Size effects on χ- to α-Al₂O₃ phase transformation

Nano-scaled χ-Al₂O₃ powders with d₅₀ mean particle sizes from 17 to 314 nm were prepared to investigate the size effect on their phase transformation. Structural properties and crystallization behavior as a function of thermal treatments of various-sized χ-Al₂O₃ particles were examined by DTA, XRD and TEM characterizations. It was confirmed that the decrease of particle size allows for stable α-Al₂O₃ formation at relatively low temperature. Furthermore, the phase transformation route of χ-Al₂O₃ to α-Al₂O₃ was also modified due to the decrease of particle size. A critical size of χ-Al₂O₃ that determines the phase transformation behavior was found to be around 40 nm. For particles larger than 40 nm, a transition phase of κ-Al₂O₃ is formed before obtaining final α-Al₂O₃ phase. Nevertheless, for those smaller than the critical size, starting χ-Al₂O₃ particles have to grow to 40 nm and then directly transform to α-Al₂O₃ bypassing κ-Al₂O₃ at a temperature as low as 1050 °C.