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Ag tapes
Physica C 296 Ž1998. 13–20
Effects of precursor powder particle size on critical current density and microstructure of Bi-2223rAg tapes J. Jiang ) , J.S. Abell School of Metallurgy and Materials, UniÕersity of Birmingham, Birmingham B15 2TT, UK Received 29 March 1997; revised 30 July 1997; accepted 20 October 1997
Abstract The effect of particle size distribution on the nature of the precursor powder and the subsequent phase formation and critical current density of silver sheathed Bi-2223 tapes has been studied. Three precursor powders with different particle size distributions were obtained by grinding the calcined powder for different times. The effects of precursor powder particle size were identified by a combination of XRD, SEM and EDX analysis. The results indicate that the phase formation, microstructure evolution and critical current density are sensitive to the precursor particle size distribution. It has been shown that a small particle size promotes the formation of the Bi-2223 phase, and that coarse powder leads to Ca 2 CuO 3 and Ca 2 PbO4 remaining in the final tape. It has been found that too fine a powder may cause the formation of large alkaline–earth cuprate grains wŽSr,Ca.14Cu 14O41 and ŽCa,Sr. 2 CuO 3 x. The critical current density tends to increase when the amount and size of non-superconducting phase decrease in the final tapes q 1998 Elsevier Science B.V. PACS: 74.72.Hs; 74.62.Bf; 85.25.Kx; 81.40.-z Keywords: Critical current density; Superconducting tape; Microstructure
1. Introduction S uperconducting Ž B i,P b . 2 S r 2 C a 2 C u 3 O x ŽBi-2223.rAg tapes fabricated by the oxide-powderin-tube ŽOPIT. technique have demonstrated the capacity to support large critical currents at liquid nitrogen temperature for large scale power engineering applications w1–7x, but problems of reproducibility and maintaining the high critical current values on scaling up to practical lengths still remain. The
)
Corresponding author. Fax: q44 121 414 5232; E-mail: [email protected]
best route to the development of a well-aligned single phase superconducting core within these tapes is yet to be established, and the phase assemblage of the precursor powders incorporated into the silver sheath, together with the optimum combination of drawing and pressing and heat treatments is the subject of much intense research. More complex issues such as the phase evolution routes associated with the extended heat treatment stages remain to be resolved w8–16x. The complexities of the phase diagram are well reported w17,18x, and the possible role of liquid phases in the phase evolution route remain obscure w10,11,16,19x. The Bi-2223 phase can equilibrate with Ca 2 CuO 3 , ŽSr,Ca.14 Cu 24 O41yx , CuO,
0921-4534r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 9 2 1 - 4 5 3 4 Ž 9 7 . 0 1 8 1 8 - 2
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J. Jiang, J.S. Abell r Physica C 296 (1998) 13–20
Pb-rich phase, Bi-2212 and the liquid w18x. One of the keys to preparing Bi-2223rAg tapes that exhibit high critical current density Ž Jc . is to control the size and distribution of the non-superconducting phases remaining in the superconducting core matrix after thermomechanical processing. Manipulation and control of the liquid phase have been reported to be crucial because the liquid phase is believed to promote the growth of Bi-2223 phase, and to heal the cracks caused by intermediate rolling and pressing w20x. The formation of the liquid phase during the heat treatment depends on the sintering temperature and the composition, the phase assemblage and the particle size distribution of the precursor powder w2,21,22x. The particle size distribution is one of the important parameters for controlling the quality of the precursor. Li et al. w2x suggested that a small particle size of the raw powders was extremely important for obtaining high Jc tapes. But, there have been few detailed studies of the effects of the particle size distribution reported in the literature. In this work, the effects of precursor powder particle size were identified by a combination of XRD, SEM, and EDX analysis. We found that an optimised particle size distribution is important for high Jc values of the Bi-2223rAg tapes.
2. Experimental Powders were prepared by the solid state reaction method. The raw materials used in this study were commercial reagents. The purity of Bi 2 O 3 , PbO, SrCO 3 , CaCO 3 , and CuO is 99.9%, 99.9%, 98%, 99% and 98%, respectively. The nominal composition of the precursor powder was Bi 1.8 Pb 0.4 Sr2.0Ca 2 .2 Cu 3 O x . The appropriate amounts of oxidercarbonate powders were mixed, pelletized and calcined in air at 8208C for 24 h w23x. The calcined pellets were ground in an automatic agate pulveriser. After grinding for 3, 4, and 5 h, the powder was taken as powder SP-1, SP-2 and SP-3, respectively. All powders were analysed by X-ray diffraction analysis. The particle size distributions of powders were analysed by a laser diffraction technique in a Coulter LS Particle Size Analyser. Powders were in an agglomerate state. In order to disperse the ag-
glomerates, the powders were subjected to ultrasonic agitation in glycerol prior to inserting in the sample cell. The powders were processed by the powder-intube method with the same processing parameters. High purity Ag tube Žfrom Advent Research Materials Ltd. was used, the dimensions of which were 6 mm outer diameter and 4 mm inner diameter. The precursor powder was loaded into the Ag tubes and densified by means of a steel piston. The powder density inside the tubes before deformation was of the order of 3 grcm3 , ; 50% of the theoretical density. The tubes were closed, and were deformed using two two-hammer swaging machines, down to a diameter of about 1 mm. The tapelike shape was given by rolling in several steps. Rolling cylinders of 120 mm in diameter and a rolling speed of about 1 mrmin have been used. The as-rolled tapes have a width of 2.2 mm and an overall thickness of 0.2 mm. Tapes made from powder SP-1, SP-2 and SP-3 were named as TP-1, TP-2 and TP-3, respectively. The tape samples were obtained by cutting as-rolled tapes into short sections of 3 cm. The tapes were sintered up to four times in air at a temperature in the range of 834–8428C for a total time between 180 and 240 h. The sintered tapes were pressed uniaxially at room temperature between sintering periods. The optimised Jc values were achieved by using intermediate pressing with a pressure of 2 GPa w24x. In this work, all the tapes were deformed with this optimised pressure between sintering.
