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      Understanding the Formation of CaAl 2Si 2O 8 in Melilite-Based Glass-Ceramics: Combined Diffraction and Spectroscopic Studies


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          An assessment is undertaken for the formation of anorthite crystalline phase in a melilite-based glass composition (CMAS: 38.7CaO–9.7MgO–12.9Al 2O 3–38.7SiO 2 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 presence of anorthite along with the melilite crystalline phase. Rietveld refinement of XRD data indicated a high fraction of glassy phase (∼67%) after the formation of crystalline phases. The 29Si 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 27Al MAS-NMR spectra revealed that most Al 3+ 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 °C owing to the interaction between the gehlenite crystals and the remaining glassy phase.

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                Author and article information

                ACS Omega
                ACS Omega
                ACS Omega
                American Chemical Society
                28 September 2017
                30 September 2017
                : 2
                : 9
                : 6233-6243
                []Glass Science and Technology Section, CSIR—Central Glass and Ceramic Research Institute , 700032 Kolkata, India
                []Department of Materials and Ceramics Engineering, CICECO, University of Aveiro , 3810-193 Aveiro, Portugal
                [§ ]Turin Polytechnic University in Tashkent , 17, Small Ring, 100095 Tashkent, Uzbekistan
                []Instituto de Cerámica y Vidrio (CSIC) , C/Kelsen 5, Campus de Cantoblanco, 28049 Madrid, Spain
                []Centre for Energy Research , Konkoly-Thege Street 29-33, 1121 Budapest, Hungary
                #Inorganic Chemistry III and Inorganic Chemistry I, University of Bayreuth , 95440 Bayreuth, Germany
                []Institute of Solid State Physics RAS , 142432 Chernogolovka, Moscow District, Russia
                Author notes
                [* ]E-mail: aareddy@ 123456cgcri.res.in . Phone.: +351-234-370242. Fax: +351-234-370204 (A.R.A.).
                [* ]E-mail: jmf@ 123456ua.pt (J.M.F.F.).
                Copyright © 2017 American Chemical Society

                This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.

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