Thermal properties, density and structure of percalcic and peraluminus CaO–Al2O3–SiO2 glasses
Introduction
Aluminosilicate glasses are of interest for excellent mechanical and thermal properties. Both SiO2 and Al2O3 constituents contribute to the formation of network in the glasses. It is well known that Al3 + can play different roles and can be in different coordination number, CN = 4, 5 and 6. The CN of Al depends on glass composition [1], [2], [3], [4], [5], temperature condition [4], [6], [7], pressure condition [8], [9], [10], [11], [12] and quenching rate [4], [7]. Therefore, the structural role of Al is essential for determining the properties of aluminosilicate glasses.
The role of Al depends on [Al2O3]/[R2O, R′O] molar ratio where R2O is the alkali oxide and R′O the alkaline earth oxide in terms of simple stoichiometric considerations. In order to link and share the corner between AlO4 tetrahedra, charge compensating cations such as R+ or R2 + are needed. When the charge compensating cation exists enough for Al in [Al2O3]/[R2O, R′O] ≤ 1.0, all Al ideally behave as a network former for the formation of AlO4 tetrahedra. On the other hand, when the cation is insufficient for Al in [Al2O3]/[R2O, R′O] > 1.0, a part of Al may behave as a modifier, e.g., AlO6 octahedra. Although some exceptions were reported e.g., Ca-aluminosilicate [3], [13] and aluminate glasses [14], at [Al2O3]/[R2O, R′O] = 1.0, it has been ideally thought that the tectrosilicate glasses have fully polymerized network structure of SiO4 and AlO4 terahedra without any non-bridging oxygens.
Riebling [15] reported that the viscosity at high temperature of Na2O–Al2O3–SiO2 melt increased with increasing [Al2O3]/[Na2O] molar ratio in persodic region, exhibited a maximum at [Al2O3]/[Na2O] = 1.0 and then decreased in the peraluminus region. He explained that the decrease of melt viscosity in the peraluminus region is due to the formation of some AlO6 octahedra based on analogy of the corresponding high-temperature crystalline phase in the Al2O3–SiO2 binary systems. Toplis et al. [16] observed maxima in melt viscosity at the peraluminus region of [Al2O3]/[Na2O] > 1.0 in Na2O–Al2O3–SiO2 melt with > 50 mol% SiO2. They explained that the result is due to the existence of a tricluster. In the tricluster, an oxygen atom is shared between three SiO4 and AlO4 tetrahedral groups [17], [18]. Le Losq et al. [19] obtained the glass transition temperature Tg and the configuration entropy at Tg from measured viscosity data for Na2O–Al2O3–SiO2 glasses and melts with 75 mol% SiO2. They explained their compositional dependences by the connectivity of network relating to the AlO5 species with threefold coordinated oxygen atoms.
The role of Al on the short- and medium-range structures of aluminosilicate glasses has been analyzed by infrared [20], [21], [22], Raman [3], [23] and NMR [3], [24] spectroscopy. Neuville et al. reported that AlO5 and AlO6 species exist in the peraluminus region of [Al2O3]/[CaO] > 1.0 [3], [25] and the glass neutrality is maintained by Al coordination number change in CaO–Al2O3–SiO2 (CAS) glasses.
In our previous study the effect of Al2O3 addition on thermal properties and structure was studied for percalcic CAS glasses with [Al2O3]/[CaO] < 1.0 [26]. In this paper CaO–SiO2 (CS) binary and CAS ternary glasses with 10–76 mol% SiO2 were prepared in the wide [Al2O3]/[CaO] range from 0.10 to 4.00. The glass transition temperature, the coefficient of linear expansion and density were evaluated for the CS and CAS glasses. The ionic packing factor [27] was derived from density measurements. The network morphologies of SiO4 tetrahedra, mixed-anion (Si, Al)O4 and AlOx species (x = 4, 5, 6) were examined by IR spectra. The proportions of AlOx evaluated by 27Al MQ-MAS NMR in a previous study [3] were used to understand the variation of AlOx species as a function of [Al2O3]/[CaO] molar ratio (ACMR hereinafter). We provide the first direct comparison among measured Al coordination information, thermal properties and the ionic packing factor.
Section snippets
Experimental
Fig. 1 shows the CS and CAS glass compositions studied in this work. Glass compositions examined in previous studies [21], [28] are also appeared for comparison. Moesgaard et al. [28] evaluated the intermediate-range order heterogeneity for the distribution of SiO4 unit by using 29Si NMR in the vicinity of eutectic compositions for anorthite–gehlenite–wollastonite and anorthite–wollastonite–tridymite points. A part of our glass series with 45, 50, 55 and 60 mol% SiO2 has similar compositions of
Properties
Table 1 summarizes the measured values of Tg, α and ρ, and calculated ones of Vm and Vp in 50CaO∙50SiO2 and CAS glasses with various SiO2 contents. Tg tends to increase with increasing [Al2O3]/[CaO] molar ratio (ACMR) in all CAS glass series (Fig. 2). Moreover, Tg increases slowly for [Al2O3]/[CaO] ≥ 1.00. As shown in Fig. 3, α decreases with increasing both ACMR and SiO2 content in the percalcic region and has almost constant values (4.5–5.0 × 10− 6/°C) in the peraluminus region. These
Glass structure
For a variety of CAS glass series, differences of peak positions relating to various network components were observed in IR spectra (Figs. 6(a)–(g) and 7). The structure of CAS glasses is evaluated by IR spectra (Fig. 6) and 27Al MQ NMR spectra measured in a previous study [3]. Fig. 8 shows variations of the proportions of AlOx (x = 4, 5, 6) species as a function of ACMR for CAS glass series with 10, 33 and 50 mol% SiO2 from the NMR data [3]. Previous NMR studies also showed the presence of
Conclusions
The CaO–SiO2 (CS) binary and CAS ternary glasses with 10–76 mol% SiO2 were prepared in the wide range of [Al2O3]/[CaO] = 0.10–4.00. The glass transition temperature Tg increases, both the coefficient of linear expansion, α, and the ionic packing factor Vp calculated from measured density decrease due to the contribution of AlO4 formation in the percalcic region with [Al2O3]/[CaO] < 1.00. The result of IR spectroscopy reveals that in the percalcic region Al forms AlO4 tetrahedra with Al–O–Al linkages
Acknowledgment
This research is partly supported by the New Energy and Industrial Technology Development Organization (NEDO), Innovative Zero-emission Coal Gasification Power Generation Project.
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