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Influence of the heating ramp on the superconducting properties of YBa2Cu3O7 − δ films using chemical solution deposition in a direct sintering method
Thin Solid Films 548 (2013) 498–501
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Influence of the heating ramp on the superconducting properties of YBa2Cu3O7 − δ films using chemical solution deposition in a direct sintering method Pieter Vermeir a,b, Iwein Cardinael a, Glenn Pollefeyt a, Jonas Feys a, Joseph Schaubroeck b, Isabel Van Driessche a,⁎ a b
Ghent University, SCRiPTS, Faculty of Sciences, Department of Inorganic and Physical Chemistry, Ghent, Belgium Ghent University, INKAT, Faculty of Engineering and Architecture, Department of Industrial Technology and Construction, Ghent, Belgium
a r t i c l e
i n f o
Article history: Received 8 May 2013 Received in revised form 29 August 2013 Accepted 11 September 2013 Available online 17 September 2013 Keywords: Superconductivity Chemical solution deposition Thin films YBa2Cu3O7 − δ
a b s t r a c t YBa2Cu3O7 − δ films were produced on (00l) SrTiO3 single crystal substrates by a sustainable fluorine-free chemical solution deposition method. Using water as the primary solvent and low-cost precursors, a direct sintering process without calcination could be obtained. Variation of the heating rate towards the sintering temperature showed a strong effect on texture, surface morphology, phase purity and concurrent superconducting properties. Using an optimized heating rate of 5 °C min−1, superconducting films were produced with an onset critical temperature of 92 K, a difference in critical temperature during transition of 1.8 K, a critical current density of 1.1 MA cm−2 and a critical current of 50 A cm−1 width at 77 K and self-field. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Within long length industrial film production, great potential is given to chemical solution deposition (CSD) methods. CSD techniques show favorable features compared to the traditionally used vacuum techniques including a low investment cost, fast deposition with high yield and most importantly it is an easily scalable and continuous process [1–5]. As high-temperature superconducting films are of great potential for industrial implementation in various applications, such as power and electronic devices [6–8], CSD is gaining more and more interest. Within the CSD approach for YBa2Cu3O7 − δ (YBCO) film production, tri-fluoro-acetate (TFA) precursors are well-established [9,10]. Nevertheless, the main drawback for this method is the high gas generation rate during processing, which is in turn determined by the fluorine content [11]. Consequently it is observed that small variations in thermal treatment, especially calcination temperature and heating rates, have a great influence on microstructure, morphology, film density and most importantly on superconducting properties [12]. In order to obtain a more easier thermal treatment increasing the film quality, fluorine-free [13–17] as well as low fluorine [11,18] precursors are established. Our research group developed a fluorine-free CSD process starting from metal-acetates dissolved in water and acetic acid using ⁎ Corresponding author at: Krijgslaan 281 (S3), B9000 Ghent, Belgium. Tel.: +32 92644433; fax: +32 92644983. E-mail address: [email protected] (I. Van Driessche). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.09.039
tri-ethanolamine as coordinating ligand (AWAT) for high-temperature superconducting film preparation [19]. This sustainable CSD approach is also appealing for other classes of materials such as buffer layers in the coated conductor design [4,20], catalysis [21] and materials with specific thermal expansion [22]. It has been shown that the method can be easily combined with multiple coating technologies such as dip coating, spin coating and ink-jet printing [23–25]. For the AWAT process a direct sintering method without a specific calcination step is applied. In this work the influence of the heating rate up to the sinter temperature was examined towards texture evolution, surface morphology, phase purity and concurrent superconducting properties.
2. Experimental 2.1. Preparation of the precursor solution and SrTiO3 (STO) substrate Precursor solutions were fabricated from metal-acetates, water, acetic acid and tri-ethanolamine [26,27]. A 4/1 ratio of water/acetic acid in combination with a ligand to metal ratio of 2.5/1 was used to prepare a precursor solution of 0.6 M. A pH adjustment to 6.25 was performed using ammonia. By dip coating this solution on polished (100) STO single crystal substrates films were produced. Previously, it was found that the introduction of water-based precursors had no restrictions towards wettability if a judiciously chosen cleaning procedure was applied [28].
