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Physics Procedia 36 (2012) 557 – 562
Superconductivity Centennial Conference
Investigation of microstructure and superconducting properties in the Fe–B magnetic particles doped GdBa2Cu3O7δ /Ag2O and GdBa2Cu3O7-δ/nano–Ag superconducting bulks Kun Xu*, Difan Zhou, Beizhan Li, Shogo Hara, Mitsuru Izumi Laboratory of Applied Physics, Tokyo University of Marine Science and Technology, 2-1-6 Etchujima, Koto-ku, Tokyo 135-8533, Japan
Abstract Fe–B magnetic particles doped GdBa2Cu3O7-δ superconducting bulks were fabricated by using the top-seeded melt growth method. 10 wt% Ag2O or equivalent 9.31 wt% nano–Ag particles (50 nm in diameter) were added into the precursors before the sample preparation. The Fe–B doped GdBa2Cu3O7-δ/nano–Ag showed poorer mechanical properties with an obviously porous/cracked structure than the GdBa2Cu3O7-δ/Ag2O one especially at the bottom part of the bulk. In the GdBa2Cu3O7-δ/Ag2O bulk, doped Fe element is mainly wrapped or attached by Ag particles. In contrast, the Fe element prefers to substitute on the Cu sites in the GdBa2Cu3O7-δ/nano–Ag bulk due to less protection from Ag particle. The microstructure and superconducting properties were investigated in these two series samples.
©2012 2011Published Published by byElsevier Elsevier B.V. Ltd. Selection Rogalla and © Selection and/or and/or peer-review peer-review under under responsibility responsibility of of Horst the Guest Editors. Peter Kes. Open access under CC BY-NC-ND license. Keywords: Superconducting bulk; critical current; trapped field; artificial pinning center
1. Introduction RE–Ba–Cu–O superconducting bulks have received considerable attentions since their wide practical applications [1, 2]. Over last decade, various types of additives were attempted to be as artificial pinning centers in order to improve the flux trapping capability and critical current density, JC [3-5]. Recently, we found Fe–B magnetic particles with small amounts of Cu–Nb–Si–Cr have a positive effect on the superconducting properties in Gd–Ba–Cu–O single domain bulk [6-11]. Xu et al reported that Fe–B
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1875-3892 © 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of the Guest Editors. Open access under CC BY-NC-ND license. doi:10.1016/j.phpro.2012.06.083
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magnetic particles could homogeneously improve the critical current density over the whole bulk [9]. By the study of microstructure, the Fe–B particles were reported to be oxidized into Fe3O4 ferrimagnetic particles embedded in or adjacent to the Ag particles and could potentially refine the Gd2BaCuO5 (Gd211) particles [10]. Due to this close relationship between Fe–B and Ag particles, we employed Ag nanoparticles instead of Ag2O in order to understand the underlying mechanism in this paper. 2. Experimental details Commercially available Gd-123, Gd-211, Ag2O, Ag, Pt, and amorphous Fe–B alloy particles (Fe0.687B0.121Si0.138Nb0.033Cr0.011Cu0.010) were used as starting materials in this study. 9.31 wt% nano–Ag 50 nm in diameter (Series A) or corresponding 10 wt% Ag2O (Series B, for comparison) were added in the mixed precursor powders to enhance the mechanical strength of the bulk. Different molar amounts of Fe– B magnetic particles with emphasis on 0.4 mol% were also added. The composition of the precursor was expressed as follows: 100 mol% Gd-123 + 40 mol% Gd-211 + x mol% Fe–B + 10 wt% Ag2O (or 9.31 wt% nano–Ag) + 0.5 wt% Pt. The mixed powders were uniaxially pressed into 20 mm in diameter pellets. The green pressed pellets were placed onto Y-stabilized ZrO2 supporting rods inside a conventional box furnace. The arrangement was heated to 1100 oC in 10 h, maintained at this temperature for 1 h, then rapidly cooled down to 1030 oC and subsequently held for 1 h. Before cooling down to 1015 oC, a melttextured NdBa2Cu3O7-δ (001) seed crystal was placed on the surface of the pellet to control the crystallization. During the growth, 164 h undercooling with a rate of 0.3 oC/ h was employed. The process of post-annealing in oxygen gas was performed between 300 to 450 oC for 210 h. Finally, the single domain bulk with 17 mm in diameter and 8 mm in thickness was successfully fabricated. The microstructure observation was performed by using a scanning electron microscopy (SEM; JSM5600). The local stoichiometry was determined by energy dispersive x-ray spectroscopy (EDX). For the trapped field measurements, the melt-grown bulk was initially field cooled to 77 K using liquid nitrogen in the presence of a 1 T magnetic field perpendicular to the a–b plane. Thirty minutes after removal of the external field, the trapped field distribution was measured by an automatic Hall probe scanning system with a sensor (F W Bell, BHT-921). The sensor was positioned 0.5 mm above the sample surface. DC magnetization measurements were carried out using a Quantum Design superconducting quantum interference device (MPMS-XL). The small rectangular specimens labelled as C1, C2, B1, B2, A1, A2 with dimensions of 2 mm × 2 mm × 1 mm were cut from the parent grain as indicated schematically in figure 1. The transition temperature (TC) and magnetic properties were measured with the magnetic field 10 Oe applied parallel to the c-axis of the samples. JC–B curves were calculated using an extended Bean critical state model [12].
