- Email: [email protected]
Ag superconducting wires
AppliedSuperconducfiuiry Vol. I, No. 1/2,pp. 25-31,1993 Pnnted in Great Britain. All rights reserved
Copyright
0964s1807/93 $5.00+0.00 Q 1992 Pergamon Press Ltd
EFFECT OF A CONTROLLED MELT PROCESS ON PHASE TRANSFORMATION AND ELECTROMAGNETIC PROPERTIES OF BiPbSrCaCuO/Ag SUPERCONDUCTING WIRES Y. C. Guo, H. K. Lru and S. X. Dou School of Materials Science and Engineering, The University of New South Wales, P.O. Box 1, Kensington, NSW 2033, Australia Abstract-The melt process has been successfully used in fabricating Ag-sheathed YBa,Cu,O, wires and (Bi,Pb),Sr,CaCu,O, (2212) wires. However, difficulties were encountered in applying the melt process to the (Bi,Pb),Sr,Ca,Cu,O, (2223) system, because the high T, (2223) phase decomposed during the melting and was difficult to recover by further heat treatment. In this paper, we present a high r, phase formation-decomposition-recovery process (PFDR) through the use of a short-period partial melt, with which the decomposed (2223) phase in the core of the wires can be recovered with subsequent annealing. The resultant tapes show a significant enhancement in critical current density (J,) and improved J,-magnetic field (H) dependence; in particular, a much slower drop of .I, in the low magnetic field regime, indicating the weak links between grains has been greatly improved. A J, of 40,000 A/cm2 at 77 K and zero field, and 9000 A/cm2 at 77 K and 1 T has been achieved in the PFDR processed tapes. a.c. susceptibility measurements under a d.c. magnetic field reveal that the irreversibility lines shift to a higher temperature, suggesting an improvement of flux pinning in the PFDR processed tapes compared with the solid-state reaction (SSR) treated tapes. The high mass density, excellent grain alignment, large grain size and uniform distribution of fine impurity particles are considered to be responsible for the improvement of tapes’ electromagnetic properties.
INTRODUCTION
Since the discovery of the rare earth free Bi-Sr-Ca-Cu-0 system [I], intensive research efforts using various techniques have been directed towards the practical application of these high critical temperature (r,) superconductors. A combination of high T, (Bi,Pb),Sr,Ca,Cu,O, (2223) phase [2] and the powder-in-tube technique [3--IO]has proved to be the most promising route for producing superconducting wires. In this technique, the superconducting oxide powders are sheathed by silver tubes for fabricating metal/superconductor composites, such as wires, tapes and multifilaments. This composite geometry is desirable since the metallic outer layer protects the oxide core from chemical corrosion and mechanical abrasion. In addition, the high thermal conductivity of the silver sheath provides a means of thermal dissipation, while the strength of the sheath mechanically supports the oxide during processing, handling and operations, thus compensating for the ceramic’s brittleness. There are two major difficulties which have to be overcome before any practical applications can be carried out: weak links at grain boundaries [1 I], which lead to a low critical current density and weak flux pinning [ 121,resulting in poor magnetic field behaviour of J,. It has been well established that high mass density, a high degree of grain alignment and finely dispersed impurities all help to improve weak links between grains and flux pinning strength, thus enhancing the critical current density of the materials. The melt process has been successfully used for fabricating high J, Ag-sheathed YBazCu,O,. [13] and (Bi,Pb)zSr,Ca, Cu,O, (2212) wires [14-161. In both cases, the superconducting phases were decomposed during melting and then recovered in the following solidification process, resulting in an enhancement of J, due to improvement of texturing and densification. However, difficulties were encountered in applying this normal melt technique to the (Bi,Pb),Sr,Ca,Cu,O, (2223) system, since the long melting period causes large (2212) grain growth and severe impurity segregation, making it difficult for them to fully recover into the (2223) phase with subsequent annealing [14, 171. In order to overcome this problem, we have developed a high T, phase formation-decompositionrecovery process (PFDR), in which a short period of melting is used instead of a couple of hours as in the normal melt process, thus avoiding large impurity segregation; the resultant tapes show a significant improvement in J, and J,-magnetic field dependence.
