Nanostructured high-temperature superconductors: Creation of strong-pinning columnar defects in nanorod/superconductor c
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Nanostructured high-temperature superconductors: Creation of strong-pinning columnar defects in nanorod/superconductor composites Peidong Yang and Charles M. Lieber Division of Engineering and Applied Sciences, and Department of Chemistry, Harvard University, Cambridge, Massachusetts 02138 (Received 14 April 1997; accepted 10 June 1997)
A chemical approach to the formation of columnar defects involving the growth and incorporation of MgO nanorods into high temperature superconductors (HTS’s) has been developed. MgO nanorods were incorporated into Bi2 Sr2 CaCu2 Oz , Bi2 Sr2 Ca2 Cu3 Oz , and Tl2 Ba2 Ca2 Cu3 Oz superconductors at areal densities up to 2 3 1010ycm2 . Microstructural analyses of the composites demonstrate that the MgO nanorods create a columnar defect structure in the HTS matrices, form a compositionally sharp interface with the matrix, and self-organize into orientations perpendicular and parallel to the copper oxide planes. Measurements of the critical current density demonstrate significant enhancements in the MgO nanorod/HTS composites at elevated temperatures and magnetic fields compared with reference samples.
I. INTRODUCTION
Large critical current densities (Jc ’s) are essential to many proposed applications of HTS’s, such as wires for power transmission cables and solenoids.1,2 Consequently, enhancing Jc in HTS materials has been and remains a topic of great scientific and technological interest. In general, Jc is limited by both intrinsic and extrinsic factors.1,3,4 For example, Jc is limited by thermally activated flux flow (TAFF); that is, Jc vanishes well below the upper critical field line Hc2 sTd as a result of the motion of magnetic flux lines. This limitation is intrinsic, and arises from the short coherence lengths and large anisotropies of the HTS materials that lead to weak pinning of flux lines.3–6 In the polycrystalline materials used for large-scale applications, Jc is also limited by extrinsic factors such as the interfaces between superconducting grains and the development of microcracks during processing.3,7 Poor alignment of crystalline grains and chemical heterogeneity at their boundaries, as well as microcracks, produce weak links with low values of Jc . Over the past several years, significant progress has been made in improving the alignment of grains and consequently improving Jc ’s in wires and tapes based on Ag –Bi2 Sr2 Can–1 Cun O2n+4 (n 2, BSCCO2212; n 3, BSCCO-2223), and YBa2 Cu3 Oz (YBCO) by identifying how material processing conditions affect microstructure and transport currents.3,8–10 Despite this progress in processing, which is now yielding BSCCObased wires with Jc values sufficiently high for some commercial applications10 and exciting performance in aligned YBCO tapes,9 the intrinsic problem of TAFF J. Mater. Res., Vol. 12, No. 11, Nov 1997
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remains a limitation to the performance of HTS materials at high temperatures and magnetic fields.11–13 The tradi
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