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Fabrication of bulk Y–Ba–Cu–O superconductors with high critical current densities through the infiltration-growth process
Cryogenics xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Cryogenics journal homepage: www.elsevier.com/locate/cryogenics
Fabrication of bulk Y–Ba–Cu–O superconductors with high critical current densities through the infiltration-growth process K. Nakazato a, M. Muralidhar a,⇑, M.R. Koblischka a,b, M. Murakami a a b
Superconducting Materials Laboratory, Department of Materials Science and Engineering, Shibaura Institute of Technology, 3-7-5 Toyosu, Koto-ku, Tokyo 135-8548, Japan Institute of Experimental Physics, Saarland University, P.O. Box 151150, D-66041 Saarbrücken, Germany
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: Infiltration-growth Microstructure analysis High critical current density (Jc) Trapped field measurements
a b s t r a c t With the aim of producing bulk YBa2Cu3Oy (Y123) superconductors with high flux pinning performance, we employed an infiltration growth (IG) process, in which liquid phase Ba–Cu–O is infiltrated into Y2BaCuO5 (Y211) pellets with NdBa2Cu3Oy seed at high temperatures to produce Y123 phase. Single grain Y123 samples 27 mm in diameter and 5 mm in thickness could be produced with the seeded IG method. Trapped field measurements showed that the IG-processed Y123 sample was a single domain with the maximum trapped field of around 0.403 T at 1 mm above the surface at 77 K. Magnetization measurements demonstrated that IG-processed Y123 sample exhibits a sharp superconducting transition with an onset Tc of around 93.2 K. Microstructural observations by means of atomic force microscopy (AFM) clarified that sub-micrometer-sized Y211 particles are finely and uniformly distributed in the Y123 matrix, which is the source of high pinning performances. The critical current density (Jc) values at liquid nitrogen temperature (77 K) and 50 K in 0 T were 1.75 105 A/cm2 and 5.28 105 A/cm2, respectively. These values are the highest among bulk Y123 samples ever reported in the literature. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Bulk Y123 superconductors can trap magnetic fields over 17 T at 29 K and 10 T at 45 K [1], and thus are attractive for various high field applications The field-trapping capability of bulk Y123 is given by the equation M = AJcr, where M is the magnetization or the trapped field, A is a geometric constant, Jc is the critical current density and r is the diameter of the circulating super-current. Hence, one needs to increase Jc or the grain size (r) of bulk superconductors to enhance field-trapping abilities. A variety of meltprocessing techniques have been developed over years in order to fabricate large grain REBa2Cu3Oy (RE: rare earth elements) materials with high Jc values [2–5]. As a result, a single-grain Y123 superconductor 10 cm in diameter was produced by adopting the top-seeded melt-growth technique. The Jc values depend on the microstructure and can therefore be enhanced through microstructural control [6]. The introduction of non-superconducting secondary RE211 particles in the RE123 matrix was found to be effective in enhancing the flux pinning performances of bulk REBa2Cu3Oy materials. Furthermore, the size reduction of such non-superconducting particles also led to flux pinning enhancement [7]. The
⇑ Corresponding author. Tel.: +81 3 5859 9378; fax: +81 3 5859 7311. E-mail address: [email protected] (M. Muralidhar).
