- Email: [email protected]
Effect of precursor stoichiometry on the superconducting properties of fluorine-free sol–gel YBCO thin films
Physica C 386 (2003) 348–352 www.elsevier.com/locate/physc
Effect of precursor stoichiometry on the superconducting properties of fluorine-free sol–gel YBCO thin films B. Zhao b
a,*
, H.B. Yao a, K. Shi a, Z.H. Han a, Y.L. Xu b, D.L. Shi
b
a Applied Superconductivity Research Center, Tsinghua University, Beijing 100084, China Department of Materials Science and Engineering, University of Cincinnati Cincinnati, OH 45221-0012, USA
Abstract In the deposition of grain-textured YBa2 Cu3 Ox (YBCO) thin films for coated conductors, we have developed a fluorine-free sol–gel method with several important advantages, including precursor solution stability, improved film density, and elimination of HF during processing. In this study, the precursor solution stoichiometry was altered and its effects on superconducting properties were studied. The fluorine-free sol–gel derived films on the LaAlO3 (LAO) substrate exhibited bi-axial grain orientation. In this research, experimental details and the X-ray diffraction and scanning electron microscopy characterization of the YBCO thin films are reported. Also discussed is the underlying crystallization mechanism of the YBCO phase related to precursor stoichiometry. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 74.60.Jg; 74.12.Bk; 74.16.Bz Keywords: YBCO; Film; Fluorine-free sol–gel
1. Introduction Recently, the research on large-scale superconductivity applications has been focused on YBa2 Cu3 Ox (YBCO) coated conductors [1]. In the development of coated superconductors the sol– gel technique has been widely used as an effective processing method, especially in making longlength wires and tapes [2–6]. Deposition of YBCO thin films using the fluorine-based sol–gel method has been extensively studied in the past [3–6]. On *
Corresponding author. Address: Key Laboratory of Internal Friction and Defects in Solids, Institute of Solid Physics, Chinese Academy of Science, P.O. Box 129, Hefei 230031, China. Fax: +86-10-6278-5913. E-mail address: [email protected] (B. Zhao).
both single crystal and buffered metallic substrates, high critical current density has been obtained on the order of 106 A/cm2 at 77 K from the highquality film. However, based on previous observations there are several disadvantages in fluorine-based sol–gel process: (1) HF inevitably forms, which is severely reactive with the substrates especially with the buffer layers; (2) the microstructure of YBCO films has been observed to be rather porous, which limits the current transport properties. Considering the above-mentioned factors, we have, in our laboratory, developed a non-fluorine sol–gel method [7,8]. In the development of precursor solution, we found that the precursor stoichiometry is critical in achieving high-quality YBCO thin films. To study the sol–gel chemistry of YBCO, in this work, we have deposited the fluorine-free
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-4534(02)02196-2
B. Zhao et al. / Physica C 386 (2003) 348–352
YBCO films on single-crystal substrates of LaAlO3 (LAO) from the compositionally altered precursor solutions. We present the experimental results on the effects of precursor stoichiometry on the superconducting properties of YBCO thin films.
2. Experimental Our fluorine-free sol–gel YBCO solutions were developed in-house. For the precursor solution, stoichiometric yttrium trimethylacetate, barium hydroxide, and copper trimethylacetate powders were dissolved in a mixed propionic acid/amine solvent with an oxide concentration between 0.1 and 0.5 mol/l. The addition of amine was important because it greatly improved the solubility of the precursor powders in propionic acid. The stock solution was stable in air with a shelf life longer than 2 years. Xylenes of alcohols were used for dilution and for controlling solution viscosity at 10–100 cP. The films were deposited on (0 0 1) LAO single crystal substrates with dimensions 10 10 1 mm3 by spin coating at 3000–3500 rpm and were baked on a hot plate at 200–250 °C for several minutes. After spin coating, the films were first calcined at 400 °C in wet nitrogen with 2% oxygen for 3 h, then heat treated in a quartz furnace under the humid nitrogen mixed with 200 ppm oxygen. The temperature was increased from room temperature to 780 °C at the rate of 20 °C /min, then to 800 °C at the rate of 3 °C/min, dwelled at 800 °C for 70 min. 10 min before the end of dwelling process, a dry atmosphere was used. Then the temperature was decreased to 450 °C at the rate of 2.5 °C/min. When the temperature was lowered to 525 °C, the atmosphere of 200 ppm oxygen was switched to pure oxygen, the temperature dwelled at 450 °C for 60 min, then the samples were furnace-cooled to room temperature. A Philip X-ray diffractometer with Cu Ka radiation was used to carry out the texture investigations. Scanning electron microscopy (SEM) experiments were performed on a LEO 1530. The critical temperature ðTc Þ of the films was determined by ac susceptibility measurement. Transport resistivity measurements were carried out
349
down to liquid-nitrogen temperature using the standard four-probe method.
