Thermal Conductivities of Molten Na ₂ O‐SiO ₂ and Na ₂ O‐Al ₂ O ₃ ‐SiO ₂ Slags From the Perspective of Bonding Energies of Si‐O and Al‐O

The thermal conductivities of molten Na 2 O‐SiO 2 and Na 2 O‐Al 2 O 3 ‐SiO 2 slags were measured over the temperature range between 1073 K and 1573 K by hot‐wire method to discuss the thermal conductivities of aluminosilicate melts from the perspective of the bonding energies of Si‐O and Al‐O. The thermal conductivity monotonically decreases with increasing temperature within the measurement temperature range for all samples. The thermal resistivity due to ionic bond between silicon and nonbridging oxygen ρ Si−NBO and the thermal resistivity due to covalent bond between aluminum and bridging oxygen ρ Al−BO at a standardized temperature T / T L , where T L is the liquidus temperature, have been obtained from the temperature dependences of thermal conductivities for molten Na 2 O‐SiO 2 and Na 2 O‐Al 2 O 3 ‐SiO 2 slags assuming that the thermal resistivity due to covalent bond between silicon and bridging oxygen ρ Si−BO is 0.442 mKW −1 (reciprocal of the thermal conductivity of fused SiO 2 at temperatures higher than ca. 1400 K). It has been found that ρ Si—NBO and ρ Al—BO are 2.66 and 1.83 mKW −1 , respectively, and thereby the magnitude of thermal resistivity is in the hierarchy of ρ Si—BO < ρ Al—BO < ρ Si—NBO . It has also been found that ρ Al—BO moderately increases and ρ Si—NBO dramatically increases with increasing T / T L . This may stem from that the magnitude of bonding energy is in the hierarchy of Si−ΒΟ > Al‐BO > Si‐NBO.