Nucleation and early growth of magnetite on synthetic and natural hematite crystals

Abstract

Few data have been reported on the nucleation of magnetite in the reduction of hematite, and previous workers have used either transmission electron microscopy or hotstage optical microscopy. In this work a scanning electron microscope was used, which allows easy observation of the surface, and a series of samples were prepared with reaction times ranging from 0.5 to 20 min in the reduction with CO-CO 2 or H 2-H 2O at 650°C. Synthetic crystals, grown in the range 150–200 μm using chemical transport, display nuclei with a spherical cap shape, which grow and then become more polyhedral, with finally some open porosity when the whole surface is covered. The average radial growth rate is around 2.1·10 −9 m s −1, in reasonable agreement with the value derived from kinetic macroscopic measurements (TGA) (3.8·10 −9 m s −1). The natural crystals undergo much more sudden nucleation after a higher incubation period; the surface is then completely covered, within around 1 min, with nuclei in the range 40–300 nm. Elimination of traces of silica makes the phenomenon less abrupt. From a discussion based on the hematite defect structure it is concluded that the prevailing point defects are interstitial iron ions, Fe i .., which develop dense nuclei partly outside the matrix. Rist's model helps in understanding the transition from the dense to the porous state. The nucleation frequency is also compared for the different samples and shows a trend towards saturation of active sites at 700°C where the frequency reaches 5·10 8 m −2s −1