Book Description

This works studies the effects of compositional and temperature variations on the structure and properties of aluminoborosilicate glasses. Two groups of aluminoborosilicate glasses, one that has lower boron content and another that has higher boron content, have been studied. The structural changes were mainly observed with high-field B-11, Al-27 and Na-23 magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. In these glasses, boron is either three-coordinate (BO3) or four-coordinate (BO4); aluminum exists predominately as four-coordinate species, but there is a small amount of five-coordinate aluminum ([5]Al). The compositional study focused on the effect of the cation field strength of the network modifiers on the glass structure by varying the ratio of the two network modifiers, CaO and Na2O. Increasing the ratio of CaO to Na2O dramatically lowers the fraction of four-coordinated boron (N4), increases [5]Al, and increases the fraction of non-bridging oxygens (NBO), which was calculated based on the boron and aluminum structural information. However, variations in these fractions are not linear with respect to the average cation field strength. Na-23 spectra reveal that the ratio of bridging to non-bridging oxygens in the coordination shell of Na+ increases with an increasing ratio of CaO to Na2O in Ca-rich glasses. These changes can be understood by the tendency of higher field strength modifier cations to facilitate the concentration of negative charges on NBO in their local coordination environment, systematically converting BO4 to BO3. The effect of temperature on the structure was studied by two ways: cooling the glass-forming melts at different rates to sample the glass structure at different fictive temperature, and using high-temperature in situ NMR. The abundances of BO3 and NBO increase with increasing fictive temperature, suggesting that the reaction BO4 [logical equivalence] BO3 + NBO shifts to the right with increasing temperature. The observed temperature dependence of the abundance of BO4 species allows us to estimate the enthalpy of reaction, [Delta]H, which is closely related to the amount of NBO in the glass. In situ high-T B-11 MAS NMR was used to observe chemical exchange between BO3 and BO4 species over the timescale of microseconds to seconds. The timescale of BO3/BO4 exchange from NMR data, [lowercase Tau](NMR), appears to be "decoupled" from that of the macroscopic shear relaxation process, [lowercase Tau](s), derived from the viscosity data; however, at higher temperatures, [lowercase Tau](s) approaches [lowercase Tau](NMR). The "decoupling" at lower temperature may be related to intermediate-range compositional heterogeneities, and /or fast modifier cation diffusivities, which trigger "unsuccessful" network exchange events.