Physica C 313 Ž1999. 232–240 Refinement of secondary phase particles for high critical current densities in žNd,Eu,Gd / –Ba–Cu–O superconductors M. Muralidhar ) , M.R. Koblischka, M. Murakami Superconducting Research Laboratory, ISTEC, 1-16-25, Shibaura, Minato-ku, Tokyo, 105, Japan Received 26 August 1998; revised 8 December 1998; accepted 13 January 1999 Abstract We have prepared ŽNd 0.33 Eu 0.33 Gd 0.33 .Ba 2 Cu 3 O y Ž‘NEG’. superconductors by means of melt-processing under a partial oxygen pressure of 0.1%. In these samples, we achieved a large critical current density, Jc , of 6.8 = 10 4 Arcm2 at T s 77 K and in an applied magnetic field, Ha s 2.5 T. Furthermore, the irreversibility field, Hirr , exceeds 7 T at 77 K for fields applied parallel to the c-axis of the sample. As additional pinning sites, we introduced up to 40 mol% ŽNd,Eu,Gd. 2 BaCuO5 ŽNEG-211. particles plus Pt in analogy to YBa 2 Cu 3 O y ŽYBCO. melt-processed superconductors. Microstructural observations clarified that the Pt addition was effective in reducing the size of NEG-211 second-phase particles, which led to a dramatic increase in Jc as compared to melt-processed YBa 2 Cu 3 O y and NdBa 2 Cu 3 O y ŽNdBCO.. A pronounced secondary peak effect was observed in all magnetization curves obtained at 77 K, however, there is a considerable change in the shape of the magnetization loops as a function of NEG-211 content, which demonstrates the pinning provided by the NEG-211 particles in addition to the dTc-pinning caused by fluctuation in the transition temperature, Tc , like in ordinary NdBa 2 Cu 3 O y superconductors. q 1999 Elsevier Science B.V. All rights reserved. PACS: 74.60 Ge; 74.60 Jg; 74.72 Jt; 74.80 Bj Keywords: ŽNd,Eu,Gd.Ba 2 Cu 3 O y ; ŽNd,Eu,Gd. 2 BaCuO5 ; Melt processing; Pt addition; Microstructure; Peak effect; Flux pinning; Critical current density 1. Introduction The critical current density, Jc , is one of the most important properties for practical applications of high-temperature superconductors. Although Jc of melt-processed YBa 2 Cu 3 O y ŽYBCO. already surpassed the lower limit of 10 4 Arcm2 at T s 77 K for some applications, further improvement will be critically important to facilitate the bulk applications, ) Corresponding author. Tel.: q81-196-35-9016; Fax: q81196-35-9017; E-mail: firstname.lastname@example.org e.g., the fabrication of quasi-permanent magnets which can generate several tesla at 77 K. Yoo et al. w1x found that NdBa 2 Cu 3 O 7 ŽNdBCO. superconductors prepared by the melt process in a reduced oxygen atmosphere exhibited high Jc values in a high field region accompanied by a secondary peak effect. The secondary peak effect is ascribed to the so-called dTc-pinning w2–8x provided by RE-rich ŽRE denoting rare earths. 123 clusters with a diameter of 10–50 nm, which are uniformly distributed in the RE-123 matrix w9x. These clusters have been observed by means of scanning tunneling mi- 0921-4534r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 9 9 . 0 0 0 1 7 - 9 M. Muralidhar et al.r Physica C 313 (1999) 232–240 croscopy ŽSEM. w10x and transmission electron microscopy ŽTEM. w11x; and the different features of the dTc-pinning mechanism were discussed in detail in Refs. w5–7x. Furthermore, it was experimentally confirmed that the melt process under low oxygen pressure Ž pO 2 ., i.e., the oxygen-controlled-meltgrowth ŽOCMG. process, is also effective in achieving high Tc and Jc values for other light rare earth superconductors w12x. Thus, these light rare earth LRE-Ba 2 Cu 3 O y ŽLRE: Nd, Sm, Eu, Gd. superconductors are promising for practical applications, since in addition to an enhanced Tc of 96 K with a sharp superconducting transition, they also exhibit Jc values larger than that of good quality YBa 2 Cu 3 O 7 in a high magnetic field region particularly at high temperatures. Recently, it was found that LRE-Ba 2 Cu 3 O y superconductors in which two LRE elements were mixed together show similar features w13–15x. It was also reported that LRE-Ba 2 Cu 3 O y composites with three different LRE elements also exhibit high Tc and Jc values w16–21x. However, the peak field and Jc values were found to be dependent on the kind of rare