependence, demonstrating that (surprisingly) DCa does not depend on the glass composition in the CaO·SiO2–Na2O·SiO2 metasilicate joint. This work provides the quantitative spatial distribution of chemical components in a partially crystallized glass, shedding light on solid solution crystallization in a glass‐forming liquid and its consequences.