Françoise Fiévet-Vincent

Nucleation and growth of bimetallic CoNi and FeNi monodisperse particles prepared in polyols

Finely divided, crystalline, ferromagnetic, bimetallic powders have been synthesized by reduction or disproportionation of inorganic compounds in liquid polyols. Two different growth mechanisms are evidenced for CoNi and FeNi particles. Monodisperse, spherical particles, homogeneous in composition are obtained in the submicron size range in both cases.

CoNi and FeCoNi fine particles prepared by the polyol process: Physico-chemical characterization and dynamic magnetic properties

Spherical and monodisperse CoxNi100 − x and Fez[CoxNi100 − x]1 − z particles are synthezied by the polyol process over a wide size range (lying from a few micrometers to a few tens of nanometers). The whole physico-chemical characterizations, i.e. dark-field image by TEM, SAED, density, saturation magnetization and chemical analysis, are consistent with a “core-shell” model. In the CoxNi100 − x system, the particles are constituted by a ferromagnetic, almost pure and dense core surrounded by a thin coating composed of metal oxides and metallo-organic phases. On the contrary, in the Fe[CoxNi100 − x]1 − z system, the ferromagnetic core is polycrystalline, slightly porous and retains impurities in higher content, the superficial layer having almost the same composition as in the Co-Ni system, but being twice more thick. The microwave permeability of the CoxNi100 − x and Fez[CoxNi100 − x]1 − z particles, previously insulated by a superficial treatment and then mechanically compacted, is investigated in the 100 MHz-18 GHz frequency range. Whatever the composition, submicrometer-sized particles show several narrow resonance bands which are interpreted as non uniform exchange resonance modes. Iron-based particles have lower resonance frequencies than iron-free powders; they also have higher permeability levels despite their lower cristallinity and their higher impurity content. A mild thermal treatment allows to increase this permeability by eliminating the metallo-organic impurities without modifying the morphology of the particles.

Size dependence of microwave permeability of spherical ferromagnetic particles

Spherical, monodispersed, ferromagnetic, metallic particles of different compositions were obtained by the polyol process with a mean radius ranging from 30 nm to 1 μm. The microwave permeability of metallic particles-dielectric matrix composites were studied in the range of 0.1–18 GHz. In the wide particle size range investigated, a size dependence of the dynamic permeability was observed. Whereas the permeability of micrometer-sized particles shows a single resonance band, the permeability of submicrometer-sized particles exhibits several narrow resonance bands which are shifted to high frequencies with decreasing particle size. This latter behavior was found to be in qualitative agreement with the exchange resonance modes calculated by Aharoni. That theory, however, gives an R−2 dependence on particle radius for the resonance frequency instead of the R−0.66 dependence observed experimentally.

Preparation and microwave characterization of spherical and monodisperse Co20Ni80 particles

Spherical and monodisperse Co20Ni80 particles were prepared, in the micrometer and submicrometer size range, by the polyol process. X‐ray diffraction showed crystalline particles with a fcc structure. From electron probe microanalysis a fairly homogeneous distribution of both elements within each particle was observed. Microwave properties of metal particles dielectric matrix composites were studied in the 0.1–18 GHz range for different filler concentrations. The intrinsic permeability of the metal powders was obtained using the Bruggeman effective medium theory. The control of the particle size allowed the study of its effect upon dynamic permeability. Whereas micrometer size particle permeability presents a single resonance band at low frequencies which can be correlated to the low magnetocrystalline anisotropy of the particles, submicrometer size particle permeability exhibits an original behavior, never reported before, with several resonance bands.

Cobalt-based anisotropic particles prepared by the polyol process

Cobalt and Co80Ni20 metal particles were prepared by reduction of acetate salts in a liquid polyol. The relative amount of the hcp phase and the probability of stacking faults in this phase vary with the experimental conditions of synthesis. The formation of either isotropic or anisotropic particles appears to be closely related to the stacking faults. Several kinds of anisotropic particles were detected by TEM: platelets, rods or diabolo-like particles. For isolated diabolos and rods, HRTEM and electron diffraction showed that the crystallographic c axis is parallel to the rotation axis of the particles. Magnetic property measurements showed high values of coercivity and remanence for the anisotropic particles. The anisotropic shapes differ markedly from those previously observed for cobalt-based particles prepared by the polyol process. A mechanism that involves a solid intermediate phase as either precursor or template is proposed.

