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High quality factor cold sintered composites for microwave applications

El-Faouri, Sinan (2019) High quality factor cold sintered composites for microwave applications. PhD thesis, University of Sheffield.

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Abstract

Ceramic-ceramic composites intended for applications as substrates for microwave MW antennas have been fabricated and investigated. BaTi4O9 – BaFe12O19 (BT4-BF12) and BF12 – TiO2 composites were initially prepared using conventional sintering but it became obvious early on in this study that interaction between the end-member phases resulted in a significant deterioration of properties. As a consequence, cold sintering was attempted in which ceramics densify at <200 OC and therefore interdiffusion between the end members is negligible. To facilitate cold sintering, Li2MoO4 (LMO) was used instead of BT4 as one end member since it has been shown to densify at 120 OC using a pressure mediated hydrothermal route. Dense cold sintered ceramics have competitive MW properties, with quality factor, Qf = 30,000 and relative permittivity, er = 5.5. (1-x)LMO-xBF12 (0.00≤x≤0.15) were cold sintered at 120oC and their structure and properties characterized. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) confirmed that compositions were dual phase and had a dense microstructure. Composites in the xBF12-(1-x)LMO (0.0≤x≤0.15) series resonated at MW frequencies (~5-6GHz) with 5.6≤ permittivity(er) ≤5.8 and 16,000≤Qf≤22,000 GHz, despite the black colour of compositions with x > 0. The permeability (Mu) of the composites was measured in the X band (~8 GHz) and showed an increase from 0.94 (x=0.05) to 1.02 (x=0.15). Finite element modelling revealed that the volume fraction of BF12 dictates the conductivity of the material, with a percolation threshold at 10 vol.% BF12 but changes in permittivity (er) as a function of x were readily explained using a series mixing model. In summary, these composites are considered suitable for the fabrication of dual mode or enhanced bandwidth microstrip patch antennas. Conventionally sintered LMO-BF12 composites failed to produce high quality ceramics and were brittle with unwanted second phases. Cold sintering of several further composites was also performed, including LMO-TiO2 and LMO-BaTiO3. Of particular note however, were dense (≃95%) (1-x)K2MoO4-xBF12 composites which resonated at MW frequencies with a 5.6≤ permittivity(er)≤5.8, temperature coefficient factor of resonance, -66≤TCF≤-39 ppm/°C and Qf ≃14,000 GHz for 0.05≤x≤0.15. No interaction was noted between the two end members according to XRD, SEM and chemical mapping using energy dispersive X-ray spectroscopy. (1-x)LMO-xLi1.5Al0.5Ge1.5P3O12 (LAGP) were also fabricated but as anticipated this composite became conducting for low values of x. TCF varied from -169 to -43 ppm/°C for x=0.05≤x≤0.15 but high relative density (94%) could be achieved. Overall, cold sintering proved a successful route for the fabrication of ceramic-ceramic composites for MW applications with a number of systems showing great promise. However, tuning TCF to zero proved problematic within the systems described due to the limited range of x (<0.2) for which dense ceramics could be achieved.

Item Type: Thesis (PhD)
Academic Units: The University of Sheffield > Faculty of Engineering (Sheffield) > Materials Science and Engineering (Sheffield)
Identification Number/EthosID: uk.bl.ethos.772924
Depositing User: Mr. Sinan El-Faouri
Date Deposited: 29 Apr 2019 08:16
Last Modified: 25 Sep 2019 20:07
URI: http://etheses.whiterose.ac.uk/id/eprint/23712

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