TiO2-nucleated mid-infrared transparent MgO-BaO-CaO-Al2O3 glass-ceramics with improved mechanical properties

Abstract

Calcium aluminate glasses are particularly appealing for IR photonic applications because of their excellent transmittance from the UV to the mid-IR range (3–5 μm). However, their mechanical performance remains inferior to that of crystalline ceramics. Glass-ceramics generally have enhanced mechanical properties than their parent glasses. This study investigates whether adding TiO₂ to a MgO-BaO-CaO-Al₂O₃ glass composition can act as a nucleating agent to promote uniform nucleation and crystallization within the glass, leading to the formation of glass-ceramics, and to assess its effectiveness in enhancing mechanical properties. The parent glasses were prepared via. conventional melt-quench method. The nucleation kinetics of the glasses was analyzed by differential thermal analysis to find the optimum nucleating condition. Then the glasses were converted into glass-ceramics using two-stage heat-treatment (nucleation-crystallization). This study demonstrates that TiO₂ is an effective nucleating agent for calcium aluminate glass-ceramics. It was shown that when the TiO₂ content is around 4 wt%, the crystallization behavior primarily results in the formation of a surface crystallization layer (mainly containing the Ca₁₂Al₁₄O₃₃ phase), with very low crystallinity within the glass interior region, leading to a non-uniform microstructure. When the TiO₂ content is increased to around 8 wt%, it promotes a shift in crystallization behavior from surface crystallization of Ca₁₂Al₁₄O₃₃ to uniform internal crystallization of Ca₃Al₂O₆. The concentration of Ti is highest at the center of the crystal and gradually decreases toward the glass matrix. The T8 glass-ceramic, nucleated at 755 °C for 16 h followed by 850 °C for 4 h, is opaque to visible light but still allows up to 70 % IR transmission. Compared to the parent glass, this glass-ceramic exhibits a hardness increase of at least 14 %, a dilatometric softening point elevation of 79 °C, and a Young's modulus increase of at least 19 %, making it the glass-ceramic with the best combination of optical and mechanical properties.