earth elements and on the processing conditions. We have found that among these mixed LREBa 2 Cu 3 O y superconductors, the ternary compound ŽNd,Eu,Gd.Ba 2 Cu 3 O y ŽNEG. with a ratio of Nd: Eu:Gds 1:1:1 showed a record high Jc of 50 000 Arcm2 at an applied field of 2 T and the irreversibility field Ž Hirr . exceeding 7 T at 77 K for fields parallel to the c-axis w19,20x. This improvement of Jc was ascribed to a more uniform distribution of the dTc-pinning sites w22x. In the present study, we have succeeded in further enhancing the critical current densities of ŽNd,Eu,Gd.Ba 2 Cu 3 O y by adding NEG-211 particles to the starting composition. The size of the NEG-211 particles could be refined by Pt addition as in the case of melt-processed YBCO. We also discuss the possible sources for enhanced flux pinning. This paper is organized as follows: in Section 2, we outline some details of the preparation of NEG samples, and the experimental procedures are described. In Section 3.1, the microstructural analysis by means of scanning tunneling microscopy ŽSEM. and transmission electron microscopy ŽTEM. microcopy is presented. Section 3.2 presents the magnetic characterization of the samples, and we discuss the 233 origin of the enhanced flux pinning in our samples. Finally, in Section 4, some conclusions are drawn. 2. Experimental Powders of Nd 2 O 3 , Eu 2 O 3 , Gd 2 O 3 , BaCO 3 and CuO were weighed to have a nominal composition of ŽNd 0.33 Eu 0.33 Gd 0.33 .Ba 2 Cu 3 O 7yx . These powders were ground thoroughly and calcined at 8808C for 24 h with intermediate grinding, which was repeated three times and then pressed into pellets. These pellets were sintered at 10208C for 48 h and ground thoroughly. NEG bulk samples with a volume fraction of 20%, 30% and 40 mol% of NEG-211 second phase were prepared using a mixture of sintered NEG and commercial ŽNd,Eu,Gd.-211 powders. Here, it should be born in mind that the formed second phase differs depending on the kind of rare earth elements. For NEG composites, we have found that the second phase has the 211 structure. Since the addition of Pt is known to be effective in refining the size of Y-211 in the YBCO system w22x, we added 0.5 mol% Pt in all the samples. Differential thermal analysis ŽDTA. measurements were performed on the samples pretreated at 9508C under low oxygen partial pressure Ž0.1% O 2 . for 15 h to determine the peritectic decomposition temperature. The heat treatment profiles in the melt process for the NEG composites were scheduled on the basis of the DTA results. The precursor ŽNd,Eu,Gd.Ba 2 Cu 3 O y powders were first pressed into pellets with a diameter of 20 mm and a thickness of 15 mm, and subsequently subjected to CIP Žcold isostatic press. under a pressure of 2000 kgrcm2 . An MgO Ž100. seed was placed on the center of the pellet, which was then OCMG processed in 0.1% partial pressure of O 2 with a gas flow rate of about 300 mlrmin. The final structure is a single domain, even though we did not apply any temperature gradient. The samples could be grown into a single domain, showing that the growth of large grain bulk superconductors is possible with this system, that is, the mixture of three different LRE elements did not inhibit the grain growth of LRE-123 system, which is critically important for the future application of bulk samples. 234 M. Muralidhar et al.r Physica C 313 (1999) 232–240 The details of the heat treatment schedules can be found elsewhere w19x. Rectangular samples with similar dimensions a = b = c f 1.5 = 1.5 = 0.5 mm3 were cut from as-grown crystals in order to enable a direct comparison of the magnetic data. The oxygen annealing was performed on these small rectangles in flowing oxygen gas in the temperature range of 300–6008C w19x. Microstructural features of the samples were observed with an optical polarization microscope, SEM, and a TEM. The size and volume fraction of the NEG-211 second phase trapped in the NEG matrix were calculated from the SEM micrographs using an image processing system. The average size of the NEG-211 particles was determined assuming that the particles are spherical in shape. EDX analyses were also performed to analyze the chemical