- Email: [email protected]
Fabrication of YBCO coated conductor by TFA-MOD method in route of dissolving Y211 and Ba3Cu5O8 powders in TFA
Physica C 426–431 (2005) 973–978 www.elsevier.com/locate/physc
Fabrication of YBCO coated conductor by TFA-MOD method in route of dissolving Y211 and Ba3Cu5O8 powders in TFA Jun Hyung Lim a, Seok Hern Jang a, Jung Ho Kim a, Kyu Tae Kim a, Jinho Joo a,*, Seung-Boo Jung a, Jung-Gu Kim a, Hee Gyoun Lee b, Gye Won Hong b a
School of Metallurgical and Materials Engineering, Sungkyunkwan University, 300 cheoncheon, Jangan, Suwon, Gyeonggi 440-746, Korea b The Department of Electronic Engineering, Korea Polytechnic University, Gyeonggi 429-793, Korea Received 23 November 2004; accepted 26 January 2005 Available online 15 July 2005
Abstract We fabricated YBCO coated conductors using a new TFA-MOD method and evaluated the phase formation, texture evolution, and critical properties as a function of the firing temperature and oxygen heat treatment time, in order to explore its possible application in CCs fabrication. In the fabrication process, Y2Ba1Cu1Ox and Ba3Cu5O8 powders were used as precursors, instead of Y-, Ba- and Cu-based acetate or organics, and dissolved in trifluoroacetic acid followed by firing and oxygen heat treatment. We observed that for the films fired at 725 C, BaF2 and other second phases formed and Tc was measured to be 66.5 K. In contrast, when the film was fired at 775 C, strong and sharp (0 0 l) diffraction peaks of YBCO were observed, without the formation of the BaF2 phase. In addition, X-ray pole-figure analysis indicated that a strong biaxial texture was developed and the full-width at half-maximums of in-plane and out-of-plane textures were 4.38 and 4.26, respectively. The critical temperature and current were also increased to 85.3 K and 35.8 A/cm-width, respectively, at 77 K when the oxygen heat treatment time was increased to 20 h. Microstructure observation indicated that a crack-free surface was obtained and that there was no a-axis grain growth. 2005 Elsevier B.V. All rights reserved.
*
Corresponding author. Tel.: +82 31 290 7358; fax: +82 31 290 7371. E-mail address: [email protected] (J. Joo).
0921-4534/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2005.01.056
974
J.H. Lim et al. / Physica C 426–431 (2005) 973–978
PACS: 74.72.Bk; 74.76.Bz Keywords: Critical temperature; Pole figure; Metal organic deposition; YBCO coated conductor; Y211 process
1. Introduction Because YBCO coated conductors (CCs) have an excellent critical current density (Jc) at high magnetic field, their practical application in power systems have been widely studied. Various processing techniques such as pulsed laser deposition (PLD) [1], metal organic chemical vapor deposition (MOCVD) [2], BaF2 process [3], and metal organic deposition (MOD)[4], etc., have been investigated for the fabrication of CCs. Among them, the MOD process has several advantages of the precise controllability in metal content, wide flexibility to coating materials, and non-vacuum approach. Recently, the trifluoroacetates (TFA)MOD method is considered to be one of the most promising methods of fabricating CCs since it has an excellent Jc exceeding 3.6 · 106A/cm2 [5]. In the conventional TFA-MOD method, the acetates of Y, Ba and Cu dissolved in TFA are used as the precursor. It is known to be difficult to depress the formation of the BaF2 phase, unless the processing conditions are precisely controlled, resulting in this method having low reproducibility probably, probably due to non-equilibrium chemical reactions based on the decomposition of the precursors [6]. In addition, since acetates are expensive, it is desirable to develop a new synthetic method of more reproducible, cost-effective, and stable reaction, by selecting other precursors and/or solutions. For the fabrication of YBCO bulk, Y2Ba1Cu1Ox (Y211) and Ba3Cu5O8 powders have been used as precursors (the so called ‘‘211 process’’), instead of Y-, Ba- and Cu-based powders, in order to enhance the reaction kinetics and to control the formation of the second phases. Our approach in TFA-MOD method is based on this ‘‘211 process’’ and we found that Y211 and Ba3Cu5O8 powders are hydrated in TFA. The chemical reaction during firing is considered to be the following: Y2 Ba1 Cu1 Ox ðsÞ þ Ba3 Cu5 O8 ðsÞ ! 2Y1 Ba2 Cu3 Ox
ð1Þ
and it is expected that the YBCO phase can easily form with higher reproducibility and less formation of second phases. In our study, therefore, we fabricated the CCs by a TFA-MOD route which involved dissolving the oxide powders of Y211 and Ba3Cu5O8 in TFA. We then evaluated the effect of the firing temperature and oxygen heat treatment time on the phase formation, texture formability, and critical temperature (Tc) and current (Ic) of the YBCO film, in order to explore the viability of this method in the fabrication of CCs.
