Physica C 471 (2011) 843–845

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Physica C

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Top-seeded infiltration growth of Y–Ba–Cu–O bulk superconductors

S. Umakoshi a,⇑, Y. Ikeda a, A. Wongsatanawarid b, C.-J. Kim c, M. Murakami a

a Shibaura Institute of Technology, Superconducting Materials Laboratory, 3-7-5 Toyosu, Koto-ku, Tokyo 135-8548, Japanb King Mongkut’s University of Technology Thonburi, 126 Pracha-Utit Rd., Bangmod, Thung-Khru, Bangkok 10140, Thailandc Korea Atomic Energy Research Institute, P.O. Box 105 Yuseong, Daejeon 305-353, Republic of Korea

a r t i c l e i n f o

Article history:Available online 13 May 2011

Keywords:Bulk YBCOLiquid infiltration growthY2O3 additive

0921-4534/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.physc.2011.05.070

⇑ Corresponding author. Address: Superconductingura Institute of Technology, 3-7-5, Toyosu, Koto-ku,fax: +81 3 5859 8117.

E-mail address: m210009@shibaura-it.ac.jp (S. Um

a b s t r a c t

A top-seeded melt-growth (TSMG) process is widely used to fabricate single domain YBa2Cu3Oy (Y–Ba–Cu–O) bulk superconductors. Pores are often found in the TSMG-processed Y–Ba–Cu–O samples due tothe oxygen gas evolution during the molten stage. Recently developed liquid infiltration growth (LIG)process is known to be effective in suppressing the pore evolution and in refining the size of Y2BaCuO5

(Y211) particles dispersed in YBa2Cu3Oy matrix. The LIG process utilizes the liquid (Ba3Cu5O8) infiltrationinto a pre- sintered Y211 contact and slow cooling through a peritectic temperature. In this study, we fab-ricated bulk Y–Ba–Cu–O superconductors by the LIG process combined with top-seeding with SmBa2-

Cu3Oy seed and confirmed that a single-domain bulk can be produced. Trapped field measurementshowever showed that some distortion in the field distribution was observed in the region near the seedcrystal, which was attributed to Y211 density and its relatively large size.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Bulk YBa2Cu3Oy (Y–Ba–Cu–O) superconductors can trap highmagnetic fields and exhibit high levitation forces. Therefore, thebulk superconductors are applied to the developments of variousengineering devices like superconducting flywheel energy storagesystem and superconducting motors [1].

For such engineering applications, critical current density is akey factor in determining field trapping abilities and repulsiveforces. A bulk superconductor demonstrates high critical currentdensity without weak links, which can be achieved in a singledomain melt textured bulk. It is now commonly accepted that agood manufacturing technique to produce a single-domain bulksuperconductor is the top-seeded melt-growth (TSMG) process.However, during the TSMG process, oxygen gas is released in themolten phase above 1000 �C, and thereby pores tend to be trappedin the bulk volume during the solidification process. The poresremained inside the bulk volume are likely to suppress supercon-ductivity and degrade mechanical properties. In addition, thereaction of the molten phase with the substrate always causesthe deterioration of both superconducting and mechanical proper-ties in the bottom part, which is another problem to overcome.

Liquid infiltration growth (LIG) process is an alternative way forproducing a single-domain bulk superconductors with reduced

ll rights reserved.

Materials Laboratory, Shiba-Tokyo 135-8548, Japan. Tel./

akoshi).

amount of porosities [2–4]. The LIG process is a technique in thatliquid phase (Ba3Cu5O8) infiltrates into porous solid phase Y211(Y2BaCuO5) by capillarity to form Y–Ba–Cu–O on cooling [5,6]. Itwas found that the bulk superconductors prepared by the LIG pro-cess showed lower pore densities and less reaction with the sub-strate materials than those prepared by the TSMG process. Inaddition, Y211 second phase particles could be uniformly distrib-uted in the Y–Ba–Cu–O matrix, leading to the improvement ofthe uniformity of the microstructure [7,8].

