Computer Science
Scientific paper
Jul 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997gecoa..61.2569m&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 61, Issue 13, pp.2569-2575
Computer Science
4
Scientific paper
The effects of temperature on the structure of Na 2 Si 2 O 5 glass and supercooled liquid has been investigated by high-temperature 29 Si NMR measurements from room temperature to 719°C. The asymmetric line shape of the 29 Si NMR spectra was reproduced well by a line shape simulation concerning the chemical shift anisotropy. A three-site (Q 2 , Q 3 , and Q 4 ) abundances for the NMR spectra yields a quantitative estimation of the population of each species, where Q n represents SiO 4 tetrahedra with n bridging O atoms. The temperature dependence of a disproportionation reaction, 2Q 3 Q 2 + Q 4 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, G °, of the reaction was determined from the van't Hoff equation. G ° was evaluated to be 33 ± 5 kJ at 538°C. The temperature variation of G ° was quite small, suggesting a small entropy contribution of the above reaction. A plot of G °/ T vs. 1/ T yields an estimate of the standard enthalpy, H °, of the above equilibrium as 27 ± 5 kJ. The temperature dependence of the 29 Si NMR line shape was interpreted in terms of the exchange reaction between Q n 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.
Maekawa Hideki
Yokokawa Toshio
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