Physica C 445–448 (2006) 737–740 www.elsevier.com/locate/physc
Micro-Raman spectroscopy as a unique tool for phase characterization of Bi-2212/Ag tapes T. Nakane a
a,*
, M. Osada b, H. Kumakura
a
Superconducting Materials Center, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan b Advanced Materials Laboratory, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan Available online 30 June 2006
Abstract In this work, we present reference Raman spectra of Bi-2212/Ag tape as evidence that micro-Raman spectroscopy constitutes a rapid and sensitive evaluation tool for high Tc superconductive cuprate (HTSC) tapes. We performed the characterization for the surface and cross sectional areas of tapes, with spot diameters smaller than 1 lm. The results of this study confirm that local phase characterization is possible for Bi-2212/Ag tape. Our further attempt to carry out a micro-Raman spectroscopy mapping measurement for the Bi-2212 phase yielded meaningful phase and grain alignment distribution data, even though the measurement spacing was rough and the small observation area. With appropriate upgrades for full-scale characterization with fine measurement spacing and a large observation area, we believe that this spectroscopy will be an effective tool to detect and evaluate precise local changes in Bi-2212/Ag tape. The Raman imaging for Bi-2212/Ag may have a substantial potential to distinguish the distribution of the impurity oxide, degree of grain orientation, crystallinity, nonstoichiometry of the main phase, and other relevant characteristics. Ó 2006 Elsevier B.V. All rights reserved. PACS: 84.71.Mn; 74.72.Hs Keywords: Bi-2212/Ag tape; Raman spectroscopy; Characterization
1. Introduction Silver sheathed Bi2Sr2CaCu2O8+d tape (Bi-2212/Ag) has important applications in high performance coils, power cables, current lead, and so on. Investigations of special interest for this material pertain to the further enhancement of the critical current density, Jc, in high-applied magnetic field, B. In order to achieve this objective, researchers have developed numerous techniques for optimizing the fabrication procedure to obtain the highest quality Bi-2212/Ag tape: one with high purity, a highly textured grain alignment, and a high density [1–3]. As time passes, therefore, the optimization of these parameters becomes increasingly difficult. In particular, minor improvements are quite difficult to identify, since Jc–B *
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[email protected] (T. Nakane).
0921-4534/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2006.05.053
characterization does not sensitively response them. Moreover, the micro-characterization techniques commonly used for Bi-2212/Ag tape investigations are limited in the spatial resolution and/or in the amount of information they provide. Thus, the introduction of a new characterization technique could be one of the effective solutions needed to lead to a breakthrough in improvements of the Jc–B property of this tape, and could provide additional information from a different perspective than that of more conventional investigations. These points apply also to the other high Tc superconductive cuprate (HTSC) tape materials, such as (Bi, Pb)2Sr2Ca2Cu3O10+d (Bi-2223) tape and CuBa2YCu2O7d (Cu-1212; so-called YBCO) tapes. In general, discussions of the Jc–B property of Bi-2212/ Ag tape focus on correlations of the changes in the material’s morphological structure, phase distribution, grain connectivity, superconductive characteristics, crystallinity and density. Our own group reported recently the