Fig. 1. Schematic illustration of the cut specimens labeled as C1, C2, B1, B2, A1, A2.
Kun Xu et al. / Physics Procedia 36 (2012) 557 – 562
3. Results and discussion 3.1. Trapped field Figure 2 shows a typical trapped field distribution of 0.4 mol% Fe–B doped Gd–Ba–Cu–O/nano–Ag superconducting bulk. The maximum trapped field and integrated trapped flux are 0.254 T and 17.09 μWb respectively. It manifests that Gd–Ba–Cu–O bulks were successfully fabricated without macroscopic defects.
Fig. 2. Trapped field distribution of 0.4 mol% Fe–B doped Gd–Ba–Cu–O/nano–Ag bulk.
3.2. Microstructure As reported in Ref. 10, the Fe–B magnetic particles were oxidized into ferrimagnetic Fe3O4 embedded inside or in the adjacent of Ag particles. It might be attributed to the flowing of melted Ag to the porous formed by the oxidation process of Fe–B additives. The merit of this structure is eliminating the proximity effect between magnetic Fe3O4 and Gd-123 superconducting matrix as well as the substitution of Fe ions on Cu sites by the Ag as a buffer layer. Thus, if nano–Ag was employed instead Ag2O microns in diameter, the existence status of Fe element should be changed. In our prepared Gd–Ba–Cu–O/nano–Ag series bulk, large cracks were always observed. It infers that the effect of nano–Ag in improving mechanical properties is not as good as the normal Ag2O particles. The microstructures of C1, B1, A1, C2, B2, A2 specimens in Gd–Ba–Cu–O/nano–Ag are shown in figure 3 (a)-(f). The results show that the specimens of series A have a structure with lots of pores and cracks compared with the ones of series B shown in figure 3 (g)-(i). It manifests that the nano–Ag can’t effectively fill the pores as Ag2O several microns in diameter. For example, as shown in figure 3 (d) and (e), some pores are only partially filled by the Ag.
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Fig. 3. Microstructures of series A (a) C1, (b) B1, (c) A1, (d) C2, (e) B2, (f) A2. Series B (g) C2, (h) B2, (i) A2 for comparison. The large white particles represent Ag.
Nomilized Magnetization (a. u.)
3.3. Transition temperature and critical current density
0.0 -0.2 -0.4
Series A without Fe-B C1 C2 with 0.4 mol% Fe-B C1 C2
-0.6 -0.8 -1.0 91
92
93
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95
T (K) Fig. 4. Temperature dependence of the normalized magnetic moment of series A- C1 and C2 specimens w/o Fe–B particles.
The transition temperatures of series A specimens were measured as shown in figure 4. It is found that the TC decreased with the doping of Fe–B additives in both C1 and C2 positions. The TC of C2 specimen is always 0.2 K higher than C1, since the less polluted by Nd-123 seed.
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12 50K
8
2
JC (10 A/cm )
10
65K
4
6 4
77K
2 0 -2
85K Series A Black: without Fe-B Red: with 0.4 mol% Fe-B
0.1
B (T)
1
Fig. 5. JC versus B of Series A-C1 specimens w/o Fe–B particles at different temperatures (85 K, 77 K, 65 K, 50 K).
The JC of series A-C1 specimens w/o Fe–B additives were measured as shown in figure 5. It is found that the JC is only enhanced at mediate-field as a second peak at relative higher temperatures (85 K and 77 K). It is different from the enhancement of JC at whole magnetic field range in Gd –Ba–Cu–O/Ag2O series by Fe–B doping. This may be related to that more Fe elements substitute to Gd-123 matrix since the Fe is less protected by Ag. The decrease of TC caused a δTC pinning correspondingly. If the temperature further decreases, the enhancement of JC becomes apparent even in low field range. 3.4. Conclusions The Fe–B doped Gd–Ba–Cu–O/nano–Ag bulks were successfully prepared by the top seeded melt growth process. The nano–Ag particles were found to form large Ag after the melting process. The mechanical properties were found to be weakened by the microstructure study. The JC is only improved in the mediate-field range at 77 K, and greatly enhanced in the whole field range at lower temperatures. Acknowledgements This work was partially supported by KAKENHI (21360425) and the Sasakawa Scientific Research Grant from the Japan Science Society. This work was partly performed using facilities of the Materials Design and Characterization Laboratory, Institute for Solid State Physics, The University of Tokyo.
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