3. Results and discussion The YBCO sol–gel solutions with different stoichiometry (Y:Ba:Cu) were applied to LAO substrates via spin coating. Using the wet chemistry we measured the compositions of these solutions, which are listed in Table 1. As can be seen in this table, the solutions is altered from near-stoichiometry 1:2:3 (solution A, B, C) to off-stoichiometry 1.00:2.02:4.51 (solution D), and 1.00:4.17:7.02 (solution E). The Cu-rich compositions were investigated because the Cu organic salts are apt to volatilize during baking and calcining processes and thus it leads to Cu loss in the films [5,6], which has been confirmed by DTA-TGA measurements and EDX analysis in our experiments. Using the experiment process mentioned above we got YBCO thin films on LAO substrate. The X-ray diffraction (XRD) patterns of these thin films are showed in Fig. 1. As can be seen in Fig. 1, all of the (0 0 1) peaks of the YBCO films exhibited strong intensity, indicating a well-textured, c-axis oriented grain structure, especially for the films from solution B with a stoichiometry 1.00:2.00:3.19. Fig. 2 shows the phi scan and omega scan of the films from solution B. The (0 0 5) rocking curves showed small values of full width at half maximum (FWHM) about 0.5° (shown in the inset of Fig. 2). The (1 0 3) phi-scan showed four-fold symmetry with the FWHM about 0.6°, indicating the films were of good in-plane texture. From the (1 0 3) pole figure, shown in Fig. 3, one can see only four small poles locating on the correct positions. All of
Table 1 The composition of the solutions Solution a
Y Ba Cu a
A
B
C
D
E
1.00 2.00 3.02
1.00 2.00 3.19
1.00 2.08 3.25
1.00 2.02 4.51
1.00 4.17 7.02
Using element Y as reference.
350
B. Zhao et al. / Physica C 386 (2003) 348–352
Fig. 1. XRD spectra of YBCO films derived from solutions with different stoichiometry on LAO substrate. Fig. 3. (1 0 3) pole figure of the YBCO film from the solution B.
Fig. 2. (1 0 3) phi scan and (0 0 5) omega scan of the YBCO film from the solution B with stoichiometry Y:Ba:Cu as 1.00:2.00:3.19.
these demonstrates that the YBCO phase grows epitaxially on the LAO substrate. The surface SEM microstructures of these films are shown in Fig. 4. The YBCO films derived from near-stoichiometry solutions exhibited rather smooth surfaces with a significant surface density so that the grain boundaries could not be seen clearly. The terrace morphology of the grains implied a three-dimensional islands growth mechanism. And also there were a few large Cu-riched particles on the surface of the films and a few fine (Cu,Ba)-riched clusters embedded in the films. However, for the films derived from off-stoichiometry solutions, large amount of Cu-rich and (Cu,Ba)-riched particles with sizes about 1–2 lm
dispersed evenly on the film surfaces. And also many a-axis oriented grains with needlelike shape emerged in these off-stoichiometry films indicating their unfavorable texture. The superconducting properties were characterized by using ac susceptibility and resistivity measurements. The ac susceptibility results are shown in Fig. 5. As can be clearly seen in this figure, the film from solution B exhibited a sharpest transition with Tc near 90 K, while films from solution C and D showed quite broad transition. The film made from solution E did not show any superconductivity down to 77 K. The resistivity measurement results are shown in Fig. 6. The effect of precursor stoichiometry on transport Tc and the superconducting transitions can be seen from this figure. For the well-off-stoichiometry film (from solution E), the normal state exhibited a semiconducting behavior. The films from nearstoichiometry solutions showed sharp superconducting transition near 90 K. It is expected that the optimum stoichiometry lies within an exact precursor composition. In this study the films derived from the solution with a stoichiometry 1.00:2.00: 3.19 (Y:Ba:Cu) were optimal because of little Cu loss during baking and calcining process. Epitaxial growth of YBCO thin films on LAO is expected due to the single crystal substrate. In our previous studies [7,8], high-resolution transmission electron microscopy study has shown
B. Zhao et al. / Physica C 386 (2003) 348–352
351
Fig. 4. SEM photo micrographs showing the top surface of YBCO films on LAO.
