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Oxidation induced formation of a-b planar defects in melt-textured YBa2Cu3O7−y containing Y2BaCuO5 inclusions
N
PHYSICA
ELSEVIER
PhysicaC276 (1997) 101-108
Oxidation induced formation of a-b planar defects in melt-textured YBa 2Cu 307- y containing Y2BaCuO5 inclusions Chan-Joong Kim a,*, Yi-Sung Lee b, Hyun-Soon Park b, I1-Hyun Kuk a, Tae-Hyun Sung c, Jin Joong Kim c, Gye-Won Hong a a Superconductivity Research Laboratory, Korea Atomic Energy Research Institute, P.O. Box 105, Yusung, Taejon, 305-600, South Korea b Department of Metallurgical Engineering, Sungkyunkwan University, Soowon, Kyunggido, 440-746, South Korea c Center tbr Advanced Studies in Energy and Environment, Korea Electric Power Research Institute, 103-16, Munjidong, Yusung, Taejon, 305-380, South Korea
Received 19 September 1996; revised manuscriptreceived 9 December 1996
Abstract In order to understand the formation mechanism of the planar defects of a B a - C u - O platelet and a CuO stacking fault of melt-textured YBa2Cu307_y (123), two different types of 123 samples containing Y2BaCuOs (211) inclusions (a tetragonal and orthorhombic samples) were prepared by melt-quenching heat treatment and/or subsequent annealing in oxygen atmosphere, and the microstructures were examined. In the tetragonal samples which are high-temperature non-superconducting phases, no remarkable defect formation was observed either around the trapped 211 inclusions or in the 123 matrix: After annealing of the tetragonal samples in oxygen atmosphere for the transformation of the tetragonal phase to the superconducting orthorhombic phase, meanwhile, various types of defects were observed such as dislocations, stacking faults, twins and the BaCuO platelets. Particularly, the planar defects of the CuO stacking faults and the BaCuO platelets were found to nucleate mostly at the 123/2t 1 interface and then to grow toward the interior of the 123 matrix. In the sample with shorter oxygen annealing time, the formation of the planar defects was limited in the vicinity of the trapped 211 while in the sample with longer annealing time, they extended to the whole 123 matrix. It is concluded that the formation of the planar defects of the melt-processed Y - B a - C u - O is attributed to the low temperature annealing in oxygen atmosphere for the tetragonal-to-orthorhombic phase transformation.
1. Introduction Analyzing the melt-processed microstructures of Y - B a - C u - O samples, various types of microdefects are observed within the superconducting 123 matrix [1-11]. These are a transformation twin, a disloca-
* Corresponding author. Fax: ÷ 82 42 862 5496.
tion, a trapped 211 inclusion, a residual liquid phase, a microcrack, a stacking fault and a BaCuO platelet. The presence of these defects in the superconducting 123 matrix is very important to the practical applications of this material, since some of them may act as flux pinning sites of the 123 phase [5,6]. The defect density should, therefore, be controlled to an optim u m value, together with understanding of the formation mechanism. A m o n g them, the formation mechanism of some defects has been well clarified
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C.-J. Kim et al. / Physica C 276 (1997) 101-108
but others are not clearly understood yet. For example, twin structure is known to be developed due to the transformation stress from the tetragonal to the orthorhombic phase accompanying the oxygen ordering on the 1-dimensional C u - O chain structure which is located between two B a - O layers [12,13]. The transformation stress and the thermal stress during fabrication can also produce microcracks [ 13,14] and dislocations [13] in the 123 matrix. Whereas, the residual liquid phase and the trapped 211 inclusion are results of incomplete peritectic reaction during growth of the 123 domains in the liquid plus 211 state [8,15]. Another typical microstructural feature of the melt-processed Y - B a - C u - O is a platelet structure (so-called a textured structure) which is developed along the a - b plane of the 123 phase. In the early studies on the melt-textured microstructure [ 16-18], the platelet structure has been considered as a boundary of the aligned plate-like 123 grains [16] or a liquid phase entrapped during high temperature peritectic growth of the 123 domain [17,18], because the direction of the platelet structure was the same as the growth direction of the 123 domain and the composition of the phase observed within the platelet was similar to that of the peritectic liquid; crystalline or amorphous BaCuO and CuO phases [7,8]. In addition to this, the formation of the platelet structure seems to be quite relevant to the presence of the trapped 211 inclusion within the 123 matrix. The number of the platelet was a function of 211 density [16]. The microstructural observations have thus led to the conclusion that the planar defect is a kind of residual liquid phase which was trapped during high temperature peritectic reaction [8,17]. The correlation of the dependency of the platelet density on the number of the trapped 211 inclusion was explained in terms of the 211 trapping within the growing 123 domain which is accompanied by the entrapment of the liquid phase [8] or in terms of the nucleation of plate-like 123 grains at the 211 inclusions [16]. By the way, recent study on the melt-textured microstructure suggested a controversial viewpoint on the formation mechanism of the platelet structure and a CuO stacking fault [19]. By comparing the optical micrographs for the polished surfaces between a tetragonal and the orthorhombic samples, it was demonstrated that the tetragonal 123 phase con-
tained no platelet structure while the orthorhombic 123 phase contained many platelets [19]. Its formation mechanism was explained on the basis of the oxidation-induced decomposition of the 123 phase into 211, Ba2Cu306_ ~ and CuO phase, which was proposed by Williams et al., in the poly-grain 123 samples [20,21]. Moreover, Sandiumenge et al. showed in the melt-processed 123 sample that the density of the CuO stacking fault varied with oxygen annealing time and it affected the critical current density [4]. These results may indicate that the formation of the planar defects has no relation to the high temperature peritectic reaction. In order to clarify the exact formation mechanism of the planar defects, therefore, it is necessary to examine the microstructures before and after the oxygen annealing, especially for the regions around the trapped 211 inclusions. The aim of this study is to clarify what the dominant mechanism is for the formation of the planar defects of the melt-processed 123 sample. We report the microstructurat difference between the tetragonal phase and the orthorhombic phase of the melt-textured 123 sample containing 211 inclusions. The microstmctural relation of the planar defects and the trapped 211 inclusion and the formation mechanism are discussed in terms of the phase decomposition during oxygen annealing.
