- Email: [email protected]
Large-domain Nd-system superconductor prepared by controlling Nd–Ba substitution
Physica C 357±360 (2001) 713±715
www.elsevier.com/locate/physc
Large-domain Nd-system superconductor prepared by controlling Nd±Ba substitution S. Hatayama a, S. Kohayashi b, H. Kezuka a, S. Yoshizawa c,*, T. Yamaura a a
c
Tokyo Engineering University, 1404-1, Katakura-cho, Hachioji, Tokyo 191-0982, Japan b Dowa Mining Co. Ltd., 277-1, Tobuki-cho, Hachioji, Tokyo 191-0001, Japan Advanced Materials R&D Center, Department of Chemistry, Meisei University, 2-1-1, Hodokubo, Hino, Tokyo 191-8506, Japan Received 16 October 2000; accepted 13 February 2001
Abstract Nd-system superconductor prepared by melt growth method in a low oxygen atmosphere exhibits good superconductivity. It is dicult to fabricate a large grain of Nd-system superconductor, since the grain-growth rate in Nd-system superconductor was sensitively dependent on the degree of undercooling. In this study, we fabricated Nd-system superconductor by changing grain-growth temperature. The maximum Tc of 91 K and Jc of 3:0 104 A/cm2 were obtained in the sample grown at 945 °C. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 74.72.Bk Keywords: Nd-system; Grain-growth temperature; Degree of undercooling; Solid solution
1. Introduction NdBa2 Cu3 O7 y (Nd123) bulk superconductor fabricated by melt growth method in air exhibits poor superconductivity due to the substitution of Nd±Ba. It is known that the fabrication of Nd123 superconductor under low oxygen pressure suppresses the formation of Nd±Ba solid solution [1±3]. The characteristics of Nd-system superconductors depend sensitively on the grain-growth temperature and atmosphere. In the Nd-system superconductor the grain-growth rate was about twice and four times larger than those of Sm-
system and Y-system superconductors, respectively [4]. The melting point of Nd-system superconductor was also very sensitive to the oxygen atmosphere [4]. The optimum heat pattern to prepare high-performance Nd-system superconductor has not yet been obtained. In the present work, Nd-system superconductor was fabricated in a low oxygen atmosphere with changing the grain-growth temperature. On the basis of the lattice parameter of the superconductors measured with an X-ray diractometer, the eect of the undercooling on the superconductivity was studied. 2. Experimental
*
Corresponding author. Tel./fax: +81-42-591-7346. E-mail address: [email protected] (S. Yoshizawa).
The samples were synthesized by the melt growth method using a mixture of Nd123 and
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 3 2 7 - 6
714
S. Hatayama et al. / Physica C 357±360 (2001) 713±715
Nd422 powders in the molar ratio of Nd123: Nd422 1:0.2 with the addition of 10 wt.% of Ag and 0.5 wt.% of Pt. The samples were pressed into pellets 30 mm in diameter and 20 mm in thickness. The samples were heated to 1080°C for 24 h and held at the same temperature for 20 min, then the temperature was cooled for the grain growth to a certain temperature in the range from 991°C to 930°C. After the grain growth, the samples were cooled to room temperature. The melt growth was conducted in 0:4%O2 99:6%N2 . Oxygen annealing was performed at 700°C for 70 h and from 700°C to 250°C for 300 h. Tc and Jc values were measured with a vibrating sample magnetometer (VSM). The Jc value was estimated by using the Bean model. The sample size for magnetization measurements was about 2:5 2:5 2 mm3 . The lattice parameter of prepared sample was measured with an X-ray diractometer. The sample for the X-ray diractometer was pulverized by mortar.
3. Results and discussion All the samples were multi-grain, where the grain size was 3±5 mm in diameter. Fig. 1 shows the temperature dependence of the magnetization in Nd-system superconductors. The sample treated at 991°C did not exhibit superconductivity above 80 K and the other samples exhibit superconducting transitions above 90 K. Fig. 2 shows magnetic ®eld dependence of Jc at
Fig. 1. The temperature dependence of magnetization.
Fig. 2. Jc ±B curves of the sample at 77 K, Hkc-axis.
77 K. The highest Jc was achieved in the sample grain grown at 945°C. The grain-growth temperature dependence of the superconductivity is summarized in Fig. 3. Tc and Jc were optimized at around 945°C. Fig. 4 shows grain-growth temperature dependence of the lattice parameters. The maximum value of a-axis lattice parameter and the minimum value of b-axis lattice parameter were observed at around 945°C. For the c-axis, the lattice parameter shows the peak at around 945°C. Consequently it is found that the superconductivity of Nd-system superconductor is related to the change of the lattice parameters. It is known that depressed superconductivity was caused by the formation of Nd±Ba solid solution. The lattice parameter was dependent on the
Fig. 3. Grain-growth temperature dependence of superconductivity.
