Effects of La2O3, Nd2O3, NiO and CoO additions on the characteristics of SiO2–Al2O3–Y2O3–ZnO glass seals for intermediate temperature solid oxide fuel cells

• SiO2–Al2O3–Y2O3–ZnO glasses with additives were evaluated for sealants of SOFCs.
• Adding La2O3 and NiO in glass raised its CTE and enhanced its adhesion on SUS430.
• Amorphous nature of the YASZ-4 glass was retained even after a 500 h leakage test.
• YASZ-4 glass displayed a low leak rate at 700 °C against long periods of operation.

In this study, nine SiO2–Al2O3–Y2O3–ZnO glasses were prepared to evaluate their applications in intermediate temperature solid oxide fuel cells (IT-SOFCs). La2O3, Nd2O3, NiO and CoO were added to the SiO2–Al2O3–Y2O3–ZnO glass samples to enlarge their CTE and improve their adhesion on SUS430 stainless steel. It was found that YASZ-4 glass was the best candidate as a sealant among the glasses evaluated. The glass transition points, softening points and crystallization peak temperature of the YASZ-4 glass were determined to be 748 °C, 897 °C and 969 °C, respectively, while the coefficient of thermal expansion (CTE) and electrical resistivity at 700 °C were reported to be 8.4 × 10−6 K−1 and 4.43 × 108 Ω-cm, respectively. Partial substitution of Y2O3 with La2O3 and NiO in the SiO2–Al2O3–Y2O3–ZnO glass was observed to increase its CTE and to enhance its adhesion on SUS430. A strict and more sensitive test condition was employed in this study to evaluate the leak rate, which included a helium gas pressure of 1 psi across the glass seals and a small top-loading pressure of 1.0 psi applied on the glass. The leak rate at 700 °C for the first three thermal cycles stabilized at 0.019 sccm/cm and then, slightly decreased to 0.012 sccm/cm after a subsequent 500 h soaking, indicating a trivial degradation of the sealing efficiency against long periods of operation. The XRD pattern of the YASZ-4 glass after a 500 h leakage test revealed that the fully amorphous nature was retained. The addition of 30 mol% LaCoO3 to YASZ-4 glass was further attempted to increase its CTE value; however, results did not show an improvement in terms of a low leak rate.