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Optimisation of single La2Zr2O7 buffer layers for YBCO coated conductors prepared by chemical solution deposition
ARTICLE IN PRESS Journal of Crystal Growth 310 (2008) 4295–4300
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Optimisation of single La2Zr2O7 buffer layers for YBCO coated conductors prepared by chemical solution deposition S. Engel a,b, R. Hu¨hne a,, K. Knoth a, A. Chopra d, N.H. Kumar d, V.S. Sarma c, P.N. Santhosh d, L. Schultz a,b, B. Holzapfel a a
IFW Dresden, P.O. Box 270116, D-01171 Dresden, Germany Department of Mechanical Engineering, Institute of Material Science, Dresden University of Technology, D-01062 Dresden, Germany c Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, 600036 Chennai, India d Department of Physics, Indian Institute of Technology Madras, 600036 Chennai, India b
a r t i c l e in fo
abstract
Article history: Received 23 April 2008 Received in revised form 30 June 2008 Accepted 7 July 2008 Communicated by R. Fornari Available online 10 July 2008
La2Zr2O7 (LZO) buffer layers have been prepared on Ni-5 at%W tapes using chemical solution deposition. The local texture of the deposited layers has been studied throughout the thickness using ion-beam sputtering and electron back-scattering diffraction (EBSD). The processing conditions have been optimised afterwards based on these results leading to a highly textured surface for annealing temperatures of 1050 1C. YBa2Cu3O7x layers were deposited by pulsed laser deposition on these single LZO buffer layers and studied in detail using X-ray diffraction and EBSD. A close correlation was found between the surface texture and the superconducting properties. & 2008 Elsevier B.V. All rights reserved.
PACS: 81.15.Lm 68.55.jm 74.78.Bz Keywords: A1. Characterisation A3. Chemical solution deposition A3. Pulsed laser deposition B1. Oxides B2. Oxide superconducting materials
1. Introduction Chemical solution deposition (CSD) is regarded as a scalable and cost-effective approach for the realisation of long-length YBa2Cu3O7x (YBCO) coated conductors [1,2], which are of great interest for electric power applications. Such coated conductor architecture is typically based on a flexible metallic substrate, on which suitable buffer layers and the superconductor is deposited by a physical or chemical method. A major requirement on the superconducting layer itself is a strong biaxial texture in order to avoid the weak coupling of high-angle grain boundaries and to reach high critical current densities in these materials [3]. Textured layers can be obtained either via epitaxial growth when starting from a cube-textured metallic substrate in the rolling assisted biaxially textured substrates (RABiTS) approach [4] or by
Corresponding author.
E-mail address: [email protected] (R. Hu¨hne). 0022-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2008.07.008
inducing the required texture in one of the buffer layers with advanced methods as ion-beam assisted deposition [5]. The preparation of a coated conductor architecture based on the RABiTS approach requires the deposition of epitaxial buffer layers on these substrates. Their main function is to transfer the biaxial texture from the substrate into the superconducting layer, to avoid the detrimental effect of Ni diffusion into the YBCO, and to reduce the oxidation of the Ni substrate during processing, which in general requires a high oxygen partial pressure. Much effort was put within the last decade to develop suitable buffer architectures on RABiTS tapes using CSD processes only in order to achieve a fully scalable and cost-effective process [6]. Especially La2Zr2O7 (LZO) has attracted much attention among the different oxide materials, which have been tested as buffer within the last years, as it displays a good combination of lattice match and structural compatibility with YBCO as well as with the Ni-alloy substrate [7–9]. Furthermore, it was shown that the diffusion of Ni is contained within the first 80–120 nm of the buffer [9]. This paper presents the investigations to optimise the use of LZO as a single buffer layer on cube-textured Ni-5 at%W
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substrates. The work is based on a newly developed precursor solution route described in detail in previous publications [10,11]. There it was shown that high-quality LZO layers can be achieved at annealing temperatures well below 1000 1C. However, the preparation of YBCO layers on top of these single layers with a thickness of about 80–100 nm showed that either an additional CeO2 layer or thicker LZO buffers using multiple coating steps are necessary to achieve good superconducting properties [12,13]. Simultaneously, detailed TEM investigation on the prepared LZO layers revealed nano-pores as well as partially amorphous regions in layers processed at 900 1C [14]. Therefore, detailed electron back-scattering diffraction (EBSD) measurements on a highresolution scanning electron microscope (SEM) were performed in order to investigate the local surface texture of the LZO layers, which has a crucial role for the epitaxial growth of the subsequent layers. The annealing procedure was optimised afterwards based on these results. Finally, YBCO layers were prepared on these buffers in order to check the potential of a single buffer layer.
2. Experimental procedure
The LZO buffer layers were characterised using X-ray diffraction (XRD) to measure y2y scans (Co–Ka). A four-circle diffractometer (Cu–Ka) was used to collect pole figures, o and f scans. Additionally, EBSD in a high-resolution SEM was used to determine the local texture of the oxide buffer layer. An rf plasma source providing an Ar ion beam was used to remove the upper LZO layers in order to study the local texture throughout the thickness using EBSD. Therefore, the sample was partially covered with a protecting layer prior to sputtering. The resulting step in the LZO after the removal of this layer was measured with a profilometer in order to determine the removed thickness during the sputter process. Reflection high-energy electron diffraction (RHEED) was used to investigate the surface texture of the upper 5 nm of the layers. Therefore, an electron beam with an energy of 30 keV and beam current of about 50 mA was introduced under a grazing incidence angle of 0.5–1.51 to the substrate surface. The diffraction pattern was recorded using a CCD camera and analyzed with a computer program based on the kinematic theory of electron scattering. The microstructure of LZO buffer layers was additionally investigated using SEM and atomic force microscopy (AFM).