composition of the matrix and the second phases. Measurements of the superconducting transition temperature, Tc , were performed with a commercial SQUID magnetometer ŽQuantum Design, model MPMS-7. in an applied magnetic field of 1 mT. Magnetization loops ŽMHLs. are measured at T s 77 K in fields up to 7 T applied parallel to the c-axis. The Jc values were calculated based on the extended Bean critical state model Jc s Ž20D M .rw a 1 y arŽ3b .4x, where D M is the magnetization hysteresis during the increasing and decreasing external field processes, a and b Ž a - b . are the dimensions of the sample cross-section perpendicular to the applied magnetic field. 3. Results and discussion 3.1. Microstructural analysis Now, we will discuss the addition of second-phase particles ŽNEG-211. in order to further increase the flux pinning and hence, the critical current densities in the NEG samples. In Fig. 1a–c, we present SEM micrographs of NEG samples with addition of 20, 30, and 40 mol% of NEG-211 second phase. All the samples were melt processed in 0.1% O 2 atmosphere. The size of the 211 particles dispersed in the NEG matrix lies in the range of 1–20 mm, with even larger particles being present. This result is comparable to YBCO Fig. 1. Scanning electron micrographs of 211 second phase in OCMG processed samples fabricated without Pt addition: Ža. ŽNd,Eu,Gd.Ba 2 Cu 3 O y q20 mol% of 211; Žb. ŽNd,Eu,Gd.Ba 2 Cu 3 O y q30 mol% of 211; and ŽNd,Eu,Gd.Ba 2 Cu 3 O y q40 mol% of 211. and NdBCO samples without the refinement by addition of Pt. M. Muralidhar et al.r Physica C 313 (1999) 232–240 In contrast to Fig. 1, Fig. 2a–c shows SEM micrographs of the NEG samples which contain Fig. 2. Scanning electron micrographs of 211 second phase in OCMG processed samples fabricated with Pt addition: Ža. ŽNd,Eu,Gd.Ba 2 Cu 3 O y q20 mol% of 211; Žb. ŽNd,Eu,Gd.Ba 2 Cu 3 O y q30 mol% of 211; and ŽNd,Eu,Gd.Ba 2 Cu 3 O y q40 mol% of 211. 235 about 20, 30 and 40 mol% of NEG-211 second phase with 0.5 mol% Pt addition. All the samples were melt-processed in 0.1% O 2 atmosphere. From the figures, it is evident that the NEG-211 second-phase particles are finely distributed in the NEG matrix and their average size is smaller than 1 mm. To further highlight these observations, we calculated the volume fraction of the second phase trapped in the NEG matrix from SEM micrographs using an imageprocessing system. The results are presented in Fig. 3a–c. The average diameter of the NEG-211 particles for samples 20, 30 and 40 mol% is 0.786, 0.08 and 0.140 mm, respectively. It is evident that the Pt addition is very effective in reducing the size of the NEG-211 particles as in the case of the YBCO system w23–25x. Fig. 4a shows a TEM image of an NEG sample with 40 mol% NEG-211 and addition of 0.5% Pt. One can observe relatively large LRE-211 particles about 1 mm in diameter and some tiny LRE-211 particles smaller than 0.1 mm. EDX analysis showed that large LRE-211 inclusions contain Nd, Eu, and Gd in an even ratio, which is identical to the nominal composition of the precursor powder, i.e., NEG-211. It is then interesting to note that the very small LRE-211 particles mainly consist of Gd in the rare earth site. Therefore, the extremely fine LRE-211 particles in our NEG samples are mostly Gd-211, which must have been produced during the melt processing. To allow for a comparison with former results on melt-processed YBCO, Fig. 4b presents a TEM image of a melt-processed YBCO sample with an addition of 40 mol% 211 and 0.5 wt.% Pt. The 211 particle size is also here of the order of 1 mm. The volume fraction of the 211 phase trapped in the NEG matrix was calculated from SEM micrographs w26x. This demonstrates that Pt addition is very effective in reducing the size of the NEG-211 particles like the case of the YBCO system. In the partially melted region, there are two different kinds of LRE-211 inclusions: one added as the initial powder and the other produced by the peritectic decomposition of LRE-123. Since the decomposition temperature of Gd-123 is the lowest among the four rare earth elements w12x, Gd-211 has the chance to nucleate at relatively low temperatures, which may lead to small