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Bi-2223 bar current leads fabricated by the combination of cold isostatic pressing and hot-pressing
Physica C 320 Ž1999. 183–188
Bi-2223 bar current leads fabricated by the combination of cold isostatic pressing and hot-pressing X.K. Fu ) , V. Rouessac, Y.C. Guo, P.N. Mikheenko, H.K. Liu, S.X. Dou Institute for Superconducting and Electronic Materials, UniÕersity of Wollongong, Wollongong, NSW 2500, Australia Received 9 March 1999; received in revised form 12 May 1999; accepted 2 June 1999
Abstract A new method using a combination of cold isostatic pressing ŽCIP. and hot-pressing ŽHP. was applied to fabricate Bi-2223 bar current leads. The critical current density Ž Jc . achieved by this method reached as high as 1000 Arcm2 at 77 K and self-generated magnetic field. This value of Jc presented here is much higher than the best Jc reported for rod current leads which is 570 Arcm2 achieved by CIP technique wY. Yamada, Bi-based bulk current leads and their applications, in: H. Meada, K. Togano ŽEds.., Bismuth-based High-temperature Superconductors, 1996, p. 277.x. The phases and microstructures were analysed by XRD and SEM. The texture and weak link were studied by pole figures and AC susceptibility, respectively. The results show that the grain connectivity, matrix density and texture of the samples were improved significantly by this method. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Bi-2223; Cold isostatic pressing; Hot-pressing
1. Introduction The high-Tc current leads have attracted much attention, because the high-Tc oxides have a low thermal conductivity at temperatures below their Tc , which allows a reduction in the length of the current leads. Many attempts have already been carried out to use high-Tc oxide superconductors for current leads w2–4x. Bock et al. w5x the reported Bi-2212 current leads successfully fabricated by the melt casting process ŽMCP.. Plechacek and Hejtamanek w6x reported Bi-2223 tube current leads fabricated by cold isostatic press ŽCIP. with a critical current density of 1000 Arcm2 at 77 K. Rouessac et al. w7x applied the uniaxial sinter-forging method to fabri)
Corresponding author
cate well-textured Bi-2223 pellets with 10,000 Arcm2 . Up until now, the best Jc obtained for bulk tube current leads exceeds 1000 Arcm2 at 77 K and self-field, while the best Jc for the rod current leads by the CIP method is 570 Arcm2 at 77 K and self-field w1,6x. So far, no paper has reported on the application of hot-pressing to fabricate high-Tc current leads. In this paper, a new method of combination of CIP and hot-pressing was applied to fabricate Bi-2223 bar current leads. The critical current density achieved by this method reached as high as 1000 Arcm2 at 77 K and self-field. 2. Experimental procedure Precursor powder with a nominal composition of Bi 1.72 Pb 0.34 Sr1.83 Ca 1.97 Cu 3.13 O10 was prepared by
0921-4534r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 9 9 . 0 0 3 4 3 - 3
X.K. Fu et al.r Physica C 320 (1999) 183–188
184
spray pyrolysis of nitrate solutions. The powder was then pressed to rods with a dimension of around 0.9 cm in diameter and 15 cm in length. After CIP, the density of the rods was estimated to be 80% theoretical value. Bars were cut from the rods along longitudinal direction and sintered at 8458C for 80 h, followed by a process of hot-pressing at 8458C for 24 h with a pressure 5 MPa. The direction of hotpressing was perpendicular to the broad surfaces of the pellets. A post-annealing was conducted for the samples at 8258C for 24 h in a 7.5% O 2rbalance N2 atmosphere, followed by a slow cooling to 8008C and furnace cooling to room temperature. Transport critical current density was measured by the four probe method with a 1 mVrcm transition criterion. The microstructure of the samples was investigated and characterised by means of X-ray diffraction ŽXRD. and scanning electron microscopy ŽSEM.. The surface texture of the samples was analysed by the pole figure measurements. AC susceptibility measurements were carried out by a Model SR830 DSP lock-in amplifier and USA CTI-cryogenics. The dimensions of the samples used for AC susceptibility were 0.2 = 0.2 = 0.9 cm3.
