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Improved Jc properties of Na-doped MTG YBCO
January 2000
Materials Letters 42 Ž2000. 171–174 www.elsevier.comrlocatermatlet
Improved Jc properties of Na-doped MTG YBCO X.W. Zou, H. Zhang
)
Shanghai Institute of Metallurgy, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China Received 18 June 1999; accepted 30 July 1999
Abstract The effects of Na addition in the MTG YBCO ŽYBa 2yx Na x Cu 3 O y q 40 mol% Y2 BaCuO5 . system with x s 0.0, 0.1, 0.2 on Tc and Jc properties have been investigated. It was observed that with increase in x Ž x F 0.2., Tc,on is almost the same, but Tc,off decreases. With certain Na addition, MTG YBCO showed an improved critical current density characteristic and a second peak in the magnetization curve. These might be connected with the inhomogeneous distribution of Na, which can become pinning centers against the motion of the flux. The formation of lower Tc phase in Na-rich region induces the second peak with increasing magnetic fields. q 2000 Elsevier Science B.V. All rights reserved. Keywords: MTG YBCO; Na-doping; Jc properties
1. Introduction Since the discovery of superconductivity in YBCO, a large number of studies have been conducted to improve its microstructure and superconducting properties with a wide range of atomic substitution. For Na-doping, some reports have been published. It was clearly shown that the substitution of Na had an effect on the superconducting properties and growing kinetics. However, the influence of Na substitution on Tc was controversial. Some experimental data showed that with certain Na addition Ž- 0.2., Tc of YBCO was almost the same as that of pure YBCO sintered samples w1–3x. Mukherjee et al. w4x reported that, in Y1y a Ba 2yb Na xsaqbCu 3 O y system, the substitution of Na decreased Tc , but there is no change in 84 K of Tc,off when the dopant concentration varies from 0.05 to 0.2. Reddy et al. w5x )
Corresponding author. E-mail address: [email protected] ŽH. Zhang.
reported that the value of Tc,off of Na-doped YBCO samples could reach 95 K, 2–3 K higher than the usual value of Tc,off for an undoped YBCO samples. Siddiqi et al. w6x also found that Na-doping increased Tc of YBCO. On the other properties, it was shown that Na-doping decreased the peritectic temperature, improved the texture w1,4x, enhanced grain growth kinetics and superconducting phase formation of YBCO w6–8x. For example, in Y1y a Ba 2yb Na xsaqbCu 3 O y system, both the c-axis texture orientation index ŽI 005rI 113 . and grain size were obviously increased with Na-doping from x s 0 to 0.2 w4x. To our knowledge, most of these Na-doping experiments focused on the powder sintered samples, but melt-textured-growth YBCO bulks ŽMTG YBCO., which show a good prospect of applications at nitrogen temperature, were seldom involved. It is welldemonstrated now that MTG YBCO bulk has strong pinning and excellent critical current density characteristics at nitrogen temperature. For improving the performance of MTG Y123 bulk, the important ques-
00167-577Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 9 . 0 0 1 7 8 - 0
172
X.W. Zou, H. Zhangr Materials Letters 42 (2000) 171–174
tions are how to increase the flux pinning and how to grow a single domain in bulk. Considering the effect of Na-doping, which MTG may be processed at lower temperature, grain size and texture may be enhanced and superconducting properties may be improved, we have examined the influence of Na substitution on the superconducting properties, especially critical current density, of MTG YBCO. Indeed, our preliminary results showed that the Jc characteristics could be improved by Na substitution.
2. Experimental Samples of Na-doped YBCO ŽYBa 2y x Na x Cu 3 O y q 40 mol% Y2 BaCuO5 , x s 0.0, 0.1 and 0.2. were prepared by a MTG process. Correspondingly, samples were nominated as NY0, NY1, NY2 in sequence. The starting materials: Y2 O 3 Ž99.9%., BaCO 3 Ž99.9%., CuO Ž99.9%. and Na 2 C 2 O4 Žreagent grade. were mixed thoroughly and calcined twice at 8508C– 9008C for 24 h in air to facilitate the reactions. The mixtures were pressed into pellets of diameter 15 mm and thickness 10 mm under a pressure of 10 M Nmy2 , and calcined at 8508C–9008C for 24 h and then furnace-cooled. The sintered pellets were heated to 10408C–10508C from room temperature in 6 h and kept for half an hour, then they were quickly cooled to 10058C–10108C and followed by cooling slowly to 9408C at 18Crh rate and then were furnace-cooled to room temperature. For the measurement of superconducting properties, slab samples were cut from a single domain of the MTG pellets and oxygenated at 4008C for 120 h under flowing oxygen at 1 atm. Tc was measured in an AC susceptometer. DC magnetization curves were measured in a commercial vibrating sample magnetometer at 77.3 K in liquid nitrogen.
