Preparation and thermal evolution of vapour-condensed nanocrystalline iron

Abstract

Nanocrystalline Fe particles (mean particle diameter, 17nm) have been prepared by the inert-gas condensation method. After passivation by pure O2 and air exposure, the material was compacted into 13mm discs. Owing to passivation in air before compaction, the individual grains are covered by a layer that contains oxygenated phases and carbonaceous species. The thermal transformations and stability of the material under heating to temperatures of about 1000K have been studied by differential scanning calorimetry, thermo-magnetic gravimetry, transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. Structural relaxation processes, grain growth, sintering and crystallization of the oxide passivation layer (Λ-Fe₂O₃ and/or Fe₃O₄) are the main processes occurring during heating in an inert-gas atmosphere or under vacuum up to about 850 K. In the range 873–913 K an endothermic transformation occurs. This transformation is reversible, but the recovering of the initial situation occurs through a process with slow kinetics. The transition can probably be ascribed to the α → Λ transformation of Fe. An important lowering of the ferromagnetic-paramagnetic transition temperature of Fe is also observed in the nanostructured material.