2. Experimental Y2Ba1Cu1Ox powder was synthesized by the solid-state reaction of the constituent powders, of Y2O3, BaCO3, and CuO, while the Ba3Cu5O8 powder was supplied by PRAXAIR Co. (99.9% purity). The precursor powders were mixed in TFA in a 1:1 molar ratio and refluxed for 6 h; we observed that the solution was a transparent bluecolor without the residue, suggesting that the solution powder were dissolved in the solution. The solution was dried in air to evaporate the solvent, resulting in a blue-colored solid residue. This residue was dissolved in a methyl alcohol with a 2.2 M concentration. The gel film was deposited onto a (0 0 1) LaAlO3 single crystal by the dip coating process with a withdrawal speed of 2 cm/ min, followed by drying in air for 2 h. The gel film was calcined, by raising the temperature to 400 C in flowing 12.1% humidified oxygen. Subsequently, the calcined film was fired at 725 C and 775 C, respectively, for 4 h in 4.1% humidified Ar gas mixed with 1000 ppm oxygen and then heat treated at 450 C for 10–20 h in dry oxygen (oxygen heat treatment). The thickness of calcined film was measured to be approximately 1.0 lm. Phase identification and texture analysis were performed using X-ray diffraction (XRD) and pole-figures (BRUKER-AXS, D8 discover). In
J.H. Lim et al. / Physica C 426–431 (2005) 973–978
the measurements, CuKa1 radiation (wavelength ˚ ) was used over an area of 5 · 5 mm2. of 1.5406 A For the texture analysis, we observed the x angle in the range of 0 to 75 and the / angle in the range of 0–360 at an interval of 5 with a dwell time of 1 second. Microstructures were observed by scanning electron microscopy (SEM, XL-30, ESEM-FEG). The Tc was measured by the standard four-probe method in a cryostat, and the Ic was obtained by the same method with a 1 lV/ cm criterion in liquid nitrogen (77 K) in a zero field, for the CC films with a width of 0.5 cm.
3. Results and discussion
Intensity
(007)
(006)
LAO(100)
(005)
BaF 2 (004)
(003)
(002)
(001)
unknown
725˚C-10h (O2 treatment)
LAO(100)
To evaluate the effect of the firing temperatures and oxygen heat treatment time on phase formation, the XRD patterns were obtained. Fig. 1 shows the XRD patterns of the films fired at (a) 725 C and (b) 775 C followed by oxygen heat treatment for 10 h and at (c) 775 C followed by oxygen heat treatment for 20 h. The film fired at 725 C consists of YBCO phase as the major phase and BaF2 and unknown phases as minor phases. When the firing temperature increased to 775 C, the BaF2 and other second phase peaks were disappeared. In addition, the intensity of the (0 0 l) plane of the YBCO phase increased, suggesting that the texture (c-axis orientation) of the YBCO
775˚C-10h (O2 treatment)
775˚C-20h (O2 treatment)
10
20
30
40
50
60
2 theta Fig. 1. XRD patterns of the films fired at (a) 725 C and (b) 775 C with oxygen heat treatment for 10 h and at (c) 775 C with oxygen heat treatment for 20 h.
