- Email: [email protected]
Thermal and phase characterization of Bi-2223 superconductors
Physica B 321 (2002) 257–264
Thermal and phase characterization of Bi-2223 superconductors M.N. Khana,*, M. Khizara, B.N. Mukashevb b
a GIK Institute of Engineering Science and Technology, Topi, NWFP, 23460 Swabi, Pakistan Institute of Physics and Technology, Ministry of Sciences, Academy of Sciences, Republic of Kazakhstan
Abstract Advances in processing and fabrication of high critical current density ðJc Þ long length conductors, silver sheathed superconducting magnets and coils from silver sheathed Bi-2223 tapes, pancake coil magnets from 1 to 10 m length and mono-multifilament Bi-2223/2212 high critical temperature superconductors by powder in tube technique continue to bring these materials closer to commercial applications. In our laboratory Bi-2223 conductors doped with Sm, Nb, Ag and Gd were prepared by the heat treatment of rapidly quenched glass precursors. Activation energies and frequency factors, employing different models were evaluated. It was observed that both peritectic transition and reaction rates were dependent on ambient atmosphere. The resistivity measurements revealed that the critical temperature decreases with increasing Gd, Sm, Nb, and Ag concentration. The critical current density Jc measured at 77 K from I2V data shows an increase with silver addition. The critical current densities of silver sheathed tapes using the powder in tube were found to be in the range 7 108 A/m2. X-ray diffraction results showed that the volume fraction of the high-Tc (2223) phase decreases and that of low-Tc (2212) phase increases with increase of these rare-earth ion contents. These results are explained on the basis of possible variation of hole concentration with trivalent rare-earth ion substitution and also by considering the magnetic nature of the substituted ion in the composition. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Phase characterization; Critical current density; Silver sheated Bi-2223 tapes; Peritectic transition
1. Introduction One of the most important applications of highTc superconductors is in the form of superconducting wires. Wires may be used for power transmission, electrical magnets for MRI, scientific equipment, particle accelerators and mineral separators. In these cases, conventional designs indicate that meter-lengths of wires are required for viable applications. Thus, good performance of *Corresponding author. E-mail address: [email protected] (M.N. Khan).
long lengths of wire, i.e. high Jc at various magnetic fields, is required. Critical current densities (Jc ’s) of short length of Bi2Sr2Ca1Cu2O8+x (Bi-2212) tapes processed on silver substrates by partial-melt slow cooling (PMSC) have reached sufficiently high values (B5.9 105 A/cm at 4.2 K and zero applied field) [1] for practical applications. Recently, Bi-2212/ Ag/Bi-2212 double-sided uncovered (DSU) tap sections 1.5 m in length were prepared by dipcoating and PMSC with long-length transport critical current densities (Jc ’s) reduced to 25% of short-length values [2]. A disadvantage of the DSU
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tape configuration is that the oxide layers break significantly more under bending strain than other tape configurations such as powder in tube. Alternatively, powder-in-tube (PIT) methodology has been used to process tape sections up to 300 m in length for the (Bi,Pb)2Sr2Ca2Cu3O10+x/Ag system. Fabrication of high Jc long-length conductors, their various micro structural and processing problems and some alternative approaches are described in the literature [3,4]. Discovery of cuprate HTS with transition temperatures over 130 K has created opportunities for many microwave and antenna applications and holds out promise of future digital application [4]. A major recent development towards the commercialization of high-Tc superconductors is the use of these materials for viable superconducting motors. US Department of Energy and American Superconductor Corporation have already demonstrated a 149 kW motor [5] operating at 1800 rpm with the coils at a temperature of 27 K. For power cables, only rolled multifilamentary power-in-tube (Bi,Pb)2Sr2Ca2Cu3Ox(Bi-2223) tape currently appears practicable. The main limitation with Bi2223 is its anisotropy and difficulty in reproducing high critical current densities ðJc Þ obtained on some very short, pressed samples. This is due to the granular nature of Bi-2223 and short coherence lengths in HTS. We have made a wide range of Jc studies [6,7] for Ba2Cu3O7 and Bi2Sr2Ca2Cu3O10 (BSCCO) in both wire and bulk forms. Although high Jc ’s have been obtained for Bi2Sr2Ca2Cu3O10 by a PIT method, there is no clear evidence for effective pinning centers in the BSCCO. On the other hand, it has been pointed out [8] that Y2BaCuO5 can provide pinning centers and/or can be introduced by specific processing method. Moreover, studies have revealed that Yba2Cu3O7 has intrinsically superior characteristics for pinning centers than that of Bi- and Ti-based superconductors [9]. In this paper, we have summarized the progress made in improving the manufacturability of these brittle materials and report out results of various ion substitutions for Ca in Bi1.7Pb0.3Sr2Ca2xRx Cu3Oy (where R=Nb, Eu, Yb and Ag) on superconducting, structural, electrical and kinetic parameters.
