Impact of silver addition on the superconducting performances of Bi2Sr2Ca0.925Na0.075Cu2Oy:Ag composite fibers
Introduction
Among high temperature superconductors (HTc), one of the most studied is the Bi-Sr-Ca-Cu-O system (BSCCO), discovered by Maeda and his group [1]. It has a general chemical formula Bi2Sr2Can-1CunO2n+4+δ, where n = 1, 2, and 3, indicating the number of CuO2 layers in the crystallographic unit cell. Depending on the n value, BSCCO system consists of three different phases, Bi-2201 (n = 1), Bi-2212 (n = 2), and Bi-2223 (n = 3), with 20, 85, and 110 K critical temperatures (Tc), respectively [1], [2]. Among these phases, the Bi-2212 is the most used in technological applications due to its stability in a wide range of compositions, and processing temperatures. However, several issues may restrict their electrical transport properties: i) formation of weak links between grains [3]; ii) formation of secondary phases along the grain boundaries, and iii) weak flux pinning properties resulting from intrinsic defects in the lattice structure.
Some strategies used to overcome these issues and enhance the transport properties of these compounds are based on the addition of metallic elements which can modify the phase equilibria without reacting with the superconducting phase [4], [5], [6] and enhance density, critical current density (Jc), and critical magnetic field (Hc). Another possibility is based on the modification of charge carrier concentration in the Cu-O planes and/or enhancing the grain alignment [7], [8], [9], [10], [11], [12], which can influence critical temperature, critical current density (Jc), and critical magnetic field (Hc). The most usual techniques to produce the grain alignment in the bulk materials are spark plasma texturing [13], hot uniaxial pressing [14], the Laser Floating Zone (LFZ) [15], and the Electrically Assisted Laser Floating Zone (EALFZ) [8]. Among these techniques, the LFZ method has many advantages for obtaining long BSCCO fibers grown at high rates. In this method, the microstructure of the materials is described by a good grain orientation with their growth direction quasi-perpendicular to the c-axis [16]. These large and highly oriented grains dramatically improve the transport properties, Jc, as a result of reduction in the number of low-angle junctions [17]. On the other hand, these compounds display incongruent melting [18] and, consequently, as-grown materials must be subjected to an annealing process for obtaining nearly pure Bi-2212 phase from the secondary ones [19]. Another drawback of these materials are their poor mechanical properties, which impose severe restrictions for technological applications. However, it has been shown that Ag addition leads to the improvement of mechanical properties of BSCCO compound [19].
When evaluating transport properties in HTc superconductors, two-stage resistive transition behavior is found. The first one is the onset temperature (Tconset), where resistivity starts to decrease, while the other one is offset temperature (Tcoffset) in the tail part of the resistivity. Tconset indicates the superconducting transition of isolated grains, while Tcoffset refers to intergranular coupling. This last one decreases under external applied fields due to the Lorentz force associated to motion of flux-lines, which can be classified into three regimes depending on the applied current density: flux flow (FF) (J> Jc), thermally activated flux flow (TAFF) (J Jc), and flux creep (FC) (JJc) [20]. On the other hand, the presence of pinning centers overcomes the Lorentz force effect, and the flux-lines can be trapped. These pinning centers may appear naturally [21], or can be artificially created using chemical doping [11], [12], [22], [23], [24], [25].
This study is focused on the investigation of magneto-resistivity, irreversibility and flux pinning energy of textured Bi2Sr2Ca0.925Na0.075Cu2Oy + x wt% Ag superconducting composites produced through a sol-gel method via nitrates, followed by LFZ texturing. Textured materials will be structurally, microstructurally, and magnetically characterized, and these characteristics will be evaluated. Moreover, the experimental data obtained through the magneto-resistivity curves recorded between 0 and 6 T external applied magnetic field will be analyzed on the basis of the TAFF model to determine their pinning characteristics.
Section snippets
Experimental procedure
Bi2Sr2Ca0.925Na0.075Cu2Oy + x wt% Ag, with x = 0, 1, 3, and 5, polycrystalline samples have been produced using sol-gel method from Bi2O3 (≥ 99.9%, Aldrich), SrCO3 (98%, Panreac), CaCO3 (≥ 99%, Aldrich), Na2CO3 (≥ 99.99%, Aldrich), CuO (98%, Panreac) and Ag (≥ 99%, Aldrich) commercial powders. They were weighed in the appropriate proportions and dissolved in a mixture of HNO3 (Fluka, PA) and distilled water. Once a clear blue solution has been produced, adequate amounts of ethylene glycol (99%,
Results and discussion
Representative DTA-TGA analysis, performed on the dry gel of the 3 wt% Ag sample, is shown in Fig. S2. As it can be observed in the plot, three main zones can be distinguished in the TGA graph. The first one, between room temperature and 150 ºC corresponds to solvent evaporation, the second one, between 150 and 300 ºC can be associated to the decomposition of organic part, drastically decreasing the sample weight. This fact is confirmed by the large exothermic peak in the DTA graph shown in
Conclusion
In the present work, Bi2Sr2Ca0.925Na0.075Cu2Oy + x wt% Ag, with x = 0, 1, 3, 5 composite fibers were prepared through the sol-gel via nitrates method followed by LFZ texturing. After annealing the as-grown samples, XRD data showed that samples were mainly composed of Bi-2212 and metallic Ag, with small amounts of secondary phases. FESEM micrographs and EDX results confirmed these data, and showed good grain orientation and low porosity in these samples. Moreover, Ag particles are mainly found
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The results used in this manuscript has been partially taken from master thesis of B. Atilla named “Determination of Magnetoresistance Properties of Bi2Sr2Ca0,925Na0,075Cu2Oy Superconductors Added with Different Proportions of Silver (Ag)”, directed by Prof. Dr. B. Özçelik. M. A. Madre and A. Sotelo wish to thank the Gobierno de Aragón (Research Group T 54–20R) for funding. The authors also acknowledge the use of Servicio General de Apoyo a la Investigación-SAI, Universidad de Zaragoza.
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