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Effects of high-pressure sintering on critical current density in Co-doped BaFe2As2 wires
Physica C xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Effects of high-pressure sintering on critical current density in Co-doped BaFe2As2 wires Hiroshi Inoue a, Yuji Tsuchiya a, Shinya Tada a, Sunseng Pyon a, Hideki Kajitani b, Norikiyo Koizumi b, Tsuyoshi Tamegai a,⇑ a b
Department of Applied Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Fusion Research and Development Directorate, Japan Atomic Energy Agency (JAEA), 801-1 Mukoyama, Naka-shi, Ibaraki 311-0193, Japan
a r t i c l e
i n f o
Article history: Received 17 January 2014 Received in revised form 5 March 2014 Accepted 25 March 2014 Available online xxxx Keywords: Iron-based superconductor Superconducting wires Ba(Fe,Co)2As2 Powder-in-tube (PIT) method Textured tape Hot isostatic press (HIP)
a b s t r a c t We demonstrate that critical current density (Jc) in Ba(Fe,Co)2As2 superconducting wires is enhanced by high-pressure processes, such as hot isostatic pressing (HIP) or texturing by uniaxial pressure. These superconducting wires have been fabricated by ex situ powder-in-tube (PIT) method. Transport Jc at T = 4.2 K under self-field of the textured tape (0.4 mm thick) and HIP processed wire is 9.2 kA/cm2 and 7.8 kA/cm2, respectively. We have also observed magneto-optical images of the wire and demonstrated that the high-pressure treatment contributed to generate strong links between superconducting grains and enhance Jc. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Since the discovery of iron-based superconductors (IBSs) in 2008 [1], enormous amount of research activities have been generated. They exhibit high critical temperatures (Tc’s) [2,3], large upper critical fields (Hc2’s) [4], relatively small anisotropies [5,6], and large critical current densities (Jc’s) [7] in single crystals. For these properties, they are expected to be used for practical applications. In IBSs, there are many types of superconductors, such as ROFeAs (R: rare earths, 1111-type) [3,8], LiFeAs (111-type) [9], AFe2As2 (A: Alkali earths, 122-type) [2], FeSe (11-type) [10], and the pnictides with perovskite-type blocking layers [11]. The 122-type compound is one of the promising candidates for applications in IBSs, since high-quality samples can be grown easily. In 122-type superconductors, there are three types of superconductors; hole-doped superconductors, e.g. (Ba,K)Fe2As2 [3], electrondoped superconductors, e.g. Ba(Fe,Co)2As2 [12], and isovalently doped superconductors, e.g. BaFe2(As,P)2 [13]. In this study, we focused on the electron-doped Ba(Fe,Co)2As2. Its Tc is approximately 25 K and its intragranular Jc (T = 2 K, H = 0 Oe) is 1 MA/ cm2 [12]. Synthesis of polycrystalline sample and fabrication of the wire using Ba(Fe,Co)2As2 are easier than those for hole-doped ⇑ Corresponding author. Tel.: +81 3 5841 6846; fax: +81 3 5841 8886. E-mail address: [email protected] (T. Tamegai).
(Ba,K)Fe2As2 (Tc = 38 K), which contains unstable potassium. Although Tc and Jc are smaller than those of (Ba,K)Fe2As2, the stability of Ba(Fe,Co)2As2 can be advantageous for practical applications. One of the serious problems in the 122-type wire is that the transport Jc is low due to the weak-link behavior between polycrystalline grains. To solve this problem, various methods have been applied for (Ba,K)Fe2As2 superconducting wires, such as the combination of several times of cold press and hot press [14], addition of metals [15–17], or texturing the tape [18,19]. In this paper, we demonstrate that transport Jc of Ba(Fe,Co)2As2 flat-rolled tapes and high-pressure sintered wires is improved. Especially, high-pressure sintered wires are expected to be used for practical applications for the round shape. We also measure a distribution of the composition by SEM and EDX and distribution of Jc by magneto-optical imaging, and discuss how grain boundaries change by the texturing and HIP process [15,18–25]. 2. Experimental We have fabricated Ba(Fe0.9Co0.1)2As2 and Ba(Fe0.925Co0.075)2As2 wires by ex situ powder-in-tube (PIT) method. PIT method is a common way to make superconducting wires [25–29]. The merits of the PIT method are low-price and easy fabrication. Tc of Ba(Fe0.925Co0.075)2As2 and Tc of Ba(Fe0.9Co0.1)2As2 are 25 K and 21 K, respectively. Therefore, changing the composition of Co from
