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Methods of introducing lead into bismuth-2223 and their effects on phase development and superconducting properties
PHYSICA PhysicaC 223 (1994) 163-172
ELSEVIER
Methods of introducing lead into bismuth-2223 and their effects on phase development and superconducting properties S.E. D o r r i s *, B.C. P r o r o k , M . T . L a n a g a n , N . B . B r o w n i n g , M . R . H a g e n Energy Technology Division ~,Argonne National Laboratory, Argonne, IL 60439-4838, USA
J.A. P a r r e U , Y. F e n g , A. U m e z a w a , D . C . L a r b a l e s t i e r Applied Superconductivity Center 2, University of Wisconsin, Madison, W153706-1206, USA
Received 8 November 1993;revised manuscript received20 January 1994
Abstract The specific method of introducing lead significantlyinfluences the phase development and superconductingproperties of Pbdoped Bi2Sr2Ca2Cu3Ox(BSCCO-2223) Ag-sheathedpowder-in-tube tapes. This is demonstrated in a study of tapes made from a series of powder mixtures containing the phases BiLsPb~Sr2.oCat.oCu2.oOs(2212), Ca2PbO4, Ca2CuO3, and CuO, where the lead content of 2212, z, was varied from 0.0 to 0.4. The overall composition of each mixture was the same, (Bil.sPbo.4Sr2.oCa2.oCuxoOto), and the amounts of each phase were varied to compensate for changes in z. In an additional mixture made from lead-free 2212, Ca2CuO3, and CuO, all lead was added as PbO. A significantly more complete 2223 formation, sharper transition temperatures, and higher critical current densities were seen when all lead was incorporated in the 2212 phase, rather than being added as either PbO or Ca2PbO4.
1. Introduction Critical current densities, Jc, greater than 6 X 104 A / c m 2 (at 77 K in zero applied field) [1-3] have been measured on short lengths ( < 10 cm) of bismuth-2223 tape made by the powder-in-tube technique. To obtain such high Jc values, the superconductor cores must be made very thin ( ~, 30-50 ttm) [4]. However, second phases in bismuth-2223 tapes * Correspondingauthor. Work supported by the US Department of Energy, EnergyEfficiencyand Renewable Energy,as part of a programto develop electric powertechnology,under ContractW-3l-109-Eng-38. 2 Work supported by Electric Power Research Institute (Contract RP 8009-05) and Advanced Research Projects Agency (Contract N00014-90-J-4115).
typically have dimensions of 10-20 Ixm, so even small amounts of second phase can form a significant fraction of the cross-sectional area and thereby severely disrupt the transport current through thin cores. Therefore, it is crucial to minimize the amount and size of nonsuperconducting second phases, especially during the fabrication of long lengths of conductor. Mukai et al. [ 5 ] reported that the Jc of powder-intube samples increased from 0.5 × 104 to 1.1 X 104 and finally to 4.7 X 104 A / c m 2 as the content ofnonsuperconducting phases decreased from 30 to 20 vol.% and finally to 8 vol.%. A recently introduced so-called two-powder process [6 ] reproducibly yields a more uniform and complete reaction to the 2223 phase than does the typical mixed-phase powder process [ 1-5]. Rela-
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tively thick tapes made by the two-powder process (having core thicknesses of 60-100 Ixm ) have developedj¢ values (0 T, 77 K) as high as 30)< l0 s A / c m 2 [6]. While others not using this process have reported higher Jc, the high purity and good reproducibility provided by the two-powder process should facilitate the fabrication of long lengths of superconductor with a high Jc. In the two-powder process, intermediate precursors of lead-doped 2212 (Bil.sPbo.4SrE.oCal.oCu2.oOs) and CaCuO2 are made separately, then combined and reacted inside the silver sheath. Because CaCuO2 decomposes to Ca2CuO3+CuO above ~ 7 4 0 ° C [7], formation of 2223 by the two-powder process primarily involves reaction between BiLsPbo.4Sr2.oCaLoCu2.oO8, Ca2CuO3, and CuO. A small amount of Ca2PbO4 is also present because it exists in the lead-doped 2212 powder as a minority second phase. In the present study, we investigate one aspect of the two-powder process, namely the method by which lead is introduced. It is known that lead is crucial for the formation of 2223 [8-10], but it has yet to be shown that the method of introducing lead also influences the phase development and properties of 2223. Some have speculated that Ca~PbO4 is a particularly beneficial source of lead [ 11-13 ] because it induces melting and thereby enhances formation of the 2223 phase and heals damage caused by mechanical processing. However, if a large fraction of lead is present as Ca2PbO4, dissociation of Ca2PbO4 may control the formation of 2223; thus it is not clear that the presence of Ca2PbO4 is desirable. Others describe a process (hereafter called the PbO process) in which leadfree 2212 reacts with Ca2CuO3 and CuO, and the lead is introduced as PbO [ 14]. X-ray analysis showed that formation of 2223 was 92-100% complete (95% on average) in tapes made by the PbO process [ 14 ], suggesting that formation of 2223 was significantly less complete than in tapes made by the two-powder process. Because the main difference between these two processes is the method of introducing lead, the difference in 2223 phase purity suggests that the method significantly affects the 2223 phase development. To demonstrate that the method for introducing lead influences the phase development and properties of 2223 tapes, powder mixtures were made from
the phases Ca2PbO4, CaeCuO3, CuO, and BiLsPbzSr2.oCaLoCu2.oO8. Each mixture had the same overall composition, BiLsPbo.4SrzoCa2.oCus.oOlo, but differed in the lead content of the 2212 phase, z (0.0~
2. Experimental details The mixture compositions are listed in Table 1 in terms of molar amounts of each phase added per mole of BiLsPbzSr2.0CaLoCu2.008. The phases Bil.sPbzSr2.0Cal.oCu2.008, Ca2PbO4, and Ca2CuO3 were prepared from the appropriate amounts of Bi203, PbO, SrCO3, CaCO3, and CuO for z values of 0.0, 0.1, 0.2, 0.3, and 0.4. To ensure complete decomposition of the carbonates, the powders were calcined first at a reduced total pressure of -~ 3 Tort O2 (20 ° C / h to 750°C, followed by 6 b at 750°C). After calcining at a reduced pressure, the desired phases were formed by further calcining for 24-48 h at 840°C in CO2 free air at ambient pressure. The powders were then ball milled in isopropyl alcohol for 12-16 h and calcined again at 840°C in CO2 free air at ambient pressure. Calcinations were repeated with intermediate ball-milling steps until near-single-phase materials were obtained. All calcinations after the initial calcination at reduced total pressure were at 840°C in CO2 free air at ambient pressure. Total calcination