Fig. 1. X-ray diffraction patterns of powder SP-1, SP-2 and SP-3.
J. Jiang, J.S. Abell r Physica C 296 (1998) 13–20
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became finer and the XRD peaks of the 2212 phase became broader. The particle size distributions by volume for the powders are shown in Fig. 2, and the average particle sizes are listed in Table 1. The powders were also observed by SEM, the micrographs of which are shown in Fig. 3. The micrographs are characterised by platelets of the 2212 phase with small particles on their surface.
Fig. 2. Particle size distributions by volume of powder SP-1, SP-2 and SP-3.
X-ray ŽCu K a . diffraction was used to identify the phases. One side of the silver sheath was mechanically removed by cutting the tape sides. XRD peak intensities were determined by measuring the integrated area of the respective peaks. Microstructures of selected samples were examined by scanning electron microscopy ŽJEOL6300. in both secondary electron ŽSE. and backscattered electron ŽBE. imaging modes and electron probe microanalysis ŽEPMA.. The critical current density was measured by the standard four-probe technique using the criterion of 1 mVrcm.
3. Results 3.1. Characterisation of precursor powders The X-ray diffraction patterns of the powders are shown in Fig. 1. The major phases were Bi-2212, ŽCa,Sr. 2 PbO4 , CuO Žand ŽCa,Sr. 2 CuO 3 identified by EDX.. With extension of grinding time, the powder Table 1 Powder grinding time and average particle size Powder ID
Grinding time Žh.
Average particle size by volume Žmm.
SP-1 SP-2 SP-3
3 4 5
6.8 5.5 4.3
Fig. 3. Secondary electron image of the precursor powders. Ža. SP-1, Žb. SP-2, and Žc. SP-3.
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J. Jiang, J.S. Abell r Physica C 296 (1998) 13–20
Fig. 4 shows the backscattered electron images of the polished cross-section of as-rolled tapes. From these images we can easily identify the size of CuO and Ca 2 CuO 3 particles, which are the black particles in the micrographs. A mixture with the finest and most homogeneously distributed particles was achieved in powder SP-3. Particles of CuO and Ca 2 CuO 3 with size larger than 5 mm were often observed in powder SP-1.
Fig. 5. X-ray diffraction patterns of as-rolled tapes TP-1, TP-2 and TP-3.
3.2. Characterisation of tapes Fig. 5 shows the XRD patterns of the as-rolled tapes TP-1, TP-2 and TP-3. No significant differences were observed among the patterns. However, comparing these patterns with those in Fig. 1, it is obvious that a preferential Ž00 l . texture was formed, indicated by the reduced intensities of peak Ž115., which is the most intense peak in powder XRD patterns. This texture was formed by alignment of the 2212 grains through the mechanical deformation during the process of wire swaging and flat rolling. Fig. 6 shows the XRD patterns of the tapes sintered in air at 8368C for 60 h. After this first
Fig. 4. Backscattered electron image of cross sections of the as-rolled tapes. Ža. TP-1, Žb. TP-2, and Žc. TP-3.
Fig. 6. X-ray diffraction patterns of tapes TP-1, TP-2 and TP-3 sintered at 8368C in air for 60 h.
J. Jiang, J.S. Abell r Physica C 296 (1998) 13–20
Fig. 7. Critical current as a function of temperature for tapes TP-1, TP-2 and TP-3 after the first sintering in air for 60 h.
sintering the 2212 phase was converted partially into the 2223 phases. By comparing the relative intensity of the Ž008. 2212 and Ž0010. 2223 , we can see that tape TP-3 exhibited the highest conversion rate from 2212 to 2223 phase, while tape TP-1 has the lowest rate. Tapes were also sintered for 60 h at different temperatures from 834 to 8428C. Fig. 7 shows the dependence on sintering temperature of critical current Ž Ic . of the tapes sintered in air for 60 h. The Ic of tape TP-3 was the most sensitive to sintering temperature, while the Ic of tape TP-1 was insensitive to the temperature between 8348C and 8428C. It can be seen that the best temperature for the first sintering is 8388C.
