dc.contributor.author | Parke, Laura | |
dc.date.accessioned | 2015-09-07T09:25:32Z | |
dc.date.issued | 2015-06-01 | |
dc.description.abstract | Ferrites are a class of magnetic oxides with superior electromagnetic (EM)
properties at microwave frequencies when compared to conventional metallic
magnetic materials for use in antenna miniturization and radar absorbers.
Metamaterials are also a special group of materials, which are known to
provide EM responses not found in nature due to subwavelength structuring.
In this thesis, a range of ferrite composite materials and metamaterial
structures are exploited to develop new methods for controlling permittivity
and permeability up to 4 GHz with a view to producing high refractive
index materials and to demonstrate broadband impedance matching to free
space.
The rst section of the thesis uses composites of powdered MnZn ferrite
(as the ller) and PTFE (as the matrix), fabricated by a novel cold pressing
technique, to produce composites for a range of volume fractions of
MnZn ferrite (between 0-80% vol.). The EM properties for all composites
were determined as a function of % vol. and the results were found to be in
agreement with the Lichtenecker mixing formula. This study is the rst convincing
con rmation of the Lichtenecker mixing formula over a broad range
of volume fractions (0-80% vol.). The cold pressing method was found to
produce composites with reproducible EM properties, and was extended to
use aluminium, barium titanate (as llers) and also cellulose as an alternative
matrix. Importantly, with regard to the study of cellulose composites,
our work is the rst to explore volume fractions of up to 85% and, the rst
to con rm the Lichtenecker mixing formula with these materials.
The ferrite particle size, as well as the volume fraction of ferrite, impacts
the EM response of the composites. Both the permittivity and permeability
increase as a function of ferrite particle size; however, the permeability
increases at a much faster rate than the permittivity with particle size. It
is shown that by controlling the ferrite particle size in conjunction with the
volume fraction of ferrite, broadband impedance matching to free space can
be realised for tailored values of refractive index. This is the rst study that
demonstrates independent control of the permittivity and permeability of
ferrite composites by controlling the ferrite particle size. Alternatively, by
adding a third component to the two part composite it is demonstrated
that broadband impedance matching to free space can also be realised with
a refractive index of 16.1 (between 10-50 MHz). This is the rst time, to the
authors knowledge, that three part composites have been used to achieve
high refractive index materials that are impedance matched to free space.
The second section of this thesis takes the concept of metamaterials to
structure ferrite composite material with a further view to gain independent
control over the permittivity and permeability. By tailoring the EM
response of this metamaterial, which is comprised of anisotropic arrays of
ferrite cubes, broadband impedance matching to free space is demonstrated.
The refractive index over the impedance-matched frequency range is also
very high (9.5). The metamaterial also acts as an excellent non-re
ecting
subwavelength thickness absorber up to 200 MHz. An analytical description
of the permittivity and permeability dependence on the metamaterial
parameters is developed to predict the EM response of this metamaterial,
and of similar systems.
In the last part of this thesis, the concept of cubic metamaterials is extended
to more complex metallic meta-atoms, where the permittivity and diamagnetic
response of the metamaterial are independently tailored to demonstrate
how the refractive index can be tuned over a broad frequency range.
By understanding the role of individual cube parameters, the diamagnetic
response can be controlled between near zero and unity, which greatly alters
the refractive index. The results are the rst experimental validation
for showing `design' control of the permittivity and permeability of these
metamaterials via geometry tuning of the meta-atom design. | en_GB |
dc.description.sponsorship | EPSRC, DSTL | en_GB |
dc.identifier.citation | Heavily loaded ferrite-polymer composites to produce high refractive index materials at centimetre wavelengths', L. Parke, I. R. Hooper, R. J. Hicken, C. E. J. Dancer, P. S. Grant, I. J. Youngs, J. R. Sambles and A. P. Hibbins, APL Mater., 1, 042108 (2013) doi 10. 1063/1.4824039. | en_GB |
dc.identifier.citation | Broadband impedance-matched electromagnetic structured ferrite composite in the megahertz range', L. Parke, I. J. Youngs, A. P. Hibbins and J. R. Sambles, App. Phys. Lett., 104, 221905 (2014) doi 10. 1063/1.4881186. | en_GB |
dc.identifier.citation | `Independently controlling permittivity and diamagnetism in broadband, lowloss, isotropic metamaterials at microwave frequencies', L. Parke, I. R. Hooper, E. Edwards, N. Cole, I. J. Youngs, A. P. Hibbins and J. R. Sambles, Appl. Phys. Lett., 106, 101908 (2015) doi 10.1063/1.4915097. | en_GB |
dc.identifier.uri | http://hdl.handle.net/10871/18190 | |
dc.language.iso | en | en_GB |
dc.publisher | University of Exeter | en_GB |
dc.title | Controlling the Electromagnetic Properties of Magnetic Composites and Metamaterials | en_GB |
dc.type | Thesis or dissertation | en_GB |
dc.date.available | 2015-09-07T09:25:32Z | |
dc.contributor.advisor | Hibbins, Alastair | |
dc.contributor.advisor | Sambles, Roy | |
dc.publisher.department | Physics | en_GB |
dc.type.degreetitle | PhD in Physics | en_GB |
dc.type.qualificationlevel | Doctoral | en_GB |
dc.type.qualificationname | PhD | en_GB |