Melilite glass-ceramic sealants for solid oxide fuel cells: Effects of ZrO₂ additions assessed by microscopy, diffraction and solid-state NMR

The influence of adding 0-5 mol% zirconia (ZrO₂) to a series of melt-quenched alkaline-earth aluminosilicate glasses designed in the gehlenite (Ca₂Al₂SiO₇)-akermanite (Ca ₂MgSi₂O₇) system has been investigated for their potential application as sealants for solid oxide fuel cells (SOFCs). The work was implemented with a dual aim of improving the sintering ability of the glass system under consideration and gaining insight into the structural changes induced by ZrO₂ additions in the glasses consequentially leading to their enhanced long-term thermal stability. That the degree of condensation of SiO₄ tetrahedra increased with increasing amounts of zirconia was confirmed by ²⁹Si magic-angle (MAS) NMR. 1D ²⁷Al, ¹¹B MAS as well as two-dimensional (2D) ¹¹B MQMAS/STMAS NMR experiments gave structural insight into the number and nature of aluminum and boron sites found in the glass and glass-ceramic (GC) samples. Irrespective of the heat treatment time, increasing the zirconia content in glasses suppressed their tendency towards devitrification, while the glasses exhibited good sintering behavior resulting in mechanically strong GCs with higher amounts of residual glassy phase making them suitable for self-healing during SOFC operation. All the GCs exhibited low total electrical conductivity; appropriate coefficients of thermal expansion (CTE), good joining and minimal reactivity with SOFC metallic components at the fuel cell operating temperature, thus, qualifying them for further appraisal in SOFC stacks.