3. Results and discussion Fig. 1 shows the top surface morphology of NY2 sample. The single domain growth is evident from traces of the seeding crystal Sm123 at the center and the domain fronts emanating from the four sides of the seed to the periphery of disk. Compared with NY0 and NY1 sample, the top surface morphology
Fig. 1. View of the top surface morphology of the NY2 sample.
of NY2 sample is much better. This might be connected with improvement of the texture and increase of the crystal growth in the Ž ab . plane owing to the Na-doping, as Chen and Kumar w7,8x reported that Na-doping could enhance grain growth kinetics and superconducting phase formation. The AC susceptibility vs. the temperature curves of samples NY0, NY1 and NY2 in Fig. 2 gave the effects of Na on the Tc of MTG YBCO. It was observed that with Na content increasing, there is little change on Tc,on as x F 0.2 for these materials, but Tc,off decreases. That is, the larger the Na content in the composition, the broader is the superconducting transition regime. It implies that the substitution of Na induces YBCO composition inhomogeneities and lowers Tc phases with higher Na content. It was different from some results of the previous studies as mentioned before w1–3,6,9x. In the magnetization measurement, the larger surface of the slabs was perpendicular to the c-axis of YBCO grains as verified in X-ray diffraction patterns. The magnetic critical current density was calculated from the magnetization curve using the extended Bean model w10x. Fig. 3 gives the calculated values of Jc under applied field parallel to the c-axis. It was observed that in low magnetic field Ž H - 0.45 T., Jc,NY2 ŽI. ) Jc,NY0 ŽI. ) Jc,NY1ŽI., but in higher magnetic field Ž H ) 0.45 T., Jc,NY2 ŽI. )
X.W. Zou, H. Zhangr Materials Letters 42 (2000) 171–174
Jc,NY1ŽI. ) Jc,NY0 ŽI.. When the magnetic field is between 0.8 T and 1 T, both NY1 and NY2 samples show a second peak on the Jc –H curves, i.e., in magnetization curves and the Jc of NY2 at 77.3 K under 0.8 T reaches to 3.3 = 10 4 Arcm2 . By comparing Jc of these samples, it is concluded that certain Na addition can obviously improve the magnetic properties of MTG YBCO, especially at higher magnetic field. However, it should be noted that the critical current density under applied field perpendicular to the c-axis does not show the peak effect. As we know, there is not a universal viewpoint to understand the peak effect. Some researchers have proposed that the local inhomogeneities with weak superconductivity, which play the role of pinning centers, lead to the peak effect w11x, These inhomogeneities with weak superconductivity may be connected with secondary phases, defects or chemical inhomogeneities, which become normal with increasing magnetic fields. The oxygen-deficient phase in MTG YBCO is one of such lower Tc phases w12x. In addition, OCMG REBCO ŽRE: Nd, Sm. also showed the peak effect in magnetization curves and studies indicated that it was not due to the oxygen-deficient phase w13,14x, but could be well ascribed to the presence of RErBa spatial variation, which leads to lower Tc phase by the substitution of RE ion on Ba ion site in rich RE region. Here, MTG YBCO with
173
Fig. 3. The Jc versus the magnetic field H.
certain Na-doping shows the peak effect in fully oxygenated state, but it is not connected with the oxygen-deficient phase. It is possible that the local chemical inhomogeneous substitution of Na leads to the formation of a lower Tc phase as shown in Fig. 2. When magnetic fields increase, the lower Tc phases become normal to be pinning centers and induce the second peak effect in the Na-doping MTG YBCO system.
4. Conclusions
Fig. 2. a.c. susceptibility versus temperature.
The effects of Na addition in the MTG YBCO ŽYBa 2y x Na x Cu 3 O y q 40 mol% Y2 BaCuO5 . system with x s 0.0, 0.1, 0.2 on Tc and Jc properties have been investigated. It was observed that with x increase Ž x F 0.2., Tc,on is almost the same, but Tc,off decreases. That is, the larger the Na content in the composition, the broader is the superconducting transition regime. With certain Na addition, MTG YBCO showed an improved critical current density characteristic and a second peak in the magnetization curve. These might be connected with the inhomogeneous distribution of Na, which can become a pinning center against the motion of the flux, and, the formation of lower Tc phase in Na-rich region induces the second peak with increasing magnetic fields. At the
174
X.W. Zou, H. Zhangr Materials Letters 42 (2000) 171–174
same time, Na-doping improved the texture of MTG YBCO and resulted in an increase of the crystal growth in Ž ab . plane.
