Abstract
We report the effect of NiF2 (10 nm) additions on the transport critical current density, of (Bi1.6Pb0.4)Sr2Ca2Cu3O10/Ag sheathed superconductor tapes. Pellets of (Bi1.6Pb0.4)Sr2Ca2Cu3O10(NiF2)x (–0.05 wt.%) superconductor were prepared using the acetate coprecipitation method. The sample with 0.04 wt.% addition of NiF2 exhibited the highest . Ag-sheathed (Bi1.6Pb0.4)Sr2Ca2Cu3O10(NiF2)0.04 superconductor tapes were fabricated using the powder-in-tube (PIT) method and sintered at 845°C for 50 and 100 h. of nonadded tapes at 30 K sintered for 50 and 100 h was 6370 and 8280 A/cm2, respectively. of (Bi1.6Pb0.4)Sr2Ca2Cu3O10(NiF2)0.04/Ag tape at 30 K sintered for 50 and 100 h was 14390 and 17270 A/cm2, respectively. In magnetic fields (0 to 0.7 T), of the NiF2 added tapes was also higher compared with the nonadded tape indicating that NiF2 nanoparticles can act as effective flux pinning centers and longer sintering time improved the microstructure. A steeper increase in was observed below 60 K in the NiF2 added tapes which coincided with the Neel temperature, of nanosized NiF2 (60 K). The pronounced enhancement of was attributed to the strong interaction between flux line network and the antiferromagnetic NiF2 below .
1. Introduction
The Bi1.4Pb0.6Sr2Ca2Cu3O10+δ (Bi-2223) high temperature superconductor shows great potential for commercialization in various applications. However, weak pinning of flux lines and weak intergrain links result in low transport critical current density, , especially in magnetic fields and these limit the application of this material [1, 2]. In order to improve the flux pinning, several methods such as adding impurities to act as pinning centers have been employed. Additions of particles such as TiO2, ZrO2, Ag2CO3, and MgO in Bi-2223 have been investigated and shown to increase the transport critical current density [3–6].
Furthermore, addition of magnetic nanoparticles to enhance in Bi-2223 has also been proposed. If the average size of the nanoparticles, , is , where is the coherence length and is the penetration depth, an increase in can be expected due to enhanced flux pinning which arises from strong interaction between flux line network and magnetic system in the superconductor [7]. Previous studies with magnetic nanoparticles such as NiFe2O4 and Fe3O4 addition to Bi-2223 have also showed improvements in the transport properties [8, 9].
The coherence length of Bi-2223 is around 2.9 nm and the penetration depth is between 60 and 1000 nm [10]. The average size of NiF2 nanoparticles employed in this study is 10 nm that is between the coherence length and penetration depth of Bi-2223. This satisfies the requirement for a frozen flux superconductor [7]. NiF2 nanoparticle has a rutile crystal structure and the Néel temperature is 60 K [11]. It is interesting to investigate the of Bi-2223 added with nano NiF2 above and below 60 K where the magnetic transition occurs. The objectives of this study were to investigate the effect of NiF2 nanoparticles with average size of 10 nm on the structure, phase formation, and transport critical current density of Ag-sheathed Bi-2223 tapes with different sintering times.
2. Experimental Details
Bi1.4Pb0.6Sr2Ca2Cu3O10 superconductor pellets were prepared by the acetate coprecipitation method [12]. Nanosized NiF2 (Aldrich, 99.99% purity) with 10 nm average size was added with composition (Bi1.6Pb0.4)Sr2Ca2Cu3O10(NiF2)x (x = 0.00–0.05 wt.%). The mixed powders were ground and pressed into pellets and then sintered at 845°C for 48 h. The sample with wt.% showed the highest at 77 K. This composition was chosen to fabricate Ag-sheathed tape by the powder-in-tube method. (Bi1.6Pb0.4)Sr2Ca2Cu3O10(NiF2)0.04 powders were packed into a 6.35 mm outer diameter and 4.35 inner diameter Ag tube (Alfa Aesar, 99.9%). The tube was drawn to a 1 mm wire and then pressed into 0.12 nm thick and 1.35 nm wide tapes. The tapes were then cut into 2-3 cm sections and sintered for 50 h and 100 h at 845°C. Tapes without the addition of NiF2 were also prepared for comparison.