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Table 1 Composition o f multiphase mixtures in moles added per mole BiLaPb~Sr2.oCaLoCu2.oOs. The overall composition of each mixture is BiLsPbo.4Sr2.oCa2.oCu3.oO to z
Ca2PbO4
Ca2CuO3
CuO
Comments
0.0 0.1 0.2 0.3 0.4 0.0
0.4 0.3 0.2 0.1 0.0 0.0 (0.4 PbO)
0.1 0.2 0.3 0.4 0.5 0.5
0.9 0.8 0.7 0.6 0.5 0.5
Lead-free 2212 Two-powder Lead-free 2212
times were 96-126 h for Bil.sPb~Sr2.oCal.oCu2.oOs, 24 h for C a 2 P b O 4 , and 48 h for Ca2CuO3. Each mixture of Table l was made by first ball milling the appropriate amounts of Ca2PbO4, Ca2CuO3, and CuO in isopropyl alcohol for ~ 16 h. The appropriate amount of this intermediate mixture was then added to Bil.sPbzSr2.oCal.oCu2.oOs to obtain the final mixture. This double mix procedure was designed to improve the mixing of small amounts of three different phases with a relatively large amount of Bit.sPbzSr2.oCal.oCu2.oO8. Powders were characterized by X-ray diffraction ( X R D ) and by differential thermal analysis (DTA). DTA measurements were made in 8% 02 at a ramp rate of 5°C/min with a Harrop DT-726 instrument. XRD patterns were obtained with a Scintag XGEN4000 diffractometer over the range in 20 of 2-50 °. The powder mixtures were packed into silver tubes having an outer diameter of 0.635 cm, an inner diameter of 0.435 cm, and a length of ~ 15 cm. End caps were fashioned from the silver rod with the same outer diameter as the tubes. One end of each cap was reduced to a diameter of 0.435 cm to fit as a plug into the tube ends. After one end cap was brazed to the tube with silver solder, powder was packed into the tube by ramming the powder while vibrating the tube. With this method, ~ 25% of the theoretical density was obtained. Before the remaining end cap was attached, the tubes were heated at a reduced total pressure (6 h / 7 0 0 ° C / ~ 2 Torr 02) to remove absorbed moisture and/or CO2 before sealing the tubes. Immediately after the vacuum anneal, the remaining end cap was brazed on to seal the tube. The vacuum-annealed tube was drawn in a stepwise fashion to a diameter of 2 mm. The wire was then rolled between two 15.24 cm diameter steel rolls
with a maximum 10% reduction in area per pass. The final overall tape thickness was 0.05 cm. Samples ( ~ 4 cm long) were cut from the tapes and heat treated in 8% 02 at 815 ° C. Thermomechanical processing of the tapes began with a 50 h anneal at 815°C, after which the tapes were uniaxially cold pressed at = 2 GPa to reduce the tape thickness by ,~ 20%. Following pressing, the tapes were annealed for an additional 100 h at 815 ° C. This process of pressing followed by annealing was repeated until cumulative heat-treatment times of up to 250 h had been reached. The critical current of each tape was measured by standard four-lead techniques at 77 K in zero applied field. Jc(0 T, 77 K) values were defined with a 1 ~tV/ cm criterion and the measured transverse cross-sectional areas. The AC susceptibility of tapes in their final reaction condition was measured, i.e. after the tapes had been annealed for 250 h at 815°C in 8% 02. Both the real and imaginary components were measured using an AC susceptometer in which a primary 103 Hz field of 1 to 20 G zero-to-peak was applied normal to the plane of the tape. Microstructures of selected samples were examined both by scanning electron microscopy (SEM) in electron backscatter imaging mode (JEOL 35C) and by transmission electron microscopy (TEM) in a Philips CM 20 Ultratwin. The specimen preparation techniques have been described elsewhere [15,16].
3. Results DTA results are shown in Fig. 1 (a) for the 2212 powders with variable Pb content and in Fig. 1 (b)
S.E. Dorris et al. / Physica C 223 (1994) 163-172
166
(a) •z = ~
8
8
3
o
c
--
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.z_
. . . . . .
550
650
750
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...z ..--....O.4 . . . . .
~
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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. 550
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.
650
.
.
i
i
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,
750 Temperature
.
.
, 850
,~'fT,~°f . . . . 950
1050
(°C)
Fig. I. DTA traces for (a) BiLsPb~rz.oCal.0Cuz.oOs for 0.0~
for the 2223 multiphase mixtures of Table 1. The DTA traces for Pb doped 2212 powders (Fig. I ( a ) ) consist of a single endotherm whose position decreases by -~ 7 ° C as the lead content increases from 0.0 to 0.4. It has been reported [ 17 ] that Pb substitution into 2223 causes a similar decrease in its melting point. The DTA traces for the 2223 multiphase mixtures (Fig. 1 ( b ) ) consist of three endotherms: The highest-temperature endotherm is broad and shallow, extending from ~ 910-970°C. This endotherm is positioned well above the normal processing range for 2223 tapes, and is thought to result from a reaction involving the alkaline earth cuprate phase and per-
haps copper oxide. The absence of this endotherm from the DTA traces in Fig. 1 (a) is consistent with this idea, because neither copper oxide nor alkaline earth cuprates are present in the 2212 powders. A second endotherm appears at 877 ° C, and like the first endotherm, is independent of the mixture composition in terms of its size and position. By contrast, the size of the third endotherm depends markedly on composition; the position of the endotherm also depends on composition, but to a lesser degree, decreasing from 850°C for z = 0 . 4 to 846°C for z=0.0. The size of the third endotherm decreases significantly as z increases from 0.0 to 0.4, i.e., as the mixture changes from one in which all lead is present as Ca2PbO4 to a mixture in which essentially all lead is incorporated in the 2212. (While no Ca2PbO4 is deliberately added to the z = 0 . 4 composition, a small amount is present as a second phase, possibly because the solubility limit for lead is less than 0.4 [ 18 ]. As a result, the z = 0.3 and 0.4 mixtures contain nearly the same amount of Ca2PbO4. This was confirmed by the Ca2PbO4 peak heights in the X R D patterns of the mixtures. ) The apparent correlation between the size of the low-temperature endotherm and the amount of Ca2PbO4 in the mixture strongly suggests that Ca2PbO4 controls the lowest-temperature endotherm, and when present, initiates melting. Fig. 2 compares the Jc(0 T, 77 K) values for tapes made from the different mixtures after total heattreatment times of 50, 150, and 250 h at 815°C in 8% 02. For each heat-treatment time, Jc increases as the amount of lead incorporated in the 2212 phase increases. After 250 h total heat-treatment time and 25.0
. . . .