In this study, we aimed at synthesizing good-quality bulk Y–Ba–Cu–O superconductors with the LIG process. We also studied themicrostructure and the superconducting properties of the LIG-pro-cessed Y–Ba–Cu–O superconductors.

2. Experimental procedure

2.1. Sample preparation

The initial powders were homemade Y211 powders that wereprepared by sintering Y2O3, BaO2 and CuO in a stoichiometric ratioof Y:Ba:Cu = 2:1:1 at 900 �C for 48 h. The sintered Y211 powderswere uni-axially pressed into pellets 20 mm in diameter and12 mm in thickness. Then the compacted Y211 precursors weresubjected to sintering at 1200 �C for 1 h. The liquid source precur-sors were prepared by mixing BaO2 and CuO in a ratio ofBa:Cu = 3:5 followed by sintering at 800 �C for 24 h. Ba3Cu5O8

powders were then uni-axially pressed into the pellets 20 mm indiameter and 12 mm in thickness. Prior to the LIG process, the Ba3-

Cu5O8 compact was placed on a MgO single crystal substrate which

solid phase

liquid phase

Sm123 seed

Y211

Ba3Cu5O8

Yb2O3

MgO single crystal

Al2O3

Fig. 1. Configuration of the precursor layers for the LIG process employed in thepresent study.

Fig. 3. Trapped field distribution of bulk Y–Ba–Cu–O grown by the LIG process.

(a)

10µm

(d)

10µm

844 S. Umakoshi et al. / Physica C 471 (2011) 843–845

was layered with Yb2O3 powders to reduce the reaction betweenthe MgO substrate and the precursor. The pre-sintered Y211 blockwas then placed on the Ba3Cu5O8 precursor as shown in Fig. 1. Fi-nally, we placed a SmBa2Cu3Oy (Sm123) single crystal as a seed onthe top surface of Y211 block.

For the LIG process, the samples were heated from the ambienttemperature to a maximum temperature of 1040 �C at a rate of100 �C/h and held there for 1 h. Then, the samples were cooledfrom 1040 �C to 1015 �C at a rate of 10 �C / h, and slowly cooledfrom 1015 �C to 980 �C at a rate of 0.3 �C/h and finally cooled toroom temperature at a rate of 100 �C/h. After the LIG process, thesamples were oxygen-annealed at 500–450 �C for 200 h in flowingoxygen.

2.2. Trapped field measurements

We characterized magnetic properties of the LIG-processed Y–Ba–Cu–O bulk superconductors by measuring trapped fields at77 K. The Y–Ba–Cu–O bulk samples were placed at 1 mm gap froma Nd–Fe–B magnet with a surface flux density of 0.5 T. We thencooled the sample with liquid nitrogen and waited for 15 min.Then the external field was reduced to zero by removing the mag-net from the sample surface. The trapped fields were then mea-sured by scanning a Hall sensor at 1 mm scanning pitch and1 mm height over the sample surface.

10mm

(a)

10mm

(b)

Fig. 2. Photos of LIG-processed Y–Ba–Cu–O: (a) top surface; and (b) side surface.

2.3. Microstructure observation

After the LIG process and oxygenation, the growth morpholo-gies at the top surfaces of the samples were visually checked witheye. The samples were then polished to the surface roughness ofabout 1 lm with abrasive papers and subjected to microstructuralobservation with a polarized optical microscope. The size and dis-tribution of Y211 particles were analyzed by using an image anal-ysis software [9].

3. Results and discussion

Fig. 2 shows the photos of a top surface and a side surface of theLIG-processed Y–Ba–Cu–O superconductor. One can see that thefacet lines on the top surface are extended toward the edge of

(b)

10µm

(c)

10 m

(f)

10µm

(e)

10µm

Fig. 4. Optical micrographs for of the LIG-processed bulk Y–Ba–Cu–O supercon-ductor along vertical directions: (a) top; (b) center; and (c) bottom parts, and for thetop surface: (d) near the seed; (e) middle; and (f) edge parts.

