Structural study of bismuth borosilicate, aluminoborate and aluminoborosilicate glasses by 11B and 27Al MAS NMR spectroscopy and thermal analysis

Glasses of bismuth borosilicates, aluminoborates and aluminoborosilicates were prepared by melt quenching and characterized by density measurements, 11B and 27Al Magic Angle Spinning (MAS) Nuclear Magnetic Resonance (NMR) spectroscopy, and Differential Scanning Calorimetry (DSC). Increase in silica concentration increases density without any change in the fraction of tetrahedral borons (N4) in the borosilicate glass network, whereas increase in alumina concentration in aluminoborate and aluminoborosilicate glasses drastically reduces density and N4. 27Al MAS NMR measurements revealed that Al ions are in four, five and six fold coordination with oxygen and about 47% of these are in [5]Al and [6]Al structural units, the concentration of the latter species decreases by adding silica in the glass network. Higher cation field strength of Bi3 + generates very large concentration of non-bridging oxygens, which act as charge balancing centers in the glass network. DSC measurements found that alumina and silica incorporation in bismuth borate glasses do not produce any systematic changes in the glass transition temperature but the tendency towards crystallization decreases steadily.