REBa2Cu3Ox ((REBCO), RE = rare earth) superconductor tapes with moderate levels of dopants have been optimized for high critical current density in low magnetic fields at 77 K, but they do not exhibit exemplary performance in conditions of interest for practical applications, i.e., temperatures less than 50 K and fields of 2–30 T. Heavy doping of REBCO tapes has been avoided by researchers thus far due to deterioration in properties. Here, we report achievement of critical current densities (Jc) above 20 MA/cm2 at 30 K, 3 T in heavily doped (25 mol. % Zr-added) (Gd,Y)Ba2Cu3Ox superconductor tapes, which is more than three times higher than the Jc typically obtained in moderately doped tapes. Pinning force levels above 1000 GN/m3 have also been attained at 20 K. A composition map of lift factor in Jc (ratio of Jc at 30 K, 3 T to the Jc at 77 K, 0 T) has been developed which reveals the optimum film composition to obtain lift factors above six, which is thrice the typical value. A highly c-axis aligned BaZrO3 (BZO) nanocolumn defect density of nearly 7 × 1011 cm−2 as well as 2–3 nm sized particles rich in Cu and Zr have been found in the high Jc films.
High performance, high temperature superconductor tapes can make a substantial impact in a variety of high power and high magnetic field applications as a replacement to resistive copper conductors and low temperature superconductors. REBa2Cu3Ox ((REBCO), RE = rare earth) superconductor tapes made by epitaxial growth of REBCO thin films by vapor deposition and solution deposition methods on biaxially textured buffer layers on flexible metal substrates1,2 are now produced in lengths of 500–1000 m by several institutions.3–7 High critical current densities are being pursued in REBCO superconducting tapes in high magnetic fields (2–30 T) in a temperature range of 4.2 K–50 K for use of these materials in electric power applications such as motors, generators, superconducting magnetic energy storage (SMES) as well as in high energy particle accelerators, magnetic resonance imaging, and high-field magnets. Introduction of Y2BaCuO5,8 BaZrO3 (BZO),9–13 BaSnO3,14,15 BaHfO3,16 Ba2YNbO6,17,18 and Gd3TaO719 nanoscale defects in the REBCO films of the superconducting tapes has been proven to be a powerful method to increase critical current density (Jc) of these tapes via enhanced flux pinning by these defects. Until recently, high Jc in REBCO films with BaMO3 (M = Zr, Sn, and Hf) nanoscale defects has been obtained only with less than 10 mol. % of dopants in films made by pulsed laser deposition (PLD),9–11,14–16 metal organic deposition,12 and metal organic chemical vapor deposition (MOCVD).13 Such moderately doped REBCO films have been optimized for high Jc in low magnetic fields at 77 K and, as such, do not exhibit exemplary performance in conditions of interest for practical applications, i.e., temperatures less than 50 K and fields of 2–30 T. A sharp reduction in the superconducting transition temperature (Tc) in films with high levels of BaMO3 addition resulted in low Jc15 and hence, heavily doped REBCO films have been generally avoided by most researchers. Using a modified MOCVD process, we have been exploring REBCO films with dopant content higher than normal levels.20 In this paper, we report REBCO tapes with nearly three-fold higher level of dopant addition that has yielded record high critical current densities over 20 MA/cm2 at 30 K, 3 T and the achievement of pinning force levels above 1000 GN/m3 at 20 K in any superconductor.
The superconductor films were grown epitaxially on LaMnO3-terminated biaxially textured buffer layers of MgO fabricated by ion beam assisted deposition on Hastelloy C276 substrates, 50 μm in thickness and 12 mm in width.21 All films including the superconductor were grown by reel-to-reel thin film processes. MOCVD using a liquid precursor delivery system was used to grow (Gd,Y)BaCuO films with 25% Zr addition.22 Standard tetramethyl heptanedionate (thd) precursors were used for all components including Zr. The deposition rate of the film was approximately 0.1 μm/min controlled by the precursor molarity and precursor flow rate. The superconductor film thickness was about 0.9 μm thick, controlled by the substrate tape movement speed.
