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Ag composite tapes
PHYSlCA ELSEVIER
PhysicaC 341-348 (2000)2589-2590
, www.elsevier.nl/locate/physc
Fabrication of Heavily Pb-doped Bi2212/Ag Composite Tapes K. Sugita, K. Murakami, J. Shimoyama, K. D. Otzschi and K. Kishio Department of Superconductivity, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Heavily Pb-doped Bi2212/Ag composite tapes were fabricated by two methods, and their microstructures and superconducting properties were investigated. The amorphous-to-crystallization method was found to be effective for elimination of impurity phases, which grew largely by the conventional melt-solidification technique. 1. INTRODUCTION Bi2Sr2CaCu2Ox (Bi2212)/Ag tapes have been already well developed and they are known to show high critical current density (Jc) exceeding 105A/cm2 at low temperatures even under 20T. However, its weak flux pinning properties, which result in a drastic decrease of Jc at temperatures above 30K under magnetic fields, has been preventing its extensive applications. Recently, large amount of Pb-doping was found to dramatically improve the J¢ properties of Bi2212 single crystals, especially at high temperatures [1, 2]. This is due to the reduction of electromagnetic anisotropy [3] and generation of new pinning centers. Therefore, development of Pb-doped Bi2212/Ag tapes has been expected. In the present study, we have investigated the microstructures and superconducting properties of heavily Pbdoped tapes fabricated by two different methods. One is the conventional melt-solidification method, and the other is a two step synthesis, where amorphous Bi(Pb)2212 was prepared at first, and then annealed for crystallization. By the latter method, control of the grain size and elimination of impurity phases were expected by crystallization from a homogeneous amorphous state. 2. EXPERIMENTAL A slurry containing Pb-doped Bi2212 calcined powder with a nominal composition of Bit dPb06Sr]sCaCuEOx was cast into a 50/zm thick green sheet by the doctor-blade method. The green sheet was heated on a Ag tape (~50flm t) at 500"C in air to remove the organic binders and were uniaxially pressed after cooling down to room temperature. Following heat treatment was done
under reduced oxygen atmosphere, pO2 = 0.01atm, which is known to suppress the formation of Ca2PbO 4 phase [4]. For the melt-solidification, tapes were heated above the partial melting point, 830-860°C, kept for 10min, slowly cooled (5*C/h) to 780"C, and then cooled in the furnace. In the amorphous-to-crystallization method, amorphous Bi(Pb)2212 was prepared on Ag tape by quenching from 930-940°C in air prior to the crystallization annealing. Then it was annealed under p 0 2 = 0.01atm at 790-830°C for 20h-80h with several intermediate pressings at room temperature. X-ray diffraction analysis, electron probe microanalysis (EPMA) and scanning electron microscope (SEM) observation .were carried out to investigate the components and microstructures of the fabricated tapes. Transport dc was measured by the standard four-probe method at various temperatures under self-field. A criterion of l pV/cm was applied for the determination ofd~. 3. RESULTS AND DISCUSSION 3.1 Melt-solidified tapes Highly textured Pb-doped tapes were obtained by melt-solidification method. By optimizing the thickness of the superconducting layer (< 20pm), the tapes showed fairly good alignment of platelike grains throughout the tape. The average composition of the Bi(Pb)2212 grains was determined to be Bil.96Pbo45Sr20sCa10/Cu2o00~. Since large amount of Pb was successfully introduced, intragrain Jc was found to be largely improved compared with pure Bi2212/Ag tapes, which was confirmed by the magnetization measurement. However, the transport dc of these tapes were depressed to ~lxl04A/cm 2 (10K, 0T).