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Fig. 1. SEM study of the influence of the heating rate on the morphology of the final YBCO films at (a) 1 °C min−1, (b) 5 °C min−1 and (c) 10 °C min−1.
2.2. Dip coating and thermal process All coatings were made using a single dip at a withdrawal speed of 50 mm min−1 using a KSV dip coater unit. After dip coating, the samples were heated for 1 h at 60 °C. Subsequently, a thermal treatment in a specially designed tube in tube furnace transformed the amorphous phases into epitaxial YBCO. Without any calcination step, the films were sintered at 815 °C in a partial oxygen pressure of 200 ppm mixed with N2. After 150 min the temperature was decreased to 400 °C at a rate of 3 °C min−1. During cooling, the atmosphere was switched to flowing oxygen (industrial quality) at 525 °C. After an annealing period of 5 h, the samples were furnace-cooled to room temperature. Different heating rates towards the sinter temperature, namely 1–5–10–20 °C min−1, were applied in order to verify the influence on texture, surface morphology, phase purity and concurrent superconducting properties. 2.3. Characterization of the deposited thin layers X-ray diffraction (XRD) was used to verify the orientation (Siemens D5000, CuKα) using a θ–2θ geometry as well as the degree of biaxial texture (Siemens D8) using pole-figures. The overall morphology of the films was characterized by optical microscopy (Leitz) and Scanning Electron Microscopy combined with Energy Dispersive X-ray analysis (SEM-EDX Philips 501). SEM operations were perfomed at an accelerating potential of 20 keV in high vacuum. For EDX analysis, the accelerating potential was lowered to 10 keV and an acquisition time of 150 s was used. A custom-made four-point test device was used to determine the critical temperature (Tc) of the superconducting layer. Critical current density (Ic) measurements were performed using a Theva cryoscan setup in liquid nitrogen. 3. Results and discussion 3.1. Morphology Upon visual inspection of the samples after sintering, we found that for the lower heating rates (1–5–10 °C min−1), no traces on uncontrolled combustion could be observed and good coatings were obtained.
Yet, in the case of a heating rate of 20 °C min−1, the coating peeled off the substrates. Therefore, further SEM investigation was limited to those layers produced at 1, 5 and 10 °C min−1 (Fig. 1). For the film heated at 1 °C min−1 (Fig. 1a), impurities are present on the surface of the YBCO layer. Based on EDX-mapping (Fig. 2), these impurities are identified as CuO (Fig. 2d and e), probably formed by coagulation during heating. The presence of these CuO impurities results in an inhomogeneous metal distribution of the film leading to local defects in stoichiometry. Films heated at 5 and 10 °C min−1 did not show these impurities (Fig. 1b and c). Therefore, we conclude that a heating ramp of 5 °C min−1 or more is necessary to provide homogenous coatings that can lead to good superconducting properties.
3.2. Microstructure In order to obtain high Ic values, it is necessary to grow the YBCO film biaxially textured on the STO substrate. Fig. 3 shows the XRD diffraction patterns obtained for sintered and annealed YBCO layers (1–5–10 °C min−1) dip coated on a (100) polished STO substrate. As expected from SEM-EDX, the XRD of the film heated at 1 °C min−1 reveals the presence of a high reflection of CuO (2θ = 38.8). Moreover, only a weak c-axis orientation of the YBCO phase and several unidentified impurities are found. This can be explained by the strong tendency of the element Cu to coagulate which obviously inhibits the homogeneous distribution of the metals in the layer. This inhomogeneity causes local defects in stoichiometry which will automatically affect the YBCO reaction pathway as described before [29], resulting in the presence of impurity phases after sintering. The films heated at 5 and 10 °C min−1 are strongly c-axis oriented. For the layer heated at 10 °C min−1, a small amount of an unidentified impurity phase (2θ = 12.9° and 26.1°) is observed. To check if this impurity has an influence on the in-plane misalignment, (103) pole figures (2θ = 32.52°) were collected (Fig. 4). The average full width at half maximum (FWHM) of the reflections are 0.19° and 0.20° for films heated at 5 and 10 °C min−1, respectively. These low FWHM-values clearly proof the epitaxial growth of YBCO on STO for both films, which indicates that the presence of the impurity does not suppress the epitaxy.