Acknowledgements The work was supported by The National Center for R & D on Superconductivity in China.
References w1x H. Wu, F. Zeng, H. Zhang, Chin. J. Low Temp. Phys. 16 Ž6. Ž1994. 453. w2x Z. Drzazga, H. Broda, F. Seidler, P. Bohm, H. Geus, D. ¨ Wohlleben, J. Magn. Magn. Mater. 83 Ž1990. 515. w3x J.L. Tallon, D.M. Pooke, M.P. Staines, M.E. Bowden, N.E. Flower, R.G. Buckley, M.R. Presland, Physica C 171 Ž1990. 61.
w4x P.S. Mukherjee, A. Simon, M.S. Sarma, Solid State Communications 81 Ž1992. 253. w5x R. Ravinder Reddy, B.V. Reddi, P. Venuyopal Reddy, Physica C 226 Ž1994. 61. w6x S.A. Siddiqi, B. Akhyar, J.A.A. Khan, J. Mater. Sci. Lett. 15 Ž1996. 1733. w7x C.H. Chen, B. Kumar, Appl. Phys. Lett. 63 Ž1993. 3072. w8x C.H. Chen, B. Kumar, J. Am. Ceram. Soc. 77 Ž1994. 1025. w9x R. Job, M. Rosenbery, K. Nahm, C.K. Kim, Supercond. Sci. Technol. 4 Ž1991. 77. w10x E.M. Gyong, R.B. Van Dover, K.A. Jackson, L.F. Schneemeyer, R.J. Waszczak, Appl. Phys. Lett. 55 Ž1989. 283. w11x M. Daeumling, J. Seuntijens, D. Larblestier, Nature 346 Ž1990. 715. w12x S.K. Tolpygo, J.Y. Lin, M. Gurvitch, S.Y. Hou, J.M. Phillips, Phys. Rev. B 53 Ž1996. 12462. w13x M. Murakami, N. Sakai, N. Chikumoto, H. Koju, T. Higuchi, S.I. Yoo, Physica C 282–287 Ž1997. 884. w14x X. Obradors, R. Yu, F. Sandiumenge, B. Martinez, N. Vilalta, V. Gomis, T. Puig, S. Pinol, Supercond. Sci. Technol. 10 Ž1997. 884.
Materials Letters 42 Ž2000. 171–174 www.elsevier.comrlocatermatlet
Improved Jc properties of Na-doped MTG YBCO X.W. Zou, H. Zhang
)
Shanghai Institute of Metallurgy, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China Received 18 June 1999; accepted 30 July 1999
Abstract The effects of Na addition in the MTG YBCO ŽYBa 2yx Na x Cu 3 O y q 40 mol% Y2 BaCuO5 . system with x s 0.0, 0.1, 0.2 on Tc and Jc properties have been investigated. It was observed that with increase in x Ž x F 0.2., Tc,on is almost the same, but Tc,off decreases. With certain Na addition, MTG YBCO showed an improved critical current density characteristic and a second peak in the magnetization curve. These might be connected with the inhomogeneous distribution of Na, which can become pinning centers against the motion of the flux. The formation of lower Tc phase in Na-rich region induces the second peak with increasing magnetic fields. q 2000 Elsevier Science B.V. All rights reserved. Keywords: MTG YBCO; Na-doping; Jc properties
1. Introduction Since the discovery of superconductivity in YBCO, a large number of studies have been conducted to improve its microstructure and superconducting properties with a wide range of atomic substitution. For Na-doping, some reports have been published. It was clearly shown that the substitution of Na had an effect on the superconducting properties and growing kinetics. However, the influence of Na substitution on Tc was controversial. Some experimental data showed that with certain Na addition Ž- 0.2., Tc of YBCO was almost the same as that of pure YBCO sintered samples w1–3x. Mukherjee et al. w4x reported that, in Y1y a Ba 2yb Na xsaqbCu 3 O y system, the substitution of Na decreased Tc , but there is no change in 84 K of Tc,off when the dopant concentration varies from 0.05 to 0.2. Reddy et al. w5x )
Corresponding author. E-mail address: [email protected] ŽH. Zhang.