2. Experimental procedure A powder mixture of 123 and 211 phases used in this experiment was prepared by the conventional solid-state reaction method using Y203, BaCO 3 and CuO powder of 99.9% purity. The powders were weighed to a cation ratio of Y:Ba:Cu = 1.8:2.4:3.4, ball-milled for 20 h in alcohol and then dried in air. The powders were calcined at 900°C in air, with repeated grinding for every 10 h and then cooled in air. The calcined powder was pressed in a steel mold into pellets. In order to understand the defect generation process in the melt-textured samples, two different samples, i.e., the tetragonal and the orthorhombic 123 samples containing 211 inclusions were prepared. First, the pellets were heated to 1100°C so as to make them a 211 plus liquid state. After holding the
C.-J. Kim et al./Physica C 276 (1997) 101-108
Temp.[ t
1100°C forO.Sh
! 211+LiquidphaseI 970°C~Tetragonal123
phase[
[ Orthorhombic123 phase] V
Tetraiiiii"i / E°°°c intl°wing°xYge. T.~k~IRi
Fig. 1. Schematic of the melt-quench and oxygen annealing heat treatment in order to produce tetragonal and orthorhombic 123 samples.
pellets at this temperature for 30 min, the samples were slowly cooled to 970°C at a rate of l ° C / h and then was rapidly quenched in a liquid nitrogen bath to obtain the tetragonal phase. The powder x-ray diffraction (XRD) analysis of the quenched samples showed that the high-temperature tetragonal phase was retained. Some of the quenched samples were annealed at 500°C for various times in flowing oxygen in order to produce the orthorhombic phase. The experimental details are shown in Fig. 1. If the planar defects such as CuO stacking faults and BaCuO platelets are found in the tetragonal sample, it can be said that the defects were generated by the high temperature mechanism involving liquid entrapment ahead of the growing 123 domain during peritectic reaction [8,17]. On the contrary, if the defects are found in the orthorhombic samples, the formation of the planar defects can be explained in terms of the low temperature mechanism based on the decomposition of a 123 phase [19]. Phases formed in the samples were analyzed by powder XRD measurement. Microstructures were examined on the polished and etched surfaces by means of an optical, scanning electron (SEM) and transmission electron microscope (TEM, 2000 FX2, JEOL). For TEM investigation, the melt-textured samples were sliced by using of a diamond saw, thinned down to 80-100 microns by mechanical
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grinding on SiC papers and then further thinned by means of ion milling technique.
3. Results First, we examined the microstructure of the tetragonal and the orthorhombic samples. Fig. 2 shows SEM micrographs of the polished and etched surfaces of the melt-textured 123 samples containing 211 inclusions. Sample (a) was quenched at 970°C after the peritectic heat-treatment and sample (b) was obtained by annealing the quenched sample at 500°C for 50 h in flowing oxygen. The phases of samples (a) and (b) were identified by power XRD analysis as a tetragonal phase and an orthorhombic phase, respectively. It can be seen in sample (a) that many 211 inclusions are trapped within the 123 domain. Except the trapped 211 inclusions, no other defects are observed both around the trapped 211 inclusions and in the 123 matrix. In sample (b) which is in an orthorhombic phase, meanwhile, many straight lines
Fig. 2. SEM micrographof the melt-textured 123 sample containing 211 inclusions: (a) sample quenched at 970°C after peritectic reaction (tetragonal phase) and (b) sample annealed at 500°C for 50 h in flowing oxygen after quenching at 970°C (orthorhombic phase).
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C.-J. Kim et al./Physica C 276 (1997) 101-108
Fig. 3. Optical micrographof the melt-textured 123-211 composite of orthorhombic phase showing the relation between the number of a platelet and (a) the 211 density, and (b) the 211 size.
are observed to be developed normal to the c-axis of the 123 phase. Sometimes, microcracks due to the transformation stress or thermal shock during cooling are included within the lines, because the a - b plane is a cleavage plane of a 123 phase [13]. Referring to the TEM analysis for the microstructure around 211 inclusions by Zanota et al. [7] and Alexander et al. [8], the line-type defects are filled with inclusion phases of BaCuO and many CuO stacking faults running parallel to the BaCuO phase. Fig. 3 is an optical micrograph showing the relation between the number of the platelet and (a) the 211 density and (b) the 211 size. It can be seen in Fig. 3(a) that the number of the platelet is dependent on the 211 density, Many more platelets are developed in the region containing 211 inclusions (see the region marked by 'B'), which is comparable to the region containing no 211 inclusion (the region marked by 'A'). Carefully examining the platelet structure, it can be recognized that the platelet density is also a function of the 211 size. As can be seen in Fig. 3(b), the platelet density around the large 211
mclusions is much higher than that around the small 211 inclusions. About 2 0 - 3 0 platelets are observed to be developed around large 211 inclusions (see the particles marked by circles). Some of them are extended toward the 123 matrix while others are limited in the vicinity of the 211 inclusions. This result is comparable to the report by Jin et al. which claimed the one-to-one relation between the trapped 211 inclusion and the platelet [16]. Furthermore, it should be noted that the direction of the platelets is dependent on the curvature of the 123/211 interface. Some of the platelets appear to initiate normally to the surface of the 211 inclusion and then to change their direction along the a - b plane of the 123 matrix (see the platelets marked by arrows in Fig. 3(b)). The presence of the platelet structure only in the orthorhombic samples strongly implies that the platelet was not formed by the high temperature peritectic reaction [8,17] but by the low-temperature oxygen annealing [19]. For more complete understanding of the formation of the planar defects, we carried out TEM analysis both for the tetragonal samples and the orthorhombic samples, especially around the trapped 211 inclusion. Fig. 4 shows the TEM bright field image of the quenched 123 sample of tetragonal phase. It can be seen that many 211 inclusions are observed in the tetragonal 123 matrix. There seems to be no orientational relationship between the 211 inclusions and the 123 matrix. This is due to the fact
Fig. 4. TEM bright field image of the melt-textured 123-211 composite of a tetragonal phase. Note the clean 123 matrix and the 123/211 interfacecontaining no defects.
