Elaboration and characterization of composite materials with ferromagnetic flake-shaped particles used as microwave absorbers

Cette thèse est consacrée à la réalisation de composites constitués d’inclusions ferromagnétiques dispersées dans une matrice diélectrique en vue d’obtenir des absorbeurs hyper fréquences dans la gamme 1 GHz – 5 GHz. La principale originalité de notre étude repose sur l’utilisation de particules à forte anisotropie géométrique dites sous forme de «pétales». En raison de leurs bonnes propriétés magnétiques (perméabilité et aimantation à saturation élevées), notre choix s’est porté sur l’utilisation d’alliages NiFeMo et FeSiAl pour cette étude. Dans un premier temps, les particules sous forme de pétales sont obtenues par mécanosynthèse. Un ajustement des paramètres de broyage a permis d’obtenir des pétales avec différents rapports de forme (longueur latérale sur épaisseur). Les particules sont ensuite incorporées à une barbotine constituée d’un liant et d’un plastifiant qui est par la suite coulée en bandes selon le procédé Doctor Blade. Les propriétés structurales et magnétiques des particules broyées ainsi que l’orientation des particules dans le composite ont été caractérisées. Dans une seconde partie, les spectres de perméabilité et de permittivité des composites ont été mesurés en cellule monospire et en ligne coaxiale APC7. Les évolutions des spectres en fonction de la quantité de particules dans le composite ainsi qu’en fonction du rapport de forme des particules ont été étudiées. Il a été montré que la transformation en pétales permettait d’obtenir des niveaux élevés de perméabilité et de permittivité. Ainsi, afin d’avoir un absorbeur de qualité, il est nécessaire de limiter les niveaux de permittivité en enrobant les particules. Les pétales NiFeMo ont été enrobés par de la silice selon le procédé Stöber et ceux à base de FeSiAl par oxydation partielle. Les propriétés magnétiques et électriques dynamiques de composites chargées par des pétales enrobés ont également été étudiées. Des modèles analytiques (loi de mélange de Maxwell-Garnett pour la perméabilité et lois d’Odelevsky et de Mclachlan pour la permittivité) ont été utilisés pour reproduire les principales caractéristiques radioélectriques des spectres de perméabilité et de permittivité Enfin, les performances électromagnétiques de ces composites ont été évaluées en considérant le problème générique de la réflexiond’une onde plane sur des revêtements recouvrant un support métallique. Le passage sous forme de pétales a permis de gagner en taux de charge et en épaisseur de couche absorbante par rapport aux composites chargés par des sphères. Cependant, un taux de charge élevé en pétales entraine une augmentation importante de la permittivité réelle et nuit à la condition d’accord. L’enrobage à la silice des pétales NiFeMo a permis de réduire la permittivité et d’atteindre un niveau d’absorption plus important. Ainsi, nous avons pu réaliser des matériaux composites plus légers et plus minces, absorbants dans la gamme de fréquence recherchéeThis thesis deals with composites material containing ferromagnetic flaky-shaped particles (high length to thickness ratio) which exhibit improved microwave properties in terms of permeability spectra. It aims at obtaining effective absorbers in the 1 GHz – 5 GHz range. Because of their good magnetic properties (high permeability and saturation magnetization), NiFeMo and FeSiAl alloys were chosen as fillers. Firstly, composite sheets containing flaky-shaped particles were elaborated. Particles with different aspect ratios were obtained through a ball-milling process and then mixed with polymers in a slurry which is casted into sheets by using the Doctor Blade method. Structural and magnetic properties of milled particles and their orientation in the dielectric matrix were investigated. Then, permeability and permittivity spectra measurements were carried out using a single coil permeameter and a standard APC-7 coaxial line. Their evolutions with the particles aspect ratio and the particles volume content in the matrix were studied. It was shown that flaky shaped particles exhibit higher permeability and permittivity levels. Hence, to avoid impedance mismatch, lowering permittivity levels by using an insulating layer is necessary. Consequently,NiFeMo flakes were coated with a silica layer through the Stöber process and FeSiAl flakes with an oxide layer through oxidation. Electromagnetic properties of composites filled with coated flakes were also studied. In addition, permeabilityspectra were fitted combining the Landau-Lifshitz-Gilbert equation and the Maxwell-Garnet mixing rule. Permittivity spectra were fitted using Odelevsky and Mclachlan laws. Finally, their efficiency as a microwave absorber is estimated by calculating the reflection loss of a normal incident electromagnetic wave on an absorbing layer backed by a perfect conductor. Using flaky particles instead of spherical ones allowed the use of lighter and thinner layer. However, it resulted in high permittivity levels and lead to impedance mismatches. Insulating NiFeMo flakes with a silica layer reduced permittivity levels and allowed better absorbing properties. By using flaky shaped particles, we elaborated thinner and lighter microwave absorbers in the 1 GHz – 5 GHz range