http://dx.doi.org/10.1016/j.physc.2014.03.020 0921-4534/Ó 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: H. Inoue et al., Effects of high-pressure sintering on critical current density in Co-doped BaFe2As2 wires, Physica C (2014), http://dx.doi.org/10.1016/j.physc.2014.03.020
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H. Inoue et al. / Physica C xxx (2014) xxx–xxx
7.5% to 10% does not affect Tc much [30]. Precursors of these wires were made by Ba flakes, FeAs powders, and CoAs powders. All the handlings were conducted in a glove box. Ba was shaved by metal files to make it into powders, and Ba powders, FeAs and CoAs were mixed for 2 h in a ball mill. These mixed powders were put into an Alumina crucible and it was put into a stainless steel (SUS) tube. The SUS tube was sealed under a nitrogen atmosphere, and it was heated at 900 °C for 24 h. The reacted precursor was pulverized in a glove box for 20 min. The phase identification of the sample was conducted by powder X-ray diffraction (XRD) (M18XHF, MAC Science) with Cu Ka radiation generated at 40 kV and 200 mA. Also, magnetization of this precursor was measured by a SQUID magnetometer (MPMS-5XL, Quantum Design). The precursor was filled into a Ag tube with OD 4.5 mm and ID 3.0 mm. 15wt% of Ag was added into Ba(Fe0.925Co0.075)2As2 wire. The Ag tube was grove rolled into OD 0.8 mm. A grove rolled wire was made into a tape by flat-rolling with the final thickness of 0.4 mm. The tape was packed into evacuated quartz tubes and heated at 700 °C for 24 h. Also wires for high-pressure sintering were packed into a Cu tube and both ends of the tube were closed by arc furnace under Ar atmosphere. High-pressure sintering was conducted in 700 °C for 4 h in Ar atmosphere under pressure of 120 MPa. Transport Jc (T = 4.2 K) of these wires and tapes were measured in liquid He. We define Jc by the value of a current density when the electric field exceeds 1 lV/cm. Also, the composition of Ba(Fe,Co)2As2 was characterized by a scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM–EDX). Furthermore, for magneto-optical (MO) imaging, the wire was cut and the surface was polished with a lapping film. 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 The powder XRD patterns of Ba(Fe0.925Co0.075)2As2 and Ba(Fe0.9 Co0.1)2As2 precursors are shown in Fig. 1(a). Peaks from impurities are barely visible except for a very small amount of FeAs in both precursors. It suggests that almost single phases of Ba(Fe,Co)2As2 are obtained. Fig. 1(b) shows the temperature dependence of magnetization under magnetic field of H = 5 Oe. Diamagnetic signal is detected below 25 K in Ba(Fe0.925Co0.075)2As2 and 23 K in Ba(Fe0.9Co0.1)2As2. A broad transition around Tc may suggest the presence of distribution of Co contents x in Ba(Fe1 xCox)2As2 precursors. Fig. 2(a) shows transport Jc of a groove-rolled Ba(Fe0.9Co0.1)2As2 wire (‘‘wire’’), Ba(Fe0.9Co0.1)2As2 tape (‘‘tape’’), and high-pressure sintered Ba(Fe0.925Co0.075)2As2 wire with 15wt%. Ag (‘‘HIP wire’’). Jc values at T = 4.2 K under self-field are 4.7 kA/cm2, 9.2 kA/cm2 and 7.8 kA/cm2 for ‘‘wire’’ and ‘‘tape’’ and ‘‘HIP wire’’, respectively. Flat-rolling and high-pressure sintering made Jc of the wire twice larger than the groove-rolled wire. These results indicate that high-pressure processes for wires contribute effectively for enhancement of Jc. It may be caused by improvement of weak links between superconducting grains. Fig. 2(b) shows a field dependence of transport Jc (T = 4.2 K) of the ‘‘wire’’ and the ‘‘HIP wire’’. At H = 90 kOe, transport Jc of the ‘‘wire’’ and the ‘‘HIP wire’’ are 7.8 101 A/cm2, 1.6 102 A/cm2, respectively. It may be due to the weak link behavior. Jc’s in both wires show strong field dependences especially at low fields. It suggests that weak link between grains has not been improved much by the HIP process.
