Zr+4-controlled nucleation and microstructure in Si-Mg-Al-K-B-F glass-ceramic sealant (solid oxide fuel cell)

Abstract

This work illustrates the strong effects of Ti4+ and Zr4+ (M4+) on nucleation ability of SiO2-MgO-Al2O3-B2O3-K2O-F glass-ceramic sealant. An addition of such 5 wt.% M4+ content caused a slight increase in density value (2.60–2.61 g.cm−3). X-ray diffraction (XRD) experiment revealed no indication of any crystallinity in melt-quenched glass-sealant and that was further confirmed from transmission electron microscopy. XRD established the development of multicrystalline phases in investigated glass-sealants at solid oxide fuel cell (SOFC) operation temperature (=800 °C). The reflected XRD peaks on the heat-treated (i.e., 800 °C) SiO2-MgO-Al2O3-B2O3-K2O-F glasses were indexed as norbergite (Mg2SiO4.MgF2), humite [Mg7F2(SiO4)3], chondrodite [Mg5F2(SiO4)2], mullite (3Al2O3·2SiO2) and enstatite (MgSiO3) phases. Field-emission scanning electron microscope (FESEM) experiment indicated that the plate-like crystallite particles (size 3–10 μm) were precipitated in a mother glass matrix heated at 800 °C and restructured into droplet-type morphology when nucleation was supported by Ti4+ ions. The presence of Zr4+ ion results an interlocked type microstructure that is desirable for sealing application because of their capability to resist the growth of a microcrack caused due to thermal stress. A wide thermal expansion value (=11.06 × 10−6/K) compatible with SOFC components (electrode, electrolyte, interconnect, and so on) is achieved for Zr4+-doped SiO2-MgO-Al2O3-B2O3-K2O-F glass-ceramic. No considerable deformation in thermal expansion value up to 10 thermal cycling operations is obtained for Zr4+ doped glass and this is ascribed to the interlocked type morphology (FESEM).