Fig. 2. (a) SEM image for the YBCO layer on STO obtained by heating at 1 °C min−1 and EDX-mapping of (b) Y, (c) Ba, (d) Cu and (e) O.
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3.3. Superconductivity Fig. 5 shows the temperature dependence of the resistance for the epitaxial YBCO films prepared with a heating ramp of 5 and 10 °C min−1. For the film heated at 5 °C min−1, a transition with a onset Tc of 92 K and a difference in critical temperature during transition (ΔTc) of 1.8 K is observed. Although the onset Tc remains equal for the film treated at 10 °C min−1, a broader transition resulting in a ΔTc of 5 K is observed. Presumably, the presence of a small amount of impurity phase, affects the zero Tc. For both films the residual resistance remains essentially zero. Additionally, the influence of the heating rate on the temperature dependence of the electrical resistivity, ρ(T), in the normal state was examined. A linear behavior of ρ(T) in the normal state, is typical for high-quality, in-plane-aligned YBCO films with little grain boundary scattering [30]. This normal state behavior can be quantified by the resistivity ratio ρ(100 K)/ρ(300 K) and the extrapolation of the lowtemperature part of the resistivity lines. If it extrapolates to zero near 0 K and ρ(100 K)/ρ(300 K) ≤ 0.4, one can conclude that a good normal state behavior is obtained. As observed from Fig. 5, the resistivity ratio
Fig. 4. YBCO (103) pole figure for an YBCO film on STO heated at (a) 5 °C min−1 and (b) 10 °C min−1.
for films heated at 5 and 10 °C min−1 is 0.32 and 0.71 respectively, while the extrapolation goes only to zero for the film heated at 5 °C min−1. This indicates again the presence of a small amount of impurity phase, contributing to the increased normal-state resistivity. Magnetic inductive critical current density (Jc) measurements at 77 K in self-field are shown in Fig. 6. The film heated at 5 °C min−1 shows a Jc of 1.1 MA cm−2, while the critical current for a film heated at 10 °C min−1 is 0.55 MA cm−2. We believe that the increase in superconducting properties for a layer heated at 5 °C min−1 is correlated to the increased phase purity. Remarkable is the broad heating rate interval, going from 5 till 10 °C min−1, giving rise to superconducting properties. Combining Jc measurements (Fig. 6) with the thickness of the layer, which is approximately 450 nm based on cross-sectional SEM, the critical current can be calculated. The Ic-value of the sample at 77 K and in self-field is 25 A cm−1 width for the film heated at 10 °C min−1, whereas the film heated at 5 °C min−1 shows an Ic of 50 A cm−1 width. The latter result was used to make a comparison between our AWAT-YBCO layer on single crystal and YBCO layers made from other
Fig. 3. XRD pattern of the YBCO layer dip coated on a (l00) polished STO substrate when exposed to different heating rates. (a) 1 °C min−1, (b) 5 °C min−1 and (c) 10 °C.min−1. (°) CuO, (⋄,*) unidentified impurities.
Fig. 5. Resistance Tc measurements of the final YBCO films on STO heated at 5 and 10 °C min−1. The resistivity for both films was 40 and 100 μΩ cm respectively.
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References
Fig. 6. Inductive Jc measurements of the final YBCO films heated at 5 and 10 °C min−1.