0921-4534/00/$- see frontmatter~, 2000 ElsevierScienceB.M All rights reserved. PII S0921-4534(00)01362-9
2590
K. Sugita et aL /Physica C 341-348 (2000) 2589-2590
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(a) quenched from 940°C in air
~
(b) annealed at 828°C underp02 = 0.0latin
IN
Figure 1. Back scattering electron image of Bi(Pb)2212/Ag melt-solidified tapes. Figure 1 shows a back scattering electron image of the melt-solidified tapes. Large impurity phases (-10#m), as seen as white regions, were observed at the surface of the samples. In addition, grain size of Bi(Pb)2212 was much larger than that of pure Bi2212 tapes, and therefore, cracks or misaligned morphology were also observed frequently. 3.2 A m o r p h o u s - t o - c r y s t a l l i z e d
_j i
° I
10
i
f
i
20 30 40 2 0 / deg(Cu- Ks) Figure 2. X-ray diffraction patterns of (a) amorphous Bi(Pb)2212/Ag, (b) fully processed tape.
tapes
Figure 2 shows the X-ray diffraction patterns of amorphous Bi(Pb)2212 before and after annealing. Amorphous tapes were successfully obtained by quenching from temperatures above 930"C. The caxis oriented Bi(Pb)2212 tapes were fabricated by annealing at 828"C for 20h. The morphology of the tapes was quite sensitive to this annealing temperature. The amorphous-to-crystallized tapes contained quite small amount of impurities compared with the melt-solidified ones, as seen in Figure 3. The average composition of the Bi(Pb)2212 grains was Bi171Pbo59Srl.94Cal.29Cu20oOx. However, the grain size became rather small (~20#m), and therefore, poor grain alignment was observed inside the superconducting layer. The transport J¢ of the tapes fabricated by this method was -3xl03A/cm 2 at 10K in 0T. 4. CONCLUSION Heavily Pb-doped Bi2212/Ag tapes prepared by melt-solidification or amorphous-to-crystallization method showed completely different microstructure. Further control of internal grain alignment is crucial in order to obtain high Jc tapes.
Figure 3. Back scattering electron image of amorphous-to-crystallized Bi(Pb)2212/Ag tapes. ACKNOWLEDGEMENT This study was supported, in part, by the Original Technology R & D Promotion Program from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. REEFERENCES 1. I. Chong et al. Science 276 (1997) 770. 2. J. Shimoyama et aL Physica C281 (1997) 69. 3. T. Motohashi etal. Phys. Rev. 1559(1999) 14080. 4. J. Shimoyama et aL Proc. ISS '97 "Advances in Superconductivity X" (1998) 279.
PhysicaC 341-348 (2000)2589-2590
, www.elsevier.nl/locate/physc
Fabrication of Heavily Pb-doped Bi2212/Ag Composite Tapes K. Sugita, K. Murakami, J. Shimoyama, K. D. Otzschi and K. Kishio Department of Superconductivity, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Heavily Pb-doped Bi2212/Ag composite tapes were fabricated by two methods, and their microstructures and superconducting properties were investigated. The amorphous-to-crystallization method was found to be effective for elimination of impurity phases, which grew largely by the conventional melt-solidification technique. 1. INTRODUCTION Bi2Sr2CaCu2Ox (Bi2212)/Ag tapes have been already well developed and they are known to show high critical current density (Jc) exceeding 105A/cm2 at low temperatures even under 20T. However, its weak flux pinning properties, which result in a drastic decrease of Jc at temperatures above 30K under magnetic fields, has been preventing its extensive applications. Recently, large amount of Pb-doping was found to dramatically improve the J¢ properties of Bi2212 single crystals, especially at high temperatures [1, 2]. This is due to the reduction of electromagnetic anisotropy [3] and generation of new pinning centers. Therefore, development of Pb-doped Bi2212/Ag tapes has been expected. In the present study, we have investigated the microstructures and superconducting properties of heavily Pbdoped tapes fabricated by two different methods. One is the conventional melt-solidification method, and the other is a two step synthesis, where amorphous Bi(Pb)2212 was prepared at first, and then annealed for crystallization. By the latter method, control of the grain size and elimination of impurity phases were expected by crystallization from a homogeneous amorphous state. 2. EXPERIMENTAL A slurry containing Pb-doped Bi2212 calcined powder with a nominal composition of Bit dPb06Sr]sCaCuEOx was cast into a 50/zm thick green sheet by the doctor-blade method. The green sheet was heated on a Ag tape (~50flm t) at 500"C in air to remove the organic binders and were uniaxially pressed after cooling down to room temperature. Following heat treatment was done