3. Results and discussion 3.1. Transport property Fig. 1 is a photo of the Bi-2223 bar current lead. The dimension of this sample is 0.27 cm thick, 0.86 cm wide and 5.5 cm long. The I–V curve measured at 77 K and self-field is shown in Fig. 2. Current
Fig. 2. I – V curve for current leads fabricated by the combination of CIP and hot-pressing.
leads with various shapes and fabricated by different methods are compared in Table 1 w1,6x. It is seen that critical current Ž Ic . of 225 A and critical current density Ž Jc . 1000 Arcm2 at 77 K and self-field have been achieved by this combined CIP and hot-pressing method. These values are much higher than those of CIP and sintering method for rod production. 3.2. Microstructure Fig. 3 shows the typical XRD patterns for the sample surfaces perpendicular to hot-pressing direction before hot-pressing, after hot-pressing and after annealing in 7.5% O 2 balance with N2 . Fig. 3Ža. indicates that the samples mainly consisted of Bi2223 phase after CIP and annealing at 8458C for 80
Fig. 1. Photo of a Bi-2223 current lead fabricated by the combination of CIP and hot-pressing.
X.K. Fu et al.r Physica C 320 (1999) 183–188 Table 1 Comparison of transport properties of for Bi-2223 current leads prepared by different techniques Ž77 K, self-field. Sample shape
Fabrication method
Ic ŽA.
Jc ŽArcm2 .
Tube Rod Bar
CIP and sintering CIP and sintering combination of CIP and hot-pressing
990 220 225
1240 w6x 570 w1x 1000
h. No strong grain alignment was detected at this stage. Fig. 3Žb. shows that the degree of grain alignment of the matrix was significantly enhanced after hot-pressing, although a trace of Bi-2212 phase still existed in the samples. Fig. 3Žc. is the XRD pattern for the sample shown in Fig. 2 after annealing in low oxygen pressure. It is seen that the Bi-2212 phase was reduced and the grains further grew, indicated by the sharp diffraction peaks. The SEM images of the current leads are presented in Fig. 4. As shown in Fig. 4Ža., no substantial grain alignment is observed in the CIP processed sample, although the matrix was quite dense. Some voids were still present between the grains. Fig. 4Žb. is the typical microstructure of the sample after hot-pressing. It shows that both grain alignment and mass density were significantly improved compared
185
to the CIP processed sample. However, the texture is not as good as that in Bi-2223 tapes fabricated by the PIT method. Some misalignment angles between grains are above 158. Consequently, the grain connectivity is also not as good as that of Bi-2223 tapes. According to the railway-switch model, the smallangle c-axis tilt grain boundaries constitute strong connections and represent good paths for the supercurrent w8x. Due to the large average misalignment angle and the poor connection between grains, the critical current density of the current leads is lower than that of tape, which reaches ; 10 4 Arcm2 at 77 K and self-field. There was no substantial grain alignment for the bar current leads fabricated by CIP and sintering procedure. This has been qualitatively shown by XRD pattern in Fig. 3Ža. and SEM image in Fig. 4Ža.. To investigate the grain alignment of the hotpressed samples quantitatively, the Ž0010 . peak pole figure was measured on the sample surface and the result is shown in Fig. 5Ža.. Fig. 5Žb. is the curve of intensity vs. psi Ždegrees. calculated from Fig. 5Ža.. The average accumulative misalignment angles psi 50% and psi 90% are 98 and 328, respectively. These values indicated that the grains have a good alignment w7x.
Fig. 3. Typical XRD patterns of current leads Ža. before hot-pressing; Žb. after hot-pressing; and Žc. after annealing in 7.5% O 2 .
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X.K. Fu et al.r Physica C 320 (1999) 183–188
ture as the AC field amplitude increased from 0.307 to 1.534 Oe. In Fig. 6Ža., however, the imaginary part, x Y , appeared with two peaks in different AC field amplitudes: the first one representing the intragrain response and the second related to inter-grain coupling or second phase Bi-2212. The second peak position was about 95 K at low AC field amplitude 0.307 Oe and shifted to lower temperature near 85 K for higher AC field amplitude, indicting that it was not from the Bi-2212 phase contribution. Therefore,
Fig. 4. Typical SEM images for Bi-2223 current leads Ža. before hot-pressing and Žb. after hot-pressing.