2. Experimental A solid-state reaction scheme was chosen for the powder synthesis. PbO, Bi2O3, SrCo3 and CuO were prepared by decomposition of metal nitrate solution in the cation ratio Bi1.72Pb0.3Sr2Ca2xNbxCu3Oy. The powder was calcined at 8201C for 10 h, uniaxially pressed into pellets, and sintered at 8401C for different times in order to alter the phase assemblage. It was found that nitrate solution route provided highly reactive, homogenous and carbonate-free powders from which the 2223 phase can be formed effectively. It should be, however, pointed out that at this stage the powders contain a small fraction of the 2223 and the 2212 is the major phase. This incompletely reacted precursor will facilitate the liquid phase formation in the subsequent processes. The average particle size was 2–3 mm. The pulverized gray powder was melted in a covered alumina crucible in an electric furnace at 12501C for 0.5 h. The crucible was removed from the furnace and melt was cast quickly on to a steel plate (30 mm thick by 14 cm diameter) and sheets of glass B1 mm thick. From the same crucible the melts were pumped up into fused silica tubes (E2 mm diameter and 30 cm long) resulting in the formation of the crystal-free glassy-rod specimens. The inner glass rods were removed mechanically from the outer silica tube for the subsequent experiment. For the heat treatment the glassy rods were cut into 20-mm long pieces. The samples were placed on an alumina plate and re-heated in air at a heating rate of 21C/min to a given temperature (840–8551C) at which they were held for various times so as to be crystallized and then cooled to room temperature in the furnace. Fabrication of long-length PIT Bi2Sr2CaCu2O8+x/Ag tapes is described elsewhere [10,11]. Differential thermal analysis (DTA) was carried out in various atmospheres at different heating rates using a Perkins Elmer DSC-7. The kinetics of crystallization of the samples were investigated from non-isothermal DTA using a Perkin Elmer DTA-7 at a heating rate of 5–201C/min. Glass samples were annealed in air for various lengths of time at temperatures selected from the DTA results. Crystalline phases formed in the annealed
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specimens were identified from the powder XRD patterns, recorded at room temperature. The electrical resistance was measured from 77 to 160 K by the standard four-probe configuration with 1.0 mV/cm used as the criteria for Jc measurements.
3. Results and discussions 3.1. Improvements in the properties of PIT BSCCO tapes Advances in the processing and fabrication of mono and multifilament Bi-2223/2212 high critical temperature superconductors by the PIT technique continue to bring these materials closer to commercial application. Consistently high critical current densities ðJc Þ >104 A/cm2 have been achieved along the entire length of a 1260 m long, 35 filament Bi-2223 tape [12]. Fig. 1 shows XRD pattern of bismuth family of superconductors. Orthorhombic lattice cell parameters were calculated and they are presented in Table 1. The values are in good agreement with the available literature. Resistivity of bulk samples were measured using Van der Pauwn four-probe method and the results are shown in Fig. 2. Closed cycle He cryostat was used for resistivity measurements down to 15 K for 2212 and 2223 phases.
259
Metallic behavior and sharp transitions are observed at 77 K for 2212 and 99 K for 2223. Fig. 3 shows the DTA curve of a typical Agsheathed 2212 tape [13]. The endotherm at 8841C was due to incongruent melting of the 2212 phase. During cooling, an exotherm was observed between 8501C and 8651C. This peak, as will be discussed later, was due to solidification of 2212 phase. The delay in solidification relative to melting occurred because nucleation required under cooling to overcome the nucleation energy barrier to form critical nuclei. Fig. 4 shows X-ray diffraction patterns of the undoped (with x ¼ 0) samples annealed at 8501C for 25 h (A) and 240 h (B) in the air in the 2y range up to 701. The intensity and peak positions of (0 0 2), (0 0 1 0), (1 1 5), (1 0 9), (0 0 1 2), (1 1 9), (2 0 0) and (0 0 1 4) reflections are in good agreement with the values reported in the literature [6,12,13] for the high Tc 2223 phase. It is evident from Fig. 4 that the intensity of reflections Table 1 Lattice parameters of orthorhombic phases of bismuth family of superconductors System
( a (A)
( b (A)
( c (A)
Tc:zero (K)
2201 2212 2223
5.375 5.230 5.218
5.310 5.412 5.632
24.525 29.180 30.815
— 77 100
Fig. 1. XRD Patterns of Bi-family of superconductors (a) 2223 phase (b) 2212 phase and (c) 2201 phase.
