Improvement of properties of BSCCO superconductor tapes with thermal processing
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I.
INTRODUCTION
SINCE the discovery of high-temperature superconducting Bi-Sr-Ca-Cu-O compounds in 1988,[1] three different superconducting phases were discovered. Those include high-TC Bi2Sr2Ca2Cu3Ox (BSCCO 2223) (x ≅ 10, TC ≅ 105 K), Bi2Sr2CaCu2Ox (BSCCO 2212) (x ≅ 8, TC ≅ 80 K), and lower TC Bi2Sr2CuOx (BSCCO 2201) (x ≅ 6, TC ≅ 20 K). It is reported[2] that it is relatively easy to develop a c-axis texture in BSCCO due to its very micaceous structure, quite unlike other high-TC materials. The presence of a liquid phase by partial melting during BSCCO formation helps densification and grain alignment and repairs any damage done during the mechanical processing.[3,4,5] It is very important to align the c-axis normal to the tape surface to improve the critical current density (JC) due to the poor conductivity along the c-axis of BSCCO. High-TC BSCCO is considered for such applications as power transmission cable, transformers, electric motors, and generators that require high critical current density.[6] The powder-in-tube (PIT) process for BSCCO superconductor wires and tapes with silver and its alloys as the sheathing material is a promising and industrially scalable technique for fabricating long-length superconductors for the aforesaid types of applications. Although considerable progress has been made in the past few years in fabricating PIT tapes and enhancing their critical current density, the complexities[7–10] involved in obtaining phase purity, crystalline orientation, good intergranular connectivity, and sufficient mechanical strength in a brittle multicomponent oxide such as BSCCO have long impeded the successful commercialization of high-TC superconductors. The following research varies the thermal processing in order to improve the critical current density and also investigates the texture properties of BSCCO superconductor tapes.
H.-J. LIM, Research Associate, is with the Center for Materials Research, Stanford University, Stanford, CA 94305-4045. J.G. BYRNE, Ivor Thomas Professor of Metallurgy and Chairman, is with the Department of Metallurgical Engineering, University of Utah, Salt Lake City, UT 84112. Manuscript submitted November 26, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS B
II.
EXPERIMENTAL
Reagent-grade Bi2O3, PbO, SrCO3, CaCO3, and CuO powders were combined with cation stoichiometries of Bi, Pb, Sr, Ca, and Cu to arrive at the BSCCO group of superconducting phases. Powders were ball milled over 16 hours with isopropanol and ZrO2 as the grinding medium. The mixed powders were calcined twice in a reduced oxygen atmosphere. Tapes were prepared by the PIT method. Prepared precursor powders were poured into a silver tube. The silver tube was sealed and drawn to a 1.387-mm diameter and then rolled into tape of about 0.3-mm thickness with less than a 10 pct thickness reduction per pass. The tape was cut into 5-cm-long pieces and intermediate heat treated in a 10 pct oxygen atmosphere. Finally, tapes were rolled to about 0.15-mm thickness and partially melted, followed by different cool
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