CSD methods. Using the TFA-approach, it is possible to obtain Jc-values up to 7 MA cm−2 on LaAlO3 at 77 K in self-field [31]. Nevertheless, as soon as a slightly modified low-fluorine TFA method is applied, Jc obtained on STO drops to ~1 MA cm−2 at 77 K in self-field [32]. The Jc-value obtained for the film heated at 5 °C min−1 is lower than those for the classic TFA methods, but is competitive with those of low-fluorine containing precursors. When taking into account the scalability for our process due to the simple processing conditions, especially compared with TFA-related methods where a constant low p(H2O) is needed inside the entire furnace, it is clear that water-based YBCO deposition can be a competitive method to the currently established CSD techniques on single crystal STO. 4. Conclusion In this study, the influence of the heating rate towards the sinter temperature was verified for YBCO film production using a direct sintering method. Therefore a sustainable fluorine-free water-based CSD precursor was used to coat a single crystal STO substrate. We clearly proof the link between heating rate, phase purity, morphology and concurrent superconducting properties. If the applied heating rate is too slow (1 °C min−1), coagulation of copper results in inhomogeneous semiconducting films. If the heating rate is too high (20 °C min−1), films start to peel off. Only intermediate heating rates (5–10 °C min−1) give rise to superconducting behavior. For both intermediate heating rates (5–10 °C.min−1) similar microstructural properties are found. Nevertheless, it was observed that a very small amount of impurity phase, in the case of a 10 °C min−1 heating rate, gave rise to inferior results concerning (i) broadness of the resistive transition, (ii) normal-state resistivity behavior, (iii) critical current density and (iv) critical current per unit of width. Acknowledgments The authors would like to thank Olivier Janssens for performing XRD measurements and SEM-EDX.
[1] S.R. Foltyn, L. Civale, J.L. Macmanus-Driscoll, Q.X. Jia, B. Maiorov, H. Wang, M. Maley, Nat. Mater. 6 (2007) 631. [2] I. Van Driessche, G. Penneman, E. Bruneel, S. Hoste, Pure Appl. Chem. 74 (2002) 2101. [3] X. Obradors, T. Puig, A. Pomar, F. Sandiumenge, S. Pinol, N. Mestres, O. Castano, M. Coll, A. Cavallaro, A. Palau, J. Gazquez, J.C. Gonzalez, J. Gutierrez, N. Roma, S. Ricart, J.M. Moreto, M.D. Rossell, G. van Tenderloo, Supercond. Sci. Technol. 17 (2004) 1055. [4] G. Pollefeyt, S. Rottiers, P. Vermeir, P. Lommens, R. Hühne, K. De Buysser, I. Van Driessche, J. Mater. Chem. A 1 (2013) 3613. [5] Y. Zhao, K. Konstantopoulou, A.C. Wulff, X. Tang, H. Tian, H.L. Suo, J.Y. Pastor, J.C. Grivel, IEEE Trans. Appl. Supercond. 23 (2013) 6600104. [6] A.B. Abrahamsen, N. Mijatovic, E. Seiler, T. Zirngibl, C. Traeholt, P.B. Norgard, N.F. Pedersen, N.H. Andersen, J. Ostergard, Supercond. Sci. Technol. 23 (2010) 034019. [7] M. Bäcker, Z. Kristallogr. 226 (2011) 343. [8] W. Schmidt, B. Gamble, H.P. Kraemer, D. Madura, A. Otto, W. Romanosky, Supercond. Sci. Technol. 