resumo
Aluminosilicate glasses are considered to follow the Al-avoidance principle, which states that Al-O-Al linkages are energetically less favorable, such that, if there is a possibility for Si-O-Al linkages to occur in a glass composition, Al-O-Al linkages are not formed. The current paper shows that breaching of the Al-avoidance principle is essential for understanding the distribution of network-forming AlO4 and SiO4 structural units in alkaline-earth aluminosilicate glasses. The present study proposes a new modified random network (NMRN) model, which accepts Al-O-Al 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 non-framework cations. The NMRN model accounts for the structural changes and devitrification behavior of aluminosilicate glasses. A parent Ca- and Al-rich melilite-based CaO-MgO-Al2O3-SiO2 (CMAS) glass composition was modified by substituting MgO for CaO and SiO(2 )for Al2O3 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 glass- ceramics (GCs) were meticulously assessed by advanced characterization techniques including neutron diffraction (ND), powder X-ray diffraction (XRD), Si-29 and Al-27 magic angle spinning (MAS)-nuclear magnetic resonance (NMR), and in situ Raman spectroscopy. ND revealed the formation of SiO, and AlO4 tetrahedral units in all the glass compositions. Simulations of chemical glass compositions based on deconvolution of Si-29 MAS NMR spectral analysis indicate the preferred formation of Si-O-Al over Si-O-Si and A1-0-Al linkages and the presence of a high concentration of nonbridging oxygens leading to the formation of a separate NS-region containing both SiO, and AlO4 tetrahedra (Si/Al) (heteronuclear) in addition to the presence of Al-[4] O-Al-[4] bonds; this region coexists with a predominantly SiO4 -containing (homonuclear) NS-region. In GCs, obtained after heat treatment at 850 degrees 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 SiO4 and AlO4 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 SiO4 -rich region at higher temperatures, leading to the formation of additional crystalline phases.
palavras-chave
NUCLEAR-MAGNETIC-RESONANCE; X-RAY-DIFFRACTION; SI-29 MAS NMR; MOLECULAR-DYNAMICS SIMULATIONS; OXIDE FUEL-CELLS; SOLID-STATE NMR; AL-27 NMR; RAMAN-SPECTROSCOPY; MICROSCOPIC ORIGINS; RANGE ORDER
categoria
Chemistry
autores
Allu, AR; Gaddam, A; Ganisett, S; Balaji, S; Siegel, R; Mather, GC; Fabian, M; Pascual, MJ; Ditaranto, N; Milius, W; Senker, J; Agarkov, DA; Kharton, VV; Ferreira, JMF
nossos autores
Allu Amarnath Reddy
Estudante de doutoramentoAnuraag Gaddam
Investigador Auxiliar
José Maria da Fonte Ferreira
Professor(a) Jubilado(a)agradecimentos
A.R.A. and S.B. would like to thank Dr. Muraleedharan K., Director, CSIR-CGCRI for his strong support and encouragement. A.R.A. gratefully acknowledges the financial support of the Budapest Neutron Centre, Hungary for allotting the beam time and financial support (BRR_407) under the NMI3-II program. Part of this work was developed in the scope of the CICECO-Aveiro Institute of Materials (UID/CTM/50011/2013) project and funded by FEDER funds through the Operational Programme Competitiveness Factors (COMPETE 2020) and the Portuguese Foundation for Science and Technology (FCT). S.G. is thankful for the financial support by the German Research Foundation (DFG) through the priority program SPP 1594 "Topological Engineering of Ultra-Strong Glasses". The work of the ISSP RAS team was supported by the Russian Science Foundation (Grant 17-79-30071).