Microwave Characteristics of Particulate Magnetic Composites
Date: 5 August 2019
University of Exeter
PhD in Physics
Spherical magnetic particles have recently become of interest for applications in microwave communications devices as they exhibit magnetic absorption modes at frequencies much larger than conventional materials. These higher order modes allow composites comprising spherical magnetic particles in dielectric matrices to have relative ...
Spherical magnetic particles have recently become of interest for applications in microwave communications devices as they exhibit magnetic absorption modes at frequencies much larger than conventional materials. These higher order modes allow composites comprising spherical magnetic particles in dielectric matrices to have relative permeabilities above unity at frequencies surpassing conventional magnetic materials. The higher order magnetic modes seen in spherical iron powders are a result of the vortex domain structure of magnetic spheres and exist also in magnetic spherical shells. Composite materials containing these magnetic spheres have use in miniaturisation of communications devices and allow access to frequency bands previously unattainable for communications as the increase in permeability at high frequencies will not only increase the refractive index of the material, but also decrease the impedance of the material, improving impedance matching to air. This thesis presents work for the investigation of higher order modes in spherical iron powders, particularly carbonyl iron powders, with the intent to demonstrate how properties such as particle size distribution affects the higher order resonances exhibited. A stripline technique for broadband characterisation of dielectric and magnetic composites is presented in the thesis, which demonstrates an ability to extract the relative permittivity and permeability of composites across an unprecedented broadband frequency range of 0.2 - 50 GHz. Although the relative permittivity and permeability of composites was able to be extracted for most cases, the refractive index of materials was shown to be significantly more resistant to uncertainty across broad frequencies. This technique employed a simple method for sample manufacture by wet-casting composites for characterisation, giving a fast and reliable method for characterising small amounts of material across a frequency range that surpasses most readily available methods. The technique was used for characterisation of several carbonyl iron powder grades. The carbonyl iron grades were imaged by scanning electron microscopy (SEM) analysis to give particle size distributions that were able to be compared between grades to allow an investigation into how the particle size distribution for carbonyl iron powders affects their microwave characteristics. Particle size distribution was confirmed to be a strong factor when considering the strength and position in frequency of these higher order resonances, so a technique was developed for filtration of powders into subgrades with different particle size distributions and average particle sizes. The results of this investigation displayed that the emergence of higher order magnetic absorption modes in carbonyl iron is heavily dependent on the average size and distribution of sizes for the powder used. The filtration technique developed is able to be used for filtration of spherical particles in the single micron size regime and uses only the flow of air through a set of stainless steel tubing and glass bottles. This experimental method is not only cheap, but is simple to assemble and achieves filtration of powders smaller than most mechanical sieves are capable of filtering. Finally, an investigation into the behaviour of these higher order modes under externally applied magnetic bias fields was performed. Samples were subject to a DC magnetic field during characterisation and the complex refractive index of composites was extracted as a field dependent value across the frequency range 0.2 - 30 GHz. The results showed that the primary absorption mode of these composites, at low GHz frequencies, was strongly affected by the application of a DC bias field, changing in both intensity and position in frequency. The higher order modes showed a less strong dependence on DC bias field strength until saturation fields were reached, and the modes were suppressed. The higher order modes not being supported at high DC bias fields is indicative of the vortex domain structure being necessary for higher order spherical modes to exist in magnetic powders.
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