Optimization of glass–ceramic sealant compositions in the system MgO–BaO–SiO2 for solid oxide fuel cells (SOFC)

The glass–ceramic with composition 55 SiO2–27 BaO–18 MgO, mol.% (Mg1.5–55) presents suitable properties for application as a sealant material in IT-SOFC; good unions are achieved with the electrolyte 8YSZ and with the aluminium based interconnect material FeCr alloy. Nevertheless, sealing delaminates when applied to chromium based ferritic stainless steel Crofer 22, one of the most used interconnect material in SOFC, due to the precipitation of BaCrO4 at the interface.In this work, the union and chemical compatibility of new compositions prepared from modifications of composition Mg1.5–55 have been tested on Crofer 22. The modifications consisted on substituting SiO2 for B2O3 (Mg1.5–50–5B to Mg1.5–35–20B), for B2O3 and PbO (Mg1.5–40–5B–10Pb), and B2O3 and part of MgO for ZnO (Mg1.5–50–5B–8Zn and Mg1.5–40–15B–8Zn). The characterization of the glass–ceramic materials was carried out by dilatometry, differential thermal analysis (DTA), hot-stage microscopy (HSM), X-ray diffraction (XRD) and scanning electron microscopy (SEM).All the glasses present a suitable thermal expansion coefficient for use in SOFCs. The addition of B2O3 produces the expected decrease of viscosity and a delay in the crystallisation, which gives a better wettability of the glasses on the steel and a better union, inhibiting the direct reaction between chromium oxide and the barium rich crystalline phases to form BaCrO4. At 850 °C, barium silicates crystallize in all the glasses. Barium magnesium silicates and magnesium silicate are present in the glasses without zinc and the crystallisation of barium zinc silicate is promoted in the zinc containing glasses instead. Compositions Mg1.5–40–15B and Mg1.5–40–15B–8Zn exhibit the best glass–ceramic/Crofer 22 union for the operation temperature of 850 °C. The seals are pore-less, strongly adherent to the steel and present elongated crystals which are homogeneously distributed in the glass matrix.