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Ag composites
Physica C 377 (2002) 67–74 www.elsevier.com/locate/physc
Superconducting critical current densities and synchrotron X-ray diffraction measurements of (Bi,Pb)2Sr2Ca2Cu3Oy /Ag composites A.R. Moodenbaugh a,*, M. Suenaga a, L.H. Lewis a, D.E. Cox a, M.W. Rupich b, G.N. Riley b, Q. Li b, R. Parrella b a
Materials Science Division, Department of Applied Science, Brookhaven National Laboratory, P.O. Box 5000, Upton, NY 11973-5000, USA b American Superconductor Corporation, 2 Technology Drive, Westborough, MA 01581, USA Received 13 April 2001; accepted 28 September 2001
Abstract A series of multifilament composite conductors (tapes), (Bi,Pb)2 Sr2 Ca2 Cu3 Oy (Bi-2223)/Ag, was studied by transmission synchrotron X-ray diffraction, including c-axis texture studies, and by critical current measurements. The diffraction measurements indicate that the tapes typically consist of 90% Bi-2223 phase. Rocking curve studies of the Bi-2223 (2 0 0) peak reveal full widths at half maximum (FWHMs) in the range 13–19°. A weak correlation between rocking curve FWHMs and critical current densities suggests that the critical current values in these tapes may not be limited by the Bi-2223 c-axis orientation. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 74.72.Hs; 74.60.Jg; 61.10:i Keywords: Critical current density; Grain alignment (texture)
1. Introduction The processing of (Bi,Pb)2 Sr2 Ca2 Cu3 Oy (Bi2223) precursor powders in silver tubes by the powder-in-tube (PIT) process is by far the most successful method for the fabrication of this high Tc superconductor. In order to correlate structure
*
Corresponding author. Tel.: +1-631-344-3870; fax: +1-631344-4071. E-mail address: [email protected] (A.R. Moodenbaugh).
with processing variables and practical properties, it is desirable to develop additional analytical methods to monitor and characterize the processing and finished state of superconductor filaments reacted in silver. Transmission X-ray diffraction, using high energy X-rays (24.5 keV [1–3] and 100 keV [4–8]) has been shown to be an effective technique for monitoring the phase purity and caxis alignment (texture) of crystallites of the superconductor within the Ag sheath. Previous work has focussed on the development and alignment of the superconductor grains during processing, and
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on the mechanism for Bi-2223 growth. In this work, in conjunction with the X-ray investigation of structure and texture, we also performed extensive critical current density (Jc ) measurements on a series of production and near-production finished tapes. Some of these tapes exhibit critical current densities up to Jc 130 A/mm2 (at a temperature of 77 K with a field H ¼ 0:1 T applied normal to the tape face). The diffraction results, along with the superconducting properties of these specimens, will be compared with results on earlier experimental samples [1–8]. The rocking curves of the Bi-2223 (2 0 0) peaks obtained in this study possess full widths at half maximum (FWHM) in the range of 13–19°. In contrast to earlier results, we observe a relatively weak correlation between low FWHM (which indicates good crystallographic alignment) and higher Jc . We conclude that the degree of preferred orientation of the superconductor tapes in this study is not a limiting factor in the determination of critical current density Jc . A detailed microstructural investigation, including compositional analysis of two tapes similar to those studied here has been published [9]. Compositional inhomogeneities, impurity phases, as well as compositional variations of the Bi-2223 phase, are documented in that work. Micrographs of a tape cross-section (Fig. 2 in Ref. [9]) show typical grain dimensions, for the Bi-2223, of 1 25 0:1 lm3 and, for Ag, a typical dimension of 30 lm. The critical current density (Jc ) measurements of eighteen of these tapes provide a detailed picture of the superconducting properties. It is essential that Jc s of practical superconductors such
as these be measured in a range of applied fields H as well as in various geometries. Jc as a function of H was measured for two orientations of sample in the field. In addition, the orientation dependence of Jc was measured in a fixed field (H ¼ 0:3 T). Ultimately, we concluded that the field and orientation dependences of Jc are consistent among the present set of samples. According to Hensel et al. [10], the angular dependence of the critical currents can also be used to probe the grain alignment along the current path. In principle this measurement should be complementary to the X-ray texture measurements. However, within the present set of samples, there appears to be little correlation between the observed X-ray texture and the Jc vs. orientation as proposed by Hensel et al. [10].
2. Experimental X-ray diffraction work was performed on 40 Ag/Bi-2223 tape packages produced at American Superconductor Corp. with varying configurations. The tapes contain 19, 37 or 81 filaments of superconductor embedded in a silver matrix. Tape thicknesses range from 0.10 to 0.20 mm while tape widths vary from 2 to 4 mm. Cross-sectional photographs of two tapes are shown in Fig. 1. Eighteen tapes underwent extensive critical current (Ic ) measurements. The Bi-2223 fraction of total cross-sectional area was estimated in order to calculate the critical current density Jc for all composite conductors. The X-ray diffraction measurements were made at the National Synchrotron Light Source at
Fig. 1. Cross-sections of two typical Bi-2223 composite conductors. The light matrices are silver while the darker gray spots are the Bi-2223 conductors: (a) 19 filament tape, (b) 85 filament tape.