23 (2009) 014024. [9] X. Obradors, T. Puig, A. Pomar, F. Sandiumenge, N. Mestres, M. Coll, A. Cavallaro, N. Roma, J. Gazquez, J.C. Gonzalez, O. Castano, J. Gutierrez, A. Palau, K. Zalamova, S. Morlens, A. Hassini, M. Gibert, S. Ricart, J.M. Moreto, S. Pinol, D. Isfort, J. Bock, Supercond. Sci. Technol. 19 (2006) S13. [10] T. Araki, I. Hirabayashi, Supercond. Sci. Technol. 16 (2003) R71. [11] Y.Q. Chen, C.B. Wu, G.Y. Zhao, C.Y. You, Supercond. Sci. Technol. 25 (2012) 062001. [12] K. Zalamova, A. Pomar, A. Palau, T. Puig, X. Obradors, Supercond. Sci. Technol. 23 (2010) 014012. [13] F. Lu, F. Kametani, E.E. Hellstrom, Supercond. Sci. Technol. 26 (2013) 045016. [14] Y. Ishiwata, J. Shimoyama, T. Motoki, K. Kishio, T. Nagaishi, IEEE Trans. Appl. Supercond. 23 (2013) 7500804. [15] H. Yamasaki, I. Yamaguchi, M. Sohma, W. Kondo, H. Matsui, T. Manabe, T. Kumagai, Phys. C (Amsterdam, Neth.) 478 (2012) 19. [16] P. Vermeir, J. Feys, J. Schaubroeck, K. Verbeken, P. Lommens, I. Van Driessche, Mater. Res. Bull. 47 (2012) 4376. [17] W.T. Wang, M.H. Pu, W.W. Wang, H. Zhang, C.H. Cheng, Y. Zhao, Phys. C (Amsterdam, Neth.) 471 (2011) 951. [18] A.A. Armenio, A. Augieri, L. Ciontea, G. Contini, I. Davoli, M. Di Giovannantonio, V. Galluzzi, A. Mancini, A. Rufoloni, T. Petriso, A. Vannozzi, G. Celentano, Supercond. Sci. Technol. 24 (2011) 115008. [19] P. Vermeir, I. Cardinael, M. Backer, J. Schaubroeck, E. Schacht, S. Hoste, I. Van Driessche, Supercond. Sci. Technol. 22 (2009) 075009. [20] V. Narayanan, I. Van Driessche, Prog. Solid State Chem. 40 (2012) 57. [21] M.T. Le, W.J.M. Van Well, I. Van Driessche, S. Hoste, Appl. Catal. A 267 (2004) 227. [22] K. De Buysser, P.F. Smet, B. Schoofs, E. Bruneel, D. Poelman, S. Hoste, I. Van Driessche, J. Sol-Gel Sci. Technol. 43 (2007) 347. [23] P. Vermeir, J. Feys, J. Schaubroeck, K. Verbeken, M. Backer, I. Van Driessche, Mater. Chem. Phys. 133 (2012) 998. [24] J. Feys, P. Vermeir, P. Lommens, S.C. Hopkins, X. Granados, B.A. Glowacki, M. Baecker, E. Reich, S. Ricard, B. Holzapfel, P. Van der Voort, I. Van Driessche, J. Mater. Chem. 22 (2012) 3717. [25] D.C. Green, M.R. Lees, S.R. Hall, Chem. Commun. 49 (2013) 2974. [26] B. Schoofs, V. Cloet, P. Vermeir, J. Schaubroeck, S. Hoste, I. Van Driessche, Supercond. Sci. Technol. 19 (2006) 1178. [27] B. Schoofs, D. Van de Vyver, P. Vermeir, J. Schaubroeck, S. Hoste, G. Herman, I. Van Driessche, J. Mater. Chem. 17 (2007) 1714. [28] P. Vermeir, F. Deruyck, J. Feys, P. Lommens, J. Schaubroeck, I. Van Driessche, J. Sol-Gel Sci. Technol. 62 (2012) 378. [29] P. Vermeir, I. Cardinael, J. Schaubroeck, M. Backer, P. Lommens, W. Knaepen, J. D'haen, K. De Buysser, I. Van Driessche, Inorg. Chem. 49 (2010) 4471. [30] Y. Xu, A. Goyal, J. Lian, N.A. Rutter, I. Shi, S. Sathyamurthy, M. Paranthaman, L. Wang, P.M. Martin, D.M. Kroeger, J. Am. Ceram. 87 (2004) 1669. [31] S.V. Ghalsasi, Y.X. Zhou, J. Chen, I. Rusakova, B. Lv, K. Salama, IEEE Trans. Appl. Supercond. 19 (2009) 3379. [32] A.A. Armenio, A. Augieri, L. Ciontea, G. Contini, I. Davoli, V. Galluzzi, A. Mancini, A. Rufoloni, T. Petrisor, A. Vannozzi, G. Celentano, Supercond. Sci. Technol. 21 (2008) 125015.
