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Abstract The thesis describes electrically tunable microwave devices utilising low sintering temperature, screen printable Barium Strontium Titanate (BST) thick films. The work has been divided into two parts. In the first section, the fabrication and microwave characterisation of BST material based structures compatible with Low Temperature Cofired Ceramic technology (BST-LTCC) are presented. Three different fabrication techniques, namely: direct writing, screen printing and via filling techniques, were used for the realisation of the structures. A detailed description of these fabrication techniques is presented. The dielectric properties such as relative permittivity, static electric field dependent tunability and loss tangent of BST-LTCC structures at microwave frequencies were characterised using coplanar waveguide transmission line and capacitive element techniques. The measured dielectric properties of BST-LTCC structures realised with the different fabrication methods are presented, compared and discussed. The second section describes tunable microwave devices based on BST-LTCC structures. A frequency tunable folded slot antenna (FSA) with a screen printed, integrated BST varactor is presented. The resonant frequency of the FSA was tuned by 3.2% with the application of 200 V external bias voltage. The impact of the BST varactor on the total efficiency of the antenna was studied through comparison with a reference antenna not incorporating the BST varactor. A compact, frequency tunable ceramic planar inverted-F antenna (PIFA) utilising an integrated BST varactor for mobile terminal application is presented. The antenna's resonant frequency was tuned by 3% with an application of 200 V bias voltage. Frequency tunable antennas with a completely integrated electrically tunable BST varactor with silver metallisation are introduced in this work for the first time. The integration techniques which are described in this thesis have not been previously reported in scientific literature. The last part of the thesis presents a microwave delay line phase shifter operating at 3 GHz based on BST-LTCC structures. The figure of merit (FOM) of the phase shifter was measured to be 14.6 °/dB at 3 GHz and and the device employs a novel structure for its realisation that enabled the required bias voltage to be decreased, while still maintaining compliance with standard screen printing technology. The performance of the phase shifter is compared and discussed with other phase shifters realised with the BST thick film process. The applications of BST-LTCC structures were demonstrated through frequency tuning of antennas, varactors, and phase shifters. The low sintering temperature BST paste not only enables the use of highly conductive silver metallisation, but also makes the devices more compact and monolithic.

Abstract The thesis describes the development of a novel LTCC material system for RF and microwave telecommunication purposes. The work has been divided into three parts. In the first section, the compositional and firing properties of this novel LTCC dielectric have been studied as well as its thermomechanical and dielectric properties. The second section describes the multilayer component preparation procedure for the ceramic material including tape casting and lamination parameters and the selection of the conductor paste. In the last section, the novel LTCC material system has been used to demonstrate its properties in RF multilayer resonators and a bandpass filter. The dielectric material for the novel LTCC system was prepared using magnesium calcium titanate ceramic, the firing temperature of which was decreased to 900°C by the addition of a mixture of zinc oxide, silicon oxide and boron oxide. The powder was made without any prior glass preparation, which is an important process advantage of this composition. The fired microstructure was totally crystalline with high density (3.7 Mg m-3) and low porosity (0.5 %). The mechanical properties were virtually identical to the values of the commercial LTCCs, but the higher thermal expansivity makes it most compatible with alumina substrates. The dielectric values were also good. The permittivity was 8.5 and the dissipation factor (0.9·10-3 at 8 GHz) less than that of the commercial LTCCs. Furthermore, the temperature coefficient of the resonance frequency was demonstrated to be adjustable between the range of +8.8 ... -62 ppm/K with a simple compositional variation of titanium oxide. The slurry for the tape casting was prepared using poly(vinyl butyral) -base organic additives and the 110 μm thick tapes had a smooth surface (RA < 0.5 μm). The multilayer components were prepared using 20 MPa lamination pressure, 90°C temperature and 1 h dwell time. The most suitable conductor paste for this composition was found to be commercial silver paste (duPont 6160), which produced satisfactory inner and outer conductor patterns for multilayer components. Finally, resonators with a resonant frequency range of 1.7 ... 3.7 GHz were prepared together with a bandpass filter suitable for the next generation of telecommunication devices. This demonstration showed the potential of the developed novel LTCC material system at high RF frequencies.

The frequency shifts and Q changes of a resonant microwave cavity were utilized as a basis for determining microwave properties of solids and liquids. The method employed consisted of varying the depth of penetration of a cylindrical sample of the material into a cavity operating in the TM0 1 0 Mode. The liquid samples were contained in a thin-walled quartz tube. The perturbation of the cavity was achieved by advancing the sample into the cavity along the symmetry axis by employing a micrometer drive appropriately calibrated for depth of penetration of the sample. A differentiation method was used to obtain the half-power points of the cavity resonance profile at each depth of penetration. The perturbation techniques for resonant cavities were used to reduce the experimental data obtained to physical parameters for the samples. The probing frequency employed was near 9 gHz.