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importance of the oxygen-nonstoichiometry of the Bi-2212 phase [4]. For the evaluation of these parameters, the most commonly used methods are scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction measurement (XRD). In SEM–EDS analyses, however, only the morphological structures and phase distribution can be observed. On the other hand, XRD allows the researcher to obtain precise information on the Bi-2212 layer, i.e. the phase content, grain alignment, crystallinity and average micro-structural data of the Bi-2212 phase, but, because the incident radiation is difficult to focus, local analysis with the scale less than 1 lm is extremely challenging. Current investigations of Bi-221/Ag tape often require the precise information from micron-sized areas in many locations, and mapping it over a wide range. The frequency shifts seen in a micro-Raman scattering measurement, together with the selection rules, make it possible to employ such a measurement for the evaluation of Bi-2212 (i.e. evaluation of the phase content, grain alignment, crystallinity, etc.). The XRD pattern also provides this kind of data. However, the pattern commonly obtained is an average over other impurity phases and/or different orientations. In addition, the existence of these phases is often speculative, deduced only from the extra peak intensity of the pattern, or the full-width half maximum (FWHM). These speculations are difficult to justify, since, in the case of a highly grain-aligned tape, the crystallinity and the background also affect them. On the other hand, micro-Raman spectroscopy permits an easy focus of the incident laser light to micron-order spot sizes. Moreover, the measurement time of each Raman spectrum can be reduced to the level of seconds. Therefore, with microRaman spectroscopy, by scanning the sample surface to collect the spectrum of local area, the investigator can observe the distribution of phases and other important information, which makes this technique an effective evaluation tool for the investigation of Bi-2212/Ag tape. A number of reports exist of the Raman spectra of Birelated HTSC samples [5–7]. They state that the Raman spectra are uneven, and the frequency shifts sensitively depend on the specifics of the sample situation, such as grain orientation, phase purity, precise composition and crystallinity. Consequently, for future investigations of tape samples, collecting the spectra for the existing phases in the Bi-2212 layer is the first important issue for the practical utilization of micro-Raman spectroscopy. However, there are remarkably few studies employing that technique for the investigation of Bi-related HTSC tape [8,9]. We have therefore collected Raman spectra for the Bi-2212 layer of tape samples, and here present the use of microRaman spectroscopy as a rapid and sensitive evaluation tool for Bi-2212/Ag tapes.
coating and melt-solidification methods [1]. The tape was heated to 885 °C, maintained at that temperature for 5 min, and then slowly cooled at a rate of 2 °C/h to 835 °C, at which temperature the sample was again held for 5 h. Raman measurements were performed in a backward micro-configuration, using the 514.5 nm line from an Ar+ laser (0.5 mW) focused to a <1 lm diameter spot on the measurement plane. The scattered light was dispersed by a subtractive triple spectrometer (Jobin Yvon: T64000) and collected with a liquid-nitrogen-cooled charge-coupled device (CCD) detector. The measurement was performed for both the cross-sectional area and the surface area of tapes; for surface measurements, the silver substrates were etched away using a solution of H2O2:NH3OH: H2O = 5:5:4.
2. Experimental
Fig. 1. Typical Raman spectra of the Bi-2212 phase observed in Ag sheathed tape: (a1) light parallel to the ab-plane, measurement time 10 s; (b1) light parallel to the ab-plane, measurement time 100 s; (c1) light perpendicular to the ab-planes, measurement time 10 s; (d1) light perpendicular to the ab-plane, measurement time 100 s.
The tape fabrication procedure for a Bi-2212/Ag tape is described elsewhere [1–4]; the tape was fabricated via dip-