I
. . . .
I . . . .
d . . . .
I
. . . .
20.0 15.0 I°'°(
150 h
5.0 0 0
0. I
i
i
i
0.2
0.3
0.4
0
(PbO) z, moles of Pb in 2212 Fig. 2. J~ at 77 K and zero applied field as a function of 2212 lead content, z, after total heat-treatment times of 50, 150, and 250 h in 8% O2 at 815°C.
S.E. Dorris et al. / Physica C 223 (1994) 163-172
two intermediate pressings, J¢ for z = 0 . 4 reached 22 000 A / c m 2, approximately twice the value found for z = 0.0, whether the lead was added to the z = 0.0 composition as PbO or as Ca2PbO4. Incorporation of lead in the 2212 dearly yields properties superior to those obtained when lead is added as either PbO or Ca2PbO4. The microstructures of tapes made from the mixtures in Table 1 were examined after 250 h total heattreatment time with two intermediate pressings, i.e., in their most reacted state. Comparison of the microstructures indicates significant differences in 2223 phase development. Figs. 3 ( a - c ) show the microstructures of tapes made from the two z = 0 . 0 mixtures and the z = 0 . 4 mixture. In Fig. 3(a), all lead was added as Ca2PbO4, while in Fig. 3 (b), lead was
167
added as PbO, and in Fig. 3 (c), it was incorporated in 2212. Several distinct phases are evident in varying amounts in Figs. 3 (a-c). The black blocky phases and the light-gray needles were identified by SEM-energy dispersive spectroscopy as alkaline earth cuprates and 2212, respectively. The dark gray major phase was identified as 2223. When lead was added as Ca2PbO4 (z = 0.0), both 2212 and alkaline earth cuprates were abundant in the final microstructure, with the alkaline earth cuprates ~ 5-10 ~m in size. In tapes made from the z = 0.1-0.3 mixtures, the amounts of 2212 and alkaline earth cuprates decreased as the amount of Ca2PbO4 decreased in the mixture and the lead content of 2212 increased. When all lead was put into the 2212 ( z = 0 . 4 ) , the reaction to 2223 was the most
Fig. 3. Backscaner electron images of samples heated for 250 h in 8% 02 at 815°C: (a) z=0.0, Ca2PbO4=0.4; (b) z=0.0, PbO=0.4; and (g) z--0.4, Ca2PbO4--0.0, PbO--O.0.
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complete that we observed. The second phases were small ( < 5 ~tm) and low in concentration; however, small amounts o f a Bi rich phase were evident in some regions. When lead was added as PbO ( z = 0 . 0 ) , alkaline earth cuprate grains were larger (10-25 ~tm) and more abundant than when all lead was added as Ca2PbO,, while 2212 grains were very fine and not readily apparent. Fig. 4 compares the inductive Tc traces, taken in a field of 0. ! mT, of the most highly reacted samples of each phase mixture. The traces differ somewhat from each other in shape and onset T~ value. The sharpest transition was obtained for the most Pb rich 2212 sample (z--- 0.4) and the broadest transition for the Pb free sample ( z = 0.0), the intermediate compositions having traces between these two extremes. Small differences also exist in onset T~ values, these being slightly higher ( 108-109 K) for the Pb free and PbO powder mixtures than for the Pb-2212 powder ( 107 K). Additional insight is obtained by observing the field dependence of the transitions for the z = 0.0 and
>,
z=O ( P b ~
60
I
I
I
I
I
70
80
90
100
110
120
Z (K) Fig. 4. AC susceptibility of samples heat treated for a total of 250 h at 815 ° C in 8% 02 with two intermediate pressings. Data were taken in a field o f 0.1 mT.
z = 0 . 4 compositions, as shown in Fig. 5. There are distinct points of inflection on the transitions of the Pb free sample (Fig. 5 ( a ) ) , whatever the applied field. At temperatures lower than the main transition at 106-107 K, it is possible to define a lower-temperature transition T' by the construction shown in Fig. 5(a): this transition varies from 105 K (0.I m T ) to 79 K (2 m T ) . By contrast, the traces shown in Fig. 5(b) for the Pb-2212 sample ( z = 0 . 4 ) exhibit no inflections, except for the trace taken in the highest field. The PbO sample (Fig. 5 ( c ) ) exhibited initially quite sharp transitions but an inflection is seen in the 0.1 m T trace at ~ 75 K. The traces at fields from 0.12 m T are definitely sharper and less inflected for the PbO sample than for the lead-free 2212 sample in which all lead was added as Ca2PbO4. Of all samples, the Pb doped 2212 sample always exhibited the sharpest transitions.