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Fig. 2. Resistivity plots for 2212 and 2223 phases.
Fig. 3. DTA curve for Ag-sheathed 2212 tape [13].
corresponding to high-Tc phases increased and those corresponding to low Tc phases decreased with increasing sintering time. The ‘c’ value of ( and that of low-Tc phase is high-Tc phase is 38 A 31 A1. The peaks at 2y ¼ 4:41 and 4.61 correspond to the (0 0 2) reflections of the high- and low-Tc phases, respectively. In the sample sintered for 25 h, distinct peaks at 2y ¼ 4:41 and 4:61 appeared indicating that the sample consisted of both low- and high-Tc phases. With increasing sintering time one can notice a gradual increase in the intensity of the peak at 2y ¼ 4:41 corresponding to (0 0 2) reflection of the high-Tc phase with a decrease in the intensity of peak at 2y ¼ 4:61: For the sample sintered for 240 h the low-Tc phase peak at 2y ¼ 4:61 is almost absent and peaks corresponding to the high-Tc phase are relatively sharper. These results indicate that on heating the glass Bi2Sr2CaoCu1O8 (2201) phase crystallizes out first followed by the formation of 80 K Bi2Sr2Ca1Cu2O8 (2212) phase at the higher temperature. The 110 K Tc phase is formed at still higher temperature just below the melting point, probably by reaction between the low-Tc 2201 and 2212 phases. Fig. 5 shows XRD patterns for Bi1.7Sr2Ca2xAgxCu3Oy ðx ¼ 0Þ annealed at 8501C for 240 h in air. We obtained the values of lattice constants ( b ¼ 5:20 A ( and c ¼ 37:75 A. ( The lattice a ¼ 5:60 A, constants for the sample annealed for 25 h are
Fig. 4. X-ray diffraction patterns for Bi1.65Pb0.32AgxSr1.73Ca1.33Cu3.85Oy superconductor with A ¼ 25 h, B ¼ 240 h. The peaks denoted by (H) and (L) correspond to the high-Tc (2223) and low-Tc (2212) peaks, respectively.
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Fig. 5. XRD for Bi1.7Pb0.3Sr2Ca2AgxCu2O ðx ¼ 0Þ annealed.
Fig. 7. Annealing time dependence of the current density Jc of Bi1.7Pb0.3Sr2Ca2xAgxCu3Oy ðx ¼ 0Þ:
Fig. 6. Temperature dependence of resistance Bi1.68Pb0.32 Ag0.6Sr1.73Ca1.85Cu2.85Oy. Fig. 8. Temperature dependence of resistance Bi1.7Pb0.3Sr2 Ca2xAgxCu3Oy.
( b ¼ 5:51 A ( and c ¼ 30:82 A. ( This cona ¼ 5:39 A, firmed that the high-Tc (2223) and low-Tc (2212) phases differ mainly in the length of c-axis. Fig. 6 shows the temperature dependence of resistance of Bi1.68Pb0.32Ag0.6Sr1.73Ca1.85Cu2.85Oy. It is evident that the material exhibits a semiconducting behavior. In fact, the doping of silver in our samples decreases the Tc ð0Þ which varied from 109 to 7 K with ðx ¼ 0:020:6Þ: Fig. 7 shows the annealing time dependence of critical current density ðJc Þ at
77 K under zero magnetic fields. From Fig. 7 one can notice a clear increase in Jc with sintering time. Fig. 8 shows the temperature dependence of resistance for Bi1.7Pb0.32Sr2Ca2xAgxCu3Oy. The DTA curve (Fig. 9) of the polycrystalline material showed that the glass transition temperature was about 4961C and the crystallization temperature was 5201C. The liquid temperature of the glass is about 8601C. A typical DTA scan of (Bi1.6Pb0.4)Sr1.7Ca2.3Cu3Oy glass showing the glass
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Fig. 9. DTA of Bi1.68Pb0.32Sr1.75Ca1.85Cu2.85Oy glasses melted at 12501C.
