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

Abstract

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 °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 °C, the MBAS glasses were converted into opaque glass-ceramics with crystalline phases (XRD), containing fluorophlogopite mica [KMg₃(AlSi₃O₁₀)F₂], norbergite [Mg₂SiO₄·MgF₂] and enstatite [MgSiO₃]. The FESEM study revealed the development of rock-like and plate-like crystallite particles (average size 2–4 μm) randomly dispersed in the heat-treated MBAS microstructure, which on the addition of Sm³⁺ and Gd³⁺ ions is restructured into nanocrystalline (size = ∼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) × 10⁻⁶ K⁻¹ (50–800 °C), which increased to 11.11–11.29 × 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).