Fig. 5. Ac susceptibility versus temperature for the thin films made from solutions A, B, C, D and E.
Fig. 6. Resistivity versus temperature for the YBCO films on LAO made from solutions A, B, C, D and E. (The curve for D is 1/2 q–T and the curve for E is 1/6 q–T .)
352
B. Zhao et al. / Physica C 386 (2003) 348–352
well-established epitaxial growth in fluorine-free sol–gel YBCO thin films on both LAO and YSZ substrates. The transport critical current densities of these films have reached the values of the order of 106 A/cm2 at 77 K and 0 T. According to crystallization model, the YBCO phase forms under controlled conditions, inheriting the orientation of the LAO single crystal due to the small lattice mismatch between YBCO and LAO, which will reduce the surface tension and energetically promote the nucleation of YBCO phase. In the phase diagram, YBCO is a line compound with a 1:2:3 composition and coexists with BaCuO2 and CuO phase in Cu-riched Y–Ba–Cu system below 900 °C. If the precursor is off-stoichiometry, the first YBCO lattice layer formed will push (or reject) other elements (it is mostly Cu and Ba in this study) away from its phase region. As the crystallization proceeds, further rejection will lead to accumulation of Cu and Ba along the direction of YBCO crystallization. Nonetheless, YBCO, as a line compound, will continue to grow with the exact ratio of 1:2:3. That is why the films from solution A, B and C show rather good texture in XRD patterns and sharp transition in RT curves. The rejected Cu and Ba may first precipitate along the grain boundaries of YBCO as BaCuO2 phase. That may be the reason for many fine (Cu, Ba)-riched clusters embedded in YBCO films from solutions with a little more Ba (such as solution C). However the precipitation of BaCuO2 cannot relax the enrichment of Cu in the system studied here (solution D and E). And at last at certain critical concentration, Cu cannot be fully contained in the matrix of YBCO and be push up to the surface of the film as observed in Fig. 4(d) and (e). Consistently, in films from solution A and B where the precursors are near stoichiometry, the rejected (Cu–Ba)-riched clusters is minimum with finely dispersed small particles in the film and a few Cu-riched particles are observed. Therefore, their transitions are sharp near 90 K. However, for both solution D and E, well-offstoichiometry makes initial nucleation of YBCO difficult in terms of the composition requirement.
Large amount of (Cu–Ba)-riched and Cu-riched phases formed in films may affect the orientation of the following nucleation of YBCO, which lead to a-axis YBCO grains emerging in the films. Certainly all these will results in poor superconducting properties.
4. Conclusion In summary, we have deposited YBCO thin films on LAO substrate using a new, fluorine-free sol–gel method. The effects of precursor stoichiometry on the superconducting properties of fluorine-free sol–gel YBCO thin films were studied by alterating the initial composition of the solution. The precise stoichiometry control of the precursor solution is critical in achieving high-quality YBCO thin films. Both XRD and SEM results have shown these sol–gel YBCO films on LAO exhibit bi-axial texture. The underlying mechanism on the effects of precursor stoichiometry is discussed. Acknowledgement This work was supported by China Postdoctoral Science Foundation.
References [1] A. Sheth, H. Schmidt, V. Lasrado, Appl. Supercond. 6 (1998) 855. [2] M.L. Kullberg, M.T. Lanagan, W. Wu, R.B. Poeppel, Supercond. Sci. Technol. 4 (1989) 337. [3] J.A. Smith, M.J. Cima, N. Sonnenberg, IEEE Trans. Appl. Supercond. 9 (1999) 1531. [4] S. Sathyamaurthy, K. Salama, IEEE Trans. Appl. Supercond. 11 (2001) 2935. [5] J.T. Dawley, P.G. Clem, M.P. Siegal, D.L. Overmyer, J. Mater. Res. 16 (2001) 13. [6] T. Araki, K. Yamagiwa, I. Hirabayashi, Cryogenics 41 (2001) 675. [7] D. Shi, Y. Xu, S. McClellan, R. Buchanan, Physics C 354 (2001) 71. [8] D. Shi, Y. Xu, S.X. Wang, et al., Physica C 371 (2002) 97.
