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Synthesis of stable precursor solution for MOD-processing of YBCO coated conductors
Physica C 445–448 (2006) 574–577 www.elsevier.com/locate/physc
Synthesis of stable precursor solution for MOD-processing of YBCO coated conductors Young-Kuk Kim *, Jaimoo Yoo, Kookchae Chung, Jaewoong Ko Korea Institute of Machinery and Materials (KIMM), #66 Sangnam-dong, Changwon, Kyungnam 641-010, Republic of Korea Available online 9 June 2006
Abstract MOD precursor solutions for YBCO coated conductors were synthesized by either adding organic additives or substituting F-free Cu precursor for Cu-trifluoroacetate (TFA) to improve the stability of the precursor solution. The precursor solution without any significant change in the viscosity was synthesized by incorporating polyvinylpyrrolidone (PVP) and thickness of precursor film after calcination was enhanced. In addition, F-free Cu precursor was employed to synthesize the precursor solution with less fluorine content. Stable precursor solution without time-dependent viscosity degradation was obtained and the crack-free and 1.7 lm thick precursor film was formed after fast calcination at 400 °C in wet O2 atmosphere. A 0.5 lm-thick YBCO film was formed after annealing in wet Ar/O2 mixed gas and showed critical current density of 0.8 MA/cm2. Further optimization of processing conditions such as total pressure, oxygen partial pressure, annealing temperature, etc. is in progress to improve critical current. This chemical modification approach is a possible candidate for improving stability and performance of MOD-processing of YBCO coated conductor. Ó 2006 Elsevier B.V. All rights reserved. PACS: 74.72.Bk; 74.76.Bz Keywords: Precursor solution; MOD; YBCO
1. Introduction The YBa2Cu3O7 x (YBCO)-based coated conductor (CC) is promising high temperature superconducting (HTSC) wire architectures for electrical devices [1]. Many efforts are in progress to develop long HTSC wire with high current carrying capacity by coated conductor architecture [1–3]. Among them, metal–organic deposition using trifluoroacetates (MOD-TFA) is attracting much interest because of its potential for scale-up and cost-effectiveness. MOD-TFA is chemical solution deposition process which is intrinsically non-vacuum approach and YBCO thin films with high critical current density (Jc > 1 MA/cm2) can be routinely fabricated by this process [4–7]. One of the most important issues in the MOD-processing of YBCO coated conductor is the synthesis of MOD *
Corresponding author. Tel.: +82 55 280 3343; fax: +82 55 280 3399. E-mail address: [email protected] (Y.-K. Kim).
0921-4534/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2006.04.051
precursor solution. The precursor solution in conventional MOD-TFA process often shows time-dependent degradation of its properties such as viscosity. In particular, long length processing of coated conductor requires stable precursor solution. In addition, metal-TFA salts in the precursor solution of the conventional MOD-TFA process generate large amount of HF gas during the calcination step. As a result of this, very long processing time (20 h) is required to fabricate crack-free YBCO precursor film and the thickness of precursor film is limited to small values after single coating [7]. In order to obtain thick and dense YBCO layers to improve critical current, modifications of precursor solution are required. In this study, chemically modified precursor solutions for MOD processing were synthesized to improve the stability of process and thickness of YBCO films. Chemical modification of precursor solution was performed by either adding organic additives or substituting F-free Cu precursor for Cu-TFA. In particular, the effect of chemical
Y.-K. Kim et al. / Physica C 445–448 (2006) 574–577
modification of precursor solution on stability of precursor solution and thickness of YBCO precursor film was checked. 2. Effect of polyvinylpyrrolidone (PVP) addition In chemical solution processing of thick ceramic films, organic additives such as 1,3-propanediol, ethylene glycol, etc. are often employed to improve stability of precursor solution and fabricate crack-free thick films [8–10]. Tu et al. deposited 1 lm-thick and crack-free Pb(Zr, Ti)O3 (PZT) films using chemical solution deposition with 1,3propanediol containing precursor solution [8]. Recently, Kozuka and Takenaka, fabricated 2 lm-thick PZT films without any crack by polyvinylpyrrolidone (PVP)-assisted sol–gel process [10]. It is known that addition of PVP in precursor solution improve stability of precursor solution by inhibiting condensation reaction and increase critical thickness for crack generation [10]. Clem et al. shows that PVP can reduce film stress and increase the thickness of YBCO film prepared by PVP-assisted SANDEATM solution [11]. In this study, in order to improve stability and thickness of YBCO film, PVP was incorporated into our YBCO oxide-based precursor solution. Fig. 1 shows surface of the precursor film after calcination using PVP-assisted YBCO precursor solution. No crack was detected and uniform surface was shown.
