Grain boundary doping effect on critical current density in YBa2Cu3O7 polycrystalline materials

Grain boundary doping effect on critical current density in YBa2Cu3O7 polycrystalline materials

Physica C 386 (2003) 286–291 www.elsevier.com/locate/physc Grain boundary doping effect on critical current density in YBa2Cu3O7 polycrystalline mater...

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Physica C 386 (2003) 286–291 www.elsevier.com/locate/physc

Grain boundary doping effect on critical current density in YBa2Cu3O7 polycrystalline materials Y. Zhao *, C.H. Cheng Superconductivity Research Group, School of Materials Science and Engineering, University of New South Wales, P.O. Box 1, Sydney, NSW 2052, Australia

Abstract Diffusion method has been developed to preferentially dope the grain boundaries (GBs) in textured YBa2 Cu3 O7 bicrystals and polycrystalline materials. Ag, Ca, Fe, Pb, etc. have been used as the dopants in this study. The distribution of the dopants is found to be highly localized around the GBs. The Jc of YBa2 Cu3 O7 textured polycrystalline samples are significantly enhanced by doping Ag or Ca in the GBs. The Ca doping effect has been explained by chargecarrier-compensation in the GBs, which reduces the GB/junction resistance and thus increases the GB critical current. The Ag doping effect has been explained by the partially repairing of the broken Cu–O bonds, which transforms an extended geometric distortion in CuO2 planes to a localized electronic distortion. Four mechanisms for repairing GB of HTSs are proposed, which are charge-carrier compensation, grain boundary cleaning, weak-link to pinning-center transformation, and concurrent doping. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 74.60.Jg; 74.80.Bj Keywords: YBa2 Cu3 O7 ; Grain boundaries; Preferential doping; Critical current density

1. Introduction Weak-link effect of grain boundaries (GBs) in high-Tc superconductors (HTSs) is extremely harmful to the performance of HTSs, many efforts have been made to overcome it. Practically, it is very hard to avoid the occurrence of high-angle GBs in a kilometer-long tapes/wires. Therefore, to ‘‘repair’’ these fault parts and pursue ‘‘weak-linkfree’’ instead of ‘‘GB-free’’ conductor is a more realistic approach of realizing the large-scale ap-

*

Corresponding author. Tel.: +61-2-9385-5986; fax: +61-29385-5956. E-mail address: [email protected] (Y. Zhao).

plication of HTSs. It is reported recently [1,2] that the weak-link effect of the high-angle GBs in Y-123 multilayer films was significantly reduced by doping Ca at Y site. Similar achieved of Jc was made in a high magnetic field of 5 T [3]. All these demonstrate that the ‘‘damage’’ of superconductivity in GBs of HTSs can be ‘‘repaired’’ to a large extent by chemical doping. However, the GB problem in HTSs remains unsolved because: (1) the best GB Jc value is still much lower than the Jc necessary for applications, especially in a high magnetic field; (2) it is not clear the mechanism underlying the doping effect at GBs; (3) little has been known about the mechanisms and features of ion diffusion in the GBs of HTSs; (4) chemical doping may improve the GB Jc but at the same

0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-4534(02)02133-0

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time, it may degrade the intragrain performance of the HTSs, so it is necessary to develop a grainboundary-preferential doping technique which repairs the GBs but has little side-effect on the intragrain properties of HTSs. In this work we try to solve the problems mentioned above using the chemical doping method.

2. Experimental Samples used in this study were prepared by a modified melt-textured-growth (MTG) method [4]. In addition, several different ways were used to dope chemical elements into the GB regions of the MTG Y-123 bicrystals. One is to add the dopants in the Y211 substrate that was placed right under the potential joint (GB formation area). Because only a small area around the joint rather than the whole sample was contacted with the doped substrate, the chemical dopants were, as expected, mainly doped in the area around the GB. This method is denoted hereinafter as ‘‘high temperature preferential doping (HTPD)’’. Another method of doping GB is based on the solid or gas diffusion at a temperature much lower than the solid state reaction temperature. By coating a thin layer on the surface of the HTSs bicrystals and heating them at a certain temperature, or directly heating the sample in the vapor of the desired elements such as S, Pb, Hg, etc. the dopants gradually penetrate the crystals. Because of the preferential doping mechanism governed by GB energy [4], the dopants preferentially occupy the GB. This technique is denoted as ‘‘low temperature diffusion doping (LTDD)’’. X-ray diffraction was used to determine the average crystal structure of doped and undoped crystals. Micro-structures and compositions were analyzed by polarized optical microscope, scanning electron microscope, transmission electron microscope, energy dispersive analyses and electron probe micro-analyses (EPMA). Micro-Raman spectroscopy, together with the temperature–pressure phase diagram, was used to determine the oxygen content of the crystals. DC SQUID magnetization was used to measure the magnetization loops, irreversibility lines, and the magnetization Jc (calculated with Bean

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Model) with H jjc (for both single domain crystals, bicrystals, and c-axis aligned polycrystalline samples).

