Designing dielectric loss at microwave frequencies using multi-layered filler particles in a composite

Microwave-absorbing materials find application in telecommunications, microwave heating and for representing the behavior of biological tissue in the presence of microwave radiation. Commonly, such materials are formed using ferromagnetic filler particles and rely on the phenomenon of ferromagnetic resonance for absorption of microwaves. Dielectric loss at microwave frequencies can be engineered through creating a phase lag, with respect to the applied electric field, of the movement of free charges in a composite formed using metal-coated filler particles. These materials can be engineered to be less dense and, therefore, more lightweight than those formed with ferromagnetic fillers, which is an advantage in some applications. Furthermore, theory shows that the frequency of maximum absorption can be tailored by selecting the conductivity and thickness of the particle coating although, in practice, it may be difficult to fabricate particles with tightly controlled physical parameters. In this work, theories for calculating complex permittivity of composites with layered filler particles are reviewed, and experimental observations of dielectric relaxation in composites formed by dispersing tungsten-coated glass bubbles in paraffin wax are shown