Understanding the Formation of CaAl2Si2O8 in Melilite-Based Glass-Ceramics: Combined Diffractionand Spectroscopic Studies

摘要

An assessment is undertaken for the formation of anorthite crystalline phase in a melilite-based glass composition (CMAS: 38.7CaO–9.7MgO–12.9Al2O3–38.7SiO2 mol %), used as a sealing material in solid oxide fuel cells, in view of the detrimental effect of anorthite on the sealing properties. Several advanced characterization techniques are employed to assess the material after prolonged heat treatment, including neutron powder diffraction (ND), X-ray powder diffraction (XRD), ²⁹Si and ²⁷Al magic-angle spinning nuclear magnetic resonance (MAS-NMR), and in situ Raman spectroscopy. ND, ²⁹Si MAS-NMR, and ²⁷Al MAS-NMR results revealed that both Si and Al adopt tetrahedral coordination and participate in the formation of the network structure. In situ XRD measurements for the CMAS glass demonstrate the thermal stability of the glass structure up to 850 °C. Further heat treatment up to 900 °C initiates the precipitation of melilite, a solid solution of akermanite/gehlenite crystalline phase. Qualitative XRD data for glass-ceramics (GCs) produced after heat treatment at 850 °C for 500 h revealed the presenceof anorthite along with the melilite crystalline phase. Rietveld refinementof XRD data indicated a high fraction of glassy phase (∼67%)after the formation of crystalline phases. The ²⁹Si MAS-NMRspectra for the CMAS-GC suggest the presence of structural units inthe remaining glassy phase with a polymerization degree higher thandimer units, whereas the ²⁷Al MAS-NMR spectra revealedthat most Al³⁺ cations exhibit a 4-fold coordination. Insitu Raman spectroscopy data indicate that the formation of anorthitecrystalline phase initiated after 240 h of heat treatment at 850 °Cowing to the interaction between the gehlenite crystals and the remainingglassy phase.