The effect of borate and silicate structure on thermal conductivity in the molten Na2O–B2O3–SiO2 system

• Thermal conductivity of molten Na2O–B2O3–SiO2 system decreases at higher temperatures.
• At 1273 K, thermal conductivity initially increases as the R ratio increases, but subsequently decreases for R above 0.5.
• Increase in the K ratio results in an increase in thermal conductivity.
• To evaluate the Debye temperature of the oxide system, the one-dimensional Debye temperature was considered.

Thermal conductivity of molten Na2O–B2O3–SiO2 system was measured at various temperatures and for different compositions using the hot-wire method. It was observed that thermal conductivity decreases at higher temperatures. The calculated one-dimensional Debye temperature implies that the change of the phonon mean free path dominantly affects the thermal conductivity in the present measurements. Using solid state 11B and 29Si MAS NMR, the structure of the oxide system was confirmed. A conflicting effect of the R ratio (Na2O/B2O3) on thermal conductivity was observed. For R below 0.5, owing to the increase in 4-coordinated boron, thermal conductivity increases with increasing R ratio. However, for R above 0.5, thermal conductivity decreases with increasing R ratio as a result of the depolymerization of silicate networks and the increase in non-bridging oxygen (NBO). Finally, the effect of the K ratio (SiO2/B2O3) on thermal conductivity was considered. A higher K ratio causes an increase in thermal conductivity as a result of the increase in the total amount of the silicate structure and a more effective mixing between silicon and 4-coordinated boron.