Shown in Fig. 6 is the SEM micrograph of interface between glass#10ZnO and GDC held at 700 °C for 7 days, along with the elemental EDS line scans taken across the interface. The EDS line scans reveal that no obvious interdiffusion zone can be observed at the interface. It is worth noting that the Al/Zn ratio decreases from 1.00 for the interface between glass#10ZnO and GDC (point#1 in Fig. 6) to 0.80 for the bulk of glass–ceramic of glass#10ZnO (point#2 in Fig. 6). This further confirms the aggregation of solid solution phase at the interface. In addition, the Al/Zn ratio at the interface between glass#10ZnO and GDC increases from 1.00 to 2.33 with the heat-treatment time increases from 7 days to 30 days, consistent with the XRD results of reaction couples between glass–ceramics and GDC (in Table 2).