Preparation of spherical α‐Al 2 O 3 nanoparticles by microwave hydrothermal synthesis and addition of nano‐Al seeds

Abstract Due to their special appearance, spherical α‐Al 2 O 3 nanoparticles play an important role for obtaining high‐performance structural and functional ceramics. However, there are still problems such as easily agglomerates to form worm‐like structures at high temperatures and difficult availability of spherical nanoparticles. In this study, spherical α‐Al 2 O 3 nanoparticles with high dispersion were prepared by a combination of a microwave hydrothermal method and an addition of nano‐Al particles as seeds. First, spherical amorphous alumina precursors were synthesized by the microwave hydrothermal method at 100°C for 30 min using Al 2 (SO 4 ) 3 ·18H 2 O, Al(NO 3 ) 3 ·9H 2 O, and urea, as raw materials, and then spherical α‐Al 2 O 3 nanoparticles with a diameter of about 66 nm were acquired after calcined the precursor at 1050°C for 90 min by adding nano‐Al seeds, which reduced the calcination temperature by 50°C and holding time by 30 min compared to that without seeds. Kinetic analysis shows that 5 wt.% nano‐Al seeds can reduce the activation energy of crystalline transition of γ‐Al 2 O 3 to α‐Al 2 O 3 from 516.51 to 474.37 kJ/mol. Moreover, the microscopic mechanism of nano‐Al particles as seeds was investigated. The characterizations of sintering properties show that spherical α‐Al 2 O 3 nanoparticles facilitate the acquisition of uniform microstructure for resulting ceramic and the fracture modes include both intergranular and transgranular fractures.