C. Monty

Magnetic properties of γ‐Fe2O3 nanoparticles obtained by vaporization condensation in a solar furnace

The magnetic properties of γ‐Fe2O3 nanoparticles synthesized by vaporization condensation in a solar image furnace have been studied using both magnetic measurements and Mossbauer spectroscopy. The mean size of the particles turns out to be easily controlled by changing the pressure conditions in the growth chamber. The particles exhibit superparamagnetic behavior at room temperature. Magnetic measurements show the appearance of magnetic hysteresis in the low‐temperature range and from the evolution with temperature of the ferromagnetic ratio, MR/MS, we have determined the distribution of the blocking temperatures for the smallest particles that is fitted to a log‐normal distribution leading to a mean blocking temperature 〈TB〉=38±15 K. The size distribution of the magnetic unit is also determined from this fitting, as well as from the Mossbauer spectra, obtaining a mean particle volume of about 3.5×105 A3.

Self-diffusion in α‒Al2O3. II. Oxygen diffusion in ‘undoped’ single crystals

Abstract Oxygen self-diffusion in ‘undoped’ (i.e. unintentionally doped) alumina (Al2O3) single crystals was investigated using the gas-solid isotope exchange technique. After diffusion annealing, profiles of 18O were determined by secondary ion mass spectrometry. These showed two parts: close to the initial surface, the first part was attributed to bulk self-diffusion, while the diffusion tails were attributed to diffusion in dislocation walls. In the temperature range 1500–1720°C the bulk self-diffusion coefficients of the oxygen in Al2O3 are represented by: Do(cm2s−1) = 206 exp[-(636 kJ mol−1)/RT]. Possible diffusion mechanisms are proposed considering an extrinsic behaviour associated with silicon contamination. In the same temperature range, the oxygen diffusion coefficients in the Al2O3 subboundaries are described by: D″o(cm2 s−1) = 3.1 × 1014exp[-(896 kJ mol−1)/RT]. The high activation enthalpy observed is attributed to segregation effects on subboundaries.

Self-diffusion in α[sbnd]Al2O. IV. Oxygen grain-boundary self-diffusion in undoped and yttria-doped alumina polycrystals

Abstract Oxygen self-diffusion coefficients were determined in polycrystals of α-alumina either ‘undoped’ or doped with 500wt.ppm (225molppm) Y2O3 in the temperature range 1460-1720°C, with 18O2 and secondary-ion mass spectrometry. Oxygen grain-boundary diffusion coefficients obey the relations for ‘undoped’ alumina and for ‘yttrium-doped’ alumina. The comparison between the two materials indicates that yttrium addition decreases the oxygen grain-boundary diffusion. For both materials, it was observed that the activation energy of grain-boundary diffusion is greater than the activation energy of the bulk diffusion. Similar results were obtained in single crystals for diffusion in the bulk and in subboundaries. These results were interpreted as being due to impurity segregation along boundaries and led to a model.

Hydrothermal synthesis of zirconia nanomaterials

Abstract Yttria-stabilised zirconia powders and films have been obtained from Zr(IV) peroxides by hydrothermal crystallisation at temperatures in the range 125–200°C. The thermodynamic evaluation evidenced that formation of Zr(OH) 5 − in H 2 O 2 solutions increases solubility of Zr (IV) hydrated species and improves the kinetics of the new phase formation by the solubilisation-reprecipitation process. Powders with crystallite sizes in the range 6–22 nm have been obtained. The kinetic parameters for the crystallisation process were calculated using Avramis approach to control the nanocrystalline structure. Nanocrystalline films consisting of platelet YSZ crystallites were also hydrothermal deposited on alumina substrate of YSZ suggesting a layer to layer growing mechanism.

Self-diffusion in α-Al2O3 and growth rate of alumina scales formed by oxidation : effect of Y2O3 doping

