Relationship between Structure and Thermodynamic Properties in the CaO–SiO₂–BO_1.5 Slag System

摘要

To understand the relationship between thermodynamic properties and structures in the CaO–SiO_(2)–BO_(1.5) slag system, the thermodynamic properties of BO_(1.5) and SiO_(2) were measured by a chemical equilibrium technique and the local structures of boron(B) and silicon(Si) were investigated by ~(11)B and ~(29)Si magic angle spinning–nuclear magnetic resonance (MAS–NMR) measurements.Activity coefficients of BO_(1.5) increased with an increase in the BO_(1.5) content of the slag system. Activity coefficients of SiO_(2) increased with increasing BO_(1.5) content and decreased with increasing CaO/SiO_(2) ratio.By using ~(11)B MAS–NMR, the existence of two types of B sites was confirmed—three- and four-coordinated B sites. The relative fraction of four-coordinated B increased with an increase in the BO_(1.5) content and decreased with an increase in the CaO/SiO_(2) ratio. ~(29)Si MAS–NMR results revealed that bridging oxygen bonding to Si atoms increased with an increase in the BO_(1.5) content and decreased with decreasing CaO/SiO_(2) ratio. The number of non-bridging oxygen atoms bonded to the tetrahedrally coordinated Si atom (NBO/T) was calculated using ~(29)Si and ~(11)B MAS–NMR under specific assumptions. By comparing NBO/Ts calculated from these two methods, the number of non-bridging oxygen atoms bonded to three-coordinated B in the studied composition region was estimated to be one.Finally, by comparing the thermodynamic properties with the results of MAS–NMR measurements, it was found that the composition dependences on BO_(1.5) and SiO_(2) activity coefficients were dependent on changes in the local structure of B and Si with composition.