Abstract
The effect of different nanosized Co3O4 (10, 30, and 50 nm) addition on the Bi1.6Pb0.4Sr2Ca2Cu3O10(Co3O4)x superconductor with wt.% has been investigated using X-ray diffraction method, scanning electron microscopy, transition temperature, and critical current density measurements. The samples were prepared by the conventional solid-state reaction method. Samples with wt.% Co3O4 (10 nm) showed the highest at 102 K. The highest was observed in the wt.% Co3O4 (10 nm) and wt.% Co3O4 (30 nm) samples. At 77 K, of the 10 nm and 30 nm Co3O4 added samples was 6 and 13 times larger than the nonadded samples, respectively. Small addition of Co3O4 nanoparticles in the Bi1.6Pb0.4Sr2Ca2Cu3O10 (Bi-2223) samples enhanced the critical current density and the phase formation. The larger Co3O4 nanoparticle (50 nm) had a greater degradation affect on superconductivity of the Bi-2223 phase.
1. Introduction
The critical current density, , is one of the most significant parameters for the successful application of high- superconductors. The pinning potential is one of the important factors that determine the critical current density. can be improved by introducing efficient pinning center with size that matches the coherence length, which can improve pinning potential and suppress the flux flow [1, 2].
Extensive researches have been done to improve the flux pinning properties of the Bi1.6Pb0.4Sr2Ca2Cu3O10 (Bi-2223) system [2–19]. Elemental substitution and addition of nanosized particles in high- superconductors are an easy and efficient method to improve the transport properties. Much effort has been done to improve intergrain links and the properties of the Bi-based superconductor, such as adding Al, Cr, Mg, Cd, Ag, Ni, Nb, Li, La, and Nd into high- superconductors [2–11].
The critical current density and transition temperature of Sm added samples were found to be higher than those of the pure samples with a maximum given by wt.%, which is seven times higher than nonadded sample [12]. In Cr2O3 added Bi-2223 samples, the maximum and were observed for the sample with 0.5 wt.% [3]. In MgO added BiSr2CaCu2Oδ (Bi-2212) samples, the transition is narrow and the volume fraction increases with addition up to 9 wt.% [13]. The volume fraction of the Bi-2223 phase, lattice parameter, and zero-resistance temperature, , decrease with increasing MgCO3 content [4]. The Fe3O4 magnetic nanoparticle and SiC have the ability to enhance the critical current density in Bi-2Sr2Ca2Cu3O10/Ag tape [14, 18].
Flux pinning can be optimized if the particle size is carefully controlled. With judicious addition level, these particles can improve flux pinning [7]. The pinning strengths of the flux lines can be enhanced by direct magnetic interaction of vortices with magnetic pinning centers.
The two important characteristic lengths in superconductors are the coherence length ξ and penetration depth . The critical current density is expected to increase if the pinning center is larger than but smaller than [20]. For Bi2Sr2Ca2Cu3O10 system, nm and nm. For magnetic nanoparticles, a strong interaction between flux line network and the system can be expected if , where is the particle size [21]. The range between and is large and it is interesting to investigate the effect of different magnetic nanosized particles, that is, between 2.9 nm and 60 nm on (Bi,Pb-2223) superconductor.
Most studies on the nanosized particle addition into (Bi,Pb)-2223 have only been carried out with one average size. Preliminary results on the effect of Co3O4 on the formation of the Bi-2223 phase have been reported [22]. In this paper the effects of magnetic Co3O4 addition with average sizes 10, 30, and 50 nm in Bi1.6Pb0.4Sr2Ca2Cu3O10(Co3O4)x with were studied. These sizes are larger than but smaller than which satisfies the condition in [20]. This objective of this research was to determine the effect of different nanosized Co3O4 on the phase formation, structure, microstructure, and transport critical current properties of Bi-2223 superconductor.
2. Experimental Details
Superconducting powders precursor with nominal composition Bi1.6Pb0.4Sr2Ca2Cu3O10 were synthesized by using the coprecipitation method. The material was prepared using metal acetates of bismuth, lead, strontium, calcium and copper (purity ≥99.99%), oxalic acid, deionized water, and isopropanol. The dried-up powders were ground manually in agate mortar and calcined at 730°C for 12 h followed by an intermediate grinding before the second calcination at 845°C for 24 h. After cooling, the calcined powders were ground again and after that nanosized Co3O4 (US Research Nanomaterials Inc.) with average particle size of 10, 30, and 50 nm were added to the precursor powders with nominal composition Bi1.6Pb0.4Sr2Ca2Cu3O10(Co3O4)x with , 0.01, 0.02, 0.03, 0.04, and 0.05 wt.%. The samples were thoroughly mixed, ground, pressed into pellet of 2 mm thickness and 12.5 mm diameter, and sintered at 850°C for 48 h.
The electrical resistance-temperature measurements were carried out by the four-point probe technique in conjunction with a CTI cryogenics closed-cycle refrigerator (Model 22). The four-point probe method using the 1 μV/cm criterion was used to measure the transport critical current density between 30 K and 77 K.
The XRD diffraction patterns were recorded using a Bruker D8 Advance diffractometer with CuKα radiation. The microstructure of the samples was observed using a scanning electron microscope (SEM) Philips XL 30. The distribution of nano-Co3O4 in the sample and the presence of the different phases were analyzed by using a Philips energy dispersive X-ray analyzer (EDX) model PV99. The size of Co3O4 was determined using a Philips transmission electron microscope (TEM) model CM12.