26
Y. C. GIJOet al.
In this paper, the results on the phase stability, microstructures, magnetic field dependence of J, and irreversibility lines of Ag-sheathed (2223) superconducting wires prepared through the PFDR process, are reported. The properties of PFDR processed tapes are compared with those of the SSR processed tapes.
EXPERIMENTAL
The powders were prepared by a co-decomposition of a solution consisting of high-purity B&O,, Pb(NO,),, Sr(NO,),, Ca(NO,),*4H,O and Cu(NO,),*3H,O reagents in the cation ratios Bi:Pb:Sr:Ca:Cu = 1.8:0.4:2:2.2: 3. The powders were calcined at 830°C for 10 h, ground and pressed into pellets, and sintered at 840°C for 30 h. X-ray diffraction patterns obtained using a Siemens D5000 diffractometer showed that the major phase in the pellets was low T, (2212) phase; the high T, (2223) phase has not been formed at this stage. The pellets were crushed and ground into a fine powder with a mortar and pestle. The powders were then pressed into round bars of 50 mm length and 8 mm diameter. The bars were loaded into a silver tube of 10 mm o.d. and 7 mm i.d., and the composite was then drawn to a final diameter of 0.8-l .Omm. The wires were rolled into tapes of overall thickness 0.1-0.15 mm and width 2-3 mm. The resultant tapes were processed by two different processes: PFDR and normal SSR procedures (without melt). For the PFDR procedure, the tapes were heat treated at 830-838°C for 80-100 h in air. The tapes were uniaxially pressed, and melt-processed at 860-866°C for lo-30 min. In order to investigate the high T, phase stability, some tapes were melted at temperatures varying from 848 to 876°C for a long period of 3-30 h, followed by annealing at 834°C for 30 h, and cooled to room temperature at 1SOC/h. The tapes were pressed again and annealed at 834°C for 80-160 h. The SSR schedule was the same as the PFDR schedule except that the period of melt processing was not used during heat treatment. A small piece of the sample was cut from the tapes for X-ray analyses and microstructural observations after each step of the heat treatment. The critical current density in the presence of magnetic field was determined at 77 K by the 4-point probe d.c. technique using a criterion of 1 pm/cm. Measurements of a.c. susceptibility were obtained using a mutual inductance bridge. A d.c. magnetic field was superimposed on an a.c. field in order to compare the behaviours of the PFDR and SSR treated tapes. Microstructural and compositional analyses were performed with a Jeol-840 scanning electron microscope (SEM) equipped with a link system AN 10000 energy dispersive spectrometer (EDS).
RESULTS
AND
DISCUSSION
X-ray diffraction (XRD) results showed that a single (2223) phase was almost formed in the core of the superconducting tapes after the first cycle of heat treatment at 834°C for 80 h. A series of temperatures varying from 848 to 876°C was used in the melt processing steps. XRD results indicated that the (2223) phase was completely decomposed into low T, (2212) and non-superconducting phases at melt temperatures above 859°C while for the samples treated at temperatures below 853°C the (2223) phase remained unchanged. At melt temperatures between 854 and 859°C the (2223) and (2212) phases co-existed. But the ratios of the (2223) to the (2212) phase in these samples decreased with increasing melting temperatures. With post-annealing at 834°C for 80-160 h, the decomposed (2223) phase can be recovered for tapes treated with the “short period” melt. For the “long period” melt samples, however, the full (2223) phase recovery cannot be achieved through post-annealing. As an example, Fig. 1 shows that the (2223) phase was totally decomposed after melting at 864°C for 15 min and completely recovered with post-annealing at 834°C for 80 h. Figure 2 shows that when a tape sample was melt-treated at 864°C for 10 h, only about 40% of the (2223) phase was recovered after 120 h post-annealing at 834°C. It is evident that the longer the tape was melted for, the smaller the percentage of the (2223) phase that can be recovered. Figures 3 and 4 show the electron micrographs of polished cross sections for the PFDR processed tape and SSR processed tape, respectively. It is seen that the PFDR tape has large, well aligned and elongated grains, with a high mass density and finely dispersed (2212) and impurity particles.
A controlled
melt process
for superconducting
77
wires
Melted at 866 “C, 1Smin.
