Effect of CaO on the optical quality and microstructure of transparent MgO·1.5Al2O3 spinel ceramics prepared by reactive sintering

Transparent MgO·1.5Al2O3 spinel ceramics were successfully prepared via reactive sintering of Al2O3 and MgO raw powders followed by hot isostatic pressing (HIP) using CaO as the sintering additive. The effects of CaO on the densification process, microstructure and optical quality of samples were investigated. It was found that the amount of CaO played an important role in the sintering process. By adding 0.05 wt% CaO, the sample with high transmittance (82.3% at 400 nm), small grain size (<5 μm) and high strength (228 ± 15 MPa) was obtained after HIPing at 1550 °C. However, when the amount of CaO increased to 0.1 wt%, non-cubic and columnar-shaped grains generated at low HIP temperatures (1550-1650 °C), which severely reduced the optical quality of resulting samples. The grains were calcium aluminates, whose formation was closely related to the molar ratio of Al2O3/MgO, CaO amount and sintereing temperature.