was very slow. We note that the endothermic peak appears at 779.81C in the bulk sample. It is of particular interest to clarify the origin of this endothermic peak because the Bi2Sr2Ca2Cu3Ox phase precipitates and grows above this endothermic temperature. It is clear that the reaction among the Bi2Sr3xCaxOy ðx ¼ 1Þ CuO and Bi2Sr2CuOx phases in the interior of the sample is closely related at around 779.81C. On the basis of these results, the crystallization mechanism may be speculated as follows. The 2212 phase first precipitates out followed by the formation of 2212 phase at higher temperatures. The 110 K Tc phase is formed at 861.51C just below the melting point, probably by reaction between the low-Tc 2201 and 2212 phases and the residual calcium and copper oxides. The variable heating rate DSC method was used to evaluate the kinetics of crystallization. The values of kinetic parameters are determined by using the kinetic model of Bansal and Doremus [14] which is expressed by the relation: In½TP2 =a ¼ In½E=R In v þ E=RTP ;
ð1Þ
where TP is the peak maximum temperature, a the heating rate, E the activation energy, R the gas
Fig. 10. A typical scan of (Bi1.6Pb0.4)Sr1.7Ca2.3Cu3Oy glass in argon atmosphere at a heating rate of 101C/min.
transition ðTg Þ and crystallization temperature ðTx Þ is depicted in Fig. 10. The Tg and Tx temperatures for this system are 3801C and 4631C, respectively. Several other exothermic and endothermic peaks are also observed at temperatures above Tx : The thermogravimetry ðTg Þ curve in air for the bulk sample is also shown in Fig. 10. The weight gain begins around 7001C and the maximum gain occurs at about 8131C. These results indicate that the diffusion of oxygen into the interior of the sample and at temperatures below around 6001C
Fig. 11. DTA curves in N2 atmosphere for niobium-doped samples recorded at heating rates.
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263
from X-ray diffractions, decreases with increasing dopant concentration. The c lattice parameter is also found to decrease with increase in dopant concentration. Thus the volume fraction of the 2212 phase depends on the c lattice parameter. Our results have revealed that the Ag-sheathed tapes should be fabricated from the BPSCCO pellets having high concentration (>50%) of 2223 phase and sintered for short duration at lower temperature better Jc values. The critical current densities of silver-sheathed tapes were found to be in the range of 7 108 Am2. These results are explained on the basis of possible variation of hole concentration with trivalent rare-earth ion substitution and also by considering the magnetic nature of the substituted ion in the composition.
Fig. 12. A plot of LnðTP2 =aÞ vs ð1=TP Þ for the boron-doped sample.
Acknowledgements
constant and v the frequency factor. The kinetic parameters (E and v) are related to the reaction rate constant ðKÞ by an Arrhenius equation:
The support funded by GIK Institute of Engineering Sciences and Technology is gratefully acknowledged. The authors would like to thank Ministry of Sciences of the Republic of Kazakhstan and Ministry or Science and Technology of the Islamic Republic of Pakistan for providing financial support for this project.
K ¼ v exp ½E=RT:
ð2Þ
Eq. (1) is an extension of Johnson–Mehl–Avremi isothermal kinetic model for use in non-isothermal methods. In Eq. (1) it was assumed that the rate of reaction is maximum compensated DSC. DTA runs recorded at different heating rates between 51C/min and 201C/min in an N atmosphere are shown in Fig. 11. A plot of In½TP2 =a vs. ½1000=TP for crystallization of glass was linear as shown in Fig. 12, verifying applicability of the kinetic model. From linear-square fitting, values of the kinetic parameters were calculated.
4. Conclusions The silver addition process continues to show considerable potential for production of silversheathed tapes with confirmed HTSC technological applications. We have investigated the kinetics of the lead-doped 2223 phase superconductors. The volume fraction of the 2223 phase, determined
References [1] J. Shimoyama, N. Tomita, T. Morimoto, H. Kitaguchi, H. Kumakura, J. Appl. Phys. 31 (1992) L1328. [2] J. Shimoyama, T. Morimoto, Jpn. J. Appl. Phys. 31 (1992) L163. [3] S. Jin, in: Proceedings of the Symposium on Processing of Long Lengths of Superconductors, Pittsburgh, Pennsylvania, USA, October 17–21, 1993, pp. 3–22. [4] U. Balachandran, et al., J. Mater. 11 (4) (1996) 19. [5] R&D magazine 38 (7) (1996) 14. [6] M.N. Khan, A. Memon, S. Al-Dallal, Physica C 235–240 (1994) 491. [7] M.N. Khan, A.N. Kayani, A-ul-Haq, J. Mater. Sci. 33 (1998) 2365. [8] T.J. Doi, M. Okada, K. Higashiyama, T. Kamo, S. Matsuda, International Workshop on Superconductivity, Co-sponsored by ISTEC and MRS, 1992, p. 315. [9] M. Murakami, Cryogenics 30 (1990) 390. [10] J.Y. Huang, et al., J. Electron. Mater. 24 (1995) 1793. [11] M.N. Khan, M. Khizar, M.M. Ahmed, B.N. Mukashew, in: M. Akhavan, J. Jenson, K. Kitazawa (Eds.), Proceedings
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of the First Regional Conference on Magnetic and Superconducting Materials, Vol. A, World Scientific, Singapore, 2000, p. 153. [12] R. Flukiger, B. Hensel, A. Jeremie, M. Decroux, H. Kupfur, W. Jahn, E. Seibt, W. Goldacker, Y. Yamada, J.Q. Xu, Supercond. Sci. Technol. 5 (1992) S61.