3. Results and discussion For doped crystals, the distribution of the dopant concentration near the GB has been measured by EPMA. Fig. 1a shows the Fe concentration distribution in a Fe-doped Y-123 textured crystal. Because Fe2 O3 was added in the presintered sample before the MTG process, the Fe ions disperse uniformly in the whole crystal except for a slight accumulation of Fe ions in the GB region. This is due to the GB energy which prefers the dopant staying in the GB region [4]. For the GBs where the dopant was introduced using the HTPD technique, the distribution of the dopant is more localized in the region near the GB. Fig. 1b illustrates the distribution of the Ca concentration along a straight line crossing the GB in a Ca-doped GB using HTPD method. The Ca concentration is significantly higher in the GB region than in other areas, and the closer to the GB, the higher the Ca concentration, indicating that the HTPD technique is a feasible and effective method to preferentially dope GBs of Y-123. It is very interesting to note that the morphology of the Ca-doped GB is different from those doped with other elements, such as Fe. As shown in Fig. 1c, a Ca-rich second-phase accumulates near the Ca-doped GB, forming a hump (wall) just above where the GB is located. Such a phenomenon was not observed in the samples doped by other elements (e.g., Fe, as illustrated in Fig. 1d). For GBs doped with the LTDD method, the distribution of the dopant concentration near the GB is much more narrow than the above-shown results. Fig. 2 displays the Ag distribution near the GB of a Ag-doped Y-123 bicrystal treated with LTDD. Because LTDD method is based on the diffusion along the GB, such a sharp distribution of the dopant in the GB region is an expected result. In order to protrude the GB effect in the magnetization measurement, the melt textured

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Fig. 1. Distribution of the dopant concentration along a straight line crossing GB for Y-123 MTG bicrystal: (a) Fe-doped and (b) Cadoped using HTPD method. The corresponding micro-graphs of the GBs is shown in (c) for Ca-doped and (d) for Fe-doped bicrystals.

Fig. 2. Distribution of the Ag concentration along a straight line crossing GB for Ag-doped Y-123 MTG bicrystal.

polycrystals containing a large number of narrow and clean GBs have been doped with Ag, Ca and Fe using the same methods as described above. The distribution of the dopant concentration in

the GBs is similar to that in the bicrystals. Three typical samples are examined. First sample is the undoped Y-123 textured polycrystal prepared using the liquid-phase-removing method, and denoted as ‘‘undoped’’. The second sample was doped with Ca using the same preparation method as the first one. The Ca was added into the Y211 substrate and diffused into the GBs of the textured polycrystalline Y-123. This sample is denoted as ‘‘Ca-doped’’. The third one was prepared by coating a thin Ag film on the surface of the ‘‘undoped’’ sample, and then heating it at 850 °C for 24 h in the flowing oxygen, and finally annealing it in flowing oxygen at 450 °C for 24 h. This sample was denoted as Ag-doped. Because these samples were prepared in the similar conditions, so the results generated from them will reflect some intrinsic properties about the doping effect on GBs. Typical dimensions of the sample are 1:2  1:0  0:3 mm3 .

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Fig. 3. Jc –H curves for undoped, Ca-doped and Ag-doped Y-123 textured polycrystals.

Fig. 3 shows the Jc deduced from MðH Þ loops using the Bean model for the above-mentioned typical samples. For the undoped sample, Jc value is around 2:2  104 A/cm2 at 85 K in self-field, 5:8  104 A/cm2 at 77 K in self-field and 1  104 A/ cm2 at 77 K in 1 T. The high Jc value at 77 K indicates that the GBs in the textured polycrystalline sample is rather strongly coupled. However, the Jc decreases rapidly with increasing applied magnetic field, attributed to the degradation of Jc in the GBs. For Ca-doped sample, a significant improvement in the Jc –H dependence can be clearly seen. The improvement is more remarkable in high fields than in low fields. For example, at 60 K, the Jc was increased by a factor of 144% through Ca-doping in the zero-field whereas it was increased by a factor of three in 7 T. For Ag-doped sample, Jc was also improved compared to the undoped one. The improvement mainly exhibits in the behavior of Jc in the high field region, whereas in low fields, the Jc is almost the same as that of the undoped sample.

