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The fabrication, microstructure and transport current study of BPSCCO 2223 multi-connected tapes
PHYSICA® ELSEVIER
Physica C 267 (1996) 45-52
The fabrication, microstructure and transport current study of BPSCCO 2223 multi-connected tapes M.K. A1-Mosawi *, C. Beduz, Y. Yang, R.G. Scurlock Institute of Cryogenics, University of Southampton, Southampton, S017 1BJ, UK Received 11 November 1995; revised manuscript received 20 May 1996
Abstract
The properties of BPSCCO-2223 tapes produced by the oxide powder in tube (OPIT) method with a "multi-connected" core configuration are presented. By using large silver particles ( ~ 80 p,m) up to 50 wt.% in the precursor BPSCCO powder, a multi-connected core with a homogeneous distribution of elongated and aligned silver platelets was obtained in the processed tapes. The samples were sintered at 832°C in air for different times with two intermediate cold-rolling processes and were characterised by transport critical current measurements at 77 K (0 T < B < 0.5 T), XRD, SEM, and optical microscopy. The effect of silver addition on the formation rate of the high-temperature BiPb(2223) phase is presented. The XRD analysis shows a saturation percentage of the BiPb(2223) phase within the core that is not affected by increased sintering time. Although the initial speed of conversion of the BiPb(2223) phase is seen to increase with increasing silver addition, the saturation percentage decreases. Local phase analysis shows a homogeneous distribution of the BiPb(2223) phase throughout the multi-connected tape, in contrast to the significant phase gradient observed in the control tapes without silver addition. For similar processing parameters, the critical current density increases with increasing silver addition (Jc = 20 kAcm -2 at 77 K (B = 0) with 50 wt.% added Ag, compared to Jc = 10 kA cm -2 without Ag).
1. Introduction
Critical current densities as high as 6 × 10 4 A / c m 2 at 77 K have been reported [1] for (BiPb)ESr2Ca2Cu3Oy tapes. These high currents carrying capabilities have initiated an enormous effort in the optimisation of tape processing. For practical application requiring long lengths of tapes, the reproducibility of the tape and the homogeneity of the critical currents are important factors. In addition, the conductor strain tolerance (the irreversible strain limit) above which the critical current density degrades irreversibly must be considered. At the pre* Corresponding author. Fax: + 44 1703 593053.
sent time, monocore tapes have a strain tolerance below the required values for most applications [2]. Although multifilament tapes offer a higher strain tolerance due to a reduced core thickness [3], the fabrication of multifilament tapes is a complicated and time-consuming process. It is generally believed [4,5] that grain alignment, densification and phase formation are among the critical parameters in determining the current carrying capability of the tapes. For the tapes produced by the oxide powder in tube (OPIT) technique, these parameters are governed to a considerable extent by the mechanical processes [6] (including drawing and rolling), initial powder stoichiometry [6], precursor phase composition, particle size [1] and thermal pro-
0921-4534/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PH S 0 9 2 1 - 4 5 3 4 ( 9 6 ) 0 0 3 1 9-X
46
M.K. Al-Mosawi et a l . / Physica C 267 (1996) 45-52
cesses (sintering temperature, duration etc.). Other factors include the sheath material [7], core thickness [8,9] and core additives [10,11]. It is accepted [12,13] that an enhancement of 2212-2223 conversion and grain alignment occurs at the silver sheath-core interface. Also, a variation in the local phase and microstructure at different distances from the silver-core interface has been found [14]. Silver addition is widely used [15-19] to enhance the Jc and strain tolerance of the BiPb(2223) tapes. In these reports the Ag added BiPb(2223) monocore tapes were fabricated by using fine (2-3
,
,,
~ ¢ ", ~i
ixm) equiaxed Ag20 and/or Ag powders, and Ag flakes (10 p,m). These reports show that the optimum Ag amount to maintain relatively high Jc is around 15 vol.% beyond which the critical current density degrades. The reason for this degradation was related to the morphology of the added Ag. In this paper, we report on the processing and the properties of BiPb(2223) tapes with a multi-connected core configuration (produced by using large silver particles ( ~ 80 Ixm) up to 50 wt.% in the precursor BPSCCO powder) which gives a more homogeneous phase formation. It is also expected
i / i:i~ iiiii!i!iii?iii:ii:iiii[i~i¸I;~¸¸¸¸¸!I¸;¸
,_
50~,m ,,
Fig. 1. Optical micrographs of the sample with 30 wt.% added silver showing the (a) transverse and (b) longitudinal cross-sections.
M.K. AI-Mosawi et al. / Physica C 267 (1996) 45-52
that this tape will have a Jc-strain characteristic similar to that of the multifilament tapes, with the advantage of a single-step production as for monocore tapes. The purpose of this comparative study is to understand the role of silver addition in the superconducting core. As explained earlier, the critical current density (Jc) of a tape depends on a large number of inter-related parameters. The thermal and mechanical processes used in this work are not optimised, and the Jc obtained for the reference and Ag added samples do not represent the best obtainable values.
