T Kuga

Critical current properties in HTS tapes

Abstract The author’s recent studies on the critical current properties in Bi-2223 multifilamentary tapes as well as YBCO coated IBAD tapes have been reviewed. Extended electric field vs. current density ( E – J ) characteristics were studied over wide range of temperature, T , magnetic field, B , and angle, θ . Based on the analysis of statistic J c distribution and scaling, we derived an analytical expression for J as a function of E , T , B , and θ . This method allows us to describe and extrapolate the nonlinear E – J characteristics even in extremely low E region and/or high B region based on rather simple measurements.

Estimation of E–J characteristics in a YBCO coated conductor at low temperature and very high magnetic field

Abstract E – J characteristics in a YBCO coated conductor at low temperature and very high magnetic field up to 27 T have been estimated using experimental E – J characteristics below 12 T along with a scaling analysis based on the percolation model. 1 mm thick YBCO films was deposited on a biaxially aligned Y 2 O 3 /YSZ/Hastelloy substrate fabricated by the IBAD technique. The critical current I c for the 1 cm wide tape was 85 A at 77 K in self-field. We measured E – J characteristics in perpendicular field up to 12 T using 100 μm wide and 1 mm long bridge patterned in the tape. Statistic J c distribution and pinning properties were analyzed from the E – J characteristics. Using the pinning parameters determined by the scaling analysis, we can calculate the critical current properties as a function of temperature and magnetic field. E – J characteristics at low temperature and very high magnetic field up to 27 T were estimated analytically and compared with the experimental data. Estimated value of J c and n -value, and their error analysis were also discussed.

Percolative transition and scaling of transport E-J characteristics in YBCO coated IBAD tape

We have investigated extended electric field-vs.-current density E-J characteristics in YBCO coated IBAD tapes. The results of a Monte-Carlo study on the depinning in a random pin medium were compared with measurements. Using a low temperature scanning laser microscope, we examined the percolative behavior of the local resistive transition in the YBCO tape. It was also shown that the depinning probability, which is proportional to the dynamic resistance of the E-J curves, is scaled as a function of reduced current density with the aid of a power index. Consequently, the E-J characteristics in the tapes can be described by the combination of the two kinds of scaling laws: one is the scaling law of the depinning probability predicted in a random network and the other is the scaling law for the flux pinning force. These properties agree well with the percolation model of depinned clusters.

Angular dependence of critical current properties in YBCO coated tape under high magnetic field up to 18 T

Abstract Critical current properties in a YBCO coated tape have been studied as a function of temperature, magnetic field and field angle under high magnetic field up to 18 T. 1 μm thick YBCO film was deposited on a bi-axially aligned Y 2 O 3 /YSZ/Hastelloy substrate fabricated by the IBAD technique. The value of critical current, I c , of the 1 cm wide tape reached as high as 85 A at 77 K in self-field condition. After etching the tape into 100 μm wide and 1 mm long bridge, we measured extended E – J characteristics. The value of critical current density, J c , is maintained higher than 10 5 A/cm 2 at 70 K in 5 T perpendicular to the tape surface. Moreover, the power index n and the so called glass–liquid transition field, B GL , of the tape in perpendicular field are superior to those of epitaxial film deposited on a SrTiO 3 single crystalline substrate. J c vs. angle relationship shows broad peak around 90° where field is perpendicular to the tape surface, in addition to a sharp peak at 0°. These results suggest that the tape is highly uniform and correlated defects along c -axis direction contribute to flux pinning.

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.

Angular dependence of irreversibility field in Y-123 coated tape

It is known that the critical current density vs. field angle of an Y-123 coated tape has a broad peak around the field direction normal to the tape surface. In this work, the angular dependencies of the irreversibility field and the critical current density are analytically investigated using the flux creep-flow theory with an assumption of the angular dependence of flux pinning strength, and these results are compared with the experimental results. The angular dependence of flux pinning strength obtained so as to get a good agreement between the experimental and theoretical results suggests an existence of long pinning centers along the c-axis.

Distribution of pinning strength and scaling behavior in YBCO coated IBAD tape

Abstract Extended E – J characteristics in a YBCO coated ion beam assisted deposition tape have been studied over a wide range of perpendicular magnetic field, B , and temperature, T . The statistical distribution of the critical current density, J c , and the magnetic transition field have been analyzed within the frame work of the percolation model. It has been shown that the distribution of pinning strength in the coated tape is much sharper than that in an epitaxial thin film deposited on a SrTiO 3 single crystalline substrate. This result suggests that effective pinning sites are introduced in the coated tape. Scaling behavior of the pinning force density will also be shown. The present analysis allows us to estimate the J c value quantitatively at an arbitrary B , T and electric field criterion.

Size effect on current transport properties in the mixed state of YBCO thin film

Abstract We have investigated the influence of sample width on the transport properties in YBCO thin film using 10, 120, and 1200 μm-wide bridges formed in a same c -axis oriented YBCO film deposited on a SrTiO 3 substrate. We measured I–V characteristics of the bridges at various temperature, T , and magnetic field, B , perpendicular to the film surface. It has been found that the so called glass–liquid transition field, B GL , reduces as the sample width becomes narrower, while the critical temperature, T c , and critical current density, J c , in the glass region ( B B GL ) are almost independent of the width of the bridges. The good agreement of T c and J c in three bridges indicates high uniformity of the bridges and that the reduction of B GL does not come from the degradation of the film. The measured width dependence of B GL can be scaled with a constant critical index as can be predicted by the percolation model at all measured temperature region from 70 to 85 K. The influence of sample width on J c has also been discussed based on the scaling analysis.

Evaluation of E–J characteristics of YBCO-coated conductor in a wide range of electric field

Abstract The E – J characteristics were measured for a YBCO-coated conductor by using the four probe method and the relaxation method of DC magnetization. The result on the latter measurement at extremely low electric fields was theoretically analyzed based on the flux creep-flow model. From the result on n -value the flux lines in YBCO are considered to be in the glass state even at high fields. The apparent pinning potential U 0 ∗ estimated from the relaxation of DC magnetization is much larger than that of Bi-2223. The present theoretical analysis clarifies a much stronger flux pinning and a higher dimensionality of YBCO in comparison with Bi-2223.