Fig. 7 shows the SEM image and EDS elemental mappings of the interface between glass#10ZnO and GDC after thermal cycles for 100 times. It is clear that the interface still remains intact after thermal cycles. In addition, grain boundaries can be observed throughout the glass–ceramic sample. Combining with the micrograph (Fig. 7a) and EDS mappings (Fig. 7b), one can conclude that crystalline phases, including hardystonite phases, Sr2SiO4 and Bi4B2O9, present on the glass side after heat-treatment, consistent with the XRD results of glass–ceramics (Fig. 1).