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Influence of the heating ramp on the superconducting properties of YBa2Cu3O7 − δ films using chemical solution deposition in a direct sintering method Pieter Vermeir a,b, Iwein Cardinael a, Glenn Pollefeyt a, Jonas Feys a, Joseph Schaubroeck b, Isabel Van Driessche a,⁎ a b
Ghent University, SCRiPTS, Faculty of Sciences, Department of Inorganic and Physical Chemistry, Ghent, Belgium Ghent University, INKAT, Faculty of Engineering and Architecture, Department of Industrial Technology and Construction, Ghent, Belgium
a r t i c l e
i n f o
Article history: Received 8 May 2013 Received in revised form 29 August 2013 Accepted 11 September 2013 Available online 17 September 2013 Keywords: Superconductivity Chemical solution deposition Thin films YBa2Cu3O7 − δ
a b s t r a c t YBa2Cu3O7 − δ films were produced on (00l) SrTiO3 single crystal substrates by a sustainable fluorine-free chemical solution deposition method. Using water as the primary solvent and low-cost precursors, a direct sintering process without calcination could be obtained. Variation of the heating rate towards the sintering temperature showed a strong effect on texture, surface morphology, phase purity and concurrent superconducting properties. Using an optimized heating rate of 5 °C min−1, superconducting films were produced with an onset critical temperature of 92 K, a difference in critical temperature during transition of 1.8 K, a critical current density of 1.1 MA cm−2 and a critical current of 50 A cm−1 width at 77 K and self-field. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Within long length industrial film production, great potential is given to chemical solution deposition (CSD) methods. CSD techniques show favorable features compared to the traditionally used vacuum techniques including a low investment cost, fast deposition with high yield and most importantly it is an easily scalable and continuous process [1–5]. As high-temperature superconducting films are of great potential for industrial implementation in various applications, such as power and electronic devices [6–8], CSD is gaining more and more interest. Within the CSD approach for YBa2Cu3O7 − δ (YBCO) film production, tri-fluoro-acetate (TFA) precursors are well-established [9,10]. Nevertheless, the main drawback for this method is the high gas generation rate during processing, which is in turn determined by the fluorine content [11]. Consequently it is observed that small variations in thermal treatment, especially calcination temperature and heating rates, have a great influence on microstructure, morphology, film density and most importantly on superconducting properties [12]. In order to obtain a more easier thermal treatment increasing the film quality, fluorine-free [13–17] as well as low fluorine [11,18] precursors are established. Our research group developed a fluorine-free CSD process starting from metal-acetates dissolved in water and acetic acid using ⁎ Corresponding author at: Krijgslaan 281 (S3), B9000 Ghent, Belgium. Tel.: +32 92644433; fax: +32 92644983. E-mail address: [email protected] (I. Van Driessche). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.09.039
tri-ethanolamine as coordinating ligand (AWAT) for high-temperature superconducting film preparation [19]. This sustainable CSD approach is also appealing for other classes of materials such as buffer layers in the coated conductor design [4,20], catalysis [21] and materials with specific thermal expansion [22]. It has been shown that the method can be easily combined with multiple coating technologies such as dip coating, spin coating and ink-jet printing [23–25]. For the AWAT process a direct sintering method without a specific calcination step is applied. In this work the influence of the heating rate up to the sinter temperature was examined towards texture evolution, surface morphology, phase purity and concurrent superconducting properties.
2. Experimental 2.1. Preparation of the precursor solution and SrTiO3 (STO) substrate Precursor solutions were fabricated from metal-acetates, water, acetic acid and tri-ethanolamine [26,27]. A 4/1 ratio of water/acetic acid in combination with a ligand to metal ratio of 2.5/1 was used to prepare a precursor solution of 0.6 M. A pH adjustment to 6.25 was performed using ammonia. By dip coating this solution on polished (100) STO single crystal substrates films were produced. Previously, it was found that the introduction of water-based precursors had no restrictions towards wettability if a judiciously chosen cleaning procedure was applied [28].
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Fig. 1. SEM study of the influence of the heating rate on the morphology of the final YBCO films at (a) 1 °C min−1, (b) 5 °C min−1 and (c) 10 °C min−1.