Fig. 8. Critical current density Jc as a function of the pressingrsintering cycles for tapes sintered at 8368C in air.
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Fig. 9. Comparison of critical current density of the fully processed tapes TP-1, TP-2 and TP-3 with percent conversion to 2223 of the corresponding tapes after the first sintering at 8368C in air for 60 h.
However, optimised Jc values of these three tapes were obtained after sintering four times at 8368C for 60 h with intermediate pressing Žas shown in Fig. 8.. Additional cycles resulted in a reduction in Jc . The fractional conversion of 2212 to 2223 was calculated from Fig. 6 by using the Ž008. and Ž0010. XRD reflection intensities of the 2212 and the 2223, respectively. Fig. 9 shows the comparison of Jc values of the fully processed tapes with the percent conversion to 2223 of the corresponding tapes after the first sintering at 8368C in air for 60 h. The conversion rate after first sintering was chosen because approxi-
Fig. 10. X-ray diffraction patterns of the fully processed tapes TP-1, TP-2 and TP-3.
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J. Jiang, J.S. Abell r Physica C 296 (1998) 13–20
mately full conversion was achieved after the second cycle. Tape TP-3 exhibited the highest formation rate of the 2223 phase after the first sintering stage and the highest Jc values at the first three stages; however, the highest final Jc value was achieved in tape TP-2.
Fig. 10 shows the XRD patterns of the fully processed tapes, which indicate that the 2212 completely converted into the 2223 phase which was highly textured. The longitudinal cross sections were observed by SEM. Fig. 11 shows the backscattered images of the polished cross sections. The grey matrix was the 2223 phase. Tape TP-3 appeared to have a larger average 2223 grain size. A large number of black and white particles were observed in tape TP-1, with fewer black particles in tape TP-2. Coarse black particles were present in TP-3. EDX analysis revealed that the black particles in tape TP-1 and TP-2 were ŽCa,Sr. 2 CuO 3 , while they were ŽCa,Sr. 2 CuO 3 or ŽSr,Ca.14 Cu 24O41 in tape TP-3. Coarse white particles were present in TP-1, and the size and the quantity of these white particles were reduced significantly in tape TP-2, and disappeared in TP-3. The white particles were either ŽCa,Sr. 2 PbO4 , or 3321 phases w25,26x.
4. Discussion
Fig. 11. Backscattered electron images of cross sections of the full processed tapes. Ža. TP-1, Žb. TP-2, and Žc. TP-3.
In this work, the effects of precursor powder particle size on microstructure and superconducting properties were identified by a combination of XRD, SEM, EDX and critical current analysis. The results indicate that the critical current density and microstructure were sensitive to the precursor particle size distribution. Firstly, a small particle size promoted the formation of the 2223 phase; secondly, coarse powder led to ŽCa,Sr. 2 CuO 3 and ŽCa,Sr. 2PbO4 remaining in the final tape; thirdly, very fine powder also resulted in the formation of large alkaline–earth cuprate grains wŽSr,Ca.14 Cu 14O41 and ŽCa,Sr. 2 CuO 3 x. All the proposed Bi-2223 formation mechanisms appear to depend on the presence of a liquid phase w8,14x. The liquid can help the diffusion of atoms which are required for nucleation and growth of the 2223 phase, and on the other hand it can heal cracks resulting from the intermediate mechanical deformation. Acoustic velocity measurements of isothermal anneals of the tape showed a gradual decrease in propagation time w27x, where the transition from solid to liquid phase was accompanied by an increase in propagation time. Thus, the acoustic measurement indicated the consumption of the liquid
J. Jiang, J.S. Abell r Physica C 296 (1998) 13–20
phase during the tape annealing. The formation of the liquid phase is sensitive to the phase assemblage and particle size distribution of the precursor. Although the reaction path of the liquid phase is still not clear, TEM analysis on quenched samples indicated that it was a lead-rich phase with a metal atom composition of Bi 2.2 Pb1.6 Sr1.3 Ca 1.0 Cu 2.0 w28x. The formation of the liquid is believed to be related to Ca 2 PbO4 w29x, so the increase of Ca 2 PbO4 content in the precursor can lead to more liquid phase during the tape anneal. The formation of liquid is also sensitive to the Ca and Cu cation content w21x. A sample with less calcium and copper exhibits slow reaction rates over considerably longer heat-treatment time and slightly higher annealing temperature. On the other hand, a sample with a relatively higher CarCu content, has lower annealing temperatures and shorter heat treatment times for a high Jc , implying formation of liquid phase. Although the CarCu content was not varied in the present work, the starting composition was Ca-rich, which would be consistent with higher liquid phase formation. But we still do not know