ARTICLE IN PRESS
JID: PHYSC
[m5G;March 15, 2016;21:34]
Physica C: Superconductivity and its applications 0 0 0 (2016) 1–3
Contents lists available at ScienceDirect
Physica C: Superconductivity and its applications journal homepage: www.elsevier.com/locate/physc
Evaluation of global and local critical current densities in 122-type iron-based superconducting tapes Sunseng Pyon∗, Akinori Mine, Takahiro Suwa, Tsuyoshi Tamegai Department of Applied Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
a r t i c l e
i n f o
Article history: Received 28 January 2016 Revised 15 February 2016 Accepted 23 February 2016 Available online xxx Keywords: (Ba,K)Fe2 As2 Powder-in-tube (PIT) Superconducting tapes Cold pressing Critical current density Magneto-optical imaging
a b s t r a c t We demonstrate the fabrication of (Ba,K)Fe2 As2 superconducting pressed tapes through a powder-in-tube method using uniaxial cold pressing technique, and evaluate global and local Jc by several physical measurements. We found an intriguing anisotropy in Jc determined by magnetic measurements, which is related to microcrack structures affecting the local Jc . The maximum Jc estimated from magnetization measurements has reached ∼1.9 × 105 A/cm2 under self-field at 4.2 K. Furthermore, microcrack structures are directly observed by magneto-optical imaging technique. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Iron-based superconductors such as LaFeAsO1-x Fx (1111 type) [1] and (Ba,K)Fe2 As2 (122 type) [2] are promising candidates for high-field applications because they exhibit high critical temperature Tc , large upper critical field Hc2 , and relatively low anisotropy compared with cuprate superconductors [3]. In particular, 122-type compounds are considered to be the most attractive candidates because of their small anisotropies of 2-3, moderate Tc ’s, high Hc2 ’s, and large Jc ’s [4]. In order to apply 122-type superconducting wires for practical use, weak links between superconducting grains, which are the main factor for low Jc and their strong magnetic field dependence, should be improved [3]. Recently, very high Jc over 1 × 105 A/cm2 has been reported both in uniaxially-pressed tapes and round-shaped hot isostatic pressed (HIP) wires [5-10]. Especially, excellent Jc values of ∼1 × 105 A/cm2 at 10 T are realized in uniaxially-pressed tapes [6,7]. It is argued that the superior Jc in the pressed tape is due to not only high core density, which implies improved links between grains, but also textured grains and a change in the microcrack structure, which are suggested by X-ray diffraction (XRD) pattern and SEM image, respectively [6]. In this work, we demonstrate the fabrication of 122-type superconducting tapes and evaluate global and local Jc . Firstly we show the anisotropy in magnetic Jc , which may be originated from microcrack structures in the tape. Secondly, these microcrack ∗
Corresponding author. Tel./fax: +81 3 5841 6848. E-mail address:
[email protected] (S. Pyon).
structures are directly observed by magneto-optical imaging (MOI) technique. 2. Experimental details (Ba,K)Fe2 As2 superconducting tapes were fabricated by ex-situ powder-in-tube (PIT) method. Polycrystalline Ba0.6 K0.4 Fe2 As2 powders were prepared by the solid-state reaction using Ba pieces, K ingots, Fe powder, and As pieces as starting materials. In order to compensate the loss of elements, the starting mixture contained 15% excess K and 5% excess As. They were mixed in a nitrogen atmosphere about 10 h using a ball-milling machine and placed into a niobium tube. It was put into a stainless steel tube which was sealed in nitrogen filled glove box for heat treatment at 900 °C for 30 h. After the heat treatment, they were ground into powder using an agate mortar in nitrogen filled glove box. Ground powders were filled in a silver tube with an outer diameter 4.5 mm and an inner diameter 3 mm, then cold drawn into a square wire with diagonal dimension of approximately 2.0 mm. The wires were deformed into a tape form initially into 0.8 mm in thickness followed by intermediate annealing at 800 °C for 2 h and then into 0.40 mm in thickness. The tape was then cut and uniaxially pressed under a pressure of 0.5–1.0 GPa. The pressed tapes were finally sintered at 850 °C for 1–2 h. All heat treatments for the tape were carried out by putting the samples into a quartz tube filled with 0.05 MPa of Ar gas. The phase identification of the polycrystalline sample and tapes were carried out by means of powder X-ray diffraction (Smartlab, Rigaku) with Cu-Kα
http://dx.doi.org/10.1016/j.physc.2016.02.016 0921-4534/© 2016 Elsevier B.V. All rights reserved.
Please cite this article as: S. Pyon et al., Evaluation of global and local critical current densities in 122-type iron-based superconducting tapes, Physica C: Superconductivity and its applications (2016), http://dx.doi.org/10.1016/j.physc.2016.02.016
ARTICLE IN PRESS
JID: PHYSC 2
[m5G;March 15, 2016;21:34]
S. Pyon et al. / Physica C: Superconductivity and its applications 000 (2016) 1–3
b
20
(Ba,K)Fe2As2 pressed tape (1 0 3)
(Ba,K)Fe2As2 pressed tape
-40
(0 0 8)
-20
Ag
(0 0 4)
(0 0 2)
Intensity ( a.u. )
4πM (G)
0
(0 0 6)
a
(Ba,K)Fe2As2 powder
-60 H = 5 Oe -80 0
10
20
30
40
50
10
20
T (K)
30 40 2θ (deg.)
