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Title: Clinical modification of MgB₂ for optimisation of critical current density
Author: Chen, Soo Kien
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2006
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This work focuses on the optimisation of critical current density, Jc of bulk polycrystalline MgB2 via studying the influence of boron precursor powder, nominal Mg non-stoichiometry and finally by chemical modification. On the influence of the nature of the boron precursor on the superconducting properties of MgB2, Jc’s for the MgB2 made from high purity amorphous boron are at least a factor of three higher than typical values measured for standard MgB2 samples made from amorphous precursors. Two possible mechanisms are proposed to account for this difference. Samples made from crystalline boron powders have around an order of magnitude lower Jc’s compared to those made from amorphous precursors. X-ray, superconducting transition temperature, Tc and resistivity studies indicate that this is as a result of reduced current cross section due to the formation of (Mg)B-O phases. The samples made from amorphous B contain far fewer Mg(B)-O phases than crystalline B despite the fact that the amorphous B contains more B2O3. The different reactivity rates of the precursor powders can account for this anomaly. The influence of Mg content was investigated in a series of samples with systematic variation of nominal Mg non-stoichiometry. Jc(H) was found to be influenced significantly with variation of Mg while leaving Tc unchanged. Mg deficient samples show higher degree of disorder as inferred from the Raman spectroscopy data. The Mg deficient samples also show higher Hirr and Hc2 compared to samples with reduced Mg vacancy as the Mg nominal content increases. Based on the results from Raman spectroscopy studies, XRD, resistivity and normalised Jc(H), a phase diagram for Mgx2 – Mg is proposed. For chemical modification, different reaction conditions and amount of Cu, GaN and Dy2O3 additions into MgB­­2 during the in situ reaction enhance Jc at 6k and 20K up to 5T without changing Tc appreciably compared to undoped samples whereas ZrO2 additions combined with ball milling degrade Tc and Jc. Attempt of Cu substitution in varying annealing temperatures does not result in doping of lattice structure. The optimisation of Jc is found to rest on the competing effects between the improved grain connectivity and MgB2 phase formation versus Mg-Cu secondary phase formation which decreases Jc. Additions of Ga in the form of GaN into MgB2 produce plate-like grains and enhance Jc. Dy­2O3 additions result in the highest Jc at both 6K and 20K up to 5T among the additives studied. The best sample (only 0.5 wt.% Dy2O3) had a Jc ~ 6.5 X 105 Acm-2 at 6K, 1T and 3.5 x 105 Acm-2 at 20K, 1T, around a factor of 4 higher compared to the best pure sample, and equivalent to hot-pressed or nano-Si added MgB2 at ≤ 1T. Even distributions of nano-scale precipitates of DyB4 and MgO were observed within the grains. The room temperature resistivity decreased with Dy2O3 indicative of improved grain connectivity.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available