2.1. Preparation of nickel substrates Biaxially textured Ni-5 at%W tapes were used as substrates for the investigations. The preparation of these substrates using intense cold rolling to deformations 497% and a subsequent annealing are described in detail elsewhere [15,16]. The desired cube texture was obtained after a two-step recrystallisation treatment in a reducing atmosphere (Ar+5%H2). Prior to coating, the recrystallised tapes with a cross-section of 10 mm 80 mm were cleaned in an ultrasonic acetone bath for 20 min and additionally annealed in Ar+5%H2 at 800 1C for 20 min in order to reduce the naturally grown NiO layer built up during storage and handling on the substrate surface. 2.2. Preparation and characterization of LZO coatings The synthesis of the precursor solution to obtain LZO buffer layers was described in detail previously [10,11]. The LZO precursor solution was produced using lanthanum (III) 2,4pentanedionate and zirconium (IV) 2,4-pentanedionate (99.9% metals basis, both from Alfa Aesar) as starting substances and propionic acid (X99%, Alfa Aesar) as a solvent. The advantage of propionic acid is its ability to dissolve these pentanedionates in high concentrations. After adding propionic acid to the starting substances, the precursor solution was heated to 130 1C for 15 min to obtain a stable yellow-coloured solution. The (standard) concentration of the solution was adjusted to c(La3+) ¼ c(Zr4+) ¼ 0.2 mol l1. The precise metal contents were determined using inductively coupled plasma optical emission spectroscopy (ICP-OES) to ensure the stoichiometric ratio. The solution preparation was carried out under atmospheric conditions at room temperature. The films were prepared using dip-coating at room temperature. After dipping the substrates into the precursor solution, they were held immersed for 30 s and withdrawn with a speed of 0.2 cm/s. Before annealing in a quartz tube furnace, the samples were placed in a drying chamber at 180 1C for 20 min in air. The annealing step was carried out at various peak temperatures between 900 1C and 1050 1C using a heating rates of 10 K/min with different dwelling times in a continuous flowing forming gas (Ar+5% H2) atmosphere. After annealing, the samples were cooled in the furnace down to 500 1C and subsequently fast cooled within 30 min to room temperature.
2.3. Preparation and characterization of YBCO YBCO films were prepared on the LZO layers using pulsed laser deposition (PLD) in order to test the suitability of the prepared buffers. The deposition was carried out using a repetition rate of 5 Hz and a background pressure of 0.3 mbar oxygen at 810 1C. After deposition, the samples were cooled in 0.4 bar O2. The standard thickness of the YBCO layer was about 300 nm for all samples. YBCO films were also deposited with the same conditions on (1 0 0) SrTiO3 single crystals for comparison. More details can be found elsewhere [17,18]. The preferred orientations and the texture of the prepared samples were investigated using XRD, which allows the determination of the full-width at half-maximum (FWHM) of the out-ofplane and the in-plane distribution of all layers. Again, EBSD measurements were used to determine the local texture of the YBCO layers. SEM was used to investigate the surface structure. The quality of the deposited YBCO layers was determined with inductive measurements of the critical temperature Tc as well as of the critical current density Jc at 77 K in self-field.
3. Results and discussion 3.1. Sputter experiments on LZO The LZO buffer layers were prepared on Ni-5 at% W tapes using dip-coating with a newly developed precursor solution [11]. The main advantages of this route are the high reproducibility and stability of the solution, the application of less toxic raw materials and the low annealing temperatures necessary for perfect textured buffer layers. Annealing temperatures as low as 900 1C were found to be sufficient to achieve smooth layers with a good surface texture determined using RHEED. However, thicker LZO layers prepared by multiple coating steps or additional buffer layers as CeO2 were necessary to achieve good superconducting properties in the final YBCO layers. Therefore, the surface texture of standard LZO layers prepared at 900 1C was studied locally using high-resolution EBSD. The result of the measurement is summarised in Fig. 1a. It is noticeable that the majority of the grains is well textured, which is in good agreement with the X-ray pole figures and the RHEED
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Fig. 1. EBSD maps of a La2Zr2O7 buffer layer prepared on Ni-5 at% W substrate using an annealing temperature of 900 1C for 60 min: (a) as prepared; (b) after removal of the upper 20 nm; (c) after removal of 40 nm using ion-beam etching.
investigations on these layers published in previous papers. Especially, the grain structure of the underlying Ni substrate can be still recognised. However, two other features are striking in these measurements. Firstly, a significant number of grains seem to be not textured at the surface, i.e. these grains are marked black on the figure due to missing or unrecognised EBSD pattern. A lot of these grains are connected with each other resulting in band like structures on which an epitaxial growth of the superconductor is disturbed. Secondly, even inside the welltextured grains areas are visible, where no EBSD pattern was observed. The overall indexing rate of this EBSD measurement was 46% indicating that in more than half of the measurement points the pattern could not be recognised or identified with the LZO structure. Ion-beam etching has been used in order to study the local texture in more detail throughout the thickness of the film. Therefore, the sample was brought in a high vacuum chamber and a certain amount of material was removed form the surface using Ar+ ions. No significant change was observed after removal of the upper 10 nm resulting in a similar low indexing rate of 47%. However, the picture changed after removal of further 10 nm as apparent in Fig. 1b. The indexing rate of this measurement was with 63% significant higher compared to the previous one. The main difference arises from the reduction of the grains, in which no local texture was observed. In contrast, the number of nonindexed patterns inside well-textured grains remains more or less constant. A further improvement was observed after the removal of additional 20 nm LZO by ion-beam etching (Fig. 1c). The information received by this measurement corresponds roughly to the local texture in the middle of the buffer layer. The number of non-indexed grains was further reduced. Most of them are now connected with misoriented grains in the substrate as indicated by different colours representing a high misorientation towards the cube texture. Furthermore, also the indexing rate inside the well-textured grains improved significantly leading to an overall indexing rate of 86%. In summary, the detailed EBSD measurements indicate that the surface layer of the LZO buffer is not completely textured. Highly textured layers were observed when the upper part of the buffer layer was removed. A possible explanation for the poor crystallinity in the surface region might be in the uncompleted reaction due to the low temperature. Indication for polycrystalline and amorphous fractions was found in recent TEM investigations [14]. The remaining carbon is in that case mainly enriched in the grain boundary region leading to a reduced mobility of such boundaries during crystallisation and grain growth. A similar observation was found in detailed investigations on CeO2 layers prepared by CSD [19].
