Fabrication of large antenna substrates of monolithic spatially variable ceramics and an optimization framework for nano-antennas
Berkün, Işıl (2009) Fabrication of large antenna substrates of monolithic spatially variable ceramics and an optimization framework for nano-antennas. [Thesis]
Official URL: http://192.168.1.20/record=b1293708 (Table of Contents)
The aim of this thesis is driven by two main challenges in the antenna and propagation community: The possibility to manufacture exact replicas of spatially variable dielectric substrates for low frequency Radiofrequency (RF) applications and to achieve performance enhancements via formal optimization techniques for high frequency applications such as nano-antennas. In the RF and optics community, metamaterials have gained significant interest due to their extraordinary properties which are not accessible in nature. Textured composites with novel properties allow for the realization of state-of-the-art devices which are functionalized through spatially variable properties of dielectrics, magnetics and polymers. The possibility of spatially controlling permittivity and permeability at the preferred frequency and the capability of realizing multi-material volumetric variations is an ancient vision in the RF community. One such technique has been proposed and adopted to produce spatially variable ceramic substrates of small size (2" square) and assembled to construct a UHF SATCOM antenna substrate. In the first part of the thesis, the objective is to use earlier proposed Dry Powder Deposition (DPD) technique for producing large monolithic substrates with spatial variation of ceramic constituents that will allow for impressive performance enhancements as dictated by design results. Commercially available LTCC powders namely Calcium Magnesium Titanates (MCT) of dielectric permittivities 15, 20, 70 and loss tangent < 0.0015 are used as the ceramic constituents. Thermogravimetric analysis of each constituent powder is used to analyze efficient removal of 1-3 % binder content of spray dried MCT powders and achieve complete sintering of their textured composites. Also, a detailed analysis of the process parameters such as compaction pressure and cosintering temperature within the DPD method is carried out. As a result, smooth and large substrates with sizes up to 82mm x 82mm of monolithic dielectric textured composites were obtained by cosintering at optimal conditions. Cracks and unwanted defects such as porosities in textured composites were eliminated. Density measurements and SEM stressed that final substrates obtained were over %98 dense ceramic constituents. Microstructure characterizations of pellets made of sintered constituent material were carried out by SEM and dielectric permittivity measurements were performed. In the second part of the thesis, the objective is to develop a basic framework to optimize a nano antenna's intensity enhancement and absorbed power according to variables such as length, thickness, width and wavelength using gradient and heuristic based methods (sequential quadratic programming and genetic algorithms). This framework will allow for more effective assessment of high-frequency antenna applications subject to multiple competing performance criteria and complex design variables in the future including the effect of material substrates, hence enable novel designs with superior performance for emerging plasmonic applications as was the case for the SATCOM antenna design in the first part of the thesis.
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