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Investigation of defects in functional layer of high temperature superconducting tapes
Physica C 497 (2014) 24–29
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Investigation of defects in functional layer of high temperature superconducting tapes Marcela Pekarcˇíková a,⇑, Michal Skarba a, Pavol Konopka a, Jozef Janovec a, Mykola Solovyov b, Enric Pardo b, Fedor Gömöry a,b a b
Slovak University of Technology in Bratislava, Faculty of Materials Science and Technology in Trnava, Paulínska 16, 917 24 Trnava, Slovakia Slovak Academy of Sciences, Institute of Electrical Engineering, Dúbravska cesta 9, 84104 Bratislava, Slovakia
a r t i c l e
i n f o
Article history: Received 19 July 2013 Received in revised form 25 September 2013 Accepted 22 October 2013 Available online 5 November 2013 Keywords: Superconducting tapes (RE)BCO Critical current density EBSD Outgrowths
a b s t r a c t (RE)BCO-based superconducting tapes were studied to clarify the correlation between structure of the (RE)BCO layer and the critical current density. For this purpose, both etched and cross-sectioned samples of (RE)BCO tapes were investigated by means of SEM and EBSD techniques. The outgrowths penetrating deeply into the (RE)BCO layer were found. Their non-uniform distribution density profile across a tape width was closely related to the profile of critical current density. Grains in (RE)BCO outgrowth-free regions, outgrowths themselves and neighboring regions of outgrowths were found to differ from each other in crystal orientation. It was shown that the presence of outgrowths may cause localized changes in crystal orientation of superconducting layer and in this way it may affect the critical current density. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Tape conductors from high temperature superconductors (HTS) of the (RE)Ba2Cu3O7d (RE = rare earth metal) composition are now considered for many applications at liquid nitrogen temperatures. The brittle ceramic compound with superconducting properties, also known as (RE)BCO, has an orthorhombic crystal structure with very similar lattice parameter of a and b, where the difference between them is less than 2%, and c is nearly 3 times b. The (RE)BCO structure is layered with copper oxide planes, through which the superconducting current flows preferably and causes large anisotropy in functionally important properties such as the critical current density, jc. Thus, in practical applications with high current-carrying capability, the c-axis has to be aligned normal to the direction of current flow. Presently, the major techniques utilized to grow biaxial textured (RE)BCO films are: sputtering [1], pulsed laser deposition (PLD) [2,3], metalorganic deposition (MOD) [4], and metalorganic chemical vapor deposition (MOCVD) [5,6]. Among these techniques, MOCVD has advantages of large-area deposition, high film growth rate, ability to grow film with high crystalline quality, and relatively low production cost. Very important is the profile of jc along the width of the superconducting tape. Ideally, the jc should be spatially independent. ⇑ Corresponding author. Tel.: +421 906 068312. E-mail address: [email protected] (M. Pekarcˇíková). 0921-4534/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physc.2013.10.010
However, the degradation of current transportation properties is in some cases observed, especially at the edges of the tape. We performed structural analysis of the superconducting layer from both the top view and in the cross-section, to bring more insight into the mentioned degradation of electric properties. 2. Sample material Superconducting tapes SCS12050 (12 mm wide) and SCS4050 (4 mm wide) produced by SuperPower Inc. [7] were used for electrical and structural investigations (Fig. 1). These tapes consist of the non-magnetic Hastelloy C-276 substrate, a stack of buffer layers, the superconducting (RE)BCO layer, the silver overlayer and surrounding copper stabilizer (SCS). The layers of buffer stack were deposited on electropolished substrate using deposition techniques as ion beam assisted deposition (IBAD) and magnetron sputtering. IBAD was used for deposition of MgO layer providing template for growing of following epitaxial buffer layers (homo-epitaxial MgO layer and epitaxial LaMnO3 (LMO) layer). The deposition of superconducting layer was done by means of metalorganic chemical vapor deposition (MOCVD). Following Ag and Cu layers were deposited using sputtering and electroplating, respectively. Ag layer provides good current transfer to (RE)BCO layer and facilitates oxygen diffusion during oxygenation annealing. SCS completely encases the tape in order to protect the conductor and produce rounded edges that are beneficial for highvoltage applications. Further, the probability of failure of the device
M. Pekarcˇíková et al. / Physica C 497 (2014) 24–29
Fig. 1. Scheme of investigated (RE)BCO HTS tapes, (RE)BCO = (RE)Ba2Cu3O7d, where RE means rare earth element (Y, Gd).