[13] C.-T. Wu, K.C. Goretta, M.T. Lanagan, A.C. Biondo, R.B. Poeppel, in: U. Balachandran, E.W. Collings, A. Goyal (Eds.), Processing of Long Lengths of Conductors, The Minerals, Metals & Materials Society, 1994, p. 101. [14] N.P. Bansal, R.H. Doremus, J. Thermal Anal. 29 (1984) 115.
Thermal and phase characterization of Bi-2223 superconductors M.N. Khana,*, M. Khizara, B.N. Mukashevb b
a GIK Institute of Engineering Science and Technology, Topi, NWFP, 23460 Swabi, Pakistan Institute of Physics and Technology, Ministry of Sciences, Academy of Sciences, Republic of Kazakhstan
Abstract Advances in processing and fabrication of high critical current density ðJc Þ long length conductors, silver sheathed superconducting magnets and coils from silver sheathed Bi-2223 tapes, pancake coil magnets from 1 to 10 m length and mono-multifilament Bi-2223/2212 high critical temperature superconductors by powder in tube technique continue to bring these materials closer to commercial applications. In our laboratory Bi-2223 conductors doped with Sm, Nb, Ag and Gd were prepared by the heat treatment of rapidly quenched glass precursors. Activation energies and frequency factors, employing different models were evaluated. It was observed that both peritectic transition and reaction rates were dependent on ambient atmosphere. The resistivity measurements revealed that the critical temperature decreases with increasing Gd, Sm, Nb, and Ag concentration. The critical current density Jc measured at 77 K from I2V data shows an increase with silver addition. The critical current densities of silver sheathed tapes using the powder in tube were found to be in the range 7 108 A/m2. X-ray diffraction results showed that the volume fraction of the high-Tc (2223) phase decreases and that of low-Tc (2212) phase increases with increase of these rare-earth ion contents. These results are explained on the basis of possible variation of hole concentration with trivalent rare-earth ion substitution and also by considering the magnetic nature of the substituted ion in the composition. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Phase characterization; Critical current density; Silver sheated Bi-2223 tapes; Peritectic transition
1. Introduction One of the most important applications of highTc superconductors is in the form of superconducting wires. Wires may be used for power transmission, electrical magnets for MRI, scientific equipment, particle accelerators and mineral separators. In these cases, conventional designs indicate that meter-lengths of wires are required for viable applications. Thus, good performance of *Corresponding author. E-mail address: [email protected] (M.N. Khan).
long lengths of wire, i.e. high Jc at various magnetic fields, is required. Critical current densities (Jc ’s) of short length of Bi2Sr2Ca1Cu2O8+x (Bi-2212) tapes processed on silver substrates by partial-melt slow cooling (PMSC) have reached sufficiently high values (B5.9 105 A/cm at 4.2 K and zero applied field) [1] for practical applications. Recently, Bi-2212/ Ag/Bi-2212 double-sided uncovered (DSU) tap sections 1.5 m in length were prepared by dipcoating and PMSC with long-length transport critical current densities (Jc ’s) reduced to 25% of short-length values [2]. A disadvantage of the DSU
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tape configuration is that the oxide layers break significantly more under bending strain than other tape configurations such as powder in tube. Alternatively, powder-in-tube (PIT) methodology has been used to process tape sections up to 300 m in length for the (Bi,Pb)2Sr2Ca2Cu3O10+x/Ag system. Fabrication of high Jc long-length conductors, their various micro structural and processing problems and some alternative approaches are described in the literature [3,4]. Discovery of cuprate HTS with transition temperatures over 130 K has created opportunities for many microwave and antenna applications and holds out promise of future digital application [4]. A major recent development towards the commercialization of high-Tc superconductors is the use of these materials for viable superconducting motors. US Department of Energy and American Superconductor Corporation have already demonstrated a 149 kW motor [5] operating at 1800 rpm with the coils at a temperature of 27 K. For power cables, only rolled multifilamentary power-in-tube (Bi,Pb)2Sr2Ca2Cu3Ox(Bi-2223) tape currently appears practicable. The main limitation with Bi2223 is its anisotropy and difficulty in reproducing high critical current densities ðJc Þ obtained on some very short, pressed samples. This is due to the granular nature of Bi-2223 and short coherence lengths in HTS. We have made a wide range of Jc studies [6,7] for Ba2Cu3O7 and Bi2Sr2Ca2Cu3O10 (BSCCO) in both wire and bulk forms. Although high Jc ’s have been obtained for Bi2Sr2Ca2Cu3O10 by a PIT method, there is no clear evidence for effective pinning centers in the BSCCO. On the other hand, it has been pointed out [8] that Y2BaCuO5 can provide pinning centers and/or can be introduced by specific processing method. Moreover, studies have revealed that Yba2Cu3O7 has intrinsically superior characteristics for pinning centers than that of Bi- and Ti-based superconductors [9]. In this paper, we have summarized the progress made in improving the manufacturability of these brittle materials and report out results of various ion substitutions for Ca in Bi1.7Pb0.3Sr2Ca2xRx Cu3Oy (where R=Nb, Eu, Yb and Ag) on superconducting, structural, electrical and kinetic parameters.