0
0
I
I
Post-annealed 834”C, 80 h
ANGLE Fig. 1. X-ray
diffraction
at
(20)
patterns
of the “short-period”
melt tapes
The good texture, close contact between grains and large grain size are desirable for carrying a large current [18], and the small size impurities can be expected to act as effective pinning sites. For the SSR processed tapes, however, the mass density and grain alignment are not as good as the PFDR processed tapes. Furthermore. the SSR processed tapes contain large impurity particles and pores.
o: 2223 0: I 2212 I
Pre-sintered 834 C, 80 h
0
0
0
0
I
I
LL
at
I
JI
I
11
,I_,, 0
Melted at 864 C, 10 h
9
--L_--0
I
I
I,
I
10
I
I
I,
I
I,
Post-annealed 834 C. 120 h
0
t,,
I
I
t
(
I
I
I
20
r,
50
%GLE~’ Fig. 2. X-ray
0
0
0
I
0
0
0
diffraction
patterns
I
I
I
I,
I
60
I
I
at
I,
I
70
(28) of the “long-period”
melt tapes
Y. C. Guo et al.
28
Fig. 3. SEM micrograph
Fig. 4. SEM micrograph
Fig. 5. SEM micrograph
of the cross section
of the PFDR
of the cross section
of the cross section
processed
of the SSR processed
of the “long-period”
tape.
tape
melt tape.
A controlled melt process for superconducting
wires
29
These types of impurities and pores may simply act as weak links rather than pinning sites, and result in a reduction of effective superconducting volume. Figure 5 is the SEM micrograph of a sample after a “long-period” melt at 864°C for 10 h, followed by post-annealing at 834°C for 120 h. It can be seen that the matrix consists of (2212) and (2223) grains and large Ca-Cu-0 and Sr-Ca-Cu-O impurity particles. In contrast to the PFDR processed tapes in which the high T, phase was decomposed into small (2212) grains and finely dispersed Ca-Cu-0 and Cu-0 impurities, making it is easy for them to retransform into the (2223) phase in subsequent annealing, the (2223) phase in the “long-period” melt tapes are very difficult to recover since the (2212) phase and impurities were largely segregated. Figure 6 shows the dependence of J, on the magnetic field for the PFDR processed tapes (A-C) and the SSR processed tapes (D-F). It is seen that the PFDR processed tapes exhibit a 3- to 5-fold enhancement at 77 K and 0 T and a 6- to &fold enhancement at 77 K and 1 T in the J,, over the SSR processed tapes. A J, of 40,000 A/cm* at 77 K and 0 T, 25,000 A/cm* at 77 K and 0.1 T, and 9000 A/cm* at 77 K and 1 T has been achieved in the PFDR processed tapes. Especially in the low magnetic field regime, the J, for the PFDR processed tapes drops much more slowly than that for the SSR processed tapes. At low magnetic fields (H < 0.03-0.05 T), a rapid drop in J, is attributed to the Josephson weak links at the grain boundaries. It is noticed that J, for the SSR processed tapes loses more than 70-80% of its zero field values at 0.1 T whereas J, for the PFDR processed tapes loses 25-35% of its zero field value in the same field, indicating a significant improvement in the weak links structure through the PFDR process. In addition, the PFDR processed tapes showed an extended plateau from 0.1 to 1 T. This indicates that the PFDR process also improves the intragrain pinning of tape samples. For comparison, J, dependence on the magnetic field of the PFDR processed tapes and that reported by Sato et al. [19], which is the highest J, known to date, 54,000 A/cm’ at 77 K and 0 T. is shown in Fig. 7. At low field regimes (O-0.12 T), the J, of the former drops slightly faster than that of Sato et al. However, the two J,-H curves show a crossover at high fields, i.e. the former decreases more slowly than the latter at fields above 0.12 T. At 77 K and 0.5 T, the J, of the former loses only 55% of its initial value, whereas the latter loses 76.5% of its zero field value. Furthermore, at 77 K and 1 T the former still holds 33% of its initial J, while the latter maintains only 22% of its zero field J,. This suggests that the PFDR processed tapes have stronger flux pinning than Sato’s tapes at high magnetic fields. Flux pinning properties have been investigated widely, through measurements of the irreversibility line. The irreversibility line, defined as the disappearance of pinning at a certain temperature and
lo~~,,~,~~~,~~,,~,~~~~~~~“ ,,,,,,, 0.40
MAGNETIC Fig. 6. J,-H
0.60
FIELD
0.80
1.00
(Tesla)
curves for PFDR processed tapes (AX) and SSR processed tapes (D-F).
