Nanorods and nanoparticles are commonly introduced to REBCO coated conductors as artificial pinning centers to improve the critical current density in high magnetic fields and to reduce the anisotropy with respect to the magnetic field angle. In this article the pinning properties are theoretically analyzed based on the classic condensation energy pinning interaction mechanism (δTc pinning) of these pinning centers and the results are compared with experimental results. The agreements show that the critical current density can be designed by controlling the microstructure of pinning centers and factors such as the size, number density and morphology. It is also pointed out that the upper critical field is enhanced via the electron scattering of the interface between the pinning center and the superconducting matrix. Hence, the upper critical field can also be controlled to enhance the irreversibility field by controlling the interface area in a unit volume of the superconductor. It is expected that the high field performance can be optimized by controlling the pinning strength and the upper critical field in the future.
There are high expectations for REBa2Cu3Oy (REBCO: RE=Rare Earth) coated conductors for superconducting power applications as a substitute for intermetallic superconducting wire. Especially, GdBa2Cu3Oy coated conductors have attracted attention owing to their high superconducting properties. In this study, in order to investigate the superconducting properties and microstructures in GdBa2Cu3Oy thin films with straight BaHfO3 (BHO) nanorods introduced, we fabricated BHO doped GdBa2Cu3Oy thin films. The BHO doped GdBa2Cu3Oy thin films were fabricated on a IBAD-MgO substrate using the pulsed laser deposition method. From transmission electron microscopic images, the BHO nanorods grew continuously and straight from the substrate to the surface of the films in 1.5 vol.% BHO doped GdBa2Cu3Oy film. The mean diameter was approximately 4.4 nm. The critical temperature and Jc in the self field of the films linearly decreased with increasing BHO content. The magnetic field dependence of Jc of all the BHO doped films showed wide plateau regions in which there was little decrease of the Jc. The results showed that the Jc in the magnetic field were dramatically improved compared with those of pure GdBa2Cu3Oy thin films. These films showed high pinning force density; in particular, 2.2 vol.% doped films showed 26.8 GN/m-3 at 77 K, B = 6 T.
YBa2Cu3O7(YBCO) films containing hybrid artificial pinning centers(APCs) of BaSnO3(BSO) nanorods and Y2O3 nanoparticles were fabricated on ion beam assisted deposition(IBAD)-MgO substrates. By changing contents of BSO and Y2O3, critical current density (Jc) properties were controlled. The YBCO film containing high density of BSO and Y2O3 exhibited high Jc in high magnetic fields, and an isotropic angular dependence of Jc was obtained in a low magnetic field in a film containing moderate density of nanorods and nanoparticles, showing that hybrid APCs have the potential to improve Jc performance. BSO content is the most crucial factor for Fp,max in a magnetic field parallel to the c-axis, and Y2O3 nanoparticles can significantly improve Fp,max in a magnetic field tilted from the c-axis. Advanced control of nanorods and nanoparticles is needed for realization of higher Jc performance.
We investigated the effect of introducing artificial pinning centers (APCs) into YBCO thin films to reduce the Rs in a high magnetic field. YBCO thin films with and without APCs were deposited on MgO(100) substrates by using the pulsed laser deposition (PLD) technique. We used BaMO3 (BMO, M = Zr, Hf) doped YBCO targets. The Rs of thin films was measured at 21.8 GHz with the dielectric resonator method and a DC magnetic field of up to 5.0 T. The Rs values of 1.5 wt. % of the BHO doped films were approximately 1/2 compared with that of the Rs of the pure YBCO films. As a result, YBCO thin films with APCs could provide advantages for NMR application.
For the improvement and stable use of various superconducting applications using REBa2Cu3Oy (REBCO) coated conductors, the enhancement of the critical current density (Jc) in high magnetic fields is essential. It is known that BaMO3 (BMO; M = Zr, Sn, Hf) nanorods can improve the in-field Jc of REBCO films. Furthermore, the density and formation can be controlled by the deposition conditions such as deposition temperature and BMO content. In this study, our aim was to improve the Jc of SmBa2Cu3Oy (SmBCO) films in high magnetic fields. We tried to introduce high-density BaHfO3 (BHO) nanorods into SmBCO films by increasing the BHO content. We fabricated 2.3, 4.9 and 7.2 vol.% BHO-doped SmBCO films at the same deposition temperature, and compared the geometric formation of nanorods and flux pinning properties. From TEM observation, the densities of BHO nanorods increased with increases in BHO content, and the diameters of all nanorods were almost the same (i.e., approximately 12 nm). The highest density and matching field were 1711 /μm2 and 3.54 T in a 7.2 vol.% sample. The flux pinning force of the sample with a higher matching field was effective in higher fields in the magnetic field dependence of Jc. This was in good agreement with theories and many reports. In irreversibility field lines, a unique behavior, which probably originated in the thick diameters, was observed around the low magnetic field.
We have proposed a low-temperature growth (LTG) technique which makes it possible to fabricate SmBa2Cu3Oy (SmBCO) films with c-axis orientation in a wide range of growth temperatures without the degradation of superconducting properties. In this study, using the LTG technique, we fabricated SmBa2Cu3Oy (SmBCO) films with BaHfO3 (BHO) nano-rods at low substrate temperatures using the pulsed laser deposition (PLD) method. The effect of low substrate temperatures on BHO nano-rods in SmBCO films are discussed in terms of their microstructures and superconducting properties. The nano-rods were observed clearly in transmission electron microscopic (TEM) images of the films. We found that the morphologies of the nanorods were small in diameter, had high densities and inclined along the c-axis of SmBCO. In the case of the substrate temperature of 750°C, the diameter and density of nano-rods with 2.7 vol.% BHO-doping were 5.7 nm and 2300 /μm2, respectively. The film showed high flux pinning force, Fp, of 25 GN/m3 (77 K, B//c) in a high field, 4 T. Furthermore, in the film, the angular dependence of Jc showed a broad peak at B//c, due to the flux pinning by nano-rods growing in various directions.
