Year

2004

Degree Name

Doctor of Philosophy (PhD)

Department

Institute for Superconducting and Electronic Materials - Faculty of Engineering

Abstract

Several projects are reported in this thesis. These projects deal with the processing of MgB(subscript 2) bulk samples and metal-sheathed wires or tapes. The main focus is on the fabrication aspects. The correlation between fabrication parameters and superconductivity properties is studied extensively. MgB(subscript 2) tapes and wires were prepared by the Powder-In-Tube (PIT) method. Ag, Cu and Fe were chosen to act as sheath materials. The interaction between the sheath materials and the MgB(subscript 2) core was studied. It was found that the MgB(subscript 2) superconducting tapes sheathed in copper exhibited a higher critical current density than Fe-sheathed tapes if prepared at temperatures below 850(degrees)C. However, in term of the core composition and microstructure, the Fe sheathed tapes had the most pure MgB(subscript 2) phase only with a few micro cracks regardless of the heat treatment conditions. Increases in the overall mechanic deformation rate did not influence the J(subscript c) value of the Fe-sheathed tapes, whereas the J(subscript c) of the Cu-sheathed tapes grew by up to an order of magnitude with an increasing thickness reduction rate. The Ag-sheathed tapes usually showed very poor performance in term of the core microstructure, composition and critical current density, indicating that Ag is not appropriate for sheathing MgB(subscript 2). MgB(subscript 2) Fe-sheathed wires were made by both in-situ and ex-situ approaches. The wires made using the ex-situ method did not show such high I(subscript c) as those made by the in-situ method. In the case of 7-filament wires, the adoption of an additional mechanic deformation in in-situ method can increase J(subscript c) by a factor of 2.7 compared with the initial single filament wires. After further sintering, J(subscript c) can increase by 4.3 times. However, in 19 filament and 49 filament wires, after second sintering, I(subscript c) was blocked by micro cracks and impurities. Effects on the wire induced by exposure to air were studied, with J(subscript c) decreasing by 20% after aging in air for 4 months. The decrease was attributed to the oxidation of the remnant unreacted Mg oxidation after long time exposure to air. Different mechanical deformations were adopted to make Fe-sheathed MgB(subscript 2) wires. It was found that square rolling is more beneficial than drawing in term of improving I(subscript c). The mass density of the MgB(subscript 2) core was improved after further mechanical deformation, but the I(subscript c) did not increase accordingly. The cracks caused by mechanical deformation are very difficult to heal with post annealing. The formula (MgB[subscript 2])(subscript 1-x): (Mg+2B)(subscript x) was used to fabricate MgB(subscript 2) bulk (where x varies from 0 to 1). The critical current density as a function of the applied field is larger over the entire field range for samples with x?0. An addition to the un-reacted Mg+2B mixture to the MgB(subscript 2) powder enhances the transparency of the grain boundaries, introducing an MgB(subscript 2) matrix that is formed in-situ with embedded small MgB(subscript 2) ex-situ particles (for 0?x?1). The grain boundaries are found to be responsible for the pinning of vortices. The larger x is, the smaller the amount of pinning boundaries, so that J(subscript c)(B) decays faster. MgB(subscript 2) was doped with nanometer size SiC particles. Si and C were introduced into the MgB(subscript 2) crystal lattice due to the counter balanced atomic sizes of Si and C, on average nearly coinciding with the size of B. T(subscript c) was reduced slightly by the doping. However, the doping was shown to strongly improve J(subscript c)(H) dependence at high applied fields. A pinning mechanism explaining this improvement is suggested, in which the main intra-granular ingredients are the dominant contribution of a larger number of dislocations induced by the substitution effect and a secondary contribution due to the nano-inclusions. The processing parameters for SiC doped MgB(subscript 2) wires have been studied systematically. The sintering temperature and time have strong effects on the T(subscript c) of the superconducting material. Higher temperatures and longer dwell times are beneficial to the T(subscript c) but not to the J(subscript c). Samples sintered at 750(degrees)C and 850(degrees)C exhibit higher J(subscript c) than those sintered at 680(degrees)C and 950(degrees)C. Sloww cooling has a beneficial effect on J(subscript c) field dependence. Uniaxial pressures (250 and 680 Mpa) improve the mass density but have a negligible effect on J(subscript c). The doping effects of C, Si and C+Si particles on J(subscript c)(H) have been studied. Nano-Si, C or (Si+C) dopant particles can improve J(subscript c)(H) performance at higher fields compared with undoped MgB(subscript 2). At 20K and 4T, J(subscript c) of the nano-Si doped sample reached 10(superscript 4)A/cm(superscript 2) which is 100 times higher than that of the coarse-Si doped sample. The J(subscript c) of nano-C doped samples is 10 times higher than that of the coarse-C doped sample. The nano-(Si+C) co-doped samples show similar behaviour to the nano Si doped sample whatever the ratio of Si to C. Coarse C or Si doping decreased J(subscript c) at all fields and all temperatures measured. MgB(subscript 2) wires have been synthesised with different B and Mg powders as well as different Mg: B ratios. The effects of B powder on the J(subscript c) and T(subscript c) of this superconductor are noticeable. Impurities in the B powder can suppress both the T(subscript c) and J(subscript c). Oxidizing the Mg powder did not have a strong influence on T(subscript c), although the effects on J(subscript c) are significant when the Mg powder is severely oxidized. The influence of lightly oxidized Mg powder on both T(subscript c) and J(subscript c) is negligible. If the MgB(subscript 2) composition fluctuates from the normal stoichiometricity of 1:2, T(subscript c) decreases with excessive Mg composition. A minor Mg deficiency (x?0.1 in Mg[subscript 1-x]B[subscript 2]) did not influence T(subscript c) strongly. J(subscript c) did not change dramatically with the variations in composition. A two-step sintering was used to synthesize MgB(subscript 2). However, although the mass density achieved in higher value, the critical current density dropped dramatically after the second grinding and sintering. The higher the first sintering temperature, the smaller the critical current density. The magnetic screening of the superconducting MgB(subscript 2) filaments in single and multifilament wires by means of an iron sheath has been directly visualized. The observations showed that the magnetic flux induced inside the iron sheath is higher than the applied field. The superconducting filaments exhibit an extended Meissner state and are free from the induced magnetic flux at least up to B = 0.2 T for both kinds of wires. This was observed by the magneto-optical technique used. Global magnetization measurements showed that the screen effect could be dominant up to B= 1.5 T and that it is much more effective in increasing fields than in decreasing fields. [Note: this abstract contained scientific formulae that would not come across on this form. Please see the 01Front files abstract for the full details.]

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