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Title: Fabrication of high temperature superconducting composites for microwave applications
Author: Janurudin, Juliana M.
ISNI:       0000 0004 6500 6286
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2016
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This thesis describes the fabrication of high temperature superconducting (HTS) composites synthesised by casting and 3-D printing techniques for a variety of potential applications such as microwave devices and metamaterials. The fabrication of metamaterials, where electromagnetic (EM) waves are manipulated by varying the effective dielectric constant (εeff) and static magnetic permeability (μeff) in 3- dimensions, is a major area of research worldwide. One approach to synthesising HTS composites with a controllable spatial distribution of electromagnetic properties is the synthesis of polymer composites comprising of multiple volume fractions of superconducting particles, which results in temperature-dependent composite materials. YBa2Cu3O7 (YBCO-123) superconducting powder was used as filler in the HTS composites. The effect of particle sizes and the non-superconducting Y2BaCu2O5 (YBCO-211) phase on the magnetisation properties of the YBCO filler have been analysed using a SQUID magnetometer. The magnetisation analyses on two types of YBCO fillers proved that YBCO powder ground from top seeded melt growth (TSMG) YBCO bulks with median particle size, d50 = 59 μm possesses stronger diamagnetic properties at 77 K compared to commercially available YBCO powder with a size of d50 = 5 μm. The melt-grown YBCO powder also possesses an irreversible component to the magnetisation owing to flux trapping by the YBCO-211 phase. A custom-made polytetrafluoroethylene (PTFE) mould was designed and fabricated for the purpose of synthesising HTS composites via casting. The EM properties of the cast HTS composites were measured at room temperature and 77 K using a Vector Network Analyzer. The EM properties of these HTS composites are correlated with the homogeneity and density of YBCO particle distributions in the polymer matrix measured using scanning electron microscopy (SEM). It has been shown that the εeff values of the HTS composites were enhanced with increasing volume fraction of YBCO filler at both temperatures. A large drop of μeff values with volume fraction is observed at 77 K, owing to the diamagnetic response of the superconducting particles. Thus the refractive index of the composites could be controlled by both the volume fraction of YBCO filler and temperature. In order to realise spatial variations in EM properties, a process has been developed for the fabrication of HTS composites via 3-D printing. The extrusion process was optimised to produce 20% YBCO loaded acrylonitrile butadiene styrene (ABS) feedstock filaments. Finer YBCO filler (𝑑50 = 20 𝜇𝑚) was used to print more intricate 3-D structures such as an anisotropic material and split-ring resonators, but the EM properties were suppressed, suggesting a particle size limitation of the 3-D printing. By contrast, YBCO/ABS composites have been printed with good control of YBCO volume fraction, and good agreement with the cast HTS composites at similar volume fraction of εeff = 5.1 and μeff,77K𝐾 = 0.82 at 77 K achieved.
Supervisor: Grovenor, C. R. M. ; Speller, S. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available