- Email: [email protected]
Development of modified TFA-MOD approach for GdBa2Cu3Oy film growth
Materials Letters 94 (2013) 23–26
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Development of modified TFA-MOD approach for GdBa2Cu3Oy film growth L.H. Jin n, Y.F. Lu, J.Q. Feng, S.N. Zhang, Z.M. Yu, Y. Wang, C.S. Li Northwest Institute for Nonferrous Metal Research, Xi’an 710016, PR China
a r t i c l e i n f o
abstract
Article history: Received 28 July 2012 Accepted 7 December 2012 Available online 20 December 2012
GdBa2Cu3Oy (GdBCO) films have been fabricated by metal organic deposition with a benzoic acid modified trifluoroacetates solution. Attributed to this new solution, smooth and homogeneous surface of precursor film can been obtained in a rapid pyrolysis process, during which the pyrolysis time is less than 2 h. The crystallized multi-coated films show a desirable dense microstructure and a good c-axis texture. Both the texture degree and critical current density (Jc) of GdBCO films are greatly influenced by the thickness. Meanwhile, the decreases of Jc for multi-coated films can also be partially related to the presence of second phase and the poor texture. The GdBCO film obtained with two coatings cycles (700 nm) has demonstrated a good performance with Jc (77 K, self field) ¼ 2 MA/cm2. & 2012 Elsevier B.V. All rights reserved.
Keywords: Deposition Superconductors Thin films Ceramics
1. Introduction
2. Experimental
GdBa2Cu3Oy (GdBCO) superconductors with high critical current density (Jc) in high magnetic field have been intensively researched for the applications of the second-generation superconducting wires [1]. The trifluoroacetate metal organic deposition (TFA-MOD) method is well known as a low-cost process compared with the physical vapor deposition methods [2,3]. Iguchi et al. reported that GdBCO films using the TFA-MOD process exhibited higher Tc and Jc in high magnetic fields [4]. Kaneko et al. fabricated GdBCO films on SrTiO3 substrates by an advanced TFA-MOD method [5]. Nakamura et al. also prepared GdBCO films by the MOD technique using metal-naphthenates as precursors [6]. In our previous work, we studied the effects of oxygen partial pressure and firing temperature on both the microstructure and critical properties of GdBCO film prepared by the traditional TFA-MOD method [7]. However, the traditional TFA-MOD technique for coated conductors is very sensitive to the heating rate during the pyrolysis process. And the very slow decomposition process ( 10 h) is not suitable for the fabrication of long length coated conductors. In this study, we developed a benzoic acid modified TFA-MOD approach for GdBCO films growth with a short pyrolysis time. Multi-layers of GdBCO were deposited on LaAlO3 (LAO) substrates with different coating cycles, and the effects of thickness on the texture and superconducting properties of GdBCO films have been systematically investigated.
The modified precursor solution was formed by mixing the traditional GdBCO–TFA solution with benzoic acid. The traditional GdBCO–TFA solution was composed of the precursors (Gd(TFA)3: Ba(TFA)2:Cu(TFA)2 ¼1:2:3) and the solvent (methanol), which was prepared as in the previous reference [7]. The benzoic acid as chelate (BA:Cu¼3:1) was added to the traditional GdBCO–TFA solution. This mixture was heated at 80 1C for 2 h with continuous stirring. The mixture was dissolved in methanol and propionic acid solvents (CH3OH:C2H5COOH ¼4:1) to obtain a stable greencolored solution with a total cation concentration of 1.5 mol/L. The solution was coated onto LAO substrates with a spinning rate of 3000 rpm for 1 min. The spin-coated gel films were calcined at 400 1C in 3.1% humidified oxygen atmosphere to form oxygenfluorides films. The heating rate was kept at 5 K/min in the temperature range from 100 1C to 400 1C. The humid oxygen gas was obtained by injecting dry oxygen through deionized water (dew point 25 1C) into a furnace when the furnace temperature was above 120 1C. To make films with various thicknesses, the cycles with spin-coating and pyrolysis processes were repeated one to four times. Subsequently, the GdBCO precursor films were crystallized at 815 1C for 2 h in Ar/O2 (100 ppm O2) atmosphere with 4.2% humidity (dew point 30 1C). In the last stage the crystallized films were annealed at 450 1C for 4 h in a dry oxygen atmosphere. Optical microscopy (OM) was carried out by an Olympus PMG3 to investigate the surface quality of the pyrolyzed films. The surface roughness was investigated by atomic force microscopy (AFM SPM-9500J3). The morphology of GdBCO films was characterized by scanning electron microscopy (SEM JSM-6460). The phase purity and the texture of GdBCO films were investigated by X-ray diffraction (XRD) using CuKa radiations (Rigaku
n
Corresponding author. Tel.: þ86 29 86231079; fax: þ86 29 86224487. E-mail addresses: [email protected], [email protected] (L.H. Jin).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.12.022
24
L.H. Jin et al. / Materials Letters 94 (2013) 23–26
Fig. 1. OM and AFM images of GdBCO precursor films decomposed at 400 1C with different solutions. (a) and (c) Traditional GdBCO–TFA solution. (b) and (d) Benzoic acid modified GdBCO solution.