due to voltage breakdown is reduced in tape with SCS. Table 1 summarizes information about the thicknesses of particular layers measured by scanning electron microscopy (SEM) for both of the investigated HTS tapes. Prior to the SEM investigation, the copper and silver layers were selectively removed by chemical etching based on aqueous solution of KI3. The etching provided the bare undisturbed (RE)BCO surface for top view measurements. Cross-section samples for investigation of grain orientation were prepared by cutting the tape along its width (cross section was perpendicular to longlength of tape), followed by polishing with Ar ions using the cross-section polisher device (JEOL SM-09010). 3. Experimental, results and discussion The electrical measurements performed on two distinct tapes indicated non-uniformity of electrical properties with regard to the tape width. The jc values started to decrease in area close to the edges of superconducting layer. An example of such measurement is shown in Fig. 2. Summarizing, the measuring method consists on scanning the magnetic field above the sample by a Hall probe and using an inversion method to obtain the current density [8]. When the sample carries the critical current, the current density corresponds to jc. Self-field effects are suppressed by applying a sufficiently large DC magnetic field. In the present study, the jc value was significantly reduced in marginal regions of tape in comparison with the tape center (Fig. 2). In order to clarify this nonuniformity, it would be interesting to know how structural features as the presence of outgrowths and the grain orientation change can affect the jc profile. In the following we focused therefore our attention on investigation of the structural effects. The SEM (JEOL JSM7600F) observation of both etched (top view) and cross-sectioned samples revealed a presence of tilted particles with their tops outgrowing the surface of the (RE)BCO layer into silver overlayer. We will further refer to these objects according to [9,10] as ‘‘outgrowths’’. Particles of this kind are frequently ob-
Table 1 The thicknesses of particular layers in SCS12050 and SCS4050 tapes measured (mean and standard deviation values). Layer
SCS12050 (lm)
SCS4050 (lm)
Cu (upper) Ag (upper) (RE)BCO Buffer stack Substrate Ag (lower) Cu (lower)
18.68 ± 0.91 1.54 ± 0.45 1.01 ± 0.15 0.15 ± 0.01 49.87 ± 0.35 1.31 ± 0.50 20.72 ± 0.56
19.36 ± 0.79 2.71 ± 0.46 1.62 ± 0.07 0.15 ± 0.01 45.23 ± 0.34 0.92 ± 0.57 19.49 ± 1.16
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Fig. 2. Non-uniform profile of critical current density across the tape width. The measured distribution of magnetic field above the SCS12050 tape carrying critical current at DC background field of 250 mT is used to determine the current profile by an inversion procedure.
served in the (RE)BCO superconducting thin films, having size from a few hundred to more than thousand nm. Outgrowth nucleation and growth is dependent on deposition rate, time and temperature [11]. Two kinds of outgrowths were observed in the superconducting layer. We will further distinguish between them according to their shape: needle-like outgrowths and rounded outgrowths. In the cross-sectioned view, the rounded outgrowths often penetrated deeply into the (RE)BCO layer, but rarely touched the buffer stack as shown in Fig. 3. The energy dispersive X-ray analysis (EDX) indicated increased concentration of Cu and O (Fig. 4a) or increased concentration of Cu, O and Y (Fig. 4b) inside these outgrowths in comparison with the (RE)BCO matrix. On the other side, the relevant outgrowths were depleted by Ba. The latter analysis suggests that the outgrowths may be composed of CuO, CuYO2 or Y2Cu2O5, as it has been previously also reported by several authors in [9–15]. The needle-like shaped outgrowths did not show any significant difference in the chemical composition in comparison with the (RE)BCO matrix. According to EDX measurements, the superconducting layer contains besides Y, Ba, Cu and O also rare earth element Gd (RE = Y, Gd). Preliminary SEM investigation of the HTS layers with removed Cu and Ag overlayer (top view) showed obvious differences in density distribution of outgrowths between particular regions of the tape. Therefore, the surface ratio without outgrowths in the superconducting layer was analyzed in more detail. We compared the sequence of (RE)BCO surface micrographs taken across the tape width from one edge to another. For all micrographs the same conditions of picture recording were used. Step size between particular neighboring micrographs was approximately 100 lm. In order to quantify the occurrence of outgrowths, we calculated S, which represents a surface area not covered by outgrowths related to a total area of a particular SEM micrograph. The graph in Fig. 5 shows S as function of position on the tape, where the horizontal axis was normalized with respect to the center of tape width. A very remarkable similarity was found between the graphs in Figs. 2 and 5. Both dependences show a steep decrease in the marginal regions of the tape. From the total tape width of 12 mm, approximately 2 mm from one side and 1 mm from the other side seem to have significantly different properties compared to the central part. This very good correlation between structural and electric properties suggests that the occurrence of outgrowths can have major effect on the homogeneity of the jc distribution across the tape width. Apart from the evident difference in the surface density of outgrowths, there is also a significant change in the outgrowth morphology. Fig. 6 shows SEM images used for the data collection
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Fig. 3. SEM/SE image illustrating the outgrowths in cross section of superconductor tapes (a) SCS4050 and (b) SCS12050. The outgrowths are penetrating into Ag layer as well as (RE)BCO matrix.
Fig. 4. Top view of etched (RE)BCO surface with uncovered outgrowths analyzed with EDX: the outgrowths are in comparison with (RE)BCO matrix enriched in Cu, O and/or Y and depleted in Ba (SCS12050 tape).
information about dimensions of outgrowths in vertical axis. The height maps of surface from regions near to the tape edge and in the tape middle are shown in Fig 7. According to the latter measurement, the height of outgrowths above the (RE)BCO surface seems to be slightly different in both investigated regions. The outgrowths in the center of the tape width have the height about 400 nm and this height is relatively homogeneous in whole measured area. This result is also corresponding to the measurement of outgrowths heights in central region of superconducting tape (cross section view) using SEM. Mean value of outgrowths heights above (RE)BCO surface was 385 nm. On the other hand, the outgrowths in regions near to tape edge are significantly higher and their height is ranging from about 400 nm up to 600 nm. The surface area roughness Sa (arithmetic mean deviation of all surface height values) have been calculated assuming the following expression Fig. 5. Proportion of surface area without outgrowths (S) as function of position across the tape width with regard to the SCS12050 tape center. N
illustrated in Fig. 5. They represent typical observed distribution of outgrowths near the tape edge (a) and in the center (b) of the tape width. Besides rounded outgrowths, a lot of needle-like shaped outgrowths were found preferably in marginal regions of tapes. Measurement of surface roughness by confocal laser scanning microscopy (ZEISS LSM 700) provided us with complementary
Sa ¼
y Nx X 1 X jz xi ; yj Sc j; Nx Ny i¼1 j¼1
ð1Þ
where Nx, Ny is number of measured points in x and y direction, z (xi, yj) is height of the measured point in coordinates x and y, Sc is mean height of all surface height values.
