Material degradation during isothermal ageing and thermal cycling of hybrid mica seals under solid oxide fuel cell exposure conditions

Hybrid phlogopite mica seals with glass interlayers were evaluated in terms of materials degradation in a combined ageing and thermal cycling test. Three glass interlayers were investigated: a standard Ba–Ca–Al silicate glass (G18), a modified Ba–Ca–Al silicate glass with a nucleation agent added (G18m), and a borosilicate glass (G6). The hybrid phlogopite mica seals were aged at 800 °C for ~500 to ~1000 h in moist, dilute hydrogen fuel, and then subjected to short-term thermal cycling between~100 and 800 °C. Seals with G18 and G18m glass interlayers showed very poor thermal cycle stability after isothermal ageing. Seals with borosilicate glass interlayers showed very good thermal cycle stability with constant, low leakage (<0.01 sccm cm−1 at 0.2 psi) after ageing for 508 h followed by 56 thermal cycles. Post-mortem analyses showed degradation of the mica due to interaction with the interlayer glasses. Leak paths were identified and correlated to the measured leak rates.