D/MAX2000PC). The critical current density (Jc) was measured by a cryoscan system (THEVA) at 77 K and self-field.
3. Results and discussion During the pyrolysis process, the heating rate is critical to achieving homogeneous surface due to the large film shrinkage. The modified GdBCO solution is expected to be suitable for fast calcination profile. Fig. 1 shows the surface morphology of GdBCO precursor films pyrolyzed under the same condition (5 K/min) with two kinds of solutions (traditional solution, modified solution). The pyrolyzed film with traditional solution has a rough surface with typical wrinkle, as shown in Fig. 1a and c. This morphology is similar to the special ‘‘pencil-maze’’ pattern and buckling surface of YBCO films under fast heating rate [8,9]. In comparison with the rough surface (RRMS ¼70 nm in 20 mm 20 mm), the precursor film with modified solution shows a smooth and crack-free microstructure (Fig. 1b and d). The RMS roughness of the film (Fig. 1d) in a scan area of 20 mm 20 mm is about 2.4 nm. It proves that the modified solution is beneficial to the stress relief and minimization of the surface degradation of pyrolyzed films. When the heating rate of pyrolysis process is kept at 5 K/min, the decomposition time is reduced to 1.5–2 h, which is 5 times shorter than that of the traditional process. Multi-layers of GdBCO were prepared by repeating the cycles of spin-coating and the pyrolysis process. All the precursor films (modified solution) were crystallized at the same condition (815 1C, 100 ppm O2) to investigate the effect of thickness on both the texture and superconducting properties. Fig. 2 shows cross sectional SEM images of crystallized GdBCO films. When the numbers of coating cycles are 1, 2, 3, and 4, the thicknesses of films are about 350 nm, 700 nm, 1000 nm, and 1300 nm, respectively. And the thickness increased almost linearly with increasing number of coating. All the films have dense microstructure with no
obvious pores. It can be confirmed that the smooth surface of precursor film (Fig. 1b) is useful for multilayer fabrication. The XRD patterns of GdBCO films with different layers are shown in Fig. 3a. The GdBCO (00l) peaks can be observed in all films. The formation of GdBCO (00l) peaks indicated that high c-axis preferred orientation grains. Meanwhile the appearance of minor impurity phase BaCuO2 þ x peaks can be observed in the films with three layers and four layers. The GdBCO peak intensity ratio was calculated by using the following equation: intensity ratio¼ SIGdBCO(00l)/S(IGdBCO(00l) þIBaCuO2 þ x). The GdBCO phase ratios of these films are 0.999 (1 Layer), 0.999 (2 Layers), 0.973 (3 Layers) and 0.981 (4 Layers). The GdBCO phase ratio decreases obviously when the numbers of coating cycles are 3 and 4, indicating that the phase purity of films decreases with increasing thickness. The phi-scans of GdBCO (102) and the omega-scans of GdBCO (005) were also carried out to characterize the in-plane and out-of-plane textures of these films. As shown in Fig. 3b, the full-width at half maximum (FWHM) values of phi and omega increase with the number of coating cycles. The FWHM values (Dj ¼2.081, Do ¼2.031) of the film prepared with four coating cycles are larger than those of other films. The increase of FWHM values indicated that the texture proportionally decreases with increasing thickness of GdBCO films. In addition, the critical current density (Jc) values of GdBCO films are 1.9 MA/cm2 (1 coating), 2.0 MA/cm2 (2 coatings), 0.36 MA/cm2 (3 coatings), and 0.14 MA/cm2 (4 coatings) at 77 K and self-field. The Jc reaches a maximum value (2 MA/cm2) with 2 coatings. Then the Jc decreases with the further increasing thickness (3–4 coatings). The change of Jc is partially related to the change of phase and texture of the GdBCO films. Based on the XRD pattern, we believe that the highest Jc value is due to the formation of pure GdBCO phase and strong biaxial texture for the sample prepared with 2 coating cycles. The result is partly in accord with the trend of YBCO films [10]. This also suggests that there is a certain critical thickness for the growth of GdBCO film. The reason for the decrease in Jc value of films can be attributed
L.H. Jin et al. / Materials Letters 94 (2013) 23–26
25
Fig. 2. Cross-sectional SEM images of GdBCO films with different coating numbers: (a) one coating, (b) two coatings, (c) three coatings, and (d) four coatings.
