Role of B₂O₃ on the viscosity and structure in the CaO-Al₂O₃-Na₂O-based system

The effect of B₂O₃ on the viscosity and structure in the calcium-aluminate melt flux system containing Na₂O was studied. An increase in the B₂O₃ content at fixed CaO/Al ₂O₃ ratio lowered the viscosity. Higher CaO/Al ₂O₃ ratio at fixed B₂O₃ content also decreased the viscosity. The alumino-borate structures were confirmed through Fourier transformed infrared (FTIR) and Raman spectroscopy and consisted of [AlO₄]-tetrahedral structural units, [BO₃]-triangular structural units, and [BO₄]-tetrahedral structural units, which could be correlated to the viscosity. At fixed CaO/Al₂O₃ ratio, B₂O₃ additions decreased the [AlO₄]- tetrahedral structural units and transformed the 3-D network structures such as pentaborate and tetraborate into 2-D network structures of boroxol and boroxyl rings by breaking the bridged oxygen atoms (O⁰) to produce non-bridged oxygen atoms (O^-) leading to a decrease in the molten flux viscosity. At fixed B₂O₃ contents and higher CaO/Al₂O₃ ratio, 3-D complex network structures become 3-D simple and 2-D isolated network structures, resulting in lower viscosities. The apparent activation energy for viscous flow varied from 132 to 249 kJ/mol according to the composition of B₂O₃ and CaO/Al ₂O₃ ratio.