A. Palau

Growth, nanostructure and vortex pinning in superconducting YBa2Cu3O7 thin films based on trifluoroacetate solutions

Chemical solution deposition (CSD) is a very competitive technique to obtain epitaxial films and multilayers of high quality with controlled nanostructures. Based on the strong attractiveness from the cost point of view, the production of long length coated conductors based on the CSD approach is being extensively developed. The trifluoroacetate route (TFA) is the most widely used route to achieve epitaxial YBa2Cu3O7 (YBCO) layers with high critical currents, however a deep understanding of all the individual consecutive processing steps, as well as their mutual influence and relationship, is required to achieve superconducting materials with high performance. In this work, we review advances in the knowledge of all the steps relevant to the preparation of YBCO thin films based on TFA precursors as a CSD methodology: solution preparation and deposition, pyrolysis processes, intermediate phase evolution, nucleation and growth phenomena, microstructural evolution and its influence on percolating supercurrents, as well as vortex pinning by natural existing defects. Finally, we discuss the open issues still existing in the TFA approach, particularly that of film nanostructuration, and we provide a future outlook for this outstanding methodology.

Intermediate phase evolution in YBCO thin films grown by the TFA process

The YBCO thin film growth process from TFA precursors involves a complex reaction path which includes several oxide, fluoride and oxyfluoride intermediate phases, and the final microstructure and properties of the films are strongly influenced by the morphological and chemical evolution of these intermediate phases. In this work we present a study of the evolution of the intermediate phases involved in the TFA YBCO growth process under normal pressure conditions and we show that the oxygen partial pressure during pyrolysis of the TFA precursors is an important parameter. The Cu phase after the TFA pyrolysis can be either CuO, Cu2O or a mixture of both as the oxygen partial pressure is modified. The kinetics evolution of the intermediate phases has been determined for films pyrolysed in oxygen and nitrogen atmospheres and it is concluded that non-equilibrium phase transformations influence the reaction path towards epitaxial YBCO films and its microstructure. The intermediate phase evolution in these two series of films is summarized in kinetic phase diagrams.

High quality YBa2Cu3O7 thin films grown by trifluoroacetates metalorganic deposition

Chemical solution growth of epitaxial YBa2Cu3O7 thin films on single crystalline substrates has been investigated using trifluoroacetate precursors. The concentration of the starting solution was selected to achieve a final film thickness of 250 nm. First, it is shown that high quality films can only be achieved if an homogeneous nanocrystalline film is obtained after the spin coating deposition and pyrolysis steps. Secondary phases such as BaCuO2, Y2BaCuO5 and CuO remain after the high temperature growth process, when macrosegregation is detected after these initial processing steps. A local degradation of texture associated with macrosegregation is detected by μ-Raman spectroscopy. The influence of growth temperature on the film quality has been analysed and the reaction times have been optimized at each temperature according to the reaction kinetics. In-situ fluoride analysis and μ-Raman spectroscopy have been used to determine the advancement of the formation reaction. The most apparent microstructural modification of these thin film samples with optimized annealing times has been found to be an enhanced porosity for low processing temperatures. The influence of porosity on the normal state resistivity and the critical currents has been evidenced. Optimized processing parameters lead to samples with very high critical currents (Jabc = 3.2 × 106 A cm−2 at 77 K and 2.7 × 107 A cm−2 at 5 K) which demonstrates the capability of the trifluoroacetate metalorganic deposition method for thin film and coated conductor preparation.

Simultaneous inductive determination of grain and intergrain critical current densities of YBa2Cu3O7−x coated conductors

An inductive methodology simultaneously enabling the determination of grain- and intergrain critical current densities of YBa2Cu3O7−x coated conductors is developed. This noninvasive method is based on the identification of a clear peak in the reverse branch of the magnetization loop at a positive magnetic field, as a signature of the electromagnetic granularity inherent to these materials. A quantitative evaluation of the return magnetic field at the grain boundaries allows us to understand the existence of this magnetization peak and quantify the grain critical current density. This methodology is envisaged to sort out granularity effects from vortex pinning effects on coated conductors.

