Thermal analysis of metal organic precursors for functional oxide preparation: Thin films versus powders

1-s2.0-S0040603114005693-fx1 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.  

A review of high temperature superconductors for offshore wind power synchronous generators

1-s2.0-S1364032114003190-gr1 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.

Low Temperature Stabilization of Nanoscale Epitaxial Spinel Ferrite Thin Films by Atomic Layer Deposition

adfm201400517-fig-0003 M. Coll, J. J. Montero, J. Gazquez, K. Nielsch, X. Obradors and T. Puig Advanced Functional Materials, 24, 5368-5374 (2014) DOI: 10.1002/adfm.201400517 In this work heteroepitaxial stabilization with nanoscale control of the magnetic Co2FeO4phase at 250 °C is reported. Ultrasmooth and pure Co2FeO4 thin films (5–25 nm) with no phase segregation are obtained on perovskite SrTiO3 single crystal (100) and (110) oriented substrates by atomic layer deposition (ALD). High resolution structural and chemical analyses confirm the formation of the Co-rich spinel metastable phase. The magneto-crystalline anisotropy of the Co2FeO4 phase is not modified by stress anisotropy because the films are fully relaxed. Additionally, high coervice fields, 15 kOe, and high saturation of magnetization, 3.3 μB per formula unit (at 10 K), are preserved down to 10 nm. Therefore, the properties of the ALD-Co2FeO4 films offer many possibilities for future applications in sensors, actuators, microelectronics, and spintronics. In addition, these results are promising for the use of ALD compared to the existing thin-film deposition techniques to stabilize epitaxial multicomponent materials with nanoscale control on a wide variety of substrates for which the processing temperature is a major drawback.

Coated conductors for power applications: materials challenges

sust490440f3_online X.Obradors and T. Puig Molina Supercond. Science & Technology 27, 044003 (2014) DOI:10.1088/0953-2048/27/4/044003 This manuscript reports on the recent progress and the remaining materials challenges in the development of coated conductors (CCs) for power applications and magnets, with a particular emphasis on the different initiatives being active at present in Europe. We first summarize the scientific and technological scope where CCs have been raised as a complex technology product and then we show that there exists still much room for performance improvement. The objectives and CC architectures being explored in the scope of the European project EUROTAPES are widely described and their potential in generating novel breakthroughs emphasized. The overall goal of this project is to create synergy among academic and industrial partners to go well beyond the state of the art in several scientific issues related to CCs’ enhanced performances and to develop nanoengineered CCs with reduced costs, using high throughput manufacturing processes which incorporate quality control tools and so lead to higher yields. Three general application targets are considered which will require different conductor architectures and performances and so the strategy is to combine vacuum and chemical solution deposition approaches to achieve the targeted goals. A few examples of such approaches are described related to defining new conductor architectures and shapes, as well as vortex pinning enhancement through novel paths towards nanostructure generation. Particular emphasis is made on solution chemistry approaches. We also describe the efforts being made in transforming the CCs into assembled conductors and cables which achieve appealing mechanical and electromagnetic performances for power systems. Finally, we briefly mention some outstanding superconducting power application projects being active at present, in Europe and worldwide, to exemplify the strong advances in reaching the demands to integrate them in a new electrical engineering paradigm.

Neutron and X-ray diffraction study of ferrite nanocrystals obtained by microwave-assisted growth. A structural comparison with the thermal synthetic route.

kk5153fig3 E. Solano, C. Frontera, I. Puente Orench, T. Puig, X. Obradors, S. Ricart and J. Ros Journal of Applied Crystallography 47, 1478 (2014) DOI: 10.1107/S1600576714017452 Neutron and X-ray powder diffraction have been used to investigate the differences between the crystal growth of ferrite magnetic nanoparticles (MFe2O4 with M = Mn, Fe, Co, Ni, Zn) by two methodologies: microwave radiation and thermal decomposition routes. Rietveld refinement has been used to extract the cationic distribution, the microstructure and magnetic information. Results for the nanoparticles produced by the two procedures evidence similar cationic distribution, microstructure and magnetic properties: complete cationic disorder for M = Mn and Co, crystal size around/below 10 nm etc. It is thus proven that microwave-assisted growth is a promising eco-friendly synthetic technique for the generation of high-quality nanocrystals with comparable structure and properties to those produced by the thermal methodology, even though the microwave route needs a shorter time and lower annealing temperature to obtain the final crystal nanoparticles.