In many cases, alumina scales are assumed to grow predominantly by oxygen diffusion, but some authors have found that the growth can be controlled by aluminium diffusion. These mechanisms can be modified by active elements. The problem with alumina is that there is a lack of data about self-diffusion coefficients, and, due to the stoichiometry of alumina, diffusion data correspond to an extrinsic diffusion mechanism so that it is not possible to compare oxygen and aluminium diffusion coefficients. In order to obtain information about the alumina scale growth mechanism, oxygen (18O) and aluminium (26Al) self-diffusion coefficients in Al2O3 were determined in the same materials and in the same experimental conditions, thus allowing a direct comparison. For both isotopes, bulk and sub-boundary diffusion coefficients were determined in single crystals of undoped alumina. Grain-boundary diffusion coefficients have been computed only for oxygen diffusion in polycrystals. Oxygen diffusion has been also studied for yttria-doped α-alumina in the lattice, sub-boundaries and grain boundaries. Oxygen and aluminium bulk diffusion coefficients are of the same order of magnitude. In the sub-boundaries, aluminium diffusion is slightly faster than oxygen diffusion. Yttria doping induces a slight increase of the oxygen bulk diffusion, but decreases the grain-boundary diffusion coefficients on account of segregation phenomena. These results are compared with the oxidation constants of alumina former alloys (alloys which develop an alumina scale by oxidation). It appears that neither lattice self-diffusion nor grain boundary self-diffusion can explain the growth rate of alumina scales. Such a situation is compared to the case of Cr2O3.

Self-diffusion in cr2o3 I. Chromium diffusion in single crystals

Abstract Chromium self-diffusion coefficients in chromia (Cr2O3) single crystals were determined by both ion implantation and thick-film methods, using the 54Cr and 50Cr isotopes. The concentration profiles were established by secondary-ion mass spectrometry, and the diffusion coefficients were computed using a solution of Ficks law taking into account evaporation and exchange at the surface. Chromium diffusion was studied as a function of temperature T and oxygen pressure Po2. Both methods lead to diffusion coefficients of the same order of magnitude. The diffusion coefficients are lower than those given in the literature and do not depend on the oxygen pressure; they are well described by the relation

Self-diffusion in cr2o3 II. Oxygen diffusion in single crystals

Abstract Oxygen self-diffusion in chromia (Cr2O3) single crystals was studied as a function of oxygen pressure at 1100°C, using the gas-solid isotope exchange method. The diffusion experiments were performed in H2-H2 18O atmospheres. After the diffusion anneal, the 18O diffusion profiles were determined by secondary ion mass spectrometry and the diffusion coefficients were computed using a general solution for the Ficks second law, taking into account evaporation and exchange at the surface. Our results show that oxygen diffusion coefficients at 1100°C do not depend on the oxygen pressure and are smaller than values given in the literature. Comparison with results concerning chromium self-diffusion in the same Cr2O3 single crystals clearly indicates that oxygen diffusion is faster than chromium diffusion.

Oxygen self-diffusion in NiO single crystals

Abstract The self-diffusion of oxygen in NiO has been measured under 150 Torr of oxygen between 1100 and 1600°C. Diffusion profiles were determined with secondary-ion mass spectrometry. The values of the self-diffusion coefficients between 1200 and 1600°C are given by D (cm2 s−1)= 50 exp (-5·6 eV/kT).

Self-assembled multifunctional Fe/MgO nanospheres for magnetic resonance imaging and hyperthermia

UNLABELLED A one-step process for the production of nanoparticles presenting advanced magnetic properties can be achieved using vapor condensation. In this article, we report on the fabrication of Fe particles covered by a uniform MgO epitaxial shell. MgO has a lower surface energy than Fe, which results in a core-shell crystal formation. The particles satisfy a few of technical requirements for the practical use in real clinics, such as a high biocompatibility in living cells in-vitro, an injection through blood vessels without any clothing problems in murine model, a high absorption rate for magnetic hyperthermia at small particle concentration, and the potential to be used as contrast agent in the field of diagnostic magnetic imaging. They are also able to be used in drug delivery and magnetic-activated cell sorting. FROM THE CLINICAL EDITOR In this paper, the authors report on the synthesis of Fe particles covered by a uniform MgO epitaxial shell resulting in a core-shell crystal formation. The particles are proven to be useful as contrast agents for magnetic resonance imaging and have the potential to be useful as heating mediators for cancer therapy through hyperthermia. They also might be used in drug delivery and magnetic-activated cell sorting.

Ionic conductivity of zirconia based ceramics from single crystals to nanostructured polycrystals

Abstract The ionic conductivity of yttria doped zirconia polycrystals was measured by impedance spectroscopy in the temperature range 240–800°C in air and compared to that of a single crystal. Polycrystals with 4% mol yttria and an average grain size of 300 nm have exhibited a grain boundary contribution to the ionic conductivity (value and activation energy) higher than the volume measured in the 9.5% mol yttria doped zirconia single crystal. The bulk conductivity of polycrystals with 9.5% mol yttria was found to be smaller than the single crystal one. The interpretation of the grain boundary contribution to ionic conductivity, measured by impedance spectroscopy, is revisited.