Magnesium Diboride (MgB₂) bulk superconductor has been manufactured for use in superconducting fault current limiters (SFCLs) via in situ reaction. SFCLs have proven to be a viable means for limiting surge currents on power lines by dissipating fault energy as the superconductor quenches. As the current limiting behaviour is determined by the normal resistance (Rn), research has been conducted to evaluate an effective means to increase Rn for bulk superconducting MgB₂. Other researchers have previously looked into the improvement and optimisation of the critical current, Jc, by carbon doping and other flux pinning techniques. Carbon doping has been confirmed as a means to increase Rn, and was implemented by chemical vapour deposition (CVD) at 600 ºC in a tubular furnace apparatus. Intragranular doping was achieved by CVD on the boron precursor powder. In situ manufacturing of MgB₂ bulk was performed using the reactive liquid magnesium infiltration technique. Carbon doping provided an adequate increase in Rn, for a small decrease in the critical temperature, Tc. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2012.

PrBa₂Cu₃O₆₊_x is anomalous among the compounds which can be made by substituting different rare-earth ions for yttrium (Y) in YBa₂Cu₃O₆₊_x:it is the only compound which has the same structure as YBa₂Cu₃O₆₊_x, and yet does not superconduct. This unusual property makes it an important system to study, since the differences between the two compounds could produce theoretical insights into the mechanisms of cuprate, or high-temperature, superconductivity. This thesis describes neutron scattering investigations of the magnetic properties of PrBa₂Cu₃O₆₊_x. I have investigated the magnetic ordering in well-characterised, single-crystal samples, both with and without Al-contamination. In the data analysis, proper account is taken of the sample mosaicity and the different intrinsic peak shapes encountered. Some of the most striking new characteristics reported here are that (i), there is a clear interaction between the Pr ions and the Cu-O₂ planes, (ii), the ordered Pr moments tilt away from the c-axis (as suggested previously by Mossbauer spectroscopy), and (iii), there is two-dimensional ordering in the Al-containing crystals. I show how observed trends in the magnetic ordering, as O- and Al-contents vary, may be understood in terms of charge redistribution involving the hybridisation of the Pr ions. I have also examined the crystal field transitions of the praseodymium ion. Because of the difficulties previously encountered in measuring these in PrBa₂Cu₃O₆₊_x, I have used instead the compounds PrO₂ and Y_0.3Pr_0.7Ba₂Cu₄O₈, which contain praseodymium in a chemically similar environment. Y_0.3Pr_0.7Ba₂Cu₄O₈ shows the same broadening effects as PrBa₂Cu₃O₆₊_x; PrO₂ shows signs of mixed-valency, with temperature-dependent lifetime effects. The application of symmetry considerations and the point-charge model to the crystal field measurements is considered in some detail.