Mechanistic Insights for Controlling Electromagnetic Interference Shielding Through Microwave Absorption in Magnetic Composites
Extensive use of electronic devices in daily communication and information technology causes high (microwave) frequency electromagnetic interference (EMI). This EMI often leads to noise, data misinterpretation or malfunctioning of electronic devices such as medical equipment. To protect the device from this unwanted EMI, a shield layer is essential, which can shield the device from the unwanted radiation via either reflection or absorption. As the reflected microwave may cause further EMI, the later phenomenon is advantageous, because it forbids any further interference with neighboring devices. This absorption-based shielding is also useful in stealth technology to design radar camouflage military aircraft. Metallic shields normally reflect the microwave and are heavy. To address this issue one requires shield layers with lightweight and conducting. In this respect, conducting polymer-based or metallic nanoparticles based composites seems handy. Hence, in this thesis work, we have adopted various strategies to design composites that can address the above limitations of metallic shields. We have demonstrated that the scattering, reflection and absorption of microwave depend upon the micro and macroscopic properties of the filler particles. Such properties include concentration, size, morphology, conductivity, defects and magnetism of the filler materials. We have systematically investigated their effect on EMI shielding to validate our strategies. We used composites of conducting polymer (Polyaniline), hard ferrimagnetic hexaferrites, soft magnetic Yttrium Iron Garnet (YIG), metallic iron particles, metal (Fe/Co/Ni) doped carbonaceous materials along with microwave transparent paraffin wax or PVDF. The effect of concentration, size, morphology, conductivity, defects and magnetic properties of these fillers in these composites on EMI shielding is studied. Furthermore, to understand the atomistic mechanism of shielding through light-matter interactions, complex permittivity and permeability of composites used to demonstrate the dielectric and magnetic loss contributing to the microwave absorption. In this work, in particular, the mechanistic insight into the role of concentration of hexaferrite in hexaferrite-polyaniline-Wax composites, role of network structure of garnet particles in YIG-polyaniline-wax composite, the effect of size of carbon-coated iron/iron carbide particles and micron-sized iron particles in PVDF composite, the role of defects in carbon-coated cobalt and iron particles in scattering of microwave, the effect of improved graphitization and role of magnetism in carbon-coated cobalt and iron particles and the effect of morphology of bimetallic alloy doped carbonaceous materials in PVDF matrix, on EMI shielding behavior is studied in detail. It is demonstrated that using different strategies, the designed composite specimens are very highly effective in attenuating the microwave radiation. The mechanistic insight into microwave absorption in designing highly absorbing EMI shield layer is the highlight of this thesis. The results can directly utilize for industrial applications.
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