Physica C 468 (2008) 1350–1354

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Physica C

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Pre-sintering effects on the critical current density of YBCO bulkprepared by infiltration method

A. Mahmood a, B.H. Jun a, H.W. Park b, C.J. Kim a,*

a Nuclear Nanomaterials Development Laboratory, Korea Atomic Energy Research Institute (KAERI), 150 Dukjin-dong, Yusong-gu, Daejon 305-353, Republic of Koreab Powder Metallurgy Laboratory, Korea University of Technology and Education, Chunan 330-708, Republic of Korea

a r t i c l e i n f o

Article history:Available online 27 May 2008

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

Keywords:Sintering effectInfiltration processPore size

0921-4534/$ - see front matter � 2008 Elsevier B.V. Adoi:10.1016/j.physc.2008.05.109

* Corresponding author. Address: Nuclear Nanomatory, Korea Atomic Energy Research Institute (KAERI),Dukjin-dong, Yusong-gu, Daejon 305-353, Korea. Tel.:868 8275.

E-mail address: cjkim2@kaeri.re.kr (C.J. Kim).

a b s t r a c t

Y2BaCuO5 (Y211) pre-forms with various pore densities were prepared by sintering at various tempera-tures (900–1200 �C) in air. The prepared Y211 pre-forms were reacted with Ba3Cu5O8 to form YBa2-Cu3O7�y (Y123) phase by an infiltration method. Ba3Cu5O8 pellets were placed at the bottom of theY211 pre-forms, heated above the peritectic temperature of Y–Ba–Cu–O and then slowly cooled to980 �C. It was found that the increase of the sintering temperature significantly affected the final poredensity and its distribution in the infiltrated Y123 samples. The critical current density (Jc) at 77 K ofthe samples prepared at higher sintering temperatures was higher than that of the samples preparedat low sintering temperatures. It is concluded that the control of pore size in sintered pre-form is impor-tant for producing a homogeneous microstructure regarding the pore and Jc.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction ity in infiltration reaction process. The pores inside the Y123 grains

The infiltration growth (IG) technique is one of modifications ofthe conventional melt process [1], which utilizes the peritecticreaction of a liquid and Y2BaCuO5 (Y211) to form YBa2Cu3O7�y

(Y123) phase. Because the time to stay at the high temperature ismuch lower than that of the conventional processes, the Y211growth is highly suppressed and the resulting Y211 particles arethus quite fine.

If the parameters of the infiltration process are well controlled,we may fabricate the Y123 bulk containing very fine Y211 particlesinside of the Y123 grain. With the help of an infiltration reactionprocess, it is possible to fabricate a Y123 superconductor with lessresidual liquid phase and Y211 particles less than 5 lm which aredensely and evenly distributed in Y123 grains [2,3]. Actually it wasreported that YBCO bulk superconductors made by the infiltrationprocess have high Jc values at 77 K [3,4]. In addition to the micro-structure related to the Jc, the density of the finally solidified Y123bulk is also an important factor for the practical applications of thebulk superconductor. Up to now, however, no systematic study hasbeen done on the pore density of Y211 pre-form for the final poros-

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terials Development Labora-Neutron Science Division, 150+82 42 868 8908; fax: +82 42

were reported due to the gas evolution when precursor Y123 pow-der melts [4,5]. The difference of oxygen content between an Y123,and a liquid-forming and Y211 phase was known to be the causefor the formation of pores [5].

In this study, we controlled the porosity of Y211 pre-forms andinvestigated the effect of pre-form density on the porosity and Jc ofthe liquid infiltration processed Y123 superconductor. The sinteredpre-form density, pore distribution inside Y123 grains, and Jc wereinvestigated.

2. Experimental procedure

In this study, an infiltration method was applied to the fabrica-tion of a single grain Y123 superconductor. To make Y211pre-forms, a commercially available Y2BaCuO5 (Y211) (SolvayCompany, Germany) powder was put into a steel mold of 20 mmdiameter and unaxially pressed into pellets. To control the porosityof the Y211 pre-forms, the Y211 green compacts were sintered atvarious temperatures (900, 1000, 1100 and 1200 �C) for 2 h inthe air. For the reaction of Y211 and a liquid-forming phase (Ba3-

Cu5O8), the sintered Y211 compacts were placed on the Ba3Cu5O8

pellets. To fabricate a single grain Y123 bulk, Sm1.8Ba2.4Cu3.4Ox

(Sm1.8), which was made by the conventional melt process, wasused as a seed. The Sm1.8 seed was placed on the top of Y211pre-forms for the growth of a single Y123 grain. The samples wereheated to 1045 �C for 1 h at a rate of 100 �C/h in a box furnace. Dur-ing this stage, Ba3Cu5O8 converted to a melt and infiltrated into the

Fig. 1. Photo of Y211 pre-forms prepared at various sintering temperatures.

Fig. 2. Density of Y211 pre-form as a function of sintering temperature.

Fig. 3. SEM micrographs of fracture surfaces of Y211 pre-forms; (a) as-com

A. Mahmood et al. / Physica C 468 (2008) 1350–1354 1351

porous Y211 pre-forms. After melting of Ba3Cu5O8 samples werecooled to a peritectic reaction temperature 1020 �C at a rate of20 �C/h, slowly cooled to 980 �C at a rate of 1 �C/h, maintained atthis temperature for 80 h and finally cooled to room temperature.For the tetragonal-to-orthorhombic phase change, the oxygenannealing was carried out for all samples at 450 �C – 500 �C for150 h in flowing oxygen.

