Multi-nuclear NMR studies of borosilicophosphate glasses and microfoams

Abstract

Glasses in the ternary SiO 2–B 2O 3–P 2O 5 system have a compositional region encompassed by 40–75 mol% SiO 2, 10–40 mol% B 2O 3 and 10–35 mol% P 2O 5, in which glass-ceramics containing BPO 4 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 BPO 4 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 (BPO 4, NH 4H 2P 2O 4, Si, BN) have been performed to examine differences in the formation of the gas-ceramics. These data show that crystallization of BPO 4 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 H 2, with little evidence for hydroxyl groups associated with the glass-forming cations.