Fig. 2. RHEED pattern of a LZO buffer layer after annealing for 60 min using a temperature of: (a) 900 1C and (b) 1050 1C.
3.2. Optimisation of processing conditions In the next step, different processing parameters as temperature, dwelling time and gas flow were subsequently changed in order to improve the crystallinity of the buffer layer. A higher substrate temperature leads in general to similar texture values but to bigger grains and consequently to a high surface roughness as already shown previously [10]. Fig. 2 shows the result of RHEED investigations of films processed at different peak temperatures for the same dwelling time. Both films show a well biaxially textured surface indicating a perfect epitaxial growth of the
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buffer. However, in the RHEED pattern of the film processed at 1050 1C clearer spots up to a higher diffraction order are visible. It is known that spot shapes are sensitive to the film microstructure [20], therefore, the observed patterns are an indication for a better surface crystallinity for films processed at higher temperatures. This is also in good agreement with the higher intensity of the (0 0 ‘) reflection in the standard y2y scans for films processed at higher temperatures [10]. EBSD measurements were done on this sample in order to check the local texture in the surface area. The results summarised in Fig. 3 indicate an improved surface texture compared to the films processed at 900 1C leading to an indexing rate of 96% for this sample. Furthermore, the pole figure calculated out of the EBSD data (Fig. 3b) agrees well with the integral measurement using XRD (Fig. 3c). Additionally, the influence of an increased dwelling time at the peak temperature has been studied. Therefore, the LZO buffer layer was annealed at 1050 1C for 60 and 120 min, respectively. X-ray pole figure measurements revealed a similar perfect epitaxial growth on the Ni–5W tape as shown for the longer annealed sample in Fig. 4a and b. A detailed analysis of these films showed that the out-of-plane alignment of the LZO layer degrades with increasing processing time. A FWHM of 5.81 and 8.71 for the rolling and the transverse direction, respectively, was found for the sample annealed for 60 min, whereas a value of 7.51 and 11.21 was measured for an annealing time of 120 min. In contrast, the true in-plane values extracted from the phi-scans of the (2 2 2) LZO peak according to the formulae by Specht et al. [21] showed with 4.91 and 4.61 for 60 and 120 min, respectively, a slight improvement with longer annealing times. This behaviour might be a result of the preferential grain growth in the buffer layer due to the higher mobility of the grain boundaries at higher temperatures.
3.3. YBCO deposition Finally, YBCO layers were deposited on the prepared CSD-LZO layers using PLD in order to test the performance of the single buffer layer in the coated conductor architecture. In general, the YBCO layer revealed a perfect epitaxial growth on the prepared LZO buffer layers as shown in Fig. 4c. A significant improvement of the out-of-plane orientation was observed for the samples processed at 1050 1C. The FWHM values were measured to be 4.91 and 7.51 for the rolling and the transverse direction, respectively, for the sample annealed for 60 min, whereas a width of 6.61 and 9.91 where found for a dwelling time of 120 min. The FWHM values of the true in-plane orientation of the YBCO improved only slightly by about 0.21 compared to the values measured on LZO. These differences can be explained with the preferential growth mode of the YBCO along the c-axis. The typical surface topography measured by SEM is shown in Fig. 5a and b for the two samples. In both cases a homogeneous structure was observed with the main difference that the pinhole size and density is higher in the sample prepared on the LZO layer annealed for 60 min compared to the sample annealed for twice of the time. The local texture of the YBCO layer was investigated in detail using EBSD measurements. The results are shown in Fig. 5c and d for the layers prepared on CSD-LZO using the optimised processing temperature of 1050 1C. It is visible that the YBCO is well textured on both samples revealing a cube-on-cube orientation towards the underlying LZO buffer. An indexing rate of 63% and 88% was found for the sample, where the buffer was annealed for 60 and 120 min, respectively, indicating the better quality of the YBCO on the second sample. Additionally, a significant fraction of small misoriented grains (marked red) was found in the sample
Fig. 3. Results of the texture measurement of an optimised LZO film annealed at 1050 1C for 60 min: (a) local surface texture measured using EBSD; (b) corresponding (1 0 3) pole figure calculated out of the EBSD data; (c) (1 0 3) pole figure measured by X-ray diffraction for comparison.
Fig. 4. X-ray pole figure measurements of a coated conductor sample with a CSD-LZO layer annealed at 1050 1C for 120 min using the: (a) (111) pole of Ni-5 at% W substrate; (b) (2 2 2) pole of the LZO buffer layer; (c) (1 0 3) pole of the final YBCO layer deposited by PLD.
