2, University of Turku, Turku, , Finland
3, University of Antwerp, Antwerp, , Belgium
4, Deutsche Nanoschicht, Rheinbach, , Germany
YBa2Cu3O7-δ (YBCO) coated conductors for electric power applications are demanding highly flexible and low-cost manufacturing processes. Up to now, textured superconducting YBCO films with the desired properties for the coated conductor architecture are mostly realized via vacuum processes such as pulsed laser deposition (PLD) and metal-organic chemical vapor deposition. These require costly high-vacuum systems, which are not attractive for industrial scale. Chemical solution deposition (CSD) is a non-vacuum technique and can fulfill demands such as cost-effectiveness, e.g. through high yield, and easy scalability. However, the pathway to fabricate YBCO nanocomposite films with relevant pinning properties does not seem to be easy because of the necessity to solve several critical issues related to YBCO growth. In the case of the self-assembled nanocomposite films produced via the PLD method, it has been already shown that the precise size tuning of secondary phases in YBCO matrix leads to improved pinning properties.
In this work, two different methods to synthesize HfO2 nanocrystals were introduced to reveal the influence of nanocrystal morphology and size on the defect state of YBCO matrix. This approach comes with some new specific challenges to tune the particle size for the CSD-based YBCO nanocomposite film starting from preformed nanocrystals. The solvothermal heating-up synthesis yielded highly crystalline nanorod-like HfO2 nanocrystals with a size of 2.6 nm in diameter and 8.0 nm in length. The microwave-assisted synthesis delivered highly crystalline HfO2 nanocrystals but now are spherical with a size of 6-8 nm in diameter.
For these two types of monoclinic HfO2 nanocrystals, we have shown the possibility to deposit the YBCO nanocomposite thin films starting from preformed HfO2 nanocrystals as nano-sized defects for flux pinning in combination with environmentally benign low-fluorine YBCO precursor solution on single crystal LaAlO3 substrates using a spin-coating technique. This CSD approach has been shown that the nanocrystal size and its distribution are the key parameters for a better control of structural defects in the YBCO matrix. Small particles of 5-20 nm would tend to promote formation of small planar defects (stacking faults) surrounding the particles, yielding an increase of microstrain and leading to high pinning force density of up to 17 GN m-3. Large particles (>25 nm) in the YBCO matrix could lead to formation of larger stacking faults leading to degradation of the critical current density in the YBCO nanocomposite films.