Physica C 470 (2010) 685–688

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Physica C

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Batch-processed GdBCO–Ag bulk superconductors fabricated using generic seedswith high trapped fields

Y. Shi a,*, N. Hari Babu b, K. Iida c, W.K. Yeoh d, A.R. Dennis a, S.K. Pathak a, D.A. Cardwell a

a Superconductivity Group, Engineering Department, University of Cambridge, CB2 1PZ, UKb Brunel Centre for Advanced Solidification Technology (BCAST), Brunel University, West London UB8 3PH, UKc Superconducting Group, IFW-Dresden Helmoholtz Str. 20, D-01069 Dresden, Germanyd Australian Key Centre for Microscopy and Microanalysis, University of Sydney, NSW 2006, Australia

a r t i c l e i n f o

Article history:Received 27 May 2010Accepted 28 June 2010Available online 3 July 2010

Keywords:Batch-processingGdBCO–Ag bulk superconductorsGeneric seedTrapped field

0921-4534/$ - see front matter � 2010 Elsevier B.V. Adoi:10.1016/j.physc.2010.06.013

* Corresponding author. Tel.: +44 1223 330287; faxE-mail address: ys206@cam.ac.uk (Y. Shi).

a b s t r a c t

Large, single grains of Y–Ba–Cu–O (YBCO) have been batch-processed to date by the top seeded meltgrowth (TSMG) process using NdBCO or SmBCO seed crystals. It has proved difficult, however, to econom-ically batch-process light rare earth (LRE) LRE–Ba–Cu–O bulk high temperature superconductors, whichhave higher critical current densities and irreversibility fields than YBCO, and therefore greater potentialfor high field engineering applications. In this paper, we report a novel batch-process based on a cheap,readily available generic seed crystal, developed recently at Cambridge, and a TSMG melt processingtechnique based on cold seeding in air for the batch fabrication of Gd–Ba–Cu–O–Ag single grains. Thesuperconducting properties of the (LRE)BCO single grains fabricated by this process are, in all respects,equivalent to those processed more conventionally in a reduced oxygen atmosphere.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction

Single grain LRE–Ba–Cu–O [(LRE)BCO] (light rare earth,LRE = Nd, Sm, Eu and Gd) bulk high temperature superconductors(HTS) have considerable potential for engineering applicationsdue to their ability to trap magnetic fields that are significantlyhigher than those achievable with permanent magnets. For exam-ple, a trapped field of 17 T has been achieved to date at 29 K [1] foran arrangement of two Y–Ba–Cu–O (YBCO) samples, which is morethan an order of magnitude greater than that available from per-manent magnets (and, significantly, over 100 times the energydensity). The magnetic field trapping ability of (LRE)BCO bulksuperconductors is generally superior to that of the more com-monly melt processed YBCO due to their higher critical currentdensity, Jc and irreversibility field, Birr. A batch-process for the fab-rication of YBCO single grains [2,3] based on a top seeded meltgrowth (TSMG) technique using SmBCO seed crystals in a coldseeding process was first achieved in 2003. The lack of availabilityof a suitable seed crystal to grow large, single grain (LRE)BCO bulksuperconductors with controlled orientation on a large scale, how-ever, has hindered significantly the further development of thesepotentially important materials.

The growth of large, single grains of HTS bulk superconductorsgenerally involves placing a structurally and chemically compati-

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: +44 1223 332662.

