オキシフルオロホウ酸塩ガラスの結晶化挙動

抄録

Glasses in the system 2 B₂O₃-Na₂O-Na₂F₂ were heat-treated at 450° and 500°C in order to locate the position of F ions in the glass. The crystallization rate was measured as a function of Na₂O fraction, and precipitated phases were identified by the X-ray diffraction method. From glasses of Na₂O rich composition the diborate (2 B₂O₃⋅Na₂O) phase precipitated predominantly, and from NaF rich compositions the B₂O₃⋅NaF phase precipitated along with NaF crystal. B₂O₃⋅NaF phase was considered to have the diborate structure and to include F ions attached to 4-fold coordinated boron atoms in the diborate structure. It was implied that some fraction of NaF component is isolated in the glass network. Crystal growth rate became minimum at intermediate compositions. Theoretical analysis has shown that the fusion entropy ΔS_f becomes maximum at these compositions and that oxygen atoms and fluorine atoms are randomly distributed in the glass.