Microwave-absorption performance of compositionally-varied ferrite-carbon nanotube-polymer composite and CVD-synthesized carbon nanotube-polymer composite

Currently the research and development of radar absorbing materials (RAM) have increased where wide range of materials are used for the design, aiming to eliminate or reduce the spurious electromagnetic radiation levels more closely in different applications. The present research attempts to fabricate high absorbing material compositions suitable for microwave absorption from 8 to 18 GHz. Various microwave absorbing composites were fabricated using conventional solid state method and chemical vapor deposition method. There were various different materials being synthesized with different weight percentages, different mixed materials, different catalysts to synthesize carbon nanotube (CNT) and different thicknesses. However, only selected and outstanding results will be explained in details in this thesis. The materials being discussed were divided into four parts; Multiwalled Carbon Nanotubes (MWCNTs) mixed with Nickel-Zinc Ferrite (Ni0.5Zn0.5Fe2O4), as-synthesized CNTs catalyzed by mill scale, as-synthesized CNTs catalyzed by Nickel-Zinc Ferrite (Ni0.5Zn0.5Fe2O4) mixed with Cobalt Ferrite (CoFe2O4) and as-synthesized CNTs catalyzed by Nickel-Zinc Ferrite (Ni0.5Zn0.5Fe2O4) mixed with Carbonyl Iron. For mixed ferrite with MWCNTs, the starting raw metal oxide powder materials to produce ferrite materials were weighed and milled using the high energy ball milling technique to get nanometer starting particle size while MWCNTs was obtained from commercial source. The ferrite powders were then sintered at different sintering temperature according to different materials being synthesized. For CVD method, the catalyst powder was weighed and heated at 7000C for 30 minutes. Argon gas and ethanol was used as the carrier gasand carbon source, respectively. The prepared composite samples were then incorporated into epoxy resin as a matrix with different ratio and poured into special manufacture sample holder with thickness kept at 1, 2 and 3 mm. The crystalline phase formation of all samples prepared was further investigated with an X-ray diffractometer (XRD). The particle size was measured using a transmission electron microscope (TEM). The microstructure of the samples was picture using a field emission scanning electron microscope (FESEM) measurement. The elemental analysis was measured using electron dispersive X-ray spectroscopy (EDX). The vibrational phonon modes were determined by Raman spectroscopy. The resistivity was measured using a resistivity measurement setup. The magnetic properties were measured using a Vibrating Sample Magnetometer (VSM). The scattering parameters were measured in the X and Ku-band regions by using a Vector Network Analyzer (VNA) within the frequency range from 8 GHz to 18 GHz. The XRD results for ferrite mixed with MWCNTs showed that the diffraction peaks were slightly shifted and the dominant peaks were given by ferrite materials since the weight percentages amount being added was higher. The average particle size of synthesized ferrite and average diameter of as-synthesized CNTs were in nanometers sized region which enhanced the absorption capability. The resulting aggregated morphology of as-synthesized CNTs were due to Van der Waals forces and this was resolved by incorporated them into an insulated polymer matrix. The carbon structures forms were mostly straight, spiral, hollow tube, netlike and twisted fiber which enhanced the ability of absorption. The EDX measurement for selected samples showed the elements presence in the composite samples. Raman spectrum showed defect (D-band) was higher than graphitize (G-band) which attributed to defects in the tube ends, staging disorders, hollow tube and curved graphene layers. For ferrite mixed with MWCNTs, it was found that as the amount of MWCNTs increased, the coercivity (Hc) of composites increased while the saturation magnetization (Ms) and retentivity (Mr) both decreased. For measurement at higher frequency (X-band and Ku-band), thicker samples resulted in higher microwave absorption. As the thickness increased, reflection loss peak shifted towards lower frequency range due to shifted of matching frequency. For thickness of 3 mm, the reflection loss reached -17 dB at 9.5 GHz for 2 wt% MWCNTs- Ni0.5Zn0.5Fe2O4 /P with bandwidth 3.5 GHz. As for as-synthesized CNT by mill scale milled at 20 hours for thickness of 3 mm, the value of reflection loss was -25 dB. As for as-synthesized mixed ferrite with thickness of 3mm, the reflection loss of sample with as-synthesized (80% NZF + 20% C)/P gave the most minimum reflection loss value of -29 dB at around 12.5 GHz with bandwidth 4 GHz (10.5 – 14.5 GHz) and -26 dB at 9.5 GHz for as-synthesized (20% NZF + 80% CI)/P. The prepared as-synthesized CNT polymer composites were expected to be very useful in many application especially military applications such as radar cross section reduction and for prevention of electromagnetic interference.

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