secondary phase refinement through milling with ZrO2 balls and its beneficial effects on the pinning performances have been reported in several high Tc bulk materials like Y123, GdBa2Cu3Oy, (Nd, Eu, Gd)Ba2Cu3Oy, (Sm, Eu, Gd)Ba2Cu3Oy [8–11]. Furthermore, fine non-superconducting Y2Ba4CuMOy (Y2411) particles were successfully embedded in the Y123 superconducting matrix [12]. The advantages of the infiltration-growth (IG) technique are the fabrication of near-net-shaped products with negligible shrinkage, and uniform and fine microstructure [13,14]. Here, the refinement of the Y211 particles appears to be achieved more effectively without controlling the initially added Y211 secondary phase, or without refining the Y211 particles by Pt or CeO2 additions [15]. The refinement of the Y211 secondary phase particles in IG processed Y123 is attributed to relatively short holding time at high temperatures [16]. On the other hand, the suppressed pore formation is attributed to reduced oxygen evolution during melting, which is caused by the use of a liquid forming Ba–Cu–O powder (Ba:Cu = 3:5). In addition, the porosity of the final Y123 products could be reduced by adopting the dense Y211 pre-form [16]. Bulk Y123 superconductors fabricated with the IG method possess a fine microstructure with uniform Y211 distribution and fewer cracks, and the density is higher than those fabricated with conventional melt processes due to smaller porosities. It is also possible to minimize the reaction of bulk precursor with the substrate material in a half molten state. Hence, the IG-processed
http://dx.doi.org/10.1016/j.cryogenics.2014.04.003 0011-2275/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Nakazato K et al. Fabrication of bulk Y–Ba–Cu–O superconductors with high critical current densities through the infiltration-growth process. Cryogenics (2014), http://dx.doi.org/10.1016/j.cryogenics.2014.04.003
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K. Nakazato et al. / Cryogenics xxx (2014) xxx–xxx
Y123 materials exhibit Jc values higher than those fabricated with the conventional melt process [12–16]. Recent reports also clarified that further improvements in the performances of IG-processed Y123 materials are possible by controlling the particle size of initial Y211 powders and densifying the sintered body [16]. The quality and performances of IG-processed bulk Y123 superconductors can also be further improved by optimizing the processing conditions. The aim of the present work is to prepare single grain Y123 materials with high pinning performance. In the present study, isothermal growth techniques were first used to optimize the growth conditions in the IG processes for bulk Y123 materials. We then designed the optimum thermal pattern for the IG process, and prepared bulk Y123 27 mm in diameter and 5 mm in thickness and characterized the superconducting properties, the microstructure, and the magnetic performances. 2. Experimental details In order to synthesize Y211 precursor power, high-purity commercial powders of Y2O3, BaCO3 and CuO were mixed in a nominal ratio of Y:Ba:Cu = 2:1:1 and calcined at 900 °C for 24 h. In parallel, the powders of BaO2, and CuO were mixed in a nominal ratio of Ba:Cu = 3:5. Y211 powders were pressed into precursor pellets 20 and 30 mm in diameter and 5 mm in thickness through isostatic pressing. These pre-forms of Y211 were placed on Ba3Cu5O and then a Nd123 seed was placed on top of the Y211 pre-form for the growth of single Y123 grain. The Yb2O3 powder plate and a MgO single crystal plate were used to suppress the liquid loss and spontaneous Y123 nucleation. Fig. 1 shows schematic illustration for the assembly used in the top-seeded IG process. First, the samples 20 mm in diameter were subjected to the isothermal IG process at 990, 985, 983, 980 and 978 °C for 25 h. Based on the observation of the growth mode of isothermally treated IG samples, the thermal patterns for the growth of bulk Y123 materials were determined. After the IG process, the samples were annealed at 450 °C for 150 h in flowing pure O2 gas for oxygenation. The microstructure of the IG-processed Y123 samples was studied with an optical microscope and an atomic force microscope (AFM) operating in the tapping mode. The IG-processed samples were subjected to trapped field measurements for sample characterization. The samples were fieldcooled by liquid nitrogen in the presence of the magnetic field of a Nd–Fe–B magnet with a surface flux density of 0.5 T. After removing the external field, the trapped fields were measured by scanning a Hall sensor located at 1 mm above the sample surface. Small test specimens with dimensions of 1.5 1.5 0.5 mm3 were cut from the bulk samples and subjected to the measurements of the critical temperature (Tc) and magnetization hysteresis loops (M–H loops) in fields from –1 to +5 T at 77 K using a SQUID magnetometer (Quantum Design, model MPMS5). Jc values were estimated using the extended Bean’s critical state model for a rectangular sample [17].
Fig. 1. A typical arrangement used in the IG experiment. A source of liquid phase (Ba–Cu–O) is supported on a chemically inert, impervious substrate. A perform fabricated out of Y211 powders is kept on top of it. Nd123 seed crystal is placed on top of the pre-form.
