Oxygen stoichiometry control of nanometric oxide compounds: The case of titanium ferrites

Three techniques have been coupled with an original device, based on H2/H2O equilibrium, controlling oxygen partial pressure: XRD, TGA and DC conductivity in order to characterize very reactive compounds such as nanometric powders. From XRD, both the structure and the oxygen stoichiometry (thanks to their lattice parameter) were investigated in situ. From TGA, it was the oxygen stoichiometry (thanks to mass gain or loss) which was determined. From DC conductivity, it was both the structure and the oxygen stoichiometry (thanks to the activation energy) which were obtained. The advantages were to determine very rapidly and with a small amount of powder the equilibrium conditions (T, pO2) necessary to obtain the desired phase and stoichiometry. These methods have been evaluated for nanometric titanium ferrites. Two phenomena have been observed during the reducing process: the precipitation of a rhombohedral phase and a significant grain growth linked together.

XRD patterns of Fe3(1−δ)O4 nanometric powders obtained in situ during a reducing treatment. The stoichiometric compound was obtained by a thermal annealing at 460 °C under pO2=3×10–26 Pa (ΦDRX=100 nm). In inset, data of the experimental lattice parameter compared to the theoretical one. α represents the rhomboedrical phase which precipitates during this thermal treatment, then disappeared.Figure optionsDownload as PowerPoint slideHighlights
► The control of the deviation from oxygen stoichiometry is rarely consider in literature.
► At the nanometric scale, it is necessary to develop synthesis routes under appropriate atmosphere.
► These synthesis must be coupled with in situ characterization tools.
► It opens the door to fast and precise studies concerning the evolution of nanometric materials.