S M Lim

Fabrication of Sm1Ba2Cu3O7 coated conductors using the co-evaporation method

We fabricated a coated conductor 7.5 m long and 9 mm wide using a biaxially textured Ni tape. The standard buffer layers, CeO2/Y SZ/CeO2, were used. A thick Sm1Ba2Cu3O7 (SBCO) film was deposited using the co-evaporation method, followed by in situ deposition of an Ag protection layer. The whole length of SBCO film was deposited in a single batch process, which took 7 h. We checked the distribution of critical currents (Ic) by I–V measurements in all the sections comprising the tape. Ic at 77 K was 90–130 A in most parts of the whole length.

Comparative studies on the growth conditions of CeO2 and Y2O3 buffer layers on NiW tapes

We comparatively studied the growth conditions of Y2O3 films and CeO2 films on biaxially textured Ni and NiW tapes. We found that the windows of both substrate temperatures and oxygen partial pressures for the proper growth of CeO2 films were wider than those of Y2O3 films. XRD data showed that the qualities of in-plane and out-of-plane textures of the two types of films were almost similar. The SEM observations indicated that the surface of the CeO2 films was smoother than those of Y2O3 films. The inferiority of CeO2 films to Y2O3 films was crack formation. However, we found that if YSZ over-layers were grown on the CeO2 films using dc-reactive sputtering, the cracks were not observed on the surface of the YSZ films anymore.

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.

Analyses of a solenoid magnet made of pancakes of coated conductors

We estimated the distribution of currents and fields in a magnet which consisted of three coaxial solenoids; the solenoids were made by stacking pancakes of coated conductors (CCs). We used a model for the critical current densities which satisfied simple relations with magnitudes and directions of fields. Since various types of pinning centres result in different anisotropies in the field dependence of critical currents, their contributions to the properties of magnet were different. We found that the intrinsic pinning centres in CCs resulted in inefficient performance of the magnet as they were effective only in the pancakes in the central part of the magnet, where the fields were parallel to the ab-planes of the superconducting layers in the CCs. We estimated how much the magnet performances were improved by additions of columnar pinning centres or by addition of random point pinning centres.

Hysteretic field dependences of critical currents at grain boundaries of various high Tc superconducting films

We measured critical current densities (Jcb) at 24°, 30°, 36.8°, and 45° grain boundaries of Y1Ba2Cu3O7 and Sm1Ba2Cu3O7 films, and we compared them with the previous measurements on Nd1Ba2Cu3O7 films. We observed salient hysteretic behaviours in the field-dependent Jcb at various temperatures. The main features of these measurements were consistent with the results of our calculations, where Jcb depends on the vortex density at the grain boundary as expressed by the modified Kim model. The vortex densities were estimated from field distributions around grain boundaries, while the field distributions were obtained from the Brandt formula. By these data analyses, we estimated the four characteristic parameters of grain boundaries and compared them for the three different types of material.

Direct current sputtering conditions for the growth of yttria-stabilized ZrO2 buffer layers of coated conductors

The detailed conditions of direct current (dc) sputtering for depositions of yttria-stabilized ZrO2 (YSZ) films have been investigated, with the films grown on CeO2 template layers on biaxially textured Ni tapes. The range of oxygen pressure for proper growth of YSZ films, which is dependent on sputtering powers, was determined by sufficient oxidation of the YSZ films and the deoxidation of the target surface, which is required for rapid sputtering. The range turned out to be fairly wide under certain values of argon pressure. When the sputtering power was raised, the deposition rate increased without narrowing the range. The fabricated YSZ films showed good texture qualities and surface morphologies.

Measurements and analyses of hysteretic field dependences of critical currents in bicrystalline Nd1Ba2Cu3O7 films

Abstract We measured the critical current densities ( J cb ) of grain boundaries (GB) in the bicrystalline Nd 1 Ba 2 Cu 3 O 7 films of several misorientation angles under various applied fields. J cb ’s showed hysteresis curves as functions of fields. The main features of the data were qualitatively consistent with the calculation results. In the calculations, J cb ’s were expressed as functions of the densities of Josephson vortices, which were estimated from the field distributions around GB. We calculated the field distributions using the Brandt’s formula and expressed J cb by the modified Kim’s model.

Crossover phenomena observed in field dependent critical currents at grain boundaries of Nd1Ba2Cu3O7 films

We investigated the critical current densities (Jcb) at grain boundaries (GB) of bicrystalline Nd1Ba2Cu3O7−δ films. We observed that these sharply dropped as the applied fields were increased from zero to Oe (HJA) and then showed relatively small field dependences above HJA. These contrasting field dependences of Jcb below and above HJA clearly indicated crossover phenomena in the vortex states. Our experimental results were consistent with a theoretical model, which was based on the concept that the energy difference between Josephson (J-) vortices and Abrikosov (A-) vortices caused the generation of only J-vortices initially. This might result in an initial sharp increase of J-vortex densities at GB, as the field is increased from zero to HJA. As the field is increased further, some J-vortices penetrate from GB to intra-grain regions and become A-vortices, which might change the field dependences of Jcb.

Effects of plasma on the growth conditions of Y1Ba2Cu3O7−δ thin films in dc sputtering

Y1Ba2Cu3O7−δ (YBCO) films have been grown using hollow cathode dc sputtering with various conditions. The effects of plasma conditions on the film qualities and the deposition rates have been studied in detail. To prevent the ion flux from reducing the film quality, it should be less than ~2 × 10−2 mA cm−2 with a deposition rate of 1–2 nm min−1. When the distance from the substrate to the target is very large or the discharge power is very small, the ion flux is sufficiently small. However, in the latter case the films are quite degraded, while there are no degradations for the films of the former case. Our experiments have revealed that this is due to the large differences in the densities of oxygen radicals for the two cases. The insufficient oxidation causing the degradations corresponds with the small Cu2p3/2 satellite peaks of x-ray photoemission spectroscopy and the losses of Cu content. The deposition rates are much enhanced by decreased gas pressures, increased flow rates of gas and discharge power.

I–V characteristics of grain boundaries in Sm1Ba2Cu3O7−δ thin films

Abstract The transport properties of Sm 1 Ba 2 Cu 3 O 7− δ films, which were grown on the bi-crystalline SrTiO 3 substrates of 24°, 30°, 36.8° and 45° misorientation angles, were investigated by the measurements of the I–V curves at 77 K under various magnetic fields. When the voltages were high enough, the I–V curves became straight. The resistances estimated from the slopes were almost independent of the applied fields mostly for the high-quality films. For the slightly low-quality films, the zero field resistances were smaller than those for other fields. When the voltages were low enough, the relations between voltages and currents approximately satisfied the power law. The exponents increased by one order of magnitude as the field was changed from 0 to 1 T. The field dependences of the critical current densities varied according to the voltage of the criterion. The misorientation angle dependence of the critical current density was weaker than that of the Y 1 Ba 2 Cu 3 O 7− δ grain boundaries.