NANOTECHPINNINGHTS · Self-assembled nanotechnology of pinning centres in superconducting films, devices and coated conductors

Final Activity Report Summary - NANOTECHPINNINGHTS (Self-assembled Nanotechnology of Pinning Centres in Superconducting Films, Devices and Coated Conductors)

Energy resources are now a major issue in the global economy. In these circumstances, energy-efficient cryomagnetic devices and equipment (transformers, magnets, motors, induction furnaces, superconducting-magnet energy storage, fault-current limiters, etc...) based on superconducting materials that require significantly less energy to operate and are more environmentally-friendly will start to gradually replace their classical counterparts. For power applications in magnetic fields of superconducting films there is a worldwide effort in increasing the critical current Ic. This is important not only from technological point of view, but will increase also the economic feasibility of new, energy-efficient equipment, since the "effective cost" of the superconductor is given in Euro/kAm. Doubling the critical current will, in fact, reduce the effective cost by half. The critical current is usually strongly depressed by magnetic fields, especially at higher temperatures, due to Lorentz forces and thermal fluctuations. Strong artificial pinning centres in addition to natural ones are required for controlling the magnetic flux (vortex) dynamics. The earliest cost-effective method used for introducing artificial pinning centres was the so-called substrate decoration approach using nanodots on substrates. Later on, two other nanostructuring approaches have been used with great success: building up a layered distribution of a second phase using multilayer deposition (quasi-superlattices or quasi-multilayers) and, secondly, by the distribution of a secondary phase in the film achieved by a modified target composition. The objectives of the project consisted of fabrication by Pulsed Laser Deposition and complex characterisation of nanostructured thick films and conductors with increased current carrying capabilities in high magnetic fields and for all field orientations. In addition, fundamental studies of vortex matter, important also for practical applications, were planned. From applications point of view, the most important results are the following: (i) Shifting of the maximum in the thickness-dependence of Jc by using nano-dots of noble metals and other materials towards larger values in pure YBCO films, after an optimum thickness of few hundreds nm (depending on the deposition conditions) critical current density decrease sharply with increasing thickness, leading to small values of critical current. Using nano-dots in the quasi-multilayer approach this maximum shifts close to 1 micron, so we were able to grow thick YBCO nanostructured films (up to 5-6 microns) with better critical current, in self-field and in applied fields. (ii) We have observed that noble metal nano-dots promote a dense, closely-packed columnar growth of YBCO due to catalytic effects, leading to an increase in both Jc(B) and Ic(B) due to a high density of 2-dimensional extended defects at columns' boundaries. (iii) We have also obtained increased critical current density by using nanocrystalline targets with BaZrO3 (BZO) or Gd2Ba4CuWOy nanoinclusions. (iv) By combining the above results and discoveries we integrated for the first time Ag substrate decoration, Ag/YBCO quasi-multilayers, and the use of nanocrystalline BZO-doped YBCO target. The results shown that a 5 micron-thick Ag/(BZO-doped YBCO) quasi-multilayer has quite impressive in-field critical current at 77 K (170 A/cm-width in 1T, 90 A/cm-width in 1.5T and 60 A/cm-width in 2T), which are, to our best knowledge, the highest in-field critical currents reported so far by any European group. The same architecture was implemented on flexible metallic substrates required for practical applications in power devices, provided by our industrial partner, 3-Cs Ltd. Fundamental studies of vortex matter led to the discovery of new phases, magnetically coupled pancake vortex molecules and vortex molecules composed of fractional flux quanta glued by phase-difference solitons, as well as understanding some properties of multiband superconductors.

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