Electrospinning of parallel BSCCO-2212 X. L. Zeng1, T. Karwoth1, M. R. Koblischka1, V. Presser2,3, and U. Hartmann1 1Institute
of Experimental Physics, Saarland University, Campus C 6 3, 66123 Saarbrücken, Germany 2INM – Leibniz Institute for New Materials, Campus D 2 2, 66123 Saarbrücken, Germany 3Saarland University, Campus D 2 2, 66123 Saarbrücken, Germany Abstract Bi2Sr2CaCu2O8+x (Bi-2212) nanowires were fabricated employing electrospinning. The subsequent thermal treatment is necessary to obtain the final crystal structure and superconductivity. The final average diameter of the nanowires is around 100 nm and the overall length can reach up to 100 µm according to the electron microscopy. Phase and crystallization information is given by X-ray diffraction and transmission electron microscopy. In order to obtain elongated and parallel nanowires, a SiO2/Si wafer containing a narrow slit is employed as target. This new preparation approach of high-temperature superconductor nanowires may provide several advantages in the subsequent electric and magnetic measurements of the superconducting properties. Introduction Electrospinning BSCCO is a family of cuprate superconductor, which is also the first high-temperature superconductor without rare earth element. The well-known phases 2212 and 2223 have been discovered with the TC at 85 K and 108 K in bulk materials respectively. High critical temperature and high upper critical field make this system highly attractive. With the renewed interest in the superconductivity of nanoscale systems, BSCCO nanowire become good candidates for the investigation of the quantum confinement effect and the study of the relationship between superconductivity and reduced dimensionality[1][2][3]. The template-free character of electrospinning is a promising way to synthetize relative long superconducting ultra-fine fiber[4]. The specific orientated fibers can be fabricated by the design of the electric field. Compared to the single fiber, the parallel fibers are more advantageous for measurements with a stronger signal and a lower system resistance.
Electrospinning is used for the synthesis of ultrafine fibers and is commonly used for polymer fiber fabrication. Combined with a suitable thermal treatment, it can also be used for inorganic nanowire fabrication. The precursor is supplied by a pump to a needle tip and the high voltage between the needle and the current collector realigns the mobile charge distribution in the liquid phase. When the Coulomb force overcomes the surface tension, a jet is formed, pulled to the electrically grounded current collector, and deposited accordingly.
Schematic illustration of electrospinning
Parallel fiber fabrication In order to obtain elongated and parallel nanowires, a SiO2/Si wafer containing a narrow slit is employed as target. The electric field is redistributed on the wafer, leading to the fibers suspend over the slit. In this way, parallel fibers are easily prepared.
Theraml treatment To obtain the 2212 phase sample with the maintenance of the fiber structure, the thermal treatment is separated into several steps according to the thermal gravity analysis.
Phase analysis with X-ray diffraction For the bulk material, it is necessary to heat the sample over 825°C to obtain the superconducting phase. Thus, for the final step of thermal treatment, the sample is annealed in the case furnace at 800°C for 1 h. According to X-ray diffraction, the phase has been confirmed as BSCCO 2212. The peaks are broader than the one of single crystal, because of the nanometer-scale size of the domains (with an average of about 50 nm). 5
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x.zeng@physik.uni-saarland.de
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after 1 h, 800°C annealing
BSCCO nanowires Lorentz fitting Avg. Dia.=53.2 nm
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TEM is applied to investigate the crystal structure. The diameter of the fiber is about 200 nm. The crystallite size observed from the dark field image ranges from 20 nm to 200 nm with an everage of about 53 nm; this is in excellent agreement with the XRD result. Crystallites with larger size than the domain size consist of multiple domains.
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Transmission electron micrographs
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Bi2.1Sr1.9CaCu2O8.21
Scanning electron micrographs (SEM) The SEM images below are the comparison of fibers collect by the common aluminum foil and Si wafer with slit. The length of the fibers in both cases is over 100 µm, which is a unique advantage of electrospinning. The fibers collected by common aluminum foil are highly interconnected, which serves as a fiber network. During electric measurements, the network will introduce some inner junctions to the sample so that it will behave differently than a single fiber. The parallel fibers have less junctions, thus certain sample can be treated as a shunt circuit. The properties would be similar to the single fiber while the system is much smaller. Random located fibers Parallel fibers References: [1] A. D. Zaikin, D. S. Golubev et al. Phys. Rev. Lett. 78, 1552 (1997) [2] D. S. Golubev, A. D. Zaikin Phys. Rev. B 64, 014504(2001) [3] K. Y. Arutyunov, D. S. Golubev et al. Phys. Rep. 464, 1(2008) [4] J. M. Li, X. L. Zeng et al. Cryst. Eng. Comm. 13, 6964 (2011)