Degradation of sealing glasses under electrical load

For reliable and stable operation of planar solid oxide fuel cells (SOFC), the gas tightness of all sealings is an essential issue. Failures induced by thermal cycling, slight stack deformation or degradation of the sealing material can lead to leakage in fuel or/and air compartments. As a consequence local reactions between fuel and air can occur leading to a temperature increase and a severe damage of the SOFC stack. For reliable and long term operation of SOFC systems degradation mechanisms of sealing materials has to be understood to develop improved seals. For the study of degradation mechanisms, model samples of glass seals and metallic interconnectors materials are treated under dual atmospheres and applied electrical voltage at a temperature of 850 °C for times up to 1.000 h. The electrical resistance of the samples is monitored continuously during the experiments and changes in microstructure of the sealing materials and glass to metal interfaces were examined after testing. The evolution of resistivity is discussed on basis of microstructure changes in the bulk glass and glass to metal interfaces. Hereby the different polarisations (anodic and cathodic) of the glass to interconnector interfaces are regarded. The magnitude of applied voltage has been varied to clarify the effect of electrical field on degradation behaviour. The results of the investigations are used to develop enhanced sealing glasses with improved long term stability.