Compressed perovskite aqueous mixtures near their phase transitions show very high permittivities: New prospects for high-field MRI dielectric shimming

International audiencePurpose: Perovskites are greatly used nowadays in many technological applications because of their high permittivity, more specifically in the form of aqueous solutions, for MRI dielectric shimming. In this study, full dielectric characterizations of highly concentrated CaTiO 3 /BaTiO 3 water mixtures were carried out and new permittivity maxima was reached. Methods: Permittivity measurements were done on aqueous solutions from 0%v/v to dry powder. The permittivity dependence with pressure was investigated. Scanning electron micros-copy images were performed on a few representative solutions. BaTiO 3 pressed pads of different thicknesses, permittivities, and distances to the head were compared in a 7T MRI scanner. Results: Perovskite aqueous mixtures undergo a pressure-dependent phase transition in terms of permittivity, with increasing water content. A new relative permittivity maximum of 475 was achieved. Microscopic images revealed structural differences between phases. A B þ 1 improvement in the temporal lobe was obtained with thin, high permittivity BaTiO 3 head. Conclusions: This new preparation method allows improved pad geometry and placement, as a result of the high relative permittivity values achieved. This method has great significance for medical applications of MRI dielectric shimming, being easy to replicate and implement on a large scale

Kerker Effect in Ultrahigh-Field Magnetic Resonance Imaging

International audienceUltrahigh-field (UHF) magnetic resonance imaging (MRI) systems are getting a lot of attention as they ensure high intrinsic signal-to-noise ratio resulting in higher spatial and temporal resolutions as well as better contrast. This promises improved clinical results with regard to morphological as well as functional and metabolic capabilities. Traditionally, MRI relies on volume coils (birdcage) able to deliver a homogeneous radio frequency field exciting the nuclei magnetic spin. However, this strategy is hindered at UHF because of the rf field inhomogeneities yielded by the increased Larmor frequency. A standard approach consists of inserting passive dielectric elements within the volume coil in order to locally enhance the rf field and mitigate these inhomogeneities. However, the lack of control over their electromagnetic properties prevents the development of optimal solutions. Here, a single meta-atom is used to achieve efficient and tunable rf field control in UHF MRI. We demonstrate theoretically and experimentally a full overlap between the electric dipolar and magnetic dipolar resonances of the meta-atom. This interaction is precisely tuned to reach the so-called Kerker scattering conditions when illuminated in the near field by a birdcage coil. At these conditions, a strong enhancement or suppression of the rf field is achieved in the vicinity of the meta-atom within the MRI volume coil

Evaluation of new MR invisible silicon carbide based dielectric pads for 7 T MRI

International audiencePurposeThe use of dielectric pads to redistribute the radiofrequency fields is currently a popular solution for 7 T MRI practical applications, especially in brain imaging. In this work, we tackle several downsides of the previous generation of dielectric pads. This new silicon carbide recipe makes them MR invisible and greatly extends the performance lifespan.MethodWe produce a set of two 10x10x1cm3 dielectric pads based on silicon carbide (SiC) powder dispersed in 4-Fluoro 1, 3-dioxalan-2-one (FEC) and polyethylene Glycol (PEG). The stability of the complex permittivity and the invisibility of the pads are characterized experimentally. Numerical simulations are done to evaluate global and local SAR over the head in presence of the pads. B0, B1+ and standard imaging sequences are performed on healthy volunteers.ResultsSiC pads are compared to state-of-the-art perovskite based dielectric pads with similar dielectric properties (barium titanate). Numerical simulations confirm that head and local SAR are similar. MRI measurements confirm that the pads do not induce susceptibility artefacts and improve B1+ amplitude in the temporal lobe regions by 25% on average.ConclusionWe demonstrate the long-term performance and invisibility of these new pads in order to increase the contrast in the brain temporal lobes in a commercial 7 T MRI head coil

Enhancement of transmit and receive efficiencies with hybridized meta-atom in 7T head coil

International audienceBackground B1 homogeneity in 7T MRI remains an important challenge in order to fully benefit from the signal enhancement due to the stronger magnetization available in ultra-high field scanners. High-dielectric constant pads have been proposed and optimized for passive shimming purposes [1-3]. Common formulations of dielectric pads for 7T applications are based on BaTiO 3 mixed with water. They present some drawbacks such as performance decay over time and toxicity. While previous studies tackled directly the formulation problem introducing new dielectric materials and solvent [4], we adopted a new approach based on metamaterials [5]. We demonstrated that the hybridization of four parallel metallic wires arranged on a square unit cell provides the ability to control radiofrequency field inside a 7T head birdcage. In the present work, we show that these hybridized meta-atom (HMA) can be used to improve MRI acquisition in the presence of a head receive array routinely used for in vivo MRI protocol

Passive RF shimming for 7T head coil with hybridized meta-atom: In vivo investigation

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