Relationship between Thermal Conductivity and Structure of the CaO–BO1.5–AlO1.5 System

During the continuous casting process in steel making, the mold flux plays an important role in establishing adequate heat flow. Therefore, it is important to optimize the thermal conductivity of the flux system to control this process. Boron oxide (B2O3) is one of the components of the mold flux system and its structural complexity is well known. With the aim of revealing the relationship between the thermal conductivity and flux structure, the authors previously studied BO1.5-containing mold flux systems. In this study, the thermal conductivity of the CaO–BO1.5–AlO1.5 flux system was measured above 1500 K for various compositions using the transient hot-wire method. The composition dependence of the flux thermal conductivity was investigated in terms of its local structure, as analyzed using Raman spectrometry and MAS-NMR. The non-additive change in the thermal conductivity of the CaO–BO1.5–AlO1.5 system, which is known as the borate anomaly, is attributed to the relative fraction of the BO1.5 structural unit or the three-dimensional (3D) structural network involving the [4]A–O–[3]B bond. The results obtained by Raman spectrometry revealed that the complexation of the flux structure by the 3D AlB3O7 structure can increase the thermal conductivity at a high BO1.5 content. The formation probability for this structure was calculated based on the MAS-NMR results. Thus, the increase in thermal conductivity can be adequately explained by the formation of the AlB3O7 structure. For practical purposes, the effect of substituting SiO2 for AlO1.5 on thermal conductivity was also investigated with fixed BO1.5 and CaO concentrations.