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Synthesis and characterization of large single-domain Gd–Ba–Cu–O bulk superconductor material using a modified IG process
Materials Chemistry and Physics 124 (2010) 936–939
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Synthesis and characterization of large single-domain Gd–Ba–Cu–O bulk superconductor material using a modified IG process Guo-Zheng Li ∗ , Wan-Min Yang Department of Physics, Shaanxi Normal University, Shida Lu No. 1, Xi’an, Shaanxi, 710062, China
a r t i c l e
i n f o
Article history: Received 27 March 2010 Received in revised form 11 July 2010 Accepted 17 July 2010 Keywords: Superconductors Crystal growth Superconductivity
a b s t r a c t Large single-domain Gd–Ba–Cu–O bulk material was successfully fabricated by a modified infiltration and growth process (IG) using a liquid source composed of Gd2 O3 , BaCuO2 and CuO. The growth character, microstructure, and performance of the GdBCO product were investigated. The results indicate that, this modified IG technique can be used to fabricate well-textured and high performance GdBCO large single domains more efficiently. Moreover, an increasing number of small Gd2 BaCuO5 (Gd-211) particles and coarsening of large Gd-211 particles were both observed in the microstructure with increasing distance from the seed, which together are jointly responsible for the increasing volume fraction of Gd-211 particles during the GdBa2 Cu3 O7−x (Gd-123) growth process. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Recently, the infiltration and growth process (IG), used for fabrication of single-domain RE–Ba–Cu–O (REBCO) bulk material, has attracted considerable attention because of a number of advantages compared with the conventional melt growth method (MG). These include fewer pores, cracks and distortions, insignificant shrinkage in the final product and especially a homogeneous distribution of fine RE2 BaCuO5 (RE-211) particles in the REBa2 Cu3 O7−x (RE-123) matrix [1–7]. For this process, a RE-211 preform bulk was placed on top of one liquid source pellet (Ba and Cu rich liquid phase) at room temperature initially, then, once molten, the liquid phase infiltrated up into the porous RE-211 compact at an elevated temperature, which then reacted with the RE-211 phase to form RE-123 on cooling, described as: RE2 BaCuO5 + (3BaCuO2 + 2CuO) → 2REBa2 Cu3 O7−x
particles decomposed from the starting RE-123 phase at high temperature, so it could be seen that three kinds of precursor powders, involving RE-211, RE-123, and BaCuO2 , have to be prepared for the IG process. In Ref. [8], a mixture of Gd2 O3 , BaCuO2 , and CuO has been reported to replace the mixture of Gd-123 and Ba3 Cu5 O8 as the liquid source to fabricate single-domain Gd–Ba–Cu–O (GdBCO) bulk superconductor material of 20 mm diameter. For this process, only two precursor powders, namely Gd-211 and BaCuO2 , are needed to be prepared for the whole IG flow, thus the experimental process is simplified. In this article, large single-domain GdBCO bulk material of 30 mm in diameter was fabricated successfully using this modified IG process. The growth character, microstructure, and performance of the GdBCO product were investigated in detail.
(1) 2. Experimental
It is apparent that the Ba and Cu rich liquid phase, with a nominal composition of Ba3 Cu5 O8 , is the exact phase needed for reaction with the RE-211 phase. However, the heavy loss of liquid and the collapse of the sample have been observed when the Ba3 Cu5 O8 powder (035, i.e., a mixture of BaCuO2 and CuO powders in a molar ratio of BaCuO2 :CuO = 3:2) was employed as the liquid source, especially when the liquid source was supplied rather richly. As a result, a mixture of RE-123 and Ba3 Cu5 O8 is usually used as the liquid source in the IG process [1,6,7], and the above two processing difficulties can be effectively avoided with the support of solid RE-211
∗ Corresponding author. Tel.: +86 29 85302598. E-mail address: [email protected] (G.-Z. Li). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.07.047
The single phase precursor powders consisting of Gd-211 and BaCuO2 were prepared by a conventional solid-state reaction method in air using the raw materials of Gd2 O3 , BaCO3 and CuO. The powders of Gd-211, together with an addition of 2 wt.% CeO2 , were well mixed using a ball milling machine and then pressed into pellets of 30 mm diameter in batches of 30 g. The powders of Gd2 O3 , CuO, and BaCuO2 were weighed according to the molar ratio Gd2 O3 :CuO:BaCuO2 = 1:6:10 [8], mixed thoroughly using a ball milling machine and then pressed into pellets of 30 mm diameter in batches of 35 g, to act as the liquid source for the IG process. Furthermore, Yb2 O3 powder in batches of 4.5 g was pressed into a plate of 2 mm in thickness to be used to support the liquid phase at the elevated temperature. The Gd-211 green compact was placed on top of the liquid source pellet, which, in turn, was placed on the liquid support plate. The entire arrangement was then mounted on a ring of MgO single crystals, which are all the same height and rested on an alumina plate. Finally, a Mg-doped NdBCO seed crystal [9] of dimensions 3 mm × 3 mm × 2 mm was placed on the top surface of the Gd-211 green compact
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Fig. 2. The top surface morphology of the GdBCO sample grown by the modified IG technique.
