Growth of superconducting and non-superconducting whiskers in the Bi-Sr-Ca-Cu-O (BSCCO) system
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K. Togano Institute for Materials Research, Tohoku University, Aoba-ku, Sendai, 980-8577 Japan; and National Institute for Materials Science, Tsukuba, 305-0047 Japan (Received 18 April 2005; accepted 30 August 2005)
Growth of such non-superconducting whiskers [Sr2.24Ca0.4Al2Ox, (Ca0.8–0.85Sr0.15–0.1)2CuO3, CuO, or Bi2.44Sr2Ca1.3–2Cu6.9–9.95Al0.35–0.46Ox (Cu-rich whiskers)] formed during the growth of Bi-2212 superconducting whiskers from powder or glassy substrates, is discussed. These whiskers are likely to grow from the bottom end, and there is a tight relationship with the growth of the Bi-2212 whiskers. A general reaction-path model for the whisker growth in the BSCCO system, independent of the type of the catalytic impurity and substrate, is proposed. When whiskers are grown under magnetic fields, up to 10 T, changes in the whisker size, aspect ratio, and morphology are observed.
I. INTRODUCTION
The single-crystal nature and shape of whiskers, convenient in many cases in both measurements and applications, are attracting much attention. Whiskers are also interesting objects to study growth mechanism. Understanding of the growth mechanism allowed recent successful development of nanomaterials engineering due to resemblance between growth of microscopic whiskers and nano-objects. Nanoscale wires, tubes, cages, stars, multi-pods, cubes, springs, branched (tree) structures, and many other items have been synthesized. Usually this was successful in relatively simple systems composed of one or two elements. For multicomponent materials, however, the syntheses are far more complicated. Sometimes, in such systems, the formation of the usual bulks, films, or crystals is not satisfactorily understood, and the phase diagrams are not completely solved. This situation works not only against growth of nano-objects of a certain material but also against their present applications in daily life. High-temperature superconductors (HTS) have shown enormous potential. This potential has not been realized in practice yet, with few exemptions only. There are also no reports, to the knowledge of the authors, on nanoobjects of these materials. On the other hand, in the presence of an impurity, micro- to millimeter-range whiskers can be obtained. The most popular HTS system for the
II. EXPERIMENTAL
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0413 3358
http://journals.cambridge.org
growth of superconducting whiskers is Bi-Sr-Ca-Cu-O (BSCCO). Many excellent articles are devoted to growth of BSCCO superconducting phases in general, or of whiskers in particular, but some aspects of the growth mechanism and phase equilibrium are not yet clear. Moreover, in the BSCCO system, whiskers of nonsuperconducting phases were sometimes reported. The authors of Ref. 1 observed whiskers of the Bi-2201 phase. Calcium- and copper-rich cuboid-type whiskers, different from the usual ribbon-like Bi-based superconducting ones, were found by Uemoto et al.,2 while Yamada et al.3 found whiskers
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