through which path the excess of calcium and copper accelerate the formation of the 2223 phase. Li et al. w2x compared the powder with average particle size of 2 mm with that of 5 mm. They found that the optimised annealing temperature of the tapes appeared to decrease as the particle size decreases. Our results indicate that the critical current of the tape with the smallest particle size was most sensitive to the annealing temperature, and led to fastest formation of the 2223 phase. So it is suggested that the small particle size promotes the formation of the liquid phase, and especially accelerates the melting of the Ca 2 PbO4 phase. For tape TP-1 containing the coarse precursor SP-1, the Ca 2 PbO4 and Ca 2 CuO 3 can not be consumed completely and they grow during the annealing. By extension of the powder grinding time, the average precursor particle size was decreased, and the amount of Ca 2 PbO4 and Ca 2 CuO 3 was limited significantly in the final tapes, thus leading to an increased Jc value in tape TP-2. Further decreasing the particle size of powder SP-2 led to powder SP-3. However, the Jc did not increase further in tape TP-3, while larger 2223 grains and ŽSr,Ca.14 Cu 24 O41 and ŽCa,Sr. 2 CuO 3 phases were observed. Although aerosol spray pyrolysis is one
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attractive way to produce powders with small particle size Ž1–2 mm., the growth of large alkaline–earth cuprate grains has also been reported in the tape from this process w30x. This indicates that an optimised average particle size is important. It is suggested that an excess amount of liquid phase is formed due to the very fine particles present in tape TP-3, and that the growth of ŽSr,Ca.14 Cu 24 O41 and ŽCa,Sr. 2 CuO 3 phases is related to the presence of the excess amount of liquid phase. This is consistent with the result of Parrella et al. w31x. They observed a substantial amount of liquid in the microstructure along with the formation of ŽSr,Ca.14 Cu 24 O41 phase in the 2223 and 2212 tapes annealed in 7.5% O 2 atmosphere. Excess liquid may cause rapid melting of the Bi-2212. Sr released from the melted 2212 may not be consumed simultaneously by forming the 2223 phase since the growth of the 2223 is a slow reaction. So we speculate that this causes ŽSr, Ca.14 Cu 24 O41 to grow in tape TP-3. We therefore believe that decreasing precursor particle size promotes liquid phase formation, which aids conversion, grain growth and interconnectivity, but can also induce second phase formation and growth which can impede supercurrent flow.
5. Conclusion The phase formation, microstructure evolution and critical current density are sensitive to the precursor particle size distribution. Small particle size promotes the formation of the Bi-2223 phase, and coarse powder leads to Ca 2 CuO 3 and Ca 2 PbO4 remaining in the final tape, which limit the critical current density. Too fine a powder may cause the formation of large alkaline–earth cuprate grains wŽSr,Ca.14 Cu 14 O41 and ŽCa,Sr. 2 CuO 3 x through the liquid route.
Acknowledgements The authors acknowledge the financial support from School of Metallurgy and Materials, University of Birmingham, and would like to thank all members of the Superconducting Materials Group in the School of Metallurgy and Materials, University of Birmingham, for their advice and assistance.
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References w1x S.X. Dou, H.K. Liu, Supercond. Sci. Technol. 6 Ž1993. 297. w2x Q. Li, K. Brodersen, H.A. Hjuler, T. Freltoft, Physca C 217 Ž1993. 360. w3x D.C. Larbalestier, X.Y. Cai, Y. Feng, H. Edelman, A. Umezawa, G.N. Riley Jr., W.L. Carter, Physica C 211 Ž1994. 299. w4x G. Grasso, A. Jeremie, R. Flukiger, Supercond. Sci. Technol. ¨ 8 Ž1995. 827. w5x M. Lelovic, P. Krishnaraj, N.G. Eror, A.N. Iyer, U. Balachandran, Supercond Sci. Technol. 9 Ž1996. 201. w6x K. Sato, JOM 47 Ž8. Ž1995. 65. w7x R. Jammy, A.N. Iyer, J.Y. Huang, M. Chudzik, U. Balachandran, P. Haldar, in: U. Balachandran, P.J. McGinn, J.S. Abell ŽEds.., High Temperature Superconductors: Synthesis, Processing, and Large-Scale Application, The Minerals, Metals and Materials Society, 1996, p. 1. w8x J.S. Luo, N. Merchant, E.J. Escorcia-Aparicio, V.A. Maroni, B.S. Tani, J. Mater. Res. 9 Ž1994. 3059. w9x Y.L. Chen, R. Stevens, J. Am. Ceram. Soc. 75 Ž1992. 1150. w10x Q.Y. Hu, H.K. Liu, S.X. Dou, Physica C 250 Ž1995. 7. w11x W. Wong-Ng, C.K. Chiang, S.W. Freiman, L.P. Cook, M.D. Hill, Am. Ceram. Soc. Bull. 71 Ž1992. 1261. w12x J.S. Luo, N. Merchant, V.A. Maroni, D.M. Gruen, B.S. Tani, W.L. Carter, G.N. Riley, Appl. Supercond. 1 Ž1993. 101. w13x Y.S. Sung, E.E. Hellstrom, Physica C 253 Ž1995. 79. w14x A. Jeremie, R. Flukiger, Physica C 267 Ž1996. 10. ¨ w15x Y.-L. Wang, W. Bain, Y. Zhu, Z.-X. Cai, D.O. Welch, R.L. Sabatini, M. Suenaga, T.R. Thurston, Appl. Phys. Lett. 69 Ž1996. 580. w16x J.-C. Grivel, R. Flukiger, Supercond. Sci. Technol. 9 Ž1996. ¨ 555.