4. Discussion
Taken as a whole, the results show that there is great benefit in incorporating Pb directly into the 2212 phase, rather than introducing Pb through an additional compound such as Ca2PbO4 or PbO. The most dramatic evidence is that of Fig. 2, which shows that the Jc (0 T, 77 K) values progressively and consistently increase as the Ca2PbO4 content of the starting powder decreases. The reason is that the conversion to the 2223 phase occurs more rapidly, more uniformly, and more completely when the Pb is already incorporated into the layered cuprate 2212 structure. The micrometer-scale microsctructural evidence for this is clear in the scanning electron micrographs of Fig. 3. The inductive Tc traces of Figs. 4 and 5 support these same conclusions, although it is noteworthy that even the best transitions still exhibit significant curvature down to at least 60 K. The shape of the Tc transition is a direct measure of the ability of the sample to shield the applied field from its interior. Fully connected, bulk-scale, homogeneous samples should exhibit full shielding at temperatures almost to To, but in fact this is only rarely the case for BSCCO-2223 samples. One reason for the lack of shielding is that the formation of the 2223 phase is incomplete, thus preventing the shielding current from circulating across the whole sample. In
S.E. DorMs et al. / Physica C 223 (1994) 163-172 I
I
i
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i
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i
i
i
i
[
L
>,
~
L T' (095KmT) _T'(1 roT) 95 K
/ Z . r / / 0 " 2
mI
(a) 60
I
I
I
I
I
70
80
90
100
110
(b) 120
60
i
]
i
i
i
70
80
90
100
110
T (K)
120
T (K)
2 mT
0.5 mT
0.I mT
(c) 60
70
80
90
100
110
120
T (K)
Fig. 5. AC susceptibility of (a) the z=0.0 sample, (b) the z=0.4 sample, and (c) the PbO sample. Data were taken in fields of 0.1-2 raT for samples heat treated for a total of 250 h at 815 oC in 8% 02 with two intermediate pressings.
170
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the work of Umezawa et al. [ 16,19 ], it was shown that the presence of kinks in the Tc transition, such as those seen very clearly in Fig. 5(b), was a direct consequence of the supercurrent passing through 2212 intergrowths which linger at the [ 001 ] twist boundaries separating grains of the majority 2223 phase. Since 2212 has a Tc of 80-81 K when surrounded by 2223 phase [ 19], these intergrowths produce an extended, very field-dependent transition at temperatures between the Tc of 2212 ( ~ 80 K) and that of 2223 ( ~ 105 K). The samples examined in these earlier studies [ 15,18 ] were made by the conventional one-powder, mixed-phase route [ 1-5,18 ] and all exhibited the type of inflected transition seen in Fig. 5(a), even in a weak applied field. By contrast, the two-powder tape with the maximum Pb doping exhibits a smoother, less inflected transition (at least in weak fields), which we presume is another consequence of the more complete and uniform reaction. However, there is an inflection visible in the 2 m T transition of Fig. 5 (b) for the optimally Pb doped sample, which led us to believe that some residual intergrowths of 2212 remained within the 2223 grains. This possibility was checked by TEM investigation of the most highly reacted Pb free and Pb doped 2212 samples. In both the z = 0.0 (lead added as CaEPbO4 ) and z = 0 . 4 samples, it was possible to observe 2212 intergrowths, some being within the grains and some being at the (001) twist boundaries. This is consistent with the broadened transitions seen in Fig. 5 for both samples. A typical view of the Pb free sample is shown in Fig. 6(a): 2212 layers are not seen right at the twist boundary but one half-cell of 2212 occurs at the third layer to the left of the boundary; there are also many other 2212 layers in the left-hand grain, including a block of twelve 2212 half-cells, as indicated on the Figure. By contrast, Fig. 6 (b) is a typical micrograph taken from the z = 0 . 4 Pb doped 2212 powder sample. In the right-hand grain there are two single 2212 half-cells, as well as a thicker half-cell having the lattice spacing appropriate for a fourCuO2-1ayer half-cell. As discussed in detail elsewhere [ 19 ], these 2212 layers are easily decoupled by weak fields; therefore the colonies of 2223 grains that contain them are easily rendered resistive to the passage of a transport current. The dependence of J¢ on the degree of reaction observed in this paper is thus a
natural consequence of the reaction mechanism of the 2212-to-2223 phase conversion, as expressed both on the 1-50 ~tm scale of the BSCCO core and on the nanometer scale of the grain boundary and intergrowth structure. Finally, we should briefly note that the residual broadening of the transitions of the Pb doped (z--0.4) sample may be because the field can penetrate between the grains of the 2223 phase, even when there is no 2212 phase present. This can occur either because some grain boundaries are inherently weaklinked or because of microcracks within the sample. In either case the sample becomes subdivided and the shielding current path does not circulate around the whole sample. It seems likely that both conditions exist even in the best reacted Ag sheathed BSCCO tapes [2,20]. Developing consistently high Jc values requires attention to all these points, not just to the reaction mechanism which has been the principal subject of this work. In all cases, the field-dependent Tc transition is a very useful tool for optimizing the process conditions for BSCCO tapes.
5. Conclusions The method of introducing lead significantly affects the phase development and superconducting properties of bismuth-2223 during powder-in-tube processing. When lead is added in the form of PbO or Ca2PbO4, relatively large grains of alkaline earth cuprates or 2212 remain even after 250 h of heat treatment. When lead is incorporated in the 2212 prior to its reaction with CaECuO 3 and CuO, the formation of 2223 is more complete and uniform. This is apparent in terms of fewer second phases with sizes of l - I 0 ~tm and fewer 2212 intergrowths. As a direct result of the more homogeneous formation of 2223, tapes made from Pb doped 2212 exhibit sharper transition temperatures and higher critical current densities.
Acknowledgements The authors would like to acknowledge discussions with E.E. Hellstrom (University of Wisconsin at Madison). The work of Argonne National Labora-
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Fig. 6. Transmission electron mierographs of the same samples as studied in Fig. 5: (a) view of (001) twist boundary and its adjacent grains within a colony of BSCCO-2223 found in the z=0 sample, and (b) view of (001) twist boundary and its adjacent grains within a colony of BSCCO-2223 found in the z=0.4 sample. Half-cells of the 2212, 2223, and 2234 phases are indicated by 2, 3, and 4, respectively. Twist boundaries are indicated by the designation TB. tory was s u p p o r t e d by the U S D e p a r t m e n t o f Energy, Energy Efficiency a n d Renewable Energy, as part o f a p r o g r a m to develop electric power technology, under Contract W-31-109-Eng-38. The U n i v e r s i t y o f
Wisconsin work was s u p p o r t e d by the Electric Power Research Institute (Contract RP-8009-05) a n d the A d v a n c e d Research Projects Agency ( C o n t r a c t N00014-90-J-4115 ).
