A fast pressureless sintering method for transparent AlON ceramics by using a bimodal particle size distribution powder

Using a pure AlON powder synthesized by carbothermal reduction and nitridation, a bimodal particle size distribution (PSD) powder was prepared by planetary ball mill, and pressurelessly sintered at 1880 °C to fabricate transparent AlON ceramics. Relative density of ≥99.72% was achieved for all the obtained AlON ceramics samples held for 90-150 min. The maximum infrared transmittance for 3 mm thickness sample was up to 81.8% (93.8% of theoretical in-line transmittance). Therefore, the holding time to fabricate transparent AlON ceramics can be shortened to 90 min or 150 min, compared to 480 min holding reported in the literature. The fast sintering of transparent AlON ceramics is a combination of fast heating (40 °C/min) and shorter holding time, and should be mainly attributed to fast surface diffusion during heating due to the bimodal PSD of fine powder and fast grain boundary diffusion during both heating and holding due to the transient bimodal grain size distribution.