Characteristics of glass sealants for intermediate-temperature solid oxide fuel cell applications

In this study, seven glass sealants were explored for use in intermediate-temperature solid-oxide fuel cells (ITSOFCs): one with boron oxide and an alkaline earth metal (AEM) oxide (GS01), one with boron oxide and free of AEM oxides (GS02), two with AEM oxides but free of boron oxide (GS03 and GS06), and three free of both boron oxide and AEM oxides (GS04, GS05, and GS07). The thermal stability, adhesion, and sealing properties of the glass systems were examined with respect to SUS 430 stainless steel (SUS430). All the glass sealants had electrical resistivities greater than 10(6) Omega-cm. The glass transition points ranged from 632 degrees C to 800 degrees C, while the glass softening points ranged from 708 degrees C to 852 degrees C. The GS01 and GS06 glasses, which contained AEM oxides, had lower glass crystal temperatures (< 785 degrees C) and high coefficients of thermal expansion (CTE; > 11.6 ppm/ degrees C), comparable to those of SUS430. The glasses without AEM oxides had lower CTEs but possessed good thermal stability at 700 degrees C. The glasses exhibited different levels of crystallization after being held at a sealing temperature of 850 degrees C for 1 h. With further soaking at 700 degrees C for 200 h, no new phases were formed. Scanning electron microscopy (SEM) was used to examine cross-sections of the interfaces of the glasses with SUS430 after heat treatment at 850 degrees C. The SEM results indicate that chromate was formed at the interfaces of the glasses containing AEM oxides (GS01, GS03, and GS06) and SUS430, while the glasses that were free of AEM oxides had smooth interfaces with SUS430 but cracks along the interfaces caused by the large differences in the CTEs. No significant inter-diffusion of species across any of the glass SUS430 interfaces was observed. The leak rates of the glass seals were measured at 700 degrees C for 500 h after joining at 850 degrees C. The GS02 glass had the lowest leak rate of 0.017 sccm/cm. The glasses containing AEM oxides (GS01, GS03, and GS06) had high leak rates but good bond strength with SUS430. The GS06 glass had the highest bond strength, greater than 11 MPa.