Fig. 1. (a) Powder X-ray diffraction patterns of Ba(Fe0.9Co0.1)2As2 and Ba(Fe0.925Co0.075)2As2 precursors. (b) Magnetization versus temperature curve for Ba(Fe0.9Co0.1)2As2 and Ba(Fe0.925Co0.075)2As2 precursors.
Fig. 2. (a) Transport Jc of the ‘‘wire’’, ‘‘tape’’ and ‘‘HIP wire’’ (for the definition, see text). (b) Transport Jc versus magnetic field of the ‘‘wire’’ and ‘‘HIP wire’’.
Please cite this article in press as: H. Inoue et al., Effects of high-pressure sintering on critical current density in Co-doped BaFe2As2 wires, Physica C (2014), http://dx.doi.org/10.1016/j.physc.2014.03.020
H. Inoue et al. / Physica C xxx (2014) xxx–xxx
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Fig. 3. SEM images of a cross section of Ba(Fe0.9Co0.1)2As2 (a) ‘‘wire’’, (b) ‘‘tape’’, and (c) ‘‘HIP wire’’. White lines are 10 lm.
Fig. 4. MO images of (a) ‘‘wire’’ and (b) ‘‘HIP wire’’. Flux distributions at several temperatures along red lines in (a) and (b) are shown in (c) and (d), respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3(a)–(c) shows that SEM images of a cross section of Ba(Fe0.9Co0.1)2As2 ‘‘wire’’, ‘‘tape’’, and ‘‘HIP wire’’, respectively. The analyzed cobalt content x, detected for the whole area was 0.09. This value is almost the same as the nominal composition. However, from the local area analyses, grains with rich or poor cobalt contents, and small amounts of impurities such as BaAs were also detected. Also, clear grains can be observed in ‘‘wire’’ and the size of grains is about 2–5 lm and there are some voids. These results suggest that grains are not well connected. On the other hand, clear grains are not observed in ‘‘tape’’ and ‘‘HIP wire’’, which suggests that grains are well connected. These results indicate that the core density becomes very high and the connectivity between grains is improved by the effect of high pressure. To clarify how the connection between the grains is improved, we performed MO measurements. Fig. 4(a) and (b) show MO images of the core region of the ‘‘wire’’ and ‘‘HIP wire’’, respectively. They were taken in the remanent state after applying magnetic field of 800 Oe for 0.25 s, which was subsequently reduced down to zero at 4.2 K. The bright region corresponds to the trapped flux in the sample. The central part of the core region in the ‘‘HIP wire’’ shows uniform and fully trapped magnetic flux compared with that in the ‘‘wire’’. Flux density profiles for the ‘‘wire’’ and ‘‘HIP wire’’ are shown in Fig. 4(c) and (d), respectively. The flux profile for the ‘‘wire’’ has many valleys as shown in Fig. 4(c). This shows that there are cracks or impurities on grain boundaries. By contrast, the flux profile for the ‘‘HIP wire’’ is smooth as shown in Fig. 4(d). This shows that it is possible that high-pressure
sintering could suppress cracks and the current flow between the grains is improved. Therefore, we can conclude that the highpressure process is an effective way to make core density very high and make connectivity between grains very strong. 4. Summary We have prepared Ba(Fe,Co)2As2 polycrystals with the help of a ball milling and fabricated wires using ex situ PIT method, and investigated the effect of high-pressure sintering. We also prepared tapes of Ba(Fe0.9Co0.1)2As2 by flat-rolling the PIT wire. X-ray diffraction, magnetization, transport Jc, SEM images and MO images of these tape and wires were measured. The obtained transport Jc at T = 4.2 K under self-field for the wire, tape, and the HIP wire are 4.7 kA/cm2, 9.2 kA/cm2, and 7.8 kA/cm2, respectively. Although transport Jc of the HIP wire is slightly smaller than the tape, the round shape of HIP wire can be advantageous for practical applications. A possible reason for the improvement of a transport Jc by high-pressure processes is the suppression of cracks between grains and densification of the core. Therefore, we can conclude that high-pressure process is an effective method to improve Jc. Acknowledgements This work was partially supported by a Grant-in-Aid for Young Scientists (B) (No. 24740238) and the Japan–China Bilateral Joint