C.-J. Kim et aL / Physica C 276 (1997) 101-108
Fig. 5. TEM microstmctum showing the development of a CuO stacking faults at the 123/211 interface of the orthorhombic sample.
that the 211 inclusion is not a precipitate which nucleates on a habit plane of the matrix phase but is a merely trapped phase during peritectic reaction. The trapped 211 inclusions are round in shape as a result of the preferential dissolution of the angular surfaces of the 211 particles in the liquid phase during peritectic reaction. Sometimes, coalesced 211 inclusions are observed (see the marked 211 by an arrow). Although we carefully examined many 211 inclusions of the tetragonal sample, we could not find any significant defects generation in the tetragonal sample except the presence of some dislocations probably due to the thermal shock by quenching. As can be seen in the figure, the microstructures both around the trapped 211 inclusions and in the 123 matrix are fairly clean, and are consistent with the SEM microstructure of Fig. 2(a). Fig. 5 shows the TEM bright field image with a [001] zone axis of the orthorhombic 123 sample, heat-treated at 500°C for 30 h in flowing oxygen after quenching process. Unlike the tetragonal 123 sample, various types of defects are observed in the orthorhombic 123 sample. In the 123 matrix, many [110] twins due to the tetragonal-to-orthorhombic phase transformation are observed. It should be noticed that a planar defect arm is found to be initiated at the 211/123 interface, which has been reported as an excessive CuO layer located between two 123 phase, i.e., a local YBa2CU4Ol0 (124 phase) in a 123 matrix [22]. The boundary of the stacking fault is of
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Fig. 6. Progressed feature of the a - b planar defects at the 123/211 interface of the melt-textured sample of orthorhombic phase.
wavelike or straight shape and stretches toward the [100] direction of the 123 matrix. Fig. 6 shows a progressed feature of the planar defects at the 123/211 interface which was observed in the melt-textured 123 sample, annealed in oxygen atmosphere for 50 h. As can be seen in this figure, several finger-like planar defects which involve the stacking faults and the BaCuO platelets are developed at the round-shaped 211 inclusion and also stretched along the [100] direction of the 123 matrix (see the region marked by an arrow). This indicates
Fig. 7. Microstructure of the melt-textured 123-211 sample annealed at 500°C for 400 h in flowing oxygen showing the stacking faults developed [100] and [010] direction of the 123 matrix.
C.-J. Kim et aL / Physica C 276 (1997) 101-108
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that this orientation is energetically favorable for the defect formation. Fig. 7 shows a typical bright field image with a [001] zone axis of the melt-textured 123 sample heat-treated at 500°C for 400 h in flowing oxygen. As the oxygen annealing time was extended, the stacking faults were not limited in the vicinity of the trapped 211 inclusion but spread into the whole 123 matrix. It can be seen that the spherical 211 inclusions are dispersed within the 123 matrix where a lot of stacking faults are developed along [100] and [010] directions of the 123 matrix. The stacking fault is more than a micron in length and is about a few hundred nanometers in width. The TEM observations confirm again that the formation of the planar defects is caused by the low-temperature annealing in an oxygen atmosphere.
4. Discussion
As clearly observed in this study, the BaCuO platelet structure and the CuO stacking faults were not formed by the high temperature peritectic reaction but by the low temperature annealing in oxygen atmosphere. The tetragonal 123 sample contained no defects while the orthorhombic phase contained many defects including the a - b planar defects. This is due to the fact that the 123 phase is not stable under the applied oxygen annealing condition (500°C, 1 atm) [20,21]. During oxygen annealing, the 123 phase should be decomposed into other stable second phases. According to the work by Williams et al. [[20,21]] which investigated the decomposition nature of the poly-grain 123 sample, B a 2 C u 3 0 6 _ ~ , CuO and a 211 phase were produced phase via the decomposition reaction 4YBa2Cu307_ x + ( 1 / 2 - 3/26 + 2 x ) O 2 -~
2Y 2BaCuO 5 + 3Ba2Cu 3 0 6
- 3 -~- C u O .
( 1)
They were present as a bulky form mostly at the grain boundaries of the 123 phase. Assuming that the stability of the melt-textured 123 sample of large sized domains is similar to that of the poly-grain 123 samples, the decomposition reaction can be applied to the melt-textured 123 sample. For the large domain sample, however, the decomposition reaction
mode is different to that of the poly-grain 123 sample. Instead of the bulk form of the decomposition products, Ba2Cu306_~ and CuO were present in a form of 2-dimensional planar defects of the BaCuO platelet and the CuO stacking faults, especially around the trapped 211 inclusions within the 123 matrix. If we assume that the decomposition of the 123 matrix follows the reaction equation (1), the 211 phase produced by the decomposition reaction should be observed. But many peritectically formed 211 and excessively added 211 inclusions were already present in the 123 matrix by trapping of the 211 inclusions during the peritectic reaction, it is difficult to separate the decomposed 211 phase from the trapped 211 inclusions. Otherwise, the decomposition reaction of the 123 matrix might follow a different reaction route which does not include the formation of the solid 211 phase. Referring Wang et al.'s work [6] which examined the composition near the 123/211 interface, the Y concentration near the 123/211 interface was higher than that in the 123 matrix which is far from the 211 inclusions. Instead of the formation of the 211 phase, the decomposition reaction of the 123 matrix may produce in the yttrium-excessive 123 matrix (Y203 stacking fault) in an atomic scale via the following reaction: aYBa2Cu307_y + b O 2 ~ c B a - Cu - O phase + dCuO + eY203 +fYiBa2Cu307_ymatrix.
