Physica C 469 (2009) 1165–1168

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Physica C

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Improvement of the superconducting properties of an infiltrated YBCO bulksuperconductor by a BaCeO3 addition

A. Mahmood a, S.D. Park a, B.H. Jun a, J.S. Youn c, Y.H. Han b, T.H. Sung b, C.J. Kim a,*

a Neutron Science Division, Superconductivity Research Laboratory, Korea Atomic Energy Research Institute (KAERI), Daejon 305–353, Republic of Koreab Korea Electrical Power Research Institute, Daejon 305–380, Republic of Koreac Korea Advanced Institute of Science and Technology, Daejon 305–701, Republic of Korea

a r t i c l e i n f o a b s t r a c t

Article history:Available online 28 May 2009

PACS:74.72.Bk74.81.Bd74.25.Ha74.25.Sv

Keywords:Infiltration processBaCeO3 additionYBCO superconductorsCritical current density

0921-4534/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.physc.2009.05.016

* Corresponding author. Tel.: +82 42 868 8908; faxE-mail address: cjkim2@kaeri.re.kr (C.J. Kim).

Single grain YBa2Cu3O7�x (Y123) bulk superconductors with Y2BaCuO5 (Y211) and various amounts ofBaCeO3 (5–45 wt.% by an increment of 10 wt.%) were fabricated by a seeded infiltration process. The addi-tion of BaCeO3 was found to be effective for a modification of the microstructure and an improvement ofthe superconducting properties. The refinement effect for Y211 particles within an entire superconduc-ting YBa2Cu3O7�x (Y123) matrix was achieved by BaCeO3 additions. The critical current density (Jc) valueswere increased as the BaCeO3 contents were increased (maximum Jc at 35 wt.% BaCeO3 addition). The Jc

improvement by BaCeO3 additions might be due to the microstructure modifications associated with thefinely distributed Y211 and BaCeO3 particles. With the addition of BaCeO3 the onset Tc values decreasedslightly, indicating highly limited Ce substitution for Y site. It can be concluded that the BaCeO3 additionhas a beneficial effect on the morphology, the size and the distribution of the Y211 inclusions and themicrostructure regarding pinning improvement.

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1. Introduction

The current-carrying capability of melt-processed YBa2Cu3O7�x

(Y123) depends considerably on the microstructures developedduring fabrication process. Recently, it has been reported that aninfiltration process is appropriate for generating a uniform disper-sion of Y2BaCuO5 (Y211) secondary phase particles within a super-conducting YBa2Cu3O7�x (Y123) matrix, which leads to anenhanced flux pining [1,2]. To develop a uniform microstructureand to improve the critical current density (Jc) at high magneticfields, it is essential to control the precursor materials for the for-mation of a Y123 phase [3–5]. The important factors affecting theproperties are known to be a composition in-homogeneity, thepresence of a liquid phase at the grain boundaries, an anisotropiccurrent flow in a crystal, and microcracks induced by a phase tran-sition during a low temperature annealing in an oxygen atmo-sphere [6].

In addition to these factors, the Jc is known to be influenced bythe size of the trapped Y211 particles and other non-superconduc-ting Y211 fine particles dispersed in the Y123 matrix [7,8]. The sizeof the Y211 particles in a YBa2Cu3O7�x matrix can be reduced easily

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by the addition of second elements such as CeO2 [6,9,10], PtO2 [11]etc. Kim et al. [12] found that in order to enhance the reliability ofthe superconducting and mechanical properties of a Y123 phase itis desirable to ensure a homogeneous distribution of Y211 particleswith in a Y123 matrix. Y211 second phase inclusions form naturalpinning centres within Y123 bulk superconductors [13], eventhough there remains some debate about the exact pinningmechanism.

It has been reported that the addition of BaCeO3 is very promis-ing for generating a uniform dispersion of refined Y211 particles[14] and also BaCeO3 itself can be employed as a novel pinning cen-tre in a Y123 bulk material [15,16]. These data were obtained in thesamples prepared by the conventional melt processing. In thiswork, we have first attempted to modify the microstructure ofY123 bulk superconductors by the combination of BaCeO3 additionof various amounts (5–45 wt.% by an increment of 10 wt.%) and aninfiltration process and the microstructure and superconductingproperties are reported.

2. Experimental procedure

The detailed method for the preparation of BaCeO3 powder canbe found elsewhere [14]. The BaCeO3 powder with differentcompositions (5–45 wt.% by an increment of 10 wt.%) was mixed

Fig. 1. Schematic drawing of an infiltration process applied in this study.

