Multi-nuclear NMR studies of borosilicophosphate glasses and microfoams

Glasses in the ternary SiO2–B2O3–P2O5 system have a compositional region encompassed by 40–75 mol% SiO2, 10–40 mol% B2O3 and 10–35 mol% P2O5, in which glass-ceramics containing BPO4 crystallites can be formed. This system is also interesting in that all of the constituents are traditional glass formers. Nuclear magnetic resonance (NMR) spectroscopy has been used to examine the cation speciation and hydrogen evolution in this multi-glass former system. Spectroscopic results from 31P and 11B magic angle spinning (MAS) NMR show formation of the BPO4 crystalline phase in all heat-treated compositions, with structural differences between glasses that form traditional glass-ceramics and those that form the novel gas-ceramics. NMR studies of the microstructure of glasses with similar compositions, but different sources of glass formers (BPO4, NH4H2P2O4, Si, BN) have been performed to examine differences in the formation of the gas-ceramics. These data show that crystallization of BPO4 and a coordination change of boron, from trigonal to tetrahedral, accompany the nucleation of the hydrogen filled voids. In addition, the NMR results are consistent with hydrogen dissolved in the glasses as molecular H2, with little evidence for hydroxyl groups associated with the glass-forming cations.