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Fig. 5. YBCO films prepared by PLD on single CSD-LZO buffer layers: SEM image of the surface morphology for a YBCO layer prepared on LZO buffer annealed for (a) 60 and (b) 120 at 1050 1C; corresponding results of the EBSD measurement taken for a YBCO layer deposited on LZO buffer annealed for (c) 60 and (d) 120 min at 1050 1C.
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prepared on the optimised buffer layers reveal a significant improvement of the superconducting properties. The Tc value, determined by a 90% increase in induced voltage at the warmingup stage, was about 88 K for YBCO layers on the LZO layer processed at 1050 1C for 60 min and about 90 K for an annealing time of 120 min. The a transition width DTc was with about 6 and 3.1 K still larger compared to the typical values of 0.65 K measured for YBCO films on SrTiO3 single crystals or for superconducting layers on PLD-buffer systems, where normally a transition width of about 1 K was observed [18]. Nevertheless, the results show that a significant improvement was achieved for the single LZO buffer layer with a thickness of about 120 nm. Finally, inductive measurements of the critical current density at 77 K in self-field revealed a homogeneous Jc distribution over the complete sample area. Jc values of about 0.2 and 0.7 MA/cm2 were measured for the YBCO films prepared on the LZO layers using an annealing temperature of 1050 1C for 60 and 120 min, respectively (for comparison, a Jc of 4.1 MA/cm2 was measured for the film on SrTiO3). In general, the superconducting properties correlate well with the measured microstructure using SEM and the local texture using EBSD. It was shown in the literature that the density of pores has a direct influence on the critical current density of the superconducting layer [22]. Therefore, the denser YBCO structure observed for the sample annealed for a longer time might be responsible in addition to the improved surface texture of the buffer layer for the improved superconducting properties in this film. Further detailed studies on the influence of the local texture and the microstructure of the buffer layer surface on the growth of the YBCO are needed to further optimise the superconducting properties. Nevertheless, the achieved results are comparable to recently published data on single LZO layers with a MOCVD-YBCO layer on top having a Jc of about 0.8 MA/cm2 [23].
4. Conclusions
Fig. 6. Inductive measurement of the superconducting transition temperature for YBCO films prepared on LZO buffer layers, which were annealed at different temperatures for different time.
with shorter annealing time for the buffer. This might be a sign for a disturbed nucleation of the YBCO layer due to misoriented grains in the buffer. The superconducting properties of the YBCO layers were measured inductively on unpatterned samples. The results are summarised in Fig. 6. The YBCO layer prepared on a single LZO buffer processed at 900 1C revealed a superconducting transition below 80 K. In that case, thicker LZO layers or an additional CeO2 are necessary in order to achieve good superconducting properties as already shown previously [12,13]. The results of YBCO layers
In summary, it was shown that LZO films processed at 900 1C still show a significant number of non-textured grains at the surface. Sputter experiments in combination with EBSD measurements revealed that the crystallinity of the buffer is mainly disturbed in the surface area, whereas well-textured structure was found in the central part of the buffer. A possible explanation is that the remaining carbon content in the buffer layer is mainly enriched in the grain boundary region leading to a reduced mobility of such boundaries during crystallisation and grain growth. Higher annealing temperatures as well as longer annealing times lead to a better surface texture but also to a higher roughness. It was shown that these optimised layers are suitable templates for superconducting films. A close correlation of the surface texture measured by EBSD and the superconducting properties was found. As a result, YBCO layers prepared by PLD showed an inductively measured critical temperature of up to 90 K and a critical current density of up to 0.7 MA/cm2 at 77 K in self-field. These values indicate that LZO layers have the potential of a single buffer in a coated conductor architecture.
Acknowledgments The work and results reported in this publication was supported in part by the DAAD-DST Project Based Personnel Exchange Program under contract number D/05/57660 and the Virtual Institute: ‘Chemically deposited YBCO superconductors’ (UH-VI-126) supported by the Helmholtz Association of National Research Centres. The supply of the Ni-5 at%W substrates by the evico GmbH is gratefully acknowledged.
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References [1] K. Knoth, S. Engel, C. Apetrii, M. Falter, B. Schlobach, R. Hu¨hne, S. Oswald, L. Schultz, B. Holzapfel, Curr. Opin. Solid State Mater. Sci. 10 (2006) 205. [2] X. Obradors, et al., Supercond. Sci. Technol. 19 (2006) S13. [3] D. Dimos, P. Chaudhari, J. Mannhart, F.K. LeGoues, Phys. Rev. Lett. 61 (1988) 219. [4] D.P. Norton, et al., Science 274 (1996) 755. [5] Y. Iijima, N. Tanabe, O. Kohno, Y. Ikeno, Appl. Phys. Lett. 60 (1992) 769. [6] M.S. Bhuiyan, M. Paranthaman, K. Salama, Supercond. Sci. Technol. 19 (2006) R1. [7] T.G. Chirayil, et al., Physica C 336 (2000) 63. [8] E. Celik, Y. Akin, W. Sigmund, Y.S. Hascicek, Mater. Sci. Eng. B 106 (2004) 182. [9] S. Sathyamurthy, M. Paranthaman, L. Heatherly, P.A. Martin, E.D. Specht, A. Goyal, T. Kodenkandath, X.P. Li, M.W. Rupich, J. Mater. Res. 21 (2006) 910. [10] K. Knoth, R. Hu¨hne, S. Oswald, L. Schultz, B. Holzapfel, Acta Mater. 55 (2007) 517. [11] K. Knoth, R. Hu¨hne, S. Oswald, L. Schultz, B. Holzapfel, Supercond. Sci. Technol. 18 (2005) 334. [12] S. Engel, K. Knoth, R. Hu¨hne, L. Schultz, B. Holzapfel, Supercond. Sci. Technol. 18 (2005) 1385.