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Brookhaven National Laboratory beamline X7A. The energy of the incident radiation, as selected using a Ge(1 1 1) crystal, was set to 24.5 keV, high enough to obtain an X-ray path length comparable to the thicknesses t of the samples [1]. The photon wavelengths k corresponding to two runs of X-ray diffraction experiments were accurately determined by calibration with a CeO2 standard: k ¼ 0:05118 0:00001 nm and 0:05143 0:00001 nm. The samples were measured in the transmission geometry as illustrated in Fig. 2. The incident beam was 2–3 mm wide by 1 mm high. A position sensitive detector (PSD), utilizing flowing Kr/CO2 or Xe/CO2 , acquired data simultaneously over a 4° span in 2h for all measurements. Two types of diffraction experiments were performed. To obtain information on the phase constitution, diffraction scans were performed with the PSD being stepped in 0.25° increments through 2hav (the position of the midpoint of the PSD
Fig. 2. Diagram (not to scale) of measurement geometry for Bi-2223/Ag composite for X-ray diffraction measurements. The dimensions of the tape package are exaggerated for clarity. ki and kf are incident and diffracted X-ray paths respectively. Angles h, hav , and / are defined in the text. PSD is position sensitive detector. The directions of application of field H for the critical current measurements are also indicated (Hperp in the figure is H? ).
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Fig. 3. X-ray diffraction scans, / ¼ hav and / ¼ hav þ 45° for sample ASC026.
range defines 2hav as shown in Fig. 2). For these diffraction scans, the sample was ordinarily oriented with / ¼ hav (Fig. 2). The overlapping data segments were rebinned for analysis using standard beamline software. A diffraction scan (2° 6 2h 6 30°) typically took 40 min. Near 2h 12°, resolution in 2h is better than 0.04°, with typical observed Bi-2223 peak width of 0.06°. In order to obtain additional information on peaks whose intensities were low in the / ¼ hav data due to the strong effect of texture, one sample underwent an additional full scan for / ¼ hav þ 45° (see Fig. 3). Rocking curves, or /-scans, of the Bi-2223 (2 0 0) peak were also obtained to assess the degree of crystallite orientation in the samples. In these experiments the PSD midpoint was set to a fixed 2hav value chosen to monitor the Bi-2223 (2 0 0) peak, and / was stepped in increments of 2° over a 70° interval centered on / ¼ hav . For these samples, FWHM ranged from 13° to 19°. Since the PSD simultaneously accepts data over a range >4° in 2h, the full profile of the Bi-2223 (2 0 0) peak was available for determining the intensity as a function of /. Liu et al. [7] demonstrated the effects of large Bi-2223 crystallite size on X-ray texture using image plates. In that study, the largest grain dimension for the Bi-2223 crystallites approached 25 lm, while the typical grain dimension for the Ag was near 30 lm [4,5]. The present experimental setup,
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utilizing the PSD, was chosen to optimize the data acquisition rate. Various factors, inherent to the PSD, have the potential to affect the accuracy of the rocking curves and diffraction diagrams. The relatively large grains in the composites can produce variable, but potentially very intense, diffraction especially from the Ag grains. High counting rates for the PSD are routinely adjusted using a deadtime correction which must be applied to all data across the 4° range of the PSD. However, the correction procedure is not reliable for some of the high deadtimes encountered in these studies. In some cases the high deadtimes, due primarily to the strong silver diffraction peaks, have the potential to affect the shapes of the rocking curves and the relative intensities of the 2h scans. However, no large inconsistencies were observed in the shapes of the rocking curves, and no quantitative analysis was performed on the 2h diffraction scans. This issue will not be pursued further in this paper. The critical currents Ic s, at a temperature T ¼ 77 K, of a subset of 18 composite conductor packages were obtained across 10 mm lengths L using a four-point probe technique. Ic was recorded for both 1 and 0.1 lv criteria. Based on Ic =L as calculated for the criteria, the critical electric current gradients, Ec , are respectively 0.1 and 0.01 lV/mm. A solenoidal superconducting magnet provided an applied field H 6 3 T. The samples were mounted in a holder capable of being rotated while immersed in liquid nitrogen. Motion of the holder follows an arc, varying the orientation of the tape face with respect to the direction of the field H (represented by an angle a) while maintaining the electric current direction orthogonal to the direction of H (see Fig. 2). H? , a ¼ 90°, is defined as the orientation with the field direction parallel to a vector traversing the thin tape dimension t, in which case the tape profile intercepts the maximum magnetic flux. Hk , a ¼ 0°, is the orientation having the field direction parallel to the tape face, where the minimum flux is intercepted. Two basic types of Ic measurements were performed, one varying tape orientation in a fixed field H, the other varying the magnitude of H while the sample was held in a fixed position.