Please cite this article in press as: H. Inoue et al., Effects of high-pressure sintering on critical current density in Co-doped BaFe2As2 wires, Physica C (2014), http://dx.doi.org/10.1016/j.physc.2014.03.020
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Please cite this article in press as: H. Inoue et al., Effects of high-pressure sintering on critical current density in Co-doped BaFe2As2 wires, Physica C (2014), http://dx.doi.org/10.1016/j.physc.2014.03.020
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Effects of high-pressure sintering on critical current density in Co-doped BaFe2As2 wires Hiroshi Inoue a, Yuji Tsuchiya a, Shinya Tada a, Sunseng Pyon a, Hideki Kajitani b, Norikiyo Koizumi b, Tsuyoshi Tamegai a,⇑ a b
Department of Applied Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Fusion Research and Development Directorate, Japan Atomic Energy Agency (JAEA), 801-1 Mukoyama, Naka-shi, Ibaraki 311-0193, Japan
a r t i c l e
i n f o
Article history: Received 17 January 2014 Received in revised form 5 March 2014 Accepted 25 March 2014 Available online xxxx Keywords: Iron-based superconductor Superconducting wires Ba(Fe,Co)2As2 Powder-in-tube (PIT) method Textured tape Hot isostatic press (HIP)
a b s t r a c t We demonstrate that critical current density (Jc) in Ba(Fe,Co)2As2 superconducting wires is enhanced by high-pressure processes, such as hot isostatic pressing (HIP) or texturing by uniaxial pressure. These superconducting wires have been fabricated by ex situ powder-in-tube (PIT) method. Transport Jc at T = 4.2 K under self-field of the textured tape (0.4 mm thick) and HIP processed wire is 9.2 kA/cm2 and 7.8 kA/cm2, respectively. We have also observed magneto-optical images of the wire and demonstrated that the high-pressure treatment contributed to generate strong links between superconducting grains and enhance Jc. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Since the discovery of iron-based superconductors (IBSs) in 2008 [1], enormous amount of research activities have been generated. They exhibit high critical temperatures (Tc’s) [2,3], large upper critical fields (Hc2’s) [4], relatively small anisotropies [5,6], and large critical current densities (Jc’s) [7] in single crystals. For these properties, they are expected to be used for practical applications. In IBSs, there are many types of superconductors, such as ROFeAs (R: rare earths, 1111-type) [3,8], LiFeAs (111-type) [9], AFe2As2 (A: Alkali earths, 122-type) [2], FeSe (11-type) [10], and the pnictides with perovskite-type blocking layers [11]. The 122-type compound is one of the promising candidates for applications in IBSs, since high-quality samples can be grown easily. In 122-type superconductors, there are three types of superconductors; hole-doped superconductors, e.g. (Ba,K)Fe2As2 [3], electrondoped superconductors, e.g. Ba(Fe,Co)2As2 [12], and isovalently doped superconductors, e.g. BaFe2(As,P)2 [13]. In this study, we focused on the electron-doped Ba(Fe,Co)2As2. Its Tc is approximately 25 K and its intragranular Jc (T = 2 K, H = 0 Oe) is 1 MA/ cm2 [12]. Synthesis of polycrystalline sample and fabrication of the wire using Ba(Fe,Co)2As2 are easier than those for hole-doped ⇑ Corresponding author. Tel.: +81 3 5841 6846; fax: +81 3 5841 8886. E-mail address: [email protected] (T. Tamegai).
