Masafumi Namba

c-axis correlated pinning behavior near the irreversibility fields

The authors found that the peak at the parallel external field (B‖c) to the c axis in the angular dependence of Jc shrinks and almost disappears with increasing a magnetic field but it grows again with further increasing of a magnetic field at various temperatures for high-Jc Sm1+xBa2−xCu3Oy films. These behaviors can be explained by the flux pinning properties of the interstitial vortices, which locate in between the vortices pinned by the c-axis correlated disorders. From the obtained experimental results, the collective correlated pinned glass state is proposed in a high field region in the vortex glass states.

Flux pinning properties of TFA-MOD (Y,Gd)Ba2Cu3Ox tapes with BaZrO3 nanoparticles

A high critical current density Jc with a nearly isotropic angular dependence at 77.3 K was recently obtained by the introduction of nanoparticles to (Y,RE)Ba2Cu3Oy (RE123, RE = Sm, Gd)-coated conductors prepared by trifluoroacetate metal–organic deposition (TFA-MOD). The flux pinning properties were evaluated on the basis of detailed measurements of the upper critical field, the irreversibility field and the critical current density of the TFA-MOD RE123-coated conductor. We found that strong random pinning by the BaZrO3 nanoparticles and weak c-axis correlated pinning by twin boundaries functioned cooperatively in nanoparticles introduced to (Y, Gd)123-coated conductors prepared by TFA-MOD. The coexistence of two different pinning centers played an important role in the nearly isotropic angular dependence of Jc.

Upgrading Design to a 25 T Cryogen-Free Superconducting Magnet Based on Low Temperature and High Magnetic Field Properties of the Practical CVD Processed Coated Conductors

We evaluated critical current density and mechanical properties of the Y123 coated conductor (CC) tapes on buffered Hastelloy substrates prepared by the chemical vapor deposition method in high magnetic fields and low temperatures. The Jc values of the tape are about 1.1 MA/cm at 77.3 K, 0 T and 1.8 MA/cm2 at 4.2 K and 17 T for B//c. In addition, the hoop stress test of the single layer coil shows that the stress limit of the CVD-Y123 CC tapes on Hastelloy is over 1 GPa. On the basis of those experimental data, we designed the innermost high temperature superconducting insert coil for upgrading of the 18 T cryogen-free superconducting magnet (18 T-CSM) under the condition of the stress limit of 600 MPa. The coil, which consists of the 22 double pancake coils can generate 9.4 T with the operation current of 295 A in the backup field of 15.6 T. In this case, the central field of the 18 T-CSM can be improved up to 25 T from 18.1 T.

Flux pinning properties of correlated pinning at low temperatures in ErBCO films with inclined columnar defects

The effect of c-axis correlated pinning on the critical current density at low temperatures under high magnetic fields was investigated on the basis of the transport critical current density and irreversibility field properties of an ErBa2 Cu3 Oy film with inclined columnar defects. We found that the contribution of correlated pinning on Jc decreased drastically with decreasing temperature under high magnetic fields above the matching field, but still retained more than a 60% enhancement at 13 K. These effects of correlated pinning can be described using a cooperative model based on a combination of correlated and random pinning. At low temperatures, the presence of a matching field limited the flux pinning force for correlated pinning in comparison with that of random pinning, while the ratio of the maximum pinning force of the correlated pinning to the random one was proportional to the inverse of the irreversibility field. This suggests that the low efficiency of correlated pinning at low temperatures ...

Effects of BaMO3 (M = Zr, Sn) Nanorods on Critical Temperature of ErBa2Cu3Oy Films

In order to study the degradation of critical temperature Tc for a coated conductor using nanorods, the relationship between Tc and nanorod shape was investigated by transmission electron microscopy for ErBa2Cu3Oy (Er123) films with BaZrO3 (BZO) and BaSnO3 (BSO) nanorods. The dependence of Tc on the BZO or BSO content was different, however, the nanorods interface density dependence of Tc was the same for both. Thus, the interface density of the nanorods controls Tc of the nanorod-added films. The different degradation in Tc between the BZO and BSO nanorod-added films originated in differences in the nanorod diameter and density.

Effect of c-Axis-Correlated Disorders on the Vortex Diagram of the Pinning State

The introduction of the c-axis-correlated disorder is a promising technique for the improvement of the critical current density Jc for B\varparallelc of REBa2Cu3Oy (RE123) films and tapes. In order to understand the vortex pinning mechanism with the different variety of the c-axis-correlated disorder, the angular dependence of Jc was measured in detail for an Y123 film with BaZrO3 nano-rods, a Sm123 film with fine nano-particles and edge dislocations and an Y123 film with columnar defects introduced by the heavy-ion irradiation parallel to the c-axis. We found that the vortex pinning state for the variety of the c-axis-correlated pinning is similar in low fields below the matching field, but it shows different behaviors in high fields above the matching field. It is considered that the interstitial vortices interacted with the c-axis-correlated disorders through the vortex interaction may play an important role for the appearance of the c-axis-correlated pinning behavior in a high-field region above the matching field.