reported that the value of Tc,off of Na-doped YBCO samples could reach 95 K, 2–3 K higher than the usual value of Tc,off for an undoped YBCO samples. Siddiqi et al. w6x also found that Na-doping increased Tc of YBCO. On the other properties, it was shown that Na-doping decreased the peritectic temperature, improved the texture w1,4x, enhanced grain growth kinetics and superconducting phase formation of YBCO w6–8x. For example, in Y1y a Ba 2yb Na xsaqbCu 3 O y system, both the c-axis texture orientation index ŽI 005rI 113 . and grain size were obviously increased with Na-doping from x s 0 to 0.2 w4x. To our knowledge, most of these Na-doping experiments focused on the powder sintered samples, but melt-textured-growth YBCO bulks ŽMTG YBCO., which show a good prospect of applications at nitrogen temperature, were seldom involved. It is welldemonstrated now that MTG YBCO bulk has strong pinning and excellent critical current density characteristics at nitrogen temperature. For improving the performance of MTG Y123 bulk, the important ques-
00167-577Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 9 . 0 0 1 7 8 - 0
172
X.W. Zou, H. Zhangr Materials Letters 42 (2000) 171–174
tions are how to increase the flux pinning and how to grow a single domain in bulk. Considering the effect of Na-doping, which MTG may be processed at lower temperature, grain size and texture may be enhanced and superconducting properties may be improved, we have examined the influence of Na substitution on the superconducting properties, especially critical current density, of MTG YBCO. Indeed, our preliminary results showed that the Jc characteristics could be improved by Na substitution.
2. Experimental Samples of Na-doped YBCO ŽYBa 2y x Na x Cu 3 O y q 40 mol% Y2 BaCuO5 , x s 0.0, 0.1 and 0.2. were prepared by a MTG process. Correspondingly, samples were nominated as NY0, NY1, NY2 in sequence. The starting materials: Y2 O 3 Ž99.9%., BaCO 3 Ž99.9%., CuO Ž99.9%. and Na 2 C 2 O4 Žreagent grade. were mixed thoroughly and calcined twice at 8508C– 9008C for 24 h in air to facilitate the reactions. The mixtures were pressed into pellets of diameter 15 mm and thickness 10 mm under a pressure of 10 M Nmy2 , and calcined at 8508C–9008C for 24 h and then furnace-cooled. The sintered pellets were heated to 10408C–10508C from room temperature in 6 h and kept for half an hour, then they were quickly cooled to 10058C–10108C and followed by cooling slowly to 9408C at 18Crh rate and then were furnace-cooled to room temperature. For the measurement of superconducting properties, slab samples were cut from a single domain of the MTG pellets and oxygenated at 4008C for 120 h under flowing oxygen at 1 atm. Tc was measured in an AC susceptometer. DC magnetization curves were measured in a commercial vibrating sample magnetometer at 77.3 K in liquid nitrogen.
3. Results and discussion Fig. 1 shows the top surface morphology of NY2 sample. The single domain growth is evident from traces of the seeding crystal Sm123 at the center and the domain fronts emanating from the four sides of the seed to the periphery of disk. Compared with NY0 and NY1 sample, the top surface morphology
Fig. 1. View of the top surface morphology of the NY2 sample.
of NY2 sample is much better. This might be connected with improvement of the texture and increase of the crystal growth in the Ž ab . plane owing to the Na-doping, as Chen and Kumar w7,8x reported that Na-doping could enhance grain growth kinetics and superconducting phase formation. The AC susceptibility vs. the temperature curves of samples NY0, NY1 and NY2 in Fig. 2 gave the effects of Na on the Tc of MTG YBCO. It was observed that with Na content increasing, there is little change on Tc,on as x F 0.2 for these materials, but Tc,off decreases. That is, the larger the Na content in the composition, the broader is the superconducting transition regime. It implies that the substitution of Na induces YBCO composition inhomogeneities and lowers Tc phases with higher Na content. It was different from some results of the previous studies as mentioned before w1–3,6,9x. In the magnetization measurement, the larger surface of the slabs was perpendicular to the c-axis of YBCO grains as verified in X-ray diffraction patterns. The magnetic critical current density was calculated from the magnetization curve using the extended Bean model w10x. Fig. 3 gives the calculated values of Jc under applied field parallel to the c-axis. It was observed that in low magnetic field Ž H - 0.45 T., Jc,NY2 ŽI. ) Jc,NY0 ŽI. ) Jc,NY1ŽI., but in higher magnetic field Ž H ) 0.45 T., Jc,NY2 ŽI. )
X.W. Zou, H. Zhangr Materials Letters 42 (2000) 171–174
Jc,NY1ŽI. ) Jc,NY0 ŽI.. When the magnetic field is between 0.8 T and 1 T, both NY1 and NY2 samples show a second peak on the Jc –H curves, i.e., in magnetization curves and the Jc of NY2 at 77.3 K under 0.8 T reaches to 3.3 = 10 4 Arcm2 . By comparing Jc of these samples, it is concluded that certain Na addition can obviously improve the magnetic properties of MTG YBCO, especially at higher magnetic field. However, it should be noted that the critical current density under applied field perpendicular to the c-axis does not show the peak effect. As we know, there is not a universal viewpoint to understand the peak effect. Some researchers have proposed that the local inhomogeneities with weak superconductivity, which play the role of pinning centers, lead to the peak effect w11x, These inhomogeneities with weak superconductivity may be connected with secondary phases, defects or chemical inhomogeneities, which become normal with increasing magnetic fields. The oxygen-deficient phase in MTG YBCO is one of such lower Tc phases w12x. In addition, OCMG REBCO ŽRE: Nd, Sm. also showed the peak effect in magnetization curves and studies indicated that it was not due to the oxygen-deficient phase w13,14x, but could be well ascribed to the presence of RErBa spatial variation, which leads to lower Tc phase by the substitution of RE ion on Ba ion site in rich RE region. Here, MTG YBCO with
173
Fig. 3. The Jc versus the magnetic field H.
