Large Grain Bulk Superconductors for High Field Motor Applications

This PhD project will focus on investigating the physical properties of bulk high temperature superconductors, which have shown great technological potential in recent years. Superconductors are materials that exhibit zero electrical resistance when cooled to below a characteristic transition temperature, allowing dissipation-less electrical transport and very high current densities to be sustained as a result. In addition to this unique property, the Re-Ba-Cu-O family of bulk melt processed high temperature superconductors [where Re denotes rare earth elements, i.e. Y-Ba-Cu-O, Gd-Ba-Cu-O, and Sm-Ba-Cu-O] have the ability to "trap" and maintain tremendous magnetic fields, as they currently hold a record trapped field of 17.6 T (a record set by the Bulk Superconductivity Group, University of Cambridge, 2014) between two 25 mm diameter 12 mm thick samples. This field is an order of magnitude greater than the best Nd-Fe-B permanent magnets available, enabling the realisation of an array of applications that aim to utilise such strong fields over a compact volume/weight to achieve superior power density, energy efficiency and reduced energy wastage that conventional machine counterparts simply cannot achieve. Most notably, it has been proposed that superconductivity may be the enabling technology to new generations of electric motors and generators, flywheel energy storage systems, stable levitation devices, and other high field applications, including MRI scanners and water purification devices. Ultimately, economical, technological and environmental benefits should follow from the accomplishment of such novel innovative concepts.
Re-Ba-Cu-O bulk superconductors are ceramic materials, and therefore are brittle in nature. Their ability to trap immense fields due to their high current density is not only of great technological value, but also great detriment, as it puts them under considerable magnetic tensile stresses. These stresses scale with the field they trap. These electromagnetic stresses have been reported to be equivalent to three tonnes of force over a 25 mm sample and have been shown to literally tear samples apart when operating in extremely high fields and low temperatures, making the mechanical properties of these functional materials the limiting factor. Furthermore, unlike permanent magnets which are fundamentally limited in terms of the maximum achievable field or electromagnets where performance often scales with size, the superconducting/field-trapping performance of superconductors can be improved simply by lowering the temperature. Therefore, at lower temperatures, there is the opportunity to achieve even greater fields than the current record of 17.6 T provided samples are mechanically stable. In addition, to be compatible with proposed applications such as rotating electric machines, the response of these materials to the high tensile stresses present in high frequency rotational motion, has to be comprehensively studied as well.
However, despite the importance of mechanical strength in determining the performance of these functional materials, little study has been carried out on characterising them, predominantly due to the difficulty and complexity in characterising ceramic materials in general. A common issue is the large distribution/scatter of strengths associated with these materials that implies a very large sample size, and thus repetition, is required to effectively characterise them.
To reliably implement this functional ceramic in commercial or industrial applications, they will need to undergo rigorous physical characterisation, in particular mechanical strength testing. Mechanical testing, a standard and crucial aspect of any design and manufacturing process, is used to first, achieve assurance of the sample performance (by determining what stress these superconductors break at, we can validate the relationship between sample strength and maximum trapped field, as well as estim