2. Invisible cloaking

220 Pages·2015·6.92 MB·English

Institut de la Matière Condensée et des Nanosciences (IMCN)

Institute of Information and Communication Technologies,
Electronics and Applied Mathematics (ICTEAM)

HIERARCHICALLY ORGANISED
MATERIALS BASED ON POLYMERS AND
CONDUCTIVE ONE-DIMENSIONAL
NANOSTRUCTURES FOR THE CONTROL
OF ELECTROMAGNETIC PROPAGATION

Thesis submitted for the
Ph.D. degree in Engineering Sciences

by Yann Danlée on 20th March 2015

Jury members
Prof. Isabelle Huynen, supervisor
Prof. Christian Bailly, supervisor
Prof. André de Lustrac
Dr. Jean-Michel Thomassin
Prof. Luc Piraux, secretary
Prof. Thomas Pardoen
Prof. Jean-Pierre Raskin, chairperson

Jury members

Prof. Isabelle Huynen, UCL – ICTEAM (Belgium)
Prof. Christian Bailly, UCL – IMCN (Belgium)
Prof. André de Lustrac, U-Psud – IEF (France)
Dr. Jean-Michel Thomassin, ULg – CERM (Belgium)
Prof. Luc Piraux, UCL – IMCN (Belgium)
Prof. Thomas Pardoen, UCL – iMMC (Belgium)
Prof. Jean-Pierre Raskin, UCL – ICTEAM (Belgium)

III

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
V
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.
VI
Table of contents

Abstract V
Table of contents VII
Author's publications XI
Journal papers XI
International conference proceedings XII
Scientific communications XII
Acknowledgement XV
List of the abbreviations XIX
Chapter I Overview & context 1
Chapter II State of the art 7
1. Development of broadband absorbers 9
1.1. Impedance matching 9
1.2. Resonant absorbers 11
1.3. Hybrid structures 13
1.4. Active materials 14
1.5. Originality of the developed structures for broadband
absorption 14
2. Development of electromagnetic bandgap filters 17
2.1. Electromagnetic bandgap metamaterials 17
2.2. Frequency selective surface 19
3. Development of polarisation controllers 20
VII
3.1. Linear polariser 20
3.2. Twist polariser 22
4. Development of invisible cloaking 23
Chapter III Theoretical concepts 25
1. EMI shielding by absorption and bandgap multilayers 27
1.1. Principles of electromagnetic wave propagation 27
1.2. Limit and classification of absorbing structures 31
1.3. Electromagnetic bandgap (EBG) absorption/reflection
by EM resonance 34
1.4. Operating on S-parameters 37
1.5. Simulation by chain matrix 38
1.6. Extraction of physical parameters from raw S-parameters 40
1.7. Nanoparticles-polymer composites 46
2. Invisible cloaking 55
2.1. Working principle 55
2.2. Mathematical derivation 57
2.3. Practical approach 66
2.4. Implementation of -only controlled multilayer
invisibility cloak 71
Chapter IV Materials and methods 73
1. Nanocharges for composites 75
1.1. Origin of nanocharges 75
1.2. Production of metallic nanowires 76
2. Processing of nanocomposites 80
2.1. Compounded polymer-carbonaceous nanofillers
composites 80
2.2. CNT ink based composites 82
2.3. Structured patterns in CNT 83
2.4. Metallic nanowires composites 84
2.5. Machining of dielectric spacers for cloak of invisibility 86
VIII
3. Electrical characterisation 88
3.1. DC characterisation 88
3.2. AC/microwave characterisation 89
3.3. Set up for wave propagation mapping 90
Chapter V Experimental results and discussion 91
1. Broadband absorber based on CNT 93
1.1. Electrical and morphological characterisation 93
1.2. Structure for wide band EMI shielding 98
1.3. Performance assessment through the Rozanov method 102
1.4. Optimisation of the gradient stacks for the Rozanov
optimum 103
1.5. Conclusion 106
2. Nanowires based composites for EMI shielding 107
2.1. Gold nanowire-polycarbonate composites 107
2.2. Nickel nanowires-polycarbonate composites 112
2.3. Conclusion 119
3. Anisotropic composites for polarisation 120
3.1. Electrical and morphological characterisation 120
3.2. Polarisation-selective surface 122
3.3. Microwave twist polarisation 127
3.4. Conclusion 129
4. Electromagnetic bandgap structures 130
4.1. Frequency-selective filter with narrow bandgap 130
4.2. Frequency-selective filter with absorbing peak 132
4.3. Conclusion 135
5. Cylindrical invisible cloaking 137
5.1. Bilayers for controlled permittivity 137
5.2. Stacks of bilayers for the invisibility cloak 143
5.3. Mapping of near field around cloaked zone 148
IX
5.4. Discussion of experimental results and potential
enhancements 152
5.5. Prospects and alternatives for invisible cloaking 154
5.6. Conclusion 157
Chapter VI General conclusion & prospects 159
1. Broadband absorption 160
2. Electromagnetic bandgap filtering 162
3. Invisible cloaking 164
References 167
Annex A Plasma treatment for adhesion of CNT ink on PC 181
1. Experimental results 182
2. AFM topography 184
3. X-ray photoelectron spectroscopy (XPS) 186
Annex B Gel permeation chromatography (GPC) of
PC-NW composites 189
Annex C Summary of the multilayers 193
1. Building blocks 194
2. Characteristic of the multilayers 196
Multilayer 1 196
Multilayer 2 197
Multilayer 3 197
Multilayer 4 198
Multilayer 5 198
Multilayer 6 199
Multilayer 7 199
Multilayer 8 200
Multilayer 9 200
Multilayer 10 200
X