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This thesis investigates the dielectric properties of metal-oxide ceramics at microwave frequencies. It also demonstrates for the first time that a theory of harmonic phonon coupling can effectively predict the complex permittivity of metal oxides as a function of temperature and frequency. Dielectric ceramics are an important class of materials for radio-frequency, microwave and emergent terahertz technologies. Their key property is complex permittivity, the real part of which permits the miniaturisation of devices and the imaginary part of which is responsible for the absorption of electromagnetic energy. Absorption limits the practical performance of many microwave devices such as filters, oscillators, passive circuits and antennas. Complex permittivity as a function of temperature for low-loss dielectrics is determined by measuring the resonant frequency of dielectric resonators and using the radial mode matching technique to extract the dielectric properties. There have been only a handful of publications on the theory of dielectric loss, and their predictions have often been unfortunately unsatisfactory when compared to measurements of real crystals, sometimes differing by whole orders of magnitude. The main reason for this is the lack of accurate data for a harmonic coupling coefficient and phonon eigenfrequencies at arbitrary q vectors in the Brillouin zone. Here, a quantum field theory of losses in dielectrics is applied, using results from density functional perturbation theory, to predict from first principles the complex permittivity of metal oxides as functions of frequency and temperature. .

Materials science. --- Magnetism. --- Magnetic materials. --- Microwaves. --- Optical engineering. --- Materials Science. --- Ceramics, Glass, Composites, Natural Methods. --- Microwaves, RF and Optical Engineering. --- Magnetism, Magnetic Materials. --- Dielectrics --- Magnetic properties. --- Ceramics, Glass, Composites, Natural Materials. --- Mathematical physics --- Physics --- Electricity --- Magnetics --- Hertzian waves --- Electric waves --- Electromagnetic waves --- Geomagnetic micropulsations --- Radio waves --- Shortwave radio --- Ceramics. --- Glass. --- Composites (Materials). --- Composite materials. --- Materials --- Mechanical engineering --- Composites (Materials) --- Multiphase materials --- Reinforced solids --- Solids, Reinforced --- Two phase materials --- Amorphous substances --- Ceramics --- Glazing --- Ceramic technology --- Industrial ceramics --- Keramics --- Building materials --- Chemistry, Technical --- Clay

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This brief is based on computations performed on unary neutral and charged iron clusters, binary iron clusters, and iron clusters interacting with carbon and oxygen atoms as well as with a number of diatomics and water. The author considers geometrical structure, thermodynamic stability and electronic properties which are compared with experimental data. Special attention is paid to the dependence of total spin magnetic moments of iron clusters on their size, charge and interactions with dopant and absorbed atoms. In the dopant case, species such as 3d-metal, 4d-metal, Al, and Gd atoms are considered. In the adsorption case interactions of carbon atoms with iron clusters as the initial stage of catalyzed carbon nanotube growth are presented. Interactions of iron clusters with oxygen atoms are presented and the superexchange mechanism is discussed. Of special interest is the tracking of changes due to the evolution from a few atoms to a nanocluster.

Physical & Theoretical Chemistry --- Chemistry --- Physical Sciences & Mathematics --- Iron. --- Iron --- Magnetic properties. --- Native element minerals --- Transition metals --- Siderophile elements --- Chemistry, Physical organic. --- Nanotechnology. --- Chemistry. --- Chemical engineering. --- Physical Chemistry. --- Theoretical and Computational Chemistry. --- Industrial Chemistry/Chemical Engineering. --- Chemistry, Physical organic --- Chemistry, Organic --- Chemistry, Physical and theoretical --- Physical sciences --- Molecular technology --- Nanoscale technology --- High technology --- Chemistry, Industrial --- Engineering, Chemical --- Industrial chemistry --- Engineering --- Chemistry, Technical --- Metallurgy --- Physical chemistry. --- Chemistry, Physical and theoretical. --- Chemistry, Theoretical --- Physical chemistry --- Theoretical chemistry