CRN synthesis mechanism of high sinterability MgAlON powder for fast fabrication of highly transparent ceramics

Utilizing γ-Al₂O₃, MgO, and black carbon powders as starting materials, single-phase magnesium aluminum oxynitride (MgAlON) powders with excellent sinterability were successfully synthesized by carbothermal reduction and nitridation (CRN) method, and the CRN synthesis mechanism was investigated through analyzing phase assemblage and morphology of the powders during heating. As heating to 1700°C, single-phase MgAlON is obtained, while its O/N ratio can be adjusted by controlling the composition of starting mixture and the CRN process. The MgAlON powders with O/N ratio of 9.52–11.39 (using the mixture with 8.6 wt.% MgO and 5.2 wt.% carbon black, and prepared at 1700°C for 90–150 min or 1720°C for 120 min) exhibit high sinterability. These powders were successfully fast pressureless sintered into ceramics with high transmittance (>80%@3750 nm) at 1880°C for 2.5 h. During the heating process for preparing MgAlON powder, magnesium aluminate spinel (MAS) is first formed at <1500°C, then at 1500–1600°C, along with formation of AlN via CRN and its instantaneous solid solution with MAS, MgAlON is obtained. The phase assemblage and morphology comparison between the Al₂O₃–MgO–C, Al₂O₃–MgO, and Al₂O₃–C powders during heating indicates that carbon and MAS act in concert to inhibit the growth of Al₂O₃, which contribute to the rapid formation MgAlON and high sinterability of prepared MgAlON powders.