Effect of precursor stoichiometry on the superconducting properties of fluorine-free sol–gel YBCO thin films B. Zhao b
a,*
, H.B. Yao a, K. Shi a, Z.H. Han a, Y.L. Xu b, D.L. Shi
b
a Applied Superconductivity Research Center, Tsinghua University, Beijing 100084, China Department of Materials Science and Engineering, University of Cincinnati Cincinnati, OH 45221-0012, USA
Abstract In the deposition of grain-textured YBa2 Cu3 Ox (YBCO) thin films for coated conductors, we have developed a fluorine-free sol–gel method with several important advantages, including precursor solution stability, improved film density, and elimination of HF during processing. In this study, the precursor solution stoichiometry was altered and its effects on superconducting properties were studied. The fluorine-free sol–gel derived films on the LaAlO3 (LAO) substrate exhibited bi-axial grain orientation. In this research, experimental details and the X-ray diffraction and scanning electron microscopy characterization of the YBCO thin films are reported. Also discussed is the underlying crystallization mechanism of the YBCO phase related to precursor stoichiometry. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 74.60.Jg; 74.12.Bk; 74.16.Bz Keywords: YBCO; Film; Fluorine-free sol–gel
1. Introduction Recently, the research on large-scale superconductivity applications has been focused on YBa2 Cu3 Ox (YBCO) coated conductors [1]. In the development of coated superconductors the sol– gel technique has been widely used as an effective processing method, especially in making longlength wires and tapes [2–6]. Deposition of YBCO thin films using the fluorine-based sol–gel method has been extensively studied in the past [3–6]. On *
Corresponding author. Address: Key Laboratory of Internal Friction and Defects in Solids, Institute of Solid Physics, Chinese Academy of Science, P.O. Box 129, Hefei 230031, China. Fax: +86-10-6278-5913. E-mail address: [email protected] (B. Zhao).
both single crystal and buffered metallic substrates, high critical current density has been obtained on the order of 106 A/cm2 at 77 K from the highquality film. However, based on previous observations there are several disadvantages in fluorine-based sol–gel process: (1) HF inevitably forms, which is severely reactive with the substrates especially with the buffer layers; (2) the microstructure of YBCO films has been observed to be rather porous, which limits the current transport properties. Considering the above-mentioned factors, we have, in our laboratory, developed a non-fluorine sol–gel method [7,8]. In the development of precursor solution, we found that the precursor stoichiometry is critical in achieving high-quality YBCO thin films. To study the sol–gel chemistry of YBCO, in this work, we have deposited the fluorine-free
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-4534(02)02196-2
B. Zhao et al. / Physica C 386 (2003) 348–352
YBCO films on single-crystal substrates of LaAlO3 (LAO) from the compositionally altered precursor solutions. We present the experimental results on the effects of precursor stoichiometry on the superconducting properties of YBCO thin films.
2. Experimental Our fluorine-free sol–gel YBCO solutions were developed in-house. For the precursor solution, stoichiometric yttrium trimethylacetate, barium hydroxide, and copper trimethylacetate powders were dissolved in a mixed propionic acid/amine solvent with an oxide concentration between 0.1 and 0.5 mol/l. The addition of amine was important because it greatly improved the solubility of the precursor powders in propionic acid. The stock solution was stable in air with a shelf life longer than 2 years. Xylenes of alcohols were used for dilution and for controlling solution viscosity at 10–100 cP. The films were deposited on (0 0 1) LAO single crystal substrates with dimensions 10 10 1 mm3 by spin coating at 3000–3500 rpm and were baked on a hot plate at 200–250 °C for several minutes. After spin coating, the films were first calcined at 400 °C in wet nitrogen with 2% oxygen for 3 h, then heat treated in a quartz furnace under the humid nitrogen mixed with 200 ppm oxygen. The temperature was increased from room temperature to 780 °C at the rate of 20 °C /min, then to 800 °C at the rate of 3 °C/min, dwelled at 800 °C for 70 min. 10 min before the end of dwelling process, a dry atmosphere was used. Then the temperature was decreased to 450 °C at the rate of 2.5 °C/min. When the temperature was lowered to 525 °C, the atmosphere of 200 ppm oxygen was switched to pure oxygen, the temperature dwelled at 450 °C for 60 min, then the samples were furnace-cooled to room temperature. A Philip X-ray diffractometer with Cu Ka radiation was used to carry out the texture investigations. Scanning electron microscopy (SEM) experiments were performed on a LEO 1530. The critical temperature ðTc Þ of the films was determined by ac susceptibility measurement. Transport resistivity measurements were carried out
349
down to liquid-nitrogen temperature using the standard four-probe method.