Vortex pinning phase diagram for various kinds of c-axis correlated disorders in RE123 films

The critical current density Jc and resistivity ρ of REBa2Cu3Ox (RE123) films were evaluated in high magnetic field and wide temperature regions. We use three different samples such as a heavy ion irradiated Y123 film, a BaZrO3 (BZO) added Y123 film and a low temperature growth Sm1-xBa2+xCu3Oy (LTG-Sm123) film. In these films, there exist the different c-axis correlated disorders like columnar shaped fission tracks for the heavy ion irradiated Y123 film, columnar shaped BZO precipitates for the BZO added Y123 film and edge dislocations at grain boundaries for the LTG-Sm123 films. The large peak of the angular dependence of Jc for B//c direction was observed for all samples measured in this study in a low field region, suggesting that the c-axis correlated pinning works well for these samples. However, we found that the behavior of the Jc peak for B//c strongly depends on a sort of c-axis correlated pinning centers. In the case of the columnar shaped fission tracks in the Y123 film, the Jc peak for B//c increases monotonically with increasing a magnetic field and seems to be connected with the dip structure in the angular dependence of ρ. In the case of the columnar shaped BZO precipitates, however, the Jc peak for B//c vanishes in a high field region above a few tesla and no dip behavior of ρ was observed. On the contrary, in the case of the edge dislocations for LTG-Sm123 film, the Jc peak for B//c shrinks with increasing field and almost vanishes. But it grows again with further increasing a magnetic field and the dip of the angular dependence of ρ is also observed. Hence the vortex pinning phase diagram strongly depends on a sort of c-axis correlated pinning centers. It is considered that these different behaviors of the Jc peak for B//c are related to the competition of the random and c-axis correlated pinnings.

Flux pinning properties in BaMO3 (M=Zr,Sn) nanorod-introduced ErBa2Cu3Ox films

Flux pinning properties on the ErBa2Ca3O7 films with the nanoscaled and columnar-shaped BaMO3 (BMO,M=Zr,Sn) precipitates, i.e., nanorods, were investigated systematically based on the microstructure, critical current density Jc and irreversibility field Bi properties in high magnetic fields. We found that the in-field Jc and Bi increase monotonically with increasing the nanorod density until 3.5 wt % BZO and 6 wt % BSO additions, respectively, although the critical temperature Tc is reduced. According to the flux pinning analysis assuming of the linear summation, the BSO doped samples have the higher performance of the flux pinning than the BZO doped samples quantitatively even after the reduction in the Tc and size of the nanorods. It is considered that those differences originate from the effective pinning length of the nanorods and/or the condensation energy due to the difference of the carrier density.

In-field current transport properties of 600 a-class GdBa 2Cu3O7-δ coated conductor utilizing IBAD template

We have investigated critical current I_c properties of 600 A/cm-class GdBa₂Cu₃O_7-δ (GdBCO) coated conductor (CC) deposited on IBAD template. 2.5 μm thick GdBCO layer has been obtained by pulse laser deposition without degradation of critical current density by an improved heating condition in reel-to-reel deposition process. This enables stable production of long length CC with I_c (77 K, self-field) of 600 A/cm-w. In-filed I_c has also shown high value e.g., I_c@77 K, 3 T=33 A/cm-w, I_c@20 K, 27 T=107 A/cm-w. These experimental results obtained in a wide range of bias conditions of temperature T, magnetic field B and field angle θ are useful as a fundamental data for design of superconducting devices such as coil winding and magnet. Furthermore, we have also shown that our proposed analytical expression of electric field vs. current density (E-J) characteristics agrees well with the experimental results. This analytical expression allows us to predictI_c-B-T property as well as E-J characteristics at arbitrary conditions of B and T.

Cooperative Behavior of the Random and Correlated Pinning in Er123 Films With Columnar Defects

The flux pinning behaviors for the heavy ion irradiated Er123 films are investigated based on the detailed J_c(B,T,θ) properties. The double peaks of F_p curves for B//c appear in case of the matching field of 0.3 T along c-axis and 1.7 T tilted from c-axis. In addition, the peaks on J_c(θ) at θ = 0°(B//c), which originate from the columnar defects, decrease with increasing magnetic field but increase again near the irreversibility field. We found that the cooperation model of the c-axis correlated and random pinning centers can describe the observed double peak behavior of F_p(B) and the angular dependence of J_c related to the correlated pinning.