Ye-Hyun Jung

Effects of the vortex line shape on the critical current density in high Tc superconducting film with nanorod pinning centers

We studied the critical current density distribution in a coated conductor comprised of (Gd,Y)1Ba2Cu3O7−δ–BaZrO3 film. Transmission electron microscopy measurements showed that nanorod pinning centers tilt by ~13° from the c-axis. Magneto-optical image (MOI) measurements showed interesting asymmetric distributions of magnetic flux density. From MOIs we calculated the asymmetric distributions of the critical current density, which is associated with the properties of vortex pinning. We were able to explain these results through the geometrical relationship of the tilted rod pinning centers and the curved vortex lines.

Scanning Hall probe measurements of field distributions of a coated conductor under applied fields

We measured the field profiles near the surface of a coated conductor (CC) under various applied fields by using the scanning Hall probe method. The field, applied in the normal direction, was increased from zero to 171.5 Oe and then decreased to −58.8 Oe. We could not analyse our data completely by the direct use of Brandts calculation but by a modification with unusual field dependences of the introduced parameters. Since Brandts original calculation was based on homogeneous films, it was not suitable for CCs with coarse granular structures. The modified calculations with appropriate parameters are related to the coarse granular structures. Those parameters, D, Jc, and R, represent the three characteristics of the flux penetration network: the average distance of flux penetrations, the density of critical sheet currents, and the range of meandering of the flux penetration front, respectively. The external field dependences of these parameters were different from those of the classical critical state model.

Pushing Coated Conductor Critical Currents Beyond 1 kA per cm Width: Stacks of YBCO Layers

For a number of superconducting power applications, a high value of the engineering critical current density (Je) for the wire is crucial. The superconducting layer in the coated conductor is typically a small portion of the overall cross-section, so increasing the superconductor fraction will directly result in an increase of Je . However, as the thickness of the superconductor is increased, Jc eventually drops. We describe a way to increase Je by making a stack of superconducting layers using sequential Ion-Beam Assisted Deposition (IBAD)/Superconductor deposition. Reactive Co-Evaporation by Cyclic Deposition and Reaction (RCE-CDR) is used for superconductor. An IBAD-textured layer resets the crystalline structure after each superconducting layer and we use IBAD-MgO for this purpose. However, IBAD-MgO texturing requires an extremely smooth starting surface (about 1 nm root mean square roughness), whereas the YBCO layer is typically 10-100 times rougher. We employ the Solution Deposition Planarization (SDP) process to planarize the rough surface of YBCO. The SDP layer is insulating and it provides for an easy way to separate the superconducting layers electrically. We discuss unique features of the stacking structure that allow for high Ic, low ac-losses in applied fields, as well as high Je .

The effects of a magnetic field dependent critical current on the hysteresis loss of a coated conductor

We calculated the current distribution in a coated conductor (CC) from magnetic induction profiles measured using the scanning Hall probe method. An external magnetic field was applied in the normal direction with respect to the tape surface and was decreased stepwise from Hpeak to −Hpeak. From the hysteretic behavior of the current distributions, the energy loss, QM, was calculated for various values of Hpeak. Comparison of the present results with Brandts theoretical calculations revealed that his formula is still valid if one substitutes the magnetic field dependent critical current density into his equation. The theoretical results obtained were consistent with the experimental QM values obtained.

Comparative Study on the Profiles of Intra-Granular Critical Current Density in SmBCO and YBCO Coated Conductors

We measured the magnetic hysteresis loops (M-H loop) of SmBCO coated conductor (CC) and YBCO-CC by using SQUID-magnetometer. These samples were fabricated by RABiTS method and IBAD method. We found the peak shifts of their M-B loops were qualitatively different. To understand this, we measured the magnetic field profiles, H(x), of SmBCO-CC and YBCO-CC by using scanning Hall probe method and calculated their current profiles, J(x). We found that the contrasting difference of M-B loops of the two types of CC was partly due to the inhomogeneous distributions of field and current, which depend on the sample shapes.

A simple model for the estimation of hysteresis energy loss in a coated conductor under the simultaneous application of a magnetic field and current

A simple model for estimating the hysteresis energy loss of coated conductors under a general load line was studied. We took advantage of the characteristic line Ib(Ha) to determine the major parameters used in this model. The value of Ib(Ha) was based on the scanning Hall probe measurements (SHP) on a Sm1Ba2Cu3O7 − δ coated conductor. During SHP measurement, a magnetic field (Ha) and current (Ia) were applied simultaneously and were varied along 11 different load lines. From the values of SHP measurements, the current density profiles, J(x, Ha, Ia), were calculated using a numerical inversion method. We define the quantity Ib = ∫ − ww|J(x, Ha, Ia)| dx and we calculated Ib at many points (Ha, Ia) in every load line. We found that when Ia is less than Ib and the flux trap regions are absent, the values of Ib for all points (Ha, Ia) constitute a single line Ib(Ha), which can be easily extrapolated to a high field. This line provided a major parameter for our model.

Estimation of hysteresis loss in Sm1Ba2Cu3O7-δ coated conductor via scanning Hall probe measurements with simultaneously applied field and current

The hysteresis loss in a Sm1Ba2Cu3O7−δ coated conductor was estimated from magnetic field profiles measured by the scanning Hall probe method. Current, Ia, and magnetic field, Ba, were applied simultaneously; Ba was applied in the normal direction with respect to the tape surface. Ba and Ia were varied from Bpeak to −Bpeak and from Ipeak to −Ipeak, respectively, with the ratio α = Ia/Ba fixed during the variation. Three values of α were taken for the three load lines. The values of Bpeak/Ipeak were varied from 0 mT/0 A to 10.7 mT/116 A, 99.1 mT/50 A, and 298.2 mT/46.1 A, respectively, for the three load lines. From the measured values of magnetic field profiles, the current profiles were calculated by the iterative inversion method. From the current profiles, the flux density profiles and the hysteresis loss, Q, were then calculated for various values of Ipeak(= αBpeak) in each load line. The results were compared with theoretical calculations based on Brandts model. When Bpeak was about 300 mT, the estimated values of Q were several times smaller than the theoretical values of Q with the self-field Ic0. The low value of Q in this case is due to the field dependent Ic and the saturation effect of the current profiles, which results in significant reduction of the induced magnetic moment, M.

Critical Currents at the Grain Boundary of

We measured critical current densities (Jcb) at a [001]-tilt grain boundary (GB) of a bicrystalline Sm1Ba2Cu3O7 film under various magnetic fields (Halpha) applied obliquely. The mis-orientation angle, thetas, of the [001] tilt GB was 30deg. Halpha was varied between -0.65 KOe and +0.65 KOe. The angle, phi, between the applied fields and the film surface was varied from 0deg to 90deg. The curves of Jcb vs. Halpha at the different phi s were qualitatively explained by a simple formula suggested in [8]. The formula was expressed by multiplication of the two factors which include the effects of Hperp and Hpar. One fitting parameter, alpha, in this formula was large for the sample of thetas = 30deg in our previous paper. On the contrast, alpha for the present sample in this paper is almost zero. The absolute values of Jcb for this sample are larger than those for the previous sample by one order of magnitude. These results indicate that the parameter, alpha, closely related to the quality of GB: alpha gets smaller as the film quality at GB become better.