975
grains enhanced. When the oxygen heat treatment time was prolonged to 20 h, the XRD pattern was similar to that of the film treated for 10 h and the intensity of the (0 0 l) plane increased further. In general, three parameters influence the chemical reaction rate of fluorides during firing, i.e., the reaction time and temperature and the total amount of humidity in the gas [7]. Based on the XRD data, it is considered that the reaction of the Y211 and Ba3Cu5O8 phases to produce YBCO is almost completed without the formation of BaF2 phase, when the films are fired at 775 C and oxygen treated for 10–20 h. In order to evaluate the texture components of the YBCO films, a detailed analysis of the (0 0 3), (1 0 3) and (1 1 3) pole-figures was performed for the films fired at 725 C and 775 C followed by oxygen heat treatment for 10 h, as presented in Fig. 2. We observed that the textures of the two films were similar and formed a mainly biaxial texture, whereas, the intensity of the major and minor texture components differed from each other. For the film fired at 725 C, the major texture was a biaxial texture and other minor components were present, as shown in Fig. 2(a). The full-width at half-maximums (FWHMs) of inplane and out-of-plane textures were estimated to be 6.28 and 5.15, respectively. When the firing temperature was 775 C, it can be seen that a stronger biaxial texture was developed without any minor texture components, and that the poles became more symmetric (Fig. 2(b)). The FWHMs of in-plane and out-of-plane textures were 4.38 and 4.25, respectively. Fig. 3 (a) and (b) show the high magnifications (·8000) SEM images of the films fired at 725 C and 775 C, respectively. Both films had a crackfree surface, but different morphologies of the abplane: the film fired at 725 C had larger pores and a more irregular surface than the films at 775 C. EDX analysis reveals that there are very small CuO particles in both films, although no CuO peak appeared in the XRD analysis shown in Fig. 1, and more CuO particles were present in the film fired at 725 C than in that at 775 C. It is also to be noted that both films had a similar grain size, indicating that no a-axis grain growth occurred when the firing temperature increased.
976
J.H. Lim et al. / Physica C 426–431 (2005) 973–978 Measured PF 003
Measured PF 013
Measured PF 003
Measured PF 013
Measured PF 113
a Measured PF 113
b Fig. 2. (0 0 3), (1 0 3) and (1 1 3) pole-figures of the film fired at (a) 725 C and (b) 775 C.
Fig. 4(a) shows the Tc curves for the films showing that the Tc improved as the firing temperature and oxygen heat treatment time increased. For the film fired at 725 C, the Tc–zero (Tc) and Tc-onset were measured to be 66.5 K and 77.5 K, respectively. It is considered that these low Tc values and broad transition can be attributed to the formation of BaF2 and CuO phases and to the low oxygen content in the YBCO structure. When the firing temperature increased to 775 C, the corresponding Tc values were increased to 74.5 K and 85.5 K, respectively. In addition, as the oxygen heat treatment time increased to 20 h, the corresponding Tc values were further increased to 85.3 K and 88.5 K, respectively, with a sharp transition range of 3.2 K. Because the Tc of YBCO is sensitively dependent on the oxygen contents/ ordering [8], it is estimated that the oxygen content increased from approximately from 6.85 to 6.91 when the oxygen heat treatment time increased from 10 h to 20 h at 775 C. Fig. 4 (b) shows the V–I curve for the film fired at 775 C followed by oxygen heat treatment for 20 h. From this figure, the Ic was measured to be 17.9 A (Jc 0.36 MA/cm2), which corresponds to 35.8 A/cm-width in a zero field with a 1 lV/cm criterion. Since this measurement was done at the
liquid nitrogen temperature (77 K), we could not measure the Ic for the other two films which had Tc lower than 77 K. In this study, we fabricated CCs by a new TFAMOD method which involved dissolving Y211 and Ba3Cu5O8 powders in TFA. We found that the BaF2 phase was effectively reduced and that a sharp and strong biaxial texture was formed, resulting in Tc of 85.3 K and Ic of 35.8 A/cmwidth. To further improve the critical properties, it is necessary to optimize the processing conditions in detail and to relate the processing conditions to the oxygen stoichiometry and grain connectivity using Raman micro-spectroscopy, which is currently in progress.