Y. C. Guo et al.
30
s
k >= cp z
-THIS -SAT0
-0.1
WORK ET AL
MAGNETIC ‘&ELD
(Tesla)
t
Fig. I. Normalized J,-H curves for tapes in the present work and those reported by Sato
et al.
(191.
certain magnetic field, is determined from the position of the loss peak in an imaginary part of the a.c. susceptibility with a fixed a.c. field of 0.3 Oe and a frequency of 1000 Hz. The irreversibility lines for typical PFDR and SSR processed tapes determined from a.c. susceptibility measurements under d.c. fields are presented in Fig. 8. As seen in Fig. 8, the irreversibility line for the PFDR processed tape is positioned at a higher temperature than that for the SSR processed sample, 0.08 QQQQ0 PFDR Processed UMKU SSR Processed
Tape Tape
0.06 0.05 E =d 0.04 0.03 0.02 0.01
Fig. 8. Irreversibility lines for the PFDR and SSR processed tapes.
A controlled melt process for superconducting wires
31
suggesting that a “short-period” melt process effectively enhanced flux pinning in the tape. This is consistent with the J,-H results. CONCLUSION The high T, (2223) phase in Ag-sheathed BPSCCO superconducting wires sensitively depends on the sintering temperature during heat treatment. The (2223) phase can be completely decomposed, partially decomposed and remained unchanged at sintering temperatures above 859°C between 85%854°C and below 853°C respectively. With subsequent annealing, the decomposed (2223) phase can be fully recovered for the “short-period” melt samples. However, complete (2223) phase recovery cannot be achieved for the “long-period” melt samples, due to severe phase segregation. A high critical current density and improved J,-magnetic field dependence have been achieved through the high T, (2223) phase formationdecomposition-recovery process. A J, of 40,000 A/cm2 at 77 K and 0 T, 25,000 A/cm* at 0.1 T and 9000 A/cm2 at 1 T has been obtained in the PFDR processed tape. The slow drop of J, in low magnetic field regime shows that the weak links in the PFDR processed tapes have been largely eliminated. The irreversibility lines determined using a.c. susceptibility measurement under dc. fields confirmed the significant improvement of flux pinning in the PFDR processed tapes. The high mass density, excellent grain alignment, large grain size and uniform distribution of small impurity particles derived through the PFDR process are considered to be responsible for J, enhancement, and electromagnetic property improvement of tapes. Acknowledgements-The authors are grateful to Metal Manufacturers Ltd, and the Commonwealth Department of Industry, Technology and Commerce for financial support. We would also like to thank Dr C. C. Sorrel1 and Dr G. Secrett for helpful discussions and Mr M. Day for wire drawing.
REFERENCES I. H. Maeda, Y. Tanaka, M. Fukutomi and T. Asano, Jpn. J. uppl. Phys. 27, L209 (1988). 2. M. Tanano, J. Takada, K. Oda, H. Kitaguchi, Y. Miura, Y. Ikeda, Y. Tomii and H. Mazaki, Jpn. J. appl. Phys. 27, L1041 (1988). 3. T. Hikata, K. Sato and H. Hitotsuyanagi, Jpn. J. appl. Phys. 28, L82 (1989). 4. K. Heine, J. Tenbrink and M. Thoner, Appl. Phys. L&t. 55, 2441 (1989). 5. T. Hikata, H. Mukai, K. Sato and H. Hitotsuyanagai, Sumitomo Electric Tech. Rec. 29, 45 (1990). 6. Y. Yamada, B. Obst and R. Flukiger, Supercond. Sci. Technol. 4, 165 (1991). 7. M. Ueyama, T. Hikata, T. Kato and K. Sato, Jpn. J. uppl. Phys. 30, L1384 (1991). 8. W. Ciao, R. Parrella and J. V. Sande, Physico 192C, 173 (1992). 9. R. Flukiger, B. Hensel, A. Jeremie, M. Decroux, H. Kupfer, W. Jahn, E. Seibt, W. Goldacker, Y. Yamada and J. Q. Xu, 10. II. 12. 13. 14. 15. 16. 17. 18. 19.
