Structural Analysis of Molten Na2O-NaF-SiO2 System by Raman Spectroscopy and Molecular Dynamics Simulation

Abstract

To determine the effect of F ions on the structure of the molten alkali silicate systems, quenched Na 2 OSiO 2 -NaF systems were investigated by Raman spectroscopy and molecular dynamics simulation. The systematic increase of 1 100 cm Ϫ1 band intensity in the Raman spectra of the silicate melts accompanying the replacement of O by F provides the evidence for concomitant polymerization of melts. From the molecular dynamics simulation, it was confirmed that most of substituted F was mainly coordinated to Na ϩ ions but not Si 4ϩ ions at least up to 12.5 mol% of F ion content. A small amount of F was found to be coordinated to Si as a non-bridging ion from the molecular dynamics simulation, although there was no recognizable evidence from Raman spectroscopy. These results were consistent with the mechanism in which F associated with otherwise network-modifying Na rather than with network-forming Si. Since F was associated to Na ϩ ions, the replace of O ion by two F ions promote the polymerization of silicate melts.KEY WORDS: fluorine; molten flux; slag structure; molecular dynamics simulation; Raman spectroscopy. much information on the structural details such as bond angle distributions or pair functions that are difficult to obtain by Raman measurements. 5) So, in this investigation, the molecular dynamics simulations were also employed to investigate the structural role of F Ϫ ions in molten alkali-silicate melts and simulated results were compared and discussed with the measured Raman spectra. Actually using the molecular dynamics simulation for these structural analyses has a great advantage, since it is generally difficult to carry out the precise experiments for the melts with high F content due to their high volatilization rate, Raman Spectroscopy MeasurementsThe detail of Raman spectroscopic measurements was described previously. 14) Appropriate portions of reagent grade Na 2 CO 3 , SiO 2 and NaF powders were mixed together in an agate mortar in air and placed in a Pt crucible. The mixture was heated at 1 073 K for 10.8 ks to decarbonate and then fired in a muffle furnace at 1 673K for 7.2 ks. The melted mixture was quenched into a water-cooled copper flat mold and then ground to form a powder. The prepared powder was melted again as a bead (only about 5 mg) at 1 673 K on a Pt-Pt alloy thermocouple wire loop in air and quenched. Quenching was achieved by switching off the power to the hot thermocouple. The initial compositions of the samples are presented in Table 1 and also in Fig. 1. In the table and figure, Na 2 F 2 is used instead of NaF, since it is convenient to consider the substitution of 2F Ϫ to O 2Ϫ (2F Ϫ → O 2Ϫ ). As shown in Fig.1 and Table 1, the SiO 2 contents in the alkali-silicate samples were fixed to 33 and 50 mol% and 2F Ϫ → O 2Ϫ substitution was done along the lines parallel to Na 2 O-Na 2 F 2 join. Namely, total number of Si and Na cations and (2F Ϫ ϩO 2Ϫ ) anions in the samples on this join were constant but only 2F Ϫ /O 2Ϫ ratio was changed.After the samples were prepared, their conte...

Molecular Dynamics Analysis of the Effect of F on the Structure of Molten Na2O-NaF-Al2O3-SiO2 System

“…1) This behavior is essentially the same as that of Si tetrahedra observed in the present study. So that the same role of F in the silicate melts can be possibly applied to that for the Si tetrahedral polymerization in the aluminosilicate melts.…”

“…It was confirmed that the replacement of O ions by F ions promoted the polymerization of silicate melts 1) and Al ion had a strong tendency to develop the three-dimensional network structure.…”

“…In the previous works, the structural role of F ions in the Na 2 O-NaF-SiO 2 melts 1) and that of Al ions in the Na 2 OAl 2 O 3 -SiO 2 melts 2) were discussed. It was confirmed that the replacement of O ions by F ions promoted the polymerization of silicate melts 1) and Al ion had a strong tendency to develop the three-dimensional network structure.…”

Crystallization Behaviors of Mold Fluxes Containing Li₂O Using Single Hot Thermocouple Technique

“…If Li 2 O was substituted by Na 2 O, the following substitution reaction (1) should be taken into account according to Yasushi et al [31][32][33][34][35] But it was found that most of F Ϫ ions are fourfold coordinated by Ca 2ϩ rather than Na ϩ for samples of n Na /(n Na ϩn Ca )Յ0.3, and the presence of Na 2 O may not prevent the precipitation of cuspidine from the mold fluxes. [36][37][38][39] In the composition range of experimental samples, the value of n Na /(n Na ϩn Ca ) was less than 0.3, and it was confirmed by XRD that there was precipitation of cuspidine for all the samples.…”

Reply to "Discussion on the Estimation of the Iso-viscosity Lines in Molten CaF2-CaO-SiO2 System"

“…The details of MD calculation can be found in elsewhere. 6,7) Silicate melts consist of a mixture of five distinct types of structural units that have 4, 3, 2, 1 and 0 non-bridging oxygens per silicon, i.e., SiO 4Ϫ (monomer), Si 2 O 7 6Ϫ (dimer), Si 2 O 6 4Ϫ (chain unit), Si 2 O 5 2Ϫ (sheet unit) and SiO 2 (three-dimensional structure) units. 5) In the terminology commonly employed in NMR spectroscopy, these structures are referred to as Q 0 , Q 1 , Q 2 , Q 3 and Q 4 where the superscript refers to the number of bridging oxygen in a given structural unit.…”