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.