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Fig. 7. Confocal laser scanning microscopy measurement of surface roughness near to edge (top) and in the central part of SCS12050 tape (bottom) with surface area roughness Sa of 0.079 lm and 0.054 lm, respectively. Fig. 6. SEM/SE images of the SCS12050 tape with removed Cu and Ag overlayer by selective chemical etching. The top view shows the uncovered outgrowths at different positions with its typical density distribution: (a) near tape edge, (b) in the center of the tape width. The micrographs of left- and right-hand marginal regions were very similar.
The obtained values of Sa were 0.054 and 0.079 lm in the center of the tape width and in the marginal regions, respectively. Grain boundaries separating the crystals of different orientation behave as considerable obstacles to the current flow. The interconnecting current density decreases exponentially with the grain boundary angle [16,17]. We were therefore also interested in the crystal orientation within the functional (RE)BCO layer. We supposed that the outgrowths and the regions close to the outgrowths could have other than z-axis crystal orientation (z-axis is parallel with the lattice parameter c). To verify the latter assumption, we investigated in detail the cross-sectioned samples by means of electron backscattered diffraction (EBSD) technique. Today it is widely known that EBSD technique requires sufficiently smooth sample surface in order to obtain clear and high contrasted Kikuchi patterns. We applied the sample preparation techniques described in [18–20], but the best results in the sharpness of Kikuchi patterns were achieved by polishing of cross-sectioned tapes with Ar ions (see Kikuchi pattern in Fig. 8). However, this preparation technique worked well for adjacent Ag and Cu layers, but it was still not suitable enough to enable automated EBSD mapping in the superconducting tape region. In order to successfully discriminate between a-, b- and c-axis in the crystal structure of (RE)BCO superconductor with automatic route, it is necessary to detect certain critical bands in electron backscattered patterns (EBSP), which are unfortunately associated with very low intensity. Therefore, this investigation was performed manually by
Fig. 8. A Kikuchi pattern of (RE)BCO superconducting layer.
point-by-point measurement, which eliminated also picture drift problem occurring during the long-term EBSD scanning. The Kikuchi patterns were generated at the acceleration voltage of 20 kV and recorded using 4 4 binning on a Nordlys II camera for 30 ms. The samples were tilted to 70° during measurement. All investigated (RE)BCO thin films should have biaxial texture with identical (0 0 1) or c-axis orientation. This assumption was also confirmed in the outgrowth-free regions as indicated in Fig. 9. In case of regions with outgrowths we will distinguish as previously between regions with rounded outgrowths and regions with needle-like outgrowths. The rounded outgrowths consist partially of (RE)BCO superconductor and partially of non-superconducting phases. The (RE)BCO areas inside these outgrowths
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Fig. 9. SEM/SE images of cross-section of SCS4050 tape perpendicular to long-length with marked places in the superconducting layer, where the crystal orientation of (RE)BCO was identified by EBSD. In these cross sections one needle-like shaped outgrowth with a-axis orientation is observable whereas the others are rounded outgrowths and have randomly oriented grains. Examples of both cases are marked by circles. Outgrowth-free regions indicate c-axis orientation.
indicated different crystal orientation compared to (RE)BCO outgrowth-free region. The grains in the latter regions are oriented randomly. Non-superconducting part of rounded outgrowths can have the chemical composition already mentioned in the beginning of this capture. Good contrast of the micrograph enabled visibility of non-superconducting phases as darker areas in (RE)BCO layer. In these areas it was not possible to identify the grain orientation by EBSD (question marks in Fig. 9). Nonassigned indexing was caused by either overlapping of two different EBSPs or pertaining the EBSP to an unknown phase, probably copper and/or yttrium oxide as already mentioned, so finally it can be concluded that in these areas no (RE)BCO grains with c-axis orientation occurred. The needle-like shaped outgrowths consist of (RE)BCO superconductor and have two orthogonal a-axis orientation (a-axis is parallel to the surface normal) as it was already reported by Chang et al. [10]. According to the latter authors, the needle-like shaped outgrowths with a-axis orientation are prone to epitaxial growth on the c-axis film because of a proper ratio between b and c lattice parameters (b = 1/(3c)), so the a-axis oriented nuclei have their b and c directions in the surface plane. An example of the needle-like outgrowth in cross-section view in our samples is shown in Fig. 9a (marked by red circle). This outgrowth was cut in transverse section, so the cross section of ‘‘needle’’ is visible. Its crystal orientation was measured as follow: the a-axis was oriented parallel to the surface normal and the b-axis was perpendicular to the cross section surface, although according to the orientation of needle shape we could expect that no b-axis but caxis orientation is perpendicular to the cross section surface. The difference between expected c-axis and measured b-axis orientation can be explained by theory given in [10]. Thus, the authors postulate that b is the fast growth direction and c is the slowest growth direction. Therefore, shorter site of a-axis oriented needle-like shaped outgrowth possesses c-axis orientation which direction was slower to grow. The existence of faster and slower growth directions explains also the formation of needle-like shaped outgrowths. Our experimental results indicate that the outgrowths with different density, morphology as well as size were observed
dominantly in the marginal regions of tapes in comparison to the central area. There is an indication of non-homogeneous nucleation of outgrowths across the tape width during the tape preparation. Therefore, it will be interesting to investigate such tapes produced by another deposition techniques mentioned in introduction. A non-uniform profile of critical current density across the tape width accompanied by a significant decrease of jc close to edges of the superconductor tape can be related to the presence of outgrowths. The needle-like shaped outgrowths grow on the surface of (RE)BCO layer and they are usually not penetrating into (RE)BCO layer. Finally, the influence of needle-like outgrowths on critical current density is small. However, the jc can be affected by presence of rounded outgrowths. They penetrate especially in marginal regions of tape deeply into (RE)BCO layer and consist of both nonsuperconducting phases and of randomly oriented (RE)BCO grains. Therefore, the presence of rounded outgrowths results in both the higher amount of volume with non-superconducting phases (effect of reduction of superconductor volume) and the higher amount of grains with a random crystal orientation (grain boundary effect).
