N. Chikumoto

Flux pinning in melt processed REBaCuO

Abstract REBaCuO (RE: rare earth element selected from Nd, Sm, Eu, Gd) superconductors exhibit high T c and J c values when they are melt processed in a reduced oxygen atmosphere (oxygen-controlled-melt-growth: OCMG process). Large J c values in these systems can be attributed to finely dispersed RE rich 123 clusters, which can act as field-induced pinning centers, and thus leading to the second peak effect in M - H loops. In this paper, we first propose the mechanism how RE rich clusters are formed during melt processing. Second we show that flux pinning depends on oxygen annealing conditions, because the optimum oxygenation conditions are different depending on the range of NdBa substitution. We also report that the addition of ND422 is effective in raising J c , although the mechanism is rather complicated owing to the fact that ND422 also has a REBa solid solution toward Ba rich direction.

Residual Magnetic Field Measurement of BSCCO and YBCO Tapes by a Hall Probe

A serious damage can happen to the power cable manufactured from the high-temperature superconducting (HTS) tapes due to overcurrent conditions or because of technical errors during the cable assembling. To avoid the cable damage in any urgent case, a quick interruption of the transport current is necessary. Comprehensive understanding of the current cutoff time of the HTS tape is required to restart the operation of the power transmission line. In this paper, we applied 100-A transport current to BSCCO and YBCO tapes. After interrupting the current, we measured the magnetic field profiles of each HTS tape. We compare the magnetic field profiles for the fast and slow cuts of the current. The cutoff time is 0.15 ms for the fast cut and 100 s for the slow one. The current density profiles are obtained by solving the inverse problem. Our results show that the magnetic field profiles are quite different from each experimental condition and HTS tape, which are related to the hysteresis of HTS tape characteristics.

Characterization of grain boundaries in multi-seeded YBCO superconductors

Abstract We have characterized grain boundaries (GBs) in YBCO bulk superconductors grown by the multi-seeded melt-growth (MSMG) process. Microstructural analyses revealed that GBs formed in the c -growth sectors were free from residual liquid phase. Strongly coupled GBs could be formed when the crystal growth directions of two grains were along 〈1 1 0〉 directions. We found that GBs in the c -growth sector with 〈1 1 0〉 growth direction exhibited superconducting properties comparable to the matrix.

Investigation of Three-Dimensional Current Distribution at Silver Diffusion Joint of RE-123 Coated Conductors Based on Magnetic Microscopy Combined With Finite Element Method

Combining scanning Hall-probe microscopy (SHPM) with finite element method (FEM), we have investigated three-dimensional current distribution at a silver diffusion joint of RE-123 coated conductor (CC). This research aims at the understanding of electromagnetic behaviors in a jointed CC and at the establishment of an analysis model for it. Two-dimensional distribution of sheet current density in a jointed sample was visualized by a SHPM measurement with a spatial resolution of a few hundred micrometers. Then, it was found that such a distribution included information about current transfer between the jointed pieces. Furthermore, the experimental results were successfully reconstructed by a three-dimensional FEM analysis. We believe that these will be very important findings for the establishment of jointing processes on CCs and for the designs of practical applications which need such processes.

Flux pinning properties of Bi2223 and Hg(Re)1223 single crystals with columnar defects

Flux pinning properties of Bi2223 and Hg(Re)1223 were studied before and after lead ion irradiation using recently obtained single crystals. Critical current performance of these crystals was dramatically improved by lead ion irradiation. Efficiency of the columnar defects as pinning sites in high-temperature region was found to be systematically improved with lowering the electromagnetic anisotropy of the pristine crystals including previously reported Bi2212 and Bi(Pb)2212.