575
As shown in Fig. 2(a), thickness of crack-free precursor film was linearly increases to 2 lm by increasing the concentration of PVP. That is, the critical thickness for crack initiation of the precursor film is increased by addition of PVP. This means PVP act as a stress releasing agent and improve critical thickness of precursor films. In addition, time-dependent variation of solution viscosity is negligible, which indicates excellent stability of precursor solution with PVP (Fig. 2(b)). A 1.5 lm-thick precursor film was converted to YBCO films after annealing in Ar/O2 atmosphere. The thickness of YBCO film was estimated to be about 0.7 lm (Fig. 3). However, the microstructure of annealed YBCO film contains a large amount of large pores exist. As a result of this, the measured Ic value of this sample is only 2 A (Jc 0.03 MA/cm2). The origin of these large pores can be attributed to large HF generation from thick precursor film during the annealing process and possible residual carbon from additives [12]. This indicates that a new solution with low HF content is required to inhibit pore formation and improve the microstructure. 3. F-free Cu precursor solution F-free Cu precursor solution was synthesized by dissolving F-free Cu precursor into methanol with yttrium and barium TFA salt. The total content of fluorine in the
Fig. 1. Surface of as-calcined precursor film observed by (a) optical microscopy (OM) and (b) scanning electron microscopy (SEM).
Fig. 2. (a) Thickness of precursor films after calcination as a function of concentration of PVP and (b) stability of precursor solution with PVP.
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coating. X-ray diffraction profile of the calcined film shows broad reflections and indicates existence of BaF2, CuO and Y2O3 which is consistent with calcined film prepared by conventional MOD-TFA using metal TFA as starting precursors. Fig. 4(a) shows stability of precursor solutions. Compared with conventional MOD-TFA precursor solution, F-free Cu precursor solution shows only slight increase in viscosity after 15 days preservation in ambient atmosphere, which means that the stability of precursor solution was improved by substituting F-free Cu precursor
precursor solution is half of that for conventional MODTFA precursor solution synthesized with metal-TFA salts. Since the large gas generation from all TFA-based precursor solution limits the heating rate in calcination step to be about 0.05 °C/min, much faster calcination profile is expected to be applied for F-free Cu precursor solution. The calcination was performed by heating gel film to 400 °C with the rate of 4 °C/min in wet O2 atmosphere. Only 2–3 h are required to finish calcination process. In this study, buffered RABiTS tape was used as substrate for
Fig. 3. Microstructure of converted YBCO film after annealing. (a) Surface and (b) cross-section.
30 Conventional MOD-TFA solution F-free Cu precursor solution
Viscosity (cP)
(a)
(b)
20
1.7μm
10
0
2
4
6
8
10
12
14
16
Time (days) Fig. 4. (a) Stability of F-free Cu precursor solution and (b) cross-section of as-calcined precursor film using the same solution.
Fig. 5. (a) Cross-section and (b) surface of YBCO film prepared by F-free Cu precursor solution.