Magnetic and structural characterization of inkjet-printed TFAYBa2Cu3O7−x/MODCZO/ABADYSZ/SS coated conductors

sust485654f4_online E. Bartolome, R. Vlad, A. Calleja, M. Aklalouch, R. Guzman, J Arbiol, X. Granados, A. Palau, X. Obradors, T. Puig and A. Usokin Superconductor Science and Technology 26, 125004 (2013) DOI:10.1088/0953-2048/26/12/125004 The superconductor industry is demanding new methodologies to manufacture km-long, high quality coated conductors at high growth rates, using cost-effective, scalable processes. We report on the fabrication by an all-chemical deposition method of highly textured, thick (0.9 μm) inkjet-printed YBCO films, using a Ce0.9Zr0.1O2 (CZO) capping layer deposited by MOD, on top of robust, buffered ABADYSZ/SS substrates. Thinner, 0.25 μm spin-coated YBCO films were also analyzed for comparison. The structural study performed by x-ray diffraction, optical, AFM, SEM and TEM microscopy demonstrates the success of the capping layer for enhancing the planarity of the as-received tape and obtaining highly homogeneous and well-textured YBCO films. DC magnetometry granularity analysis was used to determine the mean superconducting grain diameter, ~2.5 μm, and the intra- and intergranular critical current densities of the coated conductors (CCs). For the thin, spin-coated sample, high self-field intragrain critical currents were measured (, 3.3 MA cm−2 at 5, 77 K). For the thick, inkjet-printed tape was reduced by ~30%, but, notably, the percolative critical current, , was only ~10% smaller at 5 K, thanks to good preservation of the texture. At 77 K, was achieved, implying a critical current of Ic = 117 A/cm-width. AC susceptibility measurements allowed us to demonstrate the high homogeneity of the fabricated CCs, and investigate the magnetic vortex-pinning phase diagram. Remarkably, the thick, inkjet-printed sample showed comparable irreversibility line (IL) and activation energy for thermal depinning, Ue(H), to the thin sample. The present results open new perspectives for the fabrication of high quality-to-cost ratio, all-chemical CCs with yet higher Ic values by inkjet printing multideposition of thicker YBCO layers.

Thermal analysis for low temperature synthesis of oxide thin films from chemical solutions

jp-2013-049742_0006 D. Sanchez-Rodriguez, J. Farjas, P Roura, S. Ricart, N. Mestres Andreu, X. Obradors and T. Puig Journal of Physical Chemistry C 117, 20133-20138 (2013) DOI: 10.1021/jp4049742 The possibility of synthesizing functional oxide thin films at low temperature via combustion synthesis is analyzed both experimentally and numerically. To this aim, the decomposition of several oxide precursors [copper and cerium acetates, yttrium trifluoroacetate, and In2O3 and La0.7Sr0.3MnO3 (LSMO) nitrate based precursors] has been analyzed by thermal analysis techniques. It is shown that, although these precursors decompose via combustion when they are in the form of powders, their corresponding films show no evidence of combustion. The reason for this different behavior is clearly revealed with numerical simulations. Thin films will hardly experience combustion because the precursor front extinguishes before reaching the precursor–substrate interface leaving a “cool zone” hundreds of micrometers thick. In contrast, it is argued that thin oxide films can be obtained at temperatures lower than powders because of the enhanced gas transport mechanisms that usually limit the decomposition rate.

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