3.3. Electromagnetic property Fig. 6Ža. and Žb. present the AC susceptibilities for two samples: one after CIP and the low oxygen pressure treatment, the other is after hot-pressing and low oxygen pressure treatment. The measurements were conducted with f s 117 Hz and no DC external field was applied. The intra-grain component is essentially insensitive to the change in the AC field amplitude HAC , while the coupling component is affected significantly by a change in amplitude HAC as small as 10y4 mT w9x. As seen in Fig. 6Žb., the transition of the real part, x X , is quite sharp for such poly-crystalline sample at temperature 109 K; and the imaginary part, x Y , appeared with only one peak in two different AC field amplitudes. Moreover, the peak of imaginary part, x Y , shifted to lower tempera-
Fig. 5. Ža. The Ž0010 . pole figure of Bi-2223 bar current lead fabricated by the combination of CIP and hot-pressing and Žb. average integrated intensity percentage vs. psi Ždegree., C Ž50%
X.K. Fu et al.r Physica C 320 (1999) 183–188
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the second peak was related to inter-grain coupling. This is in agreement with the results reported by other researchers w10x. This conclusion has been confirmed by the measurement of the third harmonic susceptibilities, which are shown Fig. 7Ža. and Žb.. Fig. 7Žb. indicated that the grain links are very strong. Fig. 7Ža. manifests that the first peak was intra-grain component and the second was the grain weak-link contribution. Compared to the curve of x Y vs. T in Fig. 6Žb., Fig. 6Ža. shows an additional peak at lower temperature which reflects the flux penetration into the boundaries between the grains. So, we
Fig. 7. Third harmonic susceptibility for Ža. after CIP and low oxygen pressure annealing and Žb. after hot-pressing and low oxygen annealing.
concluded that the later sample shown in Fig. 6Žb. has a much better inter-grain coupling than that shown Fig. 6Ža.. That is because the weak links were improved significantly by the hot-pressing process.
4. Conclusions
Fig. 6. AC susceptibility for Bi-2223 current leads Ža. after CIP and low oxygen annealing and Žb. after hot-pressing and low oxygen annealing.
1. Bi-2223 bar current leads were fabricated by the combination of CIP and hot-pressing method. Critical current density of the current leads produced by this method reached as high as 1000 Arcm2 at 77 K and self-field;
188
X.K. Fu et al.r Physica C 320 (1999) 183–188
2. The mass density, grain alignment and grain connectivity in the current leads tested were significantly improved after hot-pressing, compared to those current leads processed by the CIP method.
Acknowledgements The authors wish to thank the Australian Research Council, Australian Superconductor Ltd. in Metal Manufactures Ltd, and Department of Education, Training and Youth Affairs for financial support. Grateful thanks are given to Dr. T. Hardono and Dr. J.X. Jin for their help on measurement.
References w1x Y. Yamada, Bi-based bulk current leads and their applications, in: H. Meada, K. Togano ŽEds.., Bismuth-based Hightemperature Superconductors, 1996, p. 277.
w2x J.L. Wu, J.T. Dederer, P.W. Eckels, S.K. Singh, J.R. Hull, R.B. Poppel, C.A. Youngdahl, J.P. Singh, M.T. Lanagan, U. Balachandran, IEEE Trans. Magn. 27 Ž1991. 1861. w3x B. Dorri, K. Herd, E.T. Laskaris, J.E. Tkaczyk, K.W. Lay, IEEE Trans. Magn. 27 Ž1991. 1858. w4x J.R. Hull, IEEE Trans. Appl. Supercond. 3 Ž1993. 869. w5x J. Bock, H. Eckhardt, S. Elschner, in: Y. Bando, H. Yamanchi ŽEds.., Proceedings of the 5th International Symposium on Superconductivity ŽISS ’92., November 16–19, 1992, Kobe, Advances in Superconductivity V, SpringerVerlag, 1993, p. 643. w6x V. Plechacek, J. Hejtamanek, Physica C 282 Ž1997. 2577. w7x V. Rouessac, G. Poullain, J. Provost, M. Gomina, G. Desgardin, Eur. Phys. J. Appl. Phys. 2 Ž1998. 145. w8x R. Flukiger, G. Grasso, B. Hensel, M. Daumling, R. Gladshevskii, A. Jeremie, J.C. Grivel, A. Perin, Thermodynamics, microstructure, and critical current density in Bi, PbŽ2223. tapes, in: H. Meada, K. Togano ŽEds.., Bismuth-based Hightemperature Superconductors, 1996, p. 361. w9x H. Mazaki, K. Yamamoto, H. Yasuoko, Magnetic susceptibility of superconductors and other spin systems, in: R.A. Hein, T.L. Francavilla, D.H. Liebenberg ŽEds.., 1991, p. 333. w10x S. Celebi, I. Karaca, E. Ahsu, A. Gencer, Physica C 309 Ž1998. 131.
