Investigation on the link between microstructures and superconducting properties of MgB2 bulk samples

The work in this thesis is part of an EPSRC project (reference: EP/P026427/1) that involves designing and building a desktop-sized magnet applied in magnetic drug targeting. Bulk superconductors can be used to substitute conventional ferromagnets because of the higher magnetic field they can generate. High temperature superconducting materials suffer from weak link behaviour of grain boundaries, so a complicated top-seed growth method is required. Among low temperature superconductors, MgB2 has a reasonably high transition temperature (Tc = 39 K) and can be fabricated in the bulk form by simple and cheap powder processing methods, but the macroscopic Jc values (Jc,macro) require further improvement to provide a suitable magnetic field for practical applications. Improving intrinsic properties, such as upper critical field and flux pinning strength can increase the capability of superconductors generating high Jc values. To enable macroscopic current circulating the whole sample, density, grain connectivity and anisotropy need to be systematically investigated. The goal of this thesis is to enhance the superconducting performance of MgB2 bulk samples and explore how different microstructures influence magnetization and Jc values. Theoretical models are established to simulate the contribution of Jc,macro and currents flowing locally (Jc,micro) to the total ΔM (the difference between the magnetization in the increasing and the decreasing field branches) when samples have anisotropy, inhomogeneity or connectivity problems. Five types of microstructures are introduced and simulated by the duplex microstructure model as the references for the superconducting analysis in experimental chapters. The ultra-high pressure sintering method was applied to produce MgB2 bulk samples with nano-scale structure. The sample processed at 900 oC and 5 GPa shows an extraordinary ΔM(Bapp) performance, achieved 5×106 A/m at 4.2 K, 6 T, 30 times and more than 100 times higher than samples prepared at 800 oC and 1100 oC at 5.5/5 GPa. Field-assisted sintering combined with high-energy ball milling is another ex-situ method to prepare MgB2 bulk samples within a short time (5 min). 5 wt% Mg addition is demonstrated to improve the low- and medium-field ΔM performance. The hBN addition in nanoparticle form is more effective than cBN additions with large sizes. 0.5 wt% hBN+MgB2 samples with powder milled for 12 h and 1 wt% hBN +MgB2 samples with powder milled for 6 h have ΔM values around 2.6 times than pure MgB2 at 20 K, 2 T, which may result from good macroscopic connectivity as well as the increased pinning strength by Mg(N,B) nano impurities. Co-doping 5 wt% Mg and 1 wt% hBN and 6 h of milling have a synergistic effect on the improvement of ΔM(Bapp) curves. However, the combination of 5wt% Cu and 2wt% cBN is detrimental to the MgB2 performance. All results in this dissertation stressed the necessity of the wide microstructural characterisation to interpret the superconducting results reliably.