Processing, microstructures, and pinning properties of REBa2Cu3O7-δ (RE= Y and Gd) superconducting films

Abstract

For practical applications of high temperature superconductor (HTS) coated conductors (CCs), high critical current density (Jc) in high magnetic fields is desirable. To improve the HTS current-carrying capabilities in high fields, an inclusion of nano-sized particles into the HTS matrix is required. In this study, we characterized the flux pinning properties of REBa2Cu3O7-δ (REBCO, RE=Y and Gd) films by the pulsed laser deposition (PLD), metal-organic deposition (MOD), and reactive co-evaporation deposition & reaction (RCE-DR) processes. We also successfully fabricated GdBCO films with Gd2O3 addions on CeO2-buffered MgO (100) substrate by PLD for systemic study. On the basis of the results from systemic study, Gd2O3-doped GdBCO films were post-annealed in various temperatures and oxygen pressure (PO2) for improvement of their pinning properties. Finally, we summarized comparison of pinning properties and microstructures of REBCO films prepared by various methods. The major results are as the following. First, our group have successfully fabricated Sn-doped REBCO films by MOD and PLD. Also, SuNAM Co. have successfully developed a new high-throughput process called RCE-DR for the fabrication of long-length high-Jc GdBCO CCs on LaMnO3 (LMO)-buffered IBAD MgO template. However, the flux pinning properties of those REBCO films are not fully identified yet. In this study, we fully identified the flux pinning properties of REBCO films fabricated by the PLD, MOD, and RCE-DR process. As a result, the 5 vol% Sn-doped GdBCO film on CeO2-buffered IBAD MgO template by PLD exhibited the highest maximum pinning force density (Fp,max) values and the highest Jc, which is evidence that 5 vol% is the optimum content. In the case of MOD process, the Sn-doped YBCO film on SrTiO3 (STO) (100) substrate exhibited enhanced Jc and Fp,max compared with undoped YBCO film. From the fast Fourier transform (FFT) patterns from high resolution-transmission electron microscope (HR-TEM) image and scanning transmission electron microscope-energy dispersive X-ray spectroscopy (STEM-EDS) analysis, we first reported the Sn-containning nanoparticles are not BaSnO3 but YBa2SnO5.5 (YBSO). We also investigated the effect of the growth temperature on the microstructures and superconducting properties of GdBCO CCs deposited by the RCE-DR process on LMO-buffered IBAD MgO template. We employed three different growth temperatures of 840, 860 and 880°C. With decreasing the growth temperature, the Jc at 65 and 77 K were improved due to enhanced pinning properties, which are attributable to refined Gd2O3 particles trapped in the GdBCO matrix. Analyses by transmission electron microscopy (TEM) revealed that the average particle size of Gd2O3 is 126.5 nm in the 840°C sample which is smaller than that of 171.4 nm in the 860°C sample and 217.8 nm in the 880°C sample. Interestingly, although (103) misoriented GdBCO grains existed together with biaxially textured grains in the film grown at 840°C, exhibited higher Jc values in magnetic fields compared with other films composed of only biaxially textured grains, suggesting that the increase in Jc due to pinning by refined Gd2O3 particles surpasses the degradation of Jc by (103)-oriented grains present at the film surface. Second, we tried to optimize the Gd2O3 additions in GdBCO films on CeO2-buffered MgO (100) substrate. The Gd2O3-doped GdBCO films with various x mol% (x = 0, 2, 4, 6, and 10) relative to GdBCO were prepared at 800°C with the PO2 of 300 mTorr by PLD. Among all samples, a 2 mol% Gd2O3-doped GdBCO film exhibited the highest Jc value of 2.88 MA/cm2 at 77 K and 4.72 MA/cm2 at 65 K in 0 T, and also shows the Fp,max values of 37.3 GN/m3 near 2.8 T at 65 K and 11.3 GN/m3 near 1.8 T at 77 K, evidencing that 2 mol% is the optimum Gd2O3 content. We also tried to enhance flux pinning properties of Gd2O3-doped GdBCO film by post-annealing process. According to our preliminary study on GdBCO CCs by RCE-DR, the flux pinning properties of GdBCO CCs were improved by post-annealing. Since the average particle size of Gd2O3 in GdBCO film by PLD can be refined, it must be attractive to study the effect of the post-annealing process on the GdBCO films with Gd2O3 nanoparticles on a robust substrate. To identify the effect of the post-annealing process on the pinning properties of Gd2O3-doped GdBCO films, we performed post-annealing in various temperature and PO2. The Tc,zero values of GdBCO films annealed at 800 °C in the PO2 of 10 mTorr are increased up to ~91.4 K. The angular dependence Jc of GdBCO film annealed at 800°C in 10 mTorr are slightly improved along the direction parallel to the c-axis compared with non-annealed 2 mol% Gd2O3-doped GdBCO film, representing that it has c-axis correlated pinning centers. From TEM analysis, some defects along the c-axis of GdBCO are observed in the post-annealed sample. It suggested that flux pinning along c-axis (B//c) in the post-annealed sample might be improved by the GdBa2Cu4O8 (Gd124) type stacking faults filed up along the c-axis of GdBCO matrix. Third, we summarized comparison of pinning properties and microstructures of REBCO films prepared by PLD, MOD, metal-organic chemical vapor deposition (MOCVD), and RCE-DR. We compared exponent α value, Fp,max, minimum Jc (Jc,min), angular dependence of Jc, and the shape & size of pinning centers for REBCO films. As a result, the flux pinning properties of REBCO films by PLD still exhibit much higher than other deposition method. Although the GdBCO films by the RCE-DR process still indicate lower flux pinning properties, we suggest that improvement of pinning properties will be feasible by Gd2O3 refinement and post-annealing process through previous study of Gd2O3-doped GdBCO films.