Microstructure of the samples was investigated by using of anoptical microscope (OM) and scanning electron microscope (SEM)for the etched surfaces. The trapped magnetic field profiles onthe top surface of samples were measured using an Nd–B–Fe per-manent magnet at 77 K. The Jc at 77 K up to 5 T was estimated frommagnetization hysteresis curves for the rectangular samples of2 � 2.5 � 2 mm3 by a superconducting quantum interference de-vice (SQUID) magnetometer.

3. Results and discussion

Fig. 1 is the photo of Y211 pre-forms sintered at various temper-atures of 900–1200 �C. The diameter of as-compacted Y211 pelletis 20.29 mm. The diameter (20.00 mm) of 900 �C pre-form is al-most same of that of as-compacted Y211 pre-form, which indicatesno densification at this temperature. Remarkable reduction of apellet diameter is observed to begin at 1000 �C. The diameters ofthe pre-forms at 1000 �C, 1100 �C and 1200 �C are 19.33 mm,18.61 mm and 18.11 mm, respectively. The sintered density ofthe samples is plotted as a function of sintering temperature asshown in Fig. 2. As it is expected from the diameter change results,the density of the Y211 pre-form increases with increasing the sin-tering temperatures. The density of the pre-form sintered at

pacted, sintered at (b) 900 �C, (c) 1000 �C, (d) 1100 �C and (e) 1200 �C.

Fig. 5. Jc–B curves of infiltration processed Y123 samples prepared using (a) Y211pre-forms as-compacted, sintered at 1000 �C and 1200 �C.

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1200 �C is 6.593 g/cm3, which is comparable to the density (5.561g/cm3) of the as-compacted Y211 pre-form.

Fig. 3 shows SEM micrographs for the fracture surface of Y211pre-forms sintered at various temperatures. The Y211 pre-formsintered at 900 �C consists of many particles of irregular shape(Fig. 3a). The particle shape is quite similar to that of as-compactedY211 compact, due to no growth of Y211 particles at this temper-ature. As the sintering temperature increases to 1000 �C, the Y211particles begin to have the own particle shape with many facets. Asthe sintering temperature increases further, the particle size be-comes larger. In addition to the shape change of Y211 particles,the pore density seems to be remarkably reduced (see Fig. 3e). Thisresult is well consistent with the density variation shown in Fig. 2.

Fig. 4a–d are the optical micrographs of Y123 samples infiltra-tion-processed using Y211 pre-forms, (a) as-compacted, and sin-tered at (b) 1000 oC, (c) 1100 oC and (d) 1200 oC. It can be seen inFig. 4a that many spherical pores, which are reported to formdue to the gas evolution during melting [5], are observed in Y123grain. The size of the large pores is about few tens of micrometer(lm) and small ones is a few micrometer. It can be expected thatthe pore size in Y123 grains is fairly dependent on the sinteringtemperature for the Y211 pre-form. As it can be seen in theFig. 4d, pores of the sample (d) are found to be relatively smalland finely distributed inside the Y123 grain, compared with sam-ples (a) and (b). The homogeneous distribution of the pores andthe size reduction of the sample prepared using the Y211 pre-formsintered at high temperature are attributed to the lower volume ofopen porosity in the perform itself.

Fig. 5 shows the Jc–B curves at 77 K and H//c-axis of the infil-trated samples sintered at various temperatures. The Jc value in-creases with increasing the sintering temperature for the Y211pre-forms. The Jc value at 1 T of the sample sintered at 1200 �C is8.7 � 103 A/cm2, which is much higher than 3.7 � 103 A/cm2 ofthe as-compacted sample. The higher Jc value of the infiltrationprocessed sample, which was prepared using Y211 pre-form sin-tered at higher temperature, is attributed to the reduced numberof pores in Y211 pre-form (higher density) and its influence on

Fig. 4. Optical micrographs of the polished surfaces of the infiltration processed Y123 sam1100 �C and (d) 1200 �C.

the final microstructure regarding the porosity of the infiltratedsingle grain Y123 superconductors.

Fig. 6a–e show the trapped magnetic field profiles of the singlegrain YBCO samples, prepared from the Y211 pre-forms sintered atvarious temperatures. All samples show a single grain contourwhere only one peak point is located at the center of the profile.The peak values of the samples prepared using the compactedY211 pre-forms, the pre-form sintered at 900 �C, 1000 �C, 1100 �Cand 1200 �C are 1.075 kg, 1.435 kg, 2.011 kg, 2.516 kg and3.101 kg, respectively. These values were obtained when a perma-nent magnet of 3.300 kg was used. The highest trapped field valueis achieved for the sample prepared using Y211 pre-form sinteredat 1200 �C. The result is well consistent with Jc and the microstruc-ture results regarding the porosity.

ples, prepared using Y211 pre-forms; (a) as-compacted, sintered at (b) 1000 �C, (c)

Fig. 6. Trapped magnetic field profiles crossing the top surface of the infiltration processed Y123 samples, prepared using Y211 pre-forms; (a) as-compacted, sintered at (b)900 �C, (c) 1000 �C, (d) 1100 �C and (e) 1200 oC.

A. Mahmood et al. / Physica C 468 (2008) 1350–1354 1353

4. Conclusions

The pre-sintering effect for Y211 pre-forms on the pore densityand the critical current density of liquid infiltration processed Y123superconductors was investigated. It was found that the porosity ofthe Y211 pre-form decreased as the sintering temperature in-creased. The open porosity of the sintered Y211 pre-forms stronglyaffected the formation of spherical pores in the infiltration-pro-cessed Y123 bulk samples; the reduced pore size and the homoge-neous distribution, when Y211 pre-forms sintered at higher

temperature were used. The Jc and trapped magnetic field profileresults of the samples well consistent with the porosity and den-sity variation of the Y211 pre-form and the infiltration-processedY123 samples as well.

Acknowledgement

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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