(2) Let us consider the relation between the trapped 211 inclusion and the formation of the planar defects. As clearly observed in this microstructural examination, many BaCuO platelets and the CuO stacking faults were formed at the 123/211 interface. As for the driving force for the formation of the planar defects, one can think of the stress that can be evolved during fabrication. When cooling the melttextured sample, strain field may be evolved around the trapped 211, owing to the difference in thermal behavior between the trapped 211 and the 123 matrix phase. In addition, the transformation stress due to the tetragonal-to-orthorhombic phase change evolves in the 123 matrix. When the grain size of the 123 phase is less than one micron, the transformation stress can be relieved by forming of the transforma-
C.-J. Kim et aL /Physica C 276 (1997) 101-108
tion twins without formation of other defects [23]. In the case of the melt-textured sample, the domain size of the 123 phase is as large as about a few mm to a few cm. The stress evolved inside the 123 domains is relatively large so that it can not be easily relieved without defect formation. The transformation stress and the stress formed around the trapped 211 inclusions might trigger the decomposition reaction of a 123 phase which is accompanied by the formation of the various types of the defects, microcracks, stacking faults, BaCuO platelets. As observed in Fig. 3(b), more platelets were formed around the large 211 inclusions, compared to the small 211 inclusions. This may indicate that the stress generated at the 123/211 interface of the large 211 was much larger. In this work, it could be recognized that the density of the stacking faults and the platelet structure is a function of oxygen annealing time. Other defects such as twins, dislocations, oxygen deficiency, chemical and physical inhomogeneities are simultaneously generated during oxygen annealing. Among them, nano-scaled defects will be beneficial to flux pinning of the melt-textured 123 sample but large ones such as microcracks and BaCuO platelet will not be good to the flux pinning. This means that the flux pinning capability of melt-processed YBaCuO will be dependent on the oxygenation heat treatment. To obtain a better flux pinning property, therefore, the volume fraction of the superconducting phase and the density of the nano-scaled defects should increase while the formation of the harmful defects should be suppressed via optimization of the heating cycles and control of other metallurgical parameters.
5. Conclusions By a comparative examination of the microstructure of the tetragonal phase and the orthorhombic phase of the melt-textured 123 sample containing 211 inclusions, the formation mechanism of the planar defect structures of CuO stacking faults and the BaCuO platelet structure was elucidated. In the tetragonal 123 phase which is a high temperature non-superconducting phase, the CuO stacking fault and other defects were not observed around the trapped 211 inclusions and on the 123 matrix. After
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oxygen annealing of the tetragonal samples for the transformation of the tetragonal phase to the superconducting orthorhombic phase, meanwhile, many stacking faults, twins, and the BaCuO platelet structure were observed to be developed within the orthorhombic 123 matrix. The planar defects were found to nucleate at the 123/211 interface. As annealing time increased, they extended into the interior of the 123 matrix along [100]/[010] direction on the a - b plane of the 123 phase. It is concluded that the low temperature annealing for the tetragonal-toorthorhombic phase transformation causes the formation of the planar defects and this was well explained in terms of the oxidation induced decomposition of a 123 phase.
Acknowledgements The authors wish to acknowledge the financial support for this work by the Ministry of Science and Technology of Korea (MOST).
References [1] S. Jin, T. Tiefel, R. Sherwood, R. van Dover, M. Davis, G. Kammlott and R. Fastnacht, Phys. Rev. B. 37 (1988) 7850. [2] C.-J. Kim, K.-B. Kim, I.-S. Chang, D.-Y. Won, H.-C. Moon and D.-S. Suhr, J. Mater. Res. 8 (1993) 699. [3] F. Sandiumenge, S. Pinol, X. Obradors, E. Snoeck and C. Roucau, Phys. Rev. B 50 (1994) 7032. [4] F. Sandiumenge, N. Vilata, S. Pinol, B. Martinez and X. Obradors, Phys. Rev. B 51 (1995) 6645. [5] K. Yamaguchi, M. Murakami, H. Fujimoto, S. Gotoh, T. Oyama, Y. Shiohara, N. Koshizuka and S. Tanaka, J. Mater. Res. 6 (1991) 1404. [6] Z.L. Wang, A. Goyal and D.M. Kroeger, Phys. Rev. B 47 (1993) 5373. [7] A. Zanota, E.P. Kvam, D. Balkin and P.J. McGinn, J. Appl. Phys. Lett. 62 (1993) 2722. [8] K.B. Alexander, A. Goyal, D.M. Kroeger, V. Salvamanickam and K. Salama, Phys. Rev. B 45 (1992) 5622. [9] M. Mironova, D.F. Lee and K. Salama, Physica C 211 (1993) 188. [t0] D. Shi, J.G. Chert, M. Xu, H.E. Kourous, Y. Fang, Y.H. Li and M.S. Boley, Supercond. Sci. Technol. 3 (1990) 457. [11] W. Ruikun, R. Hongtao, X. Ling, H. Qing, W. Chaoqun and Y. Dingan Supercond. Sci. Technol. 3 (1990) 344. [12] Z. Hirio, M. Takano, Y. Ikeda, Y. Takeda and Y. Bando, Jpn. J. Appl. Phys. 27 (1988) LI41.
