O Castaño

The influence of growth conditions on the microstructure and critical currents of TFA-MOD YBa2Cu3O7 films

The influence of three processing parameters, temperature, gas flow rate and water pressure, on the YBa2Cu3O7 film growth on LaAlO3 single-crystal substrates from trifluoroacetate precursors has been investigated and the optimal film processing conditions to achieve high critical currents have been determined. We have found that the growth conditions maximizing the critical current density are those where the nucleation of a-axis oriented grains is minimized, as determined by μ-Raman spectroscopy. Under these conditions the normal state resistivity is very near to that of single crystals because a vanishingly small film porosity is achieved. Transmission electron microscopy analysis of films quenched from the growth temperature gives some hints for understanding the mechanism linking the film porosity with the concentration of a-axis grains. A cross-linked influence of different processing parameters, such as temperature and water pressure, or water pressure and gas flow, has been demonstrated. The optimal growth temperatures are 790–830 °C, but at these growth temperatures, the critical current density is still dependent on the gas flow rate and water pressure. The optimal processing ranges are a compromise between two different competing phenomena influencing the quality of the films: inhomogeneous film formation due to HF gas stagnancy at small nominal growth rates (low gas flow rate or water pressure) and perturbed crystallinity at high gas flow rates or water pressures.

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.

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.

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).

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 (