Critical currents (Ic s) as a function of tape orientation were obtained in applied fields H ¼ 0:3 and 0.1 T. The application of these moderate fields during the Ic measurements significantly reduces the effect of self-field on the measured critical currents [11]. In a second type of experiment, Ic s as a function of applied field were obtained for orientation angles a ¼ 0° (0 < H < 3 T) and a ¼ 90° (0 < H < 0:5 T). All Ic s were converted to critical current densities Jc s using superconductor filament cross-sectional areas calculated from measured tape dimensions multiplied by filling fractions estimated from optical studies (see Fig. 1). We estimate the uncertainty in the conversion from Ic to Jc to be 10%.
3. Results 3.1. X-ray diffraction An example of the X-ray diffraction studies done in transmission mode for tape ASC026 is represented in Fig. 3. The data are plotted as a function of d-spacing to facilitate comparison with data taken using other wavelengths. Approximate Bragg angles 2h are indicated at the top of the figure. All samples were surveyed using / ¼ hav orientation, which, due to the strong texturing of these composites, emphasizes the ðh k 0Þ peak intensities. For the sample ASC026, we also obtained a full scan for / ¼ hav þ 45° (see Fig. 3). Before plotting, the peak intensities of the ASC026 / ¼ hav þ 45° data were corrected for the greater attenuation of X-rays within the tape relative to / ¼ hav , as described by Thurston et al. [1]. Peaks are identified by Bi-2223 indices ðh k lÞ and symbols identified on the plot. A least squares fit to peak positions determined by eye, combining the / ¼ hav and / ¼ hav þ 45° data, yields Bi2223 lattice parameters a ¼ 0:5406ð5Þ nm, b ¼ 0:5404ð3Þ nm, and c ¼ 3:7081ð10Þ nm. An estimate of the Bi-2212 content, relative to Bi-2223, can be made for sample ASC026 based on a direct comparison of the relative intensities of the Bi-2212 (1 0 5) peak and the Bi-2223 (1 1 5) [1]. Because of the high degree of Bi-2223 crystallite orientation in these samples, the Bi-2223 (1 1 5)
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peak is weak in the / ¼ hav data, and the Bi-2212 (1 0 5) is unobservable. Consequently, only the / ¼ hav þ 45° orientation provides useful data on the relative intensities of these peaks. In the /av ¼ h þ 45° orientation, the Bi-2212 (1 0 5) peak intensity is <10% of that of the Bi-2223 (1 1 5) peak (see Fig. 3). Impurity peaks are relatively weak. There are several unidentified peaks observable in the / ¼ hav þ 45° data of intensity between 1% and 5% of the Bi-2223 (1 1 9) peak, with d values in nm of: 0.8141, 0.5510, 0.3458, 0.2511, 0.2284, and 0.2185. The peak at d ¼ 0:2185 nm was also observed by Poulsen et al. [4]. The rocking curve measurements on the Bi2223 (2 0 0) peaks reveal that the crystallites are relatively well aligned. The PSD measurements provide a full profile of the (2 0 0) peak whose integrated intensity was used for constructing the rocking curve. These peak profiles were fit using a least squares procedure. The integrated intensities were then plotted as a function of /. For the Bi2223 (2 0 0) peak of the composite conductor ASC026, the rocking curve is shown in Fig. 4. The intensity vs. / data was fit using the square of a Lorentzian function [1,2,4], also shown in Fig. 4. From this fit, the FWHM was extracted (for ASC026, D/ ¼ 14:6°). The accuracy of the FWHM determined using this procedure is limited by the conformance of the data to the postulated function. We estimate that the fitted FWHM uncertainty is approximately 0.3°. Among the set of
Fig. 4. (2 0 0) rocking curve for ASC026. The line through the data is a fit of a Lorentzian squared function.
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Fig. 5. Critical current densities Jc s as a function of applied field for a selection of samples, for H? (Hperp ) and Hk .
40 multifilament tapes, the FWHMs varied from 13.7° to 18.9°. 3.2. Critical current studies The critical current density data was obtained for 18 samples out of the set of 40 composite conductors on which diffraction measurements were performed. Examples of Jc as a function of H, for H k and H? are shown in Fig. 5. The specimens show consistent dependences of Jc on field. A major objective of this work is to determine whether the critical current densities Jc s of these well optimized tapes are correlated with the FWHMs of the Bi-2223 (2 0 0) peak. Such a correlation was observed in previously examined experimental tapes [1]. Fig. 6 shows Ic as a function of FWHM for a set of earlier composite conductor samples (note that the data is plotted as critical current Ic rather than current density Jc ) [1]. Fig. 6 suggests that, at least for the subject composite conductors, high Ic strongly correlates with a narrow FWHM. Also indicated in Fig. 6 is the range of FWHM observed in this study. The critical current densities obtained in this work cannot be directly compared to the earlier work since the ranges of FWHM do not overlap, and in the previous study, Ic was reported for composites of a different configuration. However we are able to determine whether the strong trend of high
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Fig. 6. Comparison of critical currents and (2 0 0) rocking curve FWHMs from Ref. [1] with the range of FWHMs obtained in the current study (shaded area).
Ic with small FWHM is sustained in a regime of much improved (reduced) Bi-2223 (2 0 0) FWHM. Fig. 7 shows Jc as a function of FWHM for the samples measured here, at conditions given on the graph, identified according to number of filaments.
Fig. 7. Critical current densities Jc plotted against (2 0 0) rocking curve FWHMs. ASC026 is indicated by the arrow.