(Ba,K)Fe2As2 (Tc = 38 K), which contains unstable potassium. Although Tc and Jc are smaller than those of (Ba,K)Fe2As2, the stability of Ba(Fe,Co)2As2 can be advantageous for practical applications. One of the serious problems in the 122-type wire is that the transport Jc is low due to the weak-link behavior between polycrystalline grains. To solve this problem, various methods have been applied for (Ba,K)Fe2As2 superconducting wires, such as the combination of several times of cold press and hot press [14], addition of metals [15–17], or texturing the tape [18,19]. In this paper, we demonstrate that transport Jc of Ba(Fe,Co)2As2 flat-rolled tapes and high-pressure sintered wires is improved. Especially, high-pressure sintered wires are expected to be used for practical applications for the round shape. We also measure a distribution of the composition by SEM and EDX and distribution of Jc by magneto-optical imaging, and discuss how grain boundaries change by the texturing and HIP process [15,18–25]. 2. Experimental We have fabricated Ba(Fe0.9Co0.1)2As2 and Ba(Fe0.925Co0.075)2As2 wires by ex situ powder-in-tube (PIT) method. PIT method is a common way to make superconducting wires [25–29]. The merits of the PIT method are low-price and easy fabrication. Tc of Ba(Fe0.925Co0.075)2As2 and Tc of Ba(Fe0.9Co0.1)2As2 are 25 K and 21 K, respectively. Therefore, changing the composition of Co from
http://dx.doi.org/10.1016/j.physc.2014.03.020 0921-4534/Ó 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: H. Inoue et al., Effects of high-pressure sintering on critical current density in Co-doped BaFe2As2 wires, Physica C (2014), http://dx.doi.org/10.1016/j.physc.2014.03.020
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7.5% to 10% does not affect Tc much [30]. Precursors of these wires were made by Ba flakes, FeAs powders, and CoAs powders. All the handlings were conducted in a glove box. Ba was shaved by metal files to make it into powders, and Ba powders, FeAs and CoAs were mixed for 2 h in a ball mill. These mixed powders were put into an Alumina crucible and it was put into a stainless steel (SUS) tube. The SUS tube was sealed under a nitrogen atmosphere, and it was heated at 900 °C for 24 h. The reacted precursor was pulverized in a glove box for 20 min. The phase identification of the sample was conducted by powder X-ray diffraction (XRD) (M18XHF, MAC Science) with Cu Ka radiation generated at 40 kV and 200 mA. Also, magnetization of this precursor was measured by a SQUID magnetometer (MPMS-5XL, Quantum Design). The precursor was filled into a Ag tube with OD 4.5 mm and ID 3.0 mm. 15wt% of Ag was added into Ba(Fe0.925Co0.075)2As2 wire. The Ag tube was grove rolled into OD 0.8 mm. A grove rolled wire was made into a tape by flat-rolling with the final thickness of 0.4 mm. The tape was packed into evacuated quartz tubes and heated at 700 °C for 24 h. Also wires for high-pressure sintering were packed into a Cu tube and both ends of the tube were closed by arc furnace under Ar atmosphere. High-pressure sintering was conducted in 700 °C for 4 h in Ar atmosphere under pressure of 120 MPa. Transport Jc (T = 4.2 K) of these wires and tapes were measured in liquid He. We define Jc by the value of a current density when the electric field exceeds 1 lV/cm. Also, the composition of Ba(Fe,Co)2As2 was characterized by a scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM–EDX). Furthermore, for magneto-optical (MO) imaging, the wire was cut and the surface was polished with a lapping film. 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 The powder XRD patterns of Ba(Fe0.925Co0.075)2As2 and Ba(Fe0.9 Co0.1)2As2 precursors are shown in Fig. 1(a). Peaks from impurities are barely visible except for a very small amount of FeAs in both precursors. It suggests that almost single phases of Ba(Fe,Co)2As2 are obtained. Fig. 1(b) shows the temperature dependence of magnetization under magnetic field of H = 5 Oe. Diamagnetic signal is detected below 25 K in Ba(Fe0.925Co0.075)2As2 and 23 K in Ba(Fe0.9Co0.1)2As2. A broad transition around Tc may suggest the presence of distribution of Co contents x in Ba(Fe1 xCox)2As2 precursors. Fig. 2(a) shows transport Jc of a groove-rolled Ba(Fe0.9Co0.1)2As2 wire (‘‘wire’’), Ba(Fe0.9Co0.1)2As2 tape (‘‘tape’’), and high-pressure sintered Ba(Fe0.925Co0.075)2As2 wire with 15wt%. Ag (‘‘HIP wire’’). Jc values at T = 4.2 K under self-field are 4.7 kA/cm2, 9.2 kA/cm2 and 7.8 kA/cm2 for ‘‘wire’’ and ‘‘tape’’ and ‘‘HIP wire’’, respectively. Flat-rolling and high-pressure sintering made Jc of the wire twice larger than the groove-rolled wire. These results indicate that high-pressure processes for wires contribute effectively for enhancement of Jc. It may be caused by improvement of weak links between superconducting grains. Fig. 2(b) shows a field dependence of transport Jc (T = 4.2 K) of the ‘‘wire’’ and the ‘‘HIP wire’’. At H = 90 kOe, transport Jc of the ‘‘wire’’ and the ‘‘HIP wire’’ are 7.8 101 A/cm2, 1.6 102 A/cm2, respectively. It may be due to the weak link behavior. Jc’s in both wires show strong field dependences especially at low fields. It suggests that weak link between grains has not been improved much by the HIP process.
Fig. 1. (a) Powder X-ray diffraction patterns of Ba(Fe0.9Co0.1)2As2 and Ba(Fe0.925Co0.075)2As2 precursors. (b) Magnetization versus temperature curve for Ba(Fe0.9Co0.1)2As2 and Ba(Fe0.925Co0.075)2As2 precursors.