47
2. Experimental The precursor powder used in this work was Merck partially calcined Bil.s4Pb0.34Srl.91Ca2.03Cu3.06Oy , which comprises BiPb(2212) as the major phase, with 0, 30, and 50 wt.% added silver particles ( ~ 80 ixm). Two sets of tapes were produced by the OPIT method, using a silver alloy tube (99.5% Ag, 0.25% Ni, 0.25% Mg). The first set was sintered at 832°C in air followed by two intermediate mechanical rollings after a total sintering time of 30 h and 60 h (hereinafter 30R + 30R), and finally sintered up to
Fig. 2. Optical micrographs of the sample with 50 wt.% added silver showing the (a) transverse and (b) longitudinal cross-sections.
48
M.K. Al-Mosawi et al. / Physica C 267 (1996) 45-52
180 h at 20 h intervals. In the second set of samples, the two intermediate cold rollings were carried out after 10 h and 20 h (hereinafter 10R + 10R). Transport current and Jc-B measurements were carried out on 50 mm long samples by using the standard four-probe method at 77 K in a magnetic field up to 0.5 T, using a voltage criterion of 1 ~V cm -~. The critical current densities were calculated excluding the area of the silver inclusions. Microstructural and phase formation analysis were conducted by using optical microscopy, SEM, and Cu K a X-ray diffractometer. The percentage of 2223 phase was estimated from the intensity of the BiPb(2223) peak and the Bi(2212) peak as follows
[2o]: a ( % 2 2 2 3 ) = 12223(0010)//(12212(008) + •2223(0010))"
3. Results and discussion
3.1. Optical and SEM observation The optical micrographs of the polished transverse and longitudinal cross-sections of the samples with 30 and 50 wt.% Ag added are shown in Figs. 1 and 2, respectively. It can be seen that the silver inclusions divide the superconducting core into thin (10-20 txm) multi-connected filaments for samples with 30 wt.% added silver and 2-10 Ixm for samples with 50 wt.% Ag. Hence, a large proportion of the ceramic core is in close vicinity to the partially aligned elongated silver platelets with an average aspect ratio of 1 : 8. The thickness of the multi-connected filaments is comparable to that in multifilamentary tapes, hence a similar strain tolerance is expected for these tapes. The study of the effects of silver addition on the Jc-strain characteristics will be reported later. Figs. 3(a) and (b) show the SEM micrographs of the longitudinal cross-sections of the samples with 0 wt.% and 50 wt.% added Ag. The samples are etched before SEM analysis by using 1 vol.% Br in methanol for 30 s [21]. Apart from the region near the sheath-core interface, the sample with 0% Ag has a lower density with a significant amount of voids which may be due to grain misalignment during grain growth. The sample with 50 wt.% added
Fig. 3. SEM micrographs showing the etched cross sections of the samples with (a) 0 wt.% Ag and (b) 50 wt.% Ag.
silver has a lower void fraction largely due to an increase in the contact area between the ceramic core and silver. Also, the void fraction may have been reduced by the deformation of the annealed silver inclusions during the intermediate roilings.
3.2. BiPb(2223) phase formation The BiPb(2223) phase formation as a function of sintering time measured by XRD for both sets of tapes is shown in Fig. 4. The layer at which the XRD were performed is approximately 10-15 ixm from the silver sheath. As will be seen later, it is important to state the depth of the layer at which the XRD measurements are conducted especially for the reference tape. The initial overall formation speed of BiPb(2223) phase increases with increasing silver
M.K. AI-Mosawi et al. / Physica C 267 (1996) 45-52
1°°I 90
ever, from Fig. 4 it can be seen that the saturation level of the BiPb(2223) after long sintering time (t > 80 h) decreases as the amount of added silver increases. This reduction in the saturation level is accompanied by an increase in the residual amount of secondary phases (CaEPbO4, CaCuO2, etc.) seen in Fig. 5. The reason for the saturation level is not yet understood. It appears from Fig. 4 that the estimated percentage of the BiPb(2223) phase for the tapes with 50 wt.% Ag is slightly decreasing, however, this reduction is too small to be explained and further work is needed.