Fig. 1. (a) The arrangement of the sample prior to infiltration and growth and (b) heat treatment pattern for GdBCO crystal growth used in this study.
with the ab-plane parallel to the surface, as shown in Fig. 1(a). Then the sample stack is put in a tube furnace with a positive radial temperature gradient, which effectively prevents the random nucleation of GdBCO grains at the edge of the sample [10].The sample stack was heated to 800 ◦ C at a rate of 160 ◦ C/h, and held for 10 h, then heated further to 1065 ◦ C at a rate of 60 ◦ C/h, held for 1 h, cooled to 1035 ◦ C at a rate of 60 ◦ C/h, then cooled slowly to 1015 ◦ C at a rate of 0.2 ◦ C/h, and finally furnacecooled to room temperature, as shown schematically in Fig. 1(b). After completion of the IG process, the single-domain material was annealed in flowing oxygen for 200 h at temperatures ranging from 430 ◦ C to 350 ◦ C.
3. Results and discussion Fig. 2 shows the top surface morphology of one typical GdBCO sample of 30 mm in diameter fabricated using the modified IG process. As can be seen in the figure, the sample exhibited clear four-fold growth sector boundaries on the top surface, indicating that it was grown in the form of a single domain. In addition, no spontaneous satellite grains were observed in the sample. As a result, it can be concluded that the modified IG technique can be used to fabricate well-textured large GdBCO single-domain material. The levitation force of the sample was measured under a zerofield cooling state at 77 K using a home-made device [11]. A magnet (Ø 25 mm) with a surface field of 0.5 T was used in the levitation force measurement. The maximum levitation force measured in this experiment was achieved at the smallest gap (0.1 mm) between the two nearest surfaces of the sample and the magnet. The levitation force values versus distance from the magnet for the sample are shown in Fig. 3. As shown in the figure, the maximum levitation force of the sample reached 63.9 N, about 9 N/cm2 . This result indicates that, based on the new liquid source, the singledomain GdBCO bulk fabricated by the modified IG technique has a satisfactory performance and therefore can be used for various applications.
Fig. 3. The levitation forces versus distance at 77 K for the single-domain GdBCO bulk sample.
In order to study the microstructure of the GdBCO bulk sample, three small specimens were cut from the locations shown schematically in Fig. 4. Scanning electron micrographs (SEM) were taken at the centre of the polished top surface of each specimen, and the results are shown in Fig. 5. It can be seen from the figure that,
Fig. 4. A schematic drawing of the locations where the small specimens were taken from the bulk sample.
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G.-Z. Li, W.-M. Yang / Materials Chemistry and Physics 124 (2010) 936–939 Table 1 The average size and volume fraction of Gd-211 second phase particles in Gd-123 matrix for each specimen, analyzed using the image processing software and assuming that the shape of the Gd-211 particles are spherical. Small specimens
Volume fraction of Gd-211 particles (%)
Average size of Gd-211 particles (m)
A B C
25.5 32.3 42.5
2.055 1.864 1.828
volume fraction of Gd-211 particles, Vf211 , increases with increasing distance from the seed, and reaches a value of 42.5% for Specimen C. This phenomenon is usually interpreted by the established pushing/trapping theory of foreign particles during solidification of RE-123 crystal [1,6,13,14]. According to this theory, the distribution of second phase particles is expected to vary as a function of growth rate, and the critical particle radius for trapping, r*, is given by the following equation: R∝
Fig. 5. SEM micrographs of the GdBCO sample grown by the modified IG technique, which were obtained at the locations A, B, and C as shown in Fig. 4.
