Repair Joining of Glass-Ceramic Sealants for SOC Stacks

Abstract

A new concept has been developed to repair leaking solid oxide cell (SOC) stacks. Conventionally, the stacks are joined with a glass-ceramic sealant before they are taken to operation. Once they start leaking through the sealant, they have become unusable. This approach aims to prevent the scrapping of valuable aggregates.A composite sealing material composed of a glass matrix “H” based on the BaO-CaO-SiO2 system with yttrium-stabilized zirconia fibers as filler material was used for this study. The sealant tends to crystallize slowly at 800 °C forming a multi-component glass-ceramic of a finely dispersed microstructure. The melting temperature of the crystallized material was investigated by means of differential scanning calorimetry (DSC). Beside of the glass matrix powder which was crystallized by heating to 850°C and dwelling for 100 h, the state-of-art sealing material was used as green foils of the composite mixture with fibers. Sandwiched samples of two Crofer22APU plates of 50x50 mm² were joined with the two types of sealing materials, followed by forcibly breaking of the samples and repair joining at different temperatures between the onset and peak temperature of melting of the glass-ceramic obtained by DSC. Gas-tightness measurement was performed by helium leakage detection at room temperature and the broken samples were investigated by optical microscopy. The optimal thermal treatment parameters were chosen on behalf of gas-tightness and porosity. Cross-sections were prepared after repair joining at 930°C for 10 h and scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS) was carried out. The microstructure shows a high amount of residual glass phase with clusters formed of zirconia-silicate needles. The reaction zone to the steel is of a similar thickness in comparison to standard joining at 850°C, but is clearly delineated from the sealing material. Close to the three-phase boundaries of metal/glass/air a homogeneously distributed closed porosity is found while the pores have gathered to flattened enlarged voids in the middle of the joining gap.The impact of these microstructural findings on the strength of the joints are now investigated by means of torsion shear tests on hour-glass shaped Crofer22H samples joined in a ring contour. The strength data will be compared to the standard joining procedure and thermally aged samples (800°C for 1000h). Meanwhile an upscaling of the joining geometry using dummy plates of the size of window frame design (H20-design) stacks was performed and the result of gas-tightness measurements will be reported. In case of successful repair of dummy plates, this promising concept will be proofed with a 20-layer stack (internal number: H2020-02), which showed measurable leakage through sealant after start-up procedure.