Kazunari Kimura

Development of REBCO Coated Conductors by TFA-MOD Method With High Characteristic in Magnetic Field

Y_xGd_{(1 - x)}Ba₂Cu₃ O_y coated conductors (CCs) with artificial pinning centers (APCs) were fabricated by the trifluoroacetates-metal organic deposition method using a batch heat-treatment process. This process has been industrially applied to fabricate long superconducting tapes that have high critical current (<i>I</i>_c). From the viewpoint of applications, the characteristics in magnetic fields become further important, and one solution has been shown previously as introduction of APCs such as BaZrO₃. We applied the technique to our CCs fabrication process. The heat-treatment conditions, specifically total gas pressure, oxygen concentration, and crystallization temperature, were optimized for the batch heat-treatment process. As a consequence, fabricated CC showed extremely high characteristic <i>Jc</i> value of 3.5 MA/cm² in self field, and 0.3 MA/cm² at 3 T in liquid N₂. Moreover, by cross-sectional transmission electron microscopy observation, it was confirmed that the APC was distributed uniformly. Furthermore, the anisotropy of the pinning center is also examined based on the evaluation result of the angular dependence of <i>Jc</i> in 3T.

Development of REBCO Coated Conductors by TFA-MOD Method With High Properties in Magnetic Fields

With technological advance of the REBaCuO (RE: Rare Earth elements, REBCO) coated conductors (CC), development of the superconducting applications for its practical use has been carried out recently in various fields. In addition, improvement of the magnetic properties of REBCO CCs makes superconducting devices feasible in the liquid nitrogen. The higher Ic values are required in high magnetic fields by the viewpoint of the requirement in conjunction with the superconducting CCs. Therefore the improvement of the Ic values in the magnetic fields is one of the most important problems to apply them to a device (e.g., NMR, MRI, others). In this work, we investigated the fabrication processing to improve the in-field properties of the YGdBaCuO (YGdBCO) CCs by introduction of artificial pinning centers (APCs). The TFA-MOD method using the batch type furnace for the conversion reaction as a cost effective process to fabricate REBCO CCs was improved by adopting the interim heat treatment during the ramp step of the conversion reaction. The improvement of the properties by the improved process was remarkable and the advanced CCs were named as nPAD-YBCO. A 124 m-long nPAD-YBCO consisted of YGdBCO CC with BZO nano particles was successfully fabricated, exhibiting the Icmin > 50 A/cm-w @77 K, 3 T (Ic: Critical current) throughout the conductor. Analyzing the characteristics of the conductor, the BZO particles introduced into the YGdBCO layer worked as effective APCs in wide ranges of temperature and magnetic fields.

Enhancement of

A modified heat-treatment procedure, which consisted of three steps, i.e., 1) calcination, 2) intermediate heat-treatment (IHT), and 3) crystallization, was developed for the metal organic deposition method using trifluoroacetates (TFA). BaZrO₃-doped Y_0.77Gd_0.23Ba₂Cu₃O_6+δ (YGdBCO) coated conductors fabricated by this modified procedure provided the minimum critical current density (J_c) value of 0.21 MA/cm² at 77 K in 3 T, which was as high as 2.1 times the value obtained by the conventional one, and a homogeneous dispersion of BaZrO₃ nanoparticles with finer size in the matrix. Furthermore, the 1-D theoretical analysis of the reaction kinetics in the IHT process, considering diffusion and growth kinetics at the interface between the precursor layer and the calcined layer, was proposed, and the estimated reaction rate was compared with experimental ones.

J_{\mathrm{c}}

A wide range of coating thicknesses was utilized in the trifluoroacetate metal-organic decomposition process for (Y, Gd) Ba₂Cu₃O_7-δ (YGdBCO) coated conductor including BaZrO₃ (BZO) particles with a thin once-coated thickness(<italic>d</italic>_once), which was developed for improving the critical current density in a magnetic field (<italic>J</italic>_c(<italic>B</italic>)). The value of <italic>J</italic>_c (<italic>B</italic>) increased as <italic>d</italic>_once decreased to 30 nm, and a sample fabricated with <italic>d</italic>_once = 30 nm exhibited high values of <italic>J</italic>_c(<italic>B</italic>) of 0.27 and 1.60 MA/cm² at 77 and 65 K, respectively, at 3 T (<italic>B</italic>//<italic>c</italic>). However, the value of <italic>J</italic>_c(<italic> B</italic>) of a sample with <italic>d</italic>_once = 17 nm was almost the same as or slightly lower than that of the sample with <italic>d</italic>_once = 30 nm. This tendency was explained by the change in the size of the BZO particles in the YGdBCO superconducting films. Additionally, the Cu elemental peak and the valleys of other elements were observed at the same position and at certain intervals in the elemental concentration line profiles along the depth direction of the precursors, fabricated under different <italic>d</italic> _once conditions from 170 to 30 nm. However, the peaks and valleys were extremely suppressed, and the wave structure was barely observed in the sample with <italic>d</italic>_once = 17 nm. The refining of BZO particles by reducing <italic>d</italic>_once could be explained by the limited diffusion due to the existence of the concentration valleys/peaks. Moreover, the effect of the diffusion barrier due to the valleys/peaks might become weaker in the sample with <italic>d</italic>_once = 17 nm.

Properties in Magnetic Fields of BaZrO 3 Doped REBa 2 Cu 3 O 6+δ Coated Conductors by a Newly Modified TFA-MOD Process

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.

Refining Process of BaZrO 3 Particles in Coated Conductors by TFA-MOD Method

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.

Current Transport Properties of TFA-MOD Processed Long-Length

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.