The composition of the films was analyzed by inductively coupled plasma (ICP-MS) mass spectroscopy. Transport critical current measurements were conducted on tapes without copper stabilizer, at 77 K, in zero applied magnetic field and in presence of magnetic fields up to 9 T over a temperature range of 20–77 K using a standard four probe method. A bridge of 1–2 mm in width and approximately 10 mm in length was used for critical current measurements to minimize the current required and sample heating. A voltage criterion of 3 μV/cm was used to determine the critical current. Plan-view and cross section view Transmission Electron Microscopy (TEM) examination of a few samples was conducted using a JEOL-2000 FX microscope. The high-resolution microscope used for this study was a scanning TEM (STEM) Nion UltraSTEM 200™ operating at 200 kV equipped with a second generation 5th order probe aberration corrector, a cold field emission electron gun, and a Gatan Enfinium electron energy loss (EEL) spectrometer.23 This microscope routinely achieves a spatial resolution of 0.8 Å and has a maximum energy resolution of 300 meV. In order to clarify the origin of the enhanced pinning in high-content Zr samples, simultaneous bright field (BF) and high angle annular dark field (HAADF) images were acquired simultaneously. Due to the theorem of reciprocity24 between TEM and STEM, BF STEM images are analogous to phase contrast TEM images and, as such, are more sensitive to strain and interference phenomena. On the other hand, owing to the incoherent nature of the image imposed by the annular detector, HAADF images are less sensitive to strain but yield an intensity proportional to ∼Z2, showing compositional and mass contrast. EEL spectra were acquired in the thinnest region of the sample in order to maximize the signal originated within the small nanoparticles. Care was taken in the choice of dwell time and step size in order to avoid radiation damage to the area being analyzed. Both low-energy loss Ba-N4,5, Y-M4,5, Zr-M4,5 and high-energy Ba-M4,5 Cu-L2,3, and Gd-M4,5 edges were recorded.
A comparison of the critical current densities at 30 K, B ⊥ tape of GdYBCO films made by MOCVD with 7.5, 15, and 25 mol. % Zr addition is displayed in Figure 1. Critical current densities significantly higher than the previous best values in 15 mol. % Zr-added tapes have been attained in the 25 mol. % Zr-added tapes. At 3 T, the Jc of the 25 mol. % Zr-added tape is 20.3 MA/cm2, which is 1.56 times higher than that previously reported in 15 mol. % Zr-added tapes.20 The corresponding calculated critical current value in a 12 mm wide tape at 30 K, 3 T (B ⊥ tape) is 2195 A, which equates to an engineering current density (Je) of 1829 A/mm2 considering a typical tape thickness of 0.1 mm with 40 μm of copper stabilizer. Such a high Je value will enable a three-fold reduction in the amount of the superconductor tape needed for superconducting devices such as a 10 MW superconducting wind generator, which in turn will greatly improve the economics for commercial applications. Since the present cost of superconductor tapes is about 60% of the cost of a 10 MW wind generator, a significant cost reduction can be attained with the high Je tape in this and other potential superconducting applications. Maximum pinning force levels of 690–700 GN/m3 at 4.5 to 6 T at 30 K and 1010–1020 GN/m3 at 5 to 7 T at 20 K have been measured in the 25 mol. % Zr-added tape as shown in the inset of Figure 1. The power law exponent α in the range of 3–9 T in the magnetic field dependence of Jc of the 25% Zr-added tape, i.e., Jc = kB−α in Figure 1, is found to be 1.0 (k is a constant). Such a high alpha value points to uncorrelated isotropic pinning in this tape.
The critical current density of the 25 mol. % Zr-added tape described in Figure 1, at 77 K in zero applied magnetic field, is 3.1 MA/cm2. Such a high critical current density at 77 K, 0 T is possible in MOCVD-grown films because of Tc values as high as 90 K even with such high levels of Zr addition.20 The critical current density of 20.3 MA/cm2 at 30 K, 3 T corresponds to a lift factor of 6.5 which compares to values of 4.2 and 2.1 in 15 mol. % and 7.5 mol. % Zr-added tapes. Such a high lift factor indicates strongly active flux pinning mechanisms at lower temperatures. It was found that the lift factor at 30 K, 3 T depended not only on the Zr content but also on the Ba and Cu contents in the superconductor film. An extensive analysis of 38 GdYBCO films with 15–25 mol. % Zr addition was conducted to develop a composition map of lift factor at 30 K, 3 T as a function of Ba, Cu, and Zr atomic fractions in the films and result is shown in Figure 2. It is seen from the figure that in addition to a Zr atomic fraction of 25%–35%, a Cu fraction less than 58% and a Ba fraction more than 38% are required to achieve lift factors above 5.4 at 30 K, 3 T. The excess Ba is apparently needed to form sufficient BaZrO3. It is found that a lift factor above 6 at 30 K, 3 T could be achieved only in films with (Ba + Zr)/Cu ratio above 0.71. We find from X-ray diffraction that films with a higher Cu content contain Cu2O. It is possible that such Cu2O formation in films with low (Ba + Zr)/Cu content can interrupt continuous growth of BZO nanocolumns which in turn can diminish the lift factor. Since the Jc at 77 K, 0 T can reduce in films with too deficient a Cu composition, a maximum threshold is expected for the (Ba + Zr)/Cu ratio to achieve a combination of a high lift factor and a high critical current density at 30 K, 3 T.