50
60
Fig. 1. (a) Temperature dependence of magnetization of (Ba,K)Fe2 As2 pressed tape under magnetic field of 5 Oe. (b) X-ray diffraction patterns of (Ba,K)Fe2 As2 polycrystalline powder and pressed tape whose surface is polished to remove Ag sheath.
a
10
b
6 4
(Ba,K)Fe2As2 pressed tape
2
Jc (A/cm )
2
10
T = 4.2 K
H // tape surface
5
H
H
H J
4 2
10
4
H ⊥ tape surface
H
J
4
Direction of microcracks
2
3
10
0
10
20 30 H (kOe)
40
50
Fig. 2. (a) Magnetic field dependence of magnetic Jc in (Ba,K)Fe2 As2 pressed tape at 4.2 K. The magnetic field was applied parallel and perpendicular to the tape surface. (b) Schematic drawings for the relation between the magnetic field H (black arrows) and the shielding current J (red arrows). Directions of expected microcracks are also shown as blue arrows. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
radiation. Bulk magnetization is measured by a superconducting quantum interference device (SQUID) magnetometer (MPMS-5XL, Quantum Design). For magneto-optical (MO) imaging, the core of pressed tape was prepared by cutting and polishing the tape. An iron-garnet indicator film was placed in direct contact with the sample and the whole assembly was attached to the cold finger of a He-flow cryostat (Microstat-HR, Oxford Instruments). MO images were acquired by using a cooled-CCD camera with 12-bit resolution (ORCA-ER, Hamamatsu). 3. Results and discussion Firstly, the fabricated tape was evaluated by magnetization measurement. Fig. 1(a) shows the temperature dependence of magnetization of the pressed tape. Although the onset Tc of 35.2 K in the pressed tape is a little lower than Tc of 38 K in synthesized powders, sharp drop of magnetization near Tc is observed. Next, the phase purity and texture of the tape were investigated by XRD analysis of the core surface, which was prepared by mechanically removing the silver sheath. Fig. 1(b) shows the XRD pattern of (Ba,K)Fe2 As2 polycrystalline powder and press tape processed by flat rolling and uniaxial pressing. As a reference, the data for randomly orientated powder is also included in the figure. The
XRD patterns have strong peaks of 122 phases and do not have peaks of impurity phases such as FeAs, except for Ag peaks from the sheath material. When compared with the randomly oriented powder, relative intensities of the (00l) peaks with respect to that of the (103) peak are strongly enhanced. It indicates a well-defined c-axis texture in the pressed tape, although the degree of texture is still lower than the former reports of Ba-122 or Sr-122 pressed tapes [6,7]. Magnetic Jc in the pressed tape was evaluated by magnetization measurement. The Jc was estimated from the irreversible magnetization by using the extended Bean model [11]. During these measurements, we found an intriguing anisotropy of Jc . Fig. 2(a) shows a typical magnetic field dependence of magnetic Jc in our pressed tapes at 4.2 K. Magnetic field was applied parallel or perpendicular to the tape surface. The magnetic Jc has reached almost 1.9 × 105 A/cm2 at self-field when magnetic field is parallel to the tape surface. Although this value is still lower than the reported maximum transport Jc value in pressed tapes [6,7], it is higher than the maximum transport Jc values in 122-type HIP wires [5,8,9]. On the other hand, when magnetic field is perpendicular to the tape surface, magnetic Jc is substantially smaller compared with the magnetic Jc when field is parallel to the tape surface. This strange anisotropy of magnetic Jc can be explained
Please cite this article as: S. Pyon et al., Evaluation of global and local critical current densities in 122-type iron-based superconducting tapes, Physica C: Superconductivity and its applications (2016), http://dx.doi.org/10.1016/j.physc.2016.02.016
ARTICLE IN PRESS
JID: PHYSC
[m5G;March 15, 2016;21:34]
S. Pyon et al. / Physica C: Superconductivity and its applications 000 (2016) 1–3
a
b
c
3
d
Fig. 3. (a) An optical micrograph of the core of (Ba,K)Fe2 As2 pressed tape. (b) An optical micrograph whose area corresponds to yellow squares in (a) and (c). Yellow arrows indicate a microcrack. (c, d) MO images of flux penetration in the core of (Ba,K)Fe2 As2 pressed tape at 5 K at (c) 50 Oe and (d) 100 Oe after zero-field cooling. Black bars in the figures show 0.5 mm length scales. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
