The investigation of phase evolution in composite ceramic superconductors using Raman microscopy techniques

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Physica C 341-348 (2000) 2243-2244 www.elsevier.nl/Iocate/phy sc

T h e i n v e s t i g a t i o n o f p h a s e e v o l u t i o n in c o m p o s i t e c e r a m i c s u p e r c o n d u c t o r s u s i n g R a m a n m i c r o s c o p y techniques v. A. Maroni a, A.K. FischeP, and K.T. Wu b aChemical Technology Division, Argonne National Laboratory 9700 South Cass Avenue, Argonne, IL 60439, USA b State University of New York, Old Westbury Campus, Old Westbury, NY 11568, USA Raman microspectroscopy and imaging techniques are being used to investigate key mechanistic features that influence the formation of layered Bi-based superconducting phases during the thermal treatment employed to produce silver-sheathed Bi-2223 composite conductors. Seminal information gained from these studies includes the identification of the constituent phases in certain nonsuperconducting second phase (NSP) agglomerations that tend to resist dissipation as high-Tc phase formation proceeds to completion. 1. INTRODUCTION Raman microspectroscopy and imaging Raman microscopy offer unique opportunities for studying the evolution and spatial distribution of chemical phases in high-critical-temperature superconducting ceramics. When applied in conjunction with powder x-ray diffraction, scanning electron microscopy, and energy dispersive x-ray spectroscopy, it is possible to gain seminal insights about the identity, size, shape, orientation, and spatial distribution of the various NSPs that form and dissipate during heat treatment of silver-sheathed Bi-2223 composite tapes and related superconducting ceramic embodiments [1-3]. The results reported herein illustrate how imaging Raman microscopy can be used to make specific chemical identifications of alkaline earth cuprate (AEC) NSPs and to map their spatial distribution with respect to the layered Bi-2223 superconducting phase. 2. EXPERIMENTAL

The experimental methods/instrumentation used in this research are reported in detail elsewhere [1,2]. In brief, heat treated Ag/Bi-2223 specimens are cast in epoxy and the casting is polished until the ceramic core of the specimen is exposed. Raman microscopy is performed on the exposed cores (usually mounted in transverse view). Figure 1 presents an example of the type of measurement we typically make using Raman microscopy techniques. The "defocused Raman spectrum" in Fig. 1 was obtained by

spreading the excitation laser over the circled region indicated in the "white light image" (WLI) of the specimen (in this case a transverse-mounted, fullyprocessed, 19-filament Ag/Bi-2223 composite) and recording the Raman spectrum in the grating mode [ 1,2]. The silver sheath regions in the WLI shown in Fig. 1 have been darkened by image processing to provide contrast with the ceramic filaments. Portions of three ceramic filaments can be seen in the WLI. 3. RESULTS AND DISCUSSION

From previous phase characterization studies [1-3] we know that the Raman features at 626, 570, and 520 cml in the defocused spectrum shown in Fig. 1 are due to Bi-2223, (Ca,Sr)14Cu24041 (14/24 AEC), and (Ca,Sr)2CuO3 (2/1 AEC), respectively. Using our imaging Raman microscope in the filter mode [1-2], we are able to determine the location within the circled area in the WLI from which the Raman scattering at each of the three frequencies is occurring, as shown by the three circled Raman images to the right of the WLI in Fig. 1. Each of the three Raman images has been corrected to remove background (BG) effects [1-2]. In these three images the white regions represent the locations of the phases scattering at the frequency indicated below each image. The image of the 626 cm -~ feature, I(626)/BG, reveals that the layered Bi-2223 phase is located in bands along the silver sheath. The images for the 570 and 520 cm -1 features show how the (Ca,Sr)14Cuz4041 and (Ca,Sr)2CuO3 NSPs tend to

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agglomerate and overlap in the mid-core region of the filament. The appearance of overlapping phase domains is associated with the penetration depth of the excitation laser. These types of measurements illustrate how NSPs tend to aggregate in the mid-core region of the filaments in Ag/Bi-2223 composites with short transverse filament dimensions.

This research was sponsored by the U.S. Department as part of a DOE program to develop electric power technology, under Contract W-31-109-ENG-38. REFERENCES lo

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ACKNOWLEDGEMENT The Ag/Bi-2223 multifilament conductor used in this study was supplied by American Superconductor.

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K.T. Wu et al., J. Mater. Res., 12 (1997) 1195. V.A. Maroni et al., Spectroscopy, 12 (1997) 38. V.A. Maroni et al., in Spectroscopy of Superconducting Materials, E. Faulques (ed.), ACS Symp. Series 730, Am. Chem. Soc., Washington, DC, 1999, p. 156.

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Figure 1. Raman microscopy results for a fully-processed, 19-filament Ag/Bi-2223 composite. The white domains in Raman images I(520)/BG, I(570)/BG, and I(620)/BG indicate the locations of the 2/1 AEC, the 14/24 AEC, and Bi-2223, respectively, in the circled region of the white light image.