fine Gd-211 second phase particles are uniformly distributed in the Gd-123 matrix for each specimen. The average size and volume fraction of Gd-211 particles for each specimen were analyzed qualitatively using an image processing software package (ImageJ) [12], and the results are contained in Table 1. It can be seen that the
0 r∗
(2)
where R is growth rate, 0 is the net of the interfacial energy and is the melt viscosity. Hence, during the slow-cooling growth process for one bulk from the seed, with an increase of the undercooling (T), the critical radius for trapping decreases as the growth rate increases, thus more and more smaller RE-211 particles can be trapped, which finally leads to an increase of Vf211 with increasing distance from the seed and a decrease of the average size of RE-211 particles, d211 . In other words, the increase of Vf211 was considered to be a result of the lowering threshold for the entrapment of small particles. However, Fig. 5 of this study indicates that the present situation is not consistent with that analysis entirely. From Fig. 5, the increasing number of small particles with increasing distance from the seed, i.e. from position A to position C, was indeed observed clearly, as well as an increase in the total particle number, which is consistent with the known pushing/trapping theory. However, the number of large particles, and the size of the largest particles were also observed to increase gradually. Take the largest particles, for example, in Fig. 5(a), the size of largest particles was found to be about 4 m, but in Fig. 5(b), this value increased to more than 5 m, and in Fig. 5(c), even particles with a size of 8 m were observed. Furthermore, Fig. 5(c) also exhibited significant particle coalescence (or connection). This situation indicates that the particles trapped in the later growth stage should have suffered a coarsening process. And the coarsening of large particles together with the broader entrance for small particles are jointly responsible for the increase of Vf211 in the Gd-123 matrix, and finally result in the rather high Vf211 value observed in Specimen C. On the other hand, coarsening of large particles has the effect of increasing the average size of the particles, which is opposite to the entrapment phenomenon of small particles. As a result, due to the influence of these two competitive effects, only a slight reduction in d211 was observed for Specimen C compared with Specimen B (Table 1). In addition, connected particles were counted as a single entity during the image processing, which also has the effect of increasing the d211 value of Specimen C. For typical crystal growth from a seed, it is well known that, the ungrown region near the edge of the compact will be always kept molten during the RE-123 growth process starting from the center (corresponding to the formation and directional solidification of RE-123 phase). According to our experience, the edge area will have been molten for at least 40 h before the growth front reaches the outside diameter. In terms of the acknowledged Ostwald ripening theory, the solid RE-211 particles are easily coarsened in molten liquid at high temperature, in which the smaller RE-211 particles
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4. Conclusions Large GdBCO single domains with a diameter of 30 mm have been fabricated in air by a modified IG technique. After detailed investigations of the growth character, microstructure, and superconducting properties of the grown sample, it can be concluded that the modified IG technique can be used to fabricate well-textured large GdBCO single-domain material efficiently. Additionally, increasing numbers of small Gd-211 particles together with the coarsening of large Gd-211 particles have been observed in the microstructure with increasing distance from the seed. Acknowledgments This work was supported by The National High Technology Research and Development Program of China (“863” No. 2007AA03Z241), The National Natural Science Foundation of China (No. 50872079), and The Fundamental Research Funds for the Central Universities (No. 2010ZYGX021; No. GK200901017). Fig. 6. Resistivity versus temperature plot of the GdBCO sample fabricated by the modified IG process in air.