There is no strong correlation between Jc and FWHM for these samples. For this set of samples, there does appear to be a trend where the maximum observed Jc increases with decreasing FWHM. Fig. 8 shows Jc (normalized as described below) as a function of sample angle a relative to the
Fig. 8. Scaled Jc for a selection of samples as a function of tape orientation in the applied field H (as measured by the angle a as described in the text). Data for one sample is replotted as a function of logðH sin aÞ.
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applied field for six representative composite conductors. The data were scaled by normalizing the maximum Jc value to unity, after correcting for the minimum Jc value obtained at a ¼ 90°. All peaks were shifted so as to place the maxima at a 0. The observed dependences of Jc on a are all very similar. The inset shows data for one sample replotted as a function of logðH sin aÞ, to obtain a critical angle acrit ¼ 5:3°, as described previously [10,12]. Values of a obtained for our samples range from 4° to 8.5°, and again are not correlated with critical current densities.
4. Discussion The transmission high energy X-ray diffraction survey of these composite conductors yields a consistent pattern among the samples studied here. The 2h diffraction patterns show a preponderance of Bi-2223 phase with <10% Bi-2212, plus small amounts of additional phase. The rocking curves for the Bi-2223 (2 0 0) peaks show relatively good crystallite alignment, with FWHMs between 13° and 19°. The expected correlation between FWHM and critical current densities is not clearly demonstrated, as the Bi-2223 (2 0 0) rocking curve widths do not correlate strongly with the measured critical current densities. It is possible that a few grossly misoriented crystallites (>20°) could degrade superconducting properties, while having a negligible effect on the shape of the rocking curve. Another possibility is that the average rocking curve FWHM measured in this experiment is not representative of true local structure. Fig. 2a of Ref. [9] displays a transmission electron microscope bright-field image of aligned ‘‘colonies’’ of a Bi-2223 composite similar to those measured here. The Bi-2223 alignment in the neighborhood of a colony may be much better than the average; the average local (grain to grain) misorientation may not be as great as one might estimate from the rocking curves. There is also some evidence in the same figure for a texture having developed at the boundary of the Ag/Bi-2223. If the Bi-2223 crystallites grow, accurately conforming to an imperfectly formed large-grain Ag boundary, misalignments of Bi-2223 may be difficult to avoid.
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To regularize the geometry of the Ag/Bi-2223 boundary, it might be useful to consider methods for inhibiting Ag grain growth during the preparation process. The diffraction diagrams also are similar among the samples studied here. The samples appear to be 90% Bi-2223. In an earlier study the relative Bi2223 (1 1 5) peak to Bi-2212 (1 0 5) peak intensities were used to estimate the Bi-2223:Bi-2212 compositional ratio. In the present work, due to the high degree of Bi-2223 texture, the subject peaks are greatly diminished in intensity for tape face normal to the beam. From results for a representative composite, the Bi-2212 phase concentrations relative to the Bi-2223 phase appear to be below 10%.
5. Summary A series of fabricated Bi-2223/Ag tapes was studied by transmission high energy X-ray diffraction and by extensive critical current density measurements. High energy transmission X-ray diffraction showed samples to be predominantly Bi-2223, with minority Bi-2212 and unidentified impurity phases. Rocking curves of the Bi-2223 (2 0 0) peaks yielded FWHMs ranging from 13° to 19°, substantially smaller than those observed in earlier studies [1,2,4,5]. Critical current densities, measured under similar conditions on different experimental samples, varied by a factor of two or less. Critical current density measurements showed samples to have similar dependences on applied field H and on tape orientation angle a. There appears to be no firm correlation between FWHM and Jc for this set of samples. Thus it is concluded that critical current densities in these relatively well oriented samples may not be limited by crystallographic texture.
Acknowledgements R.C. Budhani helped acquire X-ray data. Frank Perez performed the critical current measurements. Research at Brookhaven was supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences under
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contract no. DE-AC02-98CH10886. The National Synchrotron Light Source at BNL is supported by the US Department of Energy, Divisions of Materials Sciences and Chemical Sciences.