Fig. 2. Heat treatment schedule for the growth of single grain YBCO bulk.

Fig. 3. Top view of the infiltrated sample with a 35 wt.% BaCeO3 addition.

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with Y2BaCuO5 powder using a ball milling technique using ZrO2

balls and ethanol. The ball-milled powders were dried in vacuumto evaporate the solvent. The dried powder mixture was pressedunaxially in a steel mold into powder compacts. The diameterand thickness of each pellet were 20 mm and 10.50 mm respec-tively. For the fabrication of a single crystal Y123 bulk samplesSm1.8Ba2.4Cu3.4Oz (Sm1.8) seed was placed on the top of the com-pacts (see Fig. 1), and the seeded compacts were positioned atthe centre of the box furnace. For the growth of single crystal bulksuperconductors the thermal schedule has been described in Fig. 2.The thermal cycle begins at a rate of 100 �C/h from the ambienttemperature to a maximum temperature of 1045 �C with a dwellof 1 h. During this stage the source compacts firstly melt and theliquid (Ba3Cu5O8) infiltrates the porous preforms (Y211 + BaCeO3).The peritectic reaction occurring between the preforms(Y211 + BaCeO3) and the matrix (Ba3Cu5O8) during the slow cool-ing stage (1020–975 �C at 1 �C/h) results in the growth of theY123 grain. The samples were then cooled from 975 �C to roomtemperature at a rate of 100 �C/h and then were oxygenated at450–500 �C for 150 h in flowing oxygen.

The microstructure investigation was implemented by an opti-cal microscope (OM) and a scanning electron microscope (SEM) forthe polished/etched surfaces of the infiltrated samples. The magne-tization curves from the rectangular samples with dimensions ofabout 2 � 2.5 � 2 mm3 were obtained at 77 K and the Hkc-axisusing a superconducting quantum interference device (SQUID)magnetometer.

3. Results and discussion

All the samples with various compositions of BaCeO3 weregrown in the form of single grains. Fig. 3 shows a typical exampleof the top view of single grain sample with 35 wt.% of BaCeO3 addi-tion grown by an infiltration technique in air. This figure shows asuccessful growth of a single grain Y123 superconductor.

Fig. 4a–f shows the SEM micrographs of the etched surfaces ofthe infiltrated Y123 samples with various compositions of Ba-CeO3. Large Y211 particles of a few microns with a slightly darkcolor are observed inside a Y123 grain of the sample as shownin Fig. 4a. From Fig. 4b–f, it is observed that there are particleswith different two colors, one is slightly dark as observed in theFig. 4a and the other is bright. A homogeneous dispersion of

the refined particles was found for the samples with a high com-position of BaCeO3 (up to 35 wt%). As the BaCeO3 contents in-creased further to 45 wt.%, particle segregation was observed(see Fig. 4f). This particle segregation seemed to be due a particlecoarsening and coalescence at a partially molten state of the Y211and the liquid.

As already mentioned, particles with different two colors wereobserved in Y123 samples, but it was difficult to distinguish onefrom the other in the SEM images. The energy dispersive spectrom-eter (EDS) analysis was used to confirm the chemical compositionof the particles as shown in Fig. 5. We undertook a compositionanalysis on nine particles marked as S1–S9 as shown in Fig. 5and the obtained data is presented in Table 1. According to thecomposition analysis of Table 1, the small dark particles are iden-tified as Y211 and the bright particles as BaCeO3. According to Kimet al. [10] the formation of the smaller Y211 particles in the sam-ples with BaCeO3 additions seems to be from the Y211 refining ef-fect by cerium. A fairly homogeneous dispersion of the refinedY211 particles was obtained for the sample with a 35 wt.% BaCeO3

contents as shown in Fig. 4e. The resultant uniform dispersion andrefinement of the Y211 particles can be considered to be from theaddition of BaCeO3 [14,15] and by using the infiltration process[1,2].

The critical current density (Jc) values were estimated from themagnetization data with magnetic field, applied parallel to the c-axis so that the current flows in the ab- planes and perpendicularto the c-axis using Bean’s critical model. Fig. 6 shows the Jc–Bcurves at 77 K and the Hkc-axis of the infiltrated samples with var-ious compositions of BaCeO3. An improvement in Jc is observedwith an increase of the BaCeO3 contents up to 35 wt.%. The Jc valuesfor the sample with 35 wt.% BaCeO3 at 0 T is about 9.0 � 104 A/cm2,which is much higher than that of 1.50 � 104 A/cm2 for the samplewithout the BaCeO3 addition at the same magnetic field. But the Jc

values of the sample with a 45 wt.% BaCeO3 addition is slightlylower at the low magnetic region and much decreased at highmagnetic field than the sample with the 35 wt.% BaCeO3 addition.The decrease in Jc can be considered to be due to an agglomerationof the BaCeO3 particles, already observed in the SEM image ofFig. 4f and it can also be assumed to be due to a decrease in the vol-ume fraction of the Y123 superconducting phase.

