title: Influence of the hydrogen reduction time and temperature on the morphology evolution and hematite/magnetite conversion of spindle-type hematite nanoparticles.
abstract: We report on the transformation via hydrogen reduction of spindle-type hematite nanoparticles into hematite/magnetite hybrid iron oxide particles. The transformation process consists of the reduction of nanoparticles powder in an autoclave using hydrogen gas at a fixed pressure  of 11 bars. Both temperature and time of reduction are varied between 300 °C to 360 °C and 0 to 45 h. X-Ray powder diffraction data on the obtained powder and corresponding Rietveld refinement allow the amount of reduced hematite to be determined as a function of these two parameters.  Kinetics parameters are measured and an estimation of the activation energy is obtained through linearization of the Arrhenius equation. While reduction is dramatically accelerated at higher temperature, the morphology of the nanoparticles only remain qualitatively unchanged at 300 °C  as seen from transmission electron microscopy images. The mechanisms underlying morphology changes are still under study and seem to be closely related to reactor pressure.
authors.affiliation: Ecole d'Ingénieurs et d'Architectes de Fribourg, Boulevard de Pérolles 80, CP 32 CH-1705 Fribourg, Switzerland. thierry.chappuis@hefr.ch
authors.forename: Thierry
authors.initials: T
authors.lastname: Chappuis
authors.forename: Izabela
authors.initials: I
authors.lastname: Bobowska
authors.forename: Stefan
authors.initials: S
authors.lastname: Hengsberger
authors.forename: Ennio
authors.initials: E
authors.lastname: Vanoli
authors.forename: Hervé
authors.initials: H
authors.lastname: Dietsch