3. Results and discussion To prepare single-grain Y123 samples, it is important to study the optimum growth temperature for the IG process. The samples were then isothermally heated at growth temperatures in the range of 970–995 °C with the same holding time of 25 h. Fig. 2 shows the top views of bulk Y123 superconductors isothermally grown at 995, 990, 980, and 970 °C for 25 h with the IG process setup shown in Fig. 1. One can see that single-grain growth without spontaneous nucleation was achieved at 980 °C. On the other hand, the growth rate was very small and multi-nucleation was observed in the samples grown at 970 and 995 °C. The experimental results implied that 980 °C was the optimum temperature to grow single grain Y–Ba–Cu–O without secondary nucleation for the IG process. Based on these results, we designed the thermal pattern for the IG process as given below and produced the large size single grain Y123 bulk material as illustrated in Fig. 3. The samples were heated at a rate of 100 °C/h to a temperature of 880 °C and held for 15 min and again heated to the temperature of 1040 °C in 5 h and held there for 50 min. Then, the temperature was lowered to 1000 °C in 60 min, and slowly cooled to 975 °C at a rate of 0.3 °C/h. Finally, the temperature was reduced with a cooling rate of 100 °C/h to 100 °C and then the furnace was left to cool down to room temperature. Fig. 4 shows the trapped field distribution for the IG-processed Y123 sample. One can notice a cone-like field distribution with a single peak of 0.403 T, showing that the sample is a single grain without any appreciable defects including cracks. The maximum trapped field value was increased to 0.45 T when measured directly on the bulk surface. These results demonstrate that the quality of IG-processed Y–Ba–Cu–O is high.
Fig. 2. Top view of Y-123 bulk superconductors grown by IG, Isothermal solidification in air. (a) 25 h at 970 °C; (b) 25 h at 980 °C; (c) 25 h at 990 °C; and (d) 25 h at 995 °C. Note that the maximum grain growth without multi-nucleations occurs at 980 °C.
Please cite this article in press as: Nakazato K et al. Fabrication of bulk Y–Ba–Cu–O superconductors with high critical current densities through the infiltration-growth process. Cryogenics (2014), http://dx.doi.org/10.1016/j.cryogenics.2014.04.003
K. Nakazato et al. / Cryogenics xxx (2014) xxx–xxx
Fig. 3. Top views of the single grain Y–Ba–Cu–O bulk superconductor grown by the IG process with continuous slow cooling.
Fig. 4. The trapped field profile at 77 K for the 27 mm diameter Y–Ba–Cu–O single grain sample processed in air by the IG method using Nd123 as a seed crystal. Note that the maximum observed value of the trapped field reached 0.41 T at 1 mm above the sample surface.
Fig. 5 displays the temperature dependence of the dc magnetic moment for the IG processed Y123 sample measured in the zerofield-cooled (ZFC) and in an applied magnetic field of 1 mT (FC). The sample exhibited a sharp superconducting transition (around 1 K width) with an onset Tc around 93.2 K. These results clearly indicate that sample quality is very good, similar to the earlier results [18]. The Jc values at 77 K, 83 K, and 90 K with the field applied parallel to the c-axis were calculated from the M–H curves using the
Fig. 5. Temperature dependence of the DC magnetic moment for the single-grain IG-processed Y–Ba–Cu–O superconductor prepared in air.
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Fig. 6. Field dependence of the critical current density (Jc) at T = 77 K, 83 K, and 90 K with H//c-axis for the IG-processed, single domain Y–Ba–Cu–O superconductor prepared in air by cold seeding method with a Nd-123 seed crystal.
extended Bean model formula, and shown in Fig. 6. Jc values at 77 K, 83 K, and 90 K in self field were 1.75 105 A/cm2, 95 104 A/cm2 and 2.4 104 A/cm2, respectively. At all temperatures, the obtained Jc values are very high as compared to earlier reports in the literature. Fig. 7 shows the field dependence of the critical current density at lower temperatures. Here, the Jc values at lower temperatures of 50 K, 60 K and 70 K in 2 T for H//c-axis were 5.28 105 A/cm2, 3.17 105 A/cm2 and 1.52 105 A/cm2, respectively. It is notable that the high critical current densities on the level of 5 105 A/cm2 in the field of 0 to 3 T were achieved at 50 K for the first time in the Y–Ba–Cu–O system. It is interesting to note that the Jc–B curve for the IG-processed Y123 is flat in the field range of 1–3 T even at 77 K, similar to the LRE123 (LRE: light rare earth) material [19]. Such high critical current behavior has been observed in Y–Ba–Cu–O system having nano-scale defects introduced through neutron irradiation [20]. Furthermore, the present Jc values are exceeding those recently reported in the Y– Ba–Cu–O with nanometer-sized Y2411 [12]. These results clearly demonstrate that the IG-processed Y123 materials have a great potential for several industrial applications. To clarify the source of higher pinning performances at 77 K in high magnetic fields, microstructural observations were carried out using optical microscopy and atomic force microscopy (AFM).