Supercond. Sci. Technol. 5, S6l (1992). S. X. Dou, H. K. Liu. Y. C. Guo and C. C. Sorrell, Supercond. Sci. Technol. 5, S471 (1992). D. Dimos, P. Chaudiali, J. Mannart and F. LeGoues, Phys. Rev. Lert. 61, 219 (1988). S. Kambe, M. Naito, K. Kitazawa, LTanaka and H. Kojima, Physica 16OC, 243 (1989). M. Okada, T. Yuasa, T. Matsumoto, K. Aihara, M. Seido and S. Matsuda, Advance in Superconductivity II,
p. 296.
Springer-Verlag, Tokyo (1990). J. Kase, K. Togano. H. Kumakura. D. R. Dietderich, N. Irisawa, T. Morimoto and H. Maeda, Jpn. J. appl. Phys. 29, L1096 (1990). R. D. Ray, II and E. E. Hellstrom, Appl. Phys. Left. 57, 2948 (1990). S. Jin, R. B. van Dover, T. H. Tiefel, J. E. Graebner and N. D. Spencer, Appl. Phys. Lerr. 58, 868 (1991). Y. Yamada, T. Graf. E. Seibt and R. Flukiger, IEEE Trans. Magn. 27, 1495 (1991). S. Jin and J. E. Graebner, Materials Sci. Engng B7, 243 (1991). K. Sato, N. Shibuta, H. Mukai, T. Hikata, M. Ueyama and T. Kato, J. appl. Phys. 70, 6484 (1991).
Copyright
0964s1807/93 $5.00+0.00 Q 1992 Pergamon Press Ltd
EFFECT OF A CONTROLLED MELT PROCESS ON PHASE TRANSFORMATION AND ELECTROMAGNETIC PROPERTIES OF BiPbSrCaCuO/Ag SUPERCONDUCTING WIRES Y. C. Guo, H. K. Lru and S. X. Dou School of Materials Science and Engineering, The University of New South Wales, P.O. Box 1, Kensington, NSW 2033, Australia Abstract-The melt process has been successfully used in fabricating Ag-sheathed YBa,Cu,O, wires and (Bi,Pb),Sr,CaCu,O, (2212) wires. However, difficulties were encountered in applying the melt process to the (Bi,Pb),Sr,Ca,Cu,O, (2223) system, because the high T, (2223) phase decomposed during the melting and was difficult to recover by further heat treatment. In this paper, we present a high r, phase formation-decomposition-recovery process (PFDR) through the use of a short-period partial melt, with which the decomposed (2223) phase in the core of the wires can be recovered with subsequent annealing. The resultant tapes show a significant enhancement in critical current density (J,) and improved J,-magnetic field (H) dependence; in particular, a much slower drop of .I, in the low magnetic field regime, indicating the weak links between grains has been greatly improved. A J, of 40,000 A/cm2 at 77 K and zero field, and 9000 A/cm2 at 77 K and 1 T has been achieved in the PFDR processed tapes. a.c. susceptibility measurements under a d.c. magnetic field reveal that the irreversibility lines shift to a higher temperature, suggesting an improvement of flux pinning in the PFDR processed tapes compared with the solid-state reaction (SSR) treated tapes. The high mass density, excellent grain alignment, large grain size and uniform distribution of fine impurity particles are considered to be responsible for the improvement of tapes’ electromagnetic properties.