[6]
[7]
References [1] T.R. Thurston, U. Wildgruber, N. Jisrawi, P. Haldar, M. Suenaga, Y.L. Wang, J. Appl. Phys. 79 (1996) 3122. [2] T.R. Thurston, P. Haldar, Y.L. Wang, M. Suenaga, N.M. Jiswari, U. Wildgruber, J. Mater. Res. 12 (1997) 891. [3] L.-j. Wu, Y.-L. Wang, W. Bian, Y. Zhu, T.R. Thurston, R.L. Sabatini, P. Haldar, M. Suenaga, J. Mater. Res. 12 (1997) 3055. [4] H.F. Poulsen, T. Frello, N.H. Andersen, M.D. Bentzon, M. von Zimmerman, Physica C 298 (1998) 265. [5] H.F. Poulsen, L.G. Andersen, T. Grello, S. Prantontep, N.H. Andersen, S. Garbe, J. Madsen, A. Abrahamsen,
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M.D. Bentzon, M. von Zimmerman, Physica C 315 (1999) 254. T. Frello, H.F. Poulsen, L.G. Andersen, N.H. Andersen, M.D. Bentzon, J. Schmidberger, Supercond. Sci. Technol. 12 (1999) 293. Y.L. Liu, W.G. Wang, H.F. Poulsen, P. Vase, Supercond. Sci. Technol. 12 (1999) 376. L.G. Andersen, H.F. Poulsen, T. Frello, N.H. Andersen, M. von Zimmerman, IEEE Trans. Appl. Supercond. 9 (1999) 1051. T.G. Holesinger, J.F. Bingert, M. Teplitsky, Q. Li, R. Parrella, M.P. Rupich, G.N. Riley Jr., J. Mater. Res. 15 (2000) 285. B. Hensel, J.-C. Grivel, A. Jeremie, A. Perin, A. Pollini, R. Flukiger, Physica C 216 (1993) 339. S. Spreafico, L. Gherardi, S. Slesher, D. Tatelbaum, J. Leone, D. Yu, G. Snitchler, IEEE Trans. Appl. Supercond. 9 (1999) 2159. G. Grasso, B. Hensel, A. Jeremie, R. Flukiger, Physica C 241 (1995) 45.
Superconducting critical current densities and synchrotron X-ray diffraction measurements of (Bi,Pb)2Sr2Ca2Cu3Oy /Ag composites A.R. Moodenbaugh a,*, M. Suenaga a, L.H. Lewis a, D.E. Cox a, M.W. Rupich b, G.N. Riley b, Q. Li b, R. Parrella b a
Materials Science Division, Department of Applied Science, Brookhaven National Laboratory, P.O. Box 5000, Upton, NY 11973-5000, USA b American Superconductor Corporation, 2 Technology Drive, Westborough, MA 01581, USA Received 13 April 2001; accepted 28 September 2001
Abstract A series of multifilament composite conductors (tapes), (Bi,Pb)2 Sr2 Ca2 Cu3 Oy (Bi-2223)/Ag, was studied by transmission synchrotron X-ray diffraction, including c-axis texture studies, and by critical current measurements. The diffraction measurements indicate that the tapes typically consist of 90% Bi-2223 phase. Rocking curve studies of the Bi-2223 (2 0 0) peak reveal full widths at half maximum (FWHMs) in the range 13–19°. A weak correlation between rocking curve FWHMs and critical current densities suggests that the critical current values in these tapes may not be limited by the Bi-2223 c-axis orientation. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 74.72.Hs; 74.60.Jg; 61.10:i Keywords: Critical current density; Grain alignment (texture)
1. Introduction The processing of (Bi,Pb)2 Sr2 Ca2 Cu3 Oy (Bi2223) precursor powders in silver tubes by the powder-in-tube (PIT) process is by far the most successful method for the fabrication of this high Tc superconductor. In order to correlate structure
*
Corresponding author. Tel.: +1-631-344-3870; fax: +1-631344-4071. E-mail address: [email protected] (A.R. Moodenbaugh).
with processing variables and practical properties, it is desirable to develop additional analytical methods to monitor and characterize the processing and finished state of superconductor filaments reacted in silver. Transmission X-ray diffraction, using high energy X-rays (24.5 keV [1–3] and 100 keV [4–8]) has been shown to be an effective technique for monitoring the phase purity and caxis alignment (texture) of crystallites of the superconductor within the Ag sheath. Previous work has focussed on the development and alignment of the superconductor grains during processing, and
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on the mechanism for Bi-2223 growth. In this work, in conjunction with the X-ray investigation of structure and texture, we also performed extensive critical current density (Jc ) measurements on a series of production and near-production finished tapes. Some of these tapes exhibit critical current densities up to Jc 130 A/mm2 (at a temperature of 77 K with a field H ¼ 0:1 T applied normal to the tape face). The diffraction results, along with the superconducting properties of these specimens, will be compared with results on earlier experimental samples [1–8]. The rocking curves of the Bi-2223 (2 0 0) peaks obtained in this study possess full widths at half maximum (FWHM) in the range of 13–19°. In contrast to earlier results, we observe a relatively weak correlation between low FWHM (which indicates good crystallographic alignment) and higher Jc . We conclude that the degree of preferred orientation of the superconductor tapes in this study is not a limiting factor in the determination of critical current density Jc . A detailed microstructural investigation, including compositional analysis of two tapes similar to those studied here has been published [9]. Compositional inhomogeneities, impurity phases, as well as compositional variations of the Bi-2223 phase, are documented in that work. Micrographs of a tape cross-section (Fig. 2 in Ref. [9]) show typical grain dimensions, for the Bi-2223, of 1 25 0:1 lm3 and, for Ag, a typical dimension of 30 lm. The critical current density (Jc ) measurements of eighteen of these tapes provide a detailed picture of the superconducting properties. It is essential that Jc s of practical superconductors such
as these be measured in a range of applied fields H as well as in various geometries. Jc as a function of H was measured for two orientations of sample in the field. In addition, the orientation dependence of Jc was measured in a fixed field (H ¼ 0:3 T). Ultimately, we concluded that the field and orientation dependences of Jc are consistent among the present set of samples. According to Hensel et al. [10], the angular dependence of the critical currents can also be used to probe the grain alignment along the current path. In principle this measurement should be complementary to the X-ray texture measurements. However, within the present set of samples, there appears to be little correlation between the observed X-ray texture and the Jc vs. orientation as proposed by Hensel et al. [10].
