Densification of Sodium and Magnesium Aluminosilicate Glasses at Ambient Temperature: Structural Investigations by Solid-state Nuclear Magnetic Resonance and Molecular Dynamics Simulations

Abstract

Sodium and magnesium aluminosilicate glasses with compositions 20Na2O-20Al2O3-60SiO2 (NAS) and 20MgO-20Al2O3-60SiO2 (MAS) were subjected to a 12 and 25 GPa compression and decompression at room temperature, resulting in density increases from 3.7 % to 5.3 % (NAS) and from 8.2 to 8.4 % (MAS), respectively. The pressurization at 25 GPa was done on 17O enriched glasses, to facilitate characterization by 17O NMR. The structural changes associated with this process have been investigated by solid state 29Si, 27Al, 23Na, 25Mg, and 17O magic-angle spinning (MAS)-NMR and compared with the situation in thermally relaxed glasses and/or glasses prepared at ambient pressure. While in the Na aluminosilicate glass only subtle structural changes are observed in a sample densified at 12 GPa, the average coordination number of Al <CN(Al> increases moderately from 4.00 to 4.26 by pressurization at 25 GPa. In the Mg-based system, <CN(Al> increases from 4.34 to 4.57 to 4.83 in the sequence 10-4 GPa -> 12 GPa -> 25 GPa. The experimental result at 25 GPa was qualitatively confirmed by Molecular Dynamics (MD) simulations. Overall, pressurization results in more positive 29Si and 17O chemical shifts, most likely reflecting a reduction in the Si-O-Si and Si-O-Al bonding angles in the pressurized glasses. Furthermore, the results are also consistent with either an increased number of non-bridging O-atoms upon pressurization, or a larger number of Si-O-Al or Al-O-Al linkages. The significantly higher sensitivity of MAS, compared to NAS glass, to an increase in <CN(Al> upon pressurization provides a good structural rationale for their significantly higher crack initiation resistances.