4. Conclusion We fabricated CCs using a new TFA-MOD method by using so called ‘‘211 process’’, and investigated the effect of the firing temperature and oxygen heat treatment time on the phase formation, texture evolution, and critical properties. We observed that the YBCO films were deposited biaxially on the LaAlO3 substrate and that the phase formation, the degree of the texture, and
J.H. Lim et al. / Physica C 426–431 (2005) 973–978
977
725˚C-10 h (O2 treatment)
-7
4.0x10
Resistivity (Ohm-1 cm-1)
775˚C-10 h (O2 treatment) 775˚C-20 h (O 2 treatment)) -7
3.0x10
-7
2.0x10
-7
1.0x10
0.0 0
20
40
60
Voltage (mV)
a
80
100
120
140
160
Temperature (K)
1.5x10
-2
1.0x10
-2
5.0x10
-3
775˚C-20 h (@ 77K)
0.0 0
2
4
b
Fig. 3. SEM micrograph of films fired at (a) 725 C and (b) 775 C followed by oxygen heat treatment time for 10 h.
the microstructure varied with the firing temperature and oxygen heat treatment time. Microstructure observation indicated that a crack-free surface was obtained and that no a-axis grain growth occurred in the firing temperature range of 725–775 C. For the film fired at 725 C, BaF2 and other second phases formed and Tc was measured to be 66.5 K. In this case, the FWHMs of in-plane and out-of-plane textures were 6.28 and 5.15, respectively. On the other hand, the phase purity improved and BaF2 phase was effectively eliminated when the firing temperature increased to 775 C. In addition, the degree of texture improved and the FWHMs of in-plane and out-of-plane textures were measured to be 4.38 and 4.26, respectively. When the oxygen heat treatment time was increased to 20 h at the same firing temperature, the Tc and Ic values of the film
6
8
10
12
14
16
18
20
22
Critical current (A)
Fig. 4. (a) Variation of Tc with firing temperature and oxygen heat treatment time and (b) V–I curve of the film fired 775 C with oxygen heat treatment of 20 h.
were increased to 85.3 K and 35.8 A/cm-width, respectively, at 77 K in a zero field.
Acknowledgements This research (R-2004-0-194) was supported by a grant from Ministry of Commerce, Industry and Energy (MOCIE), Republic of Korea. We thank Dr. Y.A. Jee at KICET for helpful discussions.
References [1] J.A. Smith, M.J. Cima, N. Sonnenberg, J. Appl. Phys. 77 (1995) 5263. [2] H. Yamane, T. Hirai, K. Watanabe, N. Kobayashi, Y. Muto, M. Hasei, H. Kurosawa, J. Appl. Phys. 69 (1991) 7948.
978
J.H. Lim et al. / Physica C 426–431 (2005) 973–978
[3] R. Freestra, T.B. Lindemer, J.D. Budai, M.D. Galloway, J. Appl. Phys. 69 (1991) 6959. [4] P.C. McIntyre et al., J. Appl. Phys. 71 (1992) 1868. [5] T. Izume et al., Physica C 412–414 (2004) 885.
[6] P.C. McIntyre, M.J. Cima, J. Mater. Res. 9 (1990) 2219. [7] T. Araki et al., Physica C 357–360 (2000) 987. [8] W.E. Farneth et al., Solid State Commun. 66 (1998) 953.