Fig. 3. (a) y–2y scan of GdBCO films prepared with different coating numbers: one coating (1L), two coatings (2L), three coatings (3L), and four coatings (4L). (b) Dependence of FWHM values of phi and omega on the number of coatings.
to the impurity phase and poor texture. Further work is needed to optimize the crystallization conditions and to improve the performance for multi-coated films.
obtained with two coatings cycles (700 nm) has demonstrated a good performance with Jc (77 K, self-field)¼ 2 MA/cm2. Hence, this approach may be useful for fabrication of GdBCO films with fast calcination.
4. Conclusion
Acknowledgments
We have successfully developed a benzoic acid modified TFAMOD approach for GdBCO film growth with improved performance. The precursor film of GdBCO for this modified solution exhibits a smooth surface at the heating rate of 5 K/min. The whole pyrolysis time is less than 2 h. The crystallized multi-coated films show desirable dense microstructure and c-axis texture. GdBCO film
This work was financially supported by the International Science and Technology Cooperation Program of China (No. 2012DFA50780), the National High Technology Research and Development Program (No. 2009AA03Z203), and the Natural Science Basic Research Plan in Shaanxi Province of China (No.S2012JC7043).
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L.H. Jin et al. / Materials Letters 94 (2013) 23–26
References [1] Kakimoto K, Igarashi M, Hanada Y, Hayashida T, Tashita C, Morita K, et al. High-speed deposition of high-quality RE123 films by a PLD system with hotwall heating. Supercond Sci Technol 2010;23:14016–9. [2] Rupich MW, Verebelyi DT, Zhang W, Kodenkandath T, Li X. Metal organic deposition of YBCO films for second-generation high-temperature superconductor wires. Mater Res Soc Bull 2004;29:572–8. [3] Obradors X, Puig T, Pomar A, Sandiumenge F, Mestres N, Coll M, et al. Progress towards all-chemical superconducting YBa2Cu3O7 coated conductors. Supercond Sci Technol 2006;19:S13–26. [4] Iguchi T, Araki T, Yamada Y, Hirabayashi I, Ikuta H. Fabrication of Gd–Ba–Cu–O films by the metal–organic deposition method using trifluoroacetates. Supercond Sci Technol 2002;15:1415–20. [5] Kaneko A, Fuji H, Teranishi R, Tokunaga Y, Matsuda JS, Asada S, et al. Fabrication of REBa2Cu3O7 y film by advanced TFA-MOD process. Physica C 2004;926:412–4.