w17x P. Majewski, Adv. Mater. 6 Ž1994. 460. w18x P. Majewski, S. Kaesche, F. Aldiger, Adv. Mater. 8 Ž1996. 762. w19x F.H. Chen, H.S. Koo, T.Y. Tseng, Appl. Phys. Lett. 58 Ž1991. 637. w20x J.A. Parrell, A.A. Polyanskii, A.E. Pashitski, D.C. Larbalestier, Supercond. Sci. Technol. 9 Ž1996. 393. w21x P. Haldar, L.R. Motowidlo, JOM 44 Ž10. Ž1992. 54. w22x J. Jiang, J.S. Abell, in: U. Balachandran, P.J. McGinn, J.S. Abell ŽEds.., High Temperature Superconductors: Synthesis, Processing, and Large-Scale Application, The Minerals, Metals and Materials Society, 1996, p. 13. w23x J. Jiang, J.S. Abell, Supercond. Sci. Technol. 10 Ž1997. 678. w24x J. Jiang, Mphil, 1997, The University of Birmingham. w25x S.X. Dou, H.K. Liu, Y.L. Zhang, W.M. Blant, Supercond. Sci. Technol. 4 Ž1991. 203. w26x J.S. Luo, N. Merchant, V.A. Maroni, M. Hash, M. Rupich, in: U. Balachandran, P.J. McGinn, J.S. Abell ŽEds.., High Temperature Superconductors: Synthesis, Processing, and Large-Scale Application, The Minerals, Metals and Materials Society, 1996, p. 33. w27x M.T. Lanagan, D.K. Kupperman, G.A. Yaconi, S.H. Kilgore, IEEE Trans. Appl. Supercond. 5 Ž1995. 1475. w28x J.S. Luo, N. Merchant, V.A. Maroni, G.N. Riley Jr., W.L. Carter, Appl. Phys. Lett. 63 Ž1993. 690. w29x N. Kijima, H. Endo, J. Tsuchiya, A. Sumiyama, M. Mizuno, Y. Oguri, Jpn. J. Appl. Phys. 27 Ž1988. L1852. w30x Y.E. High, Y. Feng, Y.S. Sung, E.E. Hellstrom, D.C. Larbalestier, Physica C 220 Ž1994. 81. w31x R.D. Parrella, Y.S. Sung, E.E. Hellstrom, IEEE Trans. Appl. Supercond. 5 Ž1995. 1283.
Effects of precursor powder particle size on critical current density and microstructure of Bi-2223rAg tapes J. Jiang ) , J.S. Abell School of Metallurgy and Materials, UniÕersity of Birmingham, Birmingham B15 2TT, UK Received 29 March 1997; revised 30 July 1997; accepted 20 October 1997
Abstract The effect of particle size distribution on the nature of the precursor powder and the subsequent phase formation and critical current density of silver sheathed Bi-2223 tapes has been studied. Three precursor powders with different particle size distributions were obtained by grinding the calcined powder for different times. The effects of precursor powder particle size were identified by a combination of XRD, SEM and EDX analysis. The results indicate that the phase formation, microstructure evolution and critical current density are sensitive to the precursor particle size distribution. It has been shown that a small particle size promotes the formation of the Bi-2223 phase, and that coarse powder leads to Ca 2 CuO 3 and Ca 2 PbO4 remaining in the final tape. It has been found that too fine a powder may cause the formation of large alkaline–earth cuprate grains wŽSr,Ca.14Cu 14O41 and ŽCa,Sr. 2 CuO 3 x. The critical current density tends to increase when the amount and size of non-superconducting phase decrease in the final tapes q 1998 Elsevier Science B.V. PACS: 74.72.Hs; 74.62.Bf; 85.25.Kx; 81.40.-z Keywords: Critical current density; Superconducting tape; Microstructure
1. Introduction S uperconducting Ž B i,P b . 2 S r 2 C a 2 C u 3 O x ŽBi-2223.rAg tapes fabricated by the oxide-powderin-tube ŽOPIT. technique have demonstrated the capacity to support large critical currents at liquid nitrogen temperature for large scale power engineering applications w1–7x, but problems of reproducibility and maintaining the high critical current values on scaling up to practical lengths still remain. The
)
Corresponding author. Fax: q44 121 414 5232; E-mail: [email protected]
best route to the development of a well-aligned single phase superconducting core within these tapes is yet to be established, and the phase assemblage of the precursor powders incorporated into the silver sheath, together with the optimum combination of drawing and pressing and heat treatments is the subject of much intense research. More complex issues such as the phase evolution routes associated with the extended heat treatment stages remain to be resolved w8–16x. The complexities of the phase diagram are well reported w17,18x, and the possible role of liquid phases in the phase evolution route remain obscure w10,11,16,19x. The Bi-2223 phase can equilibrate with Ca 2 CuO 3 , ŽSr,Ca.14 Cu 24 O41yx , CuO,
0921-4534r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 9 2 1 - 4 5 3 4 Ž 9 7 . 0 1 8 1 8 - 2
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J. Jiang, J.S. Abell r Physica C 296 (1998) 13–20
Pb-rich phase, Bi-2212 and the liquid w18x. One of the keys to preparing Bi-2223rAg tapes that exhibit high critical current density Ž Jc . is to control the size and distribution of the non-superconducting phases remaining in the superconducting core matrix after thermomechanical processing. Manipulation and control of the liquid phase have been reported to be crucial because the liquid phase is believed to promote the growth of Bi-2223 phase, and to heal the cracks caused by intermediate rolling and pressing w20x. The formation of the liquid phase during the heat treatment depends on the sintering temperature and the composition, the phase assemblage and the particle size distribution of the precursor powder w2,21,22x. The particle size distribution is one of the important parameters for controlling the quality of the precursor. Li et al. w2x suggested that a small particle size of the raw powders was extremely important for obtaining high Jc tapes. But, there have been few detailed studies of the effects of the particle size distribution reported in the literature. In this work, the effects of precursor powder particle size were identified by a combination of XRD, SEM, and EDX analysis. We found that an optimised particle size distribution is important for high Jc values of the Bi-2223rAg tapes.