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[13] S. Nakahara, T. Boone, M.F. Yan, G.J. Fisanick and D.W. Johnson, Jr., J. Appl. Phys. 63 (1988) 451. [14] C.-J. Kim, K.-B. Kim, D.-Y. Won, Mater. Lett. (1992) 268. [15] C.-J. Kim, K.-B. Kim, D.-Y. Won and G.-W. Hong, Physica C 228 (1994) 351. [16] S. Jin, G.W. Kammlott, T.H. Tiefel, T.T. Kodas, T.L. Word and D.M. Kroeger, Physica C 181 (1990) 57. [17] A. Goyal, K.B. Alexander, D.M. Kroeger, P.D. Kunkenbusch and S.J, Bums, Physica C 210 (1993) 197. [18] G.J. Schmitz, J. Laakmann, C. Wolters, S. Rex, W. Gawalek, T. Habisteuther, G. Bruchulos and P. Gomert, J. Mater. Res. 8 (1993) 2774.
[19] C.-J. Kim. K.-B. Kim, H.-W. Park, T.-H. Sung, I.-H. Kuk and G.-W. Hong, Supercond. Sci. Technol. 9 (1996) 76. [20] R.K. Williams, K.B. Alexander, J. Brynestad, T.J. Hension, D.M. Kroeger, T.B. Lindemer, G.C. Marsh, J.O. Scarbough and E.D. Specht, J. Appl. Phys. 70 (1991) 906. [21] R.K. Williams, K.B. Alexander, J. Brynestad, T.J. Hension, D.M. Kroeger, T.B. Lindemer, G.C. Marsh and J.O. Scarbough, J. Appl. Phys. 67 (1990) 6934. [22] J. Kulk, J. Appl. Phys. 70 (1991) 4398. [23] T.M. Shaw, S.L. Shinde, D. Damos, R.F. Cook, P.R. Duncombe and C. Kroll, J. Mater. Res. 4 (1989) 248.
PHYSICA
ELSEVIER
PhysicaC276 (1997) 101-108
Oxidation induced formation of a-b planar defects in melt-textured YBa 2Cu 307- y containing Y2BaCuO5 inclusions Chan-Joong Kim a,*, Yi-Sung Lee b, Hyun-Soon Park b, I1-Hyun Kuk a, Tae-Hyun Sung c, Jin Joong Kim c, Gye-Won Hong a a Superconductivity Research Laboratory, Korea Atomic Energy Research Institute, P.O. Box 105, Yusung, Taejon, 305-600, South Korea b Department of Metallurgical Engineering, Sungkyunkwan University, Soowon, Kyunggido, 440-746, South Korea c Center tbr Advanced Studies in Energy and Environment, Korea Electric Power Research Institute, 103-16, Munjidong, Yusung, Taejon, 305-380, South Korea
Received 19 September 1996; revised manuscriptreceived 9 December 1996
Abstract In order to understand the formation mechanism of the planar defects of a B a - C u - O platelet and a CuO stacking fault of melt-textured YBa2Cu307_y (123), two different types of 123 samples containing Y2BaCuOs (211) inclusions (a tetragonal and orthorhombic samples) were prepared by melt-quenching heat treatment and/or subsequent annealing in oxygen atmosphere, and the microstructures were examined. In the tetragonal samples which are high-temperature non-superconducting phases, no remarkable defect formation was observed either around the trapped 211 inclusions or in the 123 matrix: After annealing of the tetragonal samples in oxygen atmosphere for the transformation of the tetragonal phase to the superconducting orthorhombic phase, meanwhile, various types of defects were observed such as dislocations, stacking faults, twins and the BaCuO platelets. Particularly, the planar defects of the CuO stacking faults and the BaCuO platelets were found to nucleate mostly at the 123/2t 1 interface and then to grow toward the interior of the 123 matrix. In the sample with shorter oxygen annealing time, the formation of the planar defects was limited in the vicinity of the trapped 211 while in the sample with longer annealing time, they extended to the whole 123 matrix. It is concluded that the formation of the planar defects of the melt-processed Y - B a - C u - O is attributed to the low temperature annealing in oxygen atmosphere for the tetragonal-to-orthorhombic phase transformation.
1. Introduction Analyzing the melt-processed microstructures of Y - B a - C u - O samples, various types of microdefects are observed within the superconducting 123 matrix [1-11]. These are a transformation twin, a disloca-
* Corresponding author. Fax: ÷ 82 42 862 5496.