Fabrication of YBCO coated conductor by TFA-MOD method in route of dissolving Y211 and Ba3Cu5O8 powders in TFA Jun Hyung Lim a, Seok Hern Jang a, Jung Ho Kim a, Kyu Tae Kim a, Jinho Joo a,*, Seung-Boo Jung a, Jung-Gu Kim a, Hee Gyoun Lee b, Gye Won Hong b a
School of Metallurgical and Materials Engineering, Sungkyunkwan University, 300 cheoncheon, Jangan, Suwon, Gyeonggi 440-746, Korea b The Department of Electronic Engineering, Korea Polytechnic University, Gyeonggi 429-793, Korea Received 23 November 2004; accepted 26 January 2005 Available online 15 July 2005
Abstract We fabricated YBCO coated conductors using a new TFA-MOD method and evaluated the phase formation, texture evolution, and critical properties as a function of the firing temperature and oxygen heat treatment time, in order to explore its possible application in CCs fabrication. In the fabrication process, Y2Ba1Cu1Ox and Ba3Cu5O8 powders were used as precursors, instead of Y-, Ba- and Cu-based acetate or organics, and dissolved in trifluoroacetic acid followed by firing and oxygen heat treatment. We observed that for the films fired at 725 C, BaF2 and other second phases formed and Tc was measured to be 66.5 K. In contrast, when the film was fired at 775 C, strong and sharp (0 0 l) diffraction peaks of YBCO were observed, without the formation of the BaF2 phase. In addition, X-ray pole-figure analysis indicated that a strong biaxial texture was developed and the full-width at half-maximums of in-plane and out-of-plane textures were 4.38 and 4.26, respectively. The critical temperature and current were also increased to 85.3 K and 35.8 A/cm-width, respectively, at 77 K when the oxygen heat treatment time was increased to 20 h. Microstructure observation indicated that a crack-free surface was obtained and that there was no a-axis grain growth. 2005 Elsevier B.V. All rights reserved.
*
Corresponding author. Tel.: +82 31 290 7358; fax: +82 31 290 7371. E-mail address: [email protected] (J. Joo).
0921-4534/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2005.01.056
974
J.H. Lim et al. / Physica C 426–431 (2005) 973–978
PACS: 74.72.Bk; 74.76.Bz Keywords: Critical temperature; Pole figure; Metal organic deposition; YBCO coated conductor; Y211 process
1. Introduction Because YBCO coated conductors (CCs) have an excellent critical current density (Jc) at high magnetic field, their practical application in power systems have been widely studied. Various processing techniques such as pulsed laser deposition (PLD) [1], metal organic chemical vapor deposition (MOCVD) [2], BaF2 process [3], and metal organic deposition (MOD)[4], etc., have been investigated for the fabrication of CCs. Among them, the MOD process has several advantages of the precise controllability in metal content, wide flexibility to coating materials, and non-vacuum approach. Recently, the trifluoroacetates (TFA)MOD method is considered to be one of the most promising methods of fabricating CCs since it has an excellent Jc exceeding 3.6 · 106A/cm2 [5]. In the conventional TFA-MOD method, the acetates of Y, Ba and Cu dissolved in TFA are used as the precursor. It is known to be difficult to depress the formation of the BaF2 phase, unless the processing conditions are precisely controlled, resulting in this method having low reproducibility probably, probably due to non-equilibrium chemical reactions based on the decomposition of the precursors [6]. In addition, since acetates are expensive, it is desirable to develop a new synthetic method of more reproducible, cost-effective, and stable reaction, by selecting other precursors and/or solutions. For the fabrication of YBCO bulk, Y2Ba1Cu1Ox (Y211) and Ba3Cu5O8 powders have been used as precursors (the so called ‘‘211 process’’), instead of Y-, Ba- and Cu-based powders, in order to enhance the reaction kinetics and to control the formation of the second phases. Our approach in TFA-MOD method is based on this ‘‘211 process’’ and we found that Y211 and Ba3Cu5O8 powders are hydrated in TFA. The chemical reaction during firing is considered to be the following: Y2 Ba1 Cu1 Ox ðsÞ þ Ba3 Cu5 O8 ðsÞ ! 2Y1 Ba2 Cu3 Ox
ð1Þ
and it is expected that the YBCO phase can easily form with higher reproducibility and less formation of second phases. In our study, therefore, we fabricated the CCs by a TFA-MOD route which involved dissolving the oxide powders of Y211 and Ba3Cu5O8 in TFA. We then evaluated the effect of the firing temperature and oxygen heat treatment time on the phase formation, texture formability, and critical temperature (Tc) and current (Ic) of the YBCO film, in order to explore the viability of this method in the fabrication of CCs.