certain Na-doping shows the peak effect in fully oxygenated state, but it is not connected with the oxygen-deficient phase. It is possible that the local chemical inhomogeneous substitution of Na leads to the formation of a lower Tc phase as shown in Fig. 2. When magnetic fields increase, the lower Tc phases become normal to be pinning centers and induce the second peak effect in the Na-doping MTG YBCO system.
4. Conclusions
Fig. 2. a.c. susceptibility versus temperature.
The effects of Na addition in the MTG YBCO ŽYBa 2y x Na x Cu 3 O y q 40 mol% Y2 BaCuO5 . system with x s 0.0, 0.1, 0.2 on Tc and Jc properties have been investigated. It was observed that with x increase Ž x F 0.2., Tc,on is almost the same, but Tc,off decreases. That is, the larger the Na content in the composition, the broader is the superconducting transition regime. With certain Na addition, MTG YBCO showed an improved critical current density characteristic and a second peak in the magnetization curve. These might be connected with the inhomogeneous distribution of Na, which can become a pinning center against the motion of the flux, and, the formation of lower Tc phase in Na-rich region induces the second peak with increasing magnetic fields. At the
174
X.W. Zou, H. Zhangr Materials Letters 42 (2000) 171–174
same time, Na-doping improved the texture of MTG YBCO and resulted in an increase of the crystal growth in Ž ab . plane.
Acknowledgements The work was supported by The National Center for R & D on Superconductivity in China.
References w1x H. Wu, F. Zeng, H. Zhang, Chin. J. Low Temp. Phys. 16 Ž6. Ž1994. 453. w2x Z. Drzazga, H. Broda, F. Seidler, P. Bohm, H. Geus, D. ¨ Wohlleben, J. Magn. Magn. Mater. 83 Ž1990. 515. w3x J.L. Tallon, D.M. Pooke, M.P. Staines, M.E. Bowden, N.E. Flower, R.G. Buckley, M.R. Presland, Physica C 171 Ž1990. 61.
w4x P.S. Mukherjee, A. Simon, M.S. Sarma, Solid State Communications 81 Ž1992. 253. w5x R. Ravinder Reddy, B.V. Reddi, P. Venuyopal Reddy, Physica C 226 Ž1994. 61. w6x S.A. Siddiqi, B. Akhyar, J.A.A. Khan, J. Mater. Sci. Lett. 15 Ž1996. 1733. w7x C.H. Chen, B. Kumar, Appl. Phys. Lett. 63 Ž1993. 3072. w8x C.H. Chen, B. Kumar, J. Am. Ceram. Soc. 77 Ž1994. 1025. w9x R. Job, M. Rosenbery, K. Nahm, C.K. Kim, Supercond. Sci. Technol. 4 Ž1991. 77. w10x E.M. Gyong, R.B. Van Dover, K.A. Jackson, L.F. Schneemeyer, R.J. Waszczak, Appl. Phys. Lett. 55 Ž1989. 283. w11x M. Daeumling, J. Seuntijens, D. Larblestier, Nature 346 Ž1990. 715. w12x S.K. Tolpygo, J.Y. Lin, M. Gurvitch, S.Y. Hou, J.M. Phillips, Phys. Rev. B 53 Ž1996. 12462. w13x M. Murakami, N. Sakai, N. Chikumoto, H. Koju, T. Higuchi, S.I. Yoo, Physica C 282–287 Ž1997. 884. w14x X. Obradors, R. Yu, F. Sandiumenge, B. Martinez, N. Vilalta, V. Gomis, T. Puig, S. Pinol, Supercond. Sci. Technol. 10 Ž1997. 884.