2 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
Structure and Crystallization of Alkaline-Earth Aluminosilicate Glasses: Prevention of the Alumina-Avoidance Principle
Aluminosilicate glasses are considered
to follow the Al-avoidance principle, which states that AlāOāAl
linkages are energetically less favorable, such that, if there is
a possibility for SiāOāAl linkages to occur in a glass
composition, AlāOāAl linkages are not formed. The current
paper shows that breaching of the Al-avoidance principle is essential
for understanding the distribution of network-forming AlO<sub>4</sub> and SiO<sub>4</sub> structural units in alkaline-earth aluminosilicate
glasses. The present study proposes a new modified random network
(NMRN) model, which accepts AlāOāAl linkages for aluminosilicate
glasses. The NMRN model consists of two regions, a network structure
region (NS-Region) composed of well-separated homonuclear and heteronuclear
framework species and a channel region (C-Region) of nonbridging oxygens
(NBOs) and nonframework cations. The NMRN model accounts for the structural
changes and devitrification behavior of aluminosilicate glasses. A
parent Ca- and Al-rich melilite-based CaOāMgOāAl<sub>2</sub>O<sub>3</sub>āSiO<sub>2</sub> (CMAS) glass composition
was modified by substituting MgO for CaO and SiO<sub>2</sub> for Al<sub>2</sub>O<sub>3</sub> to understand variations in the distribution
of network-forming structural units in the NS-region and devitrification
behavior upon heat treating. The structural features of the glass
and glassāceramics (GCs) were meticulously assessed by advanced
characterization techniques including neutron diffraction (ND), powder
X-ray diffraction (XRD), <sup>29</sup>Si and <sup>27</sup>Al magic
angle spinning (MAS)-nuclear magnetic resonance (NMR), and in situ
Raman spectroscopy. ND revealed the formation of SiO<sub>4</sub> and
AlO<sub>4</sub> tetrahedral units in all the glass compositions. Simulations
of chemical glass compositions based on deconvolution of <sup>29</sup>Si MAS NMR spectral analysis indicate the preferred formation of
SiāOāAl over SiāOāSi and AlāOāAl
linkages and the presence of a high concentration of nonbridging oxygens
leading to the formation of a separate NS-region containing both SiO<sub>4</sub> and AlO<sub>4</sub> tetrahedra (Si/Al) (heteronuclear) in
addition to the presence of Al<sub>[4]</sub>āOāAl<sub>[4]</sub> bonds; this region coexists with a predominantly SiO<sub>4</sub>-containing (homonuclear) NS-region. In GCs, obtained after
heat treatment at 850 Ā°C for 250 h, the formation of crystalline
phases, as revealed from Rietveld refinement of XRD data, may be understood
on the basis of the distribution of SiO<sub>4</sub> and AlO<sub>4</sub> structural units in the NS-region. The in situ Raman spectra of
the GCs confirmed the formation of a Si/Al structural region, as well
as indicating interaction between the Al/Si region and SiO<sub>4</sub>-rich region at higher temperatures, leading to the formation of
additional crystalline phases