EFFECTS OF TEMPERATURE ON SILICATE MELT STRUCTURE: A HIGH TEMPERATURE 29SI NMR STUDY OF NA2SI2O5

Abstract

The effects of temperature on the structure of Na2Si2O5 glass and supercooled liquid has been investigated by high-temperature 29Si NMR measurements from room temperature to 719°C. The asymmetric line shape of the 29Si NMR spectra was reproduced well by a line shape simulation concerning the chemical shift anisotropy. A three-site (Q2, Q3, and Q4) abundances for the NMR spectra yields a quantitative estimation of the population of each species, where Qn represents SiO4 tetrahedra with n bridging O atoms. The temperature dependence of a disproportionation reaction, 2Q3 # Q2 + Q4 was obtained from the NMR results in the temperature region up to 603°C. The equilibrium constant for the above reaction, K, and its temperature dependence were determined. The free energy change, ΔG°, of the reaction was determined from the van't Hoff equation. ΔG° was evaluated to be 33 +/- 5 kJ at 538°C. The temperature variation of ΔG° was quite small, suggesting a small entropy contribution of the above reaction. A plot of ΔG°/T vs. 1/T yields an estimate of the standard enthalpy, ΔH°, of the above equilibrium as 27 +/- 5 kJ. The temperature dependence of the 29Si NMR line shape was interpreted in terms of the exchange reaction between Qn species. A multi-site exchange model was applied to this system, and the simple Eyring model for viscosity calculation from the estimated exchange rate was compared with the measured viscosity.