are dissolved in the melt and the dissolved ions migrate to grow the biggest particles [14,15]. Thus the coarsening phenomenon of large particles observed in Fig. 5(c) can be easily understood based on this theory, because the material associated with Specimen C was held liquid for a longer time than that for the other sections. Whereas, compared with the RE-211 coarsening behavior observed in the sample, which was always melted at a temperature above the peritectic temperature (Tp ) of RE-123 [16–18], the coarsening degree of RE-211 in the sample during its slow-cooling growth process observed in this study seems much slighter. This observation could be anticipated by the peritectic solidification growth mechanism of the RE-123 crystal, because during the bulk growth process a large amount of ions dissolved in liquid were consumed by the peritectic reaction [19], and finally created RE-123 matrix over 50%, in volume fraction, of the bulk. The measurement of resistivity versus temperature was performed on Specimen C, and the result is shown in Fig. 6. From this figure, a sharp superconducting transition with an onset temperature, Tc , of 91.5 K and a transition width of 2 K was observed, confirming that the formation of Gd1+x Ba2−x Cu3 O7−␦ -type solidsolution had been suppressed effectively. This result is comparable to the Tc values of GdBCO bulk superconductors prepared under reduced oxygen partial pressure [20–22], and proves the feasibility of processing GdBCO samples in air.
References [1] K. Iida, N.H. Babu, Y. Shi, D.A. Cardwell, Supercond. Sci. Technol. 18 (2005) 1421. [2] E.S. Reddy, T. Rajasekharan, Supercond. Sci. Technol. 11 (1998) 523. [3] N.H. Babu, M. Kambara, P.J. Smith, D.A. Cardwell, Y. Shi, J. Mater. Res. 15 (2000) 1235. [4] N.H. Babu, K. Iida, Y. Shi, D.A. Cardwell, Appl. Phys. Lett. 87 (2005) 202506. [5] J.P. Mathieu, T. Koutzarova, A. Rulmont, J.F. Fagnard, P. Laurent, B. Mattivi, P. Vanderbemden, M. Ausloos, R. Cloots, Supercond. Sci. Technol. 18 (2005) S136. [6] K. Iida, N.H. Babu, Y. Shi, D.A. Cardwell, Supercond. Sci. Technol. 18 (2006) S478. [7] J.G. Noudem, S. Meslin, D. Horvath, C. Harnois, D. Chateigner, B. Ouladdiaf, S. Eve, M. Gomina, X. Chaud, M. Murakami, J. Am. Ceram. Soc. 90 (2007) 2784. [8] G.Z. Li, W.M. Yang, X.F. Cheng, J. Fan, X.D. Guo, J. Mater. Sci. 44 (2009) 6423. [9] N.H. Babu, Y. Shi, K. Iida, D.A. Cardwell, Nature Mater. 4 (2005) 476. [10] W.M. Yang, L. Zhou, Y. Feng, P.X. Zhang, C.P. Zhang, J. Alloy. Compd. 415 (2006) 276. [11] W.M. Yang, X.X. Chao, Z.B. Shu, S.H. Zhu, X.L. Wu, X.B. Bian, P. Liu, Physica C 445–448 (2006) 347. [12] http://rsbweb.nih.gov/ij/. [13] A. Endo, H.S. Chauhan, T. Egi, Y. Shiohara, J. Mater. Res. 11 (1996) 795. [14] R. Cloots, T. Koutzarova, J.P. Mathieu, M. Ausloos, Supercond. Sci. Technol. 18 (2005) R9. [15] C.J. Kim, G.W. Hong, Supercond. Sci. Technol. 12 (1999) R27. [16] T. Izumi, Y. Nakamura, Y. Shiohara, J. Mater. Res. 8 (1993) 1240. [17] S.P. Athur, V. Selvamanickam, U. Balachandran, K. Salama, J. Mater. Res. 11 (1996) 2976. [18] C.J. Kim, I.H. Kuk, G.W. Hong, T.H. Sung, S.C. Han, J.J. Kim, Mater. Lett. 34 (1998) 392. [19] C.J. Kim, K.B. Kim, G.W. Hong, J. Mater. Res. 10 (1995) 1605. [20] M. Muralidhar, M. Murakami, Supercond. Sci. Technol. 15 (2002) 683. [21] K. Iida, N.H. Babu, Y.H. Shi, D.A. Cardwell, M. Murakami, Supercond. Sci. Technol. 19 (2006) 641. [22] M. Oda, X. Yao, Y. Yoshida, H. Ikuta, Supercond. Sci. Technol. 22 (2009) 075012.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Synthesis and characterization of large single-domain Gd–Ba–Cu–O bulk superconductor material using a modified IG process Guo-Zheng Li ∗ , Wan-Min Yang Department of Physics, Shaanxi Normal University, Shida Lu No. 1, Xi’an, Shaanxi, 710062, China
a r t i c l e
i n f o
Article history: Received 27 March 2010 Received in revised form 11 July 2010 Accepted 17 July 2010 Keywords: Superconductors Crystal growth Superconductivity