3. Results and Discussion
TEM micrograph of the Co3O4 nanoparticles with average size around 30 nm is shown in Figure 1. Figures 2, 3, and 4 show the XRD patterns of the sample added with 10, 30, and 50 nm Co3O4, respectively. The samples consist of a mixture of Bi-2223 and Bi-2212 phase. H and L indicate the high- Bi-2223 phase and low- Bi-2212 phase, respectively. The volume fraction of the phases for all the samples is given in Table 1. Samples with wt.% (10 nm), wt.% (30 nm), and wt.% (50 nm) showed the highest percentage of Bi-2223 phase, that is, 72, 74, and 72%, respectively. Further increase of Co3O4 decreased the percentage of the Bi-2223 phase and increased the percentage of the Bi-2212. The intensity of the peaks corresponding to the Bi-2223 phase decreases and the intensities of peaks corresponding to the Bi-2212 phase increase with further increase in Co3O4. The lattice parameters for the samples were Å and Å. A slight increase in the lattice parameter with almost no change in the and parameters was observed in the nano-Co3O4 added samples. The Co3O4 reflections were not observed in the XRD patterns due to the fact that the values are too small to be detected. Addition of 10 and 30 nm Co3O4 from to 0.04 wt.% increased the Bi-2223 phase volume fraction.
Figure 5 shows the electrical resistance as a function of temperature between 60 and 200 K for all the samples. The dc electrical resistance measurements show metallic behavior in the normal state and a well-defined superconducting transitions for all samples. All the samples show the zero electrical resistance within the range of 91 to 102 K (Table 1), while onset temperature is between 83 and 115 K. The nonadded sample showed at 100 K. Samples with wt.% Co3O4 (10 nm) showed the highest at 102 K. The high for the wt.% sample may be due to homogeneity in the sample. Excessive Co3O4 degraded the superconductivity of Bi-2223, which can affect the transport properties in this type of material. The lower with increasing Co3O4 content also could be interpreted as a result of the suppression of superconductivity by Co3O4 [3].
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Figure 6 shows the values at various temperatures for the 10, 30, and 50 nm Co3O4 added samples. The magnitude of in these samples is similar to those reported in the Y-based [23] and Bi-based [24] polycrystalline superconductors. It is clear that the addition of a small amount of Co3O4 enhanced the current-carrying capacity of the Bi-2223 ceramics. For the undoped sample, the at 77 K is about 26 mA/cm2. The highest value of was observed in the wt.% Co3O4 (10 nm) sample and wt.% Co3O4 (30 nm) sample. At 77 K, of the 10 nm and 30 nm Co3O4 added samples was 6 and 13 times larger than the nonadded samples, respectively. However, the addition of Co3O4 (50 nm) decreased the at 77 K (0.01–0.05 wt.%) compared to the nonadded sample. The of the sample increased to 358 (mA/cm2) in the 0.03 wt.% Co3O4 (30 nm) sample at 77 K. The results, showed that nano-Co3O4 (30 nm) with wt.% is the optimum amount for the highest . The added Co3O4 nanoparticles towards enhancing the critical current density for low concentration and for higher concentration it acts to suppress . The enhancing contribution is due to their magnetic and core vortex pinning forces. The suppressing contribution is due to their effect on the grain size and shape [7].
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Figure 7 shows the scanning electron micrograph of wt.% (10 nm), wt.% (30 nm), and 0.01 wt.% (50 nm) samples. For the nonadded sample, clean and flaky grains are observed which is the typical structure of the Bi-2223 system. The grain morphology of all the added samples is more or less identical except for minor variations in texture and porosity. The undoped sample showed the largest grain size. XRD analysis indicated that all samples with nano-Co3O4 showed the Bi-2212 peaks, which correspond to the low phase. However, the micrograph of wt.% of Co3O4 (50 nm) showed a different surface morphology compared to nonadded sample. The microstructures reveal a minor difference in the porosity levels with lower levels of Co3O4. Thus, the addition of Co3O4 enhanced the sample morphology slightly. Thus, the Co3O4 nanoparticles have a clear effect on the microstructure of the sample. The decrease of the porosity level enhanced the critical current density [7]. The distribution Co3O4 in the samples with (10 nm), (30 nm), and (50 nm) is shown as white dots. The nanoparticles were dispersed on the surface of the grains and filled the weak links between the grains. For sample with (50 nm), there are many voids and pores. Moreover, the grain texture of this sample is also reduced compared to other samples.
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In conclusion, the effect of nano-Co3O4 addition on flux pinning capability of bulk superconductor (Bi1.6Pb0.4)Sr2Ca2Cu3O10 was investigated. The larger Co3O4 nanoparticle (50 nm) had a greater degradation effect on superconductivity of the Bi-based materials. The highest was observed in sample with wt.% Co3O4 (30 nm). Our results showed that suppression factors of the critical current density may dominate the enhancing factors for higher Co3O4 concentrations. With judicious amounts of Co3O4 nanoparticle of size 10 or 30 nm, the critical current density can be improved in the Bi-2223 superconductor.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
This research has been supported by the Ministry of Education of Malaysia under Grant no. FRGS/2/2013/SG02/UKM/01/1 and Universiti Kebangsaan Malaysia under Grant nos. DLP-2011-018 and DIP-2012-032.