[6] Nakamura T, Kita R, Miura O, Ichinose A, Matsumoto K, Yoshida Y, et al. Fabrication of GdBa2Cu3Oy films by metal–organic deposition using metalnaphthenates. Physica C 2007;463:540–3. [7] Jin LH, Lu YF, Yan WW, Yu ZM, Wang Y, Li CS. Fabrication of GdBa2Cu3O7 x films by TFA-MOD process. J Alloys Compd 2011;509:3353–6. [8] Zalamova K, Roma N, Pomar A, Morlens S, Puig T, Ga´zquez J, et al. Smooth stress relief of trifluoroacetate metal–organic solutions for YBa2Cu3O7 film growth. Chem Mater 2006;402:143–52. [9] Dawley JT, Clem PG, Siegal MP, Tallant DR, Overmyer DL. Improving sol–gel YBa2Cu3O7 d film morphology using high-boiling-point solvents. J Mater Res 2002;17:1900–3. [10] Jang SH, Lim JH, Yoon KM, Lee SY, Kim KT, Park EC, et al. The effects of the humidity and thickness on YBCO film prepared using the TFA-MOD. IEEE Trans Appl Supercond 2007;17:3298–301.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Development of modified TFA-MOD approach for GdBa2Cu3Oy film growth L.H. Jin n, Y.F. Lu, J.Q. Feng, S.N. Zhang, Z.M. Yu, Y. Wang, C.S. Li Northwest Institute for Nonferrous Metal Research, Xi’an 710016, PR China
a r t i c l e i n f o
abstract
Article history: Received 28 July 2012 Accepted 7 December 2012 Available online 20 December 2012
GdBa2Cu3Oy (GdBCO) films have been fabricated by metal organic deposition with a benzoic acid modified trifluoroacetates solution. Attributed to this new solution, smooth and homogeneous surface of precursor film can been obtained in a rapid pyrolysis process, during which the pyrolysis time is less than 2 h. The crystallized multi-coated films show a desirable dense microstructure and a good c-axis texture. Both the texture degree and critical current density (Jc) of GdBCO films are greatly influenced by the thickness. Meanwhile, the decreases of Jc for multi-coated films can also be partially related to the presence of second phase and the poor texture. The GdBCO film obtained with two coatings cycles (700 nm) has demonstrated a good performance with Jc (77 K, self field) ¼ 2 MA/cm2. & 2012 Elsevier B.V. All rights reserved.
Keywords: Deposition Superconductors Thin films Ceramics
1. Introduction
2. Experimental
GdBa2Cu3Oy (GdBCO) superconductors with high critical current density (Jc) in high magnetic field have been intensively researched for the applications of the second-generation superconducting wires [1]. The trifluoroacetate metal organic deposition (TFA-MOD) method is well known as a low-cost process compared with the physical vapor deposition methods [2,3]. Iguchi et al. reported that GdBCO films using the TFA-MOD process exhibited higher Tc and Jc in high magnetic fields [4]. Kaneko et al. fabricated GdBCO films on SrTiO3 substrates by an advanced TFA-MOD method [5]. Nakamura et al. also prepared GdBCO films by the MOD technique using metal-naphthenates as precursors [6]. In our previous work, we studied the effects of oxygen partial pressure and firing temperature on both the microstructure and critical properties of GdBCO film prepared by the traditional TFA-MOD method [7]. However, the traditional TFA-MOD technique for coated conductors is very sensitive to the heating rate during the pyrolysis process. And the very slow decomposition process ( 10 h) is not suitable for the fabrication of long length coated conductors. In this study, we developed a benzoic acid modified TFA-MOD approach for GdBCO films growth with a short pyrolysis time. Multi-layers of GdBCO were deposited on LaAlO3 (LAO) substrates with different coating cycles, and the effects of thickness on the texture and superconducting properties of GdBCO films have been systematically investigated.
The modified precursor solution was formed by mixing the traditional GdBCO–TFA solution with benzoic acid. The traditional GdBCO–TFA solution was composed of the precursors (Gd(TFA)3: Ba(TFA)2:Cu(TFA)2 ¼1:2:3) and the solvent (methanol), which was prepared as in the previous reference [7]. The benzoic acid as chelate (BA:Cu¼3:1) was added to the traditional GdBCO–TFA solution. This mixture was heated at 80 1C for 2 h with continuous stirring. The mixture was dissolved in methanol and propionic acid solvents (CH3OH:C2H5COOH ¼4:1) to obtain a stable greencolored solution with a total cation concentration of 1.5 mol/L. The solution was coated onto LAO substrates with a spinning rate of 3000 rpm for 1 min. The spin-coated gel films were calcined at 400 1C in 3.1% humidified oxygen atmosphere to form oxygenfluorides films. The heating rate was kept at 5 K/min in the temperature range from 100 1C to 400 1C. The humid oxygen gas was obtained by injecting dry oxygen through deionized water (dew point 25 1C) into a furnace when the furnace temperature was above 120 1C. To make films with various thicknesses, the cycles with spin-coating and pyrolysis processes were repeated one to four times. Subsequently, the GdBCO precursor films were crystallized at 815 1C for 2 h in Ar/O2 (100 ppm O2) atmosphere with 4.2% humidity (dew point 30 1C). In the last stage the crystallized films were annealed at 450 1C for 4 h in a dry oxygen atmosphere. Optical microscopy (OM) was carried out by an Olympus PMG3 to investigate the surface quality of the pyrolyzed films. The surface roughness was investigated by atomic force microscopy (AFM SPM-9500J3). The morphology of GdBCO films was characterized by scanning electron microscopy (SEM JSM-6460). The phase purity and the texture of GdBCO films were investigated by X-ray diffraction (XRD) using CuKa radiations (Rigaku
n
Corresponding author. Tel.: þ86 29 86231079; fax: þ86 29 86224487. E-mail addresses: [email protected], [email protected] (L.H. Jin).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.12.022
24
L.H. Jin et al. / Materials Letters 94 (2013) 23–26
Fig. 1. OM and AFM images of GdBCO precursor films decomposed at 400 1C with different solutions. (a) and (c) Traditional GdBCO–TFA solution. (b) and (d) Benzoic acid modified GdBCO solution.
