Kazutaka Imamura

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.

Visualizing Transport Properties in IBAD Based YBCO Coated Conductors by Multiple Analysis Techniques

Current transport properties in Y1Ba2Cu3O7-delta (YBCO) coated conductors obtained by pulsed laser deposition process on a CeO2 capped Gd2Zr2O7-IBAD template have been studied by spatially resolved measurements. We utilized low temperature scanning laser microscopy along with laser induced Seebeck effect imaging and scanning SQUID microscopy that allow us to visualize 1) distributed flux flow dissipation, 2) current blocking obstacles and 3) local current flow, respectively. Combination of those measurements leads deep insights into current limiting mechanism in the coated conductor. Our results show that non-uniform current flow due to spatially distributed obstacles is responsible for the dissipation, whereas grain connectivity in each YBCO grains is not the limiting factor. Typical period of those obstacles is several tens mum to hundreds mum. Detailed correlation between local current flow and dissipation has been observed. Present methods have great potential as tools for basic understanding of current limiting mechanisms in the coated conductors.

Enhancement of In-Field Current Transport Properties in GdBCO Coated Conductors by

We have investigated the in-field current transport property in BaHfO₃ doped GdBa₂Cu₃O_{7 - δ} coated conductors in a wide range of temperatures and magnetic fields. Significant improvement of in-field critical current <i>I</i>_c was observed, e.g., <i>I</i>_c@77 K, 3 T = 93 A/cm-w, <i>I</i>_c@20&nbsp;K, 17&nbsp;T = 700 A/cm-w, which is a comparable value to that of Nb₃Sn wire at 4.2 K. Enhancement of the irreversibility field was also observed. These results suggest that BaHfO₃ is one of the most promising materials as effective artificial pinning centers and leads to the enhancement of in-field <i>I</i>_c. Furthermore, we have also shown that our analytical expression of electric field versus current density characteristics based on the percolation transition model [1-3] agrees well with the experimental results over a wide range of magnetic fields and temperatures. This analytical expression is useful for the design of superconducting devices because this allows us to predict the current carrying capability of coated conductors not only <i>J</i>_c but also <i>n</i>-value at arbitrary operating conditions of temperature and magnetic field.

\hbox{BaHfO}_{3}

Using limited data measurements, the electric field vs. current density (E-J) characteristics of practical Bi-2223/Ag tapes were predicted in a low-temperature and high-magnetic field. This method is based on findings shown in our papers, which describe E-J characteristics according to the percolation model, as a function of temperature, magnetic field and angle of the magnetic field. This method can also estimate critical current density and n-value, given any electric field criterion.

Doping

An age-hardenable Cu-2.9%Ni-0.6%Si alloy was subjected to high-pressure torsion. Aging behavior was investigated in terms of hardness, electrical conductivity and microstructural features. Transmission electron microscopy showed that the grain size is refined to ~150 nm and the Vickers microhardness was significantly increased through the HPT process. Aging treatment of the HPT-processed alloy led to a further increase in the hardness. Electrical conductivity is also improved with the aging treatment. It was confirmed that the simultaneous strengthening by grain refinement and fine precipitation is achieved while maintaining high electrical conductivity. Three dimensional atom probe analysis revealed that fine precipitates with sizes of ~20 nm or smaller were formed in the Cu matrix and some particles consist of Ni and Si with no appreciable amount of Cu.

Prediction of E-J characteristics in Bi-2223/Ag tapes at low temperature and high magnetic field

We have characterized nonlinear current transport properties in a coated conductor as a function of temperature, magnetic field vector and mechanical strain, and then have developed a thermally-electromagnetically-structurally coupled analysis code for a high-field magnet coil. The distributions of heat generation and electromagnetic force in the coil are computed by electromagnetic analysis. Then, the temperature distribution and the strain distribution are correspondingly calculated by thermal analysis and by structural analysis. Furthermore, both of them are fed back to the electromagnetic analysis. These analyses are based on finite element method, and are repeated until the convergence. By taking a design example of a 40 T class magnet coil using a GdBCO coated conductor, we have discussed the necessity of the consideration of thermally-structurally influenced transport properties in the coil for the coil design.