second-phase particle size. As the formation temperature of Gd-123 is lowest, the small 236 M. Muralidhar et al.r Physica C 313 (1999) 232–240 Gd-211 particles may not be consumed for the growth of LRE-123 and so have the chance to survive. Although further study is necessary to clarify why Fig. 4. Ža. Transmission electron micrograph of melt-processed ŽNd,Eu,Gd.Ba 2 Cu 3 O y with 40 mol% NEG-211 and 0.5 mol% Pt. Note that the very fine LRE-211 inclusions are dispersed in the matrix, which mainly comprises Gd in the rare earth site. Žb. Transmission electron micrograph of melt-processed YBCO with 40 mol% Y-211 and 0.5 mol% Pt. only the size of Gd-211 is extremely small, it definitely provides a method to enhance flux pinning in LRE-Ba 2 Cu 3 O y with mixed LRE elements. 3.2. Magnetic characterization Fig. 3. Size distribution of NEG-211 second phase in OCMGprocessed samples fabricated with Pt addition: Ža. ŽNd,Eu,Gd.Ba 2 Cu 3 O y q20 mol% NEG-211; Žb. ŽNd,Eu,Gd.Ba 2 Cu 3 O y q30 mol% NEG-211; and ŽNd,Eu,Gd.Ba 2 Cu 3 O y q40 mol% NEG-211. Fig. 5 represents the temperature dependence of the DC-susceptibility for the NEG samples with different amounts of NEG-211 in zero-field-cooled M. Muralidhar et al.r Physica C 313 (1999) 232–240 237 formed AC susceptibility measurements on the oxygenated samples. The samples also exhibit onset Tc of 93.4 K with a sharp superconducting transition Fig. 5. Temperature dependence of the normalized susceptibility for OCMG-processed ŽNd,Eu,Gd.Ba 2 Cu 3 O y superconductors with different volume fractions of the 211 phase. ŽZFC. and field-cooled ŽFC. processes in the presence of a magnetic field of 1 mT. All the samples show an onset Tc of about 93.2 K and a superconducting transition width of 1 K. We have also per- Fig. 6. Magnetization hysteresis loops measured at T s 77 K, and the external magnetic field Ha applied parallel to the c-axis of the sample for ŽNd,Eu,Gd.Ba 2 Cu 3 O y superconductors with different volume fractions of the LRE-211 phase. 238 M. Muralidhar et al.r Physica C 313 (1999) 232–240 Ž DT s 1 K. similar to DC magnetic susceptibility. Such a sharp superconducting transition is almost comparable to other LRE-Ba 2 Cu 3 O y melt processed under low oxygen partial pressures. Fig. 6 shows magnetization loops of the NEG samples with different amounts of NEG-211 measured at T s 77 K. The external magnetic field is always applied parallel to the c-axis of the samples. All samples studied were found to exhibit a pronounced secondary peak effect. Note the position of the secondary peak, which is considerably larger than in most YBCO samples, and similar to that of NdBCO w27x. This secondary peak effect has always been observed in LRE-Ba 2 Cu 3 O y melt-processed under reduced oxygen atmosphere, and is ascribed to dTc-pinning provided by LRE-rich 123 clusters dispersed in the matrix w7,22x. The fact that our NEG samples exhibit a similar peak effect suggests that such LRE-rich 123 clusters are also dispersed in the NEG compound as discussed in Ref. w22x. The combination of three LRE elements in a sample leads evidently to a very uniform distribution of the LRErich clusters, which in turn enhances the flux pinning due to a constant pinning wavelength w22,28x. Therefore, the formation of an homogeneous NEG-123 matrix is essential for the high Jc values exhibited by the ternary compounds at elevated temperatures, i.e., 77 K. Any disturbance of the matrix, e.g., by the addition of another LRE element or by excess 211 additions, will lead to a degradation of the high peak Jc . The Jc Ž Ha . curves calculated from the MHLs of Fig. 6 are displayed in Fig. 7. The maximum Jc values for samples 20, 30 and 40 mol% are 46 000, 54 000 and 68 000 Arcm2 at the respective peak field, h 0 , of 2.4, 2.2 and 2.5 T Ž Ha 5 c, at 77 K.. 68 000 Arcm2 is the highest Jc value ever reported in the literature at 2.5 T for fields parallel to the c-axis in melt-processed RE–Ba–Cu–O bulk superconductors, which emphasizes that the ternary LRE compounds are very promising superconductors for bulk applications. This is illustrated in Fig. 8, which gives a compilation of typical data of melt-processed YBCO Žwith addition of 211 and Pt. w29x, NdBCO