S. Hatayama et al. / Physica C 357±360 (2001) 713±715
715
during cooling process, where the degree of undercooling increases with passage of time. The grain grows under the large degree of undercooling for both samples treated at 991°C and 930°C. The large degree of undercooling leads to the high grain-growth rate, which causes an increase in the amount of the solid solution. 4. Summary Nd-system superconductors were grown in 0.4% oxygen atmosphere at dierent degrees of undercooling, which was dependent on the graingrowth temperature. It was found that the highest performance was obtained in the sample grown at 945°C, with Tc of 91 K and Jc of 3:0 104 A/cm2 , the superconductivity and the lattice parameters were aected by the grain-growth rate. It is suggested that the solid solution can be controlled by optimizing the grain-growth rate. Fig. 4. Grain-growth temperature dependence of (A) a- and b-axis lattice parameter and (B) c-axis lattice parameter.
References
solid solution [5] such that the dierence of a- and b-axis lattice parameter was reduced and c-axis lattice parameter was reduced with increasing Nd± Ba substitution. The samples in this study showed a similar tendency in the superconductivity and the lattice parameters. It is thus suggested that the solid solution is caused by unoptimized graingrowth temperature. The grain of the sample treated at 991°C does not grow at 991°C but grows
[1] M. Murakami, M. Morita, K. Doi, K. Miyamoto, Jpn. J. Appl. Phys. 28 (1989) L1189. [2] S. Kohayashi, Y. Ishikawa, S. Yoshizawa, H. Kojima, Advances in Superconductivity VI, Springer, Berlin, 1993, p. 795. [3] H.S. Chauhan, T. Mochida, M. Mulidhar, M. Murakami, Advances in Superconductivity X, Springer, 1998, p. 725. [4] S. Hatayama, S. Kohayashi, S. Yoshizawa, H. Kezuka, T. Yamaura, Abstract of the IEE Physical Sensor Workshop, 22 September, 2000, PHS-00-11. [5] K. Takita, H. Katoh, H. Akinaga, Jpn. J. Appl. Phys. 27 (1988) L57.
www.elsevier.com/locate/physc
Large-domain Nd-system superconductor prepared by controlling Nd±Ba substitution S. Hatayama a, S. Kohayashi b, H. Kezuka a, S. Yoshizawa c,*, T. Yamaura a a
c
Tokyo Engineering University, 1404-1, Katakura-cho, Hachioji, Tokyo 191-0982, Japan b Dowa Mining Co. Ltd., 277-1, Tobuki-cho, Hachioji, Tokyo 191-0001, Japan Advanced Materials R&D Center, Department of Chemistry, Meisei University, 2-1-1, Hodokubo, Hino, Tokyo 191-8506, Japan Received 16 October 2000; accepted 13 February 2001
Abstract Nd-system superconductor prepared by melt growth method in a low oxygen atmosphere exhibits good superconductivity. It is dicult to fabricate a large grain of Nd-system superconductor, since the grain-growth rate in Nd-system superconductor was sensitively dependent on the degree of undercooling. In this study, we fabricated Nd-system superconductor by changing grain-growth temperature. The maximum Tc of 91 K and Jc of 3:0 104 A/cm2 were obtained in the sample grown at 945 °C. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 74.72.Bk Keywords: Nd-system; Grain-growth temperature; Degree of undercooling; Solid solution
1. Introduction NdBa2 Cu3 O7 y (Nd123) bulk superconductor fabricated by melt growth method in air exhibits poor superconductivity due to the substitution of Nd±Ba. It is known that the fabrication of Nd123 superconductor under low oxygen pressure suppresses the formation of Nd±Ba solid solution [1±3]. The characteristics of Nd-system superconductors depend sensitively on the grain-growth temperature and atmosphere. In the Nd-system superconductor the grain-growth rate was about twice and four times larger than those of Sm-
system and Y-system superconductors, respectively [4]. The melting point of Nd-system superconductor was also very sensitive to the oxygen atmosphere [4]. The optimum heat pattern to prepare high-performance Nd-system superconductor has not yet been obtained. In the present work, Nd-system superconductor was fabricated in a low oxygen atmosphere with changing the grain-growth temperature. On the basis of the lattice parameter of the superconductors measured with an X-ray diractometer, the eect of the undercooling on the superconductivity was studied. 2. Experimental
*
Corresponding author. Tel./fax: +81-42-591-7346. E-mail address: [email protected] (S. Yoshizawa).
