By fast heating the nano-sized Al 2 O 3 and carbon black mixtures at 50 • C/min to 1750 • C for 30-120 min, single-phase AlON powders were successfully obtained by a fast one-step carbothermal reduction and nitridation (CRN) method. The AlON ceramics pressureless sintered at 1880 • C for 150 min by these powders show high transmittances up to 83%-84%, which indicates that the proposed fast one-step CRN method is an effective and efficient way with strong robustness to synthesize single-phase AlON powder for highly transparent AlON ceramics. It was found that α-Al 2 O 3 particles do not have enough time to aggregate and coalesce during heating due to the tremendously shortened heating span, which significantly inhibited particle coarsening until the formation of AlON starts. The fast-formed AlON further inhibits the coarsening of α-Al 2 O 3 during dwelling. Consequently, single-phase AlON powder of small primary particles can be obtained after 30 min dwelling at 1750 • C.
Fabrication of transparent AlON ceramics is extra sensitive to both particle size of starting powder and sintering additive due to shuttling transformation between AlON and Al2O3 + AlN during heating. One possible solution is to select robust additive to suppress the shuttling transformation. In this work, three AlON powders with different median particle sizes of 0.6, 0.9, and 1.1 μm were prepared. After studying the effect of CaCO3 on densification process, AlON ceramics with the maximum transmittance of ≥81.1% were successfully fast prepared by pressureless sintering (PS) at 1880°C for only 2.5 h by using three AlON powders doped with different CaCO3 amount. Specifically, AlON ceramics prepared from 1.1 μm with 0.5–0.8 wt.% CaCO3 doping consistently showed the maximum transmittance of ≥85.3%, which indicates that CaCO3 can serve as a robust additive to enable fast fabrication of highly transparent AlON ceramics even by PS.
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