[13] K. Knoth, R. Hu¨hne, S. Oswald, L. Molina, O. Eibl, L. Schultz, B. Holzapfel, Thin Solid Films 516 (2008) 2099. [14] L. Molina, S. Engel, K. Knoth, R. Hu¨hne, B. Holzapfel, O. Eibl, J. Phys.: Conf. Ser. 97 (2008) 012108. [15] J. Eickemeyer, D. Selbmann, R. Opitz, H. Wendrock, E. Maher, U. Miller, W. Prusseit, Physica C 372–376 (2002) 814. [16] J. Eickemeyer, D. Selbmann, R. Opitz, B. de Boer, B. Holzapfel, L. Schultz, U. Miller, Supercond. Sci. Technol. 14 (2001) 152. [17] R. Hu¨hne, D. Selbmann, J. Eickemeyer, J. Ha¨nisch, B. Holzapfel, Supercond. Sci. Technol. 19 (2006) 169. [18] R. Hu¨hne, V.S. Sarma, D. Okai, T. Thersleff, L. Schultz, B. Holzapfel, Supercond. Sci. Technol. 20 (2007) 709. [19] A. Cavallaro, F. Sandiumenge, J. Gazquez, T. Puig, X. Obradors, J. Arbiol, H.C. Freyhardt, Adv. Funct. Mater. 16 (2006) 1363. [20] R.T. Brewer, H.A. Atwater, J.R. Groves, P.N. Arendt, J. Appl. Phys. 93 (2003) 205. [21] E.D. Specht, A. Goyal, D.F. Lee, F.A. List, D.M. Kroeger, M. Paranthaman, R.K. Williams, D.K. Christen, Supercond. Sci. Technol. 11 (1998) 945. [22] A. Pomar, J. Gutie´rrez, A. Palau, T. Puig, X. Obradors, Phys. Rev. B 73 (2006) 214522. [23] T. Caroff, et al., Supercond. Sci. Technol. 21 (2008) 075007.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Optimisation of single La2Zr2O7 buffer layers for YBCO coated conductors prepared by chemical solution deposition S. Engel a,b, R. Hu¨hne a,, K. Knoth a, A. Chopra d, N.H. Kumar d, V.S. Sarma c, P.N. Santhosh d, L. Schultz a,b, B. Holzapfel a a
IFW Dresden, P.O. Box 270116, D-01171 Dresden, Germany Department of Mechanical Engineering, Institute of Material Science, Dresden University of Technology, D-01062 Dresden, Germany c Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, 600036 Chennai, India d Department of Physics, Indian Institute of Technology Madras, 600036 Chennai, India b
a r t i c l e in fo
abstract
Article history: Received 23 April 2008 Received in revised form 30 June 2008 Accepted 7 July 2008 Communicated by R. Fornari Available online 10 July 2008
La2Zr2O7 (LZO) buffer layers have been prepared on Ni-5 at%W tapes using chemical solution deposition. The local texture of the deposited layers has been studied throughout the thickness using ion-beam sputtering and electron back-scattering diffraction (EBSD). The processing conditions have been optimised afterwards based on these results leading to a highly textured surface for annealing temperatures of 1050 1C. YBa2Cu3O7x layers were deposited by pulsed laser deposition on these single LZO buffer layers and studied in detail using X-ray diffraction and EBSD. A close correlation was found between the surface texture and the superconducting properties. & 2008 Elsevier B.V. All rights reserved.
PACS: 81.15.Lm 68.55.jm 74.78.Bz Keywords: A1. Characterisation A3. Chemical solution deposition A3. Pulsed laser deposition B1. Oxides B2. Oxide superconducting materials
1. Introduction Chemical solution deposition (CSD) is regarded as a scalable and cost-effective approach for the realisation of long-length YBa2Cu3O7x (YBCO) coated conductors [1,2], which are of great interest for electric power applications. Such coated conductor architecture is typically based on a flexible metallic substrate, on which suitable buffer layers and the superconductor is deposited by a physical or chemical method. A major requirement on the superconducting layer itself is a strong biaxial texture in order to avoid the weak coupling of high-angle grain boundaries and to reach high critical current densities in these materials [3]. Textured layers can be obtained either via epitaxial growth when starting from a cube-textured metallic substrate in the rolling assisted biaxially textured substrates (RABiTS) approach [4] or by
Corresponding author.
E-mail address: [email protected] (R. Hu¨hne). 0022-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2008.07.008
inducing the required texture in one of the buffer layers with advanced methods as ion-beam assisted deposition [5]. The preparation of a coated conductor architecture based on the RABiTS approach requires the deposition of epitaxial buffer layers on these substrates. Their main function is to transfer the biaxial texture from the substrate into the superconducting layer, to avoid the detrimental effect of Ni diffusion into the YBCO, and to reduce the oxidation of the Ni substrate during processing, which in general requires a high oxygen partial pressure. Much effort was put within the last decade to develop suitable buffer architectures on RABiTS tapes using CSD processes only in order to achieve a fully scalable and cost-effective process [6]. Especially La2Zr2O7 (LZO) has attracted much attention among the different oxide materials, which have been tested as buffer within the last years, as it displays a good combination of lattice match and structural compatibility with YBCO as well as with the Ni-alloy substrate [7–9]. Furthermore, it was shown that the diffusion of Ni is contained within the first 80–120 nm of the buffer [9]. This paper presents the investigations to optimise the use of LZO as a single buffer layer on cube-textured Ni-5 at%W
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substrates. The work is based on a newly developed precursor solution route described in detail in previous publications [10,11]. There it was shown that high-quality LZO layers can be achieved at annealing temperatures well below 1000 1C. However, the preparation of YBCO layers on top of these single layers with a thickness of about 80–100 nm showed that either an additional CeO2 layer or thicker LZO buffers using multiple coating steps are necessary to achieve good superconducting properties [12,13]. Simultaneously, detailed TEM investigation on the prepared LZO layers revealed nano-pores as well as partially amorphous regions in layers processed at 900 1C [14]. Therefore, detailed electron back-scattering diffraction (EBSD) measurements on a highresolution scanning electron microscope (SEM) were performed in order to investigate the local surface texture of the LZO layers, which has a crucial role for the epitaxial growth of the subsequent layers. The annealing procedure was optimised afterwards based on these results. Finally, YBCO layers were prepared on these buffers in order to check the potential of a single buffer layer.