2.2. Dip coating and thermal process All coatings were made using a single dip at a withdrawal speed of 50 mm min−1 using a KSV dip coater unit. After dip coating, the samples were heated for 1 h at 60 °C. Subsequently, a thermal treatment in a specially designed tube in tube furnace transformed the amorphous phases into epitaxial YBCO. Without any calcination step, the films were sintered at 815 °C in a partial oxygen pressure of 200 ppm mixed with N2. After 150 min the temperature was decreased to 400 °C at a rate of 3 °C min−1. During cooling, the atmosphere was switched to flowing oxygen (industrial quality) at 525 °C. After an annealing period of 5 h, the samples were furnace-cooled to room temperature. Different heating rates towards the sinter temperature, namely 1–5–10–20 °C min−1, were applied in order to verify the influence on texture, surface morphology, phase purity and concurrent superconducting properties. 2.3. Characterization of the deposited thin layers X-ray diffraction (XRD) was used to verify the orientation (Siemens D5000, CuKα) using a θ–2θ geometry as well as the degree of biaxial texture (Siemens D8) using pole-figures. The overall morphology of the films was characterized by optical microscopy (Leitz) and Scanning Electron Microscopy combined with Energy Dispersive X-ray analysis (SEM-EDX Philips 501). SEM operations were perfomed at an accelerating potential of 20 keV in high vacuum. For EDX analysis, the accelerating potential was lowered to 10 keV and an acquisition time of 150 s was used. A custom-made four-point test device was used to determine the critical temperature (Tc) of the superconducting layer. Critical current density (Ic) measurements were performed using a Theva cryoscan setup in liquid nitrogen. 3. Results and discussion 3.1. Morphology Upon visual inspection of the samples after sintering, we found that for the lower heating rates (1–5–10 °C min−1), no traces on uncontrolled combustion could be observed and good coatings were obtained.
Yet, in the case of a heating rate of 20 °C min−1, the coating peeled off the substrates. Therefore, further SEM investigation was limited to those layers produced at 1, 5 and 10 °C min−1 (Fig. 1). For the film heated at 1 °C min−1 (Fig. 1a), impurities are present on the surface of the YBCO layer. Based on EDX-mapping (Fig. 2), these impurities are identified as CuO (Fig. 2d and e), probably formed by coagulation during heating. The presence of these CuO impurities results in an inhomogeneous metal distribution of the film leading to local defects in stoichiometry. Films heated at 5 and 10 °C min−1 did not show these impurities (Fig. 1b and c). Therefore, we conclude that a heating ramp of 5 °C min−1 or more is necessary to provide homogenous coatings that can lead to good superconducting properties.
3.2. Microstructure In order to obtain high Ic values, it is necessary to grow the YBCO film biaxially textured on the STO substrate. Fig. 3 shows the XRD diffraction patterns obtained for sintered and annealed YBCO layers (1–5–10 °C min−1) dip coated on a (100) polished STO substrate. As expected from SEM-EDX, the XRD of the film heated at 1 °C min−1 reveals the presence of a high reflection of CuO (2θ = 38.8). Moreover, only a weak c-axis orientation of the YBCO phase and several unidentified impurities are found. This can be explained by the strong tendency of the element Cu to coagulate which obviously inhibits the homogeneous distribution of the metals in the layer. This inhomogeneity causes local defects in stoichiometry which will automatically affect the YBCO reaction pathway as described before [29], resulting in the presence of impurity phases after sintering. The films heated at 5 and 10 °C min−1 are strongly c-axis oriented. For the layer heated at 10 °C min−1, a small amount of an unidentified impurity phase (2θ = 12.9° and 26.1°) is observed. To check if this impurity has an influence on the in-plane misalignment, (103) pole figures (2θ = 32.52°) were collected (Fig. 4). The average full width at half maximum (FWHM) of the reflections are 0.19° and 0.20° for films heated at 5 and 10 °C min−1, respectively. These low FWHM-values clearly proof the epitaxial growth of YBCO on STO for both films, which indicates that the presence of the impurity does not suppress the epitaxy.
Fig. 2. (a) SEM image for the YBCO layer on STO obtained by heating at 1 °C min−1 and EDX-mapping of (b) Y, (c) Ba, (d) Cu and (e) O.