tion, a trapped 211 inclusion, a residual liquid phase, a microcrack, a stacking fault and a BaCuO platelet. The presence of these defects in the superconducting 123 matrix is very important to the practical applications of this material, since some of them may act as flux pinning sites of the 123 phase [5,6]. The defect density should, therefore, be controlled to an optim u m value, together with understanding of the formation mechanism. A m o n g them, the formation mechanism of some defects has been well clarified
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but others are not clearly understood yet. For example, twin structure is known to be developed due to the transformation stress from the tetragonal to the orthorhombic phase accompanying the oxygen ordering on the 1-dimensional C u - O chain structure which is located between two B a - O layers [12,13]. The transformation stress and the thermal stress during fabrication can also produce microcracks [ 13,14] and dislocations [13] in the 123 matrix. Whereas, the residual liquid phase and the trapped 211 inclusion are results of incomplete peritectic reaction during growth of the 123 domains in the liquid plus 211 state [8,15]. Another typical microstructural feature of the melt-processed Y - B a - C u - O is a platelet structure (so-called a textured structure) which is developed along the a - b plane of the 123 phase. In the early studies on the melt-textured microstructure [ 16-18], the platelet structure has been considered as a boundary of the aligned plate-like 123 grains [16] or a liquid phase entrapped during high temperature peritectic growth of the 123 domain [17,18], because the direction of the platelet structure was the same as the growth direction of the 123 domain and the composition of the phase observed within the platelet was similar to that of the peritectic liquid; crystalline or amorphous BaCuO and CuO phases [7,8]. In addition to this, the formation of the platelet structure seems to be quite relevant to the presence of the trapped 211 inclusion within the 123 matrix. The number of the platelet was a function of 211 density [16]. The microstructural observations have thus led to the conclusion that the planar defect is a kind of residual liquid phase which was trapped during high temperature peritectic reaction [8,17]. The correlation of the dependency of the platelet density on the number of the trapped 211 inclusion was explained in terms of the 211 trapping within the growing 123 domain which is accompanied by the entrapment of the liquid phase [8] or in terms of the nucleation of plate-like 123 grains at the 211 inclusions [16]. By the way, recent study on the melt-textured microstructure suggested a controversial viewpoint on the formation mechanism of the platelet structure and a CuO stacking fault [19]. By comparing the optical micrographs for the polished surfaces between a tetragonal and the orthorhombic samples, it was demonstrated that the tetragonal 123 phase con-
tained no platelet structure while the orthorhombic 123 phase contained many platelets [19]. Its formation mechanism was explained on the basis of the oxidation-induced decomposition of the 123 phase into 211, Ba2Cu306_ ~ and CuO phase, which was proposed by Williams et al., in the poly-grain 123 samples [20,21]. Moreover, Sandiumenge et al. showed in the melt-processed 123 sample that the density of the CuO stacking fault varied with oxygen annealing time and it affected the critical current density [4]. These results may indicate that the formation of the planar defects has no relation to the high temperature peritectic reaction. In order to clarify the exact formation mechanism of the planar defects, therefore, it is necessary to examine the microstructures before and after the oxygen annealing, especially for the regions around the trapped 211 inclusions. The aim of this study is to clarify what the dominant mechanism is for the formation of the planar defects of the melt-processed 123 sample. We report the microstructurat difference between the tetragonal phase and the orthorhombic phase of the melt-textured 123 sample containing 211 inclusions. The microstmctural relation of the planar defects and the trapped 211 inclusion and the formation mechanism are discussed in terms of the phase decomposition during oxygen annealing.
2. Experimental procedure A powder mixture of 123 and 211 phases used in this experiment was prepared by the conventional solid-state reaction method using Y203, BaCO 3 and CuO powder of 99.9% purity. The powders were weighed to a cation ratio of Y:Ba:Cu = 1.8:2.4:3.4, ball-milled for 20 h in alcohol and then dried in air. The powders were calcined at 900°C in air, with repeated grinding for every 10 h and then cooled in air. The calcined powder was pressed in a steel mold into pellets. In order to understand the defect generation process in the melt-textured samples, two different samples, i.e., the tetragonal and the orthorhombic 123 samples containing 211 inclusions were prepared. First, the pellets were heated to 1100°C so as to make them a 211 plus liquid state. After holding the
C.-J. Kim et al./Physica C 276 (1997) 101-108
Temp.[ t
1100°C forO.Sh
! 211+LiquidphaseI 970°C~Tetragonal123
phase[
[ Orthorhombic123 phase] V
Tetraiiiii"i / E°°°c intl°wing°xYge. T.~k~IRi
Fig. 1. Schematic of the melt-quench and oxygen annealing heat treatment in order to produce tetragonal and orthorhombic 123 samples.
pellets at this temperature for 30 min, the samples were slowly cooled to 970°C at a rate of l ° C / h and then was rapidly quenched in a liquid nitrogen bath to obtain the tetragonal phase. The powder x-ray diffraction (XRD) analysis of the quenched samples showed that the high-temperature tetragonal phase was retained. Some of the quenched samples were annealed at 500°C for various times in flowing oxygen in order to produce the orthorhombic phase. The experimental details are shown in Fig. 1. If the planar defects such as CuO stacking faults and BaCuO platelets are found in the tetragonal sample, it can be said that the defects were generated by the high temperature mechanism involving liquid entrapment ahead of the growing 123 domain during peritectic reaction [8,17]. On the contrary, if the defects are found in the orthorhombic samples, the formation of the planar defects can be explained in terms of the low temperature mechanism based on the decomposition of a 123 phase [19]. Phases formed in the samples were analyzed by powder XRD measurement. Microstructures were examined on the polished and etched surfaces by means of an optical, scanning electron (SEM) and transmission electron microscope (TEM, 2000 FX2, JEOL). For TEM investigation, the melt-textured samples were sliced by using of a diamond saw, thinned down to 80-100 microns by mechanical
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grinding on SiC papers and then further thinned by means of ion milling technique.
3. Results First, we examined the microstructure of the tetragonal and the orthorhombic samples. Fig. 2 shows SEM micrographs of the polished and etched surfaces of the melt-textured 123 samples containing 211 inclusions. Sample (a) was quenched at 970°C after the peritectic heat-treatment and sample (b) was obtained by annealing the quenched sample at 500°C for 50 h in flowing oxygen. The phases of samples (a) and (b) were identified by power XRD analysis as a tetragonal phase and an orthorhombic phase, respectively. It can be seen in sample (a) that many 211 inclusions are trapped within the 123 domain. Except the trapped 211 inclusions, no other defects are observed both around the trapped 211 inclusions and in the 123 matrix. In sample (b) which is in an orthorhombic phase, meanwhile, many straight lines
Fig. 2. SEM micrographof the melt-textured 123 sample containing 211 inclusions: (a) sample quenched at 970°C after peritectic reaction (tetragonal phase) and (b) sample annealed at 500°C for 50 h in flowing oxygen after quenching at 970°C (orthorhombic phase).