,,
80
8
I ~
o
o
.Y/
40
30 / I 1 /
20 Ill / 10 / ~
10R+IOR 30R+30R
1
~
49
A
.t
0
[:3
•
30wt%Ag
wt%A,
3.3. Phase gradient
0 0
20
40
60
80
100
120
140
160 180 200
In order to investigate the BiPb(2223) phase percentage at different planes parallel to the silver interface, the tapes were thinned down by polishing about 10 Ixm before each XRD measurement. The error in the thickness measurement is about + 3 Ixm. Fig. 6 shows the local percentage of the BiPb(2223) phase at the peak Jc as a function of the distance from the sheath-core interface. For the 0% Ag tapes, there is a significant BiPb(2223) phase gradient across the
Total Sintering T i m e (hr) Fig. 4. BiPb(2223) phase formation as a function of the sintering time measured by XRD on a layer approximately 10-15 p~m from the silver sheath for samples with different silver addition.
addition in the ceramic core. This is consistent with the finding [14] that the rate of the phase conversion is largest at the silver sheath-core interface. How-
a-
E
'
'~
'
'
~"
' O%~vt Ag
e4
b
3 O % w t Ag
c
5 0 % w t Ag
e-
l,-
-$ r,,
20
22
24
26
28
30
32
34
36
38
40
20 Fig. 5. XRD patterns of the samples with (a) 0 wt.% Ag, (b) 30 wt.% Ag and (c) 50 wt.% Ag, sintered at 832°C for 180 h; ( * ) is for secondary phases.
50
M.K. AI-Mosawi et a l . / Physica C 267 (1996) 45-52
30R+30R
90
80 --"g- ..... -~ . . . . . . . . ~ ..... n . . . . . . . . . . . . -o r~
\
70
/ /A
•
/
~
-'/
\
60
o
[]
\\
"
01,/
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_+
e wt%A$
(~140k~)
_---12-- 30wt%A| (t=14OhrJ) + 50wt%Ag (t=-100hrs) 50
a
0
I
t
10
I
1
20
I
t
30
I
40
i
I
50
Distance x, pm
Fig. 6. Local percentage of the BiPb(2223) phase at the optimum Jc as a function of the distance from the sheath-core interface.
core with the maximum at the sheath-core interface and minimum in the centre. In comparison, the tapes with Ag addition show a homogeneous distribution of the BiPb(2223) phase as shown in Fig. 6. This is likely to be the result of the increase in the interaction between silver and the ceramic core throughout the whole cross-section. 3.4. Transport current measurement
Fig. 7 shows the effect of sintering time on the transport critical current density at 77 K in zero 24000 22000 _30R+30R 20000
24000 +0wt%Ag ---o-- 30wt%Ag
~ / ~
lSOOO
- - o - - 50wt%Ag
16000 ~ 14000 12000 I0000 = 8000 6000 = 4000 200O 0 60
~ •~ ~ ~
,~77K
t 80
magnetic field for the 30R + 30R samples. The critical current densities of 0, 30, 50 wt.% Ag reach its maximum at total sintering time of 140, 140 and 100 h, respectively. The corresponding peak Jc values are 9800, 13 000 and 20200 A cm -2. Although a larger residual amount of the BiPb(2212) phase remains in the silver-added samples (processed at similar parameters) as shown in Fig. 4, the peak Jc values is enhanced by silver addition. The early peak in the J~-time curve for the sample with 50 wt.% added Ag is likely to be due to the acceleration of phase formation. The inhomogeneous distribution of the BiPb(2223) phase within the core in the 0 wt.% Ag samples means that the required time to optimise different regions will increase gradually from the silver interface to the centre of the core. Consequently, it is difficult to optimise the entire core at a given time, i.e. the material can be considered as a "multi-system". In contrast, for samples with 50 wt.% Ag, most of the superconducting core is in closer vicinity to the silver sheath a n d / o r inclusions resulting in homogeneous phase conversion and, therefore, the entire core may reach its optimum state (phase formation, grain size, cracks healing, etc.) at the same time. This may explain why the J~ values for the samples with 50 wt.% Ag have peaked sharply at an earlier sintering time than the samples without silver addition. The sharpness of the Jc-time curve means that the multi-connected tapes are more sensitive to the thermal processing time which needs to be carefully controlled in order to optimise the tape.
I 100
I 120
I 140
i 160
I 180
22000 20000 18000 16000 14000 12000
10R+IOR
Sintefing Time oars) Fig. 7. The effect of the sintering time on the critical current density at 77 K in zero magnetic field for the 30R + 30R samples.
0wt%Ag 30wt%Ag t%Ag
10000 8000 •=o 6000 L~ 4000 2000 200
+
self field,
60
77K
I
q
I
i
!
1
I
80
100
120
140
160
180
200
220
Sintering Time Oars) Fig. 8. The effect of the sintering time on the critical current density at 77 K for the 10R+ 10R samples.
M.K. AI-Mosawi et al. / Physica C 267 (1996) 45-52 1.0
a) 50wt%Ag
b) 30wt%Ag
~.\\
H
r~
--o- S0hrs
~ h
r
s
lOOhrs 120hrs! 140hrs
51
c) 0wt%Ag
-~- 16oh.
.~0.5
0.0 102
103
102
103
102
103
104
Applied Magnetic Field, (G) Fig. 9. The normalised in-plane Jc-B characteristics at 77 K for samples with (a) 50 wt.% Ag, (b) 30 wt.% Ag and (c) 0 wt.% Ag.