3. Results and discussion The YBCO sol–gel solutions with different stoichiometry (Y:Ba:Cu) were applied to LAO substrates via spin coating. Using the wet chemistry we measured the compositions of these solutions, which are listed in Table 1. As can be seen in this table, the solutions is altered from near-stoichiometry 1:2:3 (solution A, B, C) to off-stoichiometry 1.00:2.02:4.51 (solution D), and 1.00:4.17:7.02 (solution E). The Cu-rich compositions were investigated because the Cu organic salts are apt to volatilize during baking and calcining processes and thus it leads to Cu loss in the films [5,6], which has been confirmed by DTA-TGA measurements and EDX analysis in our experiments. Using the experiment process mentioned above we got YBCO thin films on LAO substrate. The X-ray diffraction (XRD) patterns of these thin films are showed in Fig. 1. As can be seen in Fig. 1, all of the (0 0 1) peaks of the YBCO films exhibited strong intensity, indicating a well-textured, c-axis oriented grain structure, especially for the films from solution B with a stoichiometry 1.00:2.00:3.19. Fig. 2 shows the phi scan and omega scan of the films from solution B. The (0 0 5) rocking curves showed small values of full width at half maximum (FWHM) about 0.5° (shown in the inset of Fig. 2). The (1 0 3) phi-scan showed four-fold symmetry with the FWHM about 0.6°, indicating the films were of good in-plane texture. From the (1 0 3) pole figure, shown in Fig. 3, one can see only four small poles locating on the correct positions. All of
Table 1 The composition of the solutions Solution a
Y Ba Cu a
A
B
C
D
E
1.00 2.00 3.02
1.00 2.00 3.19
1.00 2.08 3.25
1.00 2.02 4.51
1.00 4.17 7.02
Using element Y as reference.
350
B. Zhao et al. / Physica C 386 (2003) 348–352
Fig. 1. XRD spectra of YBCO films derived from solutions with different stoichiometry on LAO substrate. Fig. 3. (1 0 3) pole figure of the YBCO film from the solution B.
Fig. 2. (1 0 3) phi scan and (0 0 5) omega scan of the YBCO film from the solution B with stoichiometry Y:Ba:Cu as 1.00:2.00:3.19.
these demonstrates that the YBCO phase grows epitaxially on the LAO substrate. The surface SEM microstructures of these films are shown in Fig. 4. The YBCO films derived from near-stoichiometry solutions exhibited rather smooth surfaces with a significant surface density so that the grain boundaries could not be seen clearly. The terrace morphology of the grains implied a three-dimensional islands growth mechanism. And also there were a few large Cu-riched particles on the surface of the films and a few fine (Cu,Ba)-riched clusters embedded in the films. However, for the films derived from off-stoichiometry solutions, large amount of Cu-rich and (Cu,Ba)-riched particles with sizes about 1–2 lm
dispersed evenly on the film surfaces. And also many a-axis oriented grains with needlelike shape emerged in these off-stoichiometry films indicating their unfavorable texture. The superconducting properties were characterized by using ac susceptibility and resistivity measurements. The ac susceptibility results are shown in Fig. 5. As can be clearly seen in this figure, the film from solution B exhibited a sharpest transition with Tc near 90 K, while films from solution C and D showed quite broad transition. The film made from solution E did not show any superconductivity down to 77 K. The resistivity measurement results are shown in Fig. 6. The effect of precursor stoichiometry on transport Tc and the superconducting transitions can be seen from this figure. For the well-off-stoichiometry film (from solution E), the normal state exhibited a semiconducting behavior. The films from nearstoichiometry solutions showed sharp superconducting transition near 90 K. It is expected that the optimum stoichiometry lies within an exact precursor composition. In this study the films derived from the solution with a stoichiometry 1.00:2.00: 3.19 (Y:Ba:Cu) were optimal because of little Cu loss during baking and calcining process. Epitaxial growth of YBCO thin films on LAO is expected due to the single crystal substrate. In our previous studies [7,8], high-resolution transmission electron microscopy study has shown
B. Zhao et al. / Physica C 386 (2003) 348–352
351
Fig. 4. SEM photo micrographs showing the top surface of YBCO films on LAO.