qualitatively by the presence of microcracks in the core of the tape as shown in Fig. 2(b). Gao et al. argued that small cracks run parallel to the tape length in the pressed tape, while they run transverse to the tape length in the as-rolled tape [6,12]. Microcracks across the tape length are harmful for flowing current in the tape core. Especially, when the magnetic field is perpendicular to the tape surface, current flows across the expected microcracks. By contrast, current flows along the microcracks when magnetic field is parallel to the tape surface. In this case, reduction of magnetic Jc should be relatively small. This speculation explains the anisotropy of Jc qualitatively. To make it clearer, we observed crack structures in the core of the pressed tape directly using the MO imaging technique. We succeeded in confirming the suppression of local Jc directly. Core of the tape was obtained by cutting and removing the Ag sheath. The dimensions of the core are 1450 × 1350 × 40 μm3 . This is shown in an optical micrograph in Fig. 3(a) and (b). Penetrated magnetic flux into the core at 5 K under 50 Oe and 100 Oe are shown in Fig. 3(c) and (d), respectively. Dark areas correspond to the superconducting region where current flows well so that magnetic field is shielded. Bright areas are locations where magnetic flux is penetrated because of weak shielding current. When the field is increased, magnetic flux penetrates into the weak-linked areas, which correspond to microcracks. All parts of microcracks are not observed in optical micrograph as shown in Fig. 3(b). Obviously, these microcracks are harmful for current flow, so local Jc in the core should be suppressed. In principle, cracks only along the tape length do not suppress transport Jc . So control of cracks is important for increasing Jc in the pressed tape. In addition, the improvement of fabrication method to reduce the number of cracks is also important to increase Jc . For example, in our tape, directions of microcracks are not the same as shown in Fig. 3(c) and (d). This may be one of the reasons why Jc in our sample is still lower than the maximum Jc in other reported pressed tapes. Generation of microcracks can be controlled or reduced by optimizing related parameters such as sheath materials and its thickness, pressure, sintering temperature and its time. The optimization of these parameters for the tape fabrication process should be demanded.
4. Summary We have prepared polycrystalline (Ba,K)Fe2 As2 and fabricated the PIT pressed tapes by cold pressing. They were characterized by X-ray diffraction and magnetization measurements, and magneto-optical imaging. We found an intriguing anisotropy in Jc determined by magnetic measurement, which is related to microcracks in the core. The maximum Jc estimated from magnetization measurements has reached almost 1.9 × 105 A/cm2 under self-field at 4.2 K. Furthermore, microcracks are directly observed by magneto-optical imaging technique. Acknowledgments This work was partially supported by the Japan-China Bilateral Joint Research Project by the Japan Society for the Promotion of Science (JSPS), by research grant of the Sumitomo foundation, and by research grant (general research) of TEPCO Memorial Foundation. References [1] Y. Kamihara, T. Watanabe, M. Hirano, H. Hosono, J. Am. Chem. Soc. 130 (2008) 3296. [2] M. Rotter, M. Tegel, D. Johrendt, Phys. Rev. Lett. 101 (2008) 107006. [3] Y. Ma, Supercond. Sci. Technol. 25 (2012) 113001. [4] X.-L. Wang, S.R. Ghorbani, S.-I. Lee, S.X. Dou, C.T. Lin, T.H. Johansen, K.-H. Müller, Z.X. Cheng, G. Peleckis, M. Shabazi, A.J. Qviller, V.V. Yurchenko, G.L. Sun, D.L. Sun, Phys. Rev. B 82 (2010) 024525. [5] J.D. Weiss, C. Tarantini, J. Jiang, F. Kametani, A.A. Polyanskii, D.C. Larbalestier, E.E. Hellstrom, Nat. Mater. 11 (2012) 682. [6] Z. Gao, K. Togano, A. Matsumoto, H. Kumakura, Sci. Rep. 4 (2014) 4065. [7] H. Lin, C. Yao, X. Zhang, H. Zhang, D. Wang, Q. Zhang, Y.W. Ma, S. Awaji, K. Watanabe, Sci. Rep. 4 (2014) 4465. [8] S. Pyon, Y. Tsuchiya, H. Inoue, H. Kajitani, N. Koizumi, S. Awaji, K. Watanabe, T. Tamegai, Supercond. Sci. Technol. 27 (2014) 095002. [9] S. Pyon, Y. Yamasaki, H. Kajitani, N. Koizumi, S. Awaji, K. Watanabe, T. Tamegai, Supercond. Sci. Technol. 28 (2015) 125014. [10] S. Pyon, T. Taen, F. Ohtake, Y. Tsuchiya, H. Inoue, H. Akiyama, H. Kajitani, N. Koizumi, S. Okayasu, T. Tamegai, Appl. Phys. Express 6 (2013) 123101. [11] Y. Nakajima, Y. Tsuchiya, T. Taen, T. Tamegai, S. Okayasu, M. Sasase, Phys. Rev. B 80 (2009) 012510. [12] K. Togano, Z. Gao, A. Matsumoto, H. Kumakura, Supercond. Sci. Technol. 26 (2013) 115007.
Please cite this article as: S. Pyon et al., Evaluation of global and local critical current densities in 122-type iron-based superconducting tapes, Physica C: Superconductivity and its applications (2016), http://dx.doi.org/10.1016/j.physc.2016.02.016