Temperature dependence of the peak effect in NdBa2Cu3O7−δ single crystals and melt-processed (Sm0.5Eu0.5)Ba2Cu3O7−δ single domains

Abstract Magnetization loops of NdBa 2 Cu 3 O 7− δ (Nd123) single crystals grown under low oxygen partial pressure and melt-processed (Sm 0.5 Eu 0.5 )Ba 2 Cu 3 O 7− δ (SE123) crystals with post-annealing in Ar were comparatively studied in a wide temperature and field regime. The peak field at which magnetic moment displays the maximum, exhibits a linear temperature dependence in both cases. The fitting shows a slightly lower critical temperature than bulk T c at null applied field. The temperature dependence of the onset field from which the absolute value of magnetic moment starts to increase, indicates the dominant pinning at low fields in Nd123 single crystals is the surface barrier while the RE211/123 interfacial pinning in melt-processed SE123 crystals.

The modulation of the peak effect in melt-processed (Sm-Eu)123 superconductors with compositional fluctuations

The peak effect in magnetization loops of melt-processed (Sm/sub x/Eu/sub 1-x/)Ba/sub 2/Cu/sub 3/O/sub 7-/spl delta// superconductors is manipulated by varying the Sm/Eu ratio in the initial composition. In all the samples the peak field B/sub p/(T), at which the magnetization moment reaches the maximum, was linearly temperature dependent. The fitting to such a relationship unveils a slightly lower critical temperature T/sub cp/ than the bulk T/sub c/ at zero field. A kink appears in the reversible magnetization moment of the field dependence of superconducting transition. This kink temperature T/sub ck/(B) is also a linear function of applied field. Transmission electron microscopy (TEM) observation confirms the presence of nano-scale regular compositional modulation arising from local RE-Ba substitution, which is weakened with increasing Eu concentration. The peak effect is thus attributed to the scattering of local T/sub c/ due to compositional fluctuation.

Current Imbalance and AC Losses of Long-Distance DC HTS Cable

Despite intensive research in the field of applied superconductivity only now we start to use of this phenomenon for the most obvious application, namely, power transmission. At present, it can be seen an explicit shift from AC to DC HTS cable systems due to the many advantages of the DC ones. However, even at constant current transmission line is subjected to current fluctuations and, consequently, AC losses. One more reason for reducing the efficiency of HTS lines is current imbalance. It is associated with the presence of electrical resistance at the soldered connections of the superconducting tapes. Although these resistances are very low, but against the background of zero resistance of the rest of the superconductor they determine the individual currents in tapes and introduce essential nonlinearity into the dependence of these currents on the total current. In order to estimate AC losses the frequency characteristics of the ripple current were calculated taking into account current imbalance in the cable.

Observation of the flux line lattice and crystal defects in Bi-Sr-Ca-Cu-O single crystals

Abstract We have observed flux line lattice (FLL) structure in two different types of Bi 2 Sr 2 CaCu 2 O 8+ δ (Bi-2212) single crystals with different pinning by using a Bitter decoration technique. We have also observed the microstructure and analyzed local chemical composition with wave dispersive spectroscopic analyses. In a low J c sample, we found two types of line defects, one with Ca-rich and the other with Ca-deficient. These defects extended almost parallel to the growth direction, which caused a distortion in the FLL structure. In contrast, no such defects were observed in the matrix in a high- J c sample, and a relatively ordered FLL was observed.

Cooling test of the 500 m class superconducting DC power transmission system

Recently, 500 m and 1000 m class superconducting DC power transmission systems were constructed in the Ishikari area in Japan and the cooling test of the 500 m system was performed. The heat leak of the cryogenic pipe and the total heat load of the system were estimated in the cooling test. The cryogenic pipe of the system has two inner pipes in one outer pipe for circulation. The heat leak was 0.98 W/m and 0.44 W/m for each inner pipe. The total heat load of the system was 1.37 kW except for the heat load by the current feeding and the circulation pumps, while the total cooling power of the system was approximately 3 kW. The pressure drop of the circulation was measured to be 19.5 kPa at the rated flow rate of 30 L/min for the 1000 m circulation both ways in the 500 m transmission line. By the cooling test, it was confirmed that the system can be operated stably.