Max Sieger

Large pinning forces and matching effects in YBa2Cu3O7-δ thin films with Ba2Y(Nb/Ta)O6 nano-precipitates

The addition of mixed double perovskite Ba2Y(Nb/Ta)O6 (BYNTO) to YBa2Cu3O7−δ (YBCO) thin films leads to a large improvement of the in-field current carrying capability. For low deposition rates, BYNTO grows as well-oriented, densely distributed nanocolumns. We achieved a pinning force density of 25 GN/m3 at 77 K at a matching field of 2.3 T, which is among the highest values reported for YBCO. The anisotropy of the critical current density shows a complex behavior whereby additional maxima are developed at field dependent angles. This is caused by a matching effect of the magnetic fields c-axis component. The exponent N of the current-voltage characteristics (inversely proportional to the creep rate S) allows the depinning mechanism to be determined. It changes from a double-kink excitation below the matching field to pinning-potential-determined creep above it.

Local Orientation Variations in YBCO Films on Technical Substrates - A Combined SEM and EBSD Study

Scanning electron microscope imaging and electron backscatter diffraction are applied to 400 nm thick YBCO films grown on Ni-9at.%W and ABAD-YSZ tape. On Ni-9at.%W tape, the orientation of YBCO strongly varies from grain to grain, which is attributed to the different orientations of the underlying substrate grains with regard to the surface normal. On ABAD-YSZ, the structures causing the orientation variations are observed on a micrometer scale only, which is attributed to the granularity of the template. In contrast to Ni-9at.%W where no preferred misorientation axis is notable within single substrate grains, the misorientation of YBCO on the ABAD-YSZ tape is primarily caused by lattice rotations about the sample normal.

Ba 2 Y(Nb/Ta)O 6 –Doped YBCO Films on Biaxially Textured Ni–5at.% W Substrates

The incorporation of nanoscaled pinning centers in superconducting YBa₂Cu₃O_7-δ (YBCO) films is one of the core topics to enhance the critical current density J_c(B, Θ) of coated conductors. The mixed double-perovskite Ba₂Y(Nb/Ta)O₆ (BYNTO) can be grown in nanosized columns parallel to the YBCO c-axis and in steplike patterns, making it customizable to meet specific working conditions (T, B, Θ). We compare a 1.6 μm thick film of pure YBCO and a similar film with additional 5 mol% of BYNTO, grown by pulsed laser deposition with a growth rate of 1.6 nm/s on chemically buffered biaxially textured Ni-5at.% W tape. Our doped sample shows nanosized BYNTO columns parallel to c_YBCO and plates in the ab-plane containing Y, Nb, and Ta. An improved homogeneity of the critical current density J_c over the sample was evaluated from trapped field profiles measured with a scanning Hall probe microscope. The mean J_c in the rolling direction of the tape is 1.8 MA/cm² (77 K, self-field) and doubles the value of the undoped sample. Angular-dependent measurements of the critical current density, J_c(Θ), show a decreased anisotropy of the doped film for various magnetic fields at 77 K and 64 K.

Planar current anisotropy and field dependence of J c in coated conductors assessed by scanning Hall probe microscopy

The local distribution of the critical current density, J c, of coated conductors and YBa2Cu3O (YBCO) films on single crystalline substrate was investigated by scanning Hall probe microscopy. The high spatial resolution of the measurements enabled an assessment of dependence of the local J c on the local magnetic induction, B, and electric field, E. The derived J c(B)-dependence agreed well with the global J c obtained from magnetization loops and provided values of J c at very low fields, which are inaccessible to magnetization and transport measurements. The anisotropic current flow within the film plane was investigated in YBCO films on miscut SrTiO3 substrates and a GdBa2Cu3O film on an MgO buffer layer prepared by inclined substrate deposition on a Hastelloy substrate. The c-axis currents calculated from the Hall maps were significantly larger than previously reported data obtained from direct transport measurements. The planar current anisotropy at 77 K was found to be highly influenced by the microstructure which can either deteriorate the current flowing across the ab-planes or cause enhanced pinning and increase the critical current flowing parallel to the ab-planes.

Pinning Centers in ISD-MgO Coated Conductors via EB-PVD

GdBa₂Cu₃O_7-x (GdBCO) films doped with up to 13.6 mol.% BaHfO₃ (BHO) have been grown by electron beam physical vapor deposition (EB-PVD). A critical current density J_c at 30 K, 3 T of 2.34 MA/cm² has been achieved for a doping level of 10.6 mol.%. This is the first report of artificial pinning centers being successfully incorporated into REBa₂Cu₃O_7-x (REBCO, RE = Y, Gd, Dy . . .) via EB-PVD. The samples were grown on Hastelloy tapes coated with a MgO buffer layer deposited by inclined substrate deposition. The J_c of the samples was characterized at 77 K, 1 T and at 30 K, 3 T. An increase in J_c (30 K, 3 T) upon Hf addition has been observed in the whole angular range α. The level of Hf addition was found to heavily influence the kind of nanoprecipitate formed, with low levels of Hf causing a reduction in the anisotropy of J_c (77 K, 1 T) and higher levels of Hf resulting in the formation of nanoprecipitates elongated parallel to the GdBCO ab-plane, causing an increase in the anisotropy of J_c (77 K, 1 T). The largest J_c (77 K, sf) measured was 0.63 MA/cm^-2.

