high-Jc Films Research Articles

Strong c-axis correlated pinning and hybrid pinning in YBa2Cu3O7−δ films containing BaHfO3 nanorods and stacking faults

High critical current density (Jc) was achieved in low temperatures of 40 and 20 K in YBa2Cu3O7−δ films containing elongated BaHfO3 nanorods and stacking faults, which were prepared using pulsed laser deposition and post-oxygen-annealing. The YBa2Cu3O7−δ + BaHfO3 film exhibited global pinning force maximum (Fp,max) of 312 GN m−3 at 40 K and 619 GN m−3 at 20 K due to strong pinning by nanorods. These Fp,max values are comparable to those in the previously-reported high-Jc films with almost the same matching field. Thus, the present study demonstrated that YBa2Cu3O7−δ superconducting matrix realized high Jc in low temperatures like other REBa2Cu3O7−δ (RE = Gd, Sm, (Gd, Y)). Furthermore, high Jc was obtained also for magnetic field parallel to the ab-plane, indicating hybrid pinning of nanorods and stacking faults. Thus, the stacking faults significantly affected the Jc properties in the YBa2Cu3O7−δ films containing nanorods.The present film structure with strong c-axis correlated pinning and hybrid pinning is effective in simultaneous control of Jc values and Jc anisotropy in YBa2Cu3O7−δ films.

High critical currents in heavily doped (Gd,Y)Ba2Cu3Ox superconductor tapes

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.

Pulsed laser deposition of BaFe2(As,P)2 superconducting thin films with high critical current density

Superconducting BaFe2(As0.6P0.4)2 (Ba122:P) thin films were fabricated on MgO(100) substrates by a pulsed laser deposition (PLD) method using a second-harmonic Nd:YAG (yttrium aluminum garnet) laser. Structural investigation by means of x-ray diffraction confirmed both c-axis orientation and in-plane alignment, or epitaxial growth on the substrate. The film exhibited Tc(onset) = 26.5 K and Tc(zero) = 24.0 K. High Jc values of 3.5 MA cm−2 at 4.2 K in the self-field and over 1 MA cm−2 at 10 K under 1 T were also obtained. Fabrication of Ba122:P film by Nd:YAG PLD seems to be a promising approach for preparing superconducting tapes, since a high-Tc and high-Jc film can be relatively easily obtained.

Critical current densities and the structural quality of 3- and 4-μm-thick superconducting YBa2Cu3O7 layers synthesized using the ex situ process

In this work, we used quantitative x-ray diffraction measurements and optical metallography to investigate the relationship between structural quality and critical current densities Jc for 3- and 4-μm-thick YBa2Cu3O7 (YBCO) films grown on CeO2-buffered Ni–W substrates by the BaF2 process. The Jc of the films was shown to be approximately: (1) proportional to the intensity of the YBCO (006) diffraction line, and (2) inversely proportional to the average grain size of the c-axis oriented YBCO as determined from optical micrographs of polished surface of the films. We conclude that to achieve Jcs well above 106 A/cm2 in self-field and at 77 K, it is critical to suppress the formation of randomly oriented YBCO grains while maintaining high crystallographic quality of the c-axis oriented part of the film. The quality of the c-axis oriented YBCO was found to be strongly dependent on the YBCO grains size—e.g., the grains, which are smaller than 10 μm, are required for high Jc films. The fine-grain high Jc films can be synthesized under processing conditions that promote a high rate of nucleation of c-axis oriented YBCO.

High critical current densities in YBa2Cu3O7−x films grown at high rates by hybrid liquid phase epitaxy

Liquid-mediated growth of YBa2Cu3O7−x has the potential to be high rate and low cost. However, the reported critical current densities (Jc) are generally lower than for films deposited by physical vapor deposition processes. We report the deposition of thick high-Jc films (1.2MAcm−2 in self-field) on (001) SrTiO3 by high-rate hybrid liquid phase epitaxy, and show angular-dependent transport critical current as a function of applied field for these films, as well as microstructural measurements by transmission electron microscopy.

High-Ic processing for YBCO coated conductors by TFA-MOD process

YBCO films were fabricated on PLD-CeO2/IBAD-Gd2Zr2 O7/Hastelloy substrates using the TFA-MOD process. In order to obtain higher Ic performance, the thicker films maintaining high-Jc values are required. Jc depended strongly on the PH2O during the crystallization in the process. The pore size was smaller in the high-Jc films crystallized under the medium PH2O condition and became larger in the low-Jc films under both lower and higher PH2O for the YBCO films with a constant thickness. Furthermore, crack generation was observed in thick films crystallized at the high PH2O condition. Consequently, both pore and crack formations limit the Jc properties since large pore causes local reduction of current paths and additionally results in local concentration of electric fields. Jc also depended strongly on the YBCO crystal grain alignment which is affected by in-plane grain alignment of the buffered substrate. Then, the effect of in-plane grain alignment of the CeO2 cap layer on Jc was investigated for thicker YBCO films. As a result, the Jc value increased with improvement of the crystal alignment of the CeO2 cap layers. Finally, a YBCO film with a high-Ic value of 413A at 77K in self-field was achieved in a film fabricated on Δϕ=4° of the CeO2 layer using the appropriate crystallization conditions for crack-free film.

Heavy ion irradiation and the vortex-glass description of the mixed state in YBa2Cu3O7 epitaxial films

We report the behavior of the Ohmic resistivity and the current-dependent dissipation in the mixed state of YBa2Cu3O7 films irradiated with 300 MeV gold ions. The transport in both irradiated and unirradiated states of the samples agrees with the model of a second order phase transition to a vortex glass state. While the zero-field critical temperature and current are reduced by the irradiation, the vortex-glass description of the mixed state remains unchanged up to a dosage of ∼9 × 1010 ions/cm2. These results indicate that in these high Jc films the vortex dynamics is controlled by pinning centers distributed at a length scale much shorter than the gold-ion induced defects.