2. Experimental Y2Ba1Cu1Ox powder was synthesized by the solid-state reaction of the constituent powders, of Y2O3, BaCO3, and CuO, while the Ba3Cu5O8 powder was supplied by PRAXAIR Co. (99.9% purity). The precursor powders were mixed in TFA in a 1:1 molar ratio and refluxed for 6 h; we observed that the solution was a transparent bluecolor without the residue, suggesting that the solution powder were dissolved in the solution. The solution was dried in air to evaporate the solvent, resulting in a blue-colored solid residue. This residue was dissolved in a methyl alcohol with a 2.2 M concentration. The gel film was deposited onto a (0 0 1) LaAlO3 single crystal by the dip coating process with a withdrawal speed of 2 cm/ min, followed by drying in air for 2 h. The gel film was calcined, by raising the temperature to 400 C in flowing 12.1% humidified oxygen. Subsequently, the calcined film was fired at 725 C and 775 C, respectively, for 4 h in 4.1% humidified Ar gas mixed with 1000 ppm oxygen and then heat treated at 450 C for 10–20 h in dry oxygen (oxygen heat treatment). The thickness of calcined film was measured to be approximately 1.0 lm. Phase identification and texture analysis were performed using X-ray diffraction (XRD) and pole-figures (BRUKER-AXS, D8 discover). In
J.H. Lim et al. / Physica C 426–431 (2005) 973–978
the measurements, CuKa1 radiation (wavelength ˚ ) was used over an area of 5 · 5 mm2. of 1.5406 A For the texture analysis, we observed the x angle in the range of 0 to 75 and the / angle in the range of 0–360 at an interval of 5 with a dwell time of 1 second. Microstructures were observed by scanning electron microscopy (SEM, XL-30, ESEM-FEG). The Tc was measured by the standard four-probe method in a cryostat, and the Ic was obtained by the same method with a 1 lV/ cm criterion in liquid nitrogen (77 K) in a zero field, for the CC films with a width of 0.5 cm.
3. Results and discussion
Intensity
(007)
(006)
LAO(100)
(005)
BaF 2 (004)
(003)
(002)
(001)
unknown
725˚C-10h (O2 treatment)
LAO(100)
To evaluate the effect of the firing temperatures and oxygen heat treatment time on phase formation, the XRD patterns were obtained. Fig. 1 shows the XRD patterns of the films fired at (a) 725 C and (b) 775 C followed by oxygen heat treatment for 10 h and at (c) 775 C followed by oxygen heat treatment for 20 h. The film fired at 725 C consists of YBCO phase as the major phase and BaF2 and unknown phases as minor phases. When the firing temperature increased to 775 C, the BaF2 and other second phase peaks were disappeared. In addition, the intensity of the (0 0 l) plane of the YBCO phase increased, suggesting that the texture (c-axis orientation) of the YBCO
775˚C-10h (O2 treatment)
775˚C-20h (O2 treatment)
10
20
30
40
50
60
2 theta Fig. 1. XRD patterns of the films fired at (a) 725 C and (b) 775 C with oxygen heat treatment for 10 h and at (c) 775 C with oxygen heat treatment for 20 h.