a b s t r a c t Large single-domain Gd–Ba–Cu–O bulk material was successfully fabricated by a modified infiltration and growth process (IG) using a liquid source composed of Gd2 O3 , BaCuO2 and CuO. The growth character, microstructure, and performance of the GdBCO product were investigated. The results indicate that, this modified IG technique can be used to fabricate well-textured and high performance GdBCO large single domains more efficiently. Moreover, an increasing number of small Gd2 BaCuO5 (Gd-211) particles and coarsening of large Gd-211 particles were both observed in the microstructure with increasing distance from the seed, which together are jointly responsible for the increasing volume fraction of Gd-211 particles during the GdBa2 Cu3 O7−x (Gd-123) growth process. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Recently, the infiltration and growth process (IG), used for fabrication of single-domain RE–Ba–Cu–O (REBCO) bulk material, has attracted considerable attention because of a number of advantages compared with the conventional melt growth method (MG). These include fewer pores, cracks and distortions, insignificant shrinkage in the final product and especially a homogeneous distribution of fine RE2 BaCuO5 (RE-211) particles in the REBa2 Cu3 O7−x (RE-123) matrix [1–7]. For this process, a RE-211 preform bulk was placed on top of one liquid source pellet (Ba and Cu rich liquid phase) at room temperature initially, then, once molten, the liquid phase infiltrated up into the porous RE-211 compact at an elevated temperature, which then reacted with the RE-211 phase to form RE-123 on cooling, described as: RE2 BaCuO5 + (3BaCuO2 + 2CuO) → 2REBa2 Cu3 O7−x
particles decomposed from the starting RE-123 phase at high temperature, so it could be seen that three kinds of precursor powders, involving RE-211, RE-123, and BaCuO2 , have to be prepared for the IG process. In Ref. [8], a mixture of Gd2 O3 , BaCuO2 , and CuO has been reported to replace the mixture of Gd-123 and Ba3 Cu5 O8 as the liquid source to fabricate single-domain Gd–Ba–Cu–O (GdBCO) bulk superconductor material of 20 mm diameter. For this process, only two precursor powders, namely Gd-211 and BaCuO2 , are needed to be prepared for the whole IG flow, thus the experimental process is simplified. In this article, large single-domain GdBCO bulk material of 30 mm in diameter was fabricated successfully using this modified IG process. The growth character, microstructure, and performance of the GdBCO product were investigated in detail.
(1) 2. Experimental
It is apparent that the Ba and Cu rich liquid phase, with a nominal composition of Ba3 Cu5 O8 , is the exact phase needed for reaction with the RE-211 phase. However, the heavy loss of liquid and the collapse of the sample have been observed when the Ba3 Cu5 O8 powder (035, i.e., a mixture of BaCuO2 and CuO powders in a molar ratio of BaCuO2 :CuO = 3:2) was employed as the liquid source, especially when the liquid source was supplied rather richly. As a result, a mixture of RE-123 and Ba3 Cu5 O8 is usually used as the liquid source in the IG process [1,6,7], and the above two processing difficulties can be effectively avoided with the support of solid RE-211
∗ Corresponding author. Tel.: +86 29 85302598. E-mail address: [email protected] (G.-Z. Li). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.07.047
The single phase precursor powders consisting of Gd-211 and BaCuO2 were prepared by a conventional solid-state reaction method in air using the raw materials of Gd2 O3 , BaCO3 and CuO. The powders of Gd-211, together with an addition of 2 wt.% CeO2 , were well mixed using a ball milling machine and then pressed into pellets of 30 mm diameter in batches of 30 g. The powders of Gd2 O3 , CuO, and BaCuO2 were weighed according to the molar ratio Gd2 O3 :CuO:BaCuO2 = 1:6:10 [8], mixed thoroughly using a ball milling machine and then pressed into pellets of 30 mm diameter in batches of 35 g, to act as the liquid source for the IG process. Furthermore, Yb2 O3 powder in batches of 4.5 g was pressed into a plate of 2 mm in thickness to be used to support the liquid phase at the elevated temperature. The Gd-211 green compact was placed on top of the liquid source pellet, which, in turn, was placed on the liquid support plate. The entire arrangement was then mounted on a ring of MgO single crystals, which are all the same height and rested on an alumina plate. Finally, a Mg-doped NdBCO seed crystal [9] of dimensions 3 mm × 3 mm × 2 mm was placed on the top surface of the Gd-211 green compact
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Fig. 2. The top surface morphology of the GdBCO sample grown by the modified IG technique.