2. Experimental Powders were prepared by the solid state reaction method. The raw materials used in this study were commercial reagents. The purity of Bi 2 O 3 , PbO, SrCO 3 , CaCO 3 , and CuO is 99.9%, 99.9%, 98%, 99% and 98%, respectively. The nominal composition of the precursor powder was Bi 1.8 Pb 0.4 Sr2.0Ca 2 .2 Cu 3 O x . The appropriate amounts of oxidercarbonate powders were mixed, pelletized and calcined in air at 8208C for 24 h w23x. The calcined pellets were ground in an automatic agate pulveriser. After grinding for 3, 4, and 5 h, the powder was taken as powder SP-1, SP-2 and SP-3, respectively. All powders were analysed by X-ray diffraction analysis. The particle size distributions of powders were analysed by a laser diffraction technique in a Coulter LS Particle Size Analyser. Powders were in an agglomerate state. In order to disperse the ag-
glomerates, the powders were subjected to ultrasonic agitation in glycerol prior to inserting in the sample cell. The powders were processed by the powder-intube method with the same processing parameters. High purity Ag tube Žfrom Advent Research Materials Ltd. was used, the dimensions of which were 6 mm outer diameter and 4 mm inner diameter. The precursor powder was loaded into the Ag tubes and densified by means of a steel piston. The powder density inside the tubes before deformation was of the order of 3 grcm3 , ; 50% of the theoretical density. The tubes were closed, and were deformed using two two-hammer swaging machines, down to a diameter of about 1 mm. The tapelike shape was given by rolling in several steps. Rolling cylinders of 120 mm in diameter and a rolling speed of about 1 mrmin have been used. The as-rolled tapes have a width of 2.2 mm and an overall thickness of 0.2 mm. Tapes made from powder SP-1, SP-2 and SP-3 were named as TP-1, TP-2 and TP-3, respectively. The tape samples were obtained by cutting as-rolled tapes into short sections of 3 cm. The tapes were sintered up to four times in air at a temperature in the range of 834–8428C for a total time between 180 and 240 h. The sintered tapes were pressed uniaxially at room temperature between sintering periods. The optimised Jc values were achieved by using intermediate pressing with a pressure of 2 GPa w24x. In this work, all the tapes were deformed with this optimised pressure between sintering.
Fig. 1. X-ray diffraction patterns of powder SP-1, SP-2 and SP-3.
J. Jiang, J.S. Abell r Physica C 296 (1998) 13–20
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became finer and the XRD peaks of the 2212 phase became broader. The particle size distributions by volume for the powders are shown in Fig. 2, and the average particle sizes are listed in Table 1. The powders were also observed by SEM, the micrographs of which are shown in Fig. 3. The micrographs are characterised by platelets of the 2212 phase with small particles on their surface.
Fig. 2. Particle size distributions by volume of powder SP-1, SP-2 and SP-3.
X-ray ŽCu K a . diffraction was used to identify the phases. One side of the silver sheath was mechanically removed by cutting the tape sides. XRD peak intensities were determined by measuring the integrated area of the respective peaks. Microstructures of selected samples were examined by scanning electron microscopy ŽJEOL6300. in both secondary electron ŽSE. and backscattered electron ŽBE. imaging modes and electron probe microanalysis ŽEPMA.. The critical current density was measured by the standard four-probe technique using the criterion of 1 mVrcm.
3. Results 3.1. Characterisation of precursor powders The X-ray diffraction patterns of the powders are shown in Fig. 1. The major phases were Bi-2212, ŽCa,Sr. 2 PbO4 , CuO Žand ŽCa,Sr. 2 CuO 3 identified by EDX.. With extension of grinding time, the powder Table 1 Powder grinding time and average particle size Powder ID
Grinding time Žh.
Average particle size by volume Žmm.
SP-1 SP-2 SP-3
3 4 5
6.8 5.5 4.3
Fig. 3. Secondary electron image of the precursor powders. Ža. SP-1, Žb. SP-2, and Žc. SP-3.