ble seed crystal on the top surface of a pressed precursor pellet.In the ‘‘cold seeding” process, the seed, which is required to havea higher melting temperature than that of the target bulk HTS com-pound, is placed on the upper surface of the precursor pellet atroom temperature. The seed-sample arrangement is then melt pro-cessed in a conventional chamber furnace, which involves initiallyheating the pellet above its peritectic decomposition temperature,Tp [4]. Values of Tp for the (LRE)Ba2Cu3O7�d (LRE-123) family ofbulk HTS materials are significantly higher than that ofYBa2Cu3O7�d (Y-123). As a result, only single grains of YBCO havebeen grown routinely to date by the cold seeding method, usingNd-123 or Sm-123 single crystals to seed the TSMG process. A‘‘hot seeding” process has been developed to overcome the diffi-culty of seeding (LRE)BCO single grains [5,6]. This involves placinga seed crystal of either NdBCO or SmBCO on the surface of a par-tially molten pellet at a temperature just above Tp, so that meltingof the seed can be avoided prior to grain nucleation. In addition,this process is usually performed under a reduced oxygen process-ing atmosphere to inhibit the substitution of the LRE ion onto theBa site of the LRE-123 superconducting phase, which impairs sig-nificantly its superconducting properties [7]. A specially designedfurnace is therefore required to process and introduce the seedto the precursor pellet at elevated temperature (typically above1000 �C) and only one sample can be fabricated practically at atime by the hot seeding process. As a result, the hot seeding meth-od is not practical for reliable batch-processing of large (LRE)BCOsingle grains.

Fig. 1. A photograph of batch-processed GdBCO–Ag single grain samples.

686 Y. Shi et al. / Physica C 470 (2010) 685–688

We have reported previously the development of a generic seedof NdBCO doped with MgO [8,9] for the melt processing of (LRE)B-CO bulk superconductors that exhibits a higher melting tempera-ture than any (RE)BCO composition (by at least 15 �C) and whichhas crystallographic lattice parameters that match closely thoseof the (LRE)BCO superconducting phase compounds (typicallywithin 1%). The generic seed crystal has enabled the full range of(LRE)BCO single grain compositions to be grown using a cold seed-ing, TSMG process.

Ag is introduced commonly to the (LRE)BCO single grain micro-structure to improve its mechanical properties [10]. The presenceof Ag, however, reduces the temperature window for the meltgrowth process and complicates further the processing of (LRE)B-CO–Ag single grains using a cold seeding method [11]. As a result,the vast majority of the GdBCO–Ag single grain processes reportedto date have been based on the hot seeding technique [12–16]. Re-cently, Cardwell et al. reported that several GdBCO–Ag singlegrains could be melt processed together using generic seeds [17],which demonstrated the potential to batch-process (LRE)BCO sin-gle grains. Muralidhar et al. [18] subsequently reported batch-pro-cessing of GdBCO single grains using Nd-123 thin-film seeds. Inthis paper, we report for the first time a successful batch-processof large GdBCO–Ag single grains using generic seeds, which is moreeconomical than using Nd-123 thin-film seeds. This process hasbeen used to fabricate single grains of GdBCO–Ag of up to 32 mmin diameter via a cold seeding technique in air. The measuredsuperconducting properties (Tc and Jc) and trapped field of theresulting GdBCO–Ag single grains are comparable to thoseachieved in samples fabricated under a reduced oxygen atmo-sphere by the hot seeding process.

2. Experimental

Generic seed crystals were manufactured as described in Refs.[8,9,19]. The chemical composition of the seeds used in this studywas (Nd-123 + 20 mol% Nd-422) + 1.0 wt.% MgO. Individual seedsfor seeding bulk pellets of normal cylindrical geometry were cho-sen from multi-grains samples prepared readily either by cuttingor cleaving large seed crystals (i.e. of dimensions up to several

mm), fabricated in a separate melt process. One multi-grainedNdBCO–MgO sample of diameter 25 mm and height 10 mm canproduce typically over one hundred seeds. This indicates the eco-nomical and practical nature of the generic seed fabricationprocess.

Melt processed GdBCO–Ag single grains were fabricated using astandard TSMG process. Powders of Gd-123 (99% purity and aver-age particle size 3 lm), Gd-211 (99% purity and average particlesize 1 lm), Ag2O (99.9% purity and average particle size 1 lm),BaO2 (97% purity) and Pt were mixed with a composition of(75 wt.% Gd-123 + 25 wt.% Gd-211) + 10 wt.% Ag2O + 1.0 wt.%BaO2 + 0.1 wt.% Pt in batches of mass 1.1 kg. A concentration ofapproximately 1 wt.% of BaO2 was used to suppress Gd/Ba substi-tution during the melt process [19,20]. Pellets were pressed uniax-ially into green bodies of diameter 40 mm (95 g) and 32 mm (52 g).A generic seed was placed on the top surface of each pellet in thebatch at room temperature to promote heterogeneous nucleationduring melt processing.

