Abstract
Hydrogen reduction conditions of α-Fe2O3 particles and surface passivation conditions of metal nanoparticles obtained through a hydrogen reduction process of acicular α-Fe2O3 particles and a slow oxidation process in a low oxygen concentration have been studied in order to determine optimum production conditions of acicular magnetic metal nanoparticles utilized for magnetic recording.
Acicular magnetic metal nanoparticles were produced by using the surface-treated acicular α-FeOOH nanoparticles containing 12.6 at%Co–9.4 at%Si–7.0 at%Al under the condition of 823 K in temperature of dehydration of α-FeOOH particles by heating, 5×10−6 m3(STP)·s−1 in flow rate, 50 mol%H2–50 mol%Ar in composition of a reducing gas, and 823 K in reduction temperature. The resulting reduced metal particles are composed of either a single crystal or polycrystals. The inside of the particles was a reduced dense metal phase in which the crystal growth with neither defect nor pore was sufficiently proceeded.
The surface oxide layer of metal particles which is formed by the slow oxidation results in a spinel structure of Fe3O4. The metallic core was entirely coated by the surface oxide layer, and it was observed that the dense crystal having no defect; pore and so forth inside the oxide layer was sufficiently grown. It is assumed that a passivation layer possessing a higher corrosion-resistant property can be obtained with a thicker surface oxide layer, since the oxide layer on the surface of the metal nanoparticle becomes thicker with higher temperature of slow oxidation.
Acicular magnetic metal nanoparticles having the excellent property of corrosion resistance were able to be produced under the condition of 338 K in oxidation temperature, 5×10−6 m3(STP)·s−1 in flow rate and 0.5 mol%O2–99.5 mol%N2 in composition of oxidizing gas.
