Crystallization of LiNbO₃ and NaNbO₃ in niobiosilicate glass–ceramics

Although glass–ceramics have been widely explored for their thermal stability and mechanical properties, they also offer unique symmetry-dependent properties such as piezoelectricity and pyroelectricity through controlled crystallization of a polar phase. This work examines crystallization of LiNbO₃ in a 35SiO₂–30Nb₂O₅–35Li₂O mol% composition and crystallization of LiNbO₃ and NaNbO₃ in a 35SiO₂–30Nb₂O₅–25Li₂O–10Na₂O mol% composition. Crystallization kinetics are examined using the Johnson–Mehl–Avrami–Kolmogorov (JMAK) theory where the Avrami exponent, n, is calculated to be 1.0–1.5. Microscopical analysis shows dendritic morphology, which when combined with the JMAK analysis, suggests diffusion-controlled one-dimensional growth. Adding Na₂O to the glass composition increases the inter-diffusivity of ions which causes LiNbO₃ to crystallize faster and lowers the activation energy of transformation from 1054 ± 217 kJ/mol in the ternary composition to 882 ± 212 kJ/mol. Time-temperature-transformation diagrams are presented which show that the temperature for maximum rate of transformation for LiNbO₃ is ∼650°C and for NaNbO₃ is ∼715°C.