A methodology of general validity to prepare epitaxial nanocomposite films based on the use of colloidal solutions containing different crystalline preformed oxide nanoparticles ( ex situ nanocomposites) is reported. The trifluoroacetate (TFA) metal–organic chemical solution deposition route is used with alcoholic solvents to grow epitaxial YBa ₂ Cu ₃ O ₇ (YBCO) films. For this reason stabilizing oxide nanoparticles in polar solvents is a challenging goal. We have used scalable nanoparticle synthetic methodologies such as thermal and microwave-assisted solvothermal techniques to prepare CeO ₂ and ZrO ₂ nanoparticles. We show that stable and homogeneous colloidal solutions with these nanoparticles can be reached using benzyl alcohol, triethyleneglycol, nonanoic acid, trifluoroacetic acid or decanoic acid as protecting ligands, thereby allowing subsequent mixing with alcoholic TFA solutions. An elaborate YBCO film growth analysis of these nanocomposites allows the identification of the different relevant growth phenomena, e.g. nanoparticles pushing towards the film surface, nanoparticle reactivity, coarsening and nanoparticle accumulation at the substrate interface. Upon mitigation of these effects, YBCO nanocomposite films with high self-field critical currents ( J _c ∼ 3–4 MA cm ⁻² at 77 K) were reached, indicating no current limitation effects associated with epitaxy perturbation, while smoothed magnetic field dependences of the critical currents at high magnetic fields and decreased effective anisotropic pinning behavior confirm the effectiveness of the novel developed approach to enhance vortex pinning. In conclusion, a novel low cost solution-derived route to high current nanocomposite superconducting films and coated conductors has been developed with very promising features.

GdBa ₂ Cu ₃ O _{7− x} -BaHfO ₃ (GdBCO-BHO) nanocomposite (NC) films containing 12 mol% BHO nanoparticles were prepared by chemical solution deposition (CSD) following the TFA route on SrTiO ₃ (STO) single crystals and buffered metallic tapes supplied by two different companies: Deutsche Nanoschicht GmbH and SuperOx. We optimized the preparation of our GdBCO-BHO solutions with acetylacetone making the film synthesis very robust and reproducible, and obtained 220 nm films with excellent superconducting properties. We show the structural, morphological and superconducting properties of the films after a careful optimization of the processing parameters (growth temperature, oxygen partial pressure and heating ramp). The films reach critical temperatures ( T _c ) of ∼94 K, self-field critical current densities ( J _c ) of >7 MA cm ^{− 2} and maximum pinning force densities ( F _p ) of ∼16 GN m ^{− 3} at 77 K on STO and T _c of ∼94.5 K and J _c > 1.5 MA cm ^{− 2} on buffered metallic tapes. The transport properties under applied magnetic fields are significantly improved with respect to the pristine GdBCO films. The GdBCO-BHO NC films on STO present epitaxial c -axis orientation with excellent out-of-plane and in-plane texture. The films are, in general, very dense with a low amount of pores and only superficial indentations. On the other hand, we present, for the first time, a systematic study of CSD-grown GdBCO-BHO NC films on buffered metallic tapes. We have used the optimized growth conditions for STO as a reference and identified some limitations on the film synthesis that should be overcome for further improvement of the films’ superconducting properties.

REBa₂Cu₃O_7-x (REBCO, RE = rare earth) compounds with different single RE elements were grown via TFA-MOD (metal-organic deposition of trifluoroacetates) to clarify their T _c values when grown by the same preparation method and their processing windows; here: the formation temperatures at a constant process gas composition (pO2, pH₂O). We focussed on the lanthanides (Ln) Nd, Sm, Gd, Dy, Ho, Er and Yb as substituents for Y in the REBCO phase and investigated their growth behaviour in terms of resulting physical (inductive T _c and J _c(77 K)) and structural properties (determined by XRD, SEM, TEM). All phases were grown as pristine films on LaAlO₃ and SrTiO₃ and compared to their respective nanocomposites with 12 mol% BaHfO₃ for in-field pinning enhancement. With regard to T _c and J _c(77 K), the optima of both values shift towards higher growth temperatures for increasing and decreasing RE ion size with respect to yttrium. Highest T _c values achieved so far do not show a trend that can solely be related to the RE ionic size. On the contrary, T _c,90 values of the LnBCO compounds from Sm to Er range between 94.0 and 95.3 K and are, therefore, significantly larger than the highest values of the average-size non-lanthanide, Y, with T _c,90 = 91.5 K. J _c,sf values seem to plateau between 5 and 6 MA cm^-² from Sm to Er and are again clearly above the maximum values we ever achieved for Y with 4.2 MA cm^-². REBCO phase formations of the very small Yb and large Nd turned out to be more difficult and require further adjustment of growth parameters. All REBCO phases investigated here show distinct dependences of T _c on the lattice parameter c.

To reduce the fabrication costs while maximizing the superconducting and pinning properties of YBa₂Cu₃O_7−δ (YBCO) nanocomposite films, the drop-on-demand ink-jet printing technique was used to deposit colloidal YBCO inks onto LaAlO₃ substrates. These inks containing preformed HfO₂ nanocrystals were carefully adjusted, prior to the jettability, as the droplet formation depends on the rheological properties of the inks themselves. After carefully adjusting printing parameters, 450-nm thick pristine YBCO films with a self-field critical current density (J_c) of 2.7 MA cm⁻² at 77 K and 500-nm thick HfO₂-YBCO nanocomposite films with a self-field J_c of 3.1 MA·cm⁻² at 77 K were achieved. The final HfO₂-YBCO nanocomposite films contained dispersed BaHfO₃ particles in a YBCO matrix due to the Ba²⁺ reactivity with the HfO₂ nanocrystals. These nanocomposite films presented a more gradual decrease of J_c with the increased magnetic field. These nanocomposite films also showed higher pinning force densities than the pristine films. This pinning enhancement was related to the favorable size and distribution of the BaHfO₃ particles in the YBCO matrix.