ZrO₂-nucleated calcium aluminate glass-ceramics with mid-infrared transparency

A silica-free MgO–CaO–BaO–Al₂O₃–ZrO₂ glass was converted into glass-ceramic by adding zirconia as a nucleating agent. The crystallization and properties of the glass-ceramics were investigated. It was found that ZrO₂ is an efficient nucleating agent for the present calcium aluminate glass. Addition of an appropriate amount of ZrO₂ changed the crystallization behavior from surface nucleation to internal nucleation. The primary crystalline phase was Ca₃Al₂O₆, which exhibited bulk nucleation. When the crystallizing time was increased, BaAl₂O₄ formed as a surface layer that grew from the surface into the interior of the sample. The hardness was increased when the parent glass was converted into glass-ceramics. The glass-ceramics without BaAl₂O₄ surface layer have visible-to-infrared transparency comparable with the parent glass. The transparency was substantially reduced after the development of the surface layer.