In-field angular pinning performances at different temperatures have been analysed on chemical solution deposited (CSD) YBa 2 Cu 3 O 7−x (YBCO) pristine films and nanocomposites. We show that with this analysis we are able to quantify the vortex pinning strength and energies, associated with different kinds of natural and artificial pinning defects, acting as efficient pinning centres at different regions of the H-T phase diagram. A good quantification of the variety of pinning defects active at different temperatures and magnetic fields provides a unique tool to design the best vortex pinning landscape under different operating conditions. We have found that by artificially introducing a unique defect in the YBCO matrix, the stacking faults, we are able to modify three different contributions to vortex pinning (isotropic-strong, anisotropic-strong, and isotropic-weak). The isotropic-strong contribution, widely studied in CSD YBCO nanocomposites, is associated with nanostrained regions induced at the partial dislocations surrounding the stacking faults. Moreover, the stacking fault itself acts as a planar defect which provides a very effective anisotropic-strong pinning at H//ab. Finally, the large presence of Cu-O cluster vacancies found in the stacking faults have been revealed as a source of isotropic-weak pinning sites, very active at low temperatures and high fields.
Superconducting nanocomposites are the best material choice to address the performances required in power applications and magnets working under high magnetic fields. However, it is still challenging to sort out how to achieve the highest superconducting performances using attractive and competitive manufacturing processes. Colloidal solutions have been recently developed as a novel and very promising low-cost route to manufacture nanocomposite coated conductors. Well dispersed and stabilized preformed nanoparticle solutions are first prepared with high concentrations and then mixed with the YBa 2 Cu 3 O 7 metalorganic precursor solutions to generate colloidal solutions to grow the nanocomposite films. Here we demonstrate, for the first time, that non-reactive BaZrO 3 and BaHfO 3 perovskite nanoparticles are suitable for growing high quality thin and thick films and coated conductors with a homogeneous distribution and controlled particle size. Additionally, we extend the nanoparticle content of the nanocomposites up to 20-25 % mol without any degradation of the superconducting properties. Thick nanocomposite films, up to 0,8 µm, have been prepared with a single deposition of low-fluorine solutions using an Ink-Jet Printing dispenser and we demonstrate that the preformed nanoparticles display only a very limited coarsening during the growth process and so high critical current densities J c (B) under high magnetic fields. These films show the highest critical currents achieved so far based on the colloidal solution approach, I c = 220 A/cm-w at 77 K and self-field, and they still have a high potential for further increase of the film thickness. Finally, we also show that nanocomposite YBa 2 Cu 3 O 7-BaZrO 3 coated conductors based on Alternating Beam Assisted Deposited YSZ buffer layer on Stainless Steel metallic substrates can be developed based on these novel colloidal solutions. Non-reactive preformed oxide perovskite nanoparticles are therefore very promising elements to further advance the colloidal solution approach in the implementation of low-cost and high-performance coated conductors for high magnetic field applications.
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