The evolution of the mold flux melt structure during the process of fluorine replacement by B2O3

This study presented the melt structure evolution of mold flux during the substitution of fluorine by B₂O₃, and a computational model for the degree of polymerization (DOP) for borosilicate structure was developed. The results showed that the reduction of fluorine content would promote the replacement of F in SiF₆]-octahedral unit by the dissociative free oxygen ions (O^2?), and release F^? ions into the melt to compensate the reduction of F^? ions. With the 2 mass% addition of B₂O₃, the original Si–O–Si bond would be disrupted, and connect with BO₃]-trihedral to form boroxol ring structure containing BO₂O^?]-trihedral and BO₃]-trihedral structural units. Then, the Si–O–B bond that BO₃]-trihedral links SiO₄]-tetrahedral in boroxol ring was destroyed with the further addition of B₂O₃, and then the BO₃]-trihedral could link with the dissociative Q¹(Si) and Q⁰(Si) structural units to transform into BO₄]-tetrahedral and form a borosilicate long chain. Finally, with 6 mass% addition of B₂O₃, the borosilicate chain would combine with simple borate and borosilicate structures, and a complex borosilicate structure containing boroxol ring with certain symmetry was formed ultimately. Besides, the calculated result of DOP suggested that the DOP of the melt structure improved during the process of fluorine replacement by B₂O₃.