Elsevier

Applied Superconductivity

Volume 6, Issues 2–5, February–May 1998, Pages 119-127
Applied Superconductivity

Critical currents in air processed NdBa2Cu3O7 melt-textured superconductors

https://doi.org/10.1016/S0964-1807(98)00092-1Get rights and content

Abstract

Single domain ceramics of NdBa2Cu3O7 with high Tc and high critical currents have been prepared in air by directional solidification techniques (Bridgman growth and top seeding growth). The temperature and field dependence of the critical currents have been investigated with the magnetic field oriented along the c-axis and within the ab-planes to analyze the microstructural features influencing the superconducting properties. The effect of Nd4Ba2Cu2O10 and Nd2BaO4 additives on the transition temperature and the critical currents has been studied in order to examine if the concentration of antisite defects may be modified with this procedure. It has been found that no noticeable difference is generated when the samples are grown in a directional solidification procedure. We discuss as well the relevance of post-processing heat treatments in Ar and O2 atmospheres on the critical currents. Finally, some clues on the general features of the magnetic phase diagram of these materials are given.

Introduction

Recently, a considerable effort has been devoted to develop new ReBa2Cu3O7 melt textured superconductors with light Re ions owing to their potentiality to enhance the critical currents, mostly in high fields. The improvement of the performances was attributed both to the fact that Tc can be slightly higher, up to 95 K in Nd123, and to the introduction of a new pinning center, of controversial origin up to now, which generates a strong fishtail anomaly at high temperatures.

The main issue from the processing point of view was the discovery of Murakami et al.1, 2that the use of oxygen-controlled growth (OCMG) could control the degree of solid solution in Nd1+xBa2−xCu3O7 and hence modify the superconducting transition temperature. Lately, however, it has been shown that the directional solidification processes, like Bridgman growth or top seeding growth for instance, enable us to prepare samples with sharp superconducting transitions at Tc=95 K even if the growth process is performed in air3, 4. It is therefore very appealing to analyze the superconducting properties of samples processed under different P(O2) and to correlate them with the changes in the microstructure.

A particularly relevant issue is the investigation of the origin of the anomalous high field behavior of the critical currents (i.e. the fishtail) in melt processed Nd123 because some controversial interpretations have been proposed. Due to the complex behavior of the thermodynamic phase diagram of this system5, 6, 7, 8, it appears necessary to correlate the critical currents with the different processing conditions. The origin of the fishtail in Nd123 has been proposed to be associated with the formation of Nd–Ba antisite defects which are distributed inhomogeneously and hence, lead to clusters with a depressed superconducting order parameter1, 2, 10, 11. The existence of these clusters involve the creation of field induced pinning centers similar to the first interpretation of this effect in Y123 single crystals. In that case, the non-superconducting clusters were associated with poorly oxygenated regions[9]. The formation of Nd-rich clusters in Nd123 was further analyzed by Nakamura et al.[11]who showed that high temperature heat treatments could strongly modify the intensity of the fishtail effect and attributed it to a spinodal decomposition of the Nd1+xBa2−xCu3O7 solid solution.

An alternative explanation for the fishtail was proposed by Wolf et al.12, 13who showed that, actually, the anomaly could also be nearly washed out through high pressure oxygen annealings, in addition to an enhancement of the irreversibility line. This allowed them to conclude that the oxygen inhomogeneities originate, in poor superconducting regions, field induced anomalies. It was also pointed out that the antisite defects increase locally the oxygen content in Nd123 and so both defects could actually be correlated.

It has been claimed by several authors, after a close examination of the crystalline phases appearing in the high temperature phase diagram of the Nd–Ba–Cu–O system, that the admixtures used as precursors, i.e. Nd4Ba2Cu2O10 (422), Nd2O3 (200) or Nd2BaO4 (210), may strongly modify the peritectic reaction leading to Nd123 solid solutions[6]. For instance, H. Kojo et al.[14]have recently confirmed that high Tc Nd123 may indeed be prepared in air[3]and the use of Ba-rich 422 phases as precursors help to obtain sharp superconducting transitions. Actually, recent high temperature analysis of the thermodynamic stability of the Nd123 solid solutions at different P(O2) has revealed that second phase precipitations could occur under well determined thermal treatments due to the differences in stability of phases with different amounts of solid solution[7]. A transmission electron microscopy (TEM) analysis of these phase precipitated samples allowed them to infere that, a coherent layered intergrowth with a composition of BaCuO2 occurs and this generates a high density of dislocations which could enhance the critical currents in high fields[15].

It appears then that the thermodynamic complexity of the Nd123 system forces a close examination of the superconducting properties observed in samples processed under well determined conditions, since the actual microstructure may differ in a subtle way. In this work, we follow up our previous investigation of the critical currents of Nd123-422 composites prepared in air. Whereas similar high Tc values are obtained, the critical currents seem to differ quantitatively from the samples processed under low oxygen pressure conditions. We will particularly focus on the influence of the additives in the precursors and the postprocessing heat treatments relationship with the critical currents and its anisotropy.

Section snippets

Experimental

In this work we have used different samples prepared by Bridgman growth and top seeding growth (TSG). Superconducting Nd123 bars of about 150 mm in length and 8 mm in diameter were directionally solidified in air at T=1120°C through a thermal gradient of 20°C/cm at a pulling rate of 1 mm/h in a vertical Bridgman furnace. Bars were prepared with different concentrations of 422 additives but most of the experimental investigation has been carried out in samples having 5 and 15 wt%. The Tc of these

Results and discussion

We have first of all investigated the influence of precursors and heat treatments in the superconducting transition temperature. It was found, as shown in Fig. 1(a), that Bridgman growth in air of Nd123 bars may lead to narrow transitions with Tc=94 K after oxygenation in 450°C and P(O2)=1 bar but without any additional heat treatment, only if the starting composition corresponds to stoichiometric Nd123. When further additives such as stoichiometric Nd422 powders with different concentrations

Conclusions

We have prepared, through two directional solidification techniques, top seeding growth and Bridgman growth, several NdBa2Cu3O7/Nd4Ba2Cu2O10 composite samples from precursor oxides including either stochiometric Nd4Ba2Cu2O10 or Nd2BaO4 powders with different concentrations. In both cases, the solidification has been carried out in air and we have shown that high Tc–high Jc values may also be obtained. Some clues concerning the processing mechanisms by which more homogeneous superconductivity in

Acknowledgements

This work has been supported by CICYT (MAT96-1052), Generalitat de Catalunya (GRQ95-8029) and Red Eléctrica de España (93-2331).

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