Controlled nucleation and growth of micrometre-size copper particles prepared by the polyol process

Fine, equiaxed copper particles have been obtained by reduction of CuO in ethylene glycol. Cu2O always exists as an intermediate solid phase. Copper particles with a narrow size distribution within the micrometee range can be obtained if the nucleation and growth steps are completely separated and if agglomeration is avoided. Addition of D-sorbitol, which acts as a protective agent, prevents particle sintering. Addition of a strong base, e.g. NaOH, enhances the solubility of the precursor CuO and of the intermediate Cu2O. Under these conditions the overall reaction appears to be controlled by the nucleation and growth steps of the metallic particles. Their mean size can be largely controlled by varying the NaOH concentration. The synthesis of copper particles in liquid polyols, which act as both solvent and reducing agent, is a simple method for producing highly pure, equiaxed, non-agglomerated monodisperse particles.

Monodisperse iron-based particles: precipitation in liquid polyols

Fine equiaxial α-Fe particles were obtained by disproportionation of iron(II) hydroxide in liquid polyols with yields in the range 5–8%. The influence of the polyols as the reactive medium upon the solid phase formation during this reaction and its effect upon the Fe formation yield are discussed. Fine polymetallic powders FexM(100–x)(M = Ni, Co; 0 ⩽x⩽ 25) and Fex[NiyCo (1 – y)](100 –x)(0 ⩽x⩽ 25; 0 ⩽y⩽ 1), were prepared from mixed hydroxides in liquid polyols, with metallic iron being generated in solution by disproportionation, and metallic nickel and/or cobalt by reduction of NiII and CoII hydroxides by the polyols. Characterization (using X-ray diffraction, energy dispersive spectroscopy and electron microscopy) showed FeNi powders, made up of spherical particles with a mean diameter (dm) in the sub-µm size range, a narrow size distribution (standard deviation σ < 10%dm) and a fairly good homogeneity of composition; FeCo powders were polyphasic and made up of polydisperse agglomerates of Fe and Co particles while FeNi powders appeared as a single phase; FeCoNi powders appeared as monodisperse and polyphasic. The growth mechanism of the particles is discussed in relation to their characteristics: for FeNi monodisperse particles a growth mode by aggregation of nm-sized primary particles is proposed.

Observations of exchange resonance modes on submicrometer sized ferromagnetic particles

Spherical, monodisperse, submicrometer-sized particles in the CoxNi(100−x) system have been synthesized by the polyol process over the whole chemical composition range. These fine particles have been insulated by a chemical treatment in solution by a superficial layer of MnO2. The microwave permeability of these passivated particles, which are mechanically compacted, has been investigated in the frequency range 0.1–18 GHz. The dynamic permeability measurements exhibit several narrow resonance bands. This experimental behavior is in qualitative agreement with Aharoni’s calculation [A. Aharoni, J. Appl. Phys. 69, 7762 (1991)] of exchange resonance modes. The influence of the chemical composition of the ferromagnetic particles upon permeability has been investigated in the microwave range. It is shown that the peaks are shifted towards the high frequencies when atomic Co/Ni ratio increases.

Preparation and microwave characterization of spherical and monodisperse CoNi particles

Abstract Spherical and monodisperse Cobalt-Nickel particles were prepared by the polyol process for different compositions. Microwave permeability of metal particles-dielectric matrix composites were measured in the 0.1–18 GHz range. The influence of size and chemical composition of the particles upon dynamic permeability were studied.

Preparation, characterization and reactivity of Pd/Nb2O5 catalysts in hexa-1,5-diene hydrogenation

Abstract Palladium/niobia catalysts are prepared by various methods involving either gas or liquid phase reduction. Although giving rise to average or low dispersion, the reduction of palladium precursors in a liquid medium (hydrazine or ethylene–glycol) appears to be a promising method since a low dispersion favors the activity in the hydrogenation of hexa-1,5-diene in liquid phase. The substitution of alumina by niobia improves the fractional selectivity and the yield of hex-1-ene in all cases. A very good global selectivity is also observed.