Bi-2223 bar current leads fabricated by the combination of cold isostatic pressing and hot-pressing X.K. Fu ) , V. Rouessac, Y.C. Guo, P.N. Mikheenko, H.K. Liu, S.X. Dou Institute for Superconducting and Electronic Materials, UniÕersity of Wollongong, Wollongong, NSW 2500, Australia Received 9 March 1999; received in revised form 12 May 1999; accepted 2 June 1999
Abstract A new method using a combination of cold isostatic pressing ŽCIP. and hot-pressing ŽHP. was applied to fabricate Bi-2223 bar current leads. The critical current density Ž Jc . achieved by this method reached as high as 1000 Arcm2 at 77 K and self-generated magnetic field. This value of Jc presented here is much higher than the best Jc reported for rod current leads which is 570 Arcm2 achieved by CIP technique wY. Yamada, Bi-based bulk current leads and their applications, in: H. Meada, K. Togano ŽEds.., Bismuth-based High-temperature Superconductors, 1996, p. 277.x. The phases and microstructures were analysed by XRD and SEM. The texture and weak link were studied by pole figures and AC susceptibility, respectively. The results show that the grain connectivity, matrix density and texture of the samples were improved significantly by this method. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Bi-2223; Cold isostatic pressing; Hot-pressing
1. Introduction The high-Tc current leads have attracted much attention, because the high-Tc oxides have a low thermal conductivity at temperatures below their Tc , which allows a reduction in the length of the current leads. Many attempts have already been carried out to use high-Tc oxide superconductors for current leads w2–4x. Bock et al. w5x the reported Bi-2212 current leads successfully fabricated by the melt casting process ŽMCP.. Plechacek and Hejtamanek w6x reported Bi-2223 tube current leads fabricated by cold isostatic press ŽCIP. with a critical current density of 1000 Arcm2 at 77 K. Rouessac et al. w7x applied the uniaxial sinter-forging method to fabri)
Corresponding author
cate well-textured Bi-2223 pellets with 10,000 Arcm2 . Up until now, the best Jc obtained for bulk tube current leads exceeds 1000 Arcm2 at 77 K and self-field, while the best Jc for the rod current leads by the CIP method is 570 Arcm2 at 77 K and self-field w1,6x. So far, no paper has reported on the application of hot-pressing to fabricate high-Tc current leads. In this paper, a new method of combination of CIP and hot-pressing was applied to fabricate Bi-2223 bar current leads. The critical current density achieved by this method reached as high as 1000 Arcm2 at 77 K and self-field. 2. Experimental procedure Precursor powder with a nominal composition of Bi 1.72 Pb 0.34 Sr1.83 Ca 1.97 Cu 3.13 O10 was prepared by
0921-4534r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 9 9 . 0 0 3 4 3 - 3
X.K. Fu et al.r Physica C 320 (1999) 183–188
184
spray pyrolysis of nitrate solutions. The powder was then pressed to rods with a dimension of around 0.9 cm in diameter and 15 cm in length. After CIP, the density of the rods was estimated to be 80% theoretical value. Bars were cut from the rods along longitudinal direction and sintered at 8458C for 80 h, followed by a process of hot-pressing at 8458C for 24 h with a pressure 5 MPa. The direction of hotpressing was perpendicular to the broad surfaces of the pellets. A post-annealing was conducted for the samples at 8258C for 24 h in a 7.5% O 2rbalance N2 atmosphere, followed by a slow cooling to 8008C and furnace cooling to room temperature. Transport critical current density was measured by the four probe method with a 1 mVrcm transition criterion. The microstructure of the samples was investigated and characterised by means of X-ray diffraction ŽXRD. and scanning electron microscopy ŽSEM.. The surface texture of the samples was analysed by the pole figure measurements. AC susceptibility measurements were carried out by a Model SR830 DSP lock-in amplifier and USA CTI-cryogenics. The dimensions of the samples used for AC susceptibility were 0.2 = 0.2 = 0.9 cm3.