4. Conclusions Very good correlation between the structure and electric properties was found in superconducting (RE)BCO thin layers of biaxial texture with z-axis orientation. Significant decrease of critical current density close to the edges of superconductor tapes is related to the presence of outgrowths. In functional (RE)BCO layer, two kinds of outgrowths were observed: needle-like shaped and rounded outgrowths. Predominantly the rounded outgrowths affected the profile of critical current density because of their higher distribution density, bigger size, and deeper penetration into (RE)BCO layer, especially in the marginal regions if compared with the outgrowths from the central region of the tape width. Besides this, it was found that the rounded outgrowths partially consist of randomly oriented (RE)BCO grains, and other parts were found to consist of non-superconducting phases e.g. CuO, CuYO2 or Y2Cu2O5. Higher density of outgrowths at the edges of superconducting
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tapes results in both the higher amount of grains with random crystal orientation (grain boundary effect) and the higher amount of volume with non-superconducting phases (reduction of superconductor volume). The homogeneity of critical current density distribution across the tape width can be affected by the above factors. Acknowledgements The authors would like to thank to the Grant Agency of the Ministry of Education of the Slovak Republic and the Slovak Academy of Sciences (VEGA) for financial support under the contract no. 1/ 0162/11 ‘‘Effects of inhomogeneities on functional properties of high-temperature superconducting wires.’’ This publication is also the result of the project implementation: Center for development and application of advanced diagnostic methods in processing of metallic and non-metallic materials, ITMS: 26220120014, supported by the Research & Development Operational Programme funded by the ERDF. References [1] J. Xiong, W. Qin, X. Cui, B. Tao, J. Tang, Y. Li, High-resolution XRD study of stress-modulated YBCO films with various thicknesses, J. Crystal Growth 300 (2007) 364–367. [2] M. Li, B. Ma, R.E. Koritala, B.L. Fisher, K. Venkataraman, V.A. Maroni, V. VlaskoVlasov, P. Berghuis, U. Welp, K.E. Gray, U. Balachandran, Pulsed laser deposition of c-axis untilted YBCO films on c-asis tilted ISD MgO-buffered metallic substrates, Physica C 387 (2003) 373–381. [3] J. Chen, X.D. Su, D.L. Zhu, J. Fan, H. Yang, Y. Chen, V.G. Harris, An approach to (103) oriented YBa2Cu3O7d thin films epitaxially grown on (100) MgO substrates, Scripta Mater. 64 (2011) 450–453. [4] J.A. Xia, N.J. Long, N.M. Strickland, P. Hoefakker, E.F. Talantsev, X. Li, W. Zhang, T. Kodenkandath, Y. Huang, M.W. Rupich, TEM observation of the microstructure of metal-organic deposited YBa2Cu3O7d with Dy additions, Supercond. Sci. Technol. 20 (2007) 880–885. [5] P. Zhao, A. Ito, T. Kato, D. Yokoe, T. Hirayama, T. Goto, High-speed growth of YBa2Cu3O7d superconducting films on multilayer-coated Hastelloy C276 tape by laser-assisted MOCVD, Supercond. Sci. Technology 26 (2013) 055020.
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[6] X. Fang, G. Li, S. Li, L. Zhao, W. Li, B. Zhang, G. Du, P. Chou, H. Li, R. Zuo, L. He, C. Chen, The effect of susceptor inclination angle on the quality of superconducting YBCO thin films prepared by a photo-assisted MOCVD system, Physica C 468 (2008) 1053–1059. [7] http://www.superpower-inc.com/. [8] M. Solovyov, J. Šouc, F. Gömöry, Investigation of superconductor uniformity in CC tapes by magnetic field mapping, Phys. Procedia 36 (2012) 617–622. [9] S. Kreiskott, M. Getta, B. Mönter, H. Piel, N. Pupeter, High Jc YBCO films on biaxially textured Ni with oxide buffer layers deposited using evaporation and sputtering techniques, Physica C 383 (2003) 306–314. [10] C.C. Chang, X.D. Wu, R. Ramesh, X.X. Xi, T.S. Ravi, T. Venkatesan, D.M. Hwang, R.E. Muenchausen, S. Foltyn, N.S. Nogar, Origin of surface roughness for c-axis oriented Y–Ba–Cu–O superconducting films, Appl. Phys. Lett. 57 (1990) 1814– 1816. [11] Z. Han, T.I. Selinder, U. Helmersson, Formation of Cu-rich particles on the surface of YBa2Cu3O7x thin film qrown by in situ off-axis sputtering, J. Appl. Phys. 75 (1994) 2020–2025. [12] J.N. Ding, Y.G. Meng, S.Z. Wen, The tribological properties of YBa2Cu3O7 films in ambient environment, Wear 250 (2001) 311–317. [13] A. Catana, J.G. Gednorz, Ch. Gerber, J. Mannhart, D.G. Schlom, Surface outgrowths on sputtered YBa2Cu3O7x films: A combined atomic force microscopy and transmission electron microscopy study, Appl. Phys. Lett. 63 (1993) 553–555. [14] P. Lu, Y.Q. Li, J. Zhao, C.S. Chern, B. Gallois, P. Norris, B. Kear, F. Cosandey, High density, ultrafine precipitates in YBa2Cu3O7x thin films prepared by plasmaenhanced metalorganic chemical vapor deposition, Appl. Phys. Lett. 60 (1992) 1265–1267. [15] O. Eibl, B. Roas, Microstructure of YBa2Cu3O7x thin films deposited by laser evaporation, J. Mater. Res. 5 (1990) 2620–2632. [16] D. Dimos, P. Chaudhari, J. Mannhart, Superconducting transport properties of grain boundaries in YBa2Cu3O7 bicrystals, Phys. Rev. B 41 (1990) 4038–4049. [17] H. Hilgenkamp, J. Mannhart, Grain boundaries in high-Tc superconductors, Rev. Mod. Phys. 74 (2002) 485–549. [18] A. Koblischka-Veneva, M.R. Koblischka, P. Simon, F. Mücklich, M. Murakami, Electron backscatter diffraction study of polycrystalline YBa2Cu3O7d ceramics, Physica C 382 (2002) 311–322. [19] J. Wang, N. Maloufi, Z. Fan, X. Xue, C. Esling, Microstructure and texture analysis of YBCO thick film with peritectic growth on unoriented silver substrate, J. Rare Earths 27 (2009) 474–479. [20] M.R. Koblishka, M. Winter, U. Hartmann, A. Koblishka-Veneva, Nanoscale flux pinning sites in high-Tc superconductors, in: A.V. Narlikar (Ed.), Studies of High Temperature Superconductors, Nova Science Publishers Inc., New York, 2006, pp. 71–88.
