Nanocrystalline microstructure in Sm3+ and Gd3+ doped K2O-MgO-Al2O3-SiO2-F glass-ceramic sealant (SOFC)

In order to demonstrate the effects of Sm3+ and Gd3+ ions on the crystalline microstructures of the magnesium-boro-alumino-silicate (MBAS) system, the K2O-MgO-B2O3-Al2O3-SiO2-F glass doped with 0-5 mol% Sm2O3 and Gd2O3 were synthesized by melt-quenching (1550 degrees C). The addition of Sm3+ and Gd3+ content was found to increase the density (2.74-2.91 g cm(-3)) of the base glass. By controlled heat-treatment at 950 degrees C, the MBAS glasses were converted into opaque glass-ceramics with crystalline phases (XRD), containing fluorophlogopite mica [KMg3(AlSi3O10)F-2], norbergite [Mg2SiO4.MgF2] and enstatite [MgSiO3]. The FESEM study revealed the development of rock-like and plate-like crystallite particles (average size 2-4 mu m) randomly dispersed in the heat-treated MBAS microstructure, which on the addition of Sm3+ and Gd3+ ions is restructured into nanocrystalline (size = similar to 50-400 nm) morphology. The substantial change in the microstructure influenced the corresponding density and thermal expansion properties. The coefficient of thermal expansion for MBAS was estimated to be 10.47(+/- 0.10) x 10(-6) K-1 (50-800 degrees C), which increased to 11.11-11.29 x 10(-6) K-1 when doped with Sm3+ and Gd3+. Such large thermal expansion makes the Sm2O3- and Gd2O3-doped K2O-MgO-B2O3-Al2O3-SiO2-F glasses suitable for high temperature sealing applications (like SOFC).