Structure transition comparison between the amorphous nanosize particles and coarse-grained polycrystalline of cobalt

Amorphous cobalt nanoparticles were synthesized by laser-inductive vaporiza tion condensation technique. The nanoparticles are in fine size dispersion and the average particle size is 20 nm. When heating the amorphous cobalt n anoparticles in O-2 ambient condition there is a sharp exothermic reaction in temperature range from 2072 degrees C to 2972 degrees C with a peak temp erature of 260 degrees C and exothermic enthalpy of 100.08 kJ/mol. When hea ting in Ar ambient condition, the amorphous cobalt nanoparticles may begin to transform from the amorphous solid to a supercooled liquid state at abou t 167 degrees C and keep the supercooled liquid state in a wide temperature span from 167 degrees C to 277 degrees C, followed by crystallization at 2 77 degrees C. The crystallization takes place in the temperature span from 277 degrees C to 477 degrees C. The corresponding exothermic heat is equal to 23.2 kJ/mol, which is larger than the heat of fusion of bulk cobalt (14. 4 kJ/mol). (C) 2000 Acta Metallurgica Inc.