Fig. 7. Field dependence of the critical current density (Jc) at T = 50 K, 60 K, and 70 K with H//c-axis for the IG-processed single-domain Y–Ba–Cu–O superconductor.
Please cite this article in press as: Nakazato K et al. Fabrication of bulk Y–Ba–Cu–O superconductors with high critical current densities through the infiltration-growth process. Cryogenics (2014), http://dx.doi.org/10.1016/j.cryogenics.2014.04.003
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studies with atomic force microscopy operating in the tapping mode, and the results of the topography measurements are presented in Fig. 9. It is evident that the size of the secondary phase particles in the Y123 matrix is sub-micrometer. The presence of small particles with a size of 100–200 nm is clearly revealed, which is not detectable with optical microscopy. These results again imply that high pinning performance of the IG-processed Y123 materials is due to fine microstructure with small Y211 distribution. 4. Conclusions We have succeeded in growing high performance single-grain Y123 superconductors by infiltrating Ba–Cu–O (Ba:Cu = 3:5) liquid source into the Y211 pre-form. The trapped field measurements gave a direct proof that the samples 27 mm in diameter and 5 mm thick are single domain with the maximum trapped field of around 0.45 T at the surface at 77 K. According to magnetization measurements, the IG sample showed a sharp superconducting transition with onset Tc of around 93.2 K. The Jc value at 77 K and 0 T was 1.75 105 A/cm2, which is the highest value reported so far in the literature for Y123 materials. In addition, the critical current densities at 50 K and 70 K and in zero field were 5.7 105 A/ cm2 and 2.3 105 A/cm2, respectively. AFM observations clearly showed that sub-micrometer-sized Y211 particles are uniformly distributed in the Y123 matrix, which is responsible for high Jc values. References [1] [2] [3] [4] Fig. 8. Optical micrographs of single grain Y–Ba–Cu–O bulk superconductor grown by the IG-process with continuous cooling method. Extremely fine and uniform Y211 phase particles are distributed in Y123 matrix.
[5] [6] [7]
Fig. 8 shows optical micrographs for a single-grain Y123 fabricated with the top-seeded IG method in a slow cooling process. It is clear that secondary phase Y211 particles are uniformly distributed in the Y123 matrix. One can also observe sub-micrometer-sized particles in the micrograph viewed with higher magnification. These results are similar to the Y2411 phase particle dispersion in the Y123 material fabricated by IG process [12]. To gain a clear picture of the sub-micrometer-sized particles, we further extended our
[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
Tomita M, Murakami M. Nature 2003;421:517–9. Salama K, Lee FD. Supercond Sci Technol 1994;7:177–93. Cardwell DA. Mater Sci Eng B 1998;53:1–10. Murakami M, Sakai N, Higuchi T, Yoo SI. Supercond Sci Technol 1996;9:1015–32. Muralidhar M, Sakai N, Nishiyama T, Jirsa M, Machi T, Murakami M. Appl Phys Lett 2003;82:943–5. Diko P, Christian W, Doris L, Thomas K, Wolfgang G. Supercond Sci Technol 1998;11:49–53. Muralidhar M, Sakai N, Jirsa M, Koshizuka N, Murakami M. Appl Phys Lett 2003;83:5005–7. Nariki S, Sakai N, Murakami M, Hirabayashi I. Physica C 2004;412– 413:557–60. Nariki S, Seo S, Sakai N, Murakami M. Supercond Sci Technol 2000;13:778–84. Muralidhar M, Sakai N, Jirsa M, Murakami M, Koshizuka N. Supercond Sci Technol 2003;16:L46–8. Muralidhar M, Sakai N, Jirsa M, Koshizuka N, Murakami M. Appl Phys Lett 2004;85:3504–6. Hari Babu N, Shi YH, Dennis AR, Pathak SK, Cardwell DA. IEEE Trans Appl Supercond 2011;21:2698–701. Sudhakar Reddy E, Rajasekharan. Supercond Sci Technol 1998;11:523–34. Cao H, Moutalbi N, Harnois C, Hu R, Li J, Zhou L, et al. Physica C 2010;470:68–74. Pavan Kumar Naik S , Devendra Kumar N, Missak Swarup Raju P, Rajasekharan T, Seshubai V. Physica C 2013;487:72–6. Mahmood A, Jun B-H, Han YH, Kim C-J. Supercond Sci Technol 2010;23:065005. 7pp. Chen DX, Goldfarb RB. J Appl Phys 1989;66:2489–500. Mahmood A, Chu YS, Sung TH. Supercond Sci Technol 2012;25:045008. 7pp. Muralidhar M, Chauhan HS, Saitoh T, Kamada K, Segawa K, Murakami M. Supercond Sci Technol 1997;10:663–8. Sauerzopf MF, Wiesinger HP, Weber HW, Crabtree GW. Phys Rev B 1995;51:6002–12.
