Electrical Transport Properties of (Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O₁₀)/Ag Tapes with Different Nanosized MgO

Abstract

MgO nanopowders with average size 20 and 40 nm were introduced into (Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O₁₀)(MgO)_x (𝑥=0--0.15 wt.%) in the pellet form. The optimum amounts for the highest transport critical current density 𝐽_𝑐 were 𝑥=0.10 and 0.01 wt.% for 20 and 40 nm MgO, respectively. These results were used to fabricate MgO added (Bi, Pb)-2223/Ag sheathed tapes using the powder-in-tube method. The tapes were sintered at 845°C for 50 h and 100 h. The structure, microstructure, and 𝐽_𝑐 of the tapes were determined. The temperature and magnetic field dependence of 𝐽_𝑐 for the MgO added tapes exhibited a significant enhancement compared with the nonadded tapes. 𝐽_𝑐 of 20 nm MgO added tape was higher compared with the 40 nm MgO added tape. A higher 𝐽_𝑐 was obtained when the tapes were sintered for 100 h. The increase in 𝐽_𝑐 can be explained as the increase of the flux pinning strength by nanosized MgO. The nanoparticle with size closer to the coherence length was more effective in enhancing 𝐽_𝑐.

1. Introduction

The transport critical current density 𝐽_𝑐 in Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O₁₀ ((Bi, Pb)-2223) high temperature superconductor is limited by the intergrain weak links, grain alignment, and the weak pinning of flux lines [1–3]. These factors suppressed the 𝐽_𝑐 and thus prevent the extensive applications of (Bi, Pb)-2223 superconductor. The weak link is also observed in silver-sheathed (Bi, Pb)-2223 tape prepared by powder-in-tube (PIT) technique [4, 5].

Nanoparticles have been used as pinning centers to improve 𝐽_𝑐 (e.g., [6–9]). Pinning center with size larger than the coherence length 𝜉 was suggested to improve 𝐽_𝑐 [10]. However, other studies have suggested that the optimum size of pinning centers should be comparable to the penetration depth 𝜆 rather than 𝜉 [11]. The 𝜉 of (Bi, Pb)-2223 system is 2.9 nm and 𝜆 is 60–1000 nm. It is expected that the interaction between flux line and the nanoparticles will be strong for a particle with size 𝐿 where 𝜉<𝐿<𝜆 [12].

In previous reports, MgO nanoparticles and nanorods (with average diameter of 30 nm) have been added into (Bi,Pb)-2223/Ag tapes [13, 14]. MgO is chemically inert with (Bi, Pb)-2223, and the transition temperature 𝑇_𝑐 did not change considerably with MgO content. In those reports, nanosized MgO additions into (Bi, Pb)-2223/Ag tapes have only been carried out with one average size [13, 14]. It is interesting to investigate the effect of different nanosized MgO on the critical current density of the (Bi, Pb)-2223 system.

Our initial study on samples in pellet form showed that at 77 K, Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O₁₀(MgO)_x (𝑥=0--0.15 wt.%) pellets exhibited the highest 𝐽_𝑐 at 𝑥=0.10 wt.% for 20 nm MgO and 𝑥=0.01 wt.% for 40 nm MgO. It is interesting to study the effect of the two different nanosized MgO with those amounts in (Bi, Pb)-2223(MgO)_x/Ag tapes. In this paper, we report on the effect of 20 nm (𝑥=0.1 wt.%) and 40 nm (𝑥=0.01 wt.%) MgO in (Bi, Pb)-2223(MgO)_x/Ag tapes. We also report the influence of sintering time (50 and 100 h) on 𝐽_𝑐.

2. Experimental Details

Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O₁₀ superconductor was prepared by the acetate coprecipitation technique. The powders were calcined at 730°C for 12 h. Followed by calcination at 845°C for 24 h. MgO nanopowders with size 20 and 40 nm (US-nano, 99+% purity) were added to Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O₁₀(MgO)_x (𝑥=0--0.15 wt.%). The mixed powders were ground and then pressed into pellets. The pellets were sintered at 845°C for 48 h. The highest 𝐽_𝑐 at 77 K for the pellets was found in the 𝑥=0.10 and 0.01 wt.% of 20 and 40 nm MgO, respectively. These amounts were used to prepare (Bi, Pb)-2223(MgO)_x/Ag tapes by powder-in-tube (PIT) method. The powders were packed into a 6.35 mm outer diameter and 4.35 mm inner diameter silver tube (99.9% metals basis, Alfa Aesar). The tubes were drawn to a 1 mm wire and then pressed into 0.30 mm thick and 1.53 mm wide tapes. The tapes were sintered for 50 h and 100 h at 845°C.

The structure and microstructure of the tapes were examined by X-ray powder diffraction using a Siemens D 5000 diffractometer with CuK_α radiation and a Philips XL 30 scanning electron microscope (SEM), respectively. The distribution of nano MgO in the tapes was determined by using a Philips energy dispersive X-ray analyzer (EDX) model PV99. The 𝐽_𝑐 was determined by the four-probe method using the 1 μV/cm criterion. Measurements of transport critical current density were done from 30 to 77 K in zero field, and at 77 K under magnetic field from 0 to 0.75 T. The size of the MgO nanopowder was confirmed by a Philips transmission electron microscope (TEM) model CM12.