2. Experimental X-ray diffraction work was performed on 40 Ag/Bi-2223 tape packages produced at American Superconductor Corp. with varying configurations. The tapes contain 19, 37 or 81 filaments of superconductor embedded in a silver matrix. Tape thicknesses range from 0.10 to 0.20 mm while tape widths vary from 2 to 4 mm. Cross-sectional photographs of two tapes are shown in Fig. 1. Eighteen tapes underwent extensive critical current (Ic ) measurements. The Bi-2223 fraction of total cross-sectional area was estimated in order to calculate the critical current density Jc for all composite conductors. The X-ray diffraction measurements were made at the National Synchrotron Light Source at
Fig. 1. Cross-sections of two typical Bi-2223 composite conductors. The light matrices are silver while the darker gray spots are the Bi-2223 conductors: (a) 19 filament tape, (b) 85 filament tape.
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Brookhaven National Laboratory beamline X7A. The energy of the incident radiation, as selected using a Ge(1 1 1) crystal, was set to 24.5 keV, high enough to obtain an X-ray path length comparable to the thicknesses t of the samples [1]. The photon wavelengths k corresponding to two runs of X-ray diffraction experiments were accurately determined by calibration with a CeO2 standard: k ¼ 0:05118 0:00001 nm and 0:05143 0:00001 nm. The samples were measured in the transmission geometry as illustrated in Fig. 2. The incident beam was 2–3 mm wide by 1 mm high. A position sensitive detector (PSD), utilizing flowing Kr/CO2 or Xe/CO2 , acquired data simultaneously over a 4° span in 2h for all measurements. Two types of diffraction experiments were performed. To obtain information on the phase constitution, diffraction scans were performed with the PSD being stepped in 0.25° increments through 2hav (the position of the midpoint of the PSD
Fig. 2. Diagram (not to scale) of measurement geometry for Bi-2223/Ag composite for X-ray diffraction measurements. The dimensions of the tape package are exaggerated for clarity. ki and kf are incident and diffracted X-ray paths respectively. Angles h, hav , and / are defined in the text. PSD is position sensitive detector. The directions of application of field H for the critical current measurements are also indicated (Hperp in the figure is H? ).
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Fig. 3. X-ray diffraction scans, / ¼ hav and / ¼ hav þ 45° for sample ASC026.
range defines 2hav as shown in Fig. 2). For these diffraction scans, the sample was ordinarily oriented with / ¼ hav (Fig. 2). The overlapping data segments were rebinned for analysis using standard beamline software. A diffraction scan (2° 6 2h 6 30°) typically took 40 min. Near 2h 12°, resolution in 2h is better than 0.04°, with typical observed Bi-2223 peak width of 0.06°. In order to obtain additional information on peaks whose intensities were low in the / ¼ hav data due to the strong effect of texture, one sample underwent an additional full scan for / ¼ hav þ 45° (see Fig. 3). Rocking curves, or /-scans, of the Bi-2223 (2 0 0) peak were also obtained to assess the degree of crystallite orientation in the samples. In these experiments the PSD midpoint was set to a fixed 2hav value chosen to monitor the Bi-2223 (2 0 0) peak, and / was stepped in increments of 2° over a 70° interval centered on / ¼ hav . For these samples, FWHM ranged from 13° to 19°. Since the PSD simultaneously accepts data over a range >4° in 2h, the full profile of the Bi-2223 (2 0 0) peak was available for determining the intensity as a function of /. Liu et al. [7] demonstrated the effects of large Bi-2223 crystallite size on X-ray texture using image plates. In that study, the largest grain dimension for the Bi-2223 crystallites approached 25 lm, while the typical grain dimension for the Ag was near 30 lm [4,5]. The present experimental setup,
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utilizing the PSD, was chosen to optimize the data acquisition rate. Various factors, inherent to the PSD, have the potential to affect the accuracy of the rocking curves and diffraction diagrams. The relatively large grains in the composites can produce variable, but potentially very intense, diffraction especially from the Ag grains. High counting rates for the PSD are routinely adjusted using a deadtime correction which must be applied to all data across the 4° range of the PSD. However, the correction procedure is not reliable for some of the high deadtimes encountered in these studies. In some cases the high deadtimes, due primarily to the strong silver diffraction peaks, have the potential to affect the shapes of the rocking curves and the relative intensities of the 2h scans. However, no large inconsistencies were observed in the shapes of the rocking curves, and no quantitative analysis was performed on the 2h diffraction scans. This issue will not be pursued further in this paper. The critical currents Ic s, at a temperature T ¼ 77 K, of a subset of 18 composite conductor packages were obtained across 10 mm lengths L using a four-point probe technique. Ic was recorded for both 1 and 0.1 lv criteria. Based on Ic =L as calculated for the criteria, the critical electric current gradients, Ec , are respectively 0.1 and 0.01 lV/mm. A solenoidal superconducting magnet provided an applied field H 6 3 T. The samples were mounted in a holder capable of being rotated while immersed in liquid nitrogen. Motion of the holder follows an arc, varying the orientation of the tape face with respect to the direction of the field H (represented by an angle a) while maintaining the electric current direction orthogonal to the direction of H (see Fig. 2). H? , a ¼ 90°, is defined as the orientation with the field direction parallel to a vector traversing the thin tape dimension t, in which case the tape profile intercepts the maximum magnetic flux. Hk , a ¼ 0°, is the orientation having the field direction parallel to the tape face, where the minimum flux is intercepted. Two basic types of Ic measurements were performed, one varying tape orientation in a fixed field H, the other varying the magnitude of H while the sample was held in a fixed position.