Y.-K. Kim et al. / Physica C 445–448 (2006) 574–577
for Cu-TFA. Fig. 4(b) presents a cross-sectional view of the as-calcined precursor film which shows crack-free 1.7 lm-thick film after single coating. Thickness of the precursor film is beyond the range of conventional MOD-TFA solution (t < 1 lm). This can be attributed to much lower HF gas generation during calcination process compared with conventional MOD-TFA process. The calcined precursor film was converted to YBCO film after annealing process in Ar/O2 atmosphere. Thickness of annealed YBCO film was estimated to be 0.5 lm from cross-sectional observation with SEM (Fig. 5(a)). DC 4probe measurement shows that critical current of YBCO film was about 0.8 MA/cm2. Microstructural investigation in Fig. 5(b) indicates that pores still exist and further removal of pores is required to improve critical current of the YBCO film. It is known that low pressure annealing will improve growth rate of YBCO film and suppress HF gas build-up during the conversion process [13]. In order to improve microstructure and critical current of YBCO film, low pressure annealing will be applied to convert precursor films to YBCO films. 4. Summary In summary, PVP-containing precursor solution was synthesized and YBCO film after annealing process shows porous microstructures possibly originated from large HF gas generation from thick films and residual carbon from additives. Reduction of fluorine content is expected to be advantageous to improve microstructures. In order to reduce fluorine content, F-free Cu-precursor solution was synthesized. A crack-free 1.7 lm-thick precursor film was fabricated through fast calcination process. Converted YBCO film with thickness of 0.5 lm shows critical current of 0.8 MA/cm2. Microstructure of YBCO film still contains pores and further reduction of pores is required for higher Ic.
577
Low pressure annealing will be applied to achieve high Ic through fast growth rate and inhibition of HF gas build-up. Acknowledgement This research was supported by a grant from the Center for Applied Superconductivity Technology of the 21st century Frontier R&D Program funded by the Ministry of Science and Technology, Republic of Korea. References [1] R.J. Soulen Jr., D.-W. Yuan, T.L. Francavilla, IEEE Trans. Appl. Supercond. 11 (2001) 2995. [2] K. Hasegawa, N. Yoshida, K. Fujino, H. Mukai, K. Hayashi, K. Sato, Y. Sato, S. Honjo, T. Ohkuma, H. Ishii, T. Hara, in: S. Nakajima, M. Murakami (Eds.), Advances in Superconductivity IX, Proceedings of the Ninth International Symposium on Superconductivity (ISS’96), Sapporo, Japan, 1996, p. 745. [3] S. Miura, K. Hashimoto, F. Wang, Y. Enomoto, T. Morishita, Physica C 278 (1997) 201. [4] S. Sathyamurthy, K. Salama, IEEE Trans. Appl. Supercond. 11 (2001) 2935. [5] D.T. Verebelyi, U. Schoop, C. Thieme, X. Li, W. Zhang, T. Kodenkandath, A.P. Malozemoff, N. Nguyen, E. Siegal, D. Buczek, Supercond. Sci. Technol. 16 (2003) L19. [6] J.T. Dawley, P.G. Clem, M.P. Siegal, D.L. Overmyer, J. Mater. Res. 16 (2001) 13. [7] Y. Tokunaga, T. Honjo, T. Izumi, Y. Shiohara, Y. Iijima, T. Saitoh, T. Goto, A. Yoshinaka, A. Yajima, Cryogenics 44 (2004) 817. [8] Y.L. Tu et al., J. Mater. Res. 11 (1996) 2566. [9] N. Sriprang, D. Kaewchinda, J.D. Kennedy, S.J. Milne, J. Am. Ceram. Soc. 83 (2000) 1914. [10] H. Kozuka, S. Takenaka, J. Am. Ceram. Soc. 85 (2002) 2696. [11] P. Clem, M.P. Siegal, J.A. Voigt, J.J. Richardson, D.L. Overmyer, in: Proceedings of the International Workshop on Coated Conductors for Applications (CCA2004), 2004, p. 6. [12] T. Araki, I. Hirabayashi, Supercond. Sci. Technol. 16 (2003) R71. [13] R. Teranishi, T. Honjo, Y. Tokunaga, H. Fuji, J. Matsuda, T. Izumi, A. Yajima, Y. Shiohara, Physica C 412–414 (2004) 920.