2. Experimental procedure
The LZO buffer layers were characterised using X-ray diffraction (XRD) to measure y2y scans (Co–Ka). A four-circle diffractometer (Cu–Ka) was used to collect pole figures, o and f scans. Additionally, EBSD in a high-resolution SEM was used to determine the local texture of the oxide buffer layer. An rf plasma source providing an Ar ion beam was used to remove the upper LZO layers in order to study the local texture throughout the thickness using EBSD. Therefore, the sample was partially covered with a protecting layer prior to sputtering. The resulting step in the LZO after the removal of this layer was measured with a profilometer in order to determine the removed thickness during the sputter process. Reflection high-energy electron diffraction (RHEED) was used to investigate the surface texture of the upper 5 nm of the layers. Therefore, an electron beam with an energy of 30 keV and beam current of about 50 mA was introduced under a grazing incidence angle of 0.5–1.51 to the substrate surface. The diffraction pattern was recorded using a CCD camera and analyzed with a computer program based on the kinematic theory of electron scattering. The microstructure of LZO buffer layers was additionally investigated using SEM and atomic force microscopy (AFM).
2.1. Preparation of nickel substrates Biaxially textured Ni-5 at%W tapes were used as substrates for the investigations. The preparation of these substrates using intense cold rolling to deformations 497% and a subsequent annealing are described in detail elsewhere [15,16]. The desired cube texture was obtained after a two-step recrystallisation treatment in a reducing atmosphere (Ar+5%H2). Prior to coating, the recrystallised tapes with a cross-section of 10 mm 80 mm were cleaned in an ultrasonic acetone bath for 20 min and additionally annealed in Ar+5%H2 at 800 1C for 20 min in order to reduce the naturally grown NiO layer built up during storage and handling on the substrate surface. 2.2. Preparation and characterization of LZO coatings The synthesis of the precursor solution to obtain LZO buffer layers was described in detail previously [10,11]. The LZO precursor solution was produced using lanthanum (III) 2,4pentanedionate and zirconium (IV) 2,4-pentanedionate (99.9% metals basis, both from Alfa Aesar) as starting substances and propionic acid (X99%, Alfa Aesar) as a solvent. The advantage of propionic acid is its ability to dissolve these pentanedionates in high concentrations. After adding propionic acid to the starting substances, the precursor solution was heated to 130 1C for 15 min to obtain a stable yellow-coloured solution. The (standard) concentration of the solution was adjusted to c(La3+) ¼ c(Zr4+) ¼ 0.2 mol l1. The precise metal contents were determined using inductively coupled plasma optical emission spectroscopy (ICP-OES) to ensure the stoichiometric ratio. The solution preparation was carried out under atmospheric conditions at room temperature. The films were prepared using dip-coating at room temperature. After dipping the substrates into the precursor solution, they were held immersed for 30 s and withdrawn with a speed of 0.2 cm/s. Before annealing in a quartz tube furnace, the samples were placed in a drying chamber at 180 1C for 20 min in air. The annealing step was carried out at various peak temperatures between 900 1C and 1050 1C using a heating rates of 10 K/min with different dwelling times in a continuous flowing forming gas (Ar+5% H2) atmosphere. After annealing, the samples were cooled in the furnace down to 500 1C and subsequently fast cooled within 30 min to room temperature.
2.3. Preparation and characterization of YBCO YBCO films were prepared on the LZO layers using pulsed laser deposition (PLD) in order to test the suitability of the prepared buffers. The deposition was carried out using a repetition rate of 5 Hz and a background pressure of 0.3 mbar oxygen at 810 1C. After deposition, the samples were cooled in 0.4 bar O2. The standard thickness of the YBCO layer was about 300 nm for all samples. YBCO films were also deposited with the same conditions on (1 0 0) SrTiO3 single crystals for comparison. More details can be found elsewhere [17,18]. The preferred orientations and the texture of the prepared samples were investigated using XRD, which allows the determination of the full-width at half-maximum (FWHM) of the out-ofplane and the in-plane distribution of all layers. Again, EBSD measurements were used to determine the local texture of the YBCO layers. SEM was used to investigate the surface structure. The quality of the deposited YBCO layers was determined with inductive measurements of the critical temperature Tc as well as of the critical current density Jc at 77 K in self-field.