3. Results and Discussion

The complete results for 𝑥=0 to 0.15 wt.% in pellet form is reported elsewhere [15]. Briefly, at 77 K, the 𝑥=0.10 wt.%, 20 nm MgO added pellet showed the highest 𝐽_𝑐 (1.73 A/cm²). For the 40 nm MgO added pellet, the 𝑥=0.01 wt.% sample showed the highest 𝐽_𝑐 (1.17 A/cm²) as shown in Table 1. The low values of 𝐽_𝑐 in these polycrystalline samples are similar in magnitude to those reported in the Y-based [16] and Bi-based [17] superconductors. A low NiO (𝑥=0.002) addition in (Bi, Pb)-2223 was also reported to elevate the flux pinning in the superconductor [6].

Table 1 

𝐽_𝑐 (at 0 T) of 𝑥=0 wt.%, 𝑥=0.10 wt.% of 20 nm MgO, and 𝑥=0.01 wt.% of 40 nm MgO added pellets and tapes sintered for 50 h and 100 h at 30 and 77 K.

Figure 1 shows the XRD patterns of (Bi, Pb)-2223(MgO)_x/Ag tapes for the nonadded and MgO added samples. Most of the peaks belong mainly to the high-𝑇_𝑐 phase (Bi-2223) with a few peaks corresponding to the low-𝑇_𝑐 phase (Bi-2212) and a small quantity of Ca₂PbO₄ phase. The Ag peak was also observed. SEM micrographs showed that the nonadded and MgO added tapes consisted of plate-like grains (Figure 2). Figures 2(b) and 2(c) show a homogeneous distribution of MgO (white dots) with additions of 20 and 40 nm MgO, respectively.

Figure 1 

XRD patterns of (Bi, Pb)-2223(MgO)x/Ag tapes for 𝑥=0 wt.%, 𝑥=0.10 wt.% of 20 nm MgO, and 𝑥=0.01 wt.% of 40 nm MgO heated for 50 h. The Ag peak is indexed to (111).

(a)

(b)

(c)

Figure 2 

SEM micrographs of (Bi, Pb)-2223(MgO)x/Ag tapes (a) 𝑥=0 wt.%, (b) 𝑥=0.10 wt.% of 20 nm MgO, and (c) 𝑥=0.01 wt.% of 40 nm MgO. The dots refer to Mg.

Figure 3 shows the 𝐽_𝑐 of the nonadded and MgO added tapes sintered for 50 and 100 h as a function of temperature. It is clear that 𝐽_𝑐 of the 20 and 40 nm MgO added tapes samples were higher compared with the nonadded tape. MgO added tapes sintered for 100 h exhibited a higher 𝐽_𝑐 than tapes sintered for 50 h (Table 1).

(a) 100 h

(b) 50 h

Figure 3 

𝐽𝑐 of 𝑥=0 wt.%, 𝑥=0.10 wt.% of 20 nm MgO, and 𝑥=0.01 wt.% of 40 nm MgO tapes sintered for (a) 100 h and (b) 50 h as a function of the temperature.

The magnetic field dependence of 𝐽_𝑐 for the nonadded and MgO added tapes at 77 K with the applied field parallel 𝐵_‖ and perpendicular 𝐵_⟂ to the surface of the tape is shown in Figure 4. The 𝐽_𝑐 of MgO added tapes were higher than the nonadded tape. It can be seen that 𝐽_𝑐(𝐵_‖) decreased slower than 𝐽_𝑐(𝐵_⟂) with increasing 𝐵. The improvement of 𝐽_𝑐 under magnetic field can be explained as the strengthening of weak links, grain boundaries and significant improvement of flux pinning centers in MgO added tapes. A high degree of intimate grain connectivity, grain alignment and flux pinning centers enhance 𝐽_𝑐 of (Bi, Pb)-2223/Ag tape under magnetic field [18, 19].

(a)

(b)

Figure 4 

𝐽𝑐 of 𝑥=0 wt.%, 𝑥=0.10 wt.% of 20 nm MgO, and 𝑥=0.01 wt.% of 40 nm MgO added tapes under magnetic field parallel 𝐵‖ and perpendicular 𝐵⟂ to the surface of the tape at 77 K.

In Figures 3 and 4 we find that 𝐽_𝑐 of the 20 nm MgO added tape was higher compared with 40 nm MgO added tape. Particles size (20 nm) which is closer to the coherence length (2.9 nm) is more effective in increasing 𝐽_𝑐. Previous reports on nano MgO addition in (Bi, Pb)-2223 pellets and tapes also showed improvement in 𝐽_𝑐 [13, 14]. In this work we showed that for low MgO content, the size of the nanoparticles also affected the 𝐽_𝑐.

In conclusion, the 𝐽_𝑐 of MgO added tapes exhibited a significant enhancement compared with the nonadded tapes. The 𝐽_𝑐 of the 20 nm MgO added samples was higher than the 40 nm MgO added pellets and tapes. A higher 𝐽_𝑐 was obtained when the tape was sintered for 100 h due to improvement in grains connectivity. The enhancement of the 𝐽_𝑐 could be the result of the increase in the flux pinning ability in (Bi, Pb)-2223(MgO)_x/Ag tapes. Our results showed that in order to enhance 𝐽_𝑐, the size of the pinning center should be closer to the coherence length.

Acknowledgments

This work has been supported by the Ministry of Education, Malaysia, under Grant no. ERGS/1/2011/STG/UKM/01/25 and Universiti Kebangsaan Malaysia, under Grant no. UKM-DIP-2012-32.