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Investigation of defects in functional layer of high temperature superconducting tapes Marcela Pekarcˇíková a,⇑, Michal Skarba a, Pavol Konopka a, Jozef Janovec a, Mykola Solovyov b, Enric Pardo b, Fedor Gömöry a,b a b
Slovak University of Technology in Bratislava, Faculty of Materials Science and Technology in Trnava, Paulínska 16, 917 24 Trnava, Slovakia Slovak Academy of Sciences, Institute of Electrical Engineering, Dúbravska cesta 9, 84104 Bratislava, Slovakia
a r t i c l e
i n f o
Article history: Received 19 July 2013 Received in revised form 25 September 2013 Accepted 22 October 2013 Available online 5 November 2013 Keywords: Superconducting tapes (RE)BCO Critical current density EBSD Outgrowths
a b s t r a c t (RE)BCO-based superconducting tapes were studied to clarify the correlation between structure of the (RE)BCO layer and the critical current density. For this purpose, both etched and cross-sectioned samples of (RE)BCO tapes were investigated by means of SEM and EBSD techniques. The outgrowths penetrating deeply into the (RE)BCO layer were found. Their non-uniform distribution density profile across a tape width was closely related to the profile of critical current density. Grains in (RE)BCO outgrowth-free regions, outgrowths themselves and neighboring regions of outgrowths were found to differ from each other in crystal orientation. It was shown that the presence of outgrowths may cause localized changes in crystal orientation of superconducting layer and in this way it may affect the critical current density. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Tape conductors from high temperature superconductors (HTS) of the (RE)Ba2Cu3O7d (RE = rare earth metal) composition are now considered for many applications at liquid nitrogen temperatures. The brittle ceramic compound with superconducting properties, also known as (RE)BCO, has an orthorhombic crystal structure with very similar lattice parameter of a and b, where the difference between them is less than 2%, and c is nearly 3 times b. The (RE)BCO structure is layered with copper oxide planes, through which the superconducting current flows preferably and causes large anisotropy in functionally important properties such as the critical current density, jc. Thus, in practical applications with high current-carrying capability, the c-axis has to be aligned normal to the direction of current flow. Presently, the major techniques utilized to grow biaxial textured (RE)BCO films are: sputtering [1], pulsed laser deposition (PLD) [2,3], metalorganic deposition (MOD) [4], and metalorganic chemical vapor deposition (MOCVD) [5,6]. Among these techniques, MOCVD has advantages of large-area deposition, high film growth rate, ability to grow film with high crystalline quality, and relatively low production cost. Very important is the profile of jc along the width of the superconducting tape. Ideally, the jc should be spatially independent. ⇑ Corresponding author. Tel.: +421 906 068312. E-mail address: [email protected] (M. Pekarcˇíková). 0921-4534/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physc.2013.10.010
However, the degradation of current transportation properties is in some cases observed, especially at the edges of the tape. We performed structural analysis of the superconducting layer from both the top view and in the cross-section, to bring more insight into the mentioned degradation of electric properties. 2. Sample material Superconducting tapes SCS12050 (12 mm wide) and SCS4050 (4 mm wide) produced by SuperPower Inc. [7] were used for electrical and structural investigations (Fig. 1). These tapes consist of the non-magnetic Hastelloy C-276 substrate, a stack of buffer layers, the superconducting (RE)BCO layer, the silver overlayer and surrounding copper stabilizer (SCS). The layers of buffer stack were deposited on electropolished substrate using deposition techniques as ion beam assisted deposition (IBAD) and magnetron sputtering. IBAD was used for deposition of MgO layer providing template for growing of following epitaxial buffer layers (homo-epitaxial MgO layer and epitaxial LaMnO3 (LMO) layer). The deposition of superconducting layer was done by means of metalorganic chemical vapor deposition (MOCVD). Following Ag and Cu layers were deposited using sputtering and electroplating, respectively. Ag layer provides good current transfer to (RE)BCO layer and facilitates oxygen diffusion during oxygenation annealing. SCS completely encases the tape in order to protect the conductor and produce rounded edges that are beneficial for highvoltage applications. Further, the probability of failure of the device
M. Pekarcˇíková et al. / Physica C 497 (2014) 24–29
Fig. 1. Scheme of investigated (RE)BCO HTS tapes, (RE)BCO = (RE)Ba2Cu3O7d, where RE means rare earth element (Y, Gd).