3. Results and discussion 3.1. Sputter experiments on LZO The LZO buffer layers were prepared on Ni-5 at% W tapes using dip-coating with a newly developed precursor solution [11]. The main advantages of this route are the high reproducibility and stability of the solution, the application of less toxic raw materials and the low annealing temperatures necessary for perfect textured buffer layers. Annealing temperatures as low as 900 1C were found to be sufficient to achieve smooth layers with a good surface texture determined using RHEED. However, thicker LZO layers prepared by multiple coating steps or additional buffer layers as CeO2 were necessary to achieve good superconducting properties in the final YBCO layers. Therefore, the surface texture of standard LZO layers prepared at 900 1C was studied locally using high-resolution EBSD. The result of the measurement is summarised in Fig. 1a. It is noticeable that the majority of the grains is well textured, which is in good agreement with the X-ray pole figures and the RHEED
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Fig. 1. EBSD maps of a La2Zr2O7 buffer layer prepared on Ni-5 at% W substrate using an annealing temperature of 900 1C for 60 min: (a) as prepared; (b) after removal of the upper 20 nm; (c) after removal of 40 nm using ion-beam etching.
investigations on these layers published in previous papers. Especially, the grain structure of the underlying Ni substrate can be still recognised. However, two other features are striking in these measurements. Firstly, a significant number of grains seem to be not textured at the surface, i.e. these grains are marked black on the figure due to missing or unrecognised EBSD pattern. A lot of these grains are connected with each other resulting in band like structures on which an epitaxial growth of the superconductor is disturbed. Secondly, even inside the welltextured grains areas are visible, where no EBSD pattern was observed. The overall indexing rate of this EBSD measurement was 46% indicating that in more than half of the measurement points the pattern could not be recognised or identified with the LZO structure. Ion-beam etching has been used in order to study the local texture in more detail throughout the thickness of the film. Therefore, the sample was brought in a high vacuum chamber and a certain amount of material was removed form the surface using Ar+ ions. No significant change was observed after removal of the upper 10 nm resulting in a similar low indexing rate of 47%. However, the picture changed after removal of further 10 nm as apparent in Fig. 1b. The indexing rate of this measurement was with 63% significant higher compared to the previous one. The main difference arises from the reduction of the grains, in which no local texture was observed. In contrast, the number of nonindexed patterns inside well-textured grains remains more or less constant. A further improvement was observed after the removal of additional 20 nm LZO by ion-beam etching (Fig. 1c). The information received by this measurement corresponds roughly to the local texture in the middle of the buffer layer. The number of non-indexed grains was further reduced. Most of them are now connected with misoriented grains in the substrate as indicated by different colours representing a high misorientation towards the cube texture. Furthermore, also the indexing rate inside the well-textured grains improved significantly leading to an overall indexing rate of 86%. In summary, the detailed EBSD measurements indicate that the surface layer of the LZO buffer is not completely textured. Highly textured layers were observed when the upper part of the buffer layer was removed. A possible explanation for the poor crystallinity in the surface region might be in the uncompleted reaction due to the low temperature. Indication for polycrystalline and amorphous fractions was found in recent TEM investigations [14]. The remaining carbon is in that case mainly enriched in the grain boundary region leading to a reduced mobility of such boundaries during crystallisation and grain growth. A similar observation was found in detailed investigations on CeO2 layers prepared by CSD [19].
Fig. 2. RHEED pattern of a LZO buffer layer after annealing for 60 min using a temperature of: (a) 900 1C and (b) 1050 1C.
3.2. Optimisation of processing conditions In the next step, different processing parameters as temperature, dwelling time and gas flow were subsequently changed in order to improve the crystallinity of the buffer layer. A higher substrate temperature leads in general to similar texture values but to bigger grains and consequently to a high surface roughness as already shown previously [10]. Fig. 2 shows the result of RHEED investigations of films processed at different peak temperatures for the same dwelling time. Both films show a well biaxially textured surface indicating a perfect epitaxial growth of the
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buffer. However, in the RHEED pattern of the film processed at 1050 1C clearer spots up to a higher diffraction order are visible. It is known that spot shapes are sensitive to the film microstructure [20], therefore, the observed patterns are an indication for a better surface crystallinity for films processed at higher temperatures. This is also in good agreement with the higher intensity of the (0 0 ‘) reflection in the standard y2y scans for films processed at higher temperatures [10]. EBSD measurements were done on this sample in order to check the local texture in the surface area. The results summarised in Fig. 3 indicate an improved surface texture compared to the films processed at 900 1C leading to an indexing rate of 96% for this sample. Furthermore, the pole figure calculated out of the EBSD data (Fig. 3b) agrees well with the integral measurement using XRD (Fig. 3c). Additionally, the influence of an increased dwelling time at the peak temperature has been studied. Therefore, the LZO buffer layer was annealed at 1050 1C for 60 and 120 min, respectively. X-ray pole figure measurements revealed a similar perfect epitaxial growth on the Ni–5W tape as shown for the longer annealed sample in Fig. 4a and b. A detailed analysis of these films showed that the out-of-plane alignment of the LZO layer degrades with increasing processing time. A FWHM of 5.81 and 8.71 for the rolling and the transverse direction, respectively, was found for the sample annealed for 60 min, whereas a value of 7.51 and 11.21 was measured for an annealing time of 120 min. In contrast, the true in-plane values extracted from the phi-scans of the (2 2 2) LZO peak according to the formulae by Specht et al. [21] showed with 4.91 and 4.61 for 60 and 120 min, respectively, a slight improvement with longer annealing times. This behaviour might be a result of the preferential grain growth in the buffer layer due to the higher mobility of the grain boundaries at higher temperatures.