Aging Behavior of Cu-Ni-Si Alloy Processed by High-Pressure Torsion

We have investigated the current transport properties of batch furnace processed TFA-MOD Y_xGd_1-xBa₂Cu₃O_y (YGdBCO) coated conductor (CC) doped with nanoscale BaZrO₃ artificial pinning centers (APCs). As the comparison with those of previous processed YGdBCO CC, it has been confirmed that the present process is very effective to improve current carrying performance of the CC at not only high-temperature but also low-temperature region, for example, the I_c at 4.2 K and 17 T is 450 A/cm-w, i.e., the engineering J_c is 4.3 × 10⁸ A/m², which is comparable value to that of Nb₃Sn wire at 4.2 K. In addition, angular dependences of the I_c clearly show the effectiveness of the APCs. Namely, the minimum I_c in the whole angle region at 77 K, 3 T increase from 7 to 35 A/cm-w, and the anisotropy of I_c is reduced. We will also discuss the analytical expression of in-field current transport property based on the percolation transition model and the scaling law of the flux pinning. By using this analysis, electric field versus current density characteristics in arbitrary conditions of temperature and magnetic field can be described, and also, the statistical distribution of J_c can be estimated. These results are important not only for the design of superconducting devices using CCs but also for understanding the pinning properties.

Coupled Analysis Method for High-Field Magnet Coil Using Coated Conductor Based on

We have investigated the current transport properties of batch furnace processed TFA-MOD Y_xGd_1-xBa₂Cu₃O_y (YGdBCO) coated conductor (CC) doped with nanoscale BaZrO₃ artificial pinning centers (APCs). As the comparison with those of previous processed YGdBCO CC, it has been confirmed that the present process is very effective to improve current carrying performance of the CC at not only high-temperature but also low-temperature region, for example, the I_c at 4.2 K and 17 T is 450 A/cm-w, i.e., the engineering J_c is 4.3 × 10⁸ A/m², which is comparable value to that of Nb₃Sn wire at 4.2 K. In addition, angular dependences of the I_c clearly show the effectiveness of the APCs. Namely, the minimum I_c in the whole angle region at 77 K, 3 T increase from 7 to 35 A/cm-w, and the anisotropy of I_c is reduced. We will also discuss the analytical expression of in-field current transport property based on the percolation transition model and the scaling law of the flux pinning. By using this analysis, electric field versus current density characteristics in arbitrary conditions of temperature and magnetic field can be described, and also, the statistical distribution of J_c can be estimated. These results are important not only for the design of superconducting devices using CCs but also for understanding the pinning properties.

J\hbox{-}E

We have investigated the current transport properties of batch furnace processed TFA-MOD Y_xGd_1-xBa₂Cu₃O_y (YGdBCO) coated conductor (CC) doped with nanoscale BaZrO₃ artificial pinning centers (APCs). As the comparison with those of previous processed YGdBCO CC, it has been confirmed that the present process is very effective to improve current carrying performance of the CC at not only high-temperature but also low-temperature region, for example, the I_c at 4.2 K and 17 T is 450 A/cm-w, i.e., the engineering J_c is 4.3 × 10⁸ A/m², which is comparable value to that of Nb₃Sn wire at 4.2 K. In addition, angular dependences of the I_c clearly show the effectiveness of the APCs. Namely, the minimum I_c in the whole angle region at 77 K, 3 T increase from 7 to 35 A/cm-w, and the anisotropy of I_c is reduced. We will also discuss the analytical expression of in-field current transport property based on the percolation transition model and the scaling law of the flux pinning. By using this analysis, electric field versus current density characteristics in arbitrary conditions of temperature and magnetic field can be described, and also, the statistical distribution of J_c can be estimated. These results are important not only for the design of superconducting devices using CCs but also for understanding the pinning properties.