Effect of Na₂O on the High-Temperature Thermal Conductivity and Structure of Na₂O-B₂O₃ Melts

The effect of Na₂O and temperature on the thermal conductivity of the Na₂O-B₂O₃ binary system has been measured using the hot-wire method to examine the relationship between the thermal conductivity and structure in high-temperature melts. The thermal conductivity of the binary melt is measured from 1173 to 1473 K in the fully liquid state. The thermal conductivity slightly increases with Na₂O content up to 20 wt%. Above 20 wt% Na₂O, the thermal conductivity decreases with increasing Na₂O. The network structure of molten glass was analyzed using Fourier transform infrared (FTIR), Raman spectroscopy, and XPS. The FTIR analysis shows that 3-D complex borate structures, such as tri-, tetra-, and pentaborate are made by [BO₄] tetrahedral units interconnected with 2-D structure boroxol rings in the low Na₂O region. Above 20 wt% Na₂O content, nonbridged oxygen in [BO₂O^-] units and diborate groups increase with increase in Na₂O. The same tendency is shown by the Raman spectroscopy and XPS analyses. The Raman analysis shows that boroxol rings disappeared with large [BO₄] groups, such as tri-, tetra-, and pentaborate structures, which increase at low Na₂O content. Isolated diborate groups and nonbridged oxygen in [BO₂O^-] units increase at high Na₂O content. It can be inferred that single structure units, such as isolated diborate groups, interfere with conduction. The XPS analysis results show that free oxygen produced by the interconnection of Na₂O in the borate structure does not cause significant changes to O^2- in the low Na₂O region, but increases the O^o and decreases the O^-. Above 20 wt% Na₂O, O^- slightly increases and O^o shows a decreasing trend.