Isabelle Huynen

Straightforward synthesis of conductive graphene/polymer nanocomposites from graphite oxide

The reduction of graphite oxide (GO) in the presence of reactive poly(methyl methacrylate) (PMMA), under mild biphasic conditions, directly affords graphene grafted with PMMA. The resulting nanocomposite shows excellent electrical conductivities resulting from the optimal dispersion and exfoliation of graphene in the polymer matrix.

Microwave properties of metallic nanowires

We report on the microwave properties of arrays of parallel magnetic nanowires constituted of nickel, cobalt, or Ni/Fe alloy embedded in nanoporous track-etched polymer membranes. The experiments consist of transmission measurements carried out on microwave stripline structures using a magnetically loaded membrane as the substrate. Measurements were performed at frequencies ranging from 100 MHz to 40 GHz and under static magnetic fields up to 5.6 kOe applied along the wires axis. Resonance phenomena have been observed in the magnitude of the complex transmission coefficient at frequencies which depend on the nature of the material and applied static magnetic field. Results are consistent with those expected for a ferromagnetic resonance (FMR) experiment and the observed behaviors are analyzed in the framework of the classical FMR theory

An unbiased integrated microstrip circulator based on magnetic nanowired substrate

A very compact planar fully integrated circulator operating at millimeter wavelength has been designed using a magnetic substrate combining a polymer membrane with an array of ferromagnetic nanowires. The original feature of this substrate, called magnetic nanowired substrate (MNWS), relies on the fact that the circulation effect is obtained without requiring any biasing dc magnetic field. This leads to a significant reduction of device dimensions since no magnetic field source is needed, and a realistic ability for integration with monolithic microwave integrated circuits. The circulator design is performed by an efficient analytical model including a self design of the impedance matching network. This model also allows a physical understanding of the circulation mechanism through the access to the electromagnetic field patterns inside the circulator substrate. Based on the excellent agreement between the theoretical and experimental results, the model is used to predict the improvement of circulator performances resulting from a reduction of dielectric and conductor losses. Insertion losses lower than 2 dB with an isolation higher than 45 dB are expected for MNWS circulators with a low-loss substrate and thick metallic layers.

Magnetic photonic band-gap material at microwave frequencies based on ferromagnetic nanowires

We present an experimental investigation of a class of microwave photonic band-gap (PBG) materials, in which the magnetic permeability μ varies periodically within the material. This material is fabricated using a periodic arrangement of arrays of magnetic nanowires. As for dielectric or metallic PBG, the band-gap behavior varies with the geometrical parameters fixing the spatial periodicity of the magnetic structure. The magnetic photonic band gap is induced by the presence of a ferromagnetic resonance effect in the vicinity of the band gap.

A wideband line-line dielectrometric method for liquids, soils, and planar substrates

A line-line (LL) method for measuring the permittivity of materials up to millimeter waves is proposed. It is based on a property of the well-known line-return-line (LRL) calibration technique developed for microwave circuits: the complex propagation constant of the two-line calibration standards can be extracted from the raw measurement of these two lines. Using this feature for a dielectrometric purpose, we combine this LL formulation with accurate models for the propagation constant to extract from the measured value the permittivity of the medium surrounding, or contained in, the LL calibration lines. Choosing the line topology (planar, coaxial, or waveguide) according to the nature and consistency of the material under consideration yields the permittivity of a wide variety of materials. In this paper, we demonstrate the efficiency of the method for three kinds of substances: organic liquids, soils, and planar dielectric substrates used for microwave planar circuits. Advantages particular to each application are detailed in the paper.

Spectral domain form of new variational expression for very fast calculation of multilayered lossy planar line parameters

A new spectral domain formulation of the propagation characteristics for planar and coplanar lines is presented. It is based on a newly established variational principle, valid for a spatial as well as for a spectral formulation. In combination with conformal mapping, it drastically reduces the complexity of the numerical computation and leads to rapidly convergent results even when higher order modes are considered. Mathieu functions are shown to be very efficient expressions for trial fields of the dominant and the higher order modes in slots. Calculation is fast: it is made on-line on a regular PC. Results obtained on open and shielded lines have been successfully checked with new experimental data and with previously published data. The method is general enough to accommodate gyrotropic substrates. The paper however is limited to isotropic media. >

Microwave circulator based on ferromagnetic nanowires in an alumina template

Unbiased planar microwave circulators were fabricated by electrodeposition of NiFe nanowires into porous alumina templates. Microwave properties of the devices are seen to depend drastically on the height of the nanowires and the newly developed devices exhibit improved features, compared to existing nanowire-based designs. Thanks to the high anisotropy of the nanowires, zero-field circulation modes may be observed in a frequency range from 10 to 30 GHz, with isolation as large as 30 dB, as well as low insertion losses - 5 dB, making it compatible with industrial needs for device applications.

A novel nanostructured microstrip device for tunable stopband filtering applications at microwaves

The authors present a novel microstrip structure using a nanoscale porous substrate filled by a ferromagnetic material, forming an array of nanowires perpendicular to the ground plane. When compared with photonic bandgap structures, the stopband behavior is created here by a gyromagnetic resonance phenomenon in the metallic nanowires. This resonance is tuned by means of a DC magnetic field parallel to the nanowires, in a very good agreement with the gyromagnetic theory. Also, tuning can be achieved over more than one octave, because the nanoscale geometry ensures that fields penetrate into the whole wire area up to 40 GHz. Other advantages are detailed in this work.

Electromagnetic Absorption Properties of Carbon Nanotube Nanocomposite Foam Filling Honeycomb Waveguide Structures

Carbon nanotube reinforced polymer foams filling a metallic honeycomb were processed and characterized for the production of hybrid materials with high electromagnetic absorption potential. Electromagnetic modeling and experimental characterization of the hybrids proved that the honeycomb, acting as a hexagonal waveguide, improves the absorption properties in the gigahertz range above the cutoff frequency. The electromagnetic absorption can be tuned by changing the hybrid material properties. The required levels of electrical conductivity are attained owing to the dispersion of low amounts (1-2 wt%) of carbon nanotubes inside the polymer matrix. The combination of the foam and honeycomb architecture contributes to decrease the real part of the relative effective permittivity Re{εr,eff }. Varying the cell shape of the honeycomb changes the frequency range for high absorption. An analytical model for the absorption has been developed, showing good agreement with the experimental results.

Thin smart multilayer microwave absorber based on hybrid structure of polymer and carbon nanotubes

We present a thin and flexible multilayer solution for effective absorption of electromagnetic waves from 8 to 40 GHz and probably higher. Submillimetric conductive and dielectric layers based on polymer and carbon nanotubes are successively stacked. A smart gradient-periodic arrangement results in a very high absorption index despite overall millimetric thickness of the system.