INTRODUCTION
Since the discovery of the rare earth free Bi-Sr-Ca-Cu-0 system [I], intensive research efforts using various techniques have been directed towards the practical application of these high critical temperature (r,) superconductors. A combination of high T, (Bi,Pb),Sr,Ca,Cu,O, (2223) phase [2] and the powder-in-tube technique [3--IO]has proved to be the most promising route for producing superconducting wires. In this technique, the superconducting oxide powders are sheathed by silver tubes for fabricating metal/superconductor composites, such as wires, tapes and multifilaments. This composite geometry is desirable since the metallic outer layer protects the oxide core from chemical corrosion and mechanical abrasion. In addition, the high thermal conductivity of the silver sheath provides a means of thermal dissipation, while the strength of the sheath mechanically supports the oxide during processing, handling and operations, thus compensating for the ceramic’s brittleness. There are two major difficulties which have to be overcome before any practical applications can be carried out: weak links at grain boundaries [1 I], which lead to a low critical current density and weak flux pinning [ 121,resulting in poor magnetic field behaviour of J,. It has been well established that high mass density, a high degree of grain alignment and finely dispersed impurities all help to improve weak links between grains and flux pinning strength, thus enhancing the critical current density of the materials. The melt process has been successfully used for fabricating high J, Ag-sheathed YBazCu,O,. [13] and (Bi,Pb)zSr,Ca, Cu,O, (2212) wires [14-161. In both cases, the superconducting phases were decomposed during melting and then recovered in the following solidification process, resulting in an enhancement of J, due to improvement of texturing and densification. However, difficulties were encountered in applying this normal melt technique to the (Bi,Pb),Sr,Ca,Cu,O, (2223) system, since the long melting period causes large (2212) grain growth and severe impurity segregation, making it difficult for them to fully recover into the (2223) phase with subsequent annealing [14, 171. In order to overcome this problem, we have developed a high T, phase formation-decompositionrecovery process (PFDR), in which a short period of melting is used instead of a couple of hours as in the normal melt process, thus avoiding large impurity segregation; the resultant tapes show a significant improvement in J, and J,-magnetic field dependence.
26
Y. C. GIJOet al.
In this paper, the results on the phase stability, microstructures, magnetic field dependence of J, and irreversibility lines of Ag-sheathed (2223) superconducting wires prepared through the PFDR process, are reported. The properties of PFDR processed tapes are compared with those of the SSR processed tapes.
EXPERIMENTAL
The powders were prepared by a co-decomposition of a solution consisting of high-purity B&O,, Pb(NO,),, Sr(NO,),, Ca(NO,),*4H,O and Cu(NO,),*3H,O reagents in the cation ratios Bi:Pb:Sr:Ca:Cu = 1.8:0.4:2:2.2: 3. The powders were calcined at 830°C for 10 h, ground and pressed into pellets, and sintered at 840°C for 30 h. X-ray diffraction patterns obtained using a Siemens D5000 diffractometer showed that the major phase in the pellets was low T, (2212) phase; the high T, (2223) phase has not been formed at this stage. The pellets were crushed and ground into a fine powder with a mortar and pestle. The powders were then pressed into round bars of 50 mm length and 8 mm diameter. The bars were loaded into a silver tube of 10 mm o.d. and 7 mm i.d., and the composite was then drawn to a final diameter of 0.8-l .Omm. The wires were rolled into tapes of overall thickness 0.1-0.15 mm and width 2-3 mm. The resultant tapes were processed by two different processes: PFDR and normal SSR procedures (without melt). For the PFDR procedure, the tapes were heat treated at 830-838°C for 80-100 h in air. The tapes were uniaxially pressed, and melt-processed at 860-866°C for lo-30 min. In order to investigate the high T, phase stability, some tapes were melted at temperatures varying from 848 to 876°C for a long period of 3-30 h, followed by annealing at 834°C for 30 h, and cooled to room temperature at 1SOC/h. The tapes were pressed again and annealed at 834°C for 80-160 h. The SSR schedule was the same as the PFDR schedule except that the period of melt processing was not used during heat treatment. A small piece of the sample was cut from the tapes for X-ray analyses and microstructural observations after each step of the heat treatment. The critical current density in the presence of magnetic field was determined at 77 K by the 4-point probe d.c. technique using a criterion of 1 pm/cm. Measurements of a.c. susceptibility were obtained using a mutual inductance bridge. A d.c. magnetic field was superimposed on an a.c. field in order to compare the behaviours of the PFDR and SSR treated tapes. Microstructural and compositional analyses were performed with a Jeol-840 scanning electron microscope (SEM) equipped with a link system AN 10000 energy dispersive spectrometer (EDS).