3. Results and discussion Fig. 1 shows typical Raman spectra of the Bi-2212 layer in the tape. Spectra (a1) and (b1) are the Raman spectra of the Bi-2212 ac/bc-plane, for measuring times are 10 and 100 s, respectively. Spectra (c1) and (d1) are the Raman spectra of the Bi-2212 ab-planes, for measuring times of 10 and 100 s, respectively. Reducing the measuring time is important for obtaining mapping images if we expect micro-Raman spectroscopy to be utilized like a SEM observation. However, the longer measuring time, the sharper the micro-Raman spectrum becomes. On the other hand, Bi-2212/Ag tape is highly c-axis grain-aligned to the tape surface. Thus, spectra (a1) and (b1) are the most common for cross-sectional measurement, and (c1) and
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(d1) are customary for surface measurement. However, spectra like (a1) and (b1) can be also observed in a surface measurement, and (c1) and (d1) spectra in a cross-sectional measurement. Moreover, some of the spectra for both measurements could be assigned as the impurity phase or as mixed spectra of (a1) and (c1) (or (b1) and (d1)). This information, difficult to obtain by XRD measurements, is the unique product of micro-Raman measurements. Additional 10s spectra obtained at a number of points on the cross-sectional area of the tape are summarized in Fig. 2. On this surface it is easier to find peaks that allow us to discriminate different phase. Spectra (a2), (b2) and (c2) show the mixed spectra of (a1) and (c1), a result indicating the existence of poorly aligned Bi-2212 grains. In these spectra, the intensity ratio of the characteristic peaks around 460 cm1 and 630 cm1 varied with the degree of grain alignment; thus, we estimated the degree of grain alignment from the absolute value of the ratio. On the other hand, spectra (d2), (e2), (f2) and (g2) originate from impurities in the Bi-2212 layer. According to previous reports [5,8], they are assigned as (Ca, Sr)14Cu24O71, Sr rich (Ca,Sr)2CuO3, the Bi-2412 phase, and the Cu-free phase, respectively. Characteristic peaks of these compounds are found around 300, 400, 530 and 550 cm1. These data indicate that micro-Raman spectroscopy can distinguish the
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phases in Bi-2212/Ag tape. Likewise, the differences in crystallinity and/or oxygen-nonstoichiometry are detectable. The crystallinity of the grains affects the peak width of the Raman spectrum in a similar way to the FWHM of an XRD pattern, and the oxygen-nonstoichiometry of the Bi-2212 shifts the peak position. However, it seems that a precise and sufficiently accurate discrimination requires an appropriately long measuring time, and, for practical use in the investigation of Bi-2212/Ag tape, micro-Raman spectroscopy measurements need to be optimized. We tried to capture a mapping image of the microRaman spectra for the cross-sectional area of the tape. Note that each point in the mapping data is generally distinguished from the highest Raman peak intensity of particular regions. However, this method is not thought to be appropriate for the observation of Bi-2212/Ag tape, because it is often affected by the background peak and/ or shoulder of the neighbor peaks [8]. Furthermore, for Bi-2212/Ag tapes, the Raman spectra of a wide-area observation require multiple criteria to discriminate the many impurity phases; thus, a simple discrimination from the
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Fig. 2. Typical Raman spectra observed in the oxide layer in Bi-2212/Ag tape. They were obtained at a number of points on the cross-sectional area of the tape with a measurement time of 10 s.
6 5 1 23 4 1. Bi-2212 (81.8%) 2. Cu-free phase (0.6%) 3. Bi-2223 (ab plane) (0.6%) 4. Ag (7%) 5. (Ca,Sr)14Cu24O71+Bi-2212 (1%) 6. No peak (crack or void) (9%)
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Fig. 3. The SEM and mapping image of Raman spectra for a crosssectional area of Bi-2212 tape. (a) SEM image of observed area. (b) Raman mapping image of phase distribution. (c) Raman mapping image for angular distribution of the grain orientation.