In the 30R + 30R samples, the average estimated percentage of BiPb(2223) phase present prior to the second intermediate cold rolling was approximately 70% which is close to the saturation level for the silver-added samples. Consequently, the second rolling process may be already too late for any cracks to be fully healed by the final sintering process. The second set of samples was produced with earlier intermediate cold rollings after sintering time of 10 h and 20 h. The overall conversion prior to the second rolling is reduced to 45% and 62% for samples with 30 wt.% and 50 wt.% added silver, respectively. The critical current density as a function of sintering time for the 10R + 10R samples is shown in Fig. 8. While there is no significant change in the peak J~ value for the 50 wt.% Ag samples, an enhancement from 13 000 A cm- 2 to 18 000 A cm- 2 for the sample with 30 wt.% Ag was obtained. The little change in the 50 wt.% Ag samples is not unexpected due to the higher level of the residual saturate liquid phases, which may be sufficient for healing microcracks at all times.
3.5. J ¢-B characteristics Fig. 9 shows the normalised in-plane Jc-B characteristics. For a particular sintering time (t), the Jc-B dependence is scaled to Jc(B, t)/Jc(B = O, t). It appears that the sample with 0% added Ag exhibits a better Jc-B dependence than the samples with silver addition; however, the difference is reduced with increasing sintering time. At the early sintering stage, the BiPb(2223) phase in the multiconnected tape is likely to develop into isolated
islands around the silver inclusions which are weakly inter-connected and extend to the sheath-core interface. With increasing sintering time the connectivity between the islands improves, as does the field dependence. At the optimum sintering time, the slight difference in Jc-B dependence is likely to be due to the reduced in-plane alignment in the multi-connected tapes compared to the monocore tapes.
4. C o n c l u s i o n s By adding large silver particles ( ~ 80 I~m) in the precursor powder, we have produced high-J c tapes with multi-connected thin filaments of 5-15 Ixm thick which are essential to improve the strain tolerance of the tape. For the same processing parameters, an enhancement of up to 200% in the optimum Jc was obtained in the silver-added samples with a reduced optimum sintering time. The results also show that silver inclusions are effective in homogenising and accelerating the formation of the BiPb(2223) phase in spite of an increase in the residual BiPb(2212) and secondary phases.
References [1] Q. Li, K. Brodersen, H.A. Hujler and T. Freltoft, Physica C 217 (1993) 360. [2] J. Yau and N. Savvides, Appl. Phys. Lett. 65 (1994) 1454. [3] J.W. Ekin, D.K. Finnemore, Q. Li, J. Tenbrink and J. Carter, Appl. Phys. Lett. 61 (1992) 858. [4] W. Gao and J.B. Vander Sander, Physica C 181 (1991) 105.
52
M.K. Al-Mosawi et al. / Physica C 267 (1996) 45-52
[5] S.X. Don and H.K. Liu, Supercond. Sci. Technol. 6 (1993) 297. [6] Y. Yamada, B. Obst and R. Fliikiger, Supercond. Sci. Technol. 4 (1991) 165. [7] D.M. Spiller, C. Beduz, Y. Yang, Z. Yi, R. Riddle and K. Pham, Physica C 235-240 (1994) 3419. [8] A. Oota, J. lwaya, P. Songsak, T. Saigou and M. Funakura, Physica C 214 (1993) 9. [9] M. Wilhelm, H. Neumiiller and G. Ries, Physica C 185-189 (1991) 2399. [10] S.X. Dou, K.H. Song, H.K. Liu, C.C. Sorrel, M.H. Apperley, A.J. Gouch, N. Savvides and D.W. Hensley, Physica C 160 (1989) 533. [11] J.C. Grivel and R. Fliikiger, Physica C 229 (1994) 177. [12] Y.E. High, Y. Feng, Y.S. Sung, E.E. Hellstrom and D.C. Larbalestier, Physica C 220 (1994) 81. [13] M. Lelovic, P. Krishnaraj, N.G. Eror and U. Balachandran, Physica C 242 (1995) 246.
[14] Z. Yi, L. Law, S. Fisher, C. Beduz, Y. Yang and R.G. Scurlock, Physica C 254 (1995) 193. [15] J.P. Singh, J. Joo, N. Vasanthamohan and R.B. Poeppel, J. Mater. Res. 8 (1993) 2458. [16] J. Joo, J.P. Singh and R.B. Poeppel, Supercond. Sci. Technol. 6 (1993) 421. [17] Y.C. Guo, H.K. Liu and S.X. Dou, Physica C 215 (1993) 291. [18] S.X. Dou, H.K. Liu, Y.C. Guo, R. Bhasale, Q.Y. Hu, E. Babic and I. Kusevic, Appl. Supercond. 2 (1994) 191. [19] Y.C. Guo, H.K. Liu and S.X. Dou, Mater. Lett. 18 (1994) 336. [20] N. Wu, T. Wei, S. Hou and S. Wong, J. Mater. Res 5 (1990) 2056. [21] R.P. Vasques, B.D. Hunt and M.C. Foote. Appl. Phys. Lett. 53 (1988) 2692.
