Network connectivity in aluminoborosilicate glasses: A high-resolution 11B, 27Al and 17O NMR study

O-17 enriched calcium and potassium aluminoborosilicate glasses with compositions similar (or analogous) to commercial E-glass were made and studied by ¹¹B, ²⁷Al and ¹⁷O NMR in order to explore the network speciation and mixing. Fractions of non-bridging oxygens associated with silicon and boron can be obtained simply from ¹⁷O magic angle spinning (MAS) NMR, while bridging oxygen populations result from complete peak assignment of high-resolution ¹⁷O triple quantum magic angle spinning (3QMAS) NMR spectra. Dramatic differences between the two samples in boron and oxygen speciation demonstrate a large effect of the charge of the modifier cation on mixing behavior and on the stabilization of non-bridging oxygens. The observed oxygen speciation is compared with that calculated from two models: random mixing of Si, B, and Al, and mixing with avoidance of linkages between tetrahedral aluminum and tetrahedral boron groups. Mixing of B and Al in the K-containing glass tends to follow the latter, while the network speciation in the Ca-containing glass is closer to the random model. The effects of Al and Si on boron speciation are discussed, and indicate that the maximum fractions of four-coordinated boron observed in a wide range of glass compositions are closely related to tetrahedral B and Al ‘avoidance’. A modified ‘Dell and Bray’ model is proposed which seems to accurately approximate boron speciation in many alkali aluminoborosilicate glasses. The higher field strength modifier cation (Ca²⁺ vs. K⁺) promotes the formation of non-bridging oxygen, trigonal over tetrahedral boron, and at least minor amounts of five-coordinated aluminum.