Thermal history dependence of the crystal structure of Co fine particles

hcp and fcc Co fine nanoparticles with different size have been synthesized by low-temperature laser pyrolysis and by reduction at different temperatures from 200 °C to 550 °C of cobalt-oxide nanoparticles prepared by the same method. The influence of thermal treatment on the crystal structure of Co particles was examined. It was found that below 420 °C, only hep Co phase particles were obtained by laser pyrolysis or by reduction. Medium temperature (around 420 °C) resulted in a mixture of hcp and fcc phases. Only when high temperatures, above 500 °C, were involved during the synthesis or annealing of Co particles, did the fcc particles form and maintain their structure to ambient temperature, without structural change of fcc-hcp. In view of the size effect on the phase transformation kinetics, the stabilization of fcc Co particles was explained.