Hydrogen-containing glass and gas-ceramic microfoams: Raman, XPS, and MAS-NMR results on the structure of precursor SiO 2B 2O 3P 2O 5 glasses

Abstract

Raman spectroscopy, photoelectron spectroscopy and magic angle spinning nuclear magnetic resonance have been used to analyze the structure of a series of glasses in the system SiO 2B 2O 3P 2O 5 which, on heat treatment, develop a cellular microstructure containing hydrogen gas. Raman spectra show that the addition of 4–21 mol% P 2O 5 to 0.6SiO 2−0.4B 2O 3 (molar) composition systematically disrupts boroxyl rings and converts some of the boron to tetrahedral coordination. The postulated structural species formed has a local structure similar to BPO 4. The Raman spectra also show that P=O bonds are present and suggest that POP linkages occur in the glass with 21 mol% P 2O 5. O1s photoelectron spectroscopy shows that the full width at half maximum does not vary between glass samples containing 4 and 11 mol% P 2O 5, suggesting that the concentration of P=O bonds does not vary between these samples. 11B, 29Si and 31P MASNMR spectra have been collected for two of the glasses (4 and 11 mol% P 2O 5). The 11B results reveal three types of boron-oxygen coordination in the low P 2O 5 glass; regular trigonal; regular tetrahedral; and irregular. Increasing P 2O 5 to 11 mol% selectively destroys the regular trigonal sites, leaving boron in regular tetrahedral sites and in irregular sites. The 29Si NMR spectrum of the 4 mol% P 2O 5 glass has peaks at −90, −95, −105 and −112 ppm indicating that silicon has, primarily, trigonal and tetrahedral boron and other silicon atoms as next nearest neighbors. The loss of the two low fields peaks in the 11 mol% P 2O 5 glass indicates the removal of tetrahedral boron as a next nearest neighbor to silicon. The 31P NMR spectrum of the low P 2O 5 glass has a dominant peak at −41 ppm, similar to branching and middle phosphate groups in compositions with highly electonegative cations. In the 11 mol% P 2O 5 glass, the peak at −33 ppm is consistent with the formation of species with the local coordination of BPO 4. { 1H− 31P} cross-polarization experiments show that molecular hydrogen is likely to be present in the 4 mol% P 2O 5 glass. In the high P 2O 5 glass, hydrogen is preferentially associated with the BPO 4 species and with another phosphate site whose next nearest neighbors are not clearly identifiable. There is no evidence for hydrogen associated with either boron or silicon.