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J. Jiang, J.S. Abell r Physica C 296 (1998) 13–20
Fig. 4 shows the backscattered electron images of the polished cross-section of as-rolled tapes. From these images we can easily identify the size of CuO and Ca 2 CuO 3 particles, which are the black particles in the micrographs. A mixture with the finest and most homogeneously distributed particles was achieved in powder SP-3. Particles of CuO and Ca 2 CuO 3 with size larger than 5 mm were often observed in powder SP-1.
Fig. 5. X-ray diffraction patterns of as-rolled tapes TP-1, TP-2 and TP-3.
3.2. Characterisation of tapes Fig. 5 shows the XRD patterns of the as-rolled tapes TP-1, TP-2 and TP-3. No significant differences were observed among the patterns. However, comparing these patterns with those in Fig. 1, it is obvious that a preferential Ž00 l . texture was formed, indicated by the reduced intensities of peak Ž115., which is the most intense peak in powder XRD patterns. This texture was formed by alignment of the 2212 grains through the mechanical deformation during the process of wire swaging and flat rolling. Fig. 6 shows the XRD patterns of the tapes sintered in air at 8368C for 60 h. After this first
Fig. 4. Backscattered electron image of cross sections of the as-rolled tapes. Ža. TP-1, Žb. TP-2, and Žc. TP-3.
Fig. 6. X-ray diffraction patterns of tapes TP-1, TP-2 and TP-3 sintered at 8368C in air for 60 h.
J. Jiang, J.S. Abell r Physica C 296 (1998) 13–20
Fig. 7. Critical current as a function of temperature for tapes TP-1, TP-2 and TP-3 after the first sintering in air for 60 h.
sintering the 2212 phase was converted partially into the 2223 phases. By comparing the relative intensity of the Ž008. 2212 and Ž0010. 2223 , we can see that tape TP-3 exhibited the highest conversion rate from 2212 to 2223 phase, while tape TP-1 has the lowest rate. Tapes were also sintered for 60 h at different temperatures from 834 to 8428C. Fig. 7 shows the dependence on sintering temperature of critical current Ž Ic . of the tapes sintered in air for 60 h. The Ic of tape TP-3 was the most sensitive to sintering temperature, while the Ic of tape TP-1 was insensitive to the temperature between 8348C and 8428C. It can be seen that the best temperature for the first sintering is 8388C.
Fig. 8. Critical current density Jc as a function of the pressingrsintering cycles for tapes sintered at 8368C in air.
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Fig. 9. Comparison of critical current density of the fully processed tapes TP-1, TP-2 and TP-3 with percent conversion to 2223 of the corresponding tapes after the first sintering at 8368C in air for 60 h.
However, optimised Jc values of these three tapes were obtained after sintering four times at 8368C for 60 h with intermediate pressing Žas shown in Fig. 8.. Additional cycles resulted in a reduction in Jc . The fractional conversion of 2212 to 2223 was calculated from Fig. 6 by using the Ž008. and Ž0010. XRD reflection intensities of the 2212 and the 2223, respectively. Fig. 9 shows the comparison of Jc values of the fully processed tapes with the percent conversion to 2223 of the corresponding tapes after the first sintering at 8368C in air for 60 h. The conversion rate after first sintering was chosen because approxi-
Fig. 10. X-ray diffraction patterns of the fully processed tapes TP-1, TP-2 and TP-3.
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J. Jiang, J.S. Abell r Physica C 296 (1998) 13–20
mately full conversion was achieved after the second cycle. Tape TP-3 exhibited the highest formation rate of the 2223 phase after the first sintering stage and the highest Jc values at the first three stages; however, the highest final Jc value was achieved in tape TP-2.
Fig. 10 shows the XRD patterns of the fully processed tapes, which indicate that the 2212 completely converted into the 2223 phase which was highly textured. The longitudinal cross sections were observed by SEM. Fig. 11 shows the backscattered images of the polished cross sections. The grey matrix was the 2223 phase. Tape TP-3 appeared to have a larger average 2223 grain size. A large number of black and white particles were observed in tape TP-1, with fewer black particles in tape TP-2. Coarse black particles were present in TP-3. EDX analysis revealed that the black particles in tape TP-1 and TP-2 were ŽCa,Sr. 2 CuO 3 , while they were ŽCa,Sr. 2 CuO 3 or ŽSr,Ca.14 Cu 24O41 in tape TP-3. Coarse white particles were present in TP-1, and the size and the quantity of these white particles were reduced significantly in tape TP-2, and disappeared in TP-3. The white particles were either ŽCa,Sr. 2 PbO4 , or 3321 phases w25,26x.
4. Discussion
Fig. 11. Backscattered electron images of cross sections of the full processed tapes. Ža. TP-1, Žb. TP-2, and Žc. TP-3.