D/MAX2000PC). The critical current density (Jc) was measured by a cryoscan system (THEVA) at 77 K and self-field.
3. Results and discussion During the pyrolysis process, the heating rate is critical to achieving homogeneous surface due to the large film shrinkage. The modified GdBCO solution is expected to be suitable for fast calcination profile. Fig. 1 shows the surface morphology of GdBCO precursor films pyrolyzed under the same condition (5 K/min) with two kinds of solutions (traditional solution, modified solution). The pyrolyzed film with traditional solution has a rough surface with typical wrinkle, as shown in Fig. 1a and c. This morphology is similar to the special ‘‘pencil-maze’’ pattern and buckling surface of YBCO films under fast heating rate [8,9]. In comparison with the rough surface (RRMS ¼70 nm in 20 mm 20 mm), the precursor film with modified solution shows a smooth and crack-free microstructure (Fig. 1b and d). The RMS roughness of the film (Fig. 1d) in a scan area of 20 mm 20 mm is about 2.4 nm. It proves that the modified solution is beneficial to the stress relief and minimization of the surface degradation of pyrolyzed films. When the heating rate of pyrolysis process is kept at 5 K/min, the decomposition time is reduced to 1.5–2 h, which is 5 times shorter than that of the traditional process. Multi-layers of GdBCO were prepared by repeating the cycles of spin-coating and the pyrolysis process. All the precursor films (modified solution) were crystallized at the same condition (815 1C, 100 ppm O2) to investigate the effect of thickness on both the texture and superconducting properties. Fig. 2 shows cross sectional SEM images of crystallized GdBCO films. When the numbers of coating cycles are 1, 2, 3, and 4, the thicknesses of films are about 350 nm, 700 nm, 1000 nm, and 1300 nm, respectively. And the thickness increased almost linearly with increasing number of coating. All the films have dense microstructure with no
obvious pores. It can be confirmed that the smooth surface of precursor film (Fig. 1b) is useful for multilayer fabrication. The XRD patterns of GdBCO films with different layers are shown in Fig. 3a. The GdBCO (00l) peaks can be observed in all films. The formation of GdBCO (00l) peaks indicated that high c-axis preferred orientation grains. Meanwhile the appearance of minor impurity phase BaCuO2 þ x peaks can be observed in the films with three layers and four layers. The GdBCO peak intensity ratio was calculated by using the following equation: intensity ratio¼ SIGdBCO(00l)/S(IGdBCO(00l) þIBaCuO2 þ x). The GdBCO phase ratios of these films are 0.999 (1 Layer), 0.999 (2 Layers), 0.973 (3 Layers) and 0.981 (4 Layers). The GdBCO phase ratio decreases obviously when the numbers of coating cycles are 3 and 4, indicating that the phase purity of films decreases with increasing thickness. The phi-scans of GdBCO (102) and the omega-scans of GdBCO (005) were also carried out to characterize the in-plane and out-of-plane textures of these films. As shown in Fig. 3b, the full-width at half maximum (FWHM) values of phi and omega increase with the number of coating cycles. The FWHM values (Dj ¼2.081, Do ¼2.031) of the film prepared with four coating cycles are larger than those of other films. The increase of FWHM values indicated that the texture proportionally decreases with increasing thickness of GdBCO films. In addition, the critical current density (Jc) values of GdBCO films are 1.9 MA/cm2 (1 coating), 2.0 MA/cm2 (2 coatings), 0.36 MA/cm2 (3 coatings), and 0.14 MA/cm2 (4 coatings) at 77 K and self-field. The Jc reaches a maximum value (2 MA/cm2) with 2 coatings. Then the Jc decreases with the further increasing thickness (3–4 coatings). The change of Jc is partially related to the change of phase and texture of the GdBCO films. Based on the XRD pattern, we believe that the highest Jc value is due to the formation of pure GdBCO phase and strong biaxial texture for the sample prepared with 2 coating cycles. The result is partly in accord with the trend of YBCO films [10]. This also suggests that there is a certain critical thickness for the growth of GdBCO film. The reason for the decrease in Jc value of films can be attributed
L.H. Jin et al. / Materials Letters 94 (2013) 23–26
25
Fig. 2. Cross-sectional SEM images of GdBCO films with different coating numbers: (a) one coating, (b) two coatings, (c) three coatings, and (d) four coatings.