Fig. 1. (a) The arrangement of the sample prior to infiltration and growth and (b) heat treatment pattern for GdBCO crystal growth used in this study.
with the ab-plane parallel to the surface, as shown in Fig. 1(a). Then the sample stack is put in a tube furnace with a positive radial temperature gradient, which effectively prevents the random nucleation of GdBCO grains at the edge of the sample [10].The sample stack was heated to 800 ◦ C at a rate of 160 ◦ C/h, and held for 10 h, then heated further to 1065 ◦ C at a rate of 60 ◦ C/h, held for 1 h, cooled to 1035 ◦ C at a rate of 60 ◦ C/h, then cooled slowly to 1015 ◦ C at a rate of 0.2 ◦ C/h, and finally furnacecooled to room temperature, as shown schematically in Fig. 1(b). After completion of the IG process, the single-domain material was annealed in flowing oxygen for 200 h at temperatures ranging from 430 ◦ C to 350 ◦ C.
3. Results and discussion Fig. 2 shows the top surface morphology of one typical GdBCO sample of 30 mm in diameter fabricated using the modified IG process. As can be seen in the figure, the sample exhibited clear four-fold growth sector boundaries on the top surface, indicating that it was grown in the form of a single domain. In addition, no spontaneous satellite grains were observed in the sample. As a result, it can be concluded that the modified IG technique can be used to fabricate well-textured large GdBCO single-domain material. The levitation force of the sample was measured under a zerofield cooling state at 77 K using a home-made device [11]. A magnet (Ø 25 mm) with a surface field of 0.5 T was used in the levitation force measurement. The maximum levitation force measured in this experiment was achieved at the smallest gap (0.1 mm) between the two nearest surfaces of the sample and the magnet. The levitation force values versus distance from the magnet for the sample are shown in Fig. 3. As shown in the figure, the maximum levitation force of the sample reached 63.9 N, about 9 N/cm2 . This result indicates that, based on the new liquid source, the singledomain GdBCO bulk fabricated by the modified IG technique has a satisfactory performance and therefore can be used for various applications.
Fig. 3. The levitation forces versus distance at 77 K for the single-domain GdBCO bulk sample.
In order to study the microstructure of the GdBCO bulk sample, three small specimens were cut from the locations shown schematically in Fig. 4. Scanning electron micrographs (SEM) were taken at the centre of the polished top surface of each specimen, and the results are shown in Fig. 5. It can be seen from the figure that,
Fig. 4. A schematic drawing of the locations where the small specimens were taken from the bulk sample.
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G.-Z. Li, W.-M. Yang / Materials Chemistry and Physics 124 (2010) 936–939 Table 1 The average size and volume fraction of Gd-211 second phase particles in Gd-123 matrix for each specimen, analyzed using the image processing software and assuming that the shape of the Gd-211 particles are spherical. Small specimens
Volume fraction of Gd-211 particles (%)
Average size of Gd-211 particles (m)
A B C
25.5 32.3 42.5
2.055 1.864 1.828
volume fraction of Gd-211 particles, Vf211 , increases with increasing distance from the seed, and reaches a value of 42.5% for Specimen C. This phenomenon is usually interpreted by the established pushing/trapping theory of foreign particles during solidification of RE-123 crystal [1,6,13,14]. According to this theory, the distribution of second phase particles is expected to vary as a function of growth rate, and the critical particle radius for trapping, r*, is given by the following equation: R∝
Fig. 5. SEM micrographs of the GdBCO sample grown by the modified IG technique, which were obtained at the locations A, B, and C as shown in Fig. 4.