2. Experimental A solid-state reaction scheme was chosen for the powder synthesis. PbO, Bi2O3, SrCo3 and CuO were prepared by decomposition of metal nitrate solution in the cation ratio Bi1.72Pb0.3Sr2Ca2xNbxCu3Oy. The powder was calcined at 8201C for 10 h, uniaxially pressed into pellets, and sintered at 8401C for different times in order to alter the phase assemblage. It was found that nitrate solution route provided highly reactive, homogenous and carbonate-free powders from which the 2223 phase can be formed effectively. It should be, however, pointed out that at this stage the powders contain a small fraction of the 2223 and the 2212 is the major phase. This incompletely reacted precursor will facilitate the liquid phase formation in the subsequent processes. The average particle size was 2–3 mm. The pulverized gray powder was melted in a covered alumina crucible in an electric furnace at 12501C for 0.5 h. The crucible was removed from the furnace and melt was cast quickly on to a steel plate (30 mm thick by 14 cm diameter) and sheets of glass B1 mm thick. From the same crucible the melts were pumped up into fused silica tubes (E2 mm diameter and 30 cm long) resulting in the formation of the crystal-free glassy-rod specimens. The inner glass rods were removed mechanically from the outer silica tube for the subsequent experiment. For the heat treatment the glassy rods were cut into 20-mm long pieces. The samples were placed on an alumina plate and re-heated in air at a heating rate of 21C/min to a given temperature (840–8551C) at which they were held for various times so as to be crystallized and then cooled to room temperature in the furnace. Fabrication of long-length PIT Bi2Sr2CaCu2O8+x/Ag tapes is described elsewhere [10,11]. Differential thermal analysis (DTA) was carried out in various atmospheres at different heating rates using a Perkins Elmer DSC-7. The kinetics of crystallization of the samples were investigated from non-isothermal DTA using a Perkin Elmer DTA-7 at a heating rate of 5–201C/min. Glass samples were annealed in air for various lengths of time at temperatures selected from the DTA results. Crystalline phases formed in the annealed
M.N. Khan et al. / Physica B 321 (2002) 257–264
specimens were identified from the powder XRD patterns, recorded at room temperature. The electrical resistance was measured from 77 to 160 K by the standard four-probe configuration with 1.0 mV/cm used as the criteria for Jc measurements.
3. Results and discussions 3.1. Improvements in the properties of PIT BSCCO tapes Advances in the processing and fabrication of mono and multifilament Bi-2223/2212 high critical temperature superconductors by the PIT technique continue to bring these materials closer to commercial application. Consistently high critical current densities ðJc Þ >104 A/cm2 have been achieved along the entire length of a 1260 m long, 35 filament Bi-2223 tape [12]. Fig. 1 shows XRD pattern of bismuth family of superconductors. Orthorhombic lattice cell parameters were calculated and they are presented in Table 1. The values are in good agreement with the available literature. Resistivity of bulk samples were measured using Van der Pauwn four-probe method and the results are shown in Fig. 2. Closed cycle He cryostat was used for resistivity measurements down to 15 K for 2212 and 2223 phases.