Critical currents (Ic s) as a function of tape orientation were obtained in applied fields H ¼ 0:3 and 0.1 T. The application of these moderate fields during the Ic measurements significantly reduces the effect of self-field on the measured critical currents [11]. In a second type of experiment, Ic s as a function of applied field were obtained for orientation angles a ¼ 0° (0 < H < 3 T) and a ¼ 90° (0 < H < 0:5 T). All Ic s were converted to critical current densities Jc s using superconductor filament cross-sectional areas calculated from measured tape dimensions multiplied by filling fractions estimated from optical studies (see Fig. 1). We estimate the uncertainty in the conversion from Ic to Jc to be 10%.
3. Results 3.1. X-ray diffraction An example of the X-ray diffraction studies done in transmission mode for tape ASC026 is represented in Fig. 3. The data are plotted as a function of d-spacing to facilitate comparison with data taken using other wavelengths. Approximate Bragg angles 2h are indicated at the top of the figure. All samples were surveyed using / ¼ hav orientation, which, due to the strong texturing of these composites, emphasizes the ðh k 0Þ peak intensities. For the sample ASC026, we also obtained a full scan for / ¼ hav þ 45° (see Fig. 3). Before plotting, the peak intensities of the ASC026 / ¼ hav þ 45° data were corrected for the greater attenuation of X-rays within the tape relative to / ¼ hav , as described by Thurston et al. [1]. Peaks are identified by Bi-2223 indices ðh k lÞ and symbols identified on the plot. A least squares fit to peak positions determined by eye, combining the / ¼ hav and / ¼ hav þ 45° data, yields Bi2223 lattice parameters a ¼ 0:5406ð5Þ nm, b ¼ 0:5404ð3Þ nm, and c ¼ 3:7081ð10Þ nm. An estimate of the Bi-2212 content, relative to Bi-2223, can be made for sample ASC026 based on a direct comparison of the relative intensities of the Bi-2212 (1 0 5) peak and the Bi-2223 (1 1 5) [1]. Because of the high degree of Bi-2223 crystallite orientation in these samples, the Bi-2223 (1 1 5)
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peak is weak in the / ¼ hav data, and the Bi-2212 (1 0 5) is unobservable. Consequently, only the / ¼ hav þ 45° orientation provides useful data on the relative intensities of these peaks. In the /av ¼ h þ 45° orientation, the Bi-2212 (1 0 5) peak intensity is <10% of that of the Bi-2223 (1 1 5) peak (see Fig. 3). Impurity peaks are relatively weak. There are several unidentified peaks observable in the / ¼ hav þ 45° data of intensity between 1% and 5% of the Bi-2223 (1 1 9) peak, with d values in nm of: 0.8141, 0.5510, 0.3458, 0.2511, 0.2284, and 0.2185. The peak at d ¼ 0:2185 nm was also observed by Poulsen et al. [4]. The rocking curve measurements on the Bi2223 (2 0 0) peaks reveal that the crystallites are relatively well aligned. The PSD measurements provide a full profile of the (2 0 0) peak whose integrated intensity was used for constructing the rocking curve. These peak profiles were fit using a least squares procedure. The integrated intensities were then plotted as a function of /. For the Bi2223 (2 0 0) peak of the composite conductor ASC026, the rocking curve is shown in Fig. 4. The intensity vs. / data was fit using the square of a Lorentzian function [1,2,4], also shown in Fig. 4. From this fit, the FWHM was extracted (for ASC026, D/ ¼ 14:6°). The accuracy of the FWHM determined using this procedure is limited by the conformance of the data to the postulated function. We estimate that the fitted FWHM uncertainty is approximately 0.3°. Among the set of
Fig. 4. (2 0 0) rocking curve for ASC026. The line through the data is a fit of a Lorentzian squared function.
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Fig. 5. Critical current densities Jc s as a function of applied field for a selection of samples, for H? (Hperp ) and Hk .