This thesis represents one phase of a joint effort of research on the properties of liquids and solids. This work is concerned primarily with the microwave properties of solids. In this investigation the properties exhibited by conductor and semiconductor materials when they are subjected to electromagnetic radiation of microwave frequency are studied. The method utilized in this experiment is the perturbation of a resonant cavity produced by introduction of a cylindrically shaped sample into it.

The aim of this study was to improve electrical properties of conductive polymer composites. For this purpose, various studies were performed using different materials in this dissertation. In order to investigate the effect of alternative composite preparation methods on electrical conductivity, nylon 6/carbon black systems were prepared by both in-situ polymerization and melt-compounding techniques. When compared with melt compounding, in-situ polymerization method provided enhancement in electrical conductivity of nylon 6 composites. Furthermore, it was aimed to improve electrical conductivity of polymer composites by modifying surface chemistry of carbon black. 1 wt. % solutions of 3-Aminopropyltriethoxysilane and formamide were tried as chemical modifier, and treated carbon black was melt mixed with low-density polyethylene (LDPE) and nylon 6. According to electron spectroscopy for chemical analysis (ESCA), chemicals used for surface treatment may have acted as doping agent and improved electrical conductivity of polymer composites more than untreated carbon black did. Formamide was more effective as dopant compared to the silane coupling agent. In order to investigate electromagnetic interference (EMI) shielding effectiveness and dielectric properties of conductive polymer composites, 1, 2 and 3 wt. % solutions of formamide were tried as chemical modifier and treated carbon black was melt mixed with poly(ethylene terephthalate) (PET). Composites containing formamide treated carbon black exhibited enhancement in electrical conductivity, EMI shielding effectiveness and dielectric constant values compared to composites with untreated carbon black. In order to enhance electrical conductivity of polymer composites, the selective localization of conductive particles in multiphase polymeric materials was aimed. For this purpose, carbon nanotubes (CNT) were melt mixed with polypropylene (PP)/PET. Grinding, a type of solid state processing technique, was applied to PP/PET/CNT systems to reduce the average domain size of blend phases and to improve interfacial adhesion between these phases. Grinding technique exhibited improvement in electrical conductivity and mechanical properties of PP/PET/CNT systems at low PET compositions. To investigate application potential of conductive polymer composites, polyaniline (Pani)/carbon nanotubes (CNT) composites were synthesized and electrochemical capacitance performances of these systems, as electrode material in electrochemical capacitors, were studied. Polyaniline/carbon nanotubes composites resulted in a higher specific capacitance than that of the composite constituents. Pseudocapacitance behavior of Pani might contribute to the double layer capacitance behavior of nanotubes. Additionally, as an alternative to Pani/CNT systems, polyaniline films were deposited on treated current collectors and electrochemical capacitance performances of these electrode systems were investigated. The highest specific capacitance of polyaniline/carbon nanotubes composites was 20 F/g and this value increased to 35.5 F/g with polyaniline film deposited on treated current collector.

Abstract A stripline structure is closed and therefore protected against surrounding EMI and it is easy to bury in multiplayer structures, which offer higher circuit density. This thesis focuses on the evaluation (and verification via actual structures) of the correct simulation of striplines and, as a new aspect, the advantages of using a dual-stripline. Multiple design methods and electromagnetic simulation systems were tested and properties of these are compared. For a reliable design it is still necessary to use at least two tools, at first a very fast tool having excellent circuit parameter optimization methods and then some electromagnetic simulator, which can be used to the complete the realizable layout. That is, because all the electromagnetic simulators suffer from the same limiting factors, the memory capacity of the computer and the unacceptable calculation time. It has been discovered through modelling that the "cat-eye" shape having many more and larger local inaccuracies at the thinned edge areas of the sintered conductor in LTCC structures increases the conductor losses. Therefore it is important to develop new manufacturing methods capable of producing better-shaped conductors. A combination of broadside coupled parallel connected striplines has been tested both in High Temperature Superconducting ( HTS ) and LTCC materials. A two-conductor stripline, a dual stripline, raises the power handling capability of a microwave bandpass HTS filter. In addition, it offers the possibility to use a normal metal protection layer at the surface of the superconductor without degradation of electrical properties, thus increasing the power handling capability even more. The dual stripline solution in LTCC would offer some preferable properties in high power filters only. The shape of the ground plane used for trimming the coupling between resonators was also found to have a remarkable influence on the quality factor of the resonator. A quite narrow ground strip can offer a much better quality factor with the same coupling level than a meshed or continuous ground plane, but it requires accurate design and manufacturing methods. It would help to design filters with lower loss in the passband without compromises in the attenuation outside the passband.