Physica C 267 (1996) 45-52
The fabrication, microstructure and transport current study of BPSCCO 2223 multi-connected tapes M.K. A1-Mosawi *, C. Beduz, Y. Yang, R.G. Scurlock Institute of Cryogenics, University of Southampton, Southampton, S017 1BJ, UK Received 11 November 1995; revised manuscript received 20 May 1996
Abstract
The properties of BPSCCO-2223 tapes produced by the oxide powder in tube (OPIT) method with a "multi-connected" core configuration are presented. By using large silver particles ( ~ 80 p,m) up to 50 wt.% in the precursor BPSCCO powder, a multi-connected core with a homogeneous distribution of elongated and aligned silver platelets was obtained in the processed tapes. The samples were sintered at 832°C in air for different times with two intermediate cold-rolling processes and were characterised by transport critical current measurements at 77 K (0 T < B < 0.5 T), XRD, SEM, and optical microscopy. The effect of silver addition on the formation rate of the high-temperature BiPb(2223) phase is presented. The XRD analysis shows a saturation percentage of the BiPb(2223) phase within the core that is not affected by increased sintering time. Although the initial speed of conversion of the BiPb(2223) phase is seen to increase with increasing silver addition, the saturation percentage decreases. Local phase analysis shows a homogeneous distribution of the BiPb(2223) phase throughout the multi-connected tape, in contrast to the significant phase gradient observed in the control tapes without silver addition. For similar processing parameters, the critical current density increases with increasing silver addition (Jc = 20 kAcm -2 at 77 K (B = 0) with 50 wt.% added Ag, compared to Jc = 10 kA cm -2 without Ag).
1. Introduction
Critical current densities as high as 6 × 10 4 A / c m 2 at 77 K have been reported [1] for (BiPb)ESr2Ca2Cu3Oy tapes. These high currents carrying capabilities have initiated an enormous effort in the optimisation of tape processing. For practical application requiring long lengths of tapes, the reproducibility of the tape and the homogeneity of the critical currents are important factors. In addition, the conductor strain tolerance (the irreversible strain limit) above which the critical current density degrades irreversibly must be considered. At the pre* Corresponding author. Fax: + 44 1703 593053.
sent time, monocore tapes have a strain tolerance below the required values for most applications [2]. Although multifilament tapes offer a higher strain tolerance due to a reduced core thickness [3], the fabrication of multifilament tapes is a complicated and time-consuming process. It is generally believed [4,5] that grain alignment, densification and phase formation are among the critical parameters in determining the current carrying capability of the tapes. For the tapes produced by the oxide powder in tube (OPIT) technique, these parameters are governed to a considerable extent by the mechanical processes [6] (including drawing and rolling), initial powder stoichiometry [6], precursor phase composition, particle size [1] and thermal pro-
0921-4534/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PH S 0 9 2 1 - 4 5 3 4 ( 9 6 ) 0 0 3 1 9-X
46
M.K. Al-Mosawi et a l . / Physica C 267 (1996) 45-52
cesses (sintering temperature, duration etc.). Other factors include the sheath material [7], core thickness [8,9] and core additives [10,11]. It is accepted [12,13] that an enhancement of 2212-2223 conversion and grain alignment occurs at the silver sheath-core interface. Also, a variation in the local phase and microstructure at different distances from the silver-core interface has been found [14]. Silver addition is widely used [15-19] to enhance the Jc and strain tolerance of the BiPb(2223) tapes. In these reports the Ag added BiPb(2223) monocore tapes were fabricated by using fine (2-3
,
,,
~ ¢ ", ~i
ixm) equiaxed Ag20 and/or Ag powders, and Ag flakes (10 p,m). These reports show that the optimum Ag amount to maintain relatively high Jc is around 15 vol.% beyond which the critical current density degrades. The reason for this degradation was related to the morphology of the added Ag. In this paper, we report on the processing and the properties of BiPb(2223) tapes with a multi-connected core configuration (produced by using large silver particles ( ~ 80 Ixm) up to 50 wt.% in the precursor BPSCCO powder) which gives a more homogeneous phase formation. It is also expected
i / i:i~ iiiii!i!iii?iii:ii:iiii[i~i¸I;~¸¸¸¸¸!I¸;¸
,_
50~,m ,,
Fig. 1. Optical micrographs of the sample with 30 wt.% added silver showing the (a) transverse and (b) longitudinal cross-sections.