Fig. 2. (a) Transport Jc of the ‘‘wire’’, ‘‘tape’’ and ‘‘HIP wire’’ (for the definition, see text). (b) Transport Jc versus magnetic field of the ‘‘wire’’ and ‘‘HIP wire’’.
Please cite this article in press as: H. Inoue et al., Effects of high-pressure sintering on critical current density in Co-doped BaFe2As2 wires, Physica C (2014), http://dx.doi.org/10.1016/j.physc.2014.03.020
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Fig. 3. SEM images of a cross section of Ba(Fe0.9Co0.1)2As2 (a) ‘‘wire’’, (b) ‘‘tape’’, and (c) ‘‘HIP wire’’. White lines are 10 lm.
Fig. 4. MO images of (a) ‘‘wire’’ and (b) ‘‘HIP wire’’. Flux distributions at several temperatures along red lines in (a) and (b) are shown in (c) and (d), respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3(a)–(c) shows that SEM images of a cross section of Ba(Fe0.9Co0.1)2As2 ‘‘wire’’, ‘‘tape’’, and ‘‘HIP wire’’, respectively. The analyzed cobalt content x, detected for the whole area was 0.09. This value is almost the same as the nominal composition. However, from the local area analyses, grains with rich or poor cobalt contents, and small amounts of impurities such as BaAs were also detected. Also, clear grains can be observed in ‘‘wire’’ and the size of grains is about 2–5 lm and there are some voids. These results suggest that grains are not well connected. On the other hand, clear grains are not observed in ‘‘tape’’ and ‘‘HIP wire’’, which suggests that grains are well connected. These results indicate that the core density becomes very high and the connectivity between grains is improved by the effect of high pressure. To clarify how the connection between the grains is improved, we performed MO measurements. Fig. 4(a) and (b) show MO images of the core region of the ‘‘wire’’ and ‘‘HIP wire’’, respectively. They were taken in the remanent state after applying magnetic field of 800 Oe for 0.25 s, which was subsequently reduced down to zero at 4.2 K. The bright region corresponds to the trapped flux in the sample. The central part of the core region in the ‘‘HIP wire’’ shows uniform and fully trapped magnetic flux compared with that in the ‘‘wire’’. Flux density profiles for the ‘‘wire’’ and ‘‘HIP wire’’ are shown in Fig. 4(c) and (d), respectively. The flux profile for the ‘‘wire’’ has many valleys as shown in Fig. 4(c). This shows that there are cracks or impurities on grain boundaries. By contrast, the flux profile for the ‘‘HIP wire’’ is smooth as shown in Fig. 4(d). This shows that it is possible that high-pressure
sintering could suppress cracks and the current flow between the grains is improved. Therefore, we can conclude that the highpressure process is an effective way to make core density very high and make connectivity between grains very strong. 4. Summary We have prepared Ba(Fe,Co)2As2 polycrystals with the help of a ball milling and fabricated wires using ex situ PIT method, and investigated the effect of high-pressure sintering. We also prepared tapes of Ba(Fe0.9Co0.1)2As2 by flat-rolling the PIT wire. X-ray diffraction, magnetization, transport Jc, SEM images and MO images of these tape and wires were measured. The obtained transport Jc at T = 4.2 K under self-field for the wire, tape, and the HIP wire are 4.7 kA/cm2, 9.2 kA/cm2, and 7.8 kA/cm2, respectively. Although transport Jc of the HIP wire is slightly smaller than the tape, the round shape of HIP wire can be advantageous for practical applications. A possible reason for the improvement of a transport Jc by high-pressure processes is the suppression of cracks between grains and densification of the core. Therefore, we can conclude that high-pressure process is an effective method to improve Jc. Acknowledgements This work was partially supported by a Grant-in-Aid for Young Scientists (B) (No. 24740238) and the Japan–China Bilateral Joint
Please cite this article in press as: H. Inoue et al., Effects of high-pressure sintering on critical current density in Co-doped BaFe2As2 wires, Physica C (2014), http://dx.doi.org/10.1016/j.physc.2014.03.020
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Please cite this article in press as: H. Inoue et al., Effects of high-pressure sintering on critical current density in Co-doped BaFe2As2 wires, Physica C (2014), http://dx.doi.org/10.1016/j.physc.2014.03.020