975
grains enhanced. When the oxygen heat treatment time was prolonged to 20 h, the XRD pattern was similar to that of the film treated for 10 h and the intensity of the (0 0 l) plane increased further. In general, three parameters influence the chemical reaction rate of fluorides during firing, i.e., the reaction time and temperature and the total amount of humidity in the gas [7]. Based on the XRD data, it is considered that the reaction of the Y211 and Ba3Cu5O8 phases to produce YBCO is almost completed without the formation of BaF2 phase, when the films are fired at 775 C and oxygen treated for 10–20 h. In order to evaluate the texture components of the YBCO films, a detailed analysis of the (0 0 3), (1 0 3) and (1 1 3) pole-figures was performed for the films fired at 725 C and 775 C followed by oxygen heat treatment for 10 h, as presented in Fig. 2. We observed that the textures of the two films were similar and formed a mainly biaxial texture, whereas, the intensity of the major and minor texture components differed from each other. For the film fired at 725 C, the major texture was a biaxial texture and other minor components were present, as shown in Fig. 2(a). The full-width at half-maximums (FWHMs) of inplane and out-of-plane textures were estimated to be 6.28 and 5.15, respectively. When the firing temperature was 775 C, it can be seen that a stronger biaxial texture was developed without any minor texture components, and that the poles became more symmetric (Fig. 2(b)). The FWHMs of in-plane and out-of-plane textures were 4.38 and 4.25, respectively. Fig. 3 (a) and (b) show the high magnifications (·8000) SEM images of the films fired at 725 C and 775 C, respectively. Both films had a crackfree surface, but different morphologies of the abplane: the film fired at 725 C had larger pores and a more irregular surface than the films at 775 C. EDX analysis reveals that there are very small CuO particles in both films, although no CuO peak appeared in the XRD analysis shown in Fig. 1, and more CuO particles were present in the film fired at 725 C than in that at 775 C. It is also to be noted that both films had a similar grain size, indicating that no a-axis grain growth occurred when the firing temperature increased.
976
J.H. Lim et al. / Physica C 426–431 (2005) 973–978 Measured PF 003
Measured PF 013
Measured PF 003
Measured PF 013
Measured PF 113
a Measured PF 113
b Fig. 2. (0 0 3), (1 0 3) and (1 1 3) pole-figures of the film fired at (a) 725 C and (b) 775 C.
Fig. 4(a) shows the Tc curves for the films showing that the Tc improved as the firing temperature and oxygen heat treatment time increased. For the film fired at 725 C, the Tc–zero (Tc) and Tc-onset were measured to be 66.5 K and 77.5 K, respectively. It is considered that these low Tc values and broad transition can be attributed to the formation of BaF2 and CuO phases and to the low oxygen content in the YBCO structure. When the firing temperature increased to 775 C, the corresponding Tc values were increased to 74.5 K and 85.5 K, respectively. In addition, as the oxygen heat treatment time increased to 20 h, the corresponding Tc values were further increased to 85.3 K and 88.5 K, respectively, with a sharp transition range of 3.2 K. Because the Tc of YBCO is sensitively dependent on the oxygen contents/ ordering [8], it is estimated that the oxygen content increased from approximately from 6.85 to 6.91 when the oxygen heat treatment time increased from 10 h to 20 h at 775 C. Fig. 4 (b) shows the V–I curve for the film fired at 775 C followed by oxygen heat treatment for 20 h. From this figure, the Ic was measured to be 17.9 A (Jc 0.36 MA/cm2), which corresponds to 35.8 A/cm-width in a zero field with a 1 lV/cm criterion. Since this measurement was done at the
liquid nitrogen temperature (77 K), we could not measure the Ic for the other two films which had Tc lower than 77 K. In this study, we fabricated CCs by a new TFAMOD method which involved dissolving Y211 and Ba3Cu5O8 powders in TFA. We found that the BaF2 phase was effectively reduced and that a sharp and strong biaxial texture was formed, resulting in Tc of 85.3 K and Ic of 35.8 A/cmwidth. To further improve the critical properties, it is necessary to optimize the processing conditions in detail and to relate the processing conditions to the oxygen stoichiometry and grain connectivity using Raman micro-spectroscopy, which is currently in progress.
