Structure and Crystallization of Alkaline-Earth Aluminosilicate Glasses: Prevention of the Alumina-Avoidance Principle

Abstract

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 AlO and SiO 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 nonframework 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-AlO-SiO (CMAS) glass composition was modified by substituting MgO for CaO and SiO for AlO 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 andAl magic angle spinning (MAS)-nuclear magnetic resonance (NMR), and in situ Raman spectroscopy. ND revealed the formation of SiO and AlO tetrahedral units in all the glass compositions. Simulations of chemical glass compositions based on deconvolution of Si MAS NMR spectral analysis indicate the preferred formation of Si-O-Al over Si-O-Si and Al-O-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 AlO tetrahedra (Si/Al) (heteronuclear) in addition to the presence of Al-O-Al bonds; this region coexists with a predominantly SiO-containing (homonuclear) NS-region. In GCs, obtained after heat treatment at 850 °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 and AlO 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-rich region at higher temperatures, leading to the formation of additional crystalline phases.

A modified random network model for P2O5–Na2O–Al2O3–SiO2 glass studied by molecular dynamics simulations

“…45,46 Recently, prevention of the Al avoidance principle have been investigated by NMR spectral analysis and it has been proposed that breaching of the principle was essential for understanding the distribution of [SiO 4 ] and [AlO 4 ] units in alkaline-earth aluminosilicate glass. 22 In our simulation, the quantity of Al-O-Al linkage decreases with P 2 O 5 content while increase with Al content, which is consistent with previous experimental and simulated results. 32,47,48 To quantify the propensity of different inter-tetrahedral connectivity, the degree of preferred connection, DPC mn (where m and n mean network-former cations, Si, Al, and P for example), is dened and calculated, which is governed by…”

“…7a), there are two regions, a network structure region composed of wellseparated aluminum-silicon and silicon-rich framework species as well as a channel region of non-bridging oxygens and network modiers inserted into aluminum-silicon region, which is consistent with the proposed model in terms of aluminosilicate glasses. 22 In the P 2 O 5 -bearing aluminosilicate glasses (Fig. 7b), the structural heterogeneity still exist and even more signicant as the fraction of homonuclear framework predominantly containing [SiO 4 ] units increase aer P 2 O 5 addition.…”

Surface hardness and flammability of Na2SiO3and nano-TiO2reinforced wood composites

“…[47][48][49] 3.3. Increase in mass due to the modication WPG of the samples increased due to the treatment.…”

The effect of water on the validity of Löwenstein's rule

“…Experimentally, Löwenstein's rule is found to be generally obeyed. However, a small number of Löwenstein-disobeying (NLöw) examples have been proposed for aluminosilicates: in the aluminate sodalites,3 zeolite stilbite,4 in bicchulite5–7 and gehlenite-type8 materials, where Si : Al ratios below 1 can be achieved, aluminosilicate glasses9,10 and metastable, disordered phases, such as cordierite11 and anorthite 12. Among related materials that have been shown to disobey Löwenstein's rule are layered Na-4-micas13 and gallobicchulite 14.…”