P. Perriat

XPS and EELS investigations of chemical homogeneity in nanometer scaled Ti-ferrites obtained by soft chemistry

Abstract Nanocrystalline Ti-ferrites with composition Fe 3− x Ti x O 4 with 0≤ x ≤1 are synthesized using the soft chemistry route. Heterogeneities in precipitate and annealed powders are investigated by a combination of XPS and EELS techniques. As-prepared powder consists in particles with spinel structure and grain size of about 15 nm. Due to high reactivity towards oxygen of nanoparticules, a significant amount of Fe 2+ cations oxidize during precipitation, so that precipitated powders present large deviation from oxygen-metal stoichiometry. Moreover precipitated particles are evidenced to exhibit a strong surface titanium enrichment and an iron richer core. Observation of such cation segregation proves that, during oxide formation process, kinetics of Ti precipitation is slower than that of Fe one. Conditions for tailoring nanocrystalline samples with satisfying composition homogeneity are finally obtained by determination of an appropriate annealing under oxidizing conditions which enables to eliminate Fe–Ti segregation.

Cationic distribution and oxidation kinetics of trivalent molybdenum ions in submicron molybdenum substituted magnetites

Abstract Valence states of molybdenum and iron ions and their cationic distribution on both octahedral (B) and tetrahedral (A) sites on the spinel structure of submicron molybdenum-substituted magnetites which are oxidized in cation deficient spinels have been performed by derivative thermogravimetry, IR spectroscopy and XPS. It was demonstrated that Fe 2+ B , Mo 3+ B , Mo 4+ B , Fe 2+ A and Mo 4+ A are successively oxidized into Fe 3+ and Mo 6+ ions below 450°C. A quantitative analysis based on this difference of reactivity of iron and molybdenum in relation to occupied sites (A or B) permits us to propose a cationic distribution for initial and oxidized phases. Kinetic studies of the oxidation process of Mo 3+ ions suggest that oxidation proceeds by way of a diffusion-controlled reaction in a cation deficient phase of variable composition with a chemical diffusion coefficient dependent on the vacancy content.

Mechanical activation conditions of the Fe2O3 and V2O3 mixture powders in order to obtain a nanometric vanadium spinel ferrite

Abstract Co-milling of iron and vanadium oxides allows to obtain an intimate oxides mixture at a nanoscale, similar to a coprecipitate elaborated by soft chemistry. Reduction of such a mixture in the same temperature and oxygen partial pressure conditions (500°C and 10−25 Pa) as the soft chemistry products leads to a nanometric vanadium ferrite with the only spinel phase. The characterization of the powders is achieved by X-ray diffraction (XRD), scanning electron microscopy, infrared (IR) spectrometry, thermogravimetry and calorimetry. Homogeneity of grain size and chemical composition is reached if the initial oxides have similar grain size.

Effect of the preparation method and grinding time of some mixed valency ferrite spinels on their cationic distribution and thermal stability toward oxygen

Abstract The reactivity in oxygen of several mixed valency ferrite spinels, namely Fe3O4, FeCr2O4, Fe3 − xTixO4 and Fe3 − xMoxO4 was investigated by derivative thermogravimetry (DTG) analysis as a function of the conditions of preparation and grinding. For all these compounds, low temperature preparation or prolonged grinding time enables small particles ( 0.5 μm), it was observed that oxidation was starting at higher temperature and that the defect spinel phases could not be retained during oxidation. This behaviour was attributed to the presence of stresses induced by the lattice parameter gradient and promoting the formation of nuclei of the α-rhombohedral phase from the superficial γ-defect phase.

Experimental set up for determining the temperature—oxygen partial pressure conditions during synthesis of spinel oxide nanoparticles

Abstract Nanometric spinel oxide powders, Fe 3− x M x O 4 , where M is a transition element, have been synthetized by soft chemistry. This method generally leads to a non-stoichiometric phase, Fe 3− x M x O 4+δ where δ is the deviation from stoichiometry so that further annealing at low temperatures around 450°C and low oxygen partial pressure around 10 −25 Pa given by N 2 /H 2 /H 2 O gas mixtures is required: this enables a stoichiometric compound to be obtained and a nanometric size to be maintained. The complete set up consisting of a gas mixer, a thermogravimetric apparatus and a preparative furnace is described. Some results concerning the conditions of temperature and oxygen partial pressure leading to stoichiometry are given in the case of vanadium ferrites containing or without Co.

Correlation between oxidation states of transition metal ions and variation of the coercivity in mixed-valence defect spinel ferrites

Abstract Due to the very high dispersion of mixed-valence spinel ferrites prepared by ‘soft chemistry’ it becomes possible to oxidize in the spinel lattice, not only ferrous ions but also different transition metal ions (Cr 3+ , Mn 2+ , Mn 3+ , Mo 3+ , Mo 4+ V 2+ , V 3+ , Cu + ), by ‘soft’ oxidation between 150 and 500°C, under formation of non-stoichiometric spinels cation vacancies. The oxidation state with the coordination and the oxidation temperature of the cations have been determined in copperue5f8manganese ferrites by derivative thermogravimetry (DTG), based on the specific solid-state reactivity of cations in the redox reaction. For Co-modified non-stoichiometric spinels, which present interesting magneto-optical properties due to their exceptional high coercivity (1000–3500 Oe) and whose cation distribution has been determined by the method described above, close correlations have been found between the oxidation state of the spinel and the variation of the coercivity.

Réactivité vis-à-vis de l'oxygène de spinelles de fer-vanadium de taille nanométrique et distribution cationique

Resume Letat de division des spinelles de fer-vanadium nanometriques V x Fe 3− x O 4 (0 ⩽ x ⩽ 2 ) permet doxyder au sein-meme de la structure spinelle les ions du fer et du vanadium. Les analyses calorimetriques et thermogravimetriques, ainsi que la spectroscopie infrarouge, montrent que les ions Fe B 2+ ,V B 3+ et Fe A 2+ sont oxydes successivement en ions Fe 3+ et V 5+ a des temperatures inferieures a 450 °C. Les spinelles lacunaires a valence mixte qui en resultent, ont un taux en lacunes croissant avec x, qui peut etre superieur a ceux determines jusqualors pour ce type doxydes.