Influence of porosity on the critical currents of trifluoroacetate-MOD YBa/sub 2/Cu/sub 3/O/sub 7/ films

The influence of porosity on the superconducting properties have been investigated on YBa/sub 2/Cu/sub 3/O/sub 7/ thin films deposited on LaAlO/sub 3/ [100] substrates by the so-called Trifluoroacetate (TFA) route. Micro-Raman spectroscopy have been used to determine the concentration of c-axis grains /spl delta/ in different samples and their influence on the final film porosity as observed from SEM imaging. This has been compared with measurements of resistivity and critical currents in the same samples. We prove that this /spl delta/ fraction is the main parameter controlling the porosity and hence the normal-state resistivity of the thin films. The optimization of the microstructure of these YBa/sub 2/Cu/sub 3/O/sub 7/ TFA films allow to have high critical currents : J/sub c/ = 3 /spl times/ 10/sup 6/ A/cm/sup 2/ at 77 K.

Isotropic and anisotropic pinning in TFA-grown YBa2Cu3O7 − x films with BaZrO3 nanoparticles

YBCO films grown by the trifluoroacetate (TFA) method with increasing number of BaZrO3 (BZO) nanoparticles have been measured by in-field angular transport measurements to investigate changes in the pinning landscape. The isotropic and anisotropic contributions to the critical current density, Jc(H), with the magnetic field applied in and orientation have been determined, allowing us to characterize the population of isotropic and correlated defects along the c axis and ab planes. First, the influence of the YBCO oxygenation process on the formation of different sorts of anisotropic defects in standard films is demonstrated. Next, we show that the addition of non-coherent BZO nanoparticles to the YBCO matrix produces an expansion of the single-vortex pinning regime toward higher fields, due to the presence of isotropic pinning centers. Moreover, by increasing the amount of isotropic defects in the BZO nanocomposites it is possible to extend the region dominated by strong isotropic pinning centers to large magnetic fields and thus enhance the irreversibility line.

Strain-driven broken twin boundary coherence in YBa2Cu3O7−δ nanocomposite thin films

In this letter we use high resolution scanning transmission electron microscopy to study epitaxial YBa2Cu3O7−δ (YBCO) nanocomposite thin films. We find that twin boundaries (TB) in YBCO nanocomposite thin films are disturbed by the presence of secondary phase nanoparticles as well as by intergrowths. Secondary phases promote the nucleation of TBs and, at the same time, result in bending, decreasing and changing the TBs spacing. On the other hand, the local strain ensuing from the partial dislocation associated to Y248 and Y125 intergrowths break the vertical coherence of TBs. This interaction results in a complex domain structure where twin boundary coherence is no longer satisfied and twin spacing is reduced down to a few nanometers precluding vortex channeling at low temperatures.

Chemical solution techniques for epitaxial growth of oxide buffer and YBa2Cu3O7 films

Abstract Chemical solution techniques have been investigated for the growth of both, oxide buffer layers suitable for coated conductors and YBa 2 Cu 3 O 7 thin films, on single crystal substrates. Growth conditions have been optimised for CeO 2 and BaZrO 3 buffer layers, leading to high quality epitaxial films (misorientation spread typically below 1°). YBa 2 Cu 3 O 7 films (thickness: 250 nm) have been grown from trifluoroacetate precursors. The kinetic hindrances for the formation of single phases have been investigated by means of Raman spectroscopy and fluorine analysis. After optimisation of the deposition and growth conditions very high critical currents have been achieved (J c ab =3.2×10 6 A/cm 2 at 77 K and 2.7×10 7 A/cm 2 at 5 K).

Characterization of superconducting rings using an in-field Hall probe magnetic mapping system

A Hall probe magnetic imaging system that works in magnetic fields in the range -1 T to 1 T has been implemented, and it has been used to characterize the superconducting behavior of single domain melt textured YBa/sub 2/Cu/sub 3/O/sub 7/ rings. We show that in addition to the analysis of the evolution of the local magnetic field distribution when the external magnetic field is cycled, the hysteretic behavior of the magnetic moment can also be investigated after integration of the local magnetic field. The critical current density has been determined through the critical state model and it has been compared to that calculated by inversion of the Biot-Savart law. A remarkable agreement is achieved with both methods.

Epitaxial nucleation and growth of buffer layers and Y123 coated conductors deposited by metal-organic decomposition

Abstract Solution-based deposition of buffer and superconductor layers offers routes to a low-cost YBCO coated conductor technology for high temperature superconductor tapes. In this work we explore the possibility of the preparation procedure of BaZrO 3 buffer and YBCO layers on SrTiO 3 (STO) single crystal substrates by metal-organic decomposition. BaZrO 3 buffer layers have been processed on single crystal substrates using a precursor solution based on barium acetate and zirconium 2,4-pentadionate in acetic acid. YBCO superconducting films were prepared on STO single crystals using trifluoroacetate precursors. X-ray diffraction and pole figure analyses denote good phase purity and high in-plane alignment (