Crystallization and microstructure of a glass seal for rapid laser sealing in the system CaO/Al₂O₃/SiO₂

A glass with the composition 61 CaO·30 Al₂O₃·9 SiO₂ was studied with respect to its crystallization behavior and its suitability as a rapidly crystallizing material for laser sealing. The glass was studied by differential scanning calorimetry; from the profiles recorded, Avrami activation energies and Avrami coefficients were calculated. The latter are in the range between 0.99 and 1.55 which is supposedly attributed to sole surface crystallization. During thermal treatment as well as during laser sealing, Ca₁₂Al₁₄O₃₃, CaAl₂O₄ and Ca₃Al₂O₆ are formed. These phases were also observed in SEM micrographs as evidenced by electron backscatter diffraction from the attributed Kikuchi patterns. Transmission electron microscopy showed a crystallized CaO- and SiO₂-enriched interface which strongly adhered to the Al₂O₃ ceramic. The porosity of the crystallized seal was in the order of few percent. The studied glass proved suitable as crystallizing seal for rapid laser sealing.