Masaru Kiuchi

Critical current properties in YBCO coated IBAD tapes

Abstract Current transport properties in YBCO coated ion beam assisted deposition tapes have been studied as a function of temperature, magnetic field and field angle under high magnetic fields up to 18 T. The value of critical current, I c , for 1 cm wide tape was 85 A at 77.3 K in self-field. Detailed properties of critical current density, J c , were studied using 100 μm wide and 1 mm long bridge patterned in the tape. The results were compared with the properties of epitaxial YBCO thin films deposited on SrTiO 3 single crystalline substrate. The value of critical current density, J c , was maintained higher than 10 5 A/cm 2 at 70 K in 5 T perpendicular to the tape surface. The value is ≈1/4 of J c value in epitaxial thin film. In higher magnetic field, the difference becomes smaller. Power E–J relationship was observed over wide range of temperature and magnetic field. The power index n were superior to those of epitaxial thin films. And, the so-called glass–liquid transition field, B GL , of the coated tape was also superior to that of epitaxial thin films. Angle dependences of J c showed a broad peak around 90° (∥ c ), in addition to a sharp peak at 0° (⊥ c ). These results suggest that correlated defects along c -axis direction contribute as pinning sites.

Influence of bending strain on Bi-2223 tape

Abstract The behavior of current–voltage ( I–V ) characteristics in Bi-2223 AgAgMg sheathed tape at various bending strain has been studied. The measured results were analyzed using the percolation model, and then the variation of critical current density, J c , has been analyzed. As increasing of bending strain, J c in the tape is reduced irreversible way. The irreversible degradation indicates that the J c reduction is caused mainly by crack formation. However, if we extract statistic J c distribution taking into account the cracking, it can be seen that the value of local J c is enlarged in the remained region as the strain is increased. Namely, the pinning strength in the remained region is enlarged as the bending strain is increased. Present analysis allows us to separate the influence on grain connectivity and pinning strength originated from the strain.

Transport E-J characteristics in Bi-2223 tape under the influence of bending strain: analysis by use of the percolation model

Abstract The influence of bending strain on the transport E – J characteristics in Bi-2223 Ag/AgMg-alloy sheathed tape is studied by using the percolation model. Magnetic field- and temperature-dependent E – J characteristics in the tape have been measured at various conditions of bending strain. As the bending radius is reduced, the apparent critical current in the tape is reduced mainly because of the reduction of geometrical cross-section area due to cracking. However, if we extract the local current density taking into account the cracking, it can be seen that the value of critical current density, J c , is enlarged due to the bending strain in the remained region. Namely, the pinning strength in the remained region is enlarged as the bending strain is increased. It will be shown that the present method allows us to separate the variation of the connectivity and the pinning strength originated from the strain.

E-J characteristics in a wide range of electric field for a Bi-2223 silver-sheathed tape wire

Abstract The E – J characteristics are measured for a Bi-2223 silver-sheathed tape wire using the four probe method and a relaxation method of DC magnetization in the high and low electric field regions, respectively. The obtained results in the two regions are approximately explained by a theoretical analysis based on the flux creep-flow model in which the distribution of flux pinning strength is taken into account. It was found that the value of the pinning force density and its peak magnetic field are largely different between the two measurements.

Effect of distribution of flux pinning strength on irreversibility line and vortex glass–liquid transition line in a Bi-2212 tape wire

Abstract The irreversibility line and the vortex glass–liquid transition line are investigated for a dip-coated Bi-2212 superconducting tape. These two characteristic lines are also analyzed using the flux creep-flow model in which the distribution of the flux pinning strength is assumed. These results are compared with the previous results for a silver-sheathed Bi-2212 tape. Good agreements were not obtained simultaneously between the experiment and the theory for the two characteristic lines in the dip-coated tape, although the theory explained the experimental results well in the silver-sheathed tape. This seems to be ascribed to the peculiar distribution of the flux pinning strength with two peaks due to a percipitation of an impurity phase in the present dip-coated tape specimen.

Angular dependence of E-J characteristics under high magnetic fields in YBCO thin films

Abstract Transport E – J characteristics in YBCO thin films have been studied as a function of temperature, T , magnetic field, B , and its angle, θ , under high magnetic fields up to 23 T. We carried out detailed measurements for the extended E – J characteristics at various T , B and θ . Analyzing the data within the framework of the percolation model, we extracted the factors related to the statistical distribution of critical current density, J c . It is shown that if we assume that the scaling property and the angular dependencies of transition field, B GL , hold down to 55 K, B GL is estimated to be as high as 96 T in parallel field. The wide range measurements allow us to reveal the high potential of this material especially in high magnetic field region.

Irreversibility field of Bi-2223 silver-sheathed tape determined with different electric field criteria

Abstract Dependence of the irreversibility field ( B i ) on the electric field criterion ( E c ) was measured for a Bi-2223 silver-sheathed tape wire using various measuring methods. It was found that B i increased monotonically with the electric field criterion. A good agreement was obtained for B i between experiments and the theoretical analysis using the flux creep-flow model. This implies that the resistive property of superconducting wires can be well described by the mechanism of flux creep and flow. The vortex glass–liquid transition field ( B g ) was also determined from E – J curves which measured in the low and high electric field regions. It was found that B g also depends on the ranges of electric field. This is incompatible with the prediction of the vortex glass–liquid transition theory.

Distribution of flux pinning strength in a superconducting Bi-2223 silver-sheathed tape

Abstract The distribution of the flux pinning strength was estimated from a measured current–voltage curve for a monofilamentary Bi-2223 silver-sheathed tape in a magnetic field parallel to the c -axis. It is found that the distribution can be fitted well to the Weibull function of a very wide distribution. This wide distribution seems to be caused by the low critical current density at weak links. The obtained Weibull function can be approximated fairly well by a simpler distribution function with which the scaling of current–voltage curves is analyzed using the flux creep–flow model. The obtained scaling parameters agree well with the experimental results.

Anisotropic transport E(J) characteristics in Bi-2223 Ag-sheathed tape as a function of temperature and magnetic field

Abstract E – J curves in Bi-2223 Ag-sheathed tape have been analyzed within the framework of the percolation model proposed by the authors. It will be shown that a scaling holds for magnetic field ( B ) dependence in both field directions when we normalize B by the transition field at each temperature. Whereas, the temperature dependence can be described accurately by the prediction of two-dimensional d-wave symmetry. Using the present model, we can describe the nonlinear E – J characteristics quantitatively in a wide range of electric field, temperature, magnetic field and its angle.

Theoretical analysis of E-J characteristics in a Bi-2223 silver-sheathed tape

Abstract From an analysis of E – J curves with the aid of the flux creep-flow model, it is found that most of the electric field is caused by the flux creep even in the range of electric field in usual resistive measurement. This result is consistent with the fact that the mechanism of flux creep generally explains various phenomena such as the critical current density, the irreversibility field and so on. On the other hand, it is much easier to use the parameters used in the percolation flow model to describe the E – J characteristics phenomenologically. The two models are compared and it is found that the two models are consistent. This gives a theoretical proof for the parameters of the percolation flow model.