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ELSEVIER
Methods of introducing lead into bismuth-2223 and their effects on phase development and superconducting properties S.E. D o r r i s *, B.C. P r o r o k , M . T . L a n a g a n , N . B . B r o w n i n g , M . R . H a g e n Energy Technology Division ~,Argonne National Laboratory, Argonne, IL 60439-4838, USA
J.A. P a r r e U , Y. F e n g , A. U m e z a w a , D . C . L a r b a l e s t i e r Applied Superconductivity Center 2, University of Wisconsin, Madison, W153706-1206, USA
Received 8 November 1993;revised manuscript received20 January 1994
Abstract The specific method of introducing lead significantlyinfluences the phase development and superconductingproperties of Pbdoped Bi2Sr2Ca2Cu3Ox(BSCCO-2223) Ag-sheathedpowder-in-tube tapes. This is demonstrated in a study of tapes made from a series of powder mixtures containing the phases BiLsPb~Sr2.oCat.oCu2.oOs(2212), Ca2PbO4, Ca2CuO3, and CuO, where the lead content of 2212, z, was varied from 0.0 to 0.4. The overall composition of each mixture was the same, (Bil.sPbo.4Sr2.oCa2.oCuxoOto), and the amounts of each phase were varied to compensate for changes in z. In an additional mixture made from lead-free 2212, Ca2CuO3, and CuO, all lead was added as PbO. A significantly more complete 2223 formation, sharper transition temperatures, and higher critical current densities were seen when all lead was incorporated in the 2212 phase, rather than being added as either PbO or Ca2PbO4.
1. Introduction Critical current densities, Jc, greater than 6 X 104 A / c m 2 (at 77 K in zero applied field) [1-3] have been measured on short lengths ( < 10 cm) of bismuth-2223 tape made by the powder-in-tube technique. To obtain such high Jc values, the superconductor cores must be made very thin ( ~, 30-50 ttm) [4]. However, second phases in bismuth-2223 tapes * Correspondingauthor. Work supported by the US Department of Energy, EnergyEfficiencyand Renewable Energy,as part of a programto develop electric powertechnology,under ContractW-3l-109-Eng-38. 2 Work supported by Electric Power Research Institute (Contract RP 8009-05) and Advanced Research Projects Agency (Contract N00014-90-J-4115).
typically have dimensions of 10-20 Ixm, so even small amounts of second phase can form a significant fraction of the cross-sectional area and thereby severely disrupt the transport current through thin cores. Therefore, it is crucial to minimize the amount and size of nonsuperconducting second phases, especially during the fabrication of long lengths of conductor. Mukai et al. [ 5 ] reported that the Jc of powder-intube samples increased from 0.5 × 104 to 1.1 X 104 and finally to 4.7 X 104 A / c m 2 as the content ofnonsuperconducting phases decreased from 30 to 20 vol.% and finally to 8 vol.%. A recently introduced so-called two-powder process [6 ] reproducibly yields a more uniform and complete reaction to the 2223 phase than does the typical mixed-phase powder process [ 1-5]. Rela-
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tively thick tapes made by the two-powder process (having core thicknesses of 60-100 Ixm ) have developedj¢ values (0 T, 77 K) as high as 30)< l0 s A / c m 2 [6]. While others not using this process have reported higher Jc, the high purity and good reproducibility provided by the two-powder process should facilitate the fabrication of long lengths of superconductor with a high Jc. In the two-powder process, intermediate precursors of lead-doped 2212 (Bil.sPbo.4SrE.oCal.oCu2.oOs) and CaCuO2 are made separately, then combined and reacted inside the silver sheath. Because CaCuO2 decomposes to Ca2CuO3+CuO above ~ 7 4 0 ° C [7], formation of 2223 by the two-powder process primarily involves reaction between BiLsPbo.4Sr2.oCaLoCu2.oO8, Ca2CuO3, and CuO. A small amount of Ca2PbO4 is also present because it exists in the lead-doped 2212 powder as a minority second phase. In the present study, we investigate one aspect of the two-powder process, namely the method by which lead is introduced. It is known that lead is crucial for the formation of 2223 [8-10], but it has yet to be shown that the method of introducing lead also influences the phase development and properties of 2223. Some have speculated that Ca~PbO4 is a particularly beneficial source of lead [ 11-13 ] because it induces melting and thereby enhances formation of the 2223 phase and heals damage caused by mechanical processing. However, if a large fraction of lead is present as Ca2PbO4, dissociation of Ca2PbO4 may control the formation of 2223; thus it is not clear that the presence of Ca2PbO4 is desirable. Others describe a process (hereafter called the PbO process) in which leadfree 2212 reacts with Ca2CuO3 and CuO, and the lead is introduced as PbO [ 14]. X-ray analysis showed that formation of 2223 was 92-100% complete (95% on average) in tapes made by the PbO process [ 14 ], suggesting that formation of 2223 was significantly less complete than in tapes made by the two-powder process. Because the main difference between these two processes is the method of introducing lead, the difference in 2223 phase purity suggests that the method significantly affects the 2223 phase development. To demonstrate that the method for introducing lead influences the phase development and properties of 2223 tapes, powder mixtures were made from
the phases Ca2PbO4, CaeCuO3, CuO, and BiLsPbzSr2.oCaLoCu2.oO8. Each mixture had the same overall composition, BiLsPbo.4SrzoCa2.oCus.oOlo, but differed in the lead content of the 2212 phase, z (0.0~
2. Experimental details The mixture compositions are listed in Table 1 in terms of molar amounts of each phase added per mole of BiLsPbzSr2.0CaLoCu2.008. The phases Bil.sPbzSr2.0Cal.oCu2.008, Ca2PbO4, and Ca2CuO3 were prepared from the appropriate amounts of Bi203, PbO, SrCO3, CaCO3, and CuO for z values of 0.0, 0.1, 0.2, 0.3, and 0.4. To ensure complete decomposition of the carbonates, the powders were calcined first at a reduced total pressure of -~ 3 Tort O2 (20 ° C / h to 750°C, followed by 6 b at 750°C). After calcining at a reduced pressure, the desired phases were formed by further calcining for 24-48 h at 840°C in CO2 free air at ambient pressure. The powders were then ball milled in isopropyl alcohol for 12-16 h and calcined again at 840°C in CO2 free air at ambient pressure. Calcinations were repeated with intermediate ball-milling steps until near-single-phase materials were obtained. All calcinations after the initial calcination at reduced total pressure were at 840°C in CO2 free air at ambient pressure. Total calcination
