HR: 1340h
AN: MR13B-1256    [Abstracts]
TI: Composition Effects on Aluminoborosilicate Glasses Structure
AU: * Wu, J
EM: jswu@stanford.edu
AF: Stanford University, Department of Geological & Environmental Sciences, Building 320, Stanford, CA 94305-2115, United States
AU: Stebbins, J F
EM: stebbins@stanford.edu
AF: Stanford University, Department of Geological & Environmental Sciences, Building 320, Stanford, CA 94305-2115, United States
AB: Quantitative determinations of the atomic-scale structure of glasses, and the effects of composition on it, are critical to the development of physically accurate models of glass and melt properties. This study is an attempt to understand the effects of composition on network connectivity of alkali and alkaline earth aluminoborosilicate glasses, used widely in technology. It is also an examination of the extent of disorder of modifier cations around non-bridging oxygens (NBO) and an evaluation of the changes in boron and aluminum speciation as a function of the ratio of modifier cations (Na/Ca). The latter outcome may also help to understand how different cation field strengths effect aluminum coordination numbers in high pressure environments. We have used B-11 and Al-27 MAS NMR to investigate the formation of different boron and aluminum species. B-coordination changes from dominantly trigonal to dominantly tetrahedral as Na/(Na+Ca) increases, but the fraction of [4]B species is not an exact linear function of composition. When multiple modifier cations are present (Na and Ca), the fraction of [4]B is lower than what would be expected from linear combinations of the appropriate end-members. [4]Al is predominant among the Al species. The small amount of [5]Al (a few percent) increases with decreasing Na/(Na+Ca). The conversion of [3]B to [4]B is also expected to convert NBO to bridging oxygens. The NBO fraction can thus be calculated directly from the compositions and the amounts of four-coordinated B and Al. The estimated NBO in these glasses thus decreases with increasing Na/(Na+Ca). We have also used Na-23 MAS NMR to investigate the Na environments. The calculated mean isotropic chemical shifts become slightly higher with increasing Na/(Na+Ca), which suggests that Na-O distances increase when more Ca substitutes for Na. Therefore, Na is in a more bridging oxygen rich environment with an increasing Ca component, with a higher proportion of NBOs surrounding Ca. Quantification of spectra from the present study shows a decrease in B- and an increase Al-coordination with increasing average field strength of the modifier cation, meanwhile the fraction of NBO is increasing. These data also suggests the Na/Ca distribution is not random, and therefore that the entropy of mixing is reduced.
DE: 1011 Thermodynamics (0766, 3611, 8411)
DE: 1099 General or miscellaneous
DE: 3611 Thermodynamics (0766, 1011, 8411)
DE: 3929 NMR, Mossbauer spectroscopy, and other magnetic techniques
DE: 3999 General or miscellaneous
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting