2019 Volume 59 Issue 6 Pages 1041-1048
Growth mechanism and structure evolution during nucleation of Calcium borosilicate (Ca11Si4B2O22) crystal in CaO–SiO2–B2O3 based fluorine-free (F-free) mold flux have been investigated in this article. The results suggested that during nucleation, the existed free oxygen ions tend to depolymerize Si–O–B units and break down borosilicate structure. Then, the transformation between [BO4]-tetrahedral and [BO3]-trihedral would further depolymerize borosilicate structure. Next, the decomposed borosilicate will connect with the formed Q2(Si) and the existed dissociative Q1(Si), and finally form a long-range ordered borosilicate structure with a certain symmetry. Finally, Ca2+ ions would associate with the borosilicate structure to form Calcium borosilicate crystal nucleus. With the formation of the crystal nucleus, the crystals first precipitated at the boundary of the thermocouple and exhibited grain-shape particles orientated in a line dispersively. Then, the grain-shape crystals at the boundary became thicker and formed dendrite structure. Subsequently, the secondary dendrites were observed to form and grew on the primary dendrites axis. Ultimately, Calcium borosilicate crystals were well distributed. The precipitated crystal phase of the F-free mold flux was β-polymorphs Calcium borosilicate. The kinetics of the isothermal crystallization at 1373 K is constant number of nuclei, 3-dimensional growth by diffusion control.