Fast detection of defects in glass-soldered fuel cell assemblies by active infrared thermography

Two interconnector plates made out of ferritic steel are joined together by a thin layer of glass-ceramics and form an airtight assembly as a core part of a solid-oxide fuel cell (SOFC). The sealant has to withstand temperatures above 800°C and has to be gas-tight and mechanically stable. The solder layer must be free of larger voids and metallic inclusions. In particular, electrical shorts between the steel plates have to be detected and localized. Flash-light excited thermography in one-side access and in transmission was employed to detect artificial and natural local voids and inclusions in the glass-ceramics. Two flash-lights with each 6.4 kJ of energy were used. The recorded thermographic image sequences were pre-processed by pulsed phase thermography (PPT). Short- and long-time reproducibility tests were performed. Sets of samples with prepared test defects (air voids and metal platelets) of different size were measured after optimization of the excitation and detection parameters. One of the steel plates consists of two tightly joint separate sheets. This may cause false alarms in thermographic testing due to small air gaps between the sheets. The experimental findings were supported by numerical FEM simulations using COMSOL Multiphysics. A POD (probability of detection) analysis was performed showing that voids with a diameter of 2.3 mm can be detected reliably. Electrical shorts between the steel sheets could be localized by a lock-in technique using a modulated electrical current. Resistive losses at the internal contact points generate heat which becomes visible as a hot-spot in the thermal image.