M.K. AI-Mosawi et al. / Physica C 267 (1996) 45-52
that this tape will have a Jc-strain characteristic similar to that of the multifilament tapes, with the advantage of a single-step production as for monocore tapes. The purpose of this comparative study is to understand the role of silver addition in the superconducting core. As explained earlier, the critical current density (Jc) of a tape depends on a large number of inter-related parameters. The thermal and mechanical processes used in this work are not optimised, and the Jc obtained for the reference and Ag added samples do not represent the best obtainable values.
47
2. Experimental The precursor powder used in this work was Merck partially calcined Bil.s4Pb0.34Srl.91Ca2.03Cu3.06Oy , which comprises BiPb(2212) as the major phase, with 0, 30, and 50 wt.% added silver particles ( ~ 80 ixm). Two sets of tapes were produced by the OPIT method, using a silver alloy tube (99.5% Ag, 0.25% Ni, 0.25% Mg). The first set was sintered at 832°C in air followed by two intermediate mechanical rollings after a total sintering time of 30 h and 60 h (hereinafter 30R + 30R), and finally sintered up to
Fig. 2. Optical micrographs of the sample with 50 wt.% added silver showing the (a) transverse and (b) longitudinal cross-sections.
48
M.K. Al-Mosawi et al. / Physica C 267 (1996) 45-52
180 h at 20 h intervals. In the second set of samples, the two intermediate cold rollings were carried out after 10 h and 20 h (hereinafter 10R + 10R). Transport current and Jc-B measurements were carried out on 50 mm long samples by using the standard four-probe method at 77 K in a magnetic field up to 0.5 T, using a voltage criterion of 1 ~V cm -~. The critical current densities were calculated excluding the area of the silver inclusions. Microstructural and phase formation analysis were conducted by using optical microscopy, SEM, and Cu K a X-ray diffractometer. The percentage of 2223 phase was estimated from the intensity of the BiPb(2223) peak and the Bi(2212) peak as follows
[2o]: a ( % 2 2 2 3 ) = 12223(0010)//(12212(008) + •2223(0010))"
3. Results and discussion
3.1. Optical and SEM observation The optical micrographs of the polished transverse and longitudinal cross-sections of the samples with 30 and 50 wt.% Ag added are shown in Figs. 1 and 2, respectively. It can be seen that the silver inclusions divide the superconducting core into thin (10-20 txm) multi-connected filaments for samples with 30 wt.% added silver and 2-10 Ixm for samples with 50 wt.% Ag. Hence, a large proportion of the ceramic core is in close vicinity to the partially aligned elongated silver platelets with an average aspect ratio of 1 : 8. The thickness of the multi-connected filaments is comparable to that in multifilamentary tapes, hence a similar strain tolerance is expected for these tapes. The study of the effects of silver addition on the Jc-strain characteristics will be reported later. Figs. 3(a) and (b) show the SEM micrographs of the longitudinal cross-sections of the samples with 0 wt.% and 50 wt.% added Ag. The samples are etched before SEM analysis by using 1 vol.% Br in methanol for 30 s [21]. Apart from the region near the sheath-core interface, the sample with 0% Ag has a lower density with a significant amount of voids which may be due to grain misalignment during grain growth. The sample with 50 wt.% added
Fig. 3. SEM micrographs showing the etched cross sections of the samples with (a) 0 wt.% Ag and (b) 50 wt.% Ag.
silver has a lower void fraction largely due to an increase in the contact area between the ceramic core and silver. Also, the void fraction may have been reduced by the deformation of the annealed silver inclusions during the intermediate roilings.
3.2. BiPb(2223) phase formation The BiPb(2223) phase formation as a function of sintering time measured by XRD for both sets of tapes is shown in Fig. 4. The layer at which the XRD were performed is approximately 10-15 ixm from the silver sheath. As will be seen later, it is important to state the depth of the layer at which the XRD measurements are conducted especially for the reference tape. The initial overall formation speed of BiPb(2223) phase increases with increasing silver
M.K. AI-Mosawi et al. / Physica C 267 (1996) 45-52
1°°I 90
ever, from Fig. 4 it can be seen that the saturation level of the BiPb(2223) after long sintering time (t > 80 h) decreases as the amount of added silver increases. This reduction in the saturation level is accompanied by an increase in the residual amount of secondary phases (CaEPbO4, CaCuO2, etc.) seen in Fig. 5. The reason for the saturation level is not yet understood. It appears from Fig. 4 that the estimated percentage of the BiPb(2223) phase for the tapes with 50 wt.% Ag is slightly decreasing, however, this reduction is too small to be explained and further work is needed.