The of the tapes was measured using the four-point probe method with the 1 μV/cm criterion. The measurements were done from 30 K to 77 K in zero fields and at 77 K under magnetic field from 0 to 0.75 T. X-ray diffraction (XRD) patterns of the tapes were recorded using a Bruker D8 Advance diffractometer with radiation. The Ag sheath was removed prior to measurement. The microstructure of the tapes was examined using a Philips XL 30 scanning electron microscope. A Philips transmission electron microscope (model CM12) was used to confirm the average size of NiF2 nanoparticles.
3. Results and Discussion
NiF2 added Bi-2223 pellet with wt.% showed the highest of 2.54 and 1.38 A/cm2 at 30 K and 77 K, respectively (Figure 1). This optimal wt.% was used to prepare (Bi1.6Pb0.4)Sr2Ca2Cu3O10(NiF2)0.04/Ag tape.
Figure 2 shows the XRD patterns of and 0.04 wt.% tapes sintered for 50 and 100 h. Most of the peaks in both nonadded and NiF2 added tapes belong to the high- phase (Bi-2223) with a few peaks corresponding to the low- phase (Bi-2212). The volume fraction of Bi-2223 phase was calculated using where and are the sum of intensities of Bi-2223 and Bi-2212 phase, respectively [13, 14]. The volume fractions are shown in Table 1. In general, the Bi-2223 peaks increased with NiF2 addition and with longer sintering times. The tape sintered for 100 h showed the highest volume fraction.
Figure 3(a) shows the TEM micrograph of the NiF2 nanoparticles employed in this study with average grain size of 10 nm. SEM micrographs of nonadded and NiF2 added tapes in Figures 3(b)–3(d) showed plate-like grains microstructure which is the typical microstructure of the Bi-2223 system. Figures 3(c) and 3(d) also show the distribution of NiF2 (white dots).
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of nonadded and NiF2 added tapes sintered for 50 and 100 h in zero fields is shown in Figure 4. Tapes with NiF2 addition showed significantly higher compared with the nonadded tapes. Furthermore, longer sintering times also contributed to a slight increase of in both nonadded and NiF2 added tapes. This could be due to improvement in connectivity between grains with longer sintering times. of all tapes decreased with an increase in temperature as a consequence of thermally activated flux creep. of NiF2 added tape sintered for 100 h was 17270 A/cm2 at 77 K, which is higher than of the tape sintered for 50 h (14390 A/cm2) as shown in Table 1. However, the is lower compared with nanosized PbO, Fe3O4, and MgO added Ag-sheathed Bi-2223 tapes sintered for 100 h (26800 A/cm2, 23130 A/cm2, and 18380 A/cm2, resp., at 30 K), but higher than of NiO added Bi-2223 tapes (15520 A/cm2 at 30 K) [9, 15–17].
Below 60 K, the of NiF2 added tapes showed a steeper increase (Figure 4) which coincides with the Néel temperature, K of NiF2 [11]. The appearance of antiferromagnetism in NiF2 below 60 K may have been the cause of the steeper increase in as suggested in a frozen flux superconductor [7].
Magnetic field dependence of for the nonadded and NiF2 added tapes at 77 K from 0 T to 0.75 T with the applied fields either parallel or perpendicular to the surface of the tape is shown in Figure 5. The of NiF2 added tapes was higher than the nonadded tapes in both applied field orientations. of all the tapes decreased with increasing field strength due to the destruction of weak links in the tapes under low magnetic fields [12]. Furthermore, the decrease in was slower for tapes under magnetic field applied parallel to the surface of the tape compared to when the field was applied perpendicular to the surface of the tape. This could be explained by the better flux pinning capability behavior along the flat surface of the tapes [18].
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4. Conclusion
In conclusion, the effect of antiferromagnetic NiF2 nanoparticles addition on phase structure, microstructure, and transport critical current density in Bi-2223 superconductor has been investigated. NiF2 added tapes showed a significant enhancement of compared with the nonadded tapes. This showed that NiF2 nanoparticles acted as effective pinning centers leading to enhancement of in the Bi-2223 system. A higher was also seen in tapes sintered for 100 h compared with tapes sintered for 50 h due to the improvement in grains connectivity. A sintering time longer than 100 h may enhance even further. A steeper increase in was observed below 60 K and this coincides with the Néel temperature ( K) of nanosized NiF2. The appearance of antiferromagnetism below 60 K may have enhanced due to strong interaction between the flux lines with the antiferromagnetic NiF2 nanoparticles.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
This research was supported by the Malaysian Ministry of Education under Grant no. FRGS/2/2013/SGD2/UKM/01/1 and Universiti Kebangsaan Malaysia under Grant no. UKM-DPP-2014-055.