3. Results and discussion 3.1. Transport property Fig. 1 is a photo of the Bi-2223 bar current lead. The dimension of this sample is 0.27 cm thick, 0.86 cm wide and 5.5 cm long. The I–V curve measured at 77 K and self-field is shown in Fig. 2. Current
Fig. 2. I – V curve for current leads fabricated by the combination of CIP and hot-pressing.
leads with various shapes and fabricated by different methods are compared in Table 1 w1,6x. It is seen that critical current Ž Ic . of 225 A and critical current density Ž Jc . 1000 Arcm2 at 77 K and self-field have been achieved by this combined CIP and hot-pressing method. These values are much higher than those of CIP and sintering method for rod production. 3.2. Microstructure Fig. 3 shows the typical XRD patterns for the sample surfaces perpendicular to hot-pressing direction before hot-pressing, after hot-pressing and after annealing in 7.5% O 2 balance with N2 . Fig. 3Ža. indicates that the samples mainly consisted of Bi2223 phase after CIP and annealing at 8458C for 80
Fig. 1. Photo of a Bi-2223 current lead fabricated by the combination of CIP and hot-pressing.
X.K. Fu et al.r Physica C 320 (1999) 183–188 Table 1 Comparison of transport properties of for Bi-2223 current leads prepared by different techniques Ž77 K, self-field. Sample shape
Fabrication method
Ic ŽA.
Jc ŽArcm2 .
Tube Rod Bar
CIP and sintering CIP and sintering combination of CIP and hot-pressing
990 220 225
1240 w6x 570 w1x 1000
h. No strong grain alignment was detected at this stage. Fig. 3Žb. shows that the degree of grain alignment of the matrix was significantly enhanced after hot-pressing, although a trace of Bi-2212 phase still existed in the samples. Fig. 3Žc. is the XRD pattern for the sample shown in Fig. 2 after annealing in low oxygen pressure. It is seen that the Bi-2212 phase was reduced and the grains further grew, indicated by the sharp diffraction peaks. The SEM images of the current leads are presented in Fig. 4. As shown in Fig. 4Ža., no substantial grain alignment is observed in the CIP processed sample, although the matrix was quite dense. Some voids were still present between the grains. Fig. 4Žb. is the typical microstructure of the sample after hot-pressing. It shows that both grain alignment and mass density were significantly improved compared
185
to the CIP processed sample. However, the texture is not as good as that in Bi-2223 tapes fabricated by the PIT method. Some misalignment angles between grains are above 158. Consequently, the grain connectivity is also not as good as that of Bi-2223 tapes. According to the railway-switch model, the smallangle c-axis tilt grain boundaries constitute strong connections and represent good paths for the supercurrent w8x. Due to the large average misalignment angle and the poor connection between grains, the critical current density of the current leads is lower than that of tape, which reaches ; 10 4 Arcm2 at 77 K and self-field. There was no substantial grain alignment for the bar current leads fabricated by CIP and sintering procedure. This has been qualitatively shown by XRD pattern in Fig. 3Ža. and SEM image in Fig. 4Ža.. To investigate the grain alignment of the hotpressed samples quantitatively, the Ž0010 . peak pole figure was measured on the sample surface and the result is shown in Fig. 5Ža.. Fig. 5Žb. is the curve of intensity vs. psi Ždegrees. calculated from Fig. 5Ža.. The average accumulative misalignment angles psi 50% and psi 90% are 98 and 328, respectively. These values indicated that the grains have a good alignment w7x.
Fig. 3. Typical XRD patterns of current leads Ža. before hot-pressing; Žb. after hot-pressing; and Žc. after annealing in 7.5% O 2 .
186
X.K. Fu et al.r Physica C 320 (1999) 183–188
ture as the AC field amplitude increased from 0.307 to 1.534 Oe. In Fig. 6Ža., however, the imaginary part, x Y , appeared with two peaks in different AC field amplitudes: the first one representing the intragrain response and the second related to inter-grain coupling or second phase Bi-2212. The second peak position was about 95 K at low AC field amplitude 0.307 Oe and shifted to lower temperature near 85 K for higher AC field amplitude, indicting that it was not from the Bi-2212 phase contribution. Therefore,
Fig. 4. Typical SEM images for Bi-2223 current leads Ža. before hot-pressing and Žb. after hot-pressing.
