Microwave Characteristics of Composite Absorbers Containing High-Aspect-Ratio Dielectric and Magnetic Fillers

Abstract

In this thesis investigations have been made on microwave characteristics of composite absorbers containing high-aspect-ratio dielectric and magnetic fillers We propose two analytical resonant cavity perturbation formulae for the complex permeability of electromagnetic materials and its accuracy is confirmed by numerical results of finite element method and the experimental data from the waveguide transmission/reflection method The proposed formula is especially useful for the electromagnetic characterization of magnetic particles and magnetic thin films with limited quantities The complex permittivity and permeability of composite samples prepared by mixing MnO2 nanoparticles MnO2 nanorods iron powder carbonyl iron powder and FeNiMo alloy powder individually with epoxy resin were measured and determined by using the transmission/reflection method whereas those of surface-modified iron nanowire/epoxy composite were measured by the resonant cavity perturbation method Based on these measured quantities the reflection loss of microwave abosrber can be predicted by using the transmission line theory Absorption properties of microwave absorbers backed by perfect conductors can be calculated from the measured complex permittivity complex permeability target frequency and the coating layer thickness The minimum reflection loss occurring at the matching frequency is found to move toward the lower frequency region with increasing real part of permittivity real and imaginary parts of permeability of the fillers Experiment results of this study show that the particle shape of fillers strongly influences the absorption properties of the microwave composite absorbers For the same thickness of absorbers flake-shaped particles achieve higher microwave absorption For the same concentration of fillers flake-shaped particles provide higher permittivity and permeability due to their high aspect-ratios in S-band and H-band than the corresponding ones of rod-shaped and sphere-shaped particles Compared with spherical particles rod-shaped particles exhibit higher shape anisotropy larger aspect ratio superior electromagnetic properties and better absorption properties in the high frequency range

Date of Award	2014 Jan 27
Original language	English
Supervisor	Siu-Tong Choi (Supervisor)