Correlation between viscosity, electrical resistivity and network connectivity of alkali-free boroalumiosilicate glasses

Abstract

Magic angle spinning nuclear magnetic resonance (MAS NMR) experiments on network formers of 29Si, 27Al and 11B were performed to determine the network connectivity of the quenched glasses. The viscosity and electrical resistivity of alkali-free boro-aluminosilicate glasses were measured at high temperatures, and this temperature dependence of both the two properties were well described by Arrhenius relation. The Mauro-Yue-Ellison-Gupta-Allan (MYEGA) equation was employed to study the viscosity and fragility of glass melts from glass transition temperature to melting temperature. With increasing in the RO substitution for the Al2O3 from 13.5 to 19.5 mol%, the melting temperature of glasses decreased from 1630.4 to 1532.6 °C, the fragility index m increased from 22.6 to 24.0 and the resistivity at 1600 °C decreased from 52.3 to 28.9 Ω·cm. It was mainly corresponding to the depolymerization of network connectivity. The NMR results showed that the number of Q4 units decreased from 27.4 to 21.1% and the value of the non-briding oxygen per tetrahedral cation (NBO/T) increased from 0.482 to 0.744. The four-fold coordinated aluminum always predominated in Al groups, and the relative fraction of [BO4] in B groups increased from 5.1% to 16.4%. It cannot be ignored that most of boron kept in [BO3] group even when the ratio of RO/(B2O3 + Al2O3) > 1, which is large different from that in alkali aluminoborosilicate glasses. It is associated with the higher field strength of alkali-earth cations who prefer to form NBO rather than [BO4].