-Doped Thick Films on Highly Alloyed Textured Ni-W Tapes

YBa₂Cu₃O_7-δ (YBCO) films with a thickness of up to 2 μm containing nano-sized BaHfO₃ (BHO) have been grown on biaxially textured metal tapes by pulsed laser deposition. Transmission electron microscopy revealed the inclusion of Y₂O₃ platelets and rod-like BHO structures. A c-axis oriented growth of the YBCO layer and biaxial oriented incorporation of BHO is confirmed by X-Ray diffraction. BHO expands the c-axis of YBCO, flattens the surface of YBCO and densifies the films. Y₂O₃ inclusions get smaller and finer distributed, possibly with a positive influence on pinning effects. A superconducting transition temperature T_c of about 89 K was determined, decreasing slightly with increasing BHO content. Transport current measurements in the maximum Lorentz force configuration showed an increased irreversibility field H_irr and higher pinning force density F_p with increasing BHO content in high magnetic fields.

Thick High YBCO Films on ABAD-YSZ Templates

In this paper, we report on high critical current density (Jc) YBCO films deposited by pulsed laser deposition on CeO2 buffered ABAD-YSZ/stainless steel templates with a thickness ranging from 0.7 μm up to 4.2 μm. In the thinnest sample, a transition temperature (Tc) of 90 K and a critical current density of 2.6 MA/cm2 was reached at 77 K. With increasing thickness, Tc drops as well as Jc. The total critical current (Ic) increases strongly up to a thickness of 2.8 μm, reaching a value of almost 600 A/cm-width. In thicker films, no further increase in Ic was observed. A higher surface roughness and misoriented YBCO grains were found in layer thicknesses above 2.8 μm and are assumed to be the main reason for the limitation of the current transport.

Reduced J_{c} anisotropy and enhanced in-field performance of thick BaHfO_{3} -doped YBa_{2}Cu_{3}O_{7-\delta} films on ABAD-YSZ templates

Pure and 6 mol% BaHfO₃ (BHO)-doped YBa₂Cu₃O_7-δ (YBCO) films with a film thickness of around 1 μm were prepared on CeO₂-buffered ABAD-YSZ templates by pulsed laser deposition. The self-field J_c at 77 K reaches 1.1 MA/cm² in the doped sample compared with 2.5 MA/cm² in pure YBCO. Above a magnetic field of 2.2 T along B||c, J_c of the BHO-doped sample exceeds the J_c of the undoped film. The maximum pinning force density (F_P,max) reaches a value of around 3 GN/cm² for both samples, but B(F_P,max) increases from 1.4 T (pure) to a value of 2.9 T (BHO:YBCO). The J_c anisotropy curves of the doped sample show a large and broad peak at B||c and a strongly reduced anisotropy at all temperatures and fields compared with the pure sample. A complex defect structure with YBa₂Cu₄O₈ intergrowths, Y₂O₃ precipitates, and BHO nanocolumns with a fan-shaped structure is observed by transmission electron microscopy investigations, which can explain the measured J_c(B, θ) behavior.

Reduced

Pure and 6 mol% BaHfO₃ (BHO)-doped YBa₂Cu₃O_7-δ (YBCO) films with a film thickness of around 1 μm were prepared on CeO₂-buffered ABAD-YSZ templates by pulsed laser deposition. The self-field J_c at 77 K reaches 1.1 MA/cm² in the doped sample compared with 2.5 MA/cm² in pure YBCO. Above a magnetic field of 2.2 T along B||c, J_c of the BHO-doped sample exceeds the J_c of the undoped film. The maximum pinning force density (F_P,max) reaches a value of around 3 GN/cm² for both samples, but B(F_P,max) increases from 1.4 T (pure) to a value of 2.9 T (BHO:YBCO). The J_c anisotropy curves of the doped sample show a large and broad peak at B||c and a strongly reduced anisotropy at all temperatures and fields compared with the pure sample. A complex defect structure with YBa₂Cu₄O₈ intergrowths, Y₂O₃ precipitates, and BHO nanocolumns with a fan-shaped structure is observed by transmission electron microscopy investigations, which can explain the measured J_c(B, θ) behavior.

J_{\mathrm{c}}

Pure and 6 mol% BaHfO₃ (BHO)-doped YBa₂Cu₃O_7-δ (YBCO) films with a film thickness of around 1 μm were prepared on CeO₂-buffered ABAD-YSZ templates by pulsed laser deposition. The self-field J_c at 77 K reaches 1.1 MA/cm² in the doped sample compared with 2.5 MA/cm² in pure YBCO. Above a magnetic field of 2.2 T along B||c, J_c of the BHO-doped sample exceeds the J_c of the undoped film. The maximum pinning force density (F_P,max) reaches a value of around 3 GN/cm² for both samples, but B(F_P,max) increases from 1.4 T (pure) to a value of 2.9 T (BHO:YBCO). The J_c anisotropy curves of the doped sample show a large and broad peak at B||c and a strongly reduced anisotropy at all temperatures and fields compared with the pure sample. A complex defect structure with YBa₂Cu₄O₈ intergrowths, Y₂O₃ precipitates, and BHO nanocolumns with a fan-shaped structure is observed by transmission electron microscopy investigations, which can explain the measured J_c(B, θ) behavior.