Formation of Bi-2212 superconducting whiskers from melt-quenched BSCCO containing alumina
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Koichi Nakamura Department of Electrical and Computer Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466, Japan
Eikichi Inukai Chubu Electric Power Co. Inc., Oodaka, Midori-ku, Nagoya 459, Japan (Received 13 September 1993; accepted 12 January 1994)
B i - S r - C a - C u - 0 (BSCCO) superconducting whiskers (2212 phase) were prepared by heating in air the compacted specimens of the mixtures of melt-quenched Bi2Srj,Ca2Cu4AliOx powders and alumina powders. Formation of the whiskers depends on the composition and the applied pressure of the compacts. The optimum composition of the melt-quenched products for preparing the long whiskers is Bi 2 Sr2Ca 2 Cu4Ali0 x . Superconducting whiskers < 1 mm in length (2212 phase) containing an excess amount of copper were grown numerously from the specimen compacted at 20 MPa; long whiskers (2212 phase) of 1-5 mm in length were obtained from that compacted at 180 MPa. These whiskers showed diamagnetic signals below Tc — 80 K.
I. INTRODUCTION A key microstructural requirement to achieving high critical current densities in high-temperature oxide superconductors is known to be the elimination of weak links between grains. For the Bi-system, therefore, some processes such as the flux method1 and the floating zone method2 have been investigated to fabricate large-sized Bi 2 Sr 2 CaiCu 2 0 j: crystals (2212 phase). Another recent approach is to prepare 2212 whiskers.3"7 Matsubara et al. reported the whiskers (denoted as 2212 whiskers) were prepared by heating glassy melt-quenched plates of the B i - S r - C a - C u - 0 system doped with Pb or Li.4'5 Abe et al. found that the formation of 2212 whiskers was enhanced drastically by the addition of A12O3 into the melt-quenched product of Bi 1 Sr 1 Ca 1 Cu 2 O^, and suggested that Al 3+ ions play an important role in the whisker formation.7 The whiskers are grown from the melt-quenched thin plates of BiiSriCaiCu 2 Al a O x , but they are more easily formed from the compacted specimens of a powder-mixture of the melt-quenched plates and alumina.7 The present objective is to investigate the effect of the composition and the applied pressure of the mixtures on the whisker formation. II. EXPERIMENTAL PROCEDURE The powders of Bi 2 O 3 , SrCO 3 , CaCO 3 , CuO, and A1(OH)3 were mixed to obtain the nominal composition "'Address all correspondence to this author.
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J. Mater. Res., Vol. 9, No. 5, May 1994
of Bi2SryCa2Cu4Al1O^ (y = 1.6 — 3) using an agate mortar. The mixture was melted in an alumina crucible at 1300 °C for 10 min in an electric furnace in air, and then it was poured onto a stainless steel plate and quenched quickly by iron-pressing, where aluminum ion was incorporated to enhance both vitrification in a casting process8'9 and whisker formation in a reheating process.7 The cast product was pulverized using a zirconia ball mill for 1.5 h. The resultant powders (
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