The improvements mentioned above may be attributed to the repair of GBs by doping Ca or Ag because the dopants mainly stay in the GB regions. Especially in the case of Ag doping with the LTDD technique, the Y-123 grains are almost not affected by the dopants. In our opinion, the mechanism of repairing GBs in HTSs can be understood from the following aspects. 3.1. Compensating charge-carriers-loss in GBs Because of the structure distortion in the GBs, the charge carrier concentration in GBs is reduced significantly. As a result, GBs become crucial barriers for the electron transport and the resistance of GBs becomes significantly higher than that in the grains. In the GB region, Ca-doping compensates the hole-loss caused by the lattice distortion and thus decreases the resistance of GBs. According to the following relationship. If each GB works as a Josephson junction, its critical current can be expressed as:

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Ic ðT Þ ¼

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p Dc ðT Þ tanh 2e Rn



Dc ðT Þ 2kB T

 ð1Þ

where Rn is the junction resistance and Dc ðT Þ is the gap parameter at the grain boundary junction. According to Eq. (1), Jc in the GB increases with decreasing GB resistance. This scenario is a good explanation for the Ca-doping effect on the GB Jc . 3.2. Cleaning GBs Because of the preferential doping mechanism governed by GB energy [4], undesired chemical impurities also have high potential to stay in the GBs and cause addition resistance or pair-breaking effect in GBs. The decrease of Jc in Fe-doped GBs in Y-123 (result is not shown here) indicates that the magnetic impurity is extremely harmful to the GB Jc . So, chemical contamination from undesired elements must be reduced to the lowest possible level to keep the GBs clean. Once the desired dopant occupies the GB, the GB energy decreases and the probability of undesired elements occupying the GB is greatly decreased.

Fig. 4. A schematic of the atom positions in the CuO2 plane of a high angle [0 0 1] GB. The black balls represent the Cu atoms, and the white represent the oxygen atoms.

structure distortion. This may remove the weaklink effect through shortening the extension of the distortion; and further turn the localized electronic distortion region to a new pinning center.

3.3. Turning weak-link to pinning center 3.4. Concurrent doping in GB The structure distortion in the GBs of HTSs is the main cause for order parameter suppression and Jc degradation [5], the larger the distorted distance, the more serious the suppression of Jc . Due to the lattice relaxation, the distortion originated from the GB area extends to a much larger distance than the size of the grain boundary itself [6]. So, reducing the extension of the distortion will effectively narrow the weak-link region. The Agdoping in GB may have played such a role because Ag replaces some of the Cu in the GB region where the Cu–O is largely weakened by lengthening the Cu–O distance. As shown in Fig. 4, as the Cu in the position 1 in the GB is replaced by Ag, the broken Cu–O bonding in the positions 1 and 2 is repaired due to the larger ionic size of Ag. Consequently, the distortion of other Cu–O bonds caused by lattice relaxation is reduced and the overall geometric distortion in the CuO2 planes near GB is reduced. So, the extended geometric distortion is transformed to a localized electronic

As mentioned above, a specific dopant may play a certain role in repairing the GB. A complete repairing of GB may be fulfilled by doping two or more elements concurrently. In this work, GB oxygenation may have worked together with other dopants.

4. Conclusions The main conclusions from this work can be drawn as follows: 1. Diffusion methods have been developed to preferentially dope Ag, Ca, Fe, Pb, etc. into the GBs in Y-123. 2. The Jc values of Y-123 textured-polycrystals are significantly enhanced by doping Ag or Ca in GBs. The Ca doping effect is attributed to the charge-carrier-compensation in the GBs and

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the Ag doping effect can be explained by the partially repairing of the broken Cu–O bonds, turn GBs into pinning centers. 3. Four mechanisms for GB repairing in HTSs have been proposed according to this work, i.e., charge-carrier compensation, grain boundary cleaning, weak-link to pinning-center transformation, and concurrent doping. Acknowledgements This work was supported in part by the Australian Research Council (ARC) and the University of New South Wales.

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