Ferromagnetic metal-based materials display properties that make them of interest for microwave applications, namely higher working frequencies and a broader working frequency band than bulk ferrimagnetic oxides. As far as microwave absorbing properties are concerned, metals have to be used as fine particles dispersed in an insulating matrix. Such composite magnetic materials exhibit magnetic losses (characterized by a non-zero imaginary part of the permeability) in the microwave range due to a gyromagnetic resonance phenomenon, their microwave properties depending on both the intrinsic characteristics of the particles and their volume concentration. The influence of the latter can be quite well described by mixture laws derived from the Bruggeman effective medium theory. [1,2] Less studied is the control of microwave properties of composite materials by altering the intrinsic properties of the magnetic particles. Two main objectives can be defined: first, the design of high-permeability composite materials with, in particular, optimal control of the resonance width; secondly, a better understanding of the dynamic properties of fine particles and a tentative correlation with their static magnetic properties. In both cases, control of the morphology of the ferromagnetic particles is needed since the gyromagnetic resonance is highly dependent on the particle shape through the effect of the demagnetizing field. Therefore, materials made up of particles with poorly defined shapes present a very broad resonance band. Moreover, materials made up of too large particles present only a weak resonance. [3,4] The polyol process, [5,6] which is known for providing monodisperse fine metal particles, afforded us the opportunity to synthesize ferromagnetic metal particles smaller than 2 mm and to investigate their dynamic properties. Our first results provided evidence of the effect of particle size on microwave properties in the 2±0.2 mm range. [7,8] The scope of this paper is to show how it has been possible recently to reduce and to control the diameter of such monodisperse particles down to the nanometer size range for various compositions and therefore to study the influence of the particle size upon the microwave permeability of monodisperse powders made up of quasi-spherical particles with a size range varying over two orders of magnitude (2.5 mm±25 nm).Polymetallic fine particles Co x Ni (100±x) and Fe z [Co x -Ni (100±x) ] (1±z) were synthesized by precipitation from metallic precursors dissolved in 1,2-propanediol with an optimized amount of sodium hydroxide according to a previously published procedure [9±11] (see Experimental section). Upon heating, as both Co II and Ni II are quantitatively reduced by the polyol itself, the Co/Ni ratio in the metallic Co x Ni (100±x) powders depends only on their initial ratio. For iron-based particles of Fe z [Co x Ni (100±x) ] (1±z) composition, Fe is generated by disproportionation of Fe II whereas Co II and Ni II are quantitatively reduced. The disproportionation of Fe II allo...

Ruthenium nanoparticles were prepared by reduction of RuCl 3 in a liquid polyol. The mean particle size was restricted to the 1-6 nm range by appropriate choice of the reduction temperature and the acetate ion concentration in the solution. Very narrow particle diameter distributions were obtained. In some samples, among nearly isotropic particles, platelets with aspect ratios as low as 1/4 were detected. Colloidal solutions in toluene were obtained by coating the metal particles with dodecane thiol. Self-assemblies of 4-nm-sized coated particles were studied on a transmission electron microscope grid. The dodecane thiol concentration in the colloidal solution was found to determine, within the particle monolayer, the formation of either columnar units made up of edgewise stacked platelets, or a hexagonal network with a mean distance between the particles of 2 nm. The stacking of hexagonal arrays of particles was also studied, and both closed-packed and noncompact stackings were found. In the noncompact stacking, moire ´images resulted from the twisting of the two hexagonal layers with respect to each other. Reconstructions of moire ´patterns were observed to favor the 6-fold and 2-fold sites.

Fine equiaxial a-Fe particles were obtained by disproportionation of iron@) 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 Fe,M~loo-x, (M =Ni, Co; 0 b x b 25) and Fe,[Ni,Co(, -y~](loo-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 Ni" and Co" 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 (d,) in the sub-pm size range, a narrow size distribution (standard deviation c < 10% d,) 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.