Grain boundaries in HTS materials
Cuprate superconductors and, in particular, YBa2Cu3O7-x (YBCO) combine large upper critical field and the potential for very high critical currents. However, the development of practical superconductors based on YBCO has revolved around the complexity of achieving high critical current densities in polycrystalline materials. The problem lies in the build-up of charge inhomogeneities and strain at the grain boundaries (GBs), regions of mismatch between crystallites with misoriented crystalline axes, which act as weak links that drastically limit the current flow. To overcome this problem in practical conductors, it has been necessary to develop expensive and sophisticated crystallographic texture fabrication processes that eliminate all but low-angle GBs.
Our goal is the understanding of the processing – structure – superconducting properties relationship for the GBs in HTS, with the underlying idea of disclosing the fundamental mechanisms limiting the current transfer across the GBs. In particular, our studies focus on single GB thin films grown by pulsed laser deposition (PLD) and devices specifically designed for field-effect or strain experiments.This activity is intended as use-inspired basic research, the final drive being to enable the control of the physical properties required for the advancement of the materials in the form of wires and tapes.
We address the issue of the vortex phase diagram in novel superconducting materials, both for investigating the basic physics of the superconductivity and for evaluating the perspectives of new materials for practical applications. We have published one of the very first papers on the magnetic and thermodynamic properties in the superconducting oxypnictides. This work evidenced the electromagnetic granularity of these systems, intrinsically related to the very high Bc2 (above 100 T), and provided a very first estimation of the variation of the critical current density Jc with the magnetic field. Recently, we are investigating vortex dynamics in low anisotropy HTS materials, giving new insights on the mechanisms behind the peak effect and on the vortex-vortex and vortex-defect interaction in these compounds.
Finally, thermally activated phenomena in the vortex matter play a key role for the applications of superconductors in high field NMR magnets. NMR magnets operate in persistent mode, which requires a low relaxation rate as a function of time for the current in the coil. The time decay of the superconducting currents has its origin in the depinning of the vortices due to the thermal activation. From this it follows the need to perform magnetic relaxation studies on both LTS and HTS top-notch industrial conductors, in view of their use in the next generation NMR magnets.
Material engineering: MgB2
Our underlying goal is to close the gap between intrinsic properties of MgB2 and present state of the conductor technology by designing and mastering new processing routes. MgB2 potential can be estimated from the data for thin films, which have shown record high values of Bc2 up to 60 T and very high values of Jc, with pinning force levels comparable to Nb3Sn. At present, Powder-In-Tube (PIT) MgB2 wires are produced in km lengths and MgB2-based prototype devices, such as low field MRI systems, have been demonstrated for operations at 20 K. Nevertheless, the commercially available wires are not yet able to satisfy the requirements of many of the envisaged applications for MgB2 (cryogen-free NMR systems, fault current limiters, large current DC cables, etc.), the performance being still well below the inherent potential of the material. Our focus is on the implementation of nanoengineering to enhance the critical current performance, addressing three issues: Bc2 enhancement, improvement of flux pinning, and improvement of grain connectivity.
Material engineering: Nb3Sn
High performance Nb3Sn wires are currently being developed and tested for the next generation of accelerator magnets, in particular for the Large Hadron Collider (LHC) upgrade with high luminosity and in view of the high-energy frontier scenarios envisaged in the Future Circular Collider (FCC) study. Further improvements of Nb3Sn performance require the simultaneous optimization of Sn composition and grain surface engineering, which can be achieved through a deeper understanding of the interplay between reaction conditions and superconducting properties. At the University of Geneva, we pioneered the use of low temperature calorimetry with the special purpose of determining the distribution of Tc in multifilamentary Nb3Sn wires. The distribution of Tc reflects the inhomogeneity of the superconducting properties in the wire and is mainly caused by the presence of Sn compositional variations within the filaments. Thanks to the Tc distribution analysis, we gain quantitative understanding about the influence of the synthesis conditions on composition, microstructure and thus on the final superconducting performance of the wires. This knowledge is transferred to the optimization of the wire processing parameters, as a support to the industrial development.