Synthesis of stable precursor solution for MOD-processing of YBCO coated conductors Young-Kuk Kim *, Jaimoo Yoo, Kookchae Chung, Jaewoong Ko Korea Institute of Machinery and Materials (KIMM), #66 Sangnam-dong, Changwon, Kyungnam 641-010, Republic of Korea Available online 9 June 2006
Abstract MOD precursor solutions for YBCO coated conductors were synthesized by either adding organic additives or substituting F-free Cu precursor for Cu-trifluoroacetate (TFA) to improve the stability of the precursor solution. The precursor solution without any significant change in the viscosity was synthesized by incorporating polyvinylpyrrolidone (PVP) and thickness of precursor film after calcination was enhanced. In addition, F-free Cu precursor was employed to synthesize the precursor solution with less fluorine content. Stable precursor solution without time-dependent viscosity degradation was obtained and the crack-free and 1.7 lm thick precursor film was formed after fast calcination at 400 °C in wet O2 atmosphere. A 0.5 lm-thick YBCO film was formed after annealing in wet Ar/O2 mixed gas and showed critical current density of 0.8 MA/cm2. Further optimization of processing conditions such as total pressure, oxygen partial pressure, annealing temperature, etc. is in progress to improve critical current. This chemical modification approach is a possible candidate for improving stability and performance of MOD-processing of YBCO coated conductor. Ó 2006 Elsevier B.V. All rights reserved. PACS: 74.72.Bk; 74.76.Bz Keywords: Precursor solution; MOD; YBCO
1. Introduction The YBa2Cu3O7 x (YBCO)-based coated conductor (CC) is promising high temperature superconducting (HTSC) wire architectures for electrical devices [1]. Many efforts are in progress to develop long HTSC wire with high current carrying capacity by coated conductor architecture [1–3]. Among them, metal–organic deposition using trifluoroacetates (MOD-TFA) is attracting much interest because of its potential for scale-up and cost-effectiveness. MOD-TFA is chemical solution deposition process which is intrinsically non-vacuum approach and YBCO thin films with high critical current density (Jc > 1 MA/cm2) can be routinely fabricated by this process [4–7]. One of the most important issues in the MOD-processing of YBCO coated conductor is the synthesis of MOD *
Corresponding author. Tel.: +82 55 280 3343; fax: +82 55 280 3399. E-mail address: [email protected] (Y.-K. Kim).
0921-4534/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2006.04.051
precursor solution. The precursor solution in conventional MOD-TFA process often shows time-dependent degradation of its properties such as viscosity. In particular, long length processing of coated conductor requires stable precursor solution. In addition, metal-TFA salts in the precursor solution of the conventional MOD-TFA process generate large amount of HF gas during the calcination step. As a result of this, very long processing time (20 h) is required to fabricate crack-free YBCO precursor film and the thickness of precursor film is limited to small values after single coating [7]. In order to obtain thick and dense YBCO layers to improve critical current, modifications of precursor solution are required. In this study, chemically modified precursor solutions for MOD processing were synthesized to improve the stability of process and thickness of YBCO films. Chemical modification of precursor solution was performed by either adding organic additives or substituting F-free Cu precursor for Cu-TFA. In particular, the effect of chemical
Y.-K. Kim et al. / Physica C 445–448 (2006) 574–577
modification of precursor solution on stability of precursor solution and thickness of YBCO precursor film was checked. 2. Effect of polyvinylpyrrolidone (PVP) addition In chemical solution processing of thick ceramic films, organic additives such as 1,3-propanediol, ethylene glycol, etc. are often employed to improve stability of precursor solution and fabricate crack-free thick films [8–10]. Tu et al. deposited 1 lm-thick and crack-free Pb(Zr, Ti)O3 (PZT) films using chemical solution deposition with 1,3propanediol containing precursor solution [8]. Recently, Kozuka and Takenaka, fabricated 2 lm-thick PZT films without any crack by polyvinylpyrrolidone (PVP)-assisted sol–gel process [10]. It is known that addition of PVP in precursor solution improve stability of precursor solution by inhibiting condensation reaction and increase critical thickness for crack generation [10]. Clem et al. shows that PVP can reduce film stress and increase the thickness of YBCO film prepared by PVP-assisted SANDEATM solution [11]. In this study, in order to improve stability and thickness of YBCO film, PVP was incorporated into our YBCO oxide-based precursor solution. Fig. 1 shows surface of the precursor film after calcination using PVP-assisted YBCO precursor solution. No crack was detected and uniform surface was shown.