fine Gd-211 second phase particles are uniformly distributed in the Gd-123 matrix for each specimen. The average size and volume fraction of Gd-211 particles for each specimen were analyzed qualitatively using an image processing software package (ImageJ) [12], and the results are contained in Table 1. It can be seen that the
0 r∗
(2)
where R is growth rate, 0 is the net of the interfacial energy and is the melt viscosity. Hence, during the slow-cooling growth process for one bulk from the seed, with an increase of the undercooling (T), the critical radius for trapping decreases as the growth rate increases, thus more and more smaller RE-211 particles can be trapped, which finally leads to an increase of Vf211 with increasing distance from the seed and a decrease of the average size of RE-211 particles, d211 . In other words, the increase of Vf211 was considered to be a result of the lowering threshold for the entrapment of small particles. However, Fig. 5 of this study indicates that the present situation is not consistent with that analysis entirely. From Fig. 5, the increasing number of small particles with increasing distance from the seed, i.e. from position A to position C, was indeed observed clearly, as well as an increase in the total particle number, which is consistent with the known pushing/trapping theory. However, the number of large particles, and the size of the largest particles were also observed to increase gradually. Take the largest particles, for example, in Fig. 5(a), the size of largest particles was found to be about 4 m, but in Fig. 5(b), this value increased to more than 5 m, and in Fig. 5(c), even particles with a size of 8 m were observed. Furthermore, Fig. 5(c) also exhibited significant particle coalescence (or connection). This situation indicates that the particles trapped in the later growth stage should have suffered a coarsening process. And the coarsening of large particles together with the broader entrance for small particles are jointly responsible for the increase of Vf211 in the Gd-123 matrix, and finally result in the rather high Vf211 value observed in Specimen C. On the other hand, coarsening of large particles has the effect of increasing the average size of the particles, which is opposite to the entrapment phenomenon of small particles. As a result, due to the influence of these two competitive effects, only a slight reduction in d211 was observed for Specimen C compared with Specimen B (Table 1). In addition, connected particles were counted as a single entity during the image processing, which also has the effect of increasing the d211 value of Specimen C. For typical crystal growth from a seed, it is well known that, the ungrown region near the edge of the compact will be always kept molten during the RE-123 growth process starting from the center (corresponding to the formation and directional solidification of RE-123 phase). According to our experience, the edge area will have been molten for at least 40 h before the growth front reaches the outside diameter. In terms of the acknowledged Ostwald ripening theory, the solid RE-211 particles are easily coarsened in molten liquid at high temperature, in which the smaller RE-211 particles
G.-Z. Li, W.-M. Yang / Materials Chemistry and Physics 124 (2010) 936–939
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4. Conclusions Large GdBCO single domains with a diameter of 30 mm have been fabricated in air by a modified IG technique. After detailed investigations of the growth character, microstructure, and superconducting properties of the grown sample, it can be concluded that the modified IG technique can be used to fabricate well-textured large GdBCO single-domain material efficiently. Additionally, increasing numbers of small Gd-211 particles together with the coarsening of large Gd-211 particles have been observed in the microstructure with increasing distance from the seed. Acknowledgments This work was supported by The National High Technology Research and Development Program of China (“863” No. 2007AA03Z241), The National Natural Science Foundation of China (No. 50872079), and The Fundamental Research Funds for the Central Universities (No. 2010ZYGX021; No. GK200901017). Fig. 6. Resistivity versus temperature plot of the GdBCO sample fabricated by the modified IG process in air.
