Hierarchically organised materials based on polymers and conductive one-dimensional nanostructures for the control of electromagnetic propagation
(2015)
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- Abstract
- Wireless communications use microwaves for fast and massive data transfer in our daily life. This thesis presents broadband absorbers, electromagnetic bandgap filters and invisibility cloaks as novel solutions to reduce electromagnetic (EM) interference phenomena responsible for many issues, ranging from simply annoying (e.g. "Wi-Fi drop out") to outright dangerous (e.g. "wrong transmission from health monitor in hospital" or "disruption of airport radar signal by wind turbine") . The studied structures consist of organised stacks of thin layers made of conductive nano-composite and dielectric polymer or ceramic substrate. As conductive fillers, we mainly use carbon nanotubes (CNT) and metallic magnetic nanowires (NW) because of their exceptional electrical properties and high aspect ratio, which allows reaching the electrical percolation threshold at very low concentration. By the nature and concentration of the fillers, we can control conductivity, permittivity and permeability of the composite films. By controlling orientation of the fillers during processing, we can impart anisotropic properties to the structures. A smart arrangement of the different layers is the key for a controlled absorption or propagation of the EM waves. Broadband absorption is obtained by a novel multilayer arrangement built from alternating films of dielectric polymer and conducting layers. The latter are stacked in a precise gradient of conductivity. The conducting layers consist of either PC-CNT nanocomposite films or a very thin CNT coating. Such multilayers effectively absorb electromagnetic waves from 8 to 67GHz and probably higher despite their overall thickness much lower than the wavelength. The efficiency can further be enhanced with the help of submillimetric multilayers based on Nickel nanowires (Ni-NW) sandwiched between PC films. The magnetic response of Ni-NW contributes to enhanced attenuation of the incoming waves. Also based on a similar gradient organisation, anisotropic multilayers provide a route to polarisation-selective absorbers. The second objective of the research is to develop frequency-selective absorbers, also called electromagnetic bandgap (EBG) filters. The multilayer is now able to absorb a specified narrow frequency band or selectively reflect desired wavelengths within the GHz-range. The structures use ultra-thin conductive layers to generate a controlled resonance in homogenous high permittivity sheets. The basic structure is still composed of CNT deposits alternating between dielectric layers. The physical properties (i.e. relative permittivity and thickness) of the dielectric spacers generate narrow high-absorption bands at defined frequencies. Finally, we investigate the ability of multilayer structures to deviate microwaves around a reflective cylindrical obstacle and reconstruct the incoming wave front pattern behind, i.e. making the obstacle invisible. A cylindrical invisibility cloak requires fine tuning the effective permittivity, which has to grow in gradient from 0 to 1 from the inner to outer radius of the cloak. We have reached this purpose by successively stacking cylindrical polymer foam - CNT composite bilayers with precise thickness of the dielectric layers and precise conductivity of the conductive layers. Parameters of each bilayer are fine-tuned to get the effective permittivity required by theory. Despite some loss, our multilayers demonstrate significant capacity to reduce the distortion of the wavefront pattern behind the obstacle.
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Danlée, Y. (2015). Hierarchically organised materials based on polymers and conductive one-dimensional nanostructures for the control of electromagnetic propagation. https://hdl.handle.net/2078.5/191492