Study of Al2O3 effect on structural change and phase separation in Na2O-B2O3-SiO2 glass by NMR

The effect of Al2O3 on the structure change and the phase separation in Na2O-B2O3-SiO2 glass was investigated using 11B nuclear magnetic resonance (NMR), 29Si MAS NMR, and 27Al MAS NMR together with infrared absorption spectroscopy and field emission scanning electron microscopy (FE-SEM). The results show that the structure change from the introduction of Al2O3 contributes greatly to the inhibition of phase separation. First, the introduction of Al2O3 imparts an ionic character to the boron-oxygen network, resulting in the formation of B-O-Al-O-Si bonds and thus increases the compatibility of the silicon network with the boron-oxygen network. Second, the addition of Al2O3 causes the sodium ion to transfer from the boron-oxygen network to AlO4 tetradedra, changing a number of four-coordinated borons into three-coordinated borons. As the bond energy of the four-coordinated boron is weaker than that of the three-coordinated boron, the -B-O-Si- bond with the four-coordinated boron in Na2O-B2O3-SiO2 glasses is easily broken and results in severe phase separation during heat treatment. However, the -B-O-Al- bond with the three-coordinated boron formed in Na2O-B2O3-SiO2-Al2O3 glasses is difficult to be broken due to the high bond energy. In addition, the silicon network in Na2O-B2O3-SiO2-Al2O3 glasses is also strengthened by the addition of Al2O3, which prevents [BO] groups from further aggregation. As a result, the tendency of the glass towards phase separation is greatly suppressed in the Na2O-B2O3-SiO2-Al2O3 system.