A large chamber furnace that can generate a uniform tempera-ture over a large volume had been designed previously to batch-process YBCO single grains, primarily to meet the requirementfor the large scale production of single grains for engineering appli-cations as a part of long term research and development strategy.The hot zone in the furnace is (height) 20 cm � (depth)60 cm � (width) 60 cm, corresponding to an active zone of dimen-sions of (height) 10 cm � (depth) 40 cm � (width) 40 cm, in whicha stable, uniform temperature is generated. No non-standardequipment is used to control the processing atmosphere in this fur-nace. Heating coils are packed relatively closely on the top, bottomand two sides of the chamber to maintain a homogeneous lateraltemperature distribution within the hot zone. The temperature ismonitored at various positions within the chamber to ensure thehomogeneity and stability of the thermal environment. This fur-nace has been used routinely for batch-processing of YBCO singlegrains and was used in the present study to demonstrate thebatch-processing of 16 GdBCO–Ag single grains in an airatmosphere.

16 GdBCO–Ag pressed pellets were loaded into the large cham-ber furnace in two batches (the first batch consisted of three sam-

Y. Shi et al. / Physica C 470 (2010) 685–688 687

ples of 40 mm in diameter and five samples 32 mm in diameter,and the second of eight samples of 32 mm in diameter). The sam-ples were heated to 1045 �C and held for 1.5 h, cooled to 1020 �C ata rate of 120 �C/h, slow cooled to 1012 �C (Tg1) at 0.3 �C/h, thencooled to 996 �C (Tg2) at 0.2 �C/h and to 984 �C (Tg3) at 0.1 �C/hand, finally, furnace cooled to room temperature. The heating pro-file was adjusted according to size and quantity of the samples ineach batch, with smaller samples requiring a shorter processingtime. The fully melt processed samples were annealed for 300 hin flowing oxygen at a temperature of between 380 �C and 420 �C.

The trapped fields of the 16 single grains were measured by ascanning Hall probe technique in a zero-field-cooled magnetiza-tion process and at a distance of 0.5 mm from the sample surface.A MPMS XL SQUID magnetometer was used to measure the mag-netic moment to determine Tc, DTc and Jc as a function of positionwithin six individual GdBCO–Ag single grain samples of variousdiameters, as described in Ref. [21], which were selected randomlyfrom the fully batch-processed single grains. The extended Beanmodel [22] was used to calculate Jc as a function of applied fieldfrom the measured magnetic moment hysteresis loops.

Fig. 2. The trapped field profile at 77 K for GdBCO–Ag single grains of diameter (a)26 mm and (b) 32 mm.

3. Results and discussion

3.1. The formation of GdBCO–Ag single grains

Fifteen of the 16 batch-processed pellets were grown fully intosingle grains. One sample of diameter 26 mm (as-grown size) had asmall secondary grain near to the side edge of the pellet. A photo-graph of the batch-processed GdBCO–Ag single grains is shown inFig. 1. It can be seen that there are three single grains of diameter32 mm (height 17 mm) and 13 of diameter 26 mm (height 14 mm).The quasi-facet growth lines apparent on the upper surface of thesamples are also present on the side of the cylindrical samples,which indicate the formation of a single grain throughout the en-tire sample and the success of the batch-TSMG process. Fig. 2aand b shows the typical distribution of trapped field in thebatch-processed samples, measured on the surface of the grainfor samples of diameter 32 mm and 26 mm, respectively. The dis-tribution of trapped field in all the samples is in the form of a singlepeak, indicating that a single, large current loop flows within thebulk grain. This is further evidence of the single grain nature ofthe melt processed samples.