575
As shown in Fig. 2(a), thickness of crack-free precursor film was linearly increases to 2 lm by increasing the concentration of PVP. That is, the critical thickness for crack initiation of the precursor film is increased by addition of PVP. This means PVP act as a stress releasing agent and improve critical thickness of precursor films. In addition, time-dependent variation of solution viscosity is negligible, which indicates excellent stability of precursor solution with PVP (Fig. 2(b)). A 1.5 lm-thick precursor film was converted to YBCO films after annealing in Ar/O2 atmosphere. The thickness of YBCO film was estimated to be about 0.7 lm (Fig. 3). However, the microstructure of annealed YBCO film contains a large amount of large pores exist. As a result of this, the measured Ic value of this sample is only 2 A (Jc 0.03 MA/cm2). The origin of these large pores can be attributed to large HF generation from thick precursor film during the annealing process and possible residual carbon from additives [12]. This indicates that a new solution with low HF content is required to inhibit pore formation and improve the microstructure. 3. F-free Cu precursor solution F-free Cu precursor solution was synthesized by dissolving F-free Cu precursor into methanol with yttrium and barium TFA salt. The total content of fluorine in the
Fig. 1. Surface of as-calcined precursor film observed by (a) optical microscopy (OM) and (b) scanning electron microscopy (SEM).
Fig. 2. (a) Thickness of precursor films after calcination as a function of concentration of PVP and (b) stability of precursor solution with PVP.
576
Y.-K. Kim et al. / Physica C 445–448 (2006) 574–577
coating. X-ray diffraction profile of the calcined film shows broad reflections and indicates existence of BaF2, CuO and Y2O3 which is consistent with calcined film prepared by conventional MOD-TFA using metal TFA as starting precursors. Fig. 4(a) shows stability of precursor solutions. Compared with conventional MOD-TFA precursor solution, F-free Cu precursor solution shows only slight increase in viscosity after 15 days preservation in ambient atmosphere, which means that the stability of precursor solution was improved by substituting F-free Cu precursor
precursor solution is half of that for conventional MODTFA precursor solution synthesized with metal-TFA salts. Since the large gas generation from all TFA-based precursor solution limits the heating rate in calcination step to be about 0.05 °C/min, much faster calcination profile is expected to be applied for F-free Cu precursor solution. The calcination was performed by heating gel film to 400 °C with the rate of 4 °C/min in wet O2 atmosphere. Only 2–3 h are required to finish calcination process. In this study, buffered RABiTS tape was used as substrate for
Fig. 3. Microstructure of converted YBCO film after annealing. (a) Surface and (b) cross-section.
30 Conventional MOD-TFA solution F-free Cu precursor solution
Viscosity (cP)
(a)
(b)
20
1.7μm
10
0
2
4
6
8
10
12
14
16
Time (days) Fig. 4. (a) Stability of F-free Cu precursor solution and (b) cross-section of as-calcined precursor film using the same solution.
Fig. 5. (a) Cross-section and (b) surface of YBCO film prepared by F-free Cu precursor solution.