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3.3. Superconductivity Fig. 5 shows the temperature dependence of the resistance for the epitaxial YBCO films prepared with a heating ramp of 5 and 10 °C min−1. For the film heated at 5 °C min−1, a transition with a onset Tc of 92 K and a difference in critical temperature during transition (ΔTc) of 1.8 K is observed. Although the onset Tc remains equal for the film treated at 10 °C min−1, a broader transition resulting in a ΔTc of 5 K is observed. Presumably, the presence of a small amount of impurity phase, affects the zero Tc. For both films the residual resistance remains essentially zero. Additionally, the influence of the heating rate on the temperature dependence of the electrical resistivity, ρ(T), in the normal state was examined. A linear behavior of ρ(T) in the normal state, is typical for high-quality, in-plane-aligned YBCO films with little grain boundary scattering [30]. This normal state behavior can be quantified by the resistivity ratio ρ(100 K)/ρ(300 K) and the extrapolation of the lowtemperature part of the resistivity lines. If it extrapolates to zero near 0 K and ρ(100 K)/ρ(300 K) ≤ 0.4, one can conclude that a good normal state behavior is obtained. As observed from Fig. 5, the resistivity ratio
Fig. 4. YBCO (103) pole figure for an YBCO film on STO heated at (a) 5 °C min−1 and (b) 10 °C min−1.
for films heated at 5 and 10 °C min−1 is 0.32 and 0.71 respectively, while the extrapolation goes only to zero for the film heated at 5 °C min−1. This indicates again the presence of a small amount of impurity phase, contributing to the increased normal-state resistivity. Magnetic inductive critical current density (Jc) measurements at 77 K in self-field are shown in Fig. 6. The film heated at 5 °C min−1 shows a Jc of 1.1 MA cm−2, while the critical current for a film heated at 10 °C min−1 is 0.55 MA cm−2. We believe that the increase in superconducting properties for a layer heated at 5 °C min−1 is correlated to the increased phase purity. Remarkable is the broad heating rate interval, going from 5 till 10 °C min−1, giving rise to superconducting properties. Combining Jc measurements (Fig. 6) with the thickness of the layer, which is approximately 450 nm based on cross-sectional SEM, the critical current can be calculated. The Ic-value of the sample at 77 K and in self-field is 25 A cm−1 width for the film heated at 10 °C min−1, whereas the film heated at 5 °C min−1 shows an Ic of 50 A cm−1 width. The latter result was used to make a comparison between our AWAT-YBCO layer on single crystal and YBCO layers made from other
Fig. 3. XRD pattern of the YBCO layer dip coated on a (l00) polished STO substrate when exposed to different heating rates. (a) 1 °C min−1, (b) 5 °C min−1 and (c) 10 °C.min−1. (°) CuO, (⋄,*) unidentified impurities.
Fig. 5. Resistance Tc measurements of the final YBCO films on STO heated at 5 and 10 °C min−1. The resistivity for both films was 40 and 100 μΩ cm respectively.
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References
Fig. 6. Inductive Jc measurements of the final YBCO films heated at 5 and 10 °C min−1.
CSD methods. Using the TFA-approach, it is possible to obtain Jc-values up to 7 MA cm−2 on LaAlO3 at 77 K in self-field [31]. Nevertheless, as soon as a slightly modified low-fluorine TFA method is applied, Jc obtained on STO drops to ~1 MA cm−2 at 77 K in self-field [32]. The Jc-value obtained for the film heated at 5 °C min−1 is lower than those for the classic TFA methods, but is competitive with those of low-fluorine containing precursors. When taking into account the scalability for our process due to the simple processing conditions, especially compared with TFA-related methods where a constant low p(H2O) is needed inside the entire furnace, it is clear that water-based YBCO deposition can be a competitive method to the currently established CSD techniques on single crystal STO. 4. Conclusion In this study, the influence of the heating rate towards the sinter temperature was verified for YBCO film production using a direct sintering method. Therefore a sustainable fluorine-free water-based CSD precursor was used to coat a single crystal STO substrate. We clearly proof the link between heating rate, phase purity, morphology and concurrent superconducting properties. If the applied heating rate is too slow (1 °C min−1), coagulation of copper results in inhomogeneous semiconducting films. If the heating rate is too high (20 °C min−1), films start to peel off. Only intermediate heating rates (5–10 °C min−1) give rise to superconducting behavior. For both intermediate heating rates (5–10 °C.min−1) similar microstructural properties are found. Nevertheless, it was observed that a very small amount of impurity phase, in the case of a 10 °C min−1 heating rate, gave rise to inferior results concerning (i) broadness of the resistive transition, (ii) normal-state resistivity behavior, (iii) critical current density and (iv) critical current per unit of width. Acknowledgments The authors would like to thank Olivier Janssens for performing XRD measurements and SEM-EDX.
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