Guillaume Viau

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.

FTIR and XPS Study of Pt Nanoparticle Functionalization and Interaction with Alumina

Platinum nanoparticles with a mean size of 1.7 nm were synthesized by reduction in sodium acetate solution in 1,2-ethanediol. The particles were then functionalized with dodecylamine, dodecanethiol, and omega-mercapto-undecanoic acid (MUDA). Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) showed important variations of the particle surface state with functionalization whereas their structure differs only slightly. Platinum-to-sulfur charge transfer inferred from XPS of thiol-coated particles enabled the identification of the formation of Pt (delta+)-S (delta-) bonds. The native carbon monoxide (CO) at the surface of the particles was a very efficient probe for following the functionalization of the particles by FTIR. The red shift of nu(CO) accounts for the nature of the ligands at the surface of the particles and also for their degree of functionalization. Immobilization on alumina substrates of particles functionalized with MUDA was realized by immersion in colloidal solutions. Free molecules, isolated particles, and aggregates of particles interconnected by hydrogen bonds at the surface of alumina were evidenced by FTIR. With successive washings, the energy variation of the CO stretch of carbon monoxide and of carboxylic acid groups and the relative intensity nu(CH2)/nu(CO) showed that the free molecules are eliminated first, followed by aggregates and less-functionalized particles. Particles presenting a high degree of functionalization by MUDA remain and interact strongly with alumina.

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.

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.

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.

Magnetic nanowires as permanent magnet materials

We present the fabrication of metallic magnetic nanowires using a low temperature chemical process. We show that pressed powders and magnetically oriented samples exhibit a very high coercivity (6.5kOe at 140K and 4.8kOe at 300K). We discuss the magnetic properties of these metamaterials and show that they have the suitable properties to realize “high temperature magnets” competitive with AlNiCo or SmCo permanent magnets. They could also be used as recording media for high density magnetic recording.

Synthesis, characterization and magnetic properties of disk-shaped particles of a cobalt alkoxide: CoII(C2H4O2)

A cobalt alkoxide, Co(OCH2CH2O), has been prepared from the reaction of cobalt acetate with ethanediol. This compound crystallizes as disk-shaped particles with diameter and thickness in the 0.4–1 µm and 100–250 nm ranges, respectively. Structural characterization (X-ray and electron diffraction) shows a layered arrangement with a brucite-like structure presenting a turbostratic disorder. The interlayer spacing and the Co⋯Co distance within the layer are c = 8.27 A and a = 3.09 A, respectively. UV-visible spectroscopy studies established that the Co(II) ions are located on octahedral sites. IR spectroscopy showed that the ethylene glycolate anions (OCH2CH2O)2− chelate the Co(II) cations. The magnetic properties of the compound were measured in the range of 2–300 K. Below 20 K, the compound exhibits 3D ferromagnetic order and the hysteresis loop shows a very high remanence-to-saturation ratio typical of a uniaxial magnetocrystalline anisotropy.

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.

One-step construction of silver nanowires in hexagonal mesoporous silica using the polyol process

An original one-step preparation of single-crystal silver nanowires in hexagonal mesoporous silica is presented. The silver precursor, silver nitrate, is reduced in ethylene glycol (EG). This procedure avoids thermal treatments which can lead to phase segregation. The absorbance spectrum of the resulting hybrid material exhibits a transverse resonance plasmon peak near 360 nm whereas the longitudinal oscillation is shifted to the near-infrared region at about 1500 nm.