Prenucleation formations in control over synthesis of CoFe2O4 nanocrystalline powders

Resumo

Nanocrystalline cobalt ferrite powders were synthesized by hydrothermal treatment of co-precipitated hydroxides in the conditions of an external heating of the autoclave and under microwave heating of the reaction medium. In the microwave-heating mode, the prenucleation clusters formed under ultrasonic treatment of a suspended mixture of cobalt and iron hydroxides is transformed into CoFe₂O₄ nanocrystals during the first minute of synthesis at a temperature satisfying the equilibrium-existence conditions of cobalt ferrite. In the case of a slow external heating of the autoclave, there is no effect of this kind, which is attributed to the disintegration of the prenucleation clusters before the dehydration of the hydroxides to give crystalline cobalt ferrite becomes thermodynamically favorable. The main factor determining the increase in the formation rate of crystallites of CoFe₂O₄ nanopowders and the decrease in their size is the generation of prenucleation centers in the starting mixture of cobalt and iron hydroxides.