In this work, the effects of precursor powder particle size on microstructure and superconducting properties were identified by a combination of XRD, SEM, EDX and critical current analysis. The results indicate that the critical current density and microstructure were sensitive to the precursor particle size distribution. Firstly, a small particle size promoted the formation of the 2223 phase; secondly, coarse powder led to ŽCa,Sr. 2 CuO 3 and ŽCa,Sr. 2PbO4 remaining in the final tape; thirdly, very fine powder also resulted in the formation of large alkaline–earth cuprate grains wŽSr,Ca.14 Cu 14O41 and ŽCa,Sr. 2 CuO 3 x. All the proposed Bi-2223 formation mechanisms appear to depend on the presence of a liquid phase w8,14x. The liquid can help the diffusion of atoms which are required for nucleation and growth of the 2223 phase, and on the other hand it can heal cracks resulting from the intermediate mechanical deformation. Acoustic velocity measurements of isothermal anneals of the tape showed a gradual decrease in propagation time w27x, where the transition from solid to liquid phase was accompanied by an increase in propagation time. Thus, the acoustic measurement indicated the consumption of the liquid
J. Jiang, J.S. Abell r Physica C 296 (1998) 13–20
phase during the tape annealing. The formation of the liquid phase is sensitive to the phase assemblage and particle size distribution of the precursor. Although the reaction path of the liquid phase is still not clear, TEM analysis on quenched samples indicated that it was a lead-rich phase with a metal atom composition of Bi 2.2 Pb1.6 Sr1.3 Ca 1.0 Cu 2.0 w28x. The formation of the liquid is believed to be related to Ca 2 PbO4 w29x, so the increase of Ca 2 PbO4 content in the precursor can lead to more liquid phase during the tape anneal. The formation of liquid is also sensitive to the Ca and Cu cation content w21x. A sample with less calcium and copper exhibits slow reaction rates over considerably longer heat-treatment time and slightly higher annealing temperature. On the other hand, a sample with a relatively higher CarCu content, has lower annealing temperatures and shorter heat treatment times for a high Jc , implying formation of liquid phase. Although the CarCu content was not varied in the present work, the starting composition was Ca-rich, which would be consistent with higher liquid phase formation. But we still do not know through which path the excess of calcium and copper accelerate the formation of the 2223 phase. Li et al. w2x compared the powder with average particle size of 2 mm with that of 5 mm. They found that the optimised annealing temperature of the tapes appeared to decrease as the particle size decreases. Our results indicate that the critical current of the tape with the smallest particle size was most sensitive to the annealing temperature, and led to fastest formation of the 2223 phase. So it is suggested that the small particle size promotes the formation of the liquid phase, and especially accelerates the melting of the Ca 2 PbO4 phase. For tape TP-1 containing the coarse precursor SP-1, the Ca 2 PbO4 and Ca 2 CuO 3 can not be consumed completely and they grow during the annealing. By extension of the powder grinding time, the average precursor particle size was decreased, and the amount of Ca 2 PbO4 and Ca 2 CuO 3 was limited significantly in the final tapes, thus leading to an increased Jc value in tape TP-2. Further decreasing the particle size of powder SP-2 led to powder SP-3. However, the Jc did not increase further in tape TP-3, while larger 2223 grains and ŽSr,Ca.14 Cu 24 O41 and ŽCa,Sr. 2 CuO 3 phases were observed. Although aerosol spray pyrolysis is one
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attractive way to produce powders with small particle size Ž1–2 mm., the growth of large alkaline–earth cuprate grains has also been reported in the tape from this process w30x. This indicates that an optimised average particle size is important. It is suggested that an excess amount of liquid phase is formed due to the very fine particles present in tape TP-3, and that the growth of ŽSr,Ca.14 Cu 24 O41 and ŽCa,Sr. 2 CuO 3 phases is related to the presence of the excess amount of liquid phase. This is consistent with the result of Parrella et al. w31x. They observed a substantial amount of liquid in the microstructure along with the formation of ŽSr,Ca.14 Cu 24 O41 phase in the 2223 and 2212 tapes annealed in 7.5% O 2 atmosphere. Excess liquid may cause rapid melting of the Bi-2212. Sr released from the melted 2212 may not be consumed simultaneously by forming the 2223 phase since the growth of the 2223 is a slow reaction. So we speculate that this causes ŽSr, Ca.14 Cu 24 O41 to grow in tape TP-3. We therefore believe that decreasing precursor particle size promotes liquid phase formation, which aids conversion, grain growth and interconnectivity, but can also induce second phase formation and growth which can impede supercurrent flow.
5. Conclusion The phase formation, microstructure evolution and critical current density are sensitive to the precursor particle size distribution. Small particle size promotes the formation of the Bi-2223 phase, and coarse powder leads to Ca 2 CuO 3 and Ca 2 PbO4 remaining in the final tape, which limit the critical current density. Too fine a powder may cause the formation of large alkaline–earth cuprate grains wŽSr,Ca.14 Cu 14 O41 and ŽCa,Sr. 2 CuO 3 x through the liquid route.
Acknowledgements The authors acknowledge the financial support from School of Metallurgy and Materials, University of Birmingham, and would like to thank all members of the Superconducting Materials Group in the School of Metallurgy and Materials, University of Birmingham, for their advice and assistance.
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