Table 1The average particle size of Y211 particles dispersed inthe Y123 matrix for the LIG-processed bulk Y–Ba–Cu–Ocorresponding to the regions (a–f) presented in Fig. 4.The optical micrographs were analyzed with an imageanalysis software [9].

Position Diameter (lm)

(a) Top 4.400(b) Center 3.533(c) Bottom 6.454(d) Near the seed 4.217(e) Middle 3.365(f) Edge 3.220

S. Umakoshi et al. / Physica C 471 (2011) 843–845 845

the sample, showing that the crystal growth was complete. In addi-tion, the facet lines are also observed at the side surface and reachthe bottom of the sample, which confirms that single grain growthoccurred in the whole bulk body. The visual observation impliesthat the LIG method is surely effective in producing a single-grainbulk superconductor.

Fig. 3 shows the magnetic field distribution trapped by the LIG-processed Y–Ba–Cu–O superconductor at liquid nitrogen tempera-ture (77 K). The peak magnetic field was 1290 G. The fields werewell trapped by the whole sample, however, the field distributionwas not symmetric and some distortion was observed in the cen-tral part. Such a deterioration in the trapped field may be causedby the presence of a Sm123 seed crystal, which may have affectedthe microstructure in the vicinity of the seed. In order to check thisfact, we slightly polished the top surface, and measured thetrapped field. After several measurements, we found that such adistortion in trapped field did not disappear, which showed thatSm contamination was not responsible for the deterioration.

Fig. 4 shows optical micrographs of the microstructure of theLIG-processed bulk Y–Ba–Cu–O along the vertical direction andthe top surface. The Fig. 4a–c correspond to the cross sectional areanear the top, center and bottom, respectively. One can see that thedensity of Y211 particles is small at the top part near Sm123 seedcrystal, which may have caused the degradation of the critical cur-rent density and thus field trapping ability. As shown in Table 1,the average particle diameter of Y211 particles in the region (a)is 4.4 lm, which is rather large compared to the average grain sizeobserved in TSMG-processed Y–Ba–Cu–O, for which the typicalaverage diameter is 1 lm.

Fig. 4d and f correspond to the microstructure of the top surfacefor the regions near the seed, middle, and the edge, respectively.One can confirm from these photos that the density of Y211 is

small for the region near the seed crystal compared to the otherregions. As shown in Table 1, the average diameter of Y211 is alsorelatively large in the region near the seed compared to the middleand the edge parts, which may be responsible for low field trappingability in this region in addition to the low Y211 density.

Another interesting feature of microstructural observation isthe distribution of relatively large Y211 particles at the bottompart (region (c) in Fig. 4). The average diameter of this region is6.4 lm and definitely large compared to the other regions as pre-sented in Table 1. Although one of attractive features of the LIGprocess is less reaction with the substrate, some reaction tookplace in this region probably due to large Y211 particle diameters.

The average particle diameter presented in Table 1 shows thatthe size of Y211 particles is still large even for the LIG-processedY–Ba–Cu–O. Hence it will be necessary to add Pt or CeO2 to the pre-cursor in order to refine the size of Y211 particles even in the caseof the LIG method.

4. Summary

We have succeeded in growing a single-domain bulk Y–Ba–Cu–O superconductor with liquid infiltration growth process. Thetrapped field distribution however showed that there was somedeteriorated region near the Sm123 seed crystal, although a signif-icant field could be trapped by the sample. Microstructural obser-vation showed that a density of Y211 is small near the seed, whichmay have caused the degradation of the field trapping ability.Overall average diameter of Y211 particles trapped in the Y123matrix is also large, which suggests that it will be necessary toadd Pt or CeO2 to refine the size of Y211 even in the LIG process.

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