3.3. YBCO deposition Finally, YBCO layers were deposited on the prepared CSD-LZO layers using PLD in order to test the performance of the single buffer layer in the coated conductor architecture. In general, the YBCO layer revealed a perfect epitaxial growth on the prepared LZO buffer layers as shown in Fig. 4c. A significant improvement of the out-of-plane orientation was observed for the samples processed at 1050 1C. The FWHM values were measured to be 4.91 and 7.51 for the rolling and the transverse direction, respectively, for the sample annealed for 60 min, whereas a width of 6.61 and 9.91 where found for a dwelling time of 120 min. The FWHM values of the true in-plane orientation of the YBCO improved only slightly by about 0.21 compared to the values measured on LZO. These differences can be explained with the preferential growth mode of the YBCO along the c-axis. The typical surface topography measured by SEM is shown in Fig. 5a and b for the two samples. In both cases a homogeneous structure was observed with the main difference that the pinhole size and density is higher in the sample prepared on the LZO layer annealed for 60 min compared to the sample annealed for twice of the time. The local texture of the YBCO layer was investigated in detail using EBSD measurements. The results are shown in Fig. 5c and d for the layers prepared on CSD-LZO using the optimised processing temperature of 1050 1C. It is visible that the YBCO is well textured on both samples revealing a cube-on-cube orientation towards the underlying LZO buffer. An indexing rate of 63% and 88% was found for the sample, where the buffer was annealed for 60 and 120 min, respectively, indicating the better quality of the YBCO on the second sample. Additionally, a significant fraction of small misoriented grains (marked red) was found in the sample
Fig. 3. Results of the texture measurement of an optimised LZO film annealed at 1050 1C for 60 min: (a) local surface texture measured using EBSD; (b) corresponding (1 0 3) pole figure calculated out of the EBSD data; (c) (1 0 3) pole figure measured by X-ray diffraction for comparison.
Fig. 4. X-ray pole figure measurements of a coated conductor sample with a CSD-LZO layer annealed at 1050 1C for 120 min using the: (a) (111) pole of Ni-5 at% W substrate; (b) (2 2 2) pole of the LZO buffer layer; (c) (1 0 3) pole of the final YBCO layer deposited by PLD.
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Fig. 5. YBCO films prepared by PLD on single CSD-LZO buffer layers: SEM image of the surface morphology for a YBCO layer prepared on LZO buffer annealed for (a) 60 and (b) 120 at 1050 1C; corresponding results of the EBSD measurement taken for a YBCO layer deposited on LZO buffer annealed for (c) 60 and (d) 120 min at 1050 1C.
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prepared on the optimised buffer layers reveal a significant improvement of the superconducting properties. The Tc value, determined by a 90% increase in induced voltage at the warmingup stage, was about 88 K for YBCO layers on the LZO layer processed at 1050 1C for 60 min and about 90 K for an annealing time of 120 min. The a transition width DTc was with about 6 and 3.1 K still larger compared to the typical values of 0.65 K measured for YBCO films on SrTiO3 single crystals or for superconducting layers on PLD-buffer systems, where normally a transition width of about 1 K was observed [18]. Nevertheless, the results show that a significant improvement was achieved for the single LZO buffer layer with a thickness of about 120 nm. Finally, inductive measurements of the critical current density at 77 K in self-field revealed a homogeneous Jc distribution over the complete sample area. Jc values of about 0.2 and 0.7 MA/cm2 were measured for the YBCO films prepared on the LZO layers using an annealing temperature of 1050 1C for 60 and 120 min, respectively (for comparison, a Jc of 4.1 MA/cm2 was measured for the film on SrTiO3). In general, the superconducting properties correlate well with the measured microstructure using SEM and the local texture using EBSD. It was shown in the literature that the density of pores has a direct influence on the critical current density of the superconducting layer [22]. Therefore, the denser YBCO structure observed for the sample annealed for a longer time might be responsible in addition to the improved surface texture of the buffer layer for the improved superconducting properties in this film. Further detailed studies on the influence of the local texture and the microstructure of the buffer layer surface on the growth of the YBCO are needed to further optimise the superconducting properties. Nevertheless, the achieved results are comparable to recently published data on single LZO layers with a MOCVD-YBCO layer on top having a Jc of about 0.8 MA/cm2 [23].
4. Conclusions
Fig. 6. Inductive measurement of the superconducting transition temperature for YBCO films prepared on LZO buffer layers, which were annealed at different temperatures for different time.
with shorter annealing time for the buffer. This might be a sign for a disturbed nucleation of the YBCO layer due to misoriented grains in the buffer. The superconducting properties of the YBCO layers were measured inductively on unpatterned samples. The results are summarised in Fig. 6. The YBCO layer prepared on a single LZO buffer processed at 900 1C revealed a superconducting transition below 80 K. In that case, thicker LZO layers or an additional CeO2 are necessary in order to achieve good superconducting properties as already shown previously [12,13]. The results of YBCO layers
In summary, it was shown that LZO films processed at 900 1C still show a significant number of non-textured grains at the surface. Sputter experiments in combination with EBSD measurements revealed that the crystallinity of the buffer is mainly disturbed in the surface area, whereas well-textured structure was found in the central part of the buffer. A possible explanation is that the remaining carbon content in the buffer layer is mainly enriched in the grain boundary region leading to a reduced mobility of such boundaries during crystallisation and grain growth. Higher annealing temperatures as well as longer annealing times lead to a better surface texture but also to a higher roughness. It was shown that these optimised layers are suitable templates for superconducting films. A close correlation of the surface texture measured by EBSD and the superconducting properties was found. As a result, YBCO layers prepared by PLD showed an inductively measured critical temperature of up to 90 K and a critical current density of up to 0.7 MA/cm2 at 77 K in self-field. These values indicate that LZO layers have the potential of a single buffer in a coated conductor architecture.
Acknowledgments The work and results reported in this publication was supported in part by the DAAD-DST Project Based Personnel Exchange Program under contract number D/05/57660 and the Virtual Institute: ‘Chemically deposited YBCO superconductors’ (UH-VI-126) supported by the Helmholtz Association of National Research Centres. The supply of the Ni-5 at%W substrates by the evico GmbH is gratefully acknowledged.
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