4. Conclusion We fabricated CCs using a new TFA-MOD method by using so called ‘‘211 process’’, and investigated the effect of the firing temperature and oxygen heat treatment time on the phase formation, texture evolution, and critical properties. We observed that the YBCO films were deposited biaxially on the LaAlO3 substrate and that the phase formation, the degree of the texture, and
J.H. Lim et al. / Physica C 426–431 (2005) 973–978
977
725˚C-10 h (O2 treatment)
-7
4.0x10
Resistivity (Ohm-1 cm-1)
775˚C-10 h (O2 treatment) 775˚C-20 h (O 2 treatment)) -7
3.0x10
-7
2.0x10
-7
1.0x10
0.0 0
20
40
60
Voltage (mV)
a
80
100
120
140
160
Temperature (K)
1.5x10
-2
1.0x10
-2
5.0x10
-3
775˚C-20 h (@ 77K)
0.0 0
2
4
b
Fig. 3. SEM micrograph of films fired at (a) 725 C and (b) 775 C followed by oxygen heat treatment time for 10 h.
the microstructure varied with the firing temperature and oxygen heat treatment time. Microstructure observation indicated that a crack-free surface was obtained and that no a-axis grain growth occurred in the firing temperature range of 725–775 C. For the film fired at 725 C, BaF2 and other second phases formed and Tc was measured to be 66.5 K. In this case, the FWHMs of in-plane and out-of-plane textures were 6.28 and 5.15, respectively. On the other hand, the phase purity improved and BaF2 phase was effectively eliminated when the firing temperature increased to 775 C. In addition, the degree of texture improved and the FWHMs of in-plane and out-of-plane textures were measured to be 4.38 and 4.26, respectively. When the oxygen heat treatment time was increased to 20 h at the same firing temperature, the Tc and Ic values of the film
6
8
10
12
14
16
18
20
22
Critical current (A)
Fig. 4. (a) Variation of Tc with firing temperature and oxygen heat treatment time and (b) V–I curve of the film fired 775 C with oxygen heat treatment of 20 h.
were increased to 85.3 K and 35.8 A/cm-width, respectively, at 77 K in a zero field.
Acknowledgements This research (R-2004-0-194) was supported by a grant from Ministry of Commerce, Industry and Energy (MOCIE), Republic of Korea. We thank Dr. Y.A. Jee at KICET for helpful discussions.
References [1] J.A. Smith, M.J. Cima, N. Sonnenberg, J. Appl. Phys. 77 (1995) 5263. [2] H. Yamane, T. Hirai, K. Watanabe, N. Kobayashi, Y. Muto, M. Hasei, H. Kurosawa, J. Appl. Phys. 69 (1991) 7948.
978
J.H. Lim et al. / Physica C 426–431 (2005) 973–978
[3] R. Freestra, T.B. Lindemer, J.D. Budai, M.D. Galloway, J. Appl. Phys. 69 (1991) 6959. [4] P.C. McIntyre et al., J. Appl. Phys. 71 (1992) 1868. [5] T. Izume et al., Physica C 412–414 (2004) 885.
[6] P.C. McIntyre, M.J. Cima, J. Mater. Res. 9 (1990) 2219. [7] T. Araki et al., Physica C 357–360 (2000) 987. [8] W.E. Farneth et al., Solid State Commun. 66 (1998) 953.