P Cayado, K De Keukeleere, A Garzón, L Perez-Mirabet, A Meledin, J De Roo, F Vallés, B Mundet, H Rijckaert, G Pollefeyt, M Coll1, S Ricart, A Palau, J Gázquez, J Ros, G Van Tendeloo, I Van Driessche, T Puig and X Obradors.
IOP Publishing Ltd • Superconductor Science and Technology, Volume 28
A methodology of general validity to prepare epitaxial nanocomposite films based on the use of colloidal solutions containing different crystalline preformed oxide nanoparticles (ex situnanocomposites) is reported. The trifluoroacetate (TFA) metal–organic chemical solution deposition route is used with alcoholic solvents to grow epitaxial YBa2Cu3O7 (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 CeO2 and ZrO2 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 (Jc ~ 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.
In this work, ferroelectricity is identified in nanocrystalline BiFeO3 (BFO) thin films prepared by low-temperature atomic layer deposition. A combination of X-ray diffraction, reflection high energy electron diffraction, and scanning transmission electron microscopy analysis indicates that the as-deposited films (250 °C) consist of BFO nanocrystals embedded in an amorphous matrix. Postannealing at 650 °C for 60 min converts the sample to a crystalline film on a SrTiO3substrate. Piezoelectric force microscopy demonstrates the existence of ferroelectricity in both as-deposited and postannealed films. The ferroelectric behavior in the as-deposited stage is attributed to the presence of nanocrystals. Finally, a band gap of 2.7 eV was measured by spectroscopic ellipsometry. This study opens broad possibilities toward ferroelectric oxides on 3D substrates and also for the development of new ferroelectric perovskites prepared at low temperature.
A. Queraltó, A. Pérez del Pino, M. de la Mata, J. Arbiol, M. Tristany, A. Gómez, X. Obradors and T. Puig.
Appl. Phys. Lett. 106, 262903 (2015)
http://dx.doi.org/10.1063/1.4923376
Highly crystalline epitaxial Ba0.8Sr0.2TiO3 (BST) thin-films are grown on (001)-oriented LaNiO3-buffered LaAlO3 substrates by pulsed laser irradiation of solution derived barium-zirconium-titanium precursor layers using a UV Nd:YAG laser source at atmospheric conditions. Thestructural analyses of the obtained films, studied by X-ray diffractometry and transmission electron microscopy, demonstrate that laser processing allows the growth of tens of nm-thick BST epitaxialfilms with crystalline structure similar to that of films obtained through conventional thermal annealing methods. However, the fast pulsed nature of the laser employed leads tocrystallization kinetic evolution orders of magnitude faster than in thermal treatments. The combination of specific photothermal and photochemical mechanisms is the main responsible for the ultrafast epitaxial laser-induced crystallization. Piezoresponse microscopy measurements demonstrate equivalent ferroelectric behavior in laser and thermally annealedfilms, being the piezoelectric constant ∼25 pm V−1.
R. Ortega-Hernandez, M. Coll, J. C. Gonzalez-Rosillo, A. Palau, X. Obradors, E. Miranda, T. Puig and J. Suñé
Microelectronic Engineering, 147, 37-40 (2015)
DOI:10.1016/j.mee.2015.04.042
The resistive switching of CeO2−x/La0.8Sr0.2MnO3 bilayer structures has been studied. First, the resistive switching (RS) characteristics of La0.8Sr0.2MnO3 (LSMO) and the CeO2−x layers are studied separately. Then, the bilayer characteristics are analyzed. It has been demonstrated that inserting a thin CeO2−x layer between the LSMO film and the metal electrodes deeply modifies the resistive switching characteristics. The metal–insulator transition of the LSMO layer results from the oxygen diffusion in and out of the film. These effects are enhanced through the introduction of the CeO2−x layer due to the fact it acts as an oxygen reservoir.
A. Queraltó, A. Pérez del Pino, M. de la Mata, J. Arbiol, X. Obradors and T. Puig
Crystal Growth and Design 15, 1957-1967 (2015)
DOI: 10.1021/acs.cgd.5b00115
The epitaxial growth of Ce0.9Zr0.1O2–y (CZO) thin-films on yttria-stabilized zirconia (YSZ) (001) single crystal and YSZ (001)/stainless steel (YSZ/SS) technological substrates is investigated by pulsed laser irradiation of solution-derived cerium–zirconium precursor layers using a UV Nd:YAG laser source at atmospheric conditions. The influence of laser processing parameters on the morphological and structural properties of the obtained films is studied by atomic force and transmission electron microscopies, as well as X-ray diffractometry. The analyses performed demonstrate that laser treatments enable the epitaxial growth of tens of nanometers thick CZO films with a crystallization kinetic process several orders of magnitude faster than that of conventional thermal annealing. Fully epitaxial films are attained using stainless steel (SS) flexible tapes as a substrate. Even though photochemical mechanisms are not fully discarded, it is concluded that photothermal processes are the main contribution responsible for the fast epitaxial crystallization.