3.2. Superconducting properties

Fig. 3a shows the typical distribution of Tc and DTc along the c-direction for a single GdBCO grain (26 mm in diameter) cut intosmaller specimens. The measured values of Tc and DTc are verysimilar for the batch-processed GdBCO–Ag single grains based onmeasurements of six different samples (26 mm and 32 mm indiameter). An average, relatively constant value of the onset Tc of92.7 K (±0.1 K) is observed across the sample, and is representativeof every batch-processed sample. The transition width, DTc varieswithin only 0.5 K in the c-direction for most single grains, exceptat the position immediately beneath the seed, where it is as highas 4 K (the maximum DTc changes from 2.5 K to 4.5 K). This sug-gests that BaO2 can be used successfully to suppress significantlythe extent of Gd/Ba substitution in the entire bulk sample, exceptwithin the vicinity of the seed. Gd/Ba substitution is very sensitiveto the concentration of BaO2 addition, and this investigation sug-gests that the BaO2 content of the precursor powder needs to beoptimized for every batch of raw material. However, the broad re-gion of DTc extends only to a region of approximate diameter1.5 mm in the vicinity of seed and will not therefore affect signifi-cantly the value of trapped field for the whole sample, since cur-

rent loops of relatively small radii make only a minorcontribution to trapped field in a fully magnetized sample. Thetrapped fields of the single grains of diameter 26 mm and32 mm, such as those shown in Fig. 3, are measured consistentlyto be approximately 0.9 T and 1.2 T at 77 K.

Fig. 3b shows the variation of Jc of a 26 mm diameter sampleshown in Fig. 2 as a function of applied field for small specimenscut along the c-axis. Specimens ‘‘1ta” and ‘‘1te” correspond to sam-ples cut from positions immediately underneath the seed and atthe extrema of the single grain, respectively. It can be seen thatJc is lowest under the seed, increases with distance from the seedalong both the a- (not shown here) and c-directions within the sin-gle grain and reaches a maximum value at a position close to theedge of the sample. The so-called fish-tail effect is particularly evi-dent in samples cut from positions more distant from the seed. Thedistribution of Jc is similar to that reported in our earlier study [19].The values of Jc observed in this study at both high and low appliedfields are at least as high as those reported for samples processedin a controlled, reduced oxygen atmosphere using the hot seedingtechnique [14,15].

Table 1 shows the average trapped field of the batch-processedsamples of different sizes. It can be seen that the average value oftrapped field at 77 K for single grain samples of diameters 26 mmand 32 mm are 0.91 ± 0.05 T and 1.15 ± 0.10 T, respectively, sug-gesting that samples with high trapping field performance can be

Fig. 3. The distribution of (a) Tc and (b) Jc along the c-axis of a GdBCO single grain.

Table 1Superconducting properties of batch-processed GdBCO–Ag single grains of differentsizes at 77 K.

Average (± deviation)diameter 32 mm

Average (± deviation)diameter 26 mm

Trapped field (T) 1.15 ± 0.10 0.91 ± 0.05Tc (K) away from seed 92.7 ± 0.10 92.7 ± 0.10DTc (K) in the seed area 2.0 ± 2.5 2.0 ± 2.5DTc (K) away from seed 0.5 ± 0.3 0.5 ± 0.3

Fig. 4. Comparison of the trapped fields of large, batch-processed single grainsfabricated in air using a generic seed by the cold seeding method (star symbols) andthose of single grains fabricated mainly in reduced O2 in N2 (or Ar) by the hotseeding technique, reported in the literature.

688 Y. Shi et al. / Physica C 470 (2010) 685–688

reproduced consistently in a batch-process. The circle symbolsshown in Fig. 4 indicate the average values of trapped field forGdBCO–Ag single grain samples batch-processed in an air atmo-sphere using the generic seed (data for 20 mm diameter sampleis from [23]). The square symbols indicate the trapped fields re-ported for samples processed in air or controlled O2 (in N2 or Ar)atmospheres using the hot seeding technique [12–16,24–26]. Itcan be seen that the average values of trapped field observed inthis study lie between the higher value and lower values of thosereported in the literature for samples processed by a hot seedingtechnique either in reduced O2 or an air atmosphere. Such consis-tency indicates that the generic seed can be used reliably to meltprocess GdBCO–Ag single grains in air and that the use of an opti-mum BaO2 concentration can effectively suppress Gd/Ba substitu-tion to yield GdBCO–Ag single grains that exhibit superconductingproperties that are comparable to those processed in a reduced O2

in N2 or Ar atmosphere.

