1 research outputs found
Understanding the Formation of CaAl<sub>2</sub>Si<sub>2</sub>O<sub>8</sub> in Melilite-Based Glass-Ceramics: Combined Diffraction and Spectroscopic Studies
An
assessment is undertaken for the formation of anorthite crystalline
phase in a melilite-based glass composition (CMAS: 38.7CaOā9.7MgOā12.9Al<sub>2</sub>O<sub>3</sub>ā38.7SiO<sub>2</sub> 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), <sup>29</sup>Si and <sup>27</sup>Al magic-angle
spinning nuclear magnetic resonance (MAS-NMR), and in situ Raman spectroscopy.
ND, <sup>29</sup>Si MAS-NMR, and <sup>27</sup>Al 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 Ā°C. Further heat treatment up to 900 Ā°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 Ā°C for 500 h revealed the presence
of anorthite along with the melilite crystalline phase. Rietveld refinement
of XRD data indicated a high fraction of glassy phase (ā¼67%)
after the formation of crystalline phases. The <sup>29</sup>Si MAS-NMR
spectra for the CMAS-GC suggest the presence of structural units in
the remaining glassy phase with a polymerization degree higher than
dimer units, whereas the <sup>27</sup>Al MAS-NMR spectra revealed
that most Al<sup>3+</sup> cations exhibit a 4-fold coordination. In
situ Raman spectroscopy data indicate that the formation of anorthite
crystalline phase initiated after 240 h of heat treatment at 850 Ā°C
owing to the interaction between the gehlenite crystals and the remaining
glassy phase