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peak intensity at a particular region seems an insufficient basis from which to discuss the mapping image of these tapes. Consequently, we did not utilize a general analysis application, rather we plotted all the data points ourselves. We analyzed spectra for all points by plotting and checking each spectrum (see Appendix). For this reason, we reduce the total number of Raman measurements (only 169 spectra), and increase the measuring time to 10 s for improving the resolution. The SEM image of the observed area is shown in Fig. 3(a), and the mapping image of the same area, taken from the results of the Raman spectra analysis, in Fig. 3(b). The quantity values shown in parentheses in Fig. 3(b) give an idea of the sensitivity that this type of observation might permit for quantitative analysis. They are calculated from the total number of assigned points and the total number of measured points (=169). Since the impurity phases shown in Fig. 3(a) correspond to the detected impurity phases shown in Fig. 3(b), we consider that our discrimination of the tape surface is successful. On the other hand, although the Bi-2212 area can be seen in Fig. 3(a), the grain orientation cannot be distinguished. However, Fig. 3(c) clearly shows the angular distribution of the grain orientation. In this figure, the bright, dark, and black areas indicate, respectively, the grains with a c-axis orientation, the grains with a/b-axes orientations, and the impurity phases (not a Bi-2212 phase). This figure definitely indicates that the degree of grain alignment in Bi-2212 is not homogeneous in the tape sample. 4. Conclusions This work present the reference Raman spectra of Bi2212/Ag tape, as evidence that micro-Raman spectroscopy constitutes a rapid and sensitive evaluation tool for HTSC tapes. We performed this characterization for the surface and cross sectional areas of tapes, with spot diameters smaller than 1 lm. The results of this work show that the local phase characterization is possible for Bi-2212/Ag tape. The phase characterization involved discriminating the distribution of the impurity oxide, degree of grain orientation, crystallinity, and nonstoichiometry of the main phase. This wide range of capabilities suggests that micro-Raman spectroscopic evaluations of such tapes are comparable to XRD measurements. However, microRaman spectroscopy has advantages in short measurement times and in the micro-size local observations permitted. Our micro-Raman spectroscopic mapping measurement for the Bi-2212 phase shows meaningful phase and grain alignment distribution data for the tape, even though the measurement spacing was rough and the observation area small. We believe that this spectroscopy will be an effective tool to detect and evaluate precise local changes in Bi-2212/
Ag tapes when it is effectively upgraded for full-scale characterization with a fine measurement spacing and large observation area. It indicates that Raman imaging possesses a high potential to create a detailed picture of the phase distribution, including grain alignment, in Bi-2212/ Ag tapes. Acknowledgements The authors acknowledge Dr. T. Numata of the Horiba, Ltd. for her technical support. Appendix. In this work, the analysis of data points is basically performed by plotting and checking each spectrum. For descriptive purpose, we classified each spectrum by the following equations: rf ¼ ðI f I min Þ=ðI max I min Þ Rp ¼ Rfrf ðobserved spectraÞ rf ðreference spectraÞg The symbols f, If, Imin and Imax indicate the Raman shift, intensity at f, minimum intensity and maximum intensity, respectively. The value of Rp was calculated by summing rf at all f. This value indicates the sum of the differences at fitting process between the obtained data and the reference data. The reference data was selected from the typical spectra observed in the tape; a data point with a small Rp value is regarded as the same as the reference phase. This procedure allows convenient classification, and an approximate assignment of the peak to the correct phase. However, it does not take into account multi-phase area and the grain orientation. Some data identification needed correction, and a final checking by the observer was indispensable. For the practical utilization of micro-Raman spectroscopy for the characterization of Bi-2212/Ag tape, the criteria used to require improvement, and a further investigation to do so would be desirable. References [1] J. Kase, N. Irisawa, T. Morimoto, K. Togano, H. Kumakura, D.R. Dietderich, H. Maeda, Appl. Phys. Lett. 56 (1990) 970. [2] H. Miao, H. Kitaguchi, H. Kumakura, K. Togano, T. Hasegawa, T. Koizumi, Physica C 303 (1998) 81. [3] H. Kumakura, H. Kitaguchi, K. Togano, N. Sugiyama, J. Appl. Phys. 80 (1996) 5162. [4] T. Nakane, A. Matsumoto, H. Kitaguchi, H. Kumakura, Supercond. Sci. Technol. 17 (2004) 29. [5] M.N. Iliev, V.G. Hadjiev, Physica C 156 (1988) 193. [6] J. Prade, A.D. Kulkarni, F.W. de Wette, U. Schro¨rer, W. Kress, Phys. Rev. B 39 (1989) 2771. [7] Y.H. Shi, M.J.G. Lee, M. Moskovits, R. Carpick, A. Hsu, B.W. Statt, Z. Wang, Phys. Rev. B 45 (1992) 370. [8] K.T. Wu, A.K. Fischer, V.A. Maroni, M.W. Rupich, J. Mater. Res. 12 (1997) 1195. [9] V.A. Maroni, A.K. Fischer, K.T. Wu, Physica C 341–348 (2000) 2243.