Fig. 9. Atomic force microscope (AFM) topography picture (tapping mode) for the IG-processed Y–Ba–Cu–O. Note that nanometer-sized Y211 inclusions are finely dispersed in the Y123 matrix.
Please cite this article in press as: Nakazato K et al. Fabrication of bulk Y–Ba–Cu–O superconductors with high critical current densities through the infiltration-growth process. Cryogenics (2014), http://dx.doi.org/10.1016/j.cryogenics.2014.04.003
Contents lists available at ScienceDirect
Cryogenics journal homepage: www.elsevier.com/locate/cryogenics
Fabrication of bulk Y–Ba–Cu–O superconductors with high critical current densities through the infiltration-growth process K. Nakazato a, M. Muralidhar a,⇑, M.R. Koblischka a,b, M. Murakami a a b
Superconducting Materials Laboratory, Department of Materials Science and Engineering, Shibaura Institute of Technology, 3-7-5 Toyosu, Koto-ku, Tokyo 135-8548, Japan Institute of Experimental Physics, Saarland University, P.O. Box 151150, D-66041 Saarbrücken, Germany
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: Infiltration-growth Microstructure analysis High critical current density (Jc) Trapped field measurements
a b s t r a c t With the aim of producing bulk YBa2Cu3Oy (Y123) superconductors with high flux pinning performance, we employed an infiltration growth (IG) process, in which liquid phase Ba–Cu–O is infiltrated into Y2BaCuO5 (Y211) pellets with NdBa2Cu3Oy seed at high temperatures to produce Y123 phase. Single grain Y123 samples 27 mm in diameter and 5 mm in thickness could be produced with the seeded IG method. Trapped field measurements showed that the IG-processed Y123 sample was a single domain with the maximum trapped field of around 0.403 T at 1 mm above the surface at 77 K. Magnetization measurements demonstrated that IG-processed Y123 sample exhibits a sharp superconducting transition with an onset Tc of around 93.2 K. Microstructural observations by means of atomic force microscopy (AFM) clarified that sub-micrometer-sized Y211 particles are finely and uniformly distributed in the Y123 matrix, which is the source of high pinning performances. The critical current density (Jc) values at liquid nitrogen temperature (77 K) and 50 K in 0 T were 1.75 105 A/cm2 and 5.28 105 A/cm2, respectively. These values are the highest among bulk Y123 samples ever reported in the literature. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Bulk Y123 superconductors can trap magnetic fields over 17 T at 29 K and 10 T at 45 K [1], and thus are attractive for various high field applications The field-trapping capability of bulk Y123 is given by the equation M = AJcr, where M is the magnetization or the trapped field, A is a geometric constant, Jc is the critical current density and r is the diameter of the circulating super-current. Hence, one needs to increase Jc or the grain size (r) of bulk superconductors to enhance field-trapping abilities. A variety of meltprocessing techniques have been developed over years in order to fabricate large grain REBa2Cu3Oy (RE: rare earth elements) materials with high Jc values [2–5]. As a result, a single-grain Y123 superconductor 10 cm in diameter was produced by adopting the top-seeded melt-growth technique. The Jc values depend on the microstructure and can therefore be enhanced through microstructural control [6]. The introduction of non-superconducting secondary RE211 particles in the RE123 matrix was found to be effective in enhancing the flux pinning performances of bulk REBa2Cu3Oy materials. Furthermore, the size reduction of such non-superconducting particles also led to flux pinning enhancement [7]. The
⇑ Corresponding author. Tel.: +81 3 5859 9378; fax: +81 3 5859 7311. E-mail address: [email protected] (M. Muralidhar).
