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 Na2 O-B2O3-SiO2 glass was investigated using B-11 nuclear magnetic resonance (N MR), Si-29 MAS NMR, and Al-27 MAS NMR together with infrared absorption spe ctroscopy and field emission scanning electron microscopy (FE-SEM). The res ults show that the structure change from the introduction of Al2O3 contribu tes 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-c oordinated borons. As the bond energy of the four-coordinated boron is weak er than that of the three-coordinated boron, the -B-O-Si- bond with the fou r-coordinated boron in Na2O-B2O3-SiO2 glasses is easily broken and results in severe phase separation during heat treatment. However, the -B-O-Al- bon d with the three-coordinated boron formed in Na2O-B2O3-SiO2-Al2O3 glasses i s difficult to be broken due to the high bond energy. In addition, the sili con network in Na2O-B2O3-SiO2-Al2O3 glasses is also strengthened by the add ition of Al2O3, which prevents [BO] groups from further aggregation. As a r esult, the tendency of the glass towards phase separation is greatly suppre ssed in the Na2O-B2O3-SiO2-Al2O3 system. (C) 2000 Kluwer Academic Publisher s.