Y.-K. Kim et al. / Physica C 445–448 (2006) 574–577
for Cu-TFA. Fig. 4(b) presents a cross-sectional view of the as-calcined precursor film which shows crack-free 1.7 lm-thick film after single coating. Thickness of the precursor film is beyond the range of conventional MOD-TFA solution (t < 1 lm). This can be attributed to much lower HF gas generation during calcination process compared with conventional MOD-TFA process. The calcined precursor film was converted to YBCO film after annealing process in Ar/O2 atmosphere. Thickness of annealed YBCO film was estimated to be 0.5 lm from cross-sectional observation with SEM (Fig. 5(a)). DC 4probe measurement shows that critical current of YBCO film was about 0.8 MA/cm2. Microstructural investigation in Fig. 5(b) indicates that pores still exist and further removal of pores is required to improve critical current of the YBCO film. It is known that low pressure annealing will improve growth rate of YBCO film and suppress HF gas build-up during the conversion process [13]. In order to improve microstructure and critical current of YBCO film, low pressure annealing will be applied to convert precursor films to YBCO films. 4. Summary In summary, PVP-containing precursor solution was synthesized and YBCO film after annealing process shows porous microstructures possibly originated from large HF gas generation from thick films and residual carbon from additives. Reduction of fluorine content is expected to be advantageous to improve microstructures. In order to reduce fluorine content, F-free Cu-precursor solution was synthesized. A crack-free 1.7 lm-thick precursor film was fabricated through fast calcination process. Converted YBCO film with thickness of 0.5 lm shows critical current of 0.8 MA/cm2. Microstructure of YBCO film still contains pores and further reduction of pores is required for higher Ic.
577
Low pressure annealing will be applied to achieve high Ic through fast growth rate and inhibition of HF gas build-up. Acknowledgement This research was supported by a grant from the Center for Applied Superconductivity Technology of the 21st century Frontier R&D Program funded by the Ministry of Science and Technology, Republic of Korea. References [1] R.J. Soulen Jr., D.-W. Yuan, T.L. Francavilla, IEEE Trans. Appl. Supercond. 11 (2001) 2995. [2] K. Hasegawa, N. Yoshida, K. Fujino, H. Mukai, K. Hayashi, K. Sato, Y. Sato, S. Honjo, T. Ohkuma, H. Ishii, T. Hara, in: S. Nakajima, M. Murakami (Eds.), Advances in Superconductivity IX, Proceedings of the Ninth International Symposium on Superconductivity (ISS’96), Sapporo, Japan, 1996, p. 745. [3] S. Miura, K. Hashimoto, F. Wang, Y. Enomoto, T. Morishita, Physica C 278 (1997) 201. [4] S. Sathyamurthy, K. Salama, IEEE Trans. Appl. Supercond. 11 (2001) 2935. [5] D.T. Verebelyi, U. Schoop, C. Thieme, X. Li, W. Zhang, T. Kodenkandath, A.P. Malozemoff, N. Nguyen, E. Siegal, D. Buczek, Supercond. Sci. Technol. 16 (2003) L19. [6] J.T. Dawley, P.G. Clem, M.P. Siegal, D.L. Overmyer, J. Mater. Res. 16 (2001) 13. [7] Y. Tokunaga, T. Honjo, T. Izumi, Y. Shiohara, Y. Iijima, T. Saitoh, T. Goto, A. Yoshinaka, A. Yajima, Cryogenics 44 (2004) 817. [8] Y.L. Tu et al., J. Mater. Res. 11 (1996) 2566. [9] N. Sriprang, D. Kaewchinda, J.D. Kennedy, S.J. Milne, J. Am. Ceram. Soc. 83 (2000) 1914. [10] H. Kozuka, S. Takenaka, J. Am. Ceram. Soc. 85 (2002) 2696. [11] P. Clem, M.P. Siegal, J.A. Voigt, J.J. Richardson, D.L. Overmyer, in: Proceedings of the International Workshop on Coated Conductors for Applications (CCA2004), 2004, p. 6. [12] T. Araki, I. Hirabayashi, Supercond. Sci. Technol. 16 (2003) R71. [13] R. Teranishi, T. Honjo, Y. Tokunaga, H. Fuji, J. Matsuda, T. Izumi, A. Yajima, Y. Shiohara, Physica C 412–414 (2004) 920.