prepared by the OCMG method Žwith Nd-422 particles. w27,30,31x, the ternary compound ŽNd,Eu,Gd.Ba 2 Cu 3 O y ŽNEG. with a ratio of Nd:Eu;Gds 1:1:1 w19x and our pre- Fig. 7. Field dependence of the critical current density ŽT s 77 K, Ha parallel to the c-axis. for ŽNd,Eu,Gd.Ba 2 Cu 3 O y superconductors with different volume fractions of the NEG-211 phase. sent NEG sample with addition of 40 mol% NEG-211 and 0.5 mol% Pt. Characteristically, the meltprocessed YBCO sample does not exhibit the peak effect, only the shape of the Jc Ž Ha . curve is some- M. Muralidhar et al.r Physica C 313 (1999) 232–240 239 Fig. 8. Comparison of the field dependence of the critical current density ŽT s 77 K, Ha parallel to the c-axis. for melt-processed YBCO, OCMG-processed NdBCO, ŽNd,Eu,Gd.Ba 2 Cu 3 O y , and ŽNd,Eu,Gd.Ba 2 Cu 3 O y with addition of 40 mol% NEG-211. what deformed at intermediate fields. OCMGNdBCO presents a well-developed peak effect, which is due to the distribution of Nd-rich phase in the sample, providing the dTc-pinning. The Jc Ž Ha . curve also reflects that the Nd-422 particles are still too large Žsize f 3 mm. to act as effective pinning sites w32x. Let us now discuss the shape of the Jc Ž Ha . curves of the NEG samples. The samples with 30 and 40 mol% of second phase showed a peak Jc value similar to that of Jc in zero field. Note the considerable change of shape of the Jc Ž Ha . curves as a function of LRE-211 content. The pure NEG samples Žas studied in Ref. w22x. and the present samples with small amounts of LRE-211 additions have a peak Jc Ž h 0 . being larger than that at zero field, Jc Ž0.. In contrast, NEG q 30 mol% NEG-211 shows similar values, and for NEG q 40 mol% NEG-211, the zero-field Jc is larger than that at the secondary peak. However, the overall Jc increases continuously with increasing NEG-211 content. Pinning provided by normal inclusions is mainly expected to be active at low temperatures andror low fields. Therefore, the NEG-211 additions increase the overall Jc Ž‘background’ Jc . as well as the Jc at intermediate fields. Furthermore, as compared to samples with 20 mol% NEG-211 additions, the irreversibility field is shifted slightly towards higher fields. Note that the peak position, h 0 , is practically unaffected by the NEG-211 content, as long as the NEG-211 content is 40 mol% or less. This indicates clearly that the main mechanism for the peak formation is different from the pinning provided by the normal inclusions. The flux pinning in the present NEG samples with addition of LRE-211 is definitely caused by two different main sources, thus the scaling of pinning forces as discussed in Refs. w5–8,22x is apparently not working in a wide temperature range. This topic will be discussed in detail in a forthcoming publication w33x. 4. Conclusions ŽNd,Eu,Gd. –Ba–Cu–O melt processed under oxygen partial pressure of 0.1% exhibits a high Jc value of 68 000 Arcm2 at 77 K and 2.5 T for Ha applied parallel to the c-axis of the samples. These values could be reproduced successfully in several experiments. Further work will consider the control of the initial particle size; this will be addressed in a separate paper w34x. Microstructural observations 240 M. Muralidhar et al.r Physica C 313 (1999) 232–240 along with compositional analysis showed that the addition of Pt is effective in refining the LRE-211 size. By means of TEM investigations, we have found that extremely fine NEG-211 particles contain only Gd on the rare earth site. These particles provide, therefore, effective flux pinning centers. The samples exhibit a pronounced secondary peak effect in the magnetization loops. The shape of the Jc Ž Ha .-curves is found to change with increasing NEG-211 content; but the position of the secondary peak is practically unaffected by the NEG-211 content. This clearly demonstrates that the secondary peak effect is due to a different pinning mechanism, i.e., the dTc-pinning is also active in the NEG-system. The NEG-211 particles are mainly effective at low fields, thus filling the gap between central peak and secondary peak, which is very important for bulk applications. 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