,,
80
8
I ~
o
o
.Y/
40
30 / I 1 /
20 Ill / 10 / ~
10R+IOR 30R+30R
1
~
49
A
.t
0
[:3
•
30wt%Ag
wt%A,
3.3. Phase gradient
0 0
20
40
60
80
100
120
140
160 180 200
In order to investigate the BiPb(2223) phase percentage at different planes parallel to the silver interface, the tapes were thinned down by polishing about 10 Ixm before each XRD measurement. The error in the thickness measurement is about + 3 Ixm. Fig. 6 shows the local percentage of the BiPb(2223) phase at the peak Jc as a function of the distance from the sheath-core interface. For the 0% Ag tapes, there is a significant BiPb(2223) phase gradient across the
Total Sintering T i m e (hr) Fig. 4. BiPb(2223) phase formation as a function of the sintering time measured by XRD on a layer approximately 10-15 p~m from the silver sheath for samples with different silver addition.
addition in the ceramic core. This is consistent with the finding [14] that the rate of the phase conversion is largest at the silver sheath-core interface. How-
a-
E
'
'~
'
'
~"
' O%~vt Ag
e4
b
3 O % w t Ag
c
5 0 % w t Ag
e-
l,-
-$ r,,
20
22
24
26
28
30
32
34
36
38
40
20 Fig. 5. XRD patterns of the samples with (a) 0 wt.% Ag, (b) 30 wt.% Ag and (c) 50 wt.% Ag, sintered at 832°C for 180 h; ( * ) is for secondary phases.
50
M.K. AI-Mosawi et a l . / Physica C 267 (1996) 45-52
30R+30R
90
80 --"g- ..... -~ . . . . . . . . ~ ..... n . . . . . . . . . . . . -o r~
\
70
/ /A
•
/
~
-'/
\
60
o
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e wt%A$
(~140k~)
_---12-- 30wt%A| (t=14OhrJ) + 50wt%Ag (t=-100hrs) 50
a
0
I
t
10
I
1
20
I
t
30
I
40
i
I
50
Distance x, pm
Fig. 6. Local percentage of the BiPb(2223) phase at the optimum Jc as a function of the distance from the sheath-core interface.
core with the maximum at the sheath-core interface and minimum in the centre. In comparison, the tapes with Ag addition show a homogeneous distribution of the BiPb(2223) phase as shown in Fig. 6. This is likely to be the result of the increase in the interaction between silver and the ceramic core throughout the whole cross-section. 3.4. Transport current measurement
Fig. 7 shows the effect of sintering time on the transport critical current density at 77 K in zero 24000 22000 _30R+30R 20000
24000 +0wt%Ag ---o-- 30wt%Ag
~ / ~
lSOOO
- - o - - 50wt%Ag
16000 ~ 14000 12000 I0000 = 8000 6000 = 4000 200O 0 60
~ •~ ~ ~
,~77K
t 80
magnetic field for the 30R + 30R samples. The critical current densities of 0, 30, 50 wt.% Ag reach its maximum at total sintering time of 140, 140 and 100 h, respectively. The corresponding peak Jc values are 9800, 13 000 and 20200 A cm -2. Although a larger residual amount of the BiPb(2212) phase remains in the silver-added samples (processed at similar parameters) as shown in Fig. 4, the peak Jc values is enhanced by silver addition. The early peak in the J~-time curve for the sample with 50 wt.% added Ag is likely to be due to the acceleration of phase formation. The inhomogeneous distribution of the BiPb(2223) phase within the core in the 0 wt.% Ag samples means that the required time to optimise different regions will increase gradually from the silver interface to the centre of the core. Consequently, it is difficult to optimise the entire core at a given time, i.e. the material can be considered as a "multi-system". In contrast, for samples with 50 wt.% Ag, most of the superconducting core is in closer vicinity to the silver sheath a n d / o r inclusions resulting in homogeneous phase conversion and, therefore, the entire core may reach its optimum state (phase formation, grain size, cracks healing, etc.) at the same time. This may explain why the J~ values for the samples with 50 wt.% Ag have peaked sharply at an earlier sintering time than the samples without silver addition. The sharpness of the Jc-time curve means that the multi-connected tapes are more sensitive to the thermal processing time which needs to be carefully controlled in order to optimise the tape.
I 100
I 120
I 140
i 160
I 180
22000 20000 18000 16000 14000 12000
10R+IOR
Sintefing Time oars) Fig. 7. The effect of the sintering time on the critical current density at 77 K in zero magnetic field for the 30R + 30R samples.
0wt%Ag 30wt%Ag t%Ag
10000 8000 •=o 6000 L~ 4000 2000 200
+
self field,
60
77K
I
q
I
i
!
1
I
80
100
120
140
160
180
200
220
Sintering Time Oars) Fig. 8. The effect of the sintering time on the critical current density at 77 K for the 10R+ 10R samples.
M.K. AI-Mosawi et al. / Physica C 267 (1996) 45-52 1.0
a) 50wt%Ag
b) 30wt%Ag
~.\\
H
r~
--o- S0hrs
~ h
r
s
lOOhrs 120hrs! 140hrs
51
c) 0wt%Ag
-~- 16oh.