Fig. 5. Ac susceptibility versus temperature for the thin films made from solutions A, B, C, D and E.
Fig. 6. Resistivity versus temperature for the YBCO films on LAO made from solutions A, B, C, D and E. (The curve for D is 1/2 q–T and the curve for E is 1/6 q–T .)
352
B. Zhao et al. / Physica C 386 (2003) 348–352
well-established epitaxial growth in fluorine-free sol–gel YBCO thin films on both LAO and YSZ substrates. The transport critical current densities of these films have reached the values of the order of 106 A/cm2 at 77 K and 0 T. According to crystallization model, the YBCO phase forms under controlled conditions, inheriting the orientation of the LAO single crystal due to the small lattice mismatch between YBCO and LAO, which will reduce the surface tension and energetically promote the nucleation of YBCO phase. In the phase diagram, YBCO is a line compound with a 1:2:3 composition and coexists with BaCuO2 and CuO phase in Cu-riched Y–Ba–Cu system below 900 °C. If the precursor is off-stoichiometry, the first YBCO lattice layer formed will push (or reject) other elements (it is mostly Cu and Ba in this study) away from its phase region. As the crystallization proceeds, further rejection will lead to accumulation of Cu and Ba along the direction of YBCO crystallization. Nonetheless, YBCO, as a line compound, will continue to grow with the exact ratio of 1:2:3. That is why the films from solution A, B and C show rather good texture in XRD patterns and sharp transition in RT curves. The rejected Cu and Ba may first precipitate along the grain boundaries of YBCO as BaCuO2 phase. That may be the reason for many fine (Cu, Ba)-riched clusters embedded in YBCO films from solutions with a little more Ba (such as solution C). However the precipitation of BaCuO2 cannot relax the enrichment of Cu in the system studied here (solution D and E). And at last at certain critical concentration, Cu cannot be fully contained in the matrix of YBCO and be push up to the surface of the film as observed in Fig. 4(d) and (e). Consistently, in films from solution A and B where the precursors are near stoichiometry, the rejected (Cu–Ba)-riched clusters is minimum with finely dispersed small particles in the film and a few Cu-riched particles are observed. Therefore, their transitions are sharp near 90 K. However, for both solution D and E, well-offstoichiometry makes initial nucleation of YBCO difficult in terms of the composition requirement.
Large amount of (Cu–Ba)-riched and Cu-riched phases formed in films may affect the orientation of the following nucleation of YBCO, which lead to a-axis YBCO grains emerging in the films. Certainly all these will results in poor superconducting properties.
4. Conclusion In summary, we have deposited YBCO thin films on LAO substrate using a new, fluorine-free sol–gel method. The effects of precursor stoichiometry on the superconducting properties of fluorine-free sol–gel YBCO thin films were studied by alterating the initial composition of the solution. The precise stoichiometry control of the precursor solution is critical in achieving high-quality YBCO thin films. Both XRD and SEM results have shown these sol–gel YBCO films on LAO exhibit bi-axial texture. The underlying mechanism on the effects of precursor stoichiometry is discussed. Acknowledgement This work was supported by China Postdoctoral Science Foundation.
References [1] A. Sheth, H. Schmidt, V. Lasrado, Appl. Supercond. 6 (1998) 855. [2] M.L. Kullberg, M.T. Lanagan, W. Wu, R.B. Poeppel, Supercond. Sci. Technol. 4 (1989) 337. [3] J.A. Smith, M.J. Cima, N. Sonnenberg, IEEE Trans. Appl. Supercond. 9 (1999) 1531. [4] S. Sathyamaurthy, K. Salama, IEEE Trans. Appl. Supercond. 11 (2001) 2935. [5] J.T. Dawley, P.G. Clem, M.P. Siegal, D.L. Overmyer, J. Mater. Res. 16 (2001) 13. [6] T. Araki, K. Yamagiwa, I. Hirabayashi, Cryogenics 41 (2001) 675. [7] D. Shi, Y. Xu, S. McClellan, R. Buchanan, Physics C 354 (2001) 71. [8] D. Shi, Y. Xu, S.X. Wang, et al., Physica C 371 (2002) 97.