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Table 1 Composition o f multiphase mixtures in moles added per mole BiLaPb~Sr2.oCaLoCu2.oOs. The overall composition of each mixture is BiLsPbo.4Sr2.oCa2.oCu3.oO to z
Ca2PbO4
Ca2CuO3
CuO
Comments
0.0 0.1 0.2 0.3 0.4 0.0
0.4 0.3 0.2 0.1 0.0 0.0 (0.4 PbO)
0.1 0.2 0.3 0.4 0.5 0.5
0.9 0.8 0.7 0.6 0.5 0.5
Lead-free 2212 Two-powder Lead-free 2212
times were 96-126 h for Bil.sPb~Sr2.oCal.oCu2.oOs, 24 h for C a 2 P b O 4 , and 48 h for Ca2CuO3. Each mixture of Table l was made by first ball milling the appropriate amounts of Ca2PbO4, Ca2CuO3, and CuO in isopropyl alcohol for ~ 16 h. The appropriate amount of this intermediate mixture was then added to Bil.sPbzSr2.oCal.oCu2.oOs to obtain the final mixture. This double mix procedure was designed to improve the mixing of small amounts of three different phases with a relatively large amount of Bit.sPbzSr2.oCal.oCu2.oO8. Powders were characterized by X-ray diffraction ( X R D ) and by differential thermal analysis (DTA). DTA measurements were made in 8% 02 at a ramp rate of 5°C/min with a Harrop DT-726 instrument. XRD patterns were obtained with a Scintag XGEN4000 diffractometer over the range in 20 of 2-50 °. The powder mixtures were packed into silver tubes having an outer diameter of 0.635 cm, an inner diameter of 0.435 cm, and a length of ~ 15 cm. End caps were fashioned from the silver rod with the same outer diameter as the tubes. One end of each cap was reduced to a diameter of 0.435 cm to fit as a plug into the tube ends. After one end cap was brazed to the tube with silver solder, powder was packed into the tube by ramming the powder while vibrating the tube. With this method, ~ 25% of the theoretical density was obtained. Before the remaining end cap was attached, the tubes were heated at a reduced total pressure (6 h / 7 0 0 ° C / ~ 2 Torr 02) to remove absorbed moisture and/or CO2 before sealing the tubes. Immediately after the vacuum anneal, the remaining end cap was brazed on to seal the tube. The vacuum-annealed tube was drawn in a stepwise fashion to a diameter of 2 mm. The wire was then rolled between two 15.24 cm diameter steel rolls
with a maximum 10% reduction in area per pass. The final overall tape thickness was 0.05 cm. Samples ( ~ 4 cm long) were cut from the tapes and heat treated in 8% 02 at 815 ° C. Thermomechanical processing of the tapes began with a 50 h anneal at 815°C, after which the tapes were uniaxially cold pressed at = 2 GPa to reduce the tape thickness by ,~ 20%. Following pressing, the tapes were annealed for an additional 100 h at 815 ° C. This process of pressing followed by annealing was repeated until cumulative heat-treatment times of up to 250 h had been reached. The critical current of each tape was measured by standard four-lead techniques at 77 K in zero applied field. Jc(0 T, 77 K) values were defined with a 1 ~tV/ cm criterion and the measured transverse cross-sectional areas. The AC susceptibility of tapes in their final reaction condition was measured, i.e. after the tapes had been annealed for 250 h at 815°C in 8% 02. Both the real and imaginary components were measured using an AC susceptometer in which a primary 103 Hz field of 1 to 20 G zero-to-peak was applied normal to the plane of the tape. Microstructures of selected samples were examined both by scanning electron microscopy (SEM) in electron backscatter imaging mode (JEOL 35C) and by transmission electron microscopy (TEM) in a Philips CM 20 Ultratwin. The specimen preparation techniques have been described elsewhere [15,16].
3. Results DTA results are shown in Fig. 1 (a) for the 2212 powders with variable Pb content and in Fig. 1 (b)
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166
(a) •z = ~
8
8
3
o
c
--
f~
.....
.z_
. . . . . .
550
650
750
850
Temperature
...z ..--....O.4 . . . . .
~
950
1050
(*C)
...............................................
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. 550
.
.
.
.
.
650
.
.
i
i
.
,
750 Temperature
.
.
, 850
,~'fT,~°f . . . . 950
1050
(°C)
Fig. I. DTA traces for (a) BiLsPb~rz.oCal.0Cuz.oOs for 0.0~
for the 2223 multiphase mixtures of Table 1. The DTA traces for Pb doped 2212 powders (Fig. I ( a ) ) consist of a single endotherm whose position decreases by -~ 7 ° C as the lead content increases from 0.0 to 0.4. It has been reported [ 17 ] that Pb substitution into 2223 causes a similar decrease in its melting point. The DTA traces for the 2223 multiphase mixtures (Fig. 1 ( b ) ) consist of three endotherms: The highest-temperature endotherm is broad and shallow, extending from ~ 910-970°C. This endotherm is positioned well above the normal processing range for 2223 tapes, and is thought to result from a reaction involving the alkaline earth cuprate phase and per-
haps copper oxide. The absence of this endotherm from the DTA traces in Fig. 1 (a) is consistent with this idea, because neither copper oxide nor alkaline earth cuprates are present in the 2212 powders. A second endotherm appears at 877 ° C, and like the first endotherm, is independent of the mixture composition in terms of its size and position. By contrast, the size of the third endotherm depends markedly on composition; the position of the endotherm also depends on composition, but to a lesser degree, decreasing from 850°C for z = 0 . 4 to 846°C for z=0.0. The size of the third endotherm decreases significantly as z increases from 0.0 to 0.4, i.e., as the mixture changes from one in which all lead is present as Ca2PbO4 to a mixture in which essentially all lead is incorporated in the 2212. (While no Ca2PbO4 is deliberately added to the z = 0 . 4 composition, a small amount is present as a second phase, possibly because the solubility limit for lead is less than 0.4 [ 18 ]. As a result, the z = 0.3 and 0.4 mixtures contain nearly the same amount of Ca2PbO4. This was confirmed by the Ca2PbO4 peak heights in the X R D patterns of the mixtures. ) The apparent correlation between the size of the low-temperature endotherm and the amount of Ca2PbO4 in the mixture strongly suggests that Ca2PbO4 controls the lowest-temperature endotherm, and when present, initiates melting. Fig. 2 compares the Jc(0 T, 77 K) values for tapes made from the different mixtures after total heattreatment times of 50, 150, and 250 h at 815°C in 8% 02. For each heat-treatment time, Jc increases as the amount of lead incorporated in the 2212 phase increases. After 250 h total heat-treatment time and 25.0
. . . .
