Monodisperse Ferromagnetic Particles for Microwave Applications

Abstract

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...

Plasma dynamic synthesis of iron oxides in a discharge plasma jet with possibility to control final phase composition

“…They have numerous applications in different areas, ranging from engineering (e.g., magnetic recording media or magnetic seals) to biomedical applications (e.g., magnetic resonance imaging, drug delivery or hyperthermia) and their appealing novel properties [1][2][3][4][5]. Among various magnetic iron oxides, those that exhibit soft magnetic behavior with excellent frequency characteristics and high electrical resistivity are strongly required as core materials used in magnetic components for the ceaseless increase of the working frequency of the data transmission and multiple accesses in computer, mobile and blue-tooth devices [6,7]. The most known iron oxide phases, having outstanding characteristics of electromagnetic radiation (EMR) absorption, are magnetite Fe3O4 and relatively new epsilon phase ε-Fe2O3.…”

Collaborative Research and Development Delivery Order 0024: Synthesis, Processing, and Evaluation of Polymers for RF Applications

“…a dielectric material). [3][4][5][6][7] Here we achieve this by incorporating ferrite based magnetic nanoparticles into a polyurethane matrix.…”

“…Previous efforts have focused on bulk materials or micron-sized magnetic particles (which behave similar to bulk materials). [3][4][5][6][7] However, understanding of the structure-property relationship of magnetic nanoparticles for RF applications is still in its infancy. 2 An important objective of this study is to develop an understanding of these relationships and correlate the frequency dependent response from 2-18 GHz.…”

Self-organization on surfaces: foreword

“…A second advantage is the nearly infinite possible combination of ligands, that give many more degrees of freedom than the choice of elements for epitaxial growth. A popular chemical route for the fabrication of magnetic nanoparticles is inverse micella [49][50][51]. In this Issue Bruno Chaudret presents an original route recently opened, i.e., an organometallic approach to nanoparticles synthesis and self-organization [52].…”