due to voltage breakdown is reduced in tape with SCS. Table 1 summarizes information about the thicknesses of particular layers measured by scanning electron microscopy (SEM) for both of the investigated HTS tapes. Prior to the SEM investigation, the copper and silver layers were selectively removed by chemical etching based on aqueous solution of KI3. The etching provided the bare undisturbed (RE)BCO surface for top view measurements. Cross-section samples for investigation of grain orientation were prepared by cutting the tape along its width (cross section was perpendicular to longlength of tape), followed by polishing with Ar ions using the cross-section polisher device (JEOL SM-09010). 3. Experimental, results and discussion The electrical measurements performed on two distinct tapes indicated non-uniformity of electrical properties with regard to the tape width. The jc values started to decrease in area close to the edges of superconducting layer. An example of such measurement is shown in Fig. 2. Summarizing, the measuring method consists on scanning the magnetic field above the sample by a Hall probe and using an inversion method to obtain the current density [8]. When the sample carries the critical current, the current density corresponds to jc. Self-field effects are suppressed by applying a sufficiently large DC magnetic field. In the present study, the jc value was significantly reduced in marginal regions of tape in comparison with the tape center (Fig. 2). In order to clarify this nonuniformity, it would be interesting to know how structural features as the presence of outgrowths and the grain orientation change can affect the jc profile. In the following we focused therefore our attention on investigation of the structural effects. The SEM (JEOL JSM7600F) observation of both etched (top view) and cross-sectioned samples revealed a presence of tilted particles with their tops outgrowing the surface of the (RE)BCO layer into silver overlayer. We will further refer to these objects according to [9,10] as ‘‘outgrowths’’. Particles of this kind are frequently ob-
Table 1 The thicknesses of particular layers in SCS12050 and SCS4050 tapes measured (mean and standard deviation values). Layer
SCS12050 (lm)
SCS4050 (lm)
Cu (upper) Ag (upper) (RE)BCO Buffer stack Substrate Ag (lower) Cu (lower)
18.68 ± 0.91 1.54 ± 0.45 1.01 ± 0.15 0.15 ± 0.01 49.87 ± 0.35 1.31 ± 0.50 20.72 ± 0.56
19.36 ± 0.79 2.71 ± 0.46 1.62 ± 0.07 0.15 ± 0.01 45.23 ± 0.34 0.92 ± 0.57 19.49 ± 1.16
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Fig. 2. Non-uniform profile of critical current density across the tape width. The measured distribution of magnetic field above the SCS12050 tape carrying critical current at DC background field of 250 mT is used to determine the current profile by an inversion procedure.
served in the (RE)BCO superconducting thin films, having size from a few hundred to more than thousand nm. Outgrowth nucleation and growth is dependent on deposition rate, time and temperature [11]. Two kinds of outgrowths were observed in the superconducting layer. We will further distinguish between them according to their shape: needle-like outgrowths and rounded outgrowths. In the cross-sectioned view, the rounded outgrowths often penetrated deeply into the (RE)BCO layer, but rarely touched the buffer stack as shown in Fig. 3. The energy dispersive X-ray analysis (EDX) indicated increased concentration of Cu and O (Fig. 4a) or increased concentration of Cu, O and Y (Fig. 4b) inside these outgrowths in comparison with the (RE)BCO matrix. On the other side, the relevant outgrowths were depleted by Ba. The latter analysis suggests that the outgrowths may be composed of CuO, CuYO2 or Y2Cu2O5, as it has been previously also reported by several authors in [9–15]. The needle-like shaped outgrowths did not show any significant difference in the chemical composition in comparison with the (RE)BCO matrix. According to EDX measurements, the superconducting layer contains besides Y, Ba, Cu and O also rare earth element Gd (RE = Y, Gd). Preliminary SEM investigation of the HTS layers with removed Cu and Ag overlayer (top view) showed obvious differences in density distribution of outgrowths between particular regions of the tape. Therefore, the surface ratio without outgrowths in the superconducting layer was analyzed in more detail. We compared the sequence of (RE)BCO surface micrographs taken across the tape width from one edge to another. For all micrographs the same conditions of picture recording were used. Step size between particular neighboring micrographs was approximately 100 lm. In order to quantify the occurrence of outgrowths, we calculated S, which represents a surface area not covered by outgrowths related to a total area of a particular SEM micrograph. The graph in Fig. 5 shows S as function of position on the tape, where the horizontal axis was normalized with respect to the center of tape width. A very remarkable similarity was found between the graphs in Figs. 2 and 5. Both dependences show a steep decrease in the marginal regions of the tape. From the total tape width of 12 mm, approximately 2 mm from one side and 1 mm from the other side seem to have significantly different properties compared to the central part. This very good correlation between structural and electric properties suggests that the occurrence of outgrowths can have major effect on the homogeneity of the jc distribution across the tape width. Apart from the evident difference in the surface density of outgrowths, there is also a significant change in the outgrowth morphology. Fig. 6 shows SEM images used for the data collection
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Fig. 3. SEM/SE image illustrating the outgrowths in cross section of superconductor tapes (a) SCS4050 and (b) SCS12050. The outgrowths are penetrating into Ag layer as well as (RE)BCO matrix.