I
. . . .
I . . . .
d . . . .
I
. . . .
20.0 15.0 I°'°(
150 h
5.0 0 0
0. I
i
i
i
0.2
0.3
0.4
0
(PbO) z, moles of Pb in 2212 Fig. 2. J~ at 77 K and zero applied field as a function of 2212 lead content, z, after total heat-treatment times of 50, 150, and 250 h in 8% O2 at 815°C.
S.E. Dorris et al. / Physica C 223 (1994) 163-172
two intermediate pressings, J¢ for z = 0 . 4 reached 22 000 A / c m 2, approximately twice the value found for z = 0.0, whether the lead was added to the z = 0.0 composition as PbO or as Ca2PbO4. Incorporation of lead in the 2212 dearly yields properties superior to those obtained when lead is added as either PbO or Ca2PbO4. The microstructures of tapes made from the mixtures in Table 1 were examined after 250 h total heattreatment time with two intermediate pressings, i.e., in their most reacted state. Comparison of the microstructures indicates significant differences in 2223 phase development. Figs. 3 ( a - c ) show the microstructures of tapes made from the two z = 0 . 0 mixtures and the z = 0 . 4 mixture. In Fig. 3(a), all lead was added as Ca2PbO4, while in Fig. 3 (b), lead was
167
added as PbO, and in Fig. 3 (c), it was incorporated in 2212. Several distinct phases are evident in varying amounts in Figs. 3 (a-c). The black blocky phases and the light-gray needles were identified by SEM-energy dispersive spectroscopy as alkaline earth cuprates and 2212, respectively. The dark gray major phase was identified as 2223. When lead was added as Ca2PbO4 (z = 0.0), both 2212 and alkaline earth cuprates were abundant in the final microstructure, with the alkaline earth cuprates ~ 5-10 ~m in size. In tapes made from the z = 0.1-0.3 mixtures, the amounts of 2212 and alkaline earth cuprates decreased as the amount of Ca2PbO4 decreased in the mixture and the lead content of 2212 increased. When all lead was put into the 2212 ( z = 0 . 4 ) , the reaction to 2223 was the most
Fig. 3. Backscaner electron images of samples heated for 250 h in 8% 02 at 815°C: (a) z=0.0, Ca2PbO4=0.4; (b) z=0.0, PbO=0.4; and (g) z--0.4, Ca2PbO4--0.0, PbO--O.0.
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complete that we observed. The second phases were small ( < 5 ~tm) and low in concentration; however, small amounts o f a Bi rich phase were evident in some regions. When lead was added as PbO ( z = 0 . 0 ) , alkaline earth cuprate grains were larger (10-25 ~tm) and more abundant than when all lead was added as Ca2PbO,, while 2212 grains were very fine and not readily apparent. Fig. 4 compares the inductive Tc traces, taken in a field of 0. ! mT, of the most highly reacted samples of each phase mixture. The traces differ somewhat from each other in shape and onset T~ value. The sharpest transition was obtained for the most Pb rich 2212 sample (z--- 0.4) and the broadest transition for the Pb free sample ( z = 0.0), the intermediate compositions having traces between these two extremes. Small differences also exist in onset T~ values, these being slightly higher ( 108-109 K) for the Pb free and PbO powder mixtures than for the Pb-2212 powder ( 107 K). Additional insight is obtained by observing the field dependence of the transitions for the z = 0.0 and
>,
z=O ( P b ~
60
I
I
I
I
I
70
80
90
100
110
120
Z (K) Fig. 4. AC susceptibility of samples heat treated for a total of 250 h at 815 ° C in 8% 02 with two intermediate pressings. Data were taken in a field o f 0.1 mT.
z = 0 . 4 compositions, as shown in Fig. 5. There are distinct points of inflection on the transitions of the Pb free sample (Fig. 5 ( a ) ) , whatever the applied field. At temperatures lower than the main transition at 106-107 K, it is possible to define a lower-temperature transition T' by the construction shown in Fig. 5(a): this transition varies from 105 K (0.I m T ) to 79 K (2 m T ) . By contrast, the traces shown in Fig. 5(b) for the Pb-2212 sample ( z = 0 . 4 ) exhibit no inflections, except for the trace taken in the highest field. The PbO sample (Fig. 5 ( c ) ) exhibited initially quite sharp transitions but an inflection is seen in the 0.1 m T trace at ~ 75 K. The traces at fields from 0.12 m T are definitely sharper and less inflected for the PbO sample than for the lead-free 2212 sample in which all lead was added as Ca2PbO4. Of all samples, the Pb doped 2212 sample always exhibited the sharpest transitions.
