Structural response of a highly viscous aluminoborosilicate melt to isotropic and anisotropic compressions

Aluminoborosilicate melts of E-glass composition have been compressed at pressures up to 500 MPa and subsequently cooled (4-5 K min(-1)) under pressure from well above the glass transition to room temperature. It is found that increasing uniaxial pressure lead to anisotropic glasses with increasing permanent birefringence, while increasing isostatic pressure resulted in isotropic glasses with increasing density (compaction of 2.1% at 500 MPa). Static and magic-angle spinning nuclear magnetic resonance spectroscopy of (11)B, (23)Na, (27)Al, and (29)Si were performed to explore pressure-induced changes in the short-range structure of these glasses. NMR experiments readily detected increasing (IV)B, (V)Al, and (VI)Al concentrations with pressure as well as a decrease in the mean distance of sodium to oxygen atoms (0.7% at 500 MPa), but no detectible evidence of short-range structural orientation around these atoms in the birefringent glasses were found. Quantifying the changes in the local boron, aluminum, silicon, and sodium environments revealed that the measured increase of recovered density with pressure in E-glass can only be partly explained by increase in B and Al coordination, and that overall compression of the network and of the network modifier cation volumes must also be important. Structural changes in the intermediate range, which were not detected by NMR, are discussed as a source of birefringence in anisotropic E-glass. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3223282]