259
Metallic behavior and sharp transitions are observed at 77 K for 2212 and 99 K for 2223. Fig. 3 shows the DTA curve of a typical Agsheathed 2212 tape [13]. The endotherm at 8841C was due to incongruent melting of the 2212 phase. During cooling, an exotherm was observed between 8501C and 8651C. This peak, as will be discussed later, was due to solidification of 2212 phase. The delay in solidification relative to melting occurred because nucleation required under cooling to overcome the nucleation energy barrier to form critical nuclei. Fig. 4 shows X-ray diffraction patterns of the undoped (with x ¼ 0) samples annealed at 8501C for 25 h (A) and 240 h (B) in the air in the 2y range up to 701. The intensity and peak positions of (0 0 2), (0 0 1 0), (1 1 5), (1 0 9), (0 0 1 2), (1 1 9), (2 0 0) and (0 0 1 4) reflections are in good agreement with the values reported in the literature [6,12,13] for the high Tc 2223 phase. It is evident from Fig. 4 that the intensity of reflections Table 1 Lattice parameters of orthorhombic phases of bismuth family of superconductors System
( a (A)
( b (A)
( c (A)
Tc:zero (K)
2201 2212 2223
5.375 5.230 5.218
5.310 5.412 5.632
24.525 29.180 30.815
— 77 100
Fig. 1. XRD Patterns of Bi-family of superconductors (a) 2223 phase (b) 2212 phase and (c) 2201 phase.
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Fig. 2. Resistivity plots for 2212 and 2223 phases.
Fig. 3. DTA curve for Ag-sheathed 2212 tape [13].
corresponding to high-Tc phases increased and those corresponding to low Tc phases decreased with increasing sintering time. The ‘c’ value of ( and that of low-Tc phase is high-Tc phase is 38 A 31 A1. The peaks at 2y ¼ 4:41 and 4.61 correspond to the (0 0 2) reflections of the high- and low-Tc phases, respectively. In the sample sintered for 25 h, distinct peaks at 2y ¼ 4:41 and 4:61 appeared indicating that the sample consisted of both low- and high-Tc phases. With increasing sintering time one can notice a gradual increase in the intensity of the peak at 2y ¼ 4:41 corresponding to (0 0 2) reflection of the high-Tc phase with a decrease in the intensity of peak at 2y ¼ 4:61: For the sample sintered for 240 h the low-Tc phase peak at 2y ¼ 4:61 is almost absent and peaks corresponding to the high-Tc phase are relatively sharper. These results indicate that on heating the glass Bi2Sr2CaoCu1O8 (2201) phase crystallizes out first followed by the formation of 80 K Bi2Sr2Ca1Cu2O8 (2212) phase at the higher temperature. The 110 K Tc phase is formed at still higher temperature just below the melting point, probably by reaction between the low-Tc 2201 and 2212 phases. Fig. 5 shows XRD patterns for Bi1.7Sr2Ca2xAgxCu3Oy ðx ¼ 0Þ annealed at 8501C for 240 h in air. We obtained the values of lattice constants ( b ¼ 5:20 A ( and c ¼ 37:75 A. ( The lattice a ¼ 5:60 A, constants for the sample annealed for 25 h are
Fig. 4. X-ray diffraction patterns for Bi1.65Pb0.32AgxSr1.73Ca1.33Cu3.85Oy superconductor with A ¼ 25 h, B ¼ 240 h. The peaks denoted by (H) and (L) correspond to the high-Tc (2223) and low-Tc (2212) peaks, respectively.
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Fig. 5. XRD for Bi1.7Pb0.3Sr2Ca2AgxCu2O ðx ¼ 0Þ annealed.
Fig. 7. Annealing time dependence of the current density Jc of Bi1.7Pb0.3Sr2Ca2xAgxCu3Oy ðx ¼ 0Þ:
Fig. 6. Temperature dependence of resistance Bi1.68Pb0.32 Ag0.6Sr1.73Ca1.85Cu2.85Oy. Fig. 8. Temperature dependence of resistance Bi1.7Pb0.3Sr2 Ca2xAgxCu3Oy.