4. Discussion
Taken as a whole, the results show that there is great benefit in incorporating Pb directly into the 2212 phase, rather than introducing Pb through an additional compound such as Ca2PbO4 or PbO. The most dramatic evidence is that of Fig. 2, which shows that the Jc (0 T, 77 K) values progressively and consistently increase as the Ca2PbO4 content of the starting powder decreases. The reason is that the conversion to the 2223 phase occurs more rapidly, more uniformly, and more completely when the Pb is already incorporated into the layered cuprate 2212 structure. The micrometer-scale microsctructural evidence for this is clear in the scanning electron micrographs of Fig. 3. The inductive Tc traces of Figs. 4 and 5 support these same conclusions, although it is noteworthy that even the best transitions still exhibit significant curvature down to at least 60 K. The shape of the Tc transition is a direct measure of the ability of the sample to shield the applied field from its interior. Fully connected, bulk-scale, homogeneous samples should exhibit full shielding at temperatures almost to To, but in fact this is only rarely the case for BSCCO-2223 samples. One reason for the lack of shielding is that the formation of the 2223 phase is incomplete, thus preventing the shielding current from circulating across the whole sample. In
S.E. DorMs et al. / Physica C 223 (1994) 163-172 I
I
i
I
i
169
i
i
i
i
[
L
>,
~
L T' (095KmT) _T'(1 roT) 95 K
/ Z . r / / 0 " 2
mI
(a) 60
I
I
I
I
I
70
80
90
100
110
(b) 120
60
i
]
i
i
i
70
80
90
100
110
T (K)
120
T (K)
2 mT
0.5 mT
0.I mT
(c) 60
70
80
90
100
110
120
T (K)
Fig. 5. AC susceptibility of (a) the z=0.0 sample, (b) the z=0.4 sample, and (c) the PbO sample. Data were taken in fields of 0.1-2 raT for samples heat treated for a total of 250 h at 815 oC in 8% 02 with two intermediate pressings.
170
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the work of Umezawa et al. [ 16,19 ], it was shown that the presence of kinks in the Tc transition, such as those seen very clearly in Fig. 5(b), was a direct consequence of the supercurrent passing through 2212 intergrowths which linger at the [ 001 ] twist boundaries separating grains of the majority 2223 phase. Since 2212 has a Tc of 80-81 K when surrounded by 2223 phase [ 19], these intergrowths produce an extended, very field-dependent transition at temperatures between the Tc of 2212 ( ~ 80 K) and that of 2223 ( ~ 105 K). The samples examined in these earlier studies [ 15,18 ] were made by the conventional one-powder, mixed-phase route [ 1-5,18 ] and all exhibited the type of inflected transition seen in Fig. 5(a), even in a weak applied field. By contrast, the two-powder tape with the maximum Pb doping exhibits a smoother, less inflected transition (at least in weak fields), which we presume is another consequence of the more complete and uniform reaction. However, there is an inflection visible in the 2 m T transition of Fig. 5 (b) for the optimally Pb doped sample, which led us to believe that some residual intergrowths of 2212 remained within the 2223 grains. This possibility was checked by TEM investigation of the most highly reacted Pb free and Pb doped 2212 samples. In both the z = 0.0 (lead added as CaEPbO4 ) and z = 0 . 4 samples, it was possible to observe 2212 intergrowths, some being within the grains and some being at the (001) twist boundaries. This is consistent with the broadened transitions seen in Fig. 5 for both samples. A typical view of the Pb free sample is shown in Fig. 6(a): 2212 layers are not seen right at the twist boundary but one half-cell of 2212 occurs at the third layer to the left of the boundary; there are also many other 2212 layers in the left-hand grain, including a block of twelve 2212 half-cells, as indicated on the Figure. By contrast, Fig. 6 (b) is a typical micrograph taken from the z = 0 . 4 Pb doped 2212 powder sample. In the right-hand grain there are two single 2212 half-cells, as well as a thicker half-cell having the lattice spacing appropriate for a fourCuO2-1ayer half-cell. As discussed in detail elsewhere [ 19 ], these 2212 layers are easily decoupled by weak fields; therefore the colonies of 2223 grains that contain them are easily rendered resistive to the passage of a transport current. The dependence of J¢ on the degree of reaction observed in this paper is thus a
natural consequence of the reaction mechanism of the 2212-to-2223 phase conversion, as expressed both on the 1-50 ~tm scale of the BSCCO core and on the nanometer scale of the grain boundary and intergrowth structure. Finally, we should briefly note that the residual broadening of the transitions of the Pb doped (z--0.4) sample may be because the field can penetrate between the grains of the 2223 phase, even when there is no 2212 phase present. This can occur either because some grain boundaries are inherently weaklinked or because of microcracks within the sample. In either case the sample becomes subdivided and the shielding current path does not circulate around the whole sample. It seems likely that both conditions exist even in the best reacted Ag sheathed BSCCO tapes [2,20]. Developing consistently high Jc values requires attention to all these points, not just to the reaction mechanism which has been the principal subject of this work. In all cases, the field-dependent Tc transition is a very useful tool for optimizing the process conditions for BSCCO tapes.
5. Conclusions The method of introducing lead significantly affects the phase development and superconducting properties of bismuth-2223 during powder-in-tube processing. When lead is added in the form of PbO or Ca2PbO4, relatively large grains of alkaline earth cuprates or 2212 remain even after 250 h of heat treatment. When lead is incorporated in the 2212 prior to its reaction with CaECuO 3 and CuO, the formation of 2223 is more complete and uniform. This is apparent in terms of fewer second phases with sizes of l - I 0 ~tm and fewer 2212 intergrowths. As a direct result of the more homogeneous formation of 2223, tapes made from Pb doped 2212 exhibit sharper transition temperatures and higher critical current densities.
Acknowledgements The authors would like to acknowledge discussions with E.E. Hellstrom (University of Wisconsin at Madison). The work of Argonne National Labora-
S.E. Dorris et al. / Physica C 223 (1994) 163-172
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Fig. 6. Transmission electron mierographs of the same samples as studied in Fig. 5: (a) view of (001) twist boundary and its adjacent grains within a colony of BSCCO-2223 found in the z=0 sample, and (b) view of (001) twist boundary and its adjacent grains within a colony of BSCCO-2223 found in the z=0.4 sample. Half-cells of the 2212, 2223, and 2234 phases are indicated by 2, 3, and 4, respectively. Twist boundaries are indicated by the designation TB. tory was s u p p o r t e d by the U S D e p a r t m e n t o f Energy, Energy Efficiency a n d Renewable Energy, as part o f a p r o g r a m to develop electric power technology, under Contract W-31-109-Eng-38. The U n i v e r s i t y o f
Wisconsin work was s u p p o r t e d by the Electric Power Research Institute (Contract RP-8009-05) a n d the A d v a n c e d Research Projects Agency ( C o n t r a c t N00014-90-J-4115 ).
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