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

Abstract

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.