3.3. Electromagnetic property Fig. 6Ža. and Žb. present the AC susceptibilities for two samples: one after CIP and the low oxygen pressure treatment, the other is after hot-pressing and low oxygen pressure treatment. The measurements were conducted with f s 117 Hz and no DC external field was applied. The intra-grain component is essentially insensitive to the change in the AC field amplitude HAC , while the coupling component is affected significantly by a change in amplitude HAC as small as 10y4 mT w9x. As seen in Fig. 6Žb., the transition of the real part, x X , is quite sharp for such poly-crystalline sample at temperature 109 K; and the imaginary part, x Y , appeared with only one peak in two different AC field amplitudes. Moreover, the peak of imaginary part, x Y , shifted to lower tempera-
Fig. 5. Ža. The Ž0010 . pole figure of Bi-2223 bar current lead fabricated by the combination of CIP and hot-pressing and Žb. average integrated intensity percentage vs. psi Ždegree., C Ž50%
X.K. Fu et al.r Physica C 320 (1999) 183–188
187
the second peak was related to inter-grain coupling. This is in agreement with the results reported by other researchers w10x. This conclusion has been confirmed by the measurement of the third harmonic susceptibilities, which are shown Fig. 7Ža. and Žb.. Fig. 7Žb. indicated that the grain links are very strong. Fig. 7Ža. manifests that the first peak was intra-grain component and the second was the grain weak-link contribution. Compared to the curve of x Y vs. T in Fig. 6Žb., Fig. 6Ža. shows an additional peak at lower temperature which reflects the flux penetration into the boundaries between the grains. So, we
Fig. 7. Third harmonic susceptibility for Ža. after CIP and low oxygen pressure annealing and Žb. after hot-pressing and low oxygen annealing.
concluded that the later sample shown in Fig. 6Žb. has a much better inter-grain coupling than that shown Fig. 6Ža.. That is because the weak links were improved significantly by the hot-pressing process.
4. Conclusions
Fig. 6. AC susceptibility for Bi-2223 current leads Ža. after CIP and low oxygen annealing and Žb. after hot-pressing and low oxygen annealing.
1. Bi-2223 bar current leads were fabricated by the combination of CIP and hot-pressing method. Critical current density of the current leads produced by this method reached as high as 1000 Arcm2 at 77 K and self-field;
188
X.K. Fu et al.r Physica C 320 (1999) 183–188
2. The mass density, grain alignment and grain connectivity in the current leads tested were significantly improved after hot-pressing, compared to those current leads processed by the CIP method.
Acknowledgements The authors wish to thank the Australian Research Council, Australian Superconductor Ltd. in Metal Manufactures Ltd, and Department of Education, Training and Youth Affairs for financial support. Grateful thanks are given to Dr. T. Hardono and Dr. J.X. Jin for their help on measurement.
References w1x Y. Yamada, Bi-based bulk current leads and their applications, in: H. Meada, K. Togano ŽEds.., Bismuth-based Hightemperature Superconductors, 1996, p. 277.
w2x J.L. Wu, J.T. Dederer, P.W. Eckels, S.K. Singh, J.R. Hull, R.B. Poppel, C.A. Youngdahl, J.P. Singh, M.T. Lanagan, U. Balachandran, IEEE Trans. Magn. 27 Ž1991. 1861. w3x B. Dorri, K. Herd, E.T. Laskaris, J.E. Tkaczyk, K.W. Lay, IEEE Trans. Magn. 27 Ž1991. 1858. w4x J.R. Hull, IEEE Trans. Appl. Supercond. 3 Ž1993. 869. w5x J. Bock, H. Eckhardt, S. Elschner, in: Y. Bando, H. Yamanchi ŽEds.., Proceedings of the 5th International Symposium on Superconductivity ŽISS ’92., November 16–19, 1992, Kobe, Advances in Superconductivity V, SpringerVerlag, 1993, p. 643. w6x V. Plechacek, J. Hejtamanek, Physica C 282 Ž1997. 2577. w7x V. Rouessac, G. Poullain, J. Provost, M. Gomina, G. Desgardin, Eur. Phys. J. Appl. Phys. 2 Ž1998. 145. w8x R. Flukiger, G. Grasso, B. Hensel, M. Daumling, R. Gladshevskii, A. Jeremie, J.C. Grivel, A. Perin, Thermodynamics, microstructure, and critical current density in Bi, PbŽ2223. tapes, in: H. Meada, K. Togano ŽEds.., Bismuth-based Hightemperature Superconductors, 1996, p. 361. w9x H. Mazaki, K. Yamamoto, H. Yasuoko, Magnetic susceptibility of superconductors and other spin systems, in: R.A. Hein, T.L. Francavilla, D.H. Liebenberg ŽEds.., 1991, p. 333. w10x S. Celebi, I. Karaca, E. Ahsu, A. Gencer, Physica C 309 Ž1998. 131.