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 200degreesC to 550degreesC 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 420degreesC, only hcp Co phase particles were obtained by laser pyrolysis or by reduction. Medium temperature (around 420degreesC) resulted in a mixture of hcp and fcc phases. Only when high temperatures, above 500degreesC, 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.