Effect of additional elements in the glass precursor on the growth of the Bi 2 Sr 2 Ca 2 Cu 2 O x whiskers
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A. Tampieri CNR/Research Institute Ceramics Technology, via Granarolo 64, 48018 Faenza (RA), Italy (Received 18 April 1994; accepted 10 May 1994)
The effect of the doping on the growth and superconducting properties of whiskers in the B i - S r - C a - C u - X - O system (where X = Na, B, Ga, and Pb, respectively), prepared by heating a glassy melt-quenched plate in flowing oxygen, is reported. The thermal behavior of the glassy samples (studied by DTA, XRD, and SEM techniques) was found to be dependent on the dopant as well as on its concentration. The Bi 2 Sr 2 CaCu 2 0 x (2212 phase) whiskers were grown only from the Ga-doped system annealed at 855 °C. The dependence of the mechanism of the whiskers' growth on the dopant is also discussed.
I. INTRODUCTION Since the discovery of glass formation in the B i - S r C a - C u - 0 (BSCCO) system,1-2 many studies have been performed in the preparation of high- Tc superconducting materials by the glass-ceramic process.3'4 This method has the following advantages for the preparation of superconducting oxides: (i) homogeneous materials can be obtained, (ii) shape formation is easier,4'5 (iii) superconducting fibers may be drawn,6 (iv) the crystal orientation can be obtained,7 and (v) superconducting whiskers can be grown from the as-cast specimen.8 Precursor glasses in the Bi system are usually obtained by quenching the melts above 1000 °C of a mixture of starting materials such as oxides and carbonates. Glassy solid is easily obtained if the quenching rate is sufficiently high. Afterward it is transformed into a polycrystalline superconductor by appropriate heat treatments. Three superconducting phases have been discovered in the Bi system: Bi 2 Sr 2 Cu0^ (2201 phase),9 Bi 2 Sr 2 CaCu 2 0^ (2212 phase), and Bi 2 Sr 2 Ca 2 Cu 3 O x (2223 phase),10 with superconducting transition temperatures (Tc) around
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