PREPARATION AND THERMAL EVOLUTION OF VAPOR-CONDENSED NANOCRYSTALLINE IRON

Nanocrystalline Fe particles (mean particle diameter, 17 nm) have been prepared by the inert-gas condensation method. After passivation by p ure O-2 and air exposure, the material was compacted into 13 mm discs. Owing to passivation in air before compaction, the individual grains are covered by a layer that contains oxygenated phases and carbonaceou s species. The thermal transformations and stability of the material u nder heating to temperatures of about 1000 K have been studied by diff erential scanning calorimetry, thermomagnetic gravimetry, transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectr oscopy. Structural relaxation processes, grain growth, sintering and c rystallization of the oxide passivation layer (gamma-Fe2O3 and/or Fe3O 4) are the main processes occurring during heating in an inert-gas atm osphere or under vacuum up to about 850 K. In the range 873-913 K an e ndothermic transformation occurs. This transformation is reversible, b ut the recovering of the initial situation occurs through a process wi th slow kinetics. The transition can probably be ascribed to the alpha --> gamma transformation of Fe. An important lowering of the ferromag netic-paramagnetic transition temperature of Fe is also observed in th e nanostructured material.