Size effects on chi- to alpha-Al_2O_3 phase transformation

摘要

Nano-scaled chi-Al_2O_3 powders with d_(50) mean particle sizes from 17 to 314 nm were prepared to investigate the size effect on their phase transformation. Structural properties and cry stallization behavior as a function of thermal treatments of various-sized chi-Al_2O_3 particles were examined by DTA, XRD and TEM characterizations. It was confirmed that the decrease of particle size allows for stable alpha-Al_2O_3 formation at relatively low temperature. Furthermore, the phase transformation route of chi-Al_2O_3 to alpha-Al_2O_3 was also modified due to the decrease of particle size. A critical size of chi-Al_2O_3 that determines the phase transformation behavior was found to be around 40 nm. For particles larger than 40 nm, a transition phase of kappa-Al_2O_3 is formed before obtaining final alpha-Al_2O_3 phase. Nevertheless, for those smaller than the critical size, starting chi-Al_2O_3 particles have to grow to 40 nm and then directly transform to alpha-Al_2O_3 bypassing kappa-Al_2O_3 at a temperature as low as 1050 deg C.