secondary phase refinement through milling with ZrO2 balls and its beneficial effects on the pinning performances have been reported in several high Tc bulk materials like Y123, GdBa2Cu3Oy, (Nd, Eu, Gd)Ba2Cu3Oy, (Sm, Eu, Gd)Ba2Cu3Oy [8–11]. Furthermore, fine non-superconducting Y2Ba4CuMOy (Y2411) particles were successfully embedded in the Y123 superconducting matrix [12]. The advantages of the infiltration-growth (IG) technique are the fabrication of near-net-shaped products with negligible shrinkage, and uniform and fine microstructure [13,14]. Here, the refinement of the Y211 particles appears to be achieved more effectively without controlling the initially added Y211 secondary phase, or without refining the Y211 particles by Pt or CeO2 additions [15]. The refinement of the Y211 secondary phase particles in IG processed Y123 is attributed to relatively short holding time at high temperatures [16]. On the other hand, the suppressed pore formation is attributed to reduced oxygen evolution during melting, which is caused by the use of a liquid forming Ba–Cu–O powder (Ba:Cu = 3:5). In addition, the porosity of the final Y123 products could be reduced by adopting the dense Y211 pre-form [16]. Bulk Y123 superconductors fabricated with the IG method possess a fine microstructure with uniform Y211 distribution and fewer cracks, and the density is higher than those fabricated with conventional melt processes due to smaller porosities. It is also possible to minimize the reaction of bulk precursor with the substrate material in a half molten state. Hence, the IG-processed
http://dx.doi.org/10.1016/j.cryogenics.2014.04.003 0011-2275/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Nakazato K et al. Fabrication of bulk Y–Ba–Cu–O superconductors with high critical current densities through the infiltration-growth process. Cryogenics (2014), http://dx.doi.org/10.1016/j.cryogenics.2014.04.003
2
K. Nakazato et al. / Cryogenics xxx (2014) xxx–xxx
Y123 materials exhibit Jc values higher than those fabricated with the conventional melt process [12–16]. Recent reports also clarified that further improvements in the performances of IG-processed Y123 materials are possible by controlling the particle size of initial Y211 powders and densifying the sintered body [16]. The quality and performances of IG-processed bulk Y123 superconductors can also be further improved by optimizing the processing conditions. The aim of the present work is to prepare single grain Y123 materials with high pinning performance. In the present study, isothermal growth techniques were first used to optimize the growth conditions in the IG processes for bulk Y123 materials. We then designed the optimum thermal pattern for the IG process, and prepared bulk Y123 27 mm in diameter and 5 mm in thickness and characterized the superconducting properties, the microstructure, and the magnetic performances. 2. Experimental details In order to synthesize Y211 precursor power, high-purity commercial powders of Y2O3, BaCO3 and CuO were mixed in a nominal ratio of Y:Ba:Cu = 2:1:1 and calcined at 900 °C for 24 h. In parallel, the powders of BaO2, and CuO were mixed in a nominal ratio of Ba:Cu = 3:5. Y211 powders were pressed into precursor pellets 20 and 30 mm in diameter and 5 mm in thickness through isostatic pressing. These pre-forms of Y211 were placed on Ba3Cu5O and then a Nd123 seed was placed on top of the Y211 pre-form for the growth of single Y123 grain. The Yb2O3 powder plate and a MgO single crystal plate were used to suppress the liquid loss and spontaneous Y123 nucleation. Fig. 1 shows schematic illustration for the assembly used in the top-seeded IG process. First, the samples 20 mm in diameter were subjected to the isothermal IG process at 990, 985, 983, 980 and 978 °C for 25 h. Based on the observation of the growth mode of isothermally treated IG samples, the thermal patterns for the growth of bulk Y123 materials were determined. After the IG process, the samples were annealed at 450 °C for 150 h in flowing pure O2 gas for oxygenation. The microstructure of the IG-processed Y123 samples was studied with an optical microscope and an atomic force microscope (AFM) operating in the tapping mode. The IG-processed samples were subjected to trapped field measurements for sample characterization. The samples were fieldcooled by liquid nitrogen in the presence of the magnetic field of a Nd–Fe–B magnet with a surface flux density of 0.5 T. After removing the external field, the trapped fields were measured by scanning a Hall sensor located at 1 mm above the sample surface. Small test specimens with dimensions of 1.5 1.5 0.5 mm3 were cut from the bulk samples and subjected to the measurements of the critical temperature (Tc) and magnetization hysteresis loops (M–H loops) in fields from –1 to +5 T at 77 K using a SQUID magnetometer (Quantum Design, model MPMS5). Jc values were estimated using the extended Bean’s critical state model for a rectangular sample [17].
