SIO2-AL2O3 LIQUIDS - INSITU STUDY BY HIGH-TEMPERATURE AL-27 NMR-SPECTROSCOPY AND MOLECULAR-DYNAMICS SIMULATION

Using a recently developed NMR technique, which involves aerodynamically levitating the sample and heating it with a CO2 laser in the bore of a conventional NMR spectrometer, we have obtained Al-27 NMR spectra of liquids along the silica-alumina binary join at temperatures above 2000-degrees-C. All of the NMR spectra obtained contain a single Lorentzian shaped line with FWHM on the order of 10(2)-10(3) Hz. Isotropic chemical shifts become slightly more positive (less shielded) with increasing Al2O3 from 56.3 ppm for the 10 mol % Al2O3 liquid to 59.5 ppm for pure Al2O3 liquid. Molecular dynamics simulations of SiO2-Al2O3 liquids indicate an increase in the average coordination number of oxygen about aluminum with increasing Al2O3, with significant proportions of AlO5 groups in the alumina-rich liquids. The slight variation in chemical shift is most probably due to opposing effects of average Al coordination and Si/(Si + Al) ratio (next-nearest-neighbor environment). For the high alumina containing liqUids (mol % Al2O3 > 50), Al-27 NMR lines are fully narrowed (FWHM 100-200 Hz). Below 50 mol % Al2O3, lines broaden with increasing SiO2 content, to near 700 Hz for the 20 mol % Al2O3 liquid. This observation is interpreted as due to decreasing spin-lattice relaxation times as the silica content is increased. Correlation times (tau(c)) for spin-lattice relaxation calculated from the observed NMR linewidths are in good agreement with shear relaxation times (tau(v)) determined from viscosity measurements of these liquids especially at high alumina content.