The samples were synthesized by the melt growth method using a mixture of Nd123 and
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 3 2 7 - 6
714
S. Hatayama et al. / Physica C 357±360 (2001) 713±715
Nd422 powders in the molar ratio of Nd123: Nd422 1:0.2 with the addition of 10 wt.% of Ag and 0.5 wt.% of Pt. The samples were pressed into pellets 30 mm in diameter and 20 mm in thickness. The samples were heated to 1080°C for 24 h and held at the same temperature for 20 min, then the temperature was cooled for the grain growth to a certain temperature in the range from 991°C to 930°C. After the grain growth, the samples were cooled to room temperature. The melt growth was conducted in 0:4%O2 99:6%N2 . Oxygen annealing was performed at 700°C for 70 h and from 700°C to 250°C for 300 h. Tc and Jc values were measured with a vibrating sample magnetometer (VSM). The Jc value was estimated by using the Bean model. The sample size for magnetization measurements was about 2:5 2:5 2 mm3 . The lattice parameter of prepared sample was measured with an X-ray diractometer. The sample for the X-ray diractometer was pulverized by mortar.
3. Results and discussion All the samples were multi-grain, where the grain size was 3±5 mm in diameter. Fig. 1 shows the temperature dependence of the magnetization in Nd-system superconductors. The sample treated at 991°C did not exhibit superconductivity above 80 K and the other samples exhibit superconducting transitions above 90 K. Fig. 2 shows magnetic ®eld dependence of Jc at
Fig. 1. The temperature dependence of magnetization.
Fig. 2. Jc ±B curves of the sample at 77 K, Hkc-axis.
77 K. The highest Jc was achieved in the sample grain grown at 945°C. The grain-growth temperature dependence of the superconductivity is summarized in Fig. 3. Tc and Jc were optimized at around 945°C. Fig. 4 shows grain-growth temperature dependence of the lattice parameters. The maximum value of a-axis lattice parameter and the minimum value of b-axis lattice parameter were observed at around 945°C. For the c-axis, the lattice parameter shows the peak at around 945°C. Consequently it is found that the superconductivity of Nd-system superconductor is related to the change of the lattice parameters. It is known that depressed superconductivity was caused by the formation of Nd±Ba solid solution. The lattice parameter was dependent on the
Fig. 3. Grain-growth temperature dependence of superconductivity.
S. Hatayama et al. / Physica C 357±360 (2001) 713±715
715
during cooling process, where the degree of undercooling increases with passage of time. The grain grows under the large degree of undercooling for both samples treated at 991°C and 930°C. The large degree of undercooling leads to the high grain-growth rate, which causes an increase in the amount of the solid solution. 4. Summary Nd-system superconductors were grown in 0.4% oxygen atmosphere at dierent degrees of undercooling, which was dependent on the graingrowth temperature. It was found that the highest performance was obtained in the sample grown at 945°C, with Tc of 91 K and Jc of 3:0 104 A/cm2 , the superconductivity and the lattice parameters were aected by the grain-growth rate. It is suggested that the solid solution can be controlled by optimizing the grain-growth rate. Fig. 4. Grain-growth temperature dependence of (A) a- and b-axis lattice parameter and (B) c-axis lattice parameter.
References
solid solution [5] such that the dierence of a- and b-axis lattice parameter was reduced and c-axis lattice parameter was reduced with increasing Nd± Ba substitution. The samples in this study showed a similar tendency in the superconductivity and the lattice parameters. It is thus suggested that the solid solution is caused by unoptimized graingrowth temperature. The grain of the sample treated at 991°C does not grow at 991°C but grows
[1] M. Murakami, M. Morita, K. Doi, K. Miyamoto, Jpn. J. Appl. Phys. 28 (1989) L1189. [2] S. Kohayashi, Y. Ishikawa, S. Yoshizawa, H. Kojima, Advances in Superconductivity VI, Springer, Berlin, 1993, p. 795. [3] H.S. Chauhan, T. Mochida, M. Mulidhar, M. Murakami, Advances in Superconductivity X, Springer, 1998, p. 725. [4] S. Hatayama, S. Kohayashi, S. Yoshizawa, H. Kezuka, T. Yamaura, Abstract of the IEE Physical Sensor Workshop, 22 September, 2000, PHS-00-11. [5] K. Takita, H. Katoh, H. Akinaga, Jpn. J. Appl. Phys. 27 (1988) L57.