Fig. 4. SEM micrographs of the etched/polished surfaces of at the central region of infiltrated YBCO samples with various BaCeO3 additions; (a) 0 wt.% (b) 5 wt.% (c) 15 wt.%(d) 25 wt.% (e) 35 wt.% (f) 45 wt.%.

A. Mahmood et al. / Physica C 469 (2009) 1165–1168 1167

From the previous research work on BaCeO3 addition to YBCOprecursors [14–16] and the present work, we can conclude thatthe enhancement of the Jc values were achieved by the BaCeO3

addition to the precursor powders as suggested by S. Nariki et al.that BaCeO3 particles themselves act as novel flux pinning centresin Y123 bulk materials [15]. However, in the present work, theimprovement of the Jc values might be attributed to the refinedY211 particles by the BaCeO3 addition, the BaCeO3 particles them-selves and the infiltration method [1,2] as well. It is necessary toelucidate which one is more effective for an improvement of theJc values.

Fig. 7 shows the temperature dependency of the normalized dcsusceptibility. The onset Tc values of the infiltrated samples with

various compositions of BaCeO3 decreased slightly as comparedto the infiltrated sample without BaCeO3 addition (un-dopedsample) as shown in Table 2. The onset Tc value of the un-dopedsample was 91.59 K, which was slightly higher than the BaCeO3

doped samples. After the addition of BaCeO3, the minor changein Tc values indicates a highly limited cerium substitution for yt-trium site.

4. Conclusions

In the present work, the YBCO bulk superconductors with Ba-CeO3 addition with various compositions (5–45 wt.%) were

Fig. 5. EDS analysis of a infiltrated sample with 35 wt.% BaCeO3.

Table 1Composition analysis of the Y211 and BaCeO3 particles, obtained from the EDSanalysis for the infiltrated sample with a 35 wt.% BaCeO3 addition.

Area Y Ba Cu Ce

S1 0.47 38.28 19.26 41.99S2 1.41 37.31 5.01 56.26S3 51.66 23.47 25.21 0.34S4 53.60 22.85 23.34 0.21S5 0.24 37.59 6.79 55.08S6 52.23 23.09 24.12 0.57S7 0.52 41.78 13.22 44.48S8 1.67 36.26 5.54 56.53S9 53.50 25.36 31.09 0.25

Fig. 6. Jc-B curves at 77 K and the Hkc-axis of samples with various BaCeO3

additions.

Fig. 7. Normalized susceptibility as a function of temperature of infiltrated YBCOsamples with various BaCeO3 additions.

Table 2Onset transition temperature (Tc) of infiltrated YBCO samples with various BaCeO3

additions.

Amount of BaCeO3 (wt.%) Onset temperature Tc (K)

0 91.595 91.09

15 90.5925 90.5935 90.5945 91.09

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successfully fabricated by combined process of a seeding and infil-tration method. The microstructure revealed a fairly homogeneousdispersion and the refinement of the Y211 particles up to 35 wt.%BaCeO3 addition. The resultant uniform dispersion of refinedY211 particles might be due to the addition of BaCeO3 and the infil-tration method as well. The Jc values for the sample with 35 wt.%BaCeO3 addition over the entire magnetic fields at 77 K were thehighest as compared to those obtained for the samples with lowerBaCeO3 addition. But for the sample with 45 wt.% BaCeO3 addition,the Jc values were slightly lower at low magnetic field and muchdecreased at high magnetic field than the sample with the

35 wt.% BaCeO3 addition. The decrease in Jc might be due to anagglomeration of the BaCeO3 particles and a decrease in the vol-ume fraction of the Y123 superconducting phase as well. Finallyit can be concluded that using a top-seeded infiltration methodand a BaCeO3 addition are beneficial for an improvement of themicrostructure and superconducting properties of the single crys-tal Y123 bulk superconductors.

Acknowledgements

This research was supported by a grant (R-2005-1-393) fromElectric Power Industry Technology Evaluation and Planning(ETEP), Republic of Korea.

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