Fig. 3. (a) y–2y scan of GdBCO films prepared with different coating numbers: one coating (1L), two coatings (2L), three coatings (3L), and four coatings (4L). (b) Dependence of FWHM values of phi and omega on the number of coatings.
to the impurity phase and poor texture. Further work is needed to optimize the crystallization conditions and to improve the performance for multi-coated films.
obtained with two coatings cycles (700 nm) has demonstrated a good performance with Jc (77 K, self-field)¼ 2 MA/cm2. Hence, this approach may be useful for fabrication of GdBCO films with fast calcination.
4. Conclusion
Acknowledgments
We have successfully developed a benzoic acid modified TFAMOD approach for GdBCO film growth with improved performance. The precursor film of GdBCO for this modified solution exhibits a smooth surface at the heating rate of 5 K/min. The whole pyrolysis time is less than 2 h. The crystallized multi-coated films show desirable dense microstructure and c-axis texture. GdBCO film
This work was financially supported by the International Science and Technology Cooperation Program of China (No. 2012DFA50780), the National High Technology Research and Development Program (No. 2009AA03Z203), and the Natural Science Basic Research Plan in Shaanxi Province of China (No.S2012JC7043).
26
L.H. Jin et al. / Materials Letters 94 (2013) 23–26
References [1] Kakimoto K, Igarashi M, Hanada Y, Hayashida T, Tashita C, Morita K, et al. High-speed deposition of high-quality RE123 films by a PLD system with hotwall heating. Supercond Sci Technol 2010;23:14016–9. [2] Rupich MW, Verebelyi DT, Zhang W, Kodenkandath T, Li X. Metal organic deposition of YBCO films for second-generation high-temperature superconductor wires. Mater Res Soc Bull 2004;29:572–8. [3] Obradors X, Puig T, Pomar A, Sandiumenge F, Mestres N, Coll M, et al. Progress towards all-chemical superconducting YBa2Cu3O7 coated conductors. Supercond Sci Technol 2006;19:S13–26. [4] Iguchi T, Araki T, Yamada Y, Hirabayashi I, Ikuta H. Fabrication of Gd–Ba–Cu–O films by the metal–organic deposition method using trifluoroacetates. Supercond Sci Technol 2002;15:1415–20. [5] Kaneko A, Fuji H, Teranishi R, Tokunaga Y, Matsuda JS, Asada S, et al. Fabrication of REBa2Cu3O7 y film by advanced TFA-MOD process. Physica C 2004;926:412–4.
[6] Nakamura T, Kita R, Miura O, Ichinose A, Matsumoto K, Yoshida Y, et al. Fabrication of GdBa2Cu3Oy films by metal–organic deposition using metalnaphthenates. Physica C 2007;463:540–3. [7] Jin LH, Lu YF, Yan WW, Yu ZM, Wang Y, Li CS. Fabrication of GdBa2Cu3O7 x films by TFA-MOD process. J Alloys Compd 2011;509:3353–6. [8] Zalamova K, Roma N, Pomar A, Morlens S, Puig T, Ga´zquez J, et al. Smooth stress relief of trifluoroacetate metal–organic solutions for YBa2Cu3O7 film growth. Chem Mater 2006;402:143–52. [9] Dawley JT, Clem PG, Siegal MP, Tallant DR, Overmyer DL. Improving sol–gel YBa2Cu3O7 d film morphology using high-boiling-point solvents. J Mater Res 2002;17:1900–3. [10] Jang SH, Lim JH, Yoon KM, Lee SY, Kim KT, Park EC, et al. The effects of the humidity and thickness on YBCO film prepared using the TFA-MOD. IEEE Trans Appl Supercond 2007;17:3298–301.