Silica-calcia binary system

It has been shown that, using this simple model, a rather good description of the structure of pure silica can be obtained compared with experimental data. This approach was extended to more complex systems such as the silica-calcia binary system. For this binary system, we used a RIM based on silica [ 15] and adapted for the silica-calcia system. The accuracy of the model was checked by comparing RDFs with experimental data at r= 1973 K from Ref. [21]. A rather good agreement of our results with experimental data for the three first peaks (Si-O, Ca-O, 0-0) was obtained but we observed some discrepancies for the interatomic distance of Si-Si. 

In binary mixtures the vaporization of silica may occur and change the composition, which may lead to some density modification. 

In binary systems, the oxygen partial charge depends on the composition. In the calculations, averaged values were used instead of including the three different types oxygen usually reported in the literature [15, 20]. 

This may account for the observed differences between the measurements and predicted values. 

This work shows that the use of the RIM allows to describe the local structure of pure silica and a silica-calcia mixture for which measurements are complicated or not possible. However, it has been shown that the calculation of properties such as density exhibit some differences from measurements but the trend is correct and offers, in a first approach, a rather good estimation of it at elevated temperature. For complex systems, including multi-cations with different oxidation degrees, it appears that the polarizability of ions should be included to get more reliable results. 

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Our study was extended to the binary system silica-calcia. The parameters of the Born-Mayer potential (Equation 3.4.2) were taken from Ref. [15]. The interatomic potential was calculated from the potential used for pure silica performed by TTAM [3] and derived from the ab initio Hartree-Fock self-consistent field calculations for model clusters of silica. The parameters used in Equation 3.4.2 are summarized in Table 3.4.2. 

The partial charges of ions for the binary system silica-calcia are = 2.4, = 1.4 and Qq are given as a... 

Table 3.4.2 Parameters of the Born-Mayer pair potential used in Equation 3.4.2 for binary system silica-calcia from Seo and Tsikihashi [15]...

For the binary system silica-calcia, the calculations were carried out hundreds of degrees above the melting in order to compare our results with available experimental measurements. The accuracy of the model was checked using experimental data from Ref. [21 ]. The evolution of the RDF for the system Si02-Ca0 calculated at r= 2000 K for a molar composition of 1 1 and calculated from an isobaric-isothermal ensemble is shown in Figure 3.4.4. Our values are summarized in Table 3.4.5 and compared with experimental data from Ref. [21]. 

Figure 3.4.4 Radial distribution function calculated atJ = 2000 K at atmospheric pressure for the binary system silica-calcia at composition 1 1...

Figure 7. Calcia-alumina-silica system. This illustration compares the melt viscosity of the inviscid binary aluminate melts from 60% CaO to 75% alumina. Addition of MgO and CaO produces quaternary aluminate melts, raises the viscosity significantly and produces a fragile melt with high viscosities below the liquidus temperature and low viscosities above the liquidus temperature. Redrawn from F. T. Wallenberger and S. D. Brown, High modulus glass fibers for new transportation and infrastructure composites and for new infrared uses, Composites Science and Technology, 51.243-263 (1994).

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