| dc.description.abstract | Use of discarded material in value-added products has been a common practice
since long. Recently, two wastes have been studied for their potential use in microwave
absorber applications. One of these wastes is rice husk (RH) from the paddy fields and
is a natural source of carbon, while another is scrap rubber (SR) from discarded
automobile tires (a synthetic source of carbon black). The RH has a low bulk density
(70-110 kg/m³), and thus requires large volumes for storage. Likewise, a very expensive
and complex recycling process is required to reuse the synthetic SR. Burning of these
wastes, releases a large amount of toxic carbon mono oxide (CO) and carbon dioxide
(CO2) gases in the air. Furthermore, commercially available dielectric foam absorbers
are specially dipped in a carbon solution (graphite suspension) before, they can be used
in anechoic chambers. However, both of the wastes have already lossy carbon contents,
which has made them attractive raw materials for the fabrication of the low cost
microwave absorbers. In this thesis electromagnetic compatibility (EMC) oriented study
of the microwave absorbers composed of these two wastes in planar and geometricallytapered
(GT) hollow shapes is presented. Innovative use of these two wastes, in EMC
absorbers can make significant contributions in our efforts to cope with the waste
disposal and the problems associated with the environmental pollution. As a first step of
the study, the samples were fabricated in composite form by mixing different loadings
(wt. %) of these two wastes, with the two types of adhesives i.e., unsaturated polyester
resin (UPR) and a commercially available non-toxic glue. The samples were prepared
on laboratory scale at room temperature by using one of the easiest composite
fabrication methods i.e., hand lay-up method. In the second step, dielectric properties of
the samples were measured using an easy to use broadband co-axial sensor over the
frequency range of 2 to 20 GHz. These properties were then used, to study and
determine the characteristic impedance, wave attenuation constant, phase constant, and
depth of penetration of the incident wave in the lossy wastes. Planar microwave
absorbers composed of a single layer, impedance-graded two and three-layer designs
were studied numerically and experimentally for their effectiveness in suppressing the
discrete frequency (narrowband) and broadband electromagnetic echoes. The values of
the reflectivity were found to be well below the -20 dB at discrete frequencies with
narrowband performance, in case of single layer absorbers. Broadband performance
below -15 dB was achieved by using impedance-gradient design. The GT hollow
absorbers were studied to solve the issues related to the installation of the bulky solid
absorbers. These absorbers were designed on the basis of the wave attenuation and
depth of penetration data and their EMC performance was evaluated in terms of bistatic
reflectivity performance. The results proved that, while, these absorbers had 58%
less weight and 15% less height than the solid absorbers, they provided standard
performance, i.e., < -20 dB. The optimum value of the weight fraction of the RH for
hollow absorbers was found to be in the range of 35 to 60 %, of the overall composite,
for which less structural damage was observed. The performance of the absorbers was
found to be below -10 dB even at 100 °C in the X band of frequencies. A two-layer
pyramidal design based on the cladding concept was also studied to improve the low
frequency EMC performance of the solid absorbers composed of scrap rubber from
automobile tires. | en_US |
