Elsevier

Applied Superconductivity

Volume 6, Issues 2–5, February–May 1998, Pages 217-224
Applied Superconductivity

Effect of Nd4Ba2Cu2O10 addition on the pinning properties of Nd-Ba-Cu-O crystals

https://doi.org/10.1016/S0964-1807(98)00104-5Get rights and content

Abstract

We have studied the effects of Nd4Ba2Cu2O10 (Nd422) on the pinning characteristics of NdBa2Cu3O7−δ (Nd123) crystals. Like Y2BaCuO5 particles in the Y–Ba–Cu–O system, the addition of Nd422 could enhance critical current density Jc at lower fields. The peak field (Bpk) and the Jc value at Bpk were almost independent of Nd422 contents although a slight decrease in Jc at Bpk was observed with increasing Nd422 contents. The normalized relaxation rate (S) increased for all the samples when the field exceeded Bpk. The addition of Nd422 was effective in preventing a rapid increase in S above Bpk. The exponent μ, defined in the collective creep model, was deduced by fitting the experimental data and was found to show strong field dependence. We found that μ changes from positive to negative at around Bpk, suggesting that the flux creep mechanism is different below and above Bpk.

Introduction

It is important to achieve high critical current densities at working temperatures for practical applications of superconductors[1]. In this context, a large amount of studies have been carried out to increase Jc of high temperature superconductors. For type II superconductors, Jc values are dependent on various factors such as the amount and distribution of pinning centers, the depth of pinning potential, and the interaction with the flux line lattice. For materials development, the key to obtaining a high Jc value is the introduction of crystal defects which can function as effective pinning centers without decaying the matrix superconducting properties2, 3. The defects which are believed to act as pinning centers in high temperature superconductors are twin planes, stacking faults, point defects (or oxygen defects), second phase particles, and artificially introduced columnar defects. Among these, non-superconducting secondary phase particles finely dispersed in the superconducting matrix have widely been studied and their correlation with flux pinning has intensively been discussed, although it is still controversial whether their contribution to flux pinning enhancement is direct or indirect.

In the Y–Ba–Cu–O system, progress in melt processing has enabled us to produce large single-domain samples with high Jc values, in which fine Y211 particles are distributed in the Y123 phase matrix. In the melt-processed Y123-Y211 composites, it is generally accepted that size refinement of Y211 particles is very effective in enhancing Jc values. Later, it was clarified that Jc values in high temperature and low field regions vary as d/d of Y2111, 2, 3, in which d is the volume fraction and d is the average grain diameter, whereby d/d corresponds to the effective interfacial area of Y211/Y123. This result supports the fact that Y211/Y123 interfaces are important pinning centers. Such relationship can be deduced from the direct summation over the elementary pinning forces of the fluxoids pinned at the Y211/Y123 interface. It is known that a simple direct summation can be applied to the system where flux line lattice is very soft, which is the case for high temperature superconductors since C66 is very small due to a large κ(=λ/ξ)2, 3. However, such linear relationships of Jc vs Vf/d collapse at higher fields, because the direct summation is no longer available once the vortex density exceeds the pin density. In such a regime, the collective creep theory4, 5, 6 that takes account of more complex vortex interaction is used to analyze flux pinning.

It was earlier known that the superconducting properties of Nd–Ba–Cu–O melt-processed in air were quite poor. The Nd–Ba–Cu–O system forms a Nd1+xBa2−xCu3Oy type solid solution (Nd123ss) and a large amount of Nd ions substitute for Ba in air, which results in a depression of the carrier concentration and thus low Tc[7]. Recently, it has been shown that such Nd–Ba substitution can largely be suppressed when Nd–Ba–Cu–O is melt-processed in a reduced oxygen atmosphere (oxygen-controlled-melt-growth: OCMG process), leading to remarkably high Tc of 96 K which was the highest Tc recorded in the RE123 family8, 9. In addition, OCMG processed Nd–Ba–Cu–O samples exhibit high Jc and a high irreversibility field accompanied by an anomalous secondary peak effect in JcB curves.

The origin of the peak effect has been assigned to the presence of finely dispersed Nd-rich Nd123ss clusters, which are superconducting but are driven normal at high magnetic fields, then act as field-induced pinning centers10, 11. In OCMG-processed Nd–Ba–Cu–O samples, therefore, there are two types of effective pinning centers active at high temperatures.

For practical applications such as a magnetically levitated train and a flywheel energy storage system, a strong electromagnetic force must be preserved for a long period, which requires a small magnetic relaxation rate. Therefore, it is important to study the magnetic relaxation of Nd–Ba–Cu–O superconductors that are believed to be promising candidates for high field applications. It is also interesting to see how the two different pinning centers interact and contribute to magnetic relaxation.

In this study, we have studied the pinning characteristic of Nd–Ba–Cu–O superconductors with an emphasis placed on the effect of Nd422 addition.

Section snippets

Experimental

Nd–Ba–Cu–O samples were fabricated by the OCMG process under the oxygen partial pressure of 0.001 atm, the details of which are described in elsewhere[12]. Here, we changed the volume fraction of Nd422. The samples Nd123+10 mol% Nd422 (Nd1.4), Nd123+20 mol% Nd422 (Nd1.8) and Nd123+30 mol% Nd422 (Nd2.2) contained 16%, 23% and 29% of Nd422 in volume fraction, respectively. The average grain size of the secondary Nd422 particles for all melt-processed samples was about 2 μm in diameter. The

Results and discussion

Fig. 1 shows the field dependence of Jc at 77 K for Nd123 single crystal and melt-processed Nd–Ba–Cu–O samples with different Nd422 contents. It is clear that Jc values near zero field are dependent on the Nd422 contents such that Jc values are enhanced with increasing the volume fraction. Similar effects can be seen with the Y211 inclusions in melt-processed Y–Ba–Cu–O crystals1, 2, 3, 16, 17, 18. Based on the assumption that Y211/Y123 interfaces provide pinning, Jc is expected to be

Conclusions

We have measured magnetization hysteresis loops and magnetic relaxation for Nd123 single crystal and melt-processed Nd–Ba–Cu–O with different Nd422 contents. Although Nd422 addition suppresses the peak effect, it enhances the Jc values at lower fields. The normalized relaxation rate S become large above Bpk, however, the relaxation above the Bpk seems to be slow for the samples with Nd422 phase. The exponent μ, defined in the collective creep model, is deduced by fitting the experimental data

Acknowledgements

This work was partially supported by NEDO through R & D of the Industrial Technology Frontier Program. The authors would like to thank Dr H. Kojo and Mr K. Sawada for the sample preparation. Thanks are also due to Dr S. I. Yoo, Dr N. Sakai, Mr T. Higuchi, Mr S. Takebayashi and Dr H. S. Chauhan for useful discussions.

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