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C.-J. Kim et al./Physica C 276 (1997) 101-108
Fig. 3. Optical micrographof the melt-textured 123-211 composite of orthorhombic phase showing the relation between the number of a platelet and (a) the 211 density, and (b) the 211 size.
are observed to be developed normal to the c-axis of the 123 phase. Sometimes, microcracks due to the transformation stress or thermal shock during cooling are included within the lines, because the a - b plane is a cleavage plane of a 123 phase [13]. Referring to the TEM analysis for the microstructure around 211 inclusions by Zanota et al. [7] and Alexander et al. [8], the line-type defects are filled with inclusion phases of BaCuO and many CuO stacking faults running parallel to the BaCuO phase. Fig. 3 is an optical micrograph showing the relation between the number of the platelet and (a) the 211 density and (b) the 211 size. It can be seen in Fig. 3(a) that the number of the platelet is dependent on the 211 density, Many more platelets are developed in the region containing 211 inclusions (see the region marked by 'B'), which is comparable to the region containing no 211 inclusion (the region marked by 'A'). Carefully examining the platelet structure, it can be recognized that the platelet density is also a function of the 211 size. As can be seen in Fig. 3(b), the platelet density around the large 211
mclusions is much higher than that around the small 211 inclusions. About 2 0 - 3 0 platelets are observed to be developed around large 211 inclusions (see the particles marked by circles). Some of them are extended toward the 123 matrix while others are limited in the vicinity of the 211 inclusions. This result is comparable to the report by Jin et al. which claimed the one-to-one relation between the trapped 211 inclusion and the platelet [16]. Furthermore, it should be noted that the direction of the platelets is dependent on the curvature of the 123/211 interface. Some of the platelets appear to initiate normally to the surface of the 211 inclusion and then to change their direction along the a - b plane of the 123 matrix (see the platelets marked by arrows in Fig. 3(b)). The presence of the platelet structure only in the orthorhombic samples strongly implies that the platelet was not formed by the high temperature peritectic reaction [8,17] but by the low-temperature oxygen annealing [19]. For more complete understanding of the formation of the planar defects, we carried out TEM analysis both for the tetragonal samples and the orthorhombic samples, especially around the trapped 211 inclusion. Fig. 4 shows the TEM bright field image of the quenched 123 sample of tetragonal phase. It can be seen that many 211 inclusions are observed in the tetragonal 123 matrix. There seems to be no orientational relationship between the 211 inclusions and the 123 matrix. This is due to the fact
Fig. 4. TEM bright field image of the melt-textured 123-211 composite of a tetragonal phase. Note the clean 123 matrix and the 123/211 interfacecontaining no defects.
C.-J. Kim et aL / Physica C 276 (1997) 101-108
Fig. 5. TEM microstmctum showing the development of a CuO stacking faults at the 123/211 interface of the orthorhombic sample.
that the 211 inclusion is not a precipitate which nucleates on a habit plane of the matrix phase but is a merely trapped phase during peritectic reaction. The trapped 211 inclusions are round in shape as a result of the preferential dissolution of the angular surfaces of the 211 particles in the liquid phase during peritectic reaction. Sometimes, coalesced 211 inclusions are observed (see the marked 211 by an arrow). Although we carefully examined many 211 inclusions of the tetragonal sample, we could not find any significant defects generation in the tetragonal sample except the presence of some dislocations probably due to the thermal shock by quenching. As can be seen in the figure, the microstructures both around the trapped 211 inclusions and in the 123 matrix are fairly clean, and are consistent with the SEM microstructure of Fig. 2(a). Fig. 5 shows the TEM bright field image with a [001] zone axis of the orthorhombic 123 sample, heat-treated at 500°C for 30 h in flowing oxygen after quenching process. Unlike the tetragonal 123 sample, various types of defects are observed in the orthorhombic 123 sample. In the 123 matrix, many [110] twins due to the tetragonal-to-orthorhombic phase transformation are observed. It should be noticed that a planar defect arm is found to be initiated at the 211/123 interface, which has been reported as an excessive CuO layer located between two 123 phase, i.e., a local YBa2CU4Ol0 (124 phase) in a 123 matrix [22]. The boundary of the stacking fault is of
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Fig. 6. Progressed feature of the a - b planar defects at the 123/211 interface of the melt-textured sample of orthorhombic phase.
wavelike or straight shape and stretches toward the [100] direction of the 123 matrix. Fig. 6 shows a progressed feature of the planar defects at the 123/211 interface which was observed in the melt-textured 123 sample, annealed in oxygen atmosphere for 50 h. As can be seen in this figure, several finger-like planar defects which involve the stacking faults and the BaCuO platelets are developed at the round-shaped 211 inclusion and also stretched along the [100] direction of the 123 matrix (see the region marked by an arrow). This indicates
Fig. 7. Microstructure of the melt-textured 123-211 sample annealed at 500°C for 400 h in flowing oxygen showing the stacking faults developed [100] and [010] direction of the 123 matrix.
C.-J. Kim et aL / Physica C 276 (1997) 101-108
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that this orientation is energetically favorable for the defect formation. Fig. 7 shows a typical bright field image with a [001] zone axis of the melt-textured 123 sample heat-treated at 500°C for 400 h in flowing oxygen. As the oxygen annealing time was extended, the stacking faults were not limited in the vicinity of the trapped 211 inclusion but spread into the whole 123 matrix. It can be seen that the spherical 211 inclusions are dispersed within the 123 matrix where a lot of stacking faults are developed along [100] and [010] directions of the 123 matrix. The stacking fault is more than a micron in length and is about a few hundred nanometers in width. The TEM observations confirm again that the formation of the planar defects is caused by the low-temperature annealing in an oxygen atmosphere.