under reduced oxygen atmosphere, pO2 = 0.01atm, which is known to suppress the formation of Ca2PbO 4 phase [4]. For the melt-solidification, tapes were heated above the partial melting point, 830-860°C, kept for 10min, slowly cooled (5*C/h) to 780"C, and then cooled in the furnace. In the amorphous-to-crystallization method, amorphous Bi(Pb)2212 was prepared on Ag tape by quenching from 930-940°C in air prior to the crystallization annealing. Then it was annealed under p 0 2 = 0.01atm at 790-830°C for 20h-80h with several intermediate pressings at room temperature. X-ray diffraction analysis, electron probe microanalysis (EPMA) and scanning electron microscope (SEM) observation .were carried out to investigate the components and microstructures of the fabricated tapes. Transport dc was measured by the standard four-probe method at various temperatures under self-field. A criterion of l pV/cm was applied for the determination ofd~. 3. RESULTS AND DISCUSSION 3.1 Melt-solidified tapes Highly textured Pb-doped tapes were obtained by melt-solidification method. By optimizing the thickness of the superconducting layer (< 20pm), the tapes showed fairly good alignment of platelike grains throughout the tape. The average composition of the Bi(Pb)2212 grains was determined to be Bil.96Pbo45Sr20sCa10/Cu2o00~. Since large amount of Pb was successfully introduced, intragrain Jc was found to be largely improved compared with pure Bi2212/Ag tapes, which was confirmed by the magnetization measurement. However, the transport dc of these tapes were depressed to ~lxl04A/cm 2 (10K, 0T).
0921-4534/00/$- see frontmatter~, 2000 ElsevierScienceB.M All rights reserved. PII S0921-4534(00)01362-9
2590
K. Sugita et aL /Physica C 341-348 (2000) 2589-2590
'
I
'
I
'
I
'
(a) quenched from 940°C in air
~
(b) annealed at 828°C underp02 = 0.0latin
IN
Figure 1. Back scattering electron image of Bi(Pb)2212/Ag melt-solidified tapes. Figure 1 shows a back scattering electron image of the melt-solidified tapes. Large impurity phases (-10#m), as seen as white regions, were observed at the surface of the samples. In addition, grain size of Bi(Pb)2212 was much larger than that of pure Bi2212 tapes, and therefore, cracks or misaligned morphology were also observed frequently. 3.2 A m o r p h o u s - t o - c r y s t a l l i z e d
_j i
° I
10
i
f
i
20 30 40 2 0 / deg(Cu- Ks) Figure 2. X-ray diffraction patterns of (a) amorphous Bi(Pb)2212/Ag, (b) fully processed tape.
tapes
Figure 2 shows the X-ray diffraction patterns of amorphous Bi(Pb)2212 before and after annealing. Amorphous tapes were successfully obtained by quenching from temperatures above 930"C. The caxis oriented Bi(Pb)2212 tapes were fabricated by annealing at 828"C for 20h. The morphology of the tapes was quite sensitive to this annealing temperature. The amorphous-to-crystallized tapes contained quite small amount of impurities compared with the melt-solidified ones, as seen in Figure 3. The average composition of the Bi(Pb)2212 grains was Bi171Pbo59Srl.94Cal.29Cu20oOx. However, the grain size became rather small (~20#m), and therefore, poor grain alignment was observed inside the superconducting layer. The transport J¢ of the tapes fabricated by this method was -3xl03A/cm 2 at 10K in 0T. 4. CONCLUSION Heavily Pb-doped Bi2212/Ag tapes prepared by melt-solidification or amorphous-to-crystallization method showed completely different microstructure. Further control of internal grain alignment is crucial in order to obtain high Jc tapes.
Figure 3. Back scattering electron image of amorphous-to-crystallized Bi(Pb)2212/Ag tapes. ACKNOWLEDGEMENT This study was supported, in part, by the Original Technology R & D Promotion Program from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. REEFERENCES 1. I. Chong et al. Science 276 (1997) 770. 2. J. Shimoyama et aL Physica C281 (1997) 69. 3. T. Motohashi etal. Phys. Rev. 1559(1999) 14080. 4. J. Shimoyama et aL Proc. ISS '97 "Advances in Superconductivity X" (1998) 279.