4. Conclusions

We have demonstrated the feasibility of batch-processing oflarge single grains of GdBCO–Ag using generic seeds for the firsttime. This investigation has indicated that batch-processedGdBCO–Ag single grains with reproducible superconducting prop-erties that are similar to those processed in a reduced O2 in N2 or Aratmosphere can be batch melt processed in air practically usinggeneric seeds. As a result, the cold seeding process described in thispaper can be used as a viable and economic alternative the hotseeding technique to potentially enable large quantities of (LRE)B-CO single grains to be manufactured reliably and economically forhigh field engineering applications.

References

[1] M. Tomita, M. Murakami, Nature 421 (2003) 517.[2] W. Gawalek, T. Habisreuther, M. Zeisberger, D. Litzkendorf, O. Surzhenko, S.

Kracunovska, T.A. Prikhna, B. Oswald, L. Kovalev, W. Canders, Supercond. Sci.Technol. 17 (2004) 1185.

[3] D. Litzkendorf, T. Habisreuther, J. Bierlich, O. Surzhenko, M. Zeisberger, S.Kracunovska, W. Gawalek, Supercond. Sci. Technol. 18 (2005) S206.

[4] D.A. Cardwell, Mater. Sci. Eng., B 53 (1998) 1.[5] H.S. Chauhan, M. Murakami, Supercond. Sci. Technol. 13 (6) (2000) 672.[6] M. Kambara, N. Hari Babu, Y. Shi, D.A. Cardwell, J. Mater. Res. 16 (2001) 1163.[7] M. Murakami, N. Sakai, T. Higuchi, S.I. Yoo, Supercond. Sci. Technol. 9 (1996)

1015.[8] Y. Shi, N. Hari Babu, D.A. Cardwell, Supercond. Sci. Technol. 18 (2005) L13.[9] N. Hari Babu, Y. Shi, K. Iida, D.A. Cardwell, Nat. Mater. 4 (2005) 476.

[10] N. Sakai, D. Ishihara, K. Inoue, M. Murakami, Supercond. Sci. Technol. 15 (2002)698.

[11] X. Wu, K. Xu, P. Pan, Physica C 469 (2009) 225.[12] S. Nariki, N. Sakai, M. Murakami, Physica C 341–348 (2000) 2409.[13] S. Nariki, N. Sakai, M. Murakami, Physica C 357–360 (2001) 629.[14] S. Nariki, N. Sakai, M. Murakami, Physica C 378–381 (2002) 631.[15] S. Nariki, N. Sakai, M. Murakami, Supercond. Sci. Technol. 15 (2002) 648.[16] S. Nariki, N. Sakai, M. Murakami, Supercond. Sci. Technol. 18 (2005) S126.[17] D.A. Cardwell, Y. Shi, N. Hari Babu, S.K. Pathak, A.R. Dennis, K. Iida, Supercond.

Sci. Technol. 23 (2010) 034008.[18] M. Muralidhar, M. Tomita, K. Suzuki, M. Jirsa, Y. Fukumoto, A. Ishihara,

Supercond. Sci. Technol. 23 (2010) 045033.[19] Y. Shi, N. Hari Babu, K. Iida, D.A. Cardwell, J. Mater. Res. 21 (6) (2006) 1355.[20] Y. Shi, N. Hari Babu, K. Iida, W.K. Yeoh, A.R. Dennis, D.A. Cardwell, Supercond.

Sci. Technol. 22 (2009) 075025.[21] Y. Shi, N. Hari Babu, K. Iida, D.A. Cardwell, Supercond. Sci. Technol. 20 (2007)

38.[22] D. Chen, R. Goldfrab, J. Appl. Phys. 66 (1989) 2489.[23] Y. Shi, PhD thesis, Engineering Department, University of Cambridge, 2008.[24] M. Sawamura, M. Morita, H. Hirano, Physica C 392–396 (2003) 531.[25] M. Sawamura, M. Morita, Supercond. Sci. Technol. 15 (2002) 774.[26] S. Nariki, H. Hinai, N. Sakai, M. Murakami, M. Otsuka, Physica C 378–381

(2002) 764.