E.S. Otabe

Improvement of flux pinning performance at high magnetic fields in GdBa2Cu3Oy coated conductors with BHO nano-rods through enhancement of Bc2

Addition of BaHfO3 (BHO) nano-rods as pinning centers into a GdBa2Cu3Oy (GdBCO) coated conductor dramatically improves the critical current properties in high magnetic fields. This is partly ascribed to the strong flux pinning of these pinning centers. In this paper it is reported that this improvement is mainly caused by unexpected enhancement of the upper critical field, Bc2. The mechanism of the enhancement of Bc2 is not yet clear, although the strain around the interface between the pinning center and the superconducting matrix or the interface itself may scatter electrons. This result suggests that the critical current property of REBa2Cu3Oy (RE: rare earth elements, REBCO) coated conductors could be designed not only by tailoring the microstructure of pinning centers but also by controlling the intrinsic superconducting property. Especially, the selection of pinning material and its content that determines Bc2 is expected to be a key problem for further improvement of the pinning performance at high magnetic fields.

Pinning-force density of superconducting Bi-2223 tape wires

Abstract The pinning-force density and the irreversibility field are measured for Bi-2223 tape wires at various temperatures under the magnetic fields normal and parallel to the c -axis. It is found that the scaling laws of pinning-force density in the low- and high-temperature regions and the temperature dependence of the irreversibility field are formally the same between the two field directions in spite of some anisotropy factor, suggesting that the pinning mechanism is essentially the same for the two field directions. These results are compared with the numerical calculation based on the flux-creep model. The experimental results are well explained by the theory including the anisotropic property. The pinning potential estimated from the flux-creep theory at 80 K and 1.0 T is of the order of several 10 meV and the factor of the anisotropy is about 1.3. The value of the pinning potential under the magnetic field along the c -axis suggests that the flux lines are strongly connected along this direction. The longitudinal elastic correlation length of the flux lines is of the order of several μm and this supports the hypothesis. The pinning characteristics of a Bi-2223 tape wire to which the strong pinning centers are successfully introduced are discussed using the flux-creep theory for evaluating the potential of this material for application. The irreversibility field for the direction parallel to the c -axis is estimated to be sufficiently high even at 77.3 K and it is concluded that Bi-2223 has a sufficient potential for application.

Weak link property in superconducting Y-Ba-Cu-O prepared by QMG process

The weak link property in a QMG (quench and melt growth)-processed single-grain Y-Ba-Cu-O specimen was investigated by measuring the dependence of the critical current density on temperature, history of magnetic field application, and its enhancement under a longitudinal field geometry. The results show that weak-link regions still remain in the specimen. It is speculated that these link regions, which can transport superconducting currents at low temperatures, are degraded with increasing temperature and lose superconductivity around 20 K.<<ETX>>

Effects of Sr substitution on the magnetic-field irreversibility in superconductive Cu(Ba1−ySry)2YbCu2O6+z

Abstract The effects of the Sr-for-Ba substitution on the H irr characteristics of the Cu(Ba 1− y Sr y ) 2 YbCu 2 O 6+ z (Cu-1212) system is established at two different oxygen-content levels in the range of 0.0≤ y ≤0.4. In the fully oxygenated sample series, an optimum was seen at y =0.2 so that the H irr line of the present Cu(Ba 0.8 Sr 0.2 ) 2 YbCu 2 O 6.96 sample clearly exceeds that of the CuBa 2 YCu 2 O 6.92 compound. For the oxygen-deficient sample series, the H irr characteristics was depressed with increasing Sr content even though the unit cell decreased. Thus, the H irr characteristics is mostly controlled by other factors than the thickness of the nonsuperconductive blocking block.