.~0.5
0.0 102
103
102
103
102
103
104
Applied Magnetic Field, (G) Fig. 9. The normalised in-plane Jc-B characteristics at 77 K for samples with (a) 50 wt.% Ag, (b) 30 wt.% Ag and (c) 0 wt.% Ag.
In the 30R + 30R samples, the average estimated percentage of BiPb(2223) phase present prior to the second intermediate cold rolling was approximately 70% which is close to the saturation level for the silver-added samples. Consequently, the second rolling process may be already too late for any cracks to be fully healed by the final sintering process. The second set of samples was produced with earlier intermediate cold rollings after sintering time of 10 h and 20 h. The overall conversion prior to the second rolling is reduced to 45% and 62% for samples with 30 wt.% and 50 wt.% added silver, respectively. The critical current density as a function of sintering time for the 10R + 10R samples is shown in Fig. 8. While there is no significant change in the peak J~ value for the 50 wt.% Ag samples, an enhancement from 13 000 A cm- 2 to 18 000 A cm- 2 for the sample with 30 wt.% Ag was obtained. The little change in the 50 wt.% Ag samples is not unexpected due to the higher level of the residual saturate liquid phases, which may be sufficient for healing microcracks at all times.
3.5. J ¢-B characteristics Fig. 9 shows the normalised in-plane Jc-B characteristics. For a particular sintering time (t), the Jc-B dependence is scaled to Jc(B, t)/Jc(B = O, t). It appears that the sample with 0% added Ag exhibits a better Jc-B dependence than the samples with silver addition; however, the difference is reduced with increasing sintering time. At the early sintering stage, the BiPb(2223) phase in the multiconnected tape is likely to develop into isolated
islands around the silver inclusions which are weakly inter-connected and extend to the sheath-core interface. With increasing sintering time the connectivity between the islands improves, as does the field dependence. At the optimum sintering time, the slight difference in Jc-B dependence is likely to be due to the reduced in-plane alignment in the multi-connected tapes compared to the monocore tapes.
4. C o n c l u s i o n s By adding large silver particles ( ~ 80 I~m) in the precursor powder, we have produced high-J c tapes with multi-connected thin filaments of 5-15 Ixm thick which are essential to improve the strain tolerance of the tape. For the same processing parameters, an enhancement of up to 200% in the optimum Jc was obtained in the silver-added samples with a reduced optimum sintering time. The results also show that silver inclusions are effective in homogenising and accelerating the formation of the BiPb(2223) phase in spite of an increase in the residual BiPb(2212) and secondary phases.
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M.K. Al-Mosawi et al. / Physica C 267 (1996) 45-52
[5] S.X. Don and H.K. Liu, Supercond. Sci. Technol. 6 (1993) 297. [6] Y. Yamada, B. Obst and R. Fliikiger, Supercond. Sci. Technol. 4 (1991) 165. [7] D.M. Spiller, C. Beduz, Y. Yang, Z. Yi, R. Riddle and K. Pham, Physica C 235-240 (1994) 3419. [8] A. Oota, J. lwaya, P. Songsak, T. Saigou and M. Funakura, Physica C 214 (1993) 9. [9] M. Wilhelm, H. Neumiiller and G. Ries, Physica C 185-189 (1991) 2399. [10] S.X. Dou, K.H. Song, H.K. Liu, C.C. Sorrel, M.H. Apperley, A.J. Gouch, N. Savvides and D.W. Hensley, Physica C 160 (1989) 533. [11] J.C. Grivel and R. Fliikiger, Physica C 229 (1994) 177. [12] Y.E. High, Y. Feng, Y.S. Sung, E.E. Hellstrom and D.C. Larbalestier, Physica C 220 (1994) 81. [13] M. Lelovic, P. Krishnaraj, N.G. Eror and U. Balachandran, Physica C 242 (1995) 246.
[14] Z. Yi, L. Law, S. Fisher, C. Beduz, Y. Yang and R.G. Scurlock, Physica C 254 (1995) 193. [15] J.P. Singh, J. Joo, N. Vasanthamohan and R.B. Poeppel, J. Mater. Res. 8 (1993) 2458. [16] J. Joo, J.P. Singh and R.B. Poeppel, Supercond. Sci. Technol. 6 (1993) 421. [17] Y.C. Guo, H.K. Liu and S.X. Dou, Physica C 215 (1993) 291. [18] S.X. Dou, H.K. Liu, Y.C. Guo, R. Bhasale, Q.Y. Hu, E. Babic and I. Kusevic, Appl. Supercond. 2 (1994) 191. [19] Y.C. Guo, H.K. Liu and S.X. Dou, Mater. Lett. 18 (1994) 336. [20] N. Wu, T. Wei, S. Hou and S. Wong, J. Mater. Res 5 (1990) 2056. [21] R.P. Vasques, B.D. Hunt and M.C. Foote. Appl. Phys. Lett. 53 (1988) 2692.