Microstructural Engineering of Bulk Superconductors

The ability of bulk superconducting materials to generate higher magnetic fields than permanent magnets enables their potential application in portable, medium-field magnet systems. Both MgB₂ and (RE)BCO are promising materials for these applications. MgB₂ has a reasonably high transition temperature (T_c = 39 K) and can be fabricated in bulk form by simple and cheap powder processing methods. However, the superconducting performance of MgB₂, especially its macroscopic critical current density (J_c), requires further improvement in order to provide a suitable magnetic field for practical applications. On the other hand, (RE)BCO has much better high field properties, and can ultimately trap much higher fields, but bulk pellets need to be processed as single grains to avoid grain boundaries that are weak links. This means that a slow melt-processing method is required that is less readily scalable to larger samples and mass production. Here, we will report a summary of recent work on MgB₂ and (RE)BCO bulks that has resulted from a collaboration between Oxford and Cambridge Universities, particularly focusing on understanding how microstructure can be controlled to improve performance in these materials.<br/>Regarding MgB₂ bulks, we have investigated a variety of novel strategies for improving the macroscopic J_c in ex-situ MgB₂ bulks processed by Field Assisted Sintering Technology and ultra high pressure synthesis. The work focuses on two elements: (i) increasing the connectivity between MgB₂ particles, and (ii) improving the intrinsic J_c of the MgB₂ by optimizing the pinning landscape. The first of these traditionally requires the use of higher processing temperatures that results in the coarsening of the microstructure and a decrease in intrinsic J_c. Therefore, we have developed a new method for improving connectivity at relatively low temperatures. In addition, we have explored the effects of a variety of additions, such as Y₂O₃, hBN and cBN together with the high energy ball-milling process on the microstructure and superconducting properties of MgB₂.<br/>We will also report recent work on understanding the microstructural evolution in the melt-growth process used to fabricate high performance (RE)BCO bulks. In particular, the choice of the RE element plays a key role in determining the growth rate of single grains, the precise microstructure, mechanical properties and final superconducting properties of the bulk samples, and also the likelihood of RE substitution onto the Ba site which can degrade the performance. A number of rare elements including Nd, Sm, Eu, Gd, Dy and Y have been incorporated into (RE)BCO single crystals, and the superconducting properties of the bulks carefully studied, and it is known that the final microstructure varies with the RE element as a result of changes in the melting temperatures of the RE–123 phase and the resulting growth rates. In this work, we have studied the growth and microstructure of (RE)BCO single crystals with RE = Gd and Eu, where the degree of Ba substitution is known to be very different. We have carried out detailed microstructural characterization of the phase distribution and composition using high resolution electron microscopy to understand the effects of Gd and Eu on the uniformity of the samples, the distribution of the secondary 211 phase, porosity and chemical variations in different regions of the melt-grown single crystals. In addition to discussing processing innovations to improve uniformity and increase the size of the bulks, we will report on new experiments using Atom Probe Tomography aimed at understanding the role of grain refiners used in the melt-processing of (RE)BCO bulks.