Angular dependence of Jc in high magnetic fields provides valuable information on the anisotropy in Jc especially in superconducting films with multiple pinning mechanisms. Figure 3 shows the angular dependence of Jc of a 25% Zr-added GdYBCO tape in a magnetic field of 3 T at 77, 65, 50, and 30 K. The Jc at 77 K, 3 T shows a prominent peak at B || c (B ⊥ tape) which is 2.7 times the value of the weak peak at B || ab (B|| tape). The abundant BZO nanorods aligned along the c-axis that will be discussed next must have contributed to strong correlated pinning resulting in such a prominent peak in Jc at B || c at 77 K. A higher ratio of Jc at B || c to that at B || ab at 77 K, 3 T was recently found to predict a higher minimum Jc at 30 K, 3 T.25 The prominent peak at B || c persists at 65 K, 3 T where the ratio of Jc at B || c to that at B || ab increases to 3.3. However, at 50 K, 3 T, this ratio reduces to 1.46 and further reduces to 0.78 at 30 K, 3 T. These results indicate that in addition to correlated pinning by BZO nanocolumns along the c-axis, other types of defects are becoming more prominent pinning centers at lower temperatures. Another manifestation of uncorrelated isotropic pinning at 30 K in the 25% Zr-added tape is the increased width of the peak in Jc in the orientation of B || a-b as seen in Figure 3.
The microstructure of GdYBCO tapes with 25 mol. % Zr addition examined by cross sectional TEM reveals continuous BZO nanocolumns threading through the entire superconductor film thickness of about 0.9 μm as shown in Figures 4(a) and 4(b). The BZO nanocolumns are aligned perpendicular to the film plane except near the interface with the LaMnO3 buffer atop the metal Hastelloy C276 metal substrate. While it is understood that the BaMO3 nanocolumn growth is by a strain-mediated self-assembly process,26 a re-orientation of BaMO3 along the film plane has been predicted by a micromechanical model based on theory of elasticity in films with high levels of BaMO3 levels.27 While we have observed a mixed orientation of BZO nanocolumns (along the c-axis and the a-b plane) in GdYBCO films with more than 15 mol. % Zr addition, the highest Jc at 30 K, 3 T (B ⊥ tape) has been obtained only in films with BZO nanocolumns aligned along the c-axis.28 Plan view TEM of BZO nanocolumns shown in Figure 4(c) reveals that they are about 6 nm in diameter which confirms the prediction that they do not coarsen with increased dopant content.28 The density of these nanocolumns has been measured to be 6.9 × 1011 cm−2, which is about twice as high as that found in films with 15 mol. % Zr addition. From plan view TEM images, the volume fraction of BZO is measured to be approximately 10%.
While indirect evidence of uncorrelated isotropic pinning is found in the electromagnetic properties of the MOCVD-grown GdYBCO tapes,29 no microstructural evidence of potential sources of such pinning has been found so far. In order to investigate such sources of pinning, the 25% Zr-added GdYBCO tape discussed in Figure 1 was examined in an aberration-corrected scanning transmission electron microscope. In BF TEM images shown in Figure 4(d), the BZO nanocolumns are revealed with a lighter contrast than the superconductor film. The BZO nanocolumns appear often decorated by dark spots indicative of strain and moire` fringes due to the superimposition of the different BZO and REBCO lattices. In HAADF imaging displayed in Figure 4(e), the BZO nanocolumns appear in similar contrast as the REBCO film, but round nanoparticles surrounding the BZO appear brighter now indicating that they are precipitates of a different composition and density than either REBCO or BZO. In addition, while they are almost invisible in BF, using HAADF, these small round nanoparticles are clearly visible in the REBCO film between the BZO nanocolumns as well. The nanoparticles are only 2–3 nm in diameter and 4–5 nm apart. A high resolution image of a BZO nanocolumn and surrounding nanoparticles is shown in Figure 4(f), where some of the nanoparticles are indicated by arrows. The nanoparticles are randomly distributed and could contribute to uncorrelated isotropic pinning discussed earlier. The chemical composition was analyzed using in situ EEL spectroscopy with atomic-scale spacial resolution resulting from the aberration corrected electron probe. Shown in Figure 4(g) are elemental maps obtained by plotting the integrated intensity of the respective edges, for higher energy (upper) and lower energy (lower) spectra. The EEL spectra reveal that the nanoparticles contain less Ba, Y, and Gd than the REBCO film and are rich in Cu and Zr.
Heavily doped REBCO tapes provide a pathway to achieve high pinning forces over a wide temperature range in high magnetic fields. Given that the films described in this paper are only 0.9 μm thick, much higher critical currents and high engineering current densities should be feasible with thicker heavily doped films. At such high performance levels, REBCO tapes would become cost effective for a wide range of high power and high magnetic field applications over a temperature range of 4.2–50 K.
The work at the University of Houston was supported by the Advanced Research Projects Agency-Energy (ARPA-E) Award No. DE-AR0000196 and the Office of Naval Research Award No. N00014-14-1-0182. C. Cantoni acknowledges support by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division.