RESULTS
AND
DISCUSSION
X-ray diffraction (XRD) results showed that a single (2223) phase was almost formed in the core of the superconducting tapes after the first cycle of heat treatment at 834°C for 80 h. A series of temperatures varying from 848 to 876°C was used in the melt processing steps. XRD results indicated that the (2223) phase was completely decomposed into low T, (2212) and non-superconducting phases at melt temperatures above 859°C while for the samples treated at temperatures below 853°C the (2223) phase remained unchanged. At melt temperatures between 854 and 859°C the (2223) and (2212) phases co-existed. But the ratios of the (2223) to the (2212) phase in these samples decreased with increasing melting temperatures. With post-annealing at 834°C for 80-160 h, the decomposed (2223) phase can be recovered for tapes treated with the “short period” melt. For the “long period” melt samples, however, the full (2223) phase recovery cannot be achieved through post-annealing. As an example, Fig. 1 shows that the (2223) phase was totally decomposed after melting at 864°C for 15 min and completely recovered with post-annealing at 834°C for 80 h. Figure 2 shows that when a tape sample was melt-treated at 864°C for 10 h, only about 40% of the (2223) phase was recovered after 120 h post-annealing at 834°C. It is evident that the longer the tape was melted for, the smaller the percentage of the (2223) phase that can be recovered. Figures 3 and 4 show the electron micrographs of polished cross sections for the PFDR processed tape and SSR processed tape, respectively. It is seen that the PFDR tape has large, well aligned and elongated grains, with a high mass density and finely dispersed (2212) and impurity particles.
A controlled
melt process
for superconducting
77
wires
Melted at 866 “C, 1Smin.
0
0
I
I
Post-annealed 834”C, 80 h
ANGLE Fig. 1. X-ray
diffraction
at
(20)
patterns
of the “short-period”
melt tapes
The good texture, close contact between grains and large grain size are desirable for carrying a large current [18], and the small size impurities can be expected to act as effective pinning sites. For the SSR processed tapes, however, the mass density and grain alignment are not as good as the PFDR processed tapes. Furthermore. the SSR processed tapes contain large impurity particles and pores.
o: 2223 0: I 2212 I
Pre-sintered 834 C, 80 h
0
0
0
0
I
I
LL
at
I
JI
I
11
,I_,, 0
Melted at 864 C, 10 h
9
--L_--0
I
I
I,
I
10
I
I
I,
I
I,
Post-annealed 834 C. 120 h
0
t,,
I
I
t
(
I
I
I
20
r,
50
%GLE~’ Fig. 2. X-ray
0
0
0
I
0
0
0
diffraction
patterns
I
I
I
I,
I
60
I
I
at
I,
I
70
(28) of the “long-period”
melt tapes
Y. C. Guo et al.
28
Fig. 3. SEM micrograph
Fig. 4. SEM micrograph
Fig. 5. SEM micrograph
of the cross section
of the PFDR
of the cross section
of the cross section
processed
of the SSR processed
of the “long-period”
tape.
tape
melt tape.
A controlled melt process for superconducting
wires
29
These types of impurities and pores may simply act as weak links rather than pinning sites, and result in a reduction of effective superconducting volume. Figure 5 is the SEM micrograph of a sample after a “long-period” melt at 864°C for 10 h, followed by post-annealing at 834°C for 120 h. It can be seen that the matrix consists of (2212) and (2223) grains and large Ca-Cu-0 and Sr-Ca-Cu-O impurity particles. In contrast to the PFDR processed tapes in which the high T, phase was decomposed into small (2212) grains and finely dispersed Ca-Cu-0 and Cu-0 impurities, making it is easy for them to retransform into the (2223) phase in subsequent annealing, the (2223) phase in the “long-period” melt tapes are very difficult to recover since the (2212) phase and impurities were largely segregated. Figure 6 shows the dependence of J, on the magnetic field for the PFDR processed tapes (A-C) and the SSR processed tapes (D-F). It is seen that the PFDR processed tapes exhibit a 3- to 5-fold enhancement at 77 K and 0 T and a 6- to &fold enhancement at 77 K and 1 T in the J,, over the SSR processed tapes. A J, of 40,000 A/cm* at 77 K and 0 T, 25,000 A/cm* at 77 K and 0.1 T, and 9000 A/cm* at 77 K and 1 T has been achieved in the PFDR processed tapes. Especially in the low magnetic field regime, the J, for the PFDR processed tapes drops much more slowly than that for the SSR processed tapes. At low magnetic fields (H < 0.03-0.05 T), a rapid drop in J, is attributed to the Josephson weak links at the