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

Abstract

Amorphous cobalt nanoparticles were synthesized by laser-inductive vaporization condensation technique. The nanoparticles are in fine size dispersion and the average particle size is 20 nm. When heating the amorphous cobalt nanoparticles in O₂ ambient condition there is a sharp exothermic reaction in temperature range from 2072°C to 2972°C with a peak temperature of 260°C and exothermic enthalpy of 100.08 kJ/mol. When heating in Ar ambient condition, the amorphous cobalt nanoparticles may begin to transform from the amorphous solid to a supercooled liquid state at about 167°C and keep the supercooled liquid state in a wide temperature span from 167°C to 277°C, followed by crystallization at 277°C. The crystallization takes place in the temperature span from 277°C to 477°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).