Fig. 4. Top view of etched (RE)BCO surface with uncovered outgrowths analyzed with EDX: the outgrowths are in comparison with (RE)BCO matrix enriched in Cu, O and/or Y and depleted in Ba (SCS12050 tape).
information about dimensions of outgrowths in vertical axis. The height maps of surface from regions near to the tape edge and in the tape middle are shown in Fig 7. According to the latter measurement, the height of outgrowths above the (RE)BCO surface seems to be slightly different in both investigated regions. The outgrowths in the center of the tape width have the height about 400 nm and this height is relatively homogeneous in whole measured area. This result is also corresponding to the measurement of outgrowths heights in central region of superconducting tape (cross section view) using SEM. Mean value of outgrowths heights above (RE)BCO surface was 385 nm. On the other hand, the outgrowths in regions near to tape edge are significantly higher and their height is ranging from about 400 nm up to 600 nm. The surface area roughness Sa (arithmetic mean deviation of all surface height values) have been calculated assuming the following expression Fig. 5. Proportion of surface area without outgrowths (S) as function of position across the tape width with regard to the SCS12050 tape center. N
illustrated in Fig. 5. They represent typical observed distribution of outgrowths near the tape edge (a) and in the center (b) of the tape width. Besides rounded outgrowths, a lot of needle-like shaped outgrowths were found preferably in marginal regions of tapes. Measurement of surface roughness by confocal laser scanning microscopy (ZEISS LSM 700) provided us with complementary
Sa ¼
y Nx X 1 X jz xi ; yj Sc j; Nx Ny i¼1 j¼1
ð1Þ
where Nx, Ny is number of measured points in x and y direction, z (xi, yj) is height of the measured point in coordinates x and y, Sc is mean height of all surface height values.
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Fig. 7. Confocal laser scanning microscopy measurement of surface roughness near to edge (top) and in the central part of SCS12050 tape (bottom) with surface area roughness Sa of 0.079 lm and 0.054 lm, respectively. Fig. 6. SEM/SE images of the SCS12050 tape with removed Cu and Ag overlayer by selective chemical etching. The top view shows the uncovered outgrowths at different positions with its typical density distribution: (a) near tape edge, (b) in the center of the tape width. The micrographs of left- and right-hand marginal regions were very similar.
The obtained values of Sa were 0.054 and 0.079 lm in the center of the tape width and in the marginal regions, respectively. Grain boundaries separating the crystals of different orientation behave as considerable obstacles to the current flow. The interconnecting current density decreases exponentially with the grain boundary angle [16,17]. We were therefore also interested in the crystal orientation within the functional (RE)BCO layer. We supposed that the outgrowths and the regions close to the outgrowths could have other than z-axis crystal orientation (z-axis is parallel with the lattice parameter c). To verify the latter assumption, we investigated in detail the cross-sectioned samples by means of electron backscattered diffraction (EBSD) technique. Today it is widely known that EBSD technique requires sufficiently smooth sample surface in order to obtain clear and high contrasted Kikuchi patterns. We applied the sample preparation techniques described in [18–20], but the best results in the sharpness of Kikuchi patterns were achieved by polishing of cross-sectioned tapes with Ar ions (see Kikuchi pattern in Fig. 8). However, this preparation technique worked well for adjacent Ag and Cu layers, but it was still not suitable enough to enable automated EBSD mapping in the superconducting tape region. In order to successfully discriminate between a-, b- and c-axis in the crystal structure of (RE)BCO superconductor with automatic route, it is necessary to detect certain critical bands in electron backscattered patterns (EBSP), which are unfortunately associated with very low intensity. Therefore, this investigation was performed manually by
Fig. 8. A Kikuchi pattern of (RE)BCO superconducting layer.
point-by-point measurement, which eliminated also picture drift problem occurring during the long-term EBSD scanning. The Kikuchi patterns were generated at the acceleration voltage of 20 kV and recorded using 4 4 binning on a Nordlys II camera for 30 ms. The samples were tilted to 70° during measurement. All investigated (RE)BCO thin films should have biaxial texture with identical (0 0 1) or c-axis orientation. This assumption was also confirmed in the outgrowth-free regions as indicated in Fig. 9. In case of regions with outgrowths we will distinguish as previously between regions with rounded outgrowths and regions with needle-like outgrowths. The rounded outgrowths consist partially of (RE)BCO superconductor and partially of non-superconducting phases. The (RE)BCO areas inside these outgrowths
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Fig. 9. SEM/SE images of cross-section of SCS4050 tape perpendicular to long-length with marked places in the superconducting layer, where the crystal orientation of (RE)BCO was identified by EBSD. In these cross sections one needle-like shaped outgrowth with a-axis orientation is observable whereas the others are rounded outgrowths and have randomly oriented grains. Examples of both cases are marked by circles. Outgrowth-free regions indicate c-axis orientation.
