Development and characterization of microwave absorber using rice husk/ carbon nanotube composites

Microwave absorber composites with nanomaterial have tremendous attraction in both industrial and academic world due to their improvement of microwave absorption and material properties. Therefore, the rice husk (RH) or rice husk ash (RHA) with carbon nanotubes (CNT) composites have used to improve several microwave properties of microwave absorber such as dielectric properties, microwave absorption, and shielding effectiveness. In this thesis, the EMI absorbing and shielding of microwave absorbers composted of these RH, RHA, RH/CNTs, and RHA/CNTs composites are presented. The main objectives of this thesis were to characterize the various CNT mixed with RH and RHA composites in microwave properties including dielectric properties, microwave absorption, and shielding effectiveness. It was found that the electric properties and electrical conductivity of RH/CNT and RHA/CNT composites increases with the increase of CNT loading. Innovative use of CNT in RH/CNT and RHA/CNT composites as dielectric absorbers can make significant contributions in our effort to improve the performance of previous work of rice husk absorber. First step of this study, the samples were fabricated in composite form by mixing different weight ratio filler of the RH, RHA, and CNT, with the bonding agent polyester (PE), and the samples were prepared in room temperature by using standard mould and fabrication process. Then, the dielectric properties of the samples were measured using a broadband Agilent dielectric probe and rectangular waveguide methods over the frequency range of 2-18 GHz and 8.2-18 GHz, respectively. The measured dielectric properties were used to calculate the wave impedance, wave attenuation constant, phase constant, and depth of penetration of the incident wave in the composites. These composites then used, to fabricate single layer, multi-layer, fully filled solid, and hollow pyramidal microwave absorbers. The microwave absorbers were studied experimentally for their effectiveness in microwave absorption (reflectivity) and shielding effectiveness in suppressing the broadband electromagnetic interference (EMI). In case of single layer, the reflectivity values of the absorbers were found to less than -20 dB at specific operate frequency with narrowband performance. For multi-layer absorbers, the reflectivity performance is below -10 dB with multiple bands was achieved over 2-18 GHz. The reflectivity of RH based pyramidal absorber were improved from more than -20 dB to less than -40 dB at 2-10 GHz by replace the 1 em RH base layer of pyramidal absorber to 70 wt% RH and 2 wt% CNT composite (RH/CNT -2). In case of hollow pyramidal absorbers, coating layer technique was used to improve the reflectivity performance of hollow pyramidal absorber over 4-18 GHz. The Shielding effectiveness of the 70 wt% RHA and 20 wt% CNT composite achieved more than 25 dB over 8.2-18 GHz in different angle rotation of absorber.