The high-transition-temperature superconductors (HTS) present a number of challenging materials science problems whose solutions must precede their use in applications. This article offers a view on our collective progress toward resolution of one central HTS issue, the “weak-link” character of high-angle grain boundaries.
Why Are HTS Grain Boundaries of Such Interest?
The properties of high-angle grain boundaries control the macroscopic electromagnetic character of all superconducting copper oxides that are produced in polycrystalline form. In particular, grain boundaries limit the transport critical current density (Jct) and determine its dependence on applied magnetic field (H) and temperature (T). Jct is the maximum macroscopic current density that the superconductor can support in the nondissipative state. H and T are the two most important environmental variables for applications of superconductivity. Thus, the Jct(H, T) characteristic largely dictates the types of applications for which a superconducting material can be used successfully.
High-angle grain boundaries are the immediate obstacle to further development of materials for applications that require large Jct values in high magnetic fields. The problem arises because most high-angle grain boundaries act like Josephson-coupled weak links. The characteristic properties of such junctions are a reduced zero-field Jct value and, more importantly, a strongly magnetic-field-dependent Jct that can decrease by more than an order of magnitude in even weak fields of a few millitesla (Fig. 1). Such Jct(H) characteristics are clearly a very serious problem for high-field magnet applications such as motors, generators, energy storage systems, and MRI machines.