indicated different crystal orientation compared to (RE)BCO outgrowth-free region. The grains in the latter regions are oriented randomly. Non-superconducting part of rounded outgrowths can have the chemical composition already mentioned in the beginning of this capture. Good contrast of the micrograph enabled visibility of non-superconducting phases as darker areas in (RE)BCO layer. In these areas it was not possible to identify the grain orientation by EBSD (question marks in Fig. 9). Nonassigned indexing was caused by either overlapping of two different EBSPs or pertaining the EBSP to an unknown phase, probably copper and/or yttrium oxide as already mentioned, so finally it can be concluded that in these areas no (RE)BCO grains with c-axis orientation occurred. The needle-like shaped outgrowths consist of (RE)BCO superconductor and have two orthogonal a-axis orientation (a-axis is parallel to the surface normal) as it was already reported by Chang et al. [10]. According to the latter authors, the needle-like shaped outgrowths with a-axis orientation are prone to epitaxial growth on the c-axis film because of a proper ratio between b and c lattice parameters (b = 1/(3c)), so the a-axis oriented nuclei have their b and c directions in the surface plane. An example of the needle-like outgrowth in cross-section view in our samples is shown in Fig. 9a (marked by red circle). This outgrowth was cut in transverse section, so the cross section of ‘‘needle’’ is visible. Its crystal orientation was measured as follow: the a-axis was oriented parallel to the surface normal and the b-axis was perpendicular to the cross section surface, although according to the orientation of needle shape we could expect that no b-axis but caxis orientation is perpendicular to the cross section surface. The difference between expected c-axis and measured b-axis orientation can be explained by theory given in [10]. Thus, the authors postulate that b is the fast growth direction and c is the slowest growth direction. Therefore, shorter site of a-axis oriented needle-like shaped outgrowth possesses c-axis orientation which direction was slower to grow. The existence of faster and slower growth directions explains also the formation of needle-like shaped outgrowths. Our experimental results indicate that the outgrowths with different density, morphology as well as size were observed
dominantly in the marginal regions of tapes in comparison to the central area. There is an indication of non-homogeneous nucleation of outgrowths across the tape width during the tape preparation. Therefore, it will be interesting to investigate such tapes produced by another deposition techniques mentioned in introduction. A non-uniform profile of critical current density across the tape width accompanied by a significant decrease of jc close to edges of the superconductor tape can be related to the presence of outgrowths. The needle-like shaped outgrowths grow on the surface of (RE)BCO layer and they are usually not penetrating into (RE)BCO layer. Finally, the influence of needle-like outgrowths on critical current density is small. However, the jc can be affected by presence of rounded outgrowths. They penetrate especially in marginal regions of tape deeply into (RE)BCO layer and consist of both nonsuperconducting phases and of randomly oriented (RE)BCO grains. Therefore, the presence of rounded outgrowths results in both the higher amount of volume with non-superconducting phases (effect of reduction of superconductor volume) and the higher amount of grains with a random crystal orientation (grain boundary effect).
4. Conclusions Very good correlation between the structure and electric properties was found in superconducting (RE)BCO thin layers of biaxial texture with z-axis orientation. Significant decrease of critical current density close to the edges of superconductor tapes is related to the presence of outgrowths. In functional (RE)BCO layer, two kinds of outgrowths were observed: needle-like shaped and rounded outgrowths. Predominantly the rounded outgrowths affected the profile of critical current density because of their higher distribution density, bigger size, and deeper penetration into (RE)BCO layer, especially in the marginal regions if compared with the outgrowths from the central region of the tape width. Besides this, it was found that the rounded outgrowths partially consist of randomly oriented (RE)BCO grains, and other parts were found to consist of non-superconducting phases e.g. CuO, CuYO2 or Y2Cu2O5. Higher density of outgrowths at the edges of superconducting
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tapes results in both the higher amount of grains with random crystal orientation (grain boundary effect) and the higher amount of volume with non-superconducting phases (reduction of superconductor volume). The homogeneity of critical current density distribution across the tape width can be affected by the above factors. Acknowledgements The authors would like to thank to the Grant Agency of the Ministry of Education of the Slovak Republic and the Slovak Academy of Sciences (VEGA) for financial support under the contract no. 1/ 0162/11 ‘‘Effects of inhomogeneities on functional properties of high-temperature superconducting wires.’’ This publication is also the result of the project implementation: Center for development and application of advanced diagnostic methods in processing of metallic and non-metallic materials, ITMS: 26220120014, supported by the Research & Development Operational Programme funded by the ERDF. References [1] J. Xiong, W. Qin, X. Cui, B. Tao, J. Tang, Y. Li, High-resolution XRD study of stress-modulated YBCO films with various thicknesses, J. Crystal Growth 300 (2007) 364–367. [2] M. Li, B. Ma, R.E. Koritala, B.L. Fisher, K. Venkataraman, V.A. Maroni, V. VlaskoVlasov, P. Berghuis, U. Welp, K.E. Gray, U. Balachandran, Pulsed laser deposition of c-axis untilted YBCO films on c-asis tilted ISD MgO-buffered metallic substrates, Physica C 387 (2003) 373–381. [3] J. Chen, X.D. Su, D.L. Zhu, J. Fan, H. Yang, Y. Chen, V.G. Harris, An approach to (103) oriented YBa2Cu3O7d thin films epitaxially grown on (100) MgO substrates, Scripta Mater. 64 (2011) 450–453. [4] J.A. Xia, N.J. Long, N.M. Strickland, P. Hoefakker, E.F. Talantsev, X. Li, W. Zhang, T. Kodenkandath, Y. Huang, M.W. Rupich, TEM observation of the microstructure of metal-organic deposited YBa2Cu3O7d with Dy additions, Supercond. Sci. Technol. 20 (2007) 880–885. [5] P. Zhao, A. Ito, T. Kato, D. Yokoe, T. Hirayama, T. Goto, High-speed growth of YBa2Cu3O7d superconducting films on multilayer-coated Hastelloy C276 tape by laser-assisted MOCVD, Supercond. Sci. Technology 26 (2013) 055020.
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