MgO nanopowders with average size 20 and 40 nm were introduced into (Bi1.6Pb0.4Sr2Ca2Cu3O10)(MgO)x (  wt.%) in the pellet form. The optimum amounts for the highest transport critical current density were 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 of the tapes were determined. The temperature and magnetic field dependence of for the MgO added tapes exhibited a significant enhancement compared with the nonadded tapes. of 20 nm MgO added tape was higher compared with the 40 nm MgO added tape. A higher was obtained when the tapes were sintered for 100 h. The increase in 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 .

1. Introduction

The transport critical current density in Bi1.6Pb0.4Sr2Ca2Cu3O10 ((Bi, Pb)-2223) high temperature superconductor is limited by the intergrain weak links, grain alignment, and the weak pinning of flux lines [13]. These factors suppressed the 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 (e.g., [69]). Pinning center with size larger than the coherence length was suggested to improve [10]. However, other studies have suggested that the optimum size of pinning centers should be comparable to the penetration depth rather than [11]. The of (Bi, Pb)-2223 system is 2.9 nm and is 60–1000 nm. It is expected that the interaction between flux line and the nanoparticles will be strong for a particle with size where [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 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, Bi1.6Pb0.4Sr2Ca2Cu3O10(MgO)x (  wt.%) pellets exhibited the highest at  wt.% for 20 nm MgO and  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 (  wt.%) and 40 nm (  wt.%) MgO in (Bi, Pb)-2223(MgO)x/Ag tapes. We also report the influence of sintering time (50 and 100 h) on .

2. Experimental Details

Bi1.6Pb0.4Sr2Ca2Cu3O10 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 Bi1.6Pb0.4Sr2Ca2Cu3O10(MgO)x (  wt.%). The mixed powders were ground and then pressed into pellets. The pellets were sintered at 845°C for 48 h. The highest at 77 K for the pellets was found in the 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 CuKα 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 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 to 0.15 wt.% in pellet form is reported elsewhere [15]. Briefly, at 77 K, the  wt.%, 20 nm MgO added pellet showed the highest (1.73 A/cm2). For the 40 nm MgO added pellet, the  wt.% sample showed the highest (1.17 A/cm2) as shown in Table 1. The low values of in these polycrystalline samples are similar in magnitude to those reported in the Y-based [16] and Bi-based [17] superconductors. A low NiO ( ) addition in (Bi, Pb)-2223 was also reported to elevate the flux pinning in the superconductor [6].

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- phase (Bi-2223) with a few peaks corresponding to the low- 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 3 shows the of the nonadded and MgO added tapes sintered for 50 and 100 h as a function of temperature. It is clear that 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 than tapes sintered for 50 h (Table 1).

The magnetic field dependence of for the nonadded and MgO added tapes at 77 K with the applied field parallel and perpendicular to the surface of the tape is shown in Figure 4. The of MgO added tapes were higher than the nonadded tape. It can be seen that decreased slower than with increasing . The improvement of 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 of (Bi, Pb)-2223/Ag tape under magnetic field [18, 19].

In Figures 3 and 4 we find that 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 . Previous reports on nano MgO addition in (Bi, Pb)-2223 pellets and tapes also showed improvement in [13, 14]. In this work we showed that for low MgO content, the size of the nanoparticles also affected the .

In conclusion, the of MgO added tapes exhibited a significant enhancement compared with the nonadded tapes. The of the 20 nm MgO added samples was higher than the 40 nm MgO added pellets and tapes. A higher was obtained when the tape was sintered for 100 h due to improvement in grains connectivity. The enhancement of the 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 , the size of the pinning center should be closer to the coherence length.


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.