A generalized approach to the flood-knapp structure based model for binary liquid silicates: Application and update for
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I.
INTRODUCTION
F O R a long time liquid silicates have been modeled as a mixture of complexes formed by chemical reactions among the mixed oxide components. Such models are based on various experimental measurements on electrical conductivity, viscosity, thermal expansion, spectroscopy, analytical techniques, and high temperature X-ray diffraction. Measurements of electrical conductivity and thermal expansion have also led to the conclusion that these complexes are predominantly ionic in nature. However, the maximum in viscosity at fayalite composition in the binary system FeO-SiO2 has been ascribed to a simultaneous formation of molecular (= neutral) complexes as (Fe2SiO4).. A. Experimental Observations
High temperature X-ray diffraction measurements on liquid silicates 2'3.4 have revealed that the fundamental local ordering unit in liquid silicates is the SiO4 tetrahedron. The Si-Si coordination number gradually increases by the addition of SiO:, indicating a polymerization of SiO4 tetrahedra. At the metasilicate composition, MSiO3, the Si-Si coordination number is about three in several of the systems investigated. Measurements with high temperature X-ray diffraction~ and Raman spectroscopy5'6 have indicated a close qualitative resemblance between the complexes present in liquid and glassy silicates of the same composition. Their quantitative distribution can, however, differ as was emphasized by Waseda. 7 Comparison of Raman spectra for both glassy and solid silicates 8-" shows that the same structural units as in solid silicates are to be expected in both liquid and glassy silicates. Measurements with Raman spectroscopy on glasses in the systems Na20-SiO2, CaO-SiO2, CaO-MgO-SiO2,12 and FeO-FezO3-SiOz 13 have shown that these systems can be BO BJORKMAN, GUNNAR ERIKSSON, and ERIK ROSt~N are all with the Department of Inorganic Chemistry, Ume:~ University, S-901 87 Ume~, Sweden. Manuscript submitted July 11, 1983. METALLURGICALTRANSACq'IONS B
interpreted as consisting of only a few structural units with 4, 3, 2, 1, and 0 nonbridging oxygens per silicon, respectively. Distinct compositional domains involving only three of these structural units could be found. In principle, the same results were obtained by Furukawa et al. J4 and Iwamoto et al. u for glasses in the system PbO-SiO2. The most common analytical method for the characterization of silicate anions in glassy and solid silicates consists of a trimethylsilylation of the anions and an ensuing identification of these trimethylsilyl derivatives with gas chromatography, tS-L8 This method has been criticized for giving poor yields and for giving results in disagreement with spectroscopic measurements due to polymerization of the silicate anions during the preparation of the derivative. 12 B. P r e s e n t M o d e l s
An alternative way to get information on the complexes present in a silicate melt is to apply structure based models to such quantities as the activity of a metal oxide in the melt. Such models for binary liquid silicate systems have recently
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