Atomic structure of hot compressed borosilicate glasses
LF Ding and KH Lee and TY Zhao and YJ Yang and M Bockowski and B Ziebarth and QW Wang and JJ Ren and MM Smedskjaer and JC Mauro, JOURNAL OF THE AMERICAN CERAMIC SOCIETY, 103, 6215-6225 (2020).
Borosilicate glasses have been in widespread use for over a century; however, a detailed understanding of the structural response to densification is still lacking. In this work, two commercial borosilicate glasses, viz., SCHOTT N-BK7 (R) (N-BK7) and Borofloat33 (R) (Boro33), are hot compressed up to 2 GPa with nitrogen gas, and the structural response to this densification is explored via B-11 solid- state nuclear magnetic resonance (NMR) spectroscopy and classical molecular dynamics (MD) simulations. The molar volume (V-m) of N-BK7 and Boro33 decreases similar to 5% and similar to 10%, respectively, as a result of hot compression at 2 GPa. The NMR results demonstrate the presence of three different types of fourfold coordinated boron species (B-4), which are confirmed in MD simulations to be B-1(4)(0B,4Si(4)), B-2(4)(1B(3),3Si(4)), and B-3(4)(1B(4),3Si(4)) (where subscripts represent different B-4 types and brackets indicate the next nearest neighbors (NNN)). The NMR results also show that the fraction of B-4 increases by similar to 13% in N-BK7 glass upon hot compression at 2 GPa via the trigonal boron to tetrahedral boron (B(3 )to B-4) conversion, while the fraction of B-4 in Boro33 glass only increases by similar to 2% at the same pressure, despite the fact that the V-m decrease in N-BK7 is double that of Boro33. The MD simulations capture the experimental trends in B-4 populations, despite an underestimation of the B-4 increase of N-BK7 (only similar to 6%) at 2 GPa. Moreover, the MD simulations suggest that the V-m reduction is a linear function of bond angle change and the fraction of Si-O-Si and B-4-O-Si. The modifiers and boron coordination conversion also influence the volume densification of borosilicate glasses by increasing the difficulty of bond bending, decreasing the bond lengths, and increasing the population of B-4-O-Si linkages. Finally, the B-4 to B-4 conversion, that is, B-2(4)(1B(3),3Si(4)) and B-3(4)(1B(4),3Si(4)) to B14 (0B,4Si(4)), is observed in hot compressed N-BK7 and Boro33 from NMR and qualitatively confirmed in MD.
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