Fig. 1. A typical arrangement used in the IG experiment. A source of liquid phase (Ba–Cu–O) is supported on a chemically inert, impervious substrate. A perform fabricated out of Y211 powders is kept on top of it. Nd123 seed crystal is placed on top of the pre-form.
3. Results and discussion To prepare single-grain Y123 samples, it is important to study the optimum growth temperature for the IG process. The samples were then isothermally heated at growth temperatures in the range of 970–995 °C with the same holding time of 25 h. Fig. 2 shows the top views of bulk Y123 superconductors isothermally grown at 995, 990, 980, and 970 °C for 25 h with the IG process setup shown in Fig. 1. One can see that single-grain growth without spontaneous nucleation was achieved at 980 °C. On the other hand, the growth rate was very small and multi-nucleation was observed in the samples grown at 970 and 995 °C. The experimental results implied that 980 °C was the optimum temperature to grow single grain Y–Ba–Cu–O without secondary nucleation for the IG process. Based on these results, we designed the thermal pattern for the IG process as given below and produced the large size single grain Y123 bulk material as illustrated in Fig. 3. The samples were heated at a rate of 100 °C/h to a temperature of 880 °C and held for 15 min and again heated to the temperature of 1040 °C in 5 h and held there for 50 min. Then, the temperature was lowered to 1000 °C in 60 min, and slowly cooled to 975 °C at a rate of 0.3 °C/h. Finally, the temperature was reduced with a cooling rate of 100 °C/h to 100 °C and then the furnace was left to cool down to room temperature. Fig. 4 shows the trapped field distribution for the IG-processed Y123 sample. One can notice a cone-like field distribution with a single peak of 0.403 T, showing that the sample is a single grain without any appreciable defects including cracks. The maximum trapped field value was increased to 0.45 T when measured directly on the bulk surface. These results demonstrate that the quality of IG-processed Y–Ba–Cu–O is high.
Fig. 2. Top view of Y-123 bulk superconductors grown by IG, Isothermal solidification in air. (a) 25 h at 970 °C; (b) 25 h at 980 °C; (c) 25 h at 990 °C; and (d) 25 h at 995 °C. Note that the maximum grain growth without multi-nucleations occurs at 980 °C.
Please cite this article in press as: Nakazato K et al. Fabrication of bulk Y–Ba–Cu–O superconductors with high critical current densities through the infiltration-growth process. Cryogenics (2014), http://dx.doi.org/10.1016/j.cryogenics.2014.04.003
K. Nakazato et al. / Cryogenics xxx (2014) xxx–xxx
Fig. 3. Top views of the single grain Y–Ba–Cu–O bulk superconductor grown by the IG process with continuous slow cooling.
Fig. 4. The trapped field profile at 77 K for the 27 mm diameter Y–Ba–Cu–O single grain sample processed in air by the IG method using Nd123 as a seed crystal. Note that the maximum observed value of the trapped field reached 0.41 T at 1 mm above the sample surface.
Fig. 5 displays the temperature dependence of the dc magnetic moment for the IG processed Y123 sample measured in the zerofield-cooled (ZFC) and in an applied magnetic field of 1 mT (FC). The sample exhibited a sharp superconducting transition (around 1 K width) with an onset Tc around 93.2 K. These results clearly indicate that sample quality is very good, similar to the earlier results [18]. The Jc values at 77 K, 83 K, and 90 K with the field applied parallel to the c-axis were calculated from the M–H curves using the
Fig. 5. Temperature dependence of the DC magnetic moment for the single-grain IG-processed Y–Ba–Cu–O superconductor prepared in air.
