Thermal history dependence of the crystal structure of Co fine particles

Abstract

hcp and fcc Co fine nanoparticles with different size have been synthesized by low-temperature laser pyrolysis and by reduction at different temperatures from $200\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}550\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ 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 $420\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, only hcp Co phase particles were obtained by laser pyrolysis or by reduction. Medium temperature (around $420\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$) resulted in a mixture of hcp and fcc phases. Only when high temperatures, above $500\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, 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.