The Reduction of Surface Oxide on Ultra-Fine FeNi Particles

Ultra-fine particles of two FeNi alloys have been produced by the gas evaporation technique. The particles were passivated by partial oxidation and subsequently characterized as-prepared, and during or after reduction in a hydrogen atmosphere. Different experimental techniques were used, such as Analytical Electron Microscopy (AEM), Mossbauer Spectroscopy (MS), Micro Gravimetry (μG), and Positron Annihilation Spectroscopy (PAS). The initial particle size was about 5 nm, the oxide layer being 0.5–1 nm thick and consisting mainly of Fe2O3. On heating in a flow of hydrogen, the main reaction observed was the reduction of the oxide at about 500 K. Above 500 K sintering of the particles took place (the size reaching ∼50 nm at ∼600 K). During sintering defects were generated in the growing particles, probably with a dislocation-like structure.