grain boundaries. It is noticed that J, for the SSR processed tapes loses more than 70-80% of its zero field values at 0.1 T whereas J, for the PFDR processed tapes loses 25-35% of its zero field value in the same field, indicating a significant improvement in the weak links structure through the PFDR process. In addition, the PFDR processed tapes showed an extended plateau from 0.1 to 1 T. This indicates that the PFDR process also improves the intragrain pinning of tape samples. For comparison, J, dependence on the magnetic field of the PFDR processed tapes and that reported by Sato et al. [19], which is the highest J, known to date, 54,000 A/cm’ at 77 K and 0 T. is shown in Fig. 7. At low field regimes (O-0.12 T), the J, of the former drops slightly faster than that of Sato et al. However, the two J,-H curves show a crossover at high fields, i.e. the former decreases more slowly than the latter at fields above 0.12 T. At 77 K and 0.5 T, the J, of the former loses only 55% of its initial value, whereas the latter loses 76.5% of its zero field value. Furthermore, at 77 K and 1 T the former still holds 33% of its initial J, while the latter maintains only 22% of its zero field J,. This suggests that the PFDR processed tapes have stronger flux pinning than Sato’s tapes at high magnetic fields. Flux pinning properties have been investigated widely, through measurements of the irreversibility line. The irreversibility line, defined as the disappearance of pinning at a certain temperature and
lo~~,,~,~~~,~~,,~,~~~~~~~“ ,,,,,,, 0.40
MAGNETIC Fig. 6. J,-H
0.60
FIELD
0.80
1.00
(Tesla)
curves for PFDR processed tapes (AX) and SSR processed tapes (D-F).
Y. C. Guo et al.
30
s
k >= cp z
-THIS -SAT0
-0.1
WORK ET AL
MAGNETIC ‘&ELD
(Tesla)
t
Fig. I. Normalized J,-H curves for tapes in the present work and those reported by Sato
et al.
(191.
certain magnetic field, is determined from the position of the loss peak in an imaginary part of the a.c. susceptibility with a fixed a.c. field of 0.3 Oe and a frequency of 1000 Hz. The irreversibility lines for typical PFDR and SSR processed tapes determined from a.c. susceptibility measurements under d.c. fields are presented in Fig. 8. As seen in Fig. 8, the irreversibility line for the PFDR processed tape is positioned at a higher temperature than that for the SSR processed sample, 0.08 QQQQ0 PFDR Processed UMKU SSR Processed
Tape Tape
0.06 0.05 E =d 0.04 0.03 0.02 0.01
Fig. 8. Irreversibility lines for the PFDR and SSR processed tapes.
A controlled melt process for superconducting wires
31
suggesting that a “short-period” melt process effectively enhanced flux pinning in the tape. This is consistent with the J,-H results. CONCLUSION The high T, (2223) phase in Ag-sheathed BPSCCO superconducting wires sensitively depends on the sintering temperature during heat treatment. The (2223) phase can be completely decomposed, partially decomposed and remained unchanged at sintering temperatures above 859°C between 85%854°C and below 853°C respectively. With subsequent annealing, the decomposed (2223) phase can be fully recovered for the “short-period” melt samples. However, complete (2223) phase recovery cannot be achieved for the “long-period” melt samples, due to severe phase segregation. A high critical current density and improved J,-magnetic field dependence have been achieved through the high T, (2223) phase formationdecomposition-recovery process. A J, of 40,000 A/cm2 at 77 K and 0 T, 25,000 A/cm* at 0.1 T and 9000 A/cm2 at 1 T has been obtained in the PFDR processed tape. The slow drop of J, in low magnetic field regime shows that the weak links in the PFDR processed tapes have been largely eliminated. The irreversibility lines determined using a.c. susceptibility measurement under dc. fields confirmed the significant improvement of flux pinning in the PFDR processed tapes. The high mass density, excellent grain alignment, large grain size and uniform distribution of small impurity particles derived through the PFDR process are considered to be responsible for J, enhancement, and electromagnetic property improvement of tapes. Acknowledgements-The authors are grateful to Metal Manufacturers Ltd, and the Commonwealth Department of Industry, Technology and Commerce for financial support. We would also like to thank Dr C. C. Sorrel1 and Dr G. Secrett for helpful discussions and Mr M. Day for wire drawing.
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