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Fig. 6. Field dependence of the critical current density (Jc) at T = 77 K, 83 K, and 90 K with H//c-axis for the IG-processed, single domain Y–Ba–Cu–O superconductor prepared in air by cold seeding method with a Nd-123 seed crystal.
extended Bean model formula, and shown in Fig. 6. Jc values at 77 K, 83 K, and 90 K in self field were 1.75 105 A/cm2, 95 104 A/cm2 and 2.4 104 A/cm2, respectively. At all temperatures, the obtained Jc values are very high as compared to earlier reports in the literature. Fig. 7 shows the field dependence of the critical current density at lower temperatures. Here, the Jc values at lower temperatures of 50 K, 60 K and 70 K in 2 T for H//c-axis were 5.28 105 A/cm2, 3.17 105 A/cm2 and 1.52 105 A/cm2, respectively. It is notable that the high critical current densities on the level of 5 105 A/cm2 in the field of 0 to 3 T were achieved at 50 K for the first time in the Y–Ba–Cu–O system. It is interesting to note that the Jc–B curve for the IG-processed Y123 is flat in the field range of 1–3 T even at 77 K, similar to the LRE123 (LRE: light rare earth) material [19]. Such high critical current behavior has been observed in Y–Ba–Cu–O system having nano-scale defects introduced through neutron irradiation [20]. Furthermore, the present Jc values are exceeding those recently reported in the Y– Ba–Cu–O with nanometer-sized Y2411 [12]. These results clearly demonstrate that the IG-processed Y123 materials have a great potential for several industrial applications. To clarify the source of higher pinning performances at 77 K in high magnetic fields, microstructural observations were carried out using optical microscopy and atomic force microscopy (AFM).
Fig. 7. Field dependence of the critical current density (Jc) at T = 50 K, 60 K, and 70 K with H//c-axis for the IG-processed single-domain Y–Ba–Cu–O superconductor.
Please cite this article in press as: Nakazato K et al. Fabrication of bulk Y–Ba–Cu–O superconductors with high critical current densities through the infiltration-growth process. Cryogenics (2014), http://dx.doi.org/10.1016/j.cryogenics.2014.04.003
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studies with atomic force microscopy operating in the tapping mode, and the results of the topography measurements are presented in Fig. 9. It is evident that the size of the secondary phase particles in the Y123 matrix is sub-micrometer. The presence of small particles with a size of 100–200 nm is clearly revealed, which is not detectable with optical microscopy. These results again imply that high pinning performance of the IG-processed Y123 materials is due to fine microstructure with small Y211 distribution. 4. Conclusions We have succeeded in growing high performance single-grain Y123 superconductors by infiltrating Ba–Cu–O (Ba:Cu = 3:5) liquid source into the Y211 pre-form. The trapped field measurements gave a direct proof that the samples 27 mm in diameter and 5 mm thick are single domain with the maximum trapped field of around 0.45 T at the surface at 77 K. According to magnetization measurements, the IG sample showed a sharp superconducting transition with onset Tc of around 93.2 K. The Jc value at 77 K and 0 T was 1.75 105 A/cm2, which is the highest value reported so far in the literature for Y123 materials. In addition, the critical current densities at 50 K and 70 K and in zero field were 5.7 105 A/ cm2 and 2.3 105 A/cm2, respectively. AFM observations clearly showed that sub-micrometer-sized Y211 particles are uniformly distributed in the Y123 matrix, which is responsible for high Jc values. References [1] [2] [3] [4] Fig. 8. Optical micrographs of single grain Y–Ba–Cu–O bulk superconductor grown by the IG-process with continuous cooling method. Extremely fine and uniform Y211 phase particles are distributed in Y123 matrix.
[5] [6] [7]
Fig. 8 shows optical micrographs for a single-grain Y123 fabricated with the top-seeded IG method in a slow cooling process. It is clear that secondary phase Y211 particles are uniformly distributed in the Y123 matrix. One can also observe sub-micrometer-sized particles in the micrograph viewed with higher magnification. These results are similar to the Y2411 phase particle dispersion in the Y123 material fabricated by IG process [12]. To gain a clear picture of the sub-micrometer-sized particles, we further extended our
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Fig. 9. Atomic force microscope (AFM) topography picture (tapping mode) for the IG-processed Y–Ba–Cu–O. Note that nanometer-sized Y211 inclusions are finely dispersed in the Y123 matrix.
Please cite this article in press as: Nakazato K et al. Fabrication of bulk Y–Ba–Cu–O superconductors with high critical current densities through the infiltration-growth process. Cryogenics (2014), http://dx.doi.org/10.1016/j.cryogenics.2014.04.003