Thermal Conductivity of Molten Li2O–B2O3 and K2O–B2O3 Systems

Using the transient hot‐wire method, the thermal conductivity properties of the molten Li₂O–B₂O₃ and K₂O–B₂O₃ systems were measured. The thermal conductivity increases with decreasing the temperature due to the borate structure change. In addition, calculations of the one‐dimensional Debye temperature and the phonon mean free paths as a function of temperature of the alkali borate systems were made. At a fixed temperature of 1273 K, the effect of the alkali oxide concentration on the thermal conductivity was evaluated. Within a range of 10–30 mol% Li₂O (or K₂O), a positive relationship between the thermal conductivity and 4‐coordinate boron was obtained. However, below 10 mol% Li₂O (or K₂O), the change in the intermediate‐range order of the borate structure had a more dominant effect on the thermal conductivity. Finally, the effect of cations on the thermal conductivity in the various molten R₂O–B₂O₃ (R=Li, Na and K) systems was considered. Depending on the type of cation, the change in the ionization potential had an effect on the thermal conductivity and also resulted in a change in the bond strength.