( b ¼ 5:51 A ( and c ¼ 30:82 A. ( This cona ¼ 5:39 A, firmed that the high-Tc (2223) and low-Tc (2212) phases differ mainly in the length of c-axis. Fig. 6 shows the temperature dependence of resistance of Bi1.68Pb0.32Ag0.6Sr1.73Ca1.85Cu2.85Oy. It is evident that the material exhibits a semiconducting behavior. In fact, the doping of silver in our samples decreases the Tc ð0Þ which varied from 109 to 7 K with ðx ¼ 0:020:6Þ: Fig. 7 shows the annealing time dependence of critical current density ðJc Þ at
77 K under zero magnetic fields. From Fig. 7 one can notice a clear increase in Jc with sintering time. Fig. 8 shows the temperature dependence of resistance for Bi1.7Pb0.32Sr2Ca2xAgxCu3Oy. The DTA curve (Fig. 9) of the polycrystalline material showed that the glass transition temperature was about 4961C and the crystallization temperature was 5201C. The liquid temperature of the glass is about 8601C. A typical DTA scan of (Bi1.6Pb0.4)Sr1.7Ca2.3Cu3Oy glass showing the glass
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Fig. 9. DTA of Bi1.68Pb0.32Sr1.75Ca1.85Cu2.85Oy glasses melted at 12501C.
was very slow. We note that the endothermic peak appears at 779.81C in the bulk sample. It is of particular interest to clarify the origin of this endothermic peak because the Bi2Sr2Ca2Cu3Ox phase precipitates and grows above this endothermic temperature. It is clear that the reaction among the Bi2Sr3xCaxOy ðx ¼ 1Þ CuO and Bi2Sr2CuOx phases in the interior of the sample is closely related at around 779.81C. On the basis of these results, the crystallization mechanism may be speculated as follows. The 2212 phase first precipitates out followed by the formation of 2212 phase at higher temperatures. The 110 K Tc phase is formed at 861.51C just below the melting point, probably by reaction between the low-Tc 2201 and 2212 phases and the residual calcium and copper oxides. The variable heating rate DSC method was used to evaluate the kinetics of crystallization. The values of kinetic parameters are determined by using the kinetic model of Bansal and Doremus [14] which is expressed by the relation: In½TP2 =a ¼ In½E=R In v þ E=RTP ;
ð1Þ
where TP is the peak maximum temperature, a the heating rate, E the activation energy, R the gas
Fig. 10. A typical scan of (Bi1.6Pb0.4)Sr1.7Ca2.3Cu3Oy glass in argon atmosphere at a heating rate of 101C/min.
transition ðTg Þ and crystallization temperature ðTx Þ is depicted in Fig. 10. The Tg and Tx temperatures for this system are 3801C and 4631C, respectively. Several other exothermic and endothermic peaks are also observed at temperatures above Tx : The thermogravimetry ðTg Þ curve in air for the bulk sample is also shown in Fig. 10. The weight gain begins around 7001C and the maximum gain occurs at about 8131C. These results indicate that the diffusion of oxygen into the interior of the sample and at temperatures below around 6001C
Fig. 11. DTA curves in N2 atmosphere for niobium-doped samples recorded at heating rates.
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from X-ray diffractions, decreases with increasing dopant concentration. The c lattice parameter is also found to decrease with increase in dopant concentration. Thus the volume fraction of the 2212 phase depends on the c lattice parameter. Our results have revealed that the Ag-sheathed tapes should be fabricated from the BPSCCO pellets having high concentration (>50%) of 2223 phase and sintered for short duration at lower temperature better Jc values. The critical current densities of silver-sheathed tapes were found to be in the range of 7 108 Am2. These results are explained on the basis of possible variation of hole concentration with trivalent rare-earth ion substitution and also by considering the magnetic nature of the substituted ion in the composition.
Fig. 12. A plot of LnðTP2 =aÞ vs ð1=TP Þ for the boron-doped sample.
Acknowledgements
constant and v the frequency factor. The kinetic parameters (E and v) are related to the reaction rate constant ðKÞ by an Arrhenius equation:
The support funded by GIK Institute of Engineering Sciences and Technology is gratefully acknowledged. The authors would like to thank Ministry of Sciences of the Republic of Kazakhstan and Ministry or Science and Technology of the Islamic Republic of Pakistan for providing financial support for this project.
K ¼ v exp ½E=RT:
ð2Þ
Eq. (1) is an extension of Johnson–Mehl–Avremi isothermal kinetic model for use in non-isothermal methods. In Eq. (1) it was assumed that the rate of reaction is maximum compensated DSC. DTA runs recorded at different heating rates between 51C/min and 201C/min in an N atmosphere are shown in Fig. 11. A plot of In½TP2 =a vs. ½1000=TP for crystallization of glass was linear as shown in Fig. 12, verifying applicability of the kinetic model. From linear-square fitting, values of the kinetic parameters were calculated.
4. Conclusions The silver addition process continues to show considerable potential for production of silversheathed tapes with confirmed HTSC technological applications. We have investigated the kinetics of the lead-doped 2223 phase superconductors. The volume fraction of the 2223 phase, determined
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