The Early Stages of the Microstructural Development of the Colony Structure in Bi-2223 Tapes
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The Early Stages of the Microstructural Development of the Colony Structure in Bi-2223 Tapes T.G. Holesinger Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545 U.S.A.
ABSTRACT The current protocol for processing (Bi,Pb)2Sr2Ca2Cu3O10-x (Bi-2223) multifilamentary tapes involves the in situ formation of the primary phase from a suitable mixture of precursor phases. As such, the developments during the first few minutes of heat treatment determine to a large extent the efficiency of primary phase development, competing secondary phase development, texture evolution, and grain-to-grain connectivity. This work documents the development of the liquid phase, secondary phases, defects which may affect alignment and reaction kinetics, and the precipitation of Bi-2223 from the liquid phase. INTRODUCTION It is commonly held that most of the transport current in Bi-2223 tapes is carried close to the silver interface. Hence, it is of interest to document this particular microstructure and determine the conditions during processing that shape its development. The overall structure in fully processed tapes has been documented by electron microscopy as well as by in-situ neutron and synchrotron x-ray diffraction [1-7]. Only a few of the electron microscopy studies have focused on the through-process microstructure development while in-situ XRD and neutron studies suffer from a lack of spatial resolution in tracking phase development within the filaments as a function of position. In this work, phase, microstructure, and composition development in Bi-2223 tapes were examined. An emphasis was placed on detailing the structure and composition of the Bi-2223 development during the initial stages of processing. EXPERIMENTAL Multifilament Bi-2223 tapes at various stages of processing were examined by analytical electron microscopy. The starting composition of the Bi-2223 powder used in the 19-filament tapes was Bi1.7Pb0.3Sr1.9Ca2.0Cu3.0Oy. All of the tapes used in this work were processed at 827˚C in 8% O2/N2. After ramping up to the processing temperature, the Bi-2223 tapes were held for a short dwell time and then quenched into an oil bath. The dwell times were 0, 10, 100, 300, 400, 500, and 640 minutes. The critical current densities (Jc) of fully processed tapes from this series of samples reached 70 kA/cm2 at 77 K and self-field. These tapes were examined from three principle directions of view in scanning and transmission electron microscopes (SEM and TEM): short transverse section, longitudinal transverse section, and plan view. Final TEM and SEM specimen preparations were accomplished with dimple polishing and ion-milling. Compositional measurements were performed in the TEM using energy dispersive spectroscopy (EDS). Standards consisted of
E3.4.1
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2:1 AEC
14:24 AEC
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Figure 1: Comparison of the changes in the microstructure between the as-rolled Bi-2223 tapes, (a) SEM and (c) TEM, and the microstructure of the tapes quenched after ramping up to the pr
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