The Combinatorial-PLD (C-PLD) method is an efficient way to screen the optimal composition in YBCO thin films because it is possible to create a film with a continuously changing composition across a single substrate. In this paper, we report on the quick exploration of optimal content for flux pinning materials in YBCO thin films using the C-PLD method. We optimized the content of BaSnO3 and the optimal content was almost the same as that of another report. Candidates for new pinning materials were also explored using the C-PLD method. Furthermore, we substituted a part of the Cu in YBCO films with various types of 3d-metals. As a result, upper critical fields (Hc2) were improved by substituting Co and Ni. Though Hc2 in a pure-YBCO film was about 100 T at 0 K, the YBCO films with the substitution of Co and Ni achieved an Hc2 of about 270 T at 0 K.
Columnar defects (CDs) produced by heavy-ion irradiations in high-Tc superconductors such as YBa2Cu3Oy (YBCO) are the most effective pinning centers to immobilize flux lines, leading to the enhancement of the critical current density Jc. In addition, slight dispersion (splay) in the directions of CDs can further improve the Jc. In this work, YBCO thin films were irradiated using 200 MeV Xe ions at two angles ±θi relative to the c-axis in two geometries where the direction of current is perpendicular (mode A) or parallel to the splay plane (mode B), to investigate the influence of the direction of splay plane on the flux pinning properties. In a low magnetic field parallel to the c-axis, a weaker magnetic field dependence of the critical current density Jc for mode B than that for mode A was observed and the Jc for mode B with θi=±45º was larger than any other sample in this work, regardless of larger crossing angles relative to the c-axis. This is attributed to the quasi-2D flux pinning by the splay plane perpendicular to the flux line motion for mode B. The behaviour of the angular dependence of Jc significantly depends on the direction of the splay plane: a crossover from a rectangle-shaped Jc peak to double peaks occurs in the angular dependence of Jc for mode A with θi =±45º in increasing magnetic field, whereas a remarkably large and broad Jc peak appears for mode B with θi =±45º.
YBa2Cu3Oy (YBCO) films with BaHfO3 (BHO) nanoparticles were fabricated by a metal organic deposition (MOD) method using a solution with trifluoroacetates and Hf salts. X-ray diffraction (XRD) revealed the formation of BHO in highly oriented YBCO film, and scanning transmission electron microscopy (STEM) with energy dispersive X-ray spectroscopy (EDS) showed the elemental distributions of Y, Cu, Ba, Hf and O in the film. The average size of BHO in the film was about 18 nm. Critical current density (Jc) properties of the films with BHO were higher than those of the films with BaZrO3 in magnetic fields.
BaSnO3 (BSO) doped GdBa2Cu3Oy (GdBCO) films were deposited on CeO2 (200) substrates using the pulsed laser deposition (PLD) method. Both 2D and 3D microstructural characterization were carried out by transmission electron microscope with selected area electron diffraction patterns, scanning transmission electron microscope with energy dispersive X-ray spectroscopy and electron tomography. In the GdBCO film with BSO 7.5 mol% addition, BSO nanorods were observed with an average width of 8.7 nm, average length of 43 nm, volume fraction of 3.4%, and number density of 2.7×1016 cm-3. Additionally, BSO nanorods were grown in the film with the tilted angle of approximately 0-30 degrees to the c-axis direction of GdBCO. Improvements of Jc were clearly due to the presence of these BSO nanorods.
Correlated pinning properties for GdBa2Cu3O7 (Gd123) tapes with BaHfO3 and BaZrO3 nanorods are investigated on the basis of the detailed critical current measurements in a wide temperature and magnetic field region. We found that a magnetic field dependency of Jc for the Gd123 tapes with nanorods introduced changes around the matching field B while that for the non-doped Gd123 tapes is proportional to B-0.56 in a wide field region below 20 K. These experimental results suggest that the nanorods are still effective as the c-axis correlated pinning center at low temperatures, although the Jc peaks at B//c in its angular dependency disappear below 20 K at 3 T. The cooperative model of random and correlated pinning centers indicates that the c-axis peaks in angular dependency of Jc disappear when fluctuation of the nanorods' growth direction is large. Therefore, the nanorods are effective as the c-axis correlated pinning centers even at low temperatures around 4.2 K, although their contribution becomes small.
The introduction of artificial pinning centers such as BaZrO3 (BZO), BaSnO3 (BSO) and BaHfO3 (BHO) nano-rods into REBCO film is effective to improve in-field critical currents (Ic). In particular, BHO doping into Gd1Ba2Cu3O7-δ (GdBCO) has been found to exhibit high in-field Ic values under a wide range of temperatures and magnetic fields. However, the increase of Ic values of the GdBCO + BHO system exhibited somewhat of a saturated manner, with thickening of the superconducting films to more than 3μm. In this work, a new combination of Eu1Ba2Cu3O7-δ (EuBCO) + BHO system was investigated using the IBAD/PLD process to improve in-field performance in thick films. As a result, it was found that a EuBCO + BHO thick film showed an extremely high Ic, min property of 141 A/cm-w at 77 K and 3 T. A 93.7-m-long EuBCO + BHO CC estimated Ic value of 108 A/cm-w at 77 K and 3 T was successfully fabricated.