Pinning mechanism of the peak effect in melt-processed Y-123 superconductors

It is known that the peak effect in Y-123 superconductors is caused by oxygen deficient regions with lower Tc. In order to study the flux pinning mechanism of these defects, the effect of the addition of 211 particles on the peak effect was measured. It was found that the addition of the 211 phase deteriorated the pronounced peak effect at medium magnetic fields. This suggests that the pinning mechanism of the oxygen deficient regions is repulsive and based on the kinetic energy interaction under the proximity effect. The peak effect itself is considered to be caused by a transitional variation in the elastic property of flux lines. In the low- and high-field regions, the dominant pinning centres are 211 particles. The observed results on the critical current density and the irreversibility field were compared with the theoretical predictions of the flux creep-flow model, assuming pinning by the 211 particles.

Evaluation of irreversible property in superconducting Cu(Ba0.8Sr0.2)2YbCu2O6+z powder

Abstract The irreversibility field of superconducting Cu(Ba 0.8 Sr 0.2 ) 2 YbCu 2 O 6+ z (so-called Yb-123) powder was carefully measured as a function of temperature using a SQUID magnetometer. The DC magnetic susceptibility showed a gradual transition near the critical temperature, suggesting a wide distribution of the critical temperature. For a detailed characterization of the material, the evaluation of the mean critical temperature and its distribution width is important. These are estimated by fitting the theoretical result of the DC magnetic susceptibility in field cooled and zero field cooled processes to the experimental result. The obtained result proves that this material showed a superior irreversibility field than usual Y-123 superconductor.

Current-carrying capacity of single layer cable using superconducting Bi-2223 tapes in a parallel magnetic field

It was theoretically shown by the authors that the current-carrying capacity of superconducting dc power cable can be enhanced by choosing a force-free configuration under a parallel magnetic field produced by the current flowing back in the outer shielding conductor. This was experimentally checked for a single layer cable using Bi-2223 tapes in an applied parallel magnetic field. It was found that the current-carrying capacity took on a peak value under the force-free condition for the total magnetic field, including the self-field. This shows that the proposed structure is suitable for practical dc power transmission. The possibility of the innovative dc superconducting power cable with multi-layers with higher current-carrying capacity is discussed.

Relationship between Condensation Energy and Anisotropy in Bi‐2212 Superconductor

The condensation energy of Bi‐2212 superconductor in various doped states was evaluated from the flux pinning strength of columnar defects. For the purpose the collective flux creep theory was used to estimate the virtual critical current density in creep‐free state and the summation theory was used to estimate the elementary pinning force of the columnar defects. The values of obtained thermodynamic critical field are in the range of other observations and this proves a validity of the present evaluation. The obtained condensation energy strongly depends on the anisotropy determined by the superconductivity in the block layer. It is found that the condensation energy is drastically enhanced with decreasing temperature below the critical point.

Flux Pinning Properties of BHO Pinning Centers at High Magnetic Fields in GdBCO Coated Conductors

It has been recognized that BaHfO3 (BHO) pinning centers introduced into GdBa2Cu3O6 +y (GdBCO) coated conductors show high pinning performance at high magnetic fields. This pinning property was ascribed to the strong pinning force of the BHO nano-rods and also to the improved upper critical field, Bc2, probably due to the scattering of electrons by the interface between the nano-rods and the superconducting matrix. In this paper the critical current properties of GdBCO coated conductors with different amounts of BHO nano-rods are theoretically investigated. It is found that BHO nano-rods are promising to improve the high field performance of GdBCO coated conductors.

Field Angle Dependence of Critical Current Density in YGdBCO Coated Conductor

The improvement of critical current density properties of REBCO coated conductors for applications in superconducting power devices is desired. It is known that the critical current density Jc of REBCO thin films made by the TFA-MOD method in a magnetic field is greatly improved by the introduction of BZO-nanoparticles of a non-superconducting phase as artificial pinning centers. In addition, the field angle anisotropy can also be reduced by isotropic flux pinning interactions of nanoparticles. However, a detailed report on the mechanism of flux pinning with a quantitative investigation in such coated conductors has not been given. In this study, the field angle dependence of the critical current density is measured for TFA-MOD-processed YGdBCO coated conductor with a superconducting layer of 0.65 μm thick, and the results are theoretically analysed to clarify the flux pinning mechanism.