40 multifilament tapes, the FWHMs varied from 13.7° to 18.9°. 3.2. Critical current studies The critical current density data was obtained for 18 samples out of the set of 40 composite conductors on which diffraction measurements were performed. Examples of Jc as a function of H, for H k and H? are shown in Fig. 5. The specimens show consistent dependences of Jc on field. A major objective of this work is to determine whether the critical current densities Jc s of these well optimized tapes are correlated with the FWHMs of the Bi-2223 (2 0 0) peak. Such a correlation was observed in previously examined experimental tapes [1]. Fig. 6 shows Ic as a function of FWHM for a set of earlier composite conductor samples (note that the data is plotted as critical current Ic rather than current density Jc ) [1]. Fig. 6 suggests that, at least for the subject composite conductors, high Ic strongly correlates with a narrow FWHM. Also indicated in Fig. 6 is the range of FWHM observed in this study. The critical current densities obtained in this work cannot be directly compared to the earlier work since the ranges of FWHM do not overlap, and in the previous study, Ic was reported for composites of a different configuration. However we are able to determine whether the strong trend of high
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Fig. 6. Comparison of critical currents and (2 0 0) rocking curve FWHMs from Ref. [1] with the range of FWHMs obtained in the current study (shaded area).
Ic with small FWHM is sustained in a regime of much improved (reduced) Bi-2223 (2 0 0) FWHM. Fig. 7 shows Jc as a function of FWHM for the samples measured here, at conditions given on the graph, identified according to number of filaments.
Fig. 7. Critical current densities Jc plotted against (2 0 0) rocking curve FWHMs. ASC026 is indicated by the arrow.
There is no strong correlation between Jc and FWHM for these samples. For this set of samples, there does appear to be a trend where the maximum observed Jc increases with decreasing FWHM. Fig. 8 shows Jc (normalized as described below) as a function of sample angle a relative to the
Fig. 8. Scaled Jc for a selection of samples as a function of tape orientation in the applied field H (as measured by the angle a as described in the text). Data for one sample is replotted as a function of logðH sin aÞ.
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applied field for six representative composite conductors. The data were scaled by normalizing the maximum Jc value to unity, after correcting for the minimum Jc value obtained at a ¼ 90°. All peaks were shifted so as to place the maxima at a 0. The observed dependences of Jc on a are all very similar. The inset shows data for one sample replotted as a function of logðH sin aÞ, to obtain a critical angle acrit ¼ 5:3°, as described previously [10,12]. Values of a obtained for our samples range from 4° to 8.5°, and again are not correlated with critical current densities.
4. Discussion The transmission high energy X-ray diffraction survey of these composite conductors yields a consistent pattern among the samples studied here. The 2h diffraction patterns show a preponderance of Bi-2223 phase with <10% Bi-2212, plus small amounts of additional phase. The rocking curves for the Bi-2223 (2 0 0) peaks show relatively good crystallite alignment, with FWHMs between 13° and 19°. The expected correlation between FWHM and critical current densities is not clearly demonstrated, as the Bi-2223 (2 0 0) rocking curve widths do not correlate strongly with the measured critical current densities. It is possible that a few grossly misoriented crystallites (>20°) could degrade superconducting properties, while having a negligible effect on the shape of the rocking curve. Another possibility is that the average rocking curve FWHM measured in this experiment is not representative of true local structure. Fig. 2a of Ref. [9] displays a transmission electron microscope bright-field image of aligned ‘‘colonies’’ of a Bi-2223 composite similar to those measured here. The Bi-2223 alignment in the neighborhood of a colony may be much better than the average; the average local (grain to grain) misorientation may not be as great as one might estimate from the rocking curves. There is also some evidence in the same figure for a texture having developed at the boundary of the Ag/Bi-2223. If the Bi-2223 crystallites grow, accurately conforming to an imperfectly formed large-grain Ag boundary, misalignments of Bi-2223 may be difficult to avoid.
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To regularize the geometry of the Ag/Bi-2223 boundary, it might be useful to consider methods for inhibiting Ag grain growth during the preparation process. The diffraction diagrams also are similar among the samples studied here. The samples appear to be 90% Bi-2223. In an earlier study the relative Bi2223 (1 1 5) peak to Bi-2212 (1 0 5) peak intensities were used to estimate the Bi-2223:Bi-2212 compositional ratio. In the present work, due to the high degree of Bi-2223 texture, the subject peaks are greatly diminished in intensity for tape face normal to the beam. From results for a representative composite, the Bi-2212 phase concentrations relative to the Bi-2223 phase appear to be below 10%.
5. Summary A series of fabricated Bi-2223/Ag tapes was studied by transmission high energy X-ray diffraction and by extensive critical current density measurements. High energy transmission X-ray diffraction showed samples to be predominantly Bi-2223, with minority Bi-2212 and unidentified impurity phases. Rocking curves of the Bi-2223 (2 0 0) peaks yielded FWHMs ranging from 13° to 19°, substantially smaller than those observed in earlier studies [1,2,4,5]. Critical current densities, measured under similar conditions on different experimental samples, varied by a factor of two or less. Critical current density measurements showed samples to have similar dependences on applied field H and on tape orientation angle a. There appears to be no firm correlation between FWHM and Jc for this set of samples. Thus it is concluded that critical current densities in these relatively well oriented samples may not be limited by crystallographic texture.
Acknowledgements R.C. Budhani helped acquire X-ray data. Frank Perez performed the critical current measurements. Research at Brookhaven was supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences under
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contract no. DE-AC02-98CH10886. The National Synchrotron Light Source at BNL is supported by the US Department of Energy, Divisions of Materials Sciences and Chemical Sciences.
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