are dissolved in the melt and the dissolved ions migrate to grow the biggest particles [14,15]. Thus the coarsening phenomenon of large particles observed in Fig. 5(c) can be easily understood based on this theory, because the material associated with Specimen C was held liquid for a longer time than that for the other sections. Whereas, compared with the RE-211 coarsening behavior observed in the sample, which was always melted at a temperature above the peritectic temperature (Tp ) of RE-123 [16–18], the coarsening degree of RE-211 in the sample during its slow-cooling growth process observed in this study seems much slighter. This observation could be anticipated by the peritectic solidification growth mechanism of the RE-123 crystal, because during the bulk growth process a large amount of ions dissolved in liquid were consumed by the peritectic reaction [19], and finally created RE-123 matrix over 50%, in volume fraction, of the bulk. The measurement of resistivity versus temperature was performed on Specimen C, and the result is shown in Fig. 6. From this figure, a sharp superconducting transition with an onset temperature, Tc , of 91.5 K and a transition width of 2 K was observed, confirming that the formation of Gd1+x Ba2−x Cu3 O7−␦ -type solidsolution had been suppressed effectively. This result is comparable to the Tc values of GdBCO bulk superconductors prepared under reduced oxygen partial pressure [20–22], and proves the feasibility of processing GdBCO samples in air.
References [1] K. Iida, N.H. Babu, Y. Shi, D.A. Cardwell, Supercond. Sci. Technol. 18 (2005) 1421. [2] E.S. Reddy, T. Rajasekharan, Supercond. Sci. Technol. 11 (1998) 523. [3] N.H. Babu, M. Kambara, P.J. Smith, D.A. Cardwell, Y. Shi, J. Mater. Res. 15 (2000) 1235. [4] N.H. Babu, K. Iida, Y. Shi, D.A. Cardwell, Appl. Phys. Lett. 87 (2005) 202506. [5] J.P. Mathieu, T. Koutzarova, A. Rulmont, J.F. Fagnard, P. Laurent, B. Mattivi, P. Vanderbemden, M. Ausloos, R. Cloots, Supercond. Sci. Technol. 18 (2005) S136. [6] K. Iida, N.H. Babu, Y. Shi, D.A. Cardwell, Supercond. Sci. Technol. 18 (2006) S478. [7] J.G. Noudem, S. Meslin, D. Horvath, C. Harnois, D. Chateigner, B. Ouladdiaf, S. Eve, M. Gomina, X. Chaud, M. Murakami, J. Am. Ceram. Soc. 90 (2007) 2784. [8] G.Z. Li, W.M. Yang, X.F. Cheng, J. Fan, X.D. Guo, J. Mater. Sci. 44 (2009) 6423. [9] N.H. Babu, Y. Shi, K. Iida, D.A. Cardwell, Nature Mater. 4 (2005) 476. [10] W.M. Yang, L. Zhou, Y. Feng, P.X. Zhang, C.P. Zhang, J. Alloy. Compd. 415 (2006) 276. [11] W.M. Yang, X.X. Chao, Z.B. Shu, S.H. Zhu, X.L. Wu, X.B. Bian, P. Liu, Physica C 445–448 (2006) 347. [12] http://rsbweb.nih.gov/ij/. [13] A. Endo, H.S. Chauhan, T. Egi, Y. Shiohara, J. Mater. Res. 11 (1996) 795. [14] R. Cloots, T. Koutzarova, J.P. Mathieu, M. Ausloos, Supercond. Sci. Technol. 18 (2005) R9. [15] C.J. Kim, G.W. Hong, Supercond. Sci. Technol. 12 (1999) R27. [16] T. Izumi, Y. Nakamura, Y. Shiohara, J. Mater. Res. 8 (1993) 1240. [17] S.P. Athur, V. Selvamanickam, U. Balachandran, K. Salama, J. Mater. Res. 11 (1996) 2976. [18] C.J. Kim, I.H. Kuk, G.W. Hong, T.H. Sung, S.C. Han, J.J. Kim, Mater. Lett. 34 (1998) 392. [19] C.J. Kim, K.B. Kim, G.W. Hong, J. Mater. Res. 10 (1995) 1605. [20] M. Muralidhar, M. Murakami, Supercond. Sci. Technol. 15 (2002) 683. [21] K. Iida, N.H. Babu, Y.H. Shi, D.A. Cardwell, M. Murakami, Supercond. Sci. Technol. 19 (2006) 641. [22] M. Oda, X. Yao, Y. Yoshida, H. Ikuta, Supercond. Sci. Technol. 22 (2009) 075012.