P Roura, J. Farjas, H. Eloussifi, L. Carreras, S. Ricart, T. Puig Molina and X. Obradors
Thermochimica Acta 601, 1-8 (2015)
DOI:10.1016/j.tca.2014.12.016
The thermal decomposition of several metal organic precursors, used in the preparation of YBa2Cu3O7−xsuperconducting coated conductors (Cu acetate, Cu, Y and Ba trifluoroacetates and Ce propionate) is analyzed by means of several thermoanalytical techniques (TG/DTA, MS and DSC). In all cases, the metal organic precursors deposited as thin films decompose differently than powders from the same precursors. In thin films, decomposition is facilitated by the easier transport of reactive gas from the surrounding atmosphere and by the easier out-diffusion of volatile products. Consequently, films decompose at lower temperature and are more sensitive to the presence of any residual reactive gas in the furnace. Good thermal contact with the substrate is also shown to minimize overheating in films and avoid combustion processes that are otherwise often observed during the thermal decomposition of powders. Finally, the formation and stability of intermediate products towards the oxide formation, such as metal fluorides, differs in films because of the easier gas exchange. With respect to powders, these compounds are much less stable in films, where their decomposition temperature can be lowered by several hundreds of degrees Celsius. While in some cases the behaviour of films can be predicted by analyzing varying masses of precursor powders, this is not always the case. Therefore, thermal analysis carried out on films is recommended to avoid erroneous conclusions about materials preparation drawn from powders.
J. Zabaleta, M. Jaafar, A. Asenjo, S. Agramunt-Puig, N. Del-Valle, C. Navau, A. Sanchez, T. Puig, X. Obradors and N. Mestres.
APL Materials. 2, 076111 (2014)
J. Lloberas, A. Sumper, M. Sanmarti and X. Granados
Renewable and Sustainable Energy Reviews 38, 404-414 (2014)
DOI: 10.1016/j.rser.2014.05.003
Large synchronous generators with high temperature superconductors are in constant development due to their advantages such as weight and volume reduction and the increased efficiency compared with conventional technologies. The offshore wind turbine market is growing by the day, increasing the capacity and energy production of the wind farms installed and increasing the electrical power for the electrical generators installed, consequently raising the total volume and weight for the electrical generators installed. The HTS synchronous generators (HTSSG) are an alternative to consider due to their low dimensions and low weight per megawatt. This article presents a detailed review of the geometric configurations of the large HTSSG for offshore wind energy followed by an explanation of the main non-conventional technological parts. Additionally, the experience from the most important projects – both ongoing and completed – by companies and research institutes related to the design and construction of HTSSG for offshore wind energy is reviewed.
P Cayado, K De Keukeleere, A Garzón, L Perez-Mirabet, A Meledin, J De Roo, F Vallés, B Mundet, H Rijckaert, G Pollefeyt, M Coll1, S Ricart, A Palau, J Gázquez, J Ros, G Van Tendeloo, I Van Driessche, T Puig and X Obradors.
IOP Publishing Ltd • Superconductor Science and Technology, Volume 28
A methodology of general validity to prepare epitaxial nanocomposite films based on the use of colloidal solutions containing different crystalline preformed oxide nanoparticles (ex situnanocomposites) is reported. The trifluoroacetate (TFA) metal–organic chemical solution deposition route is used with alcoholic solvents to grow epitaxial YBa2Cu3O7 (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 CeO2 and ZrO2 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 (Jc ~ 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.
R. Ortega-Hernandez, M. Coll, J. C. Gonzalez-Rosillo, A. Palau, X. Obradors, E. Miranda, T. Puig and J. Suñé
Microelectronic Engineering, 147, 37-40 (2015)
DOI:
A. Queraltó, A. Pérez del Pino, M. de la Mata, J. Arbiol, X. Obradors and T. Puig
Crystal Growth and Design 15, 1957-1967 (2015)
DOI: 