4. Discussion
As clearly observed in this study, the BaCuO platelet structure and the CuO stacking faults were not formed by the high temperature peritectic reaction but by the low temperature annealing in oxygen atmosphere. The tetragonal 123 sample contained no defects while the orthorhombic phase contained many defects including the a - b planar defects. This is due to the fact that the 123 phase is not stable under the applied oxygen annealing condition (500°C, 1 atm) [20,21]. During oxygen annealing, the 123 phase should be decomposed into other stable second phases. According to the work by Williams et al. [[20,21]] which investigated the decomposition nature of the poly-grain 123 sample, B a 2 C u 3 0 6 _ ~ , CuO and a 211 phase were produced phase via the decomposition reaction 4YBa2Cu307_ x + ( 1 / 2 - 3/26 + 2 x ) O 2 -~
2Y 2BaCuO 5 + 3Ba2Cu 3 0 6
- 3 -~- C u O .
( 1)
They were present as a bulky form mostly at the grain boundaries of the 123 phase. Assuming that the stability of the melt-textured 123 sample of large sized domains is similar to that of the poly-grain 123 samples, the decomposition reaction can be applied to the melt-textured 123 sample. For the large domain sample, however, the decomposition reaction
mode is different to that of the poly-grain 123 sample. Instead of the bulk form of the decomposition products, Ba2Cu306_~ and CuO were present in a form of 2-dimensional planar defects of the BaCuO platelet and the CuO stacking faults, especially around the trapped 211 inclusions within the 123 matrix. If we assume that the decomposition of the 123 matrix follows the reaction equation (1), the 211 phase produced by the decomposition reaction should be observed. But many peritectically formed 211 and excessively added 211 inclusions were already present in the 123 matrix by trapping of the 211 inclusions during the peritectic reaction, it is difficult to separate the decomposed 211 phase from the trapped 211 inclusions. Otherwise, the decomposition reaction of the 123 matrix might follow a different reaction route which does not include the formation of the solid 211 phase. Referring Wang et al.'s work [6] which examined the composition near the 123/211 interface, the Y concentration near the 123/211 interface was higher than that in the 123 matrix which is far from the 211 inclusions. Instead of the formation of the 211 phase, the decomposition reaction of the 123 matrix may produce in the yttrium-excessive 123 matrix (Y203 stacking fault) in an atomic scale via the following reaction: aYBa2Cu307_y + b O 2 ~ c B a - Cu - O phase + dCuO + eY203 +fYiBa2Cu307_ymatrix.
(2) Let us consider the relation between the trapped 211 inclusion and the formation of the planar defects. As clearly observed in this microstructural examination, many BaCuO platelets and the CuO stacking faults were formed at the 123/211 interface. As for the driving force for the formation of the planar defects, one can think of the stress that can be evolved during fabrication. When cooling the melttextured sample, strain field may be evolved around the trapped 211, owing to the difference in thermal behavior between the trapped 211 and the 123 matrix phase. In addition, the transformation stress due to the tetragonal-to-orthorhombic phase change evolves in the 123 matrix. When the grain size of the 123 phase is less than one micron, the transformation stress can be relieved by forming of the transforma-
C.-J. Kim et aL /Physica C 276 (1997) 101-108
tion twins without formation of other defects [23]. In the case of the melt-textured sample, the domain size of the 123 phase is as large as about a few mm to a few cm. The stress evolved inside the 123 domains is relatively large so that it can not be easily relieved without defect formation. The transformation stress and the stress formed around the trapped 211 inclusions might trigger the decomposition reaction of a 123 phase which is accompanied by the formation of the various types of the defects, microcracks, stacking faults, BaCuO platelets. As observed in Fig. 3(b), more platelets were formed around the large 211 inclusions, compared to the small 211 inclusions. This may indicate that the stress generated at the 123/211 interface of the large 211 was much larger. In this work, it could be recognized that the density of the stacking faults and the platelet structure is a function of oxygen annealing time. Other defects such as twins, dislocations, oxygen deficiency, chemical and physical inhomogeneities are simultaneously generated during oxygen annealing. Among them, nano-scaled defects will be beneficial to flux pinning of the melt-textured 123 sample but large ones such as microcracks and BaCuO platelet will not be good to the flux pinning. This means that the flux pinning capability of melt-processed YBaCuO will be dependent on the oxygenation heat treatment. To obtain a better flux pinning property, therefore, the volume fraction of the superconducting phase and the density of the nano-scaled defects should increase while the formation of the harmful defects should be suppressed via optimization of the heating cycles and control of other metallurgical parameters.
5. Conclusions By a comparative examination of the microstructure of the tetragonal phase and the orthorhombic phase of the melt-textured 123 sample containing 211 inclusions, the formation mechanism of the planar defect structures of CuO stacking faults and the BaCuO platelet structure was elucidated. In the tetragonal 123 phase which is a high temperature non-superconducting phase, the CuO stacking fault and other defects were not observed around the trapped 211 inclusions and on the 123 matrix. After
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oxygen annealing of the tetragonal samples for the transformation of the tetragonal phase to the superconducting orthorhombic phase, meanwhile, many stacking faults, twins, and the BaCuO platelet structure were observed to be developed within the orthorhombic 123 matrix. The planar defects were found to nucleate at the 123/211 interface. As annealing time increased, they extended into the interior of the 123 matrix along [100]/[010] direction on the a - b plane of the 123 phase. It is concluded that the low temperature annealing for the tetragonal-toorthorhombic phase transformation causes the formation of the planar defects and this was well explained in terms of the oxidation induced decomposition of a 123 phase.
Acknowledgements The authors wish to acknowledge the financial support for this work by the Ministry of Science and Technology of Korea (MOST).
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