- Email: [email protected]
The development of an aligned microstructure during melt processing of Bi2Sr2CaCu2Ox (BSCCO-2212) doctor-bladed films
Physica C 218 (1993) 141-152 North-Holland
The development of an aligned microstructure during melt processing of BiESrECaCuEOx(BSCCO-2212) doctor-bladed films Wei Z h a n g a n d Eric E. H e l l s t r o m Applied Superconductivity Center and Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, WI53706, USA
Received 28 July 1993 Revised manuscript received 13 September 1993
Bi2Sr2CaCu2Ox(2212) thick films have been made by the doctor-blade technique and melt processed on Ag, Au, and MgO substmtes. The growth and alignment of the 2212 phase have been investigatedusing quenched samples. The 2212 grains appear to grow directly from the liquid with random orientation, not preferentiallyat the substrate/oxide interface or at second phases. The free surface of the film plays an important role in 2212 alignment. Within about 25 tan of the free surface the 2212 grains were found to be well aligned with the free surface and the second-phaseparticles were small. Beyondabout 25 ttm from the free surface, the 2212 was not as well aligned and the second phases were larger. Well-aligned 2212 films (<25 ttm thick) were obtained by melt processingon Ag, Au, and MgO substrates.
1. Introduction
The Bi oxide-based superconductors have been of great interest since they were first reported. In oxide superconductors, the microstructure plays an important role in determining the critical current density (Jc) of the materials. It has been reported that an aligned microstructure is required to attain high J~ in Bi-based superconductors [1]. In Bi2Sr2Ca2Cu3Ox(2223) it is difficult to highly align the microstructure because of the complicated 2223 formation mechanism, whereas with Bi2Sr2CaCu2Ox (2212), alignment can be more easily obtained by partial melting, followed by slow cooling. It has been reported that 2212 not only has a relatively high transition temperature (about 70 to 90 K), but also an extremely high critical magnetic field [ 2 ]. Meltprocessed, doctor-bladed 2212 films can attain a J~ of more than l0 s A / c m 2 at 4.2 K in magnetic fields as high as 25 T [1, 3]. Since no conventional metallic superconductor can be used at such high magnetic field, the 2212 phase is a promising material for high magnetic field applications. Much work has been done on 2212 superconducting wires, tapes, and films using melt processing.
Wires and tapes are generally made by the oxidepowder-in-tube ( O P I T ) technique, which involves a multistep process of filling a silver tube with powder, sealing the ends, cold drawing, rolling to make tape, followed by thermal processing. In contrast, the doctor-blade technique is a much simpler process that does not require mechanical deformation. Even though the melt processing procedures used for developing high-Jc wires, tapes, and films are similar, it appears that the alignment and microstructure are best in the films. It is therefore useful to study the doctor-bladed films in detail to determine what factors control the microstructure and superconducting properties, and to compare these to wires and tapes. This paper presents a systematic study of phase formation and the development of alignment during melt processing of doctor-bladed films using samples that were fully processed or quenched at various points during processing. The effects of the maxim u m processing temperature (Tm~), film thickness, the free surface, and substrates on phase formation and alignment have been studied.
0921-4534/93/$06.00 © 1993 ElsevierSciencePublishers B.V. All fights reserved.
142
W. Zhang, E.E. Hellstrom / Aligned microstructure of BSCCO-2212
2. Experimental
Powder of nominal 2: 2: 1 : 2 (Bi: Sr: Ca: Cu ) composition was prepared from Bi203, SrCO3, CaCO3, and CuO. The powders were mixed, calcined in air at 780 to 800°C for 24 h then at 850°C for 200 h with several grindings at each stage. The final powder was jet milled, yielding powder with < 3 Ixm particles. The powder was added to an organic formulation consisting of solvent, binder, and dispersant (table 1 ) and mixed with a magnetic stirrer for 1.5 h. The slurry was then doctor-bladed onto glass. Samples were prepared by cutting 1 X20 mm 2 pieces of film and placing them on Ag or Au foil, or single-crystal MgO ( 100 face). Ag foil that was < 50 )xm thick bowed due to the difference in expansion between Ag and 2212. Ag foil > 100 ~tm thick or samples with 2212 on both sides of the foil were used to prevent bowing. Thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) were used to determine the proper temperature to burn out the organics and the processing temperature for the different substrates. Figure 1 shows a schematic heat treatment schedule. The composites were slowly heated to 600°C and held there to remove the organics, then heated to the maximum processing temperature Tma~ (875-890 ° C for Ag, 900-910°C for Au and MgO) and held there Table 1 Compositionof organicmixtureused for the doctor-bladedfdm Component oxide powder solvent binder dispersant
Material
Fraction (wt.%)
2212 trichlorethylene polyvinylbutyral sorbitan trioleate
22 68 6 4
for 5 to 10 min, cooled to 820°C at 10°C/h, held at 820°C for 2 h to eliminate the remaining liquid, then cooled to room temperature at 300°C/h. This processing was done in air. To reduce evaporation of Bi and Cu species during melt processing [4, 5 ], all films, except those used for quench experiments (see below), were heat treated in a covered alumina crucible that contained 2212 in a Ag crucible. The surface area of this 2212 was much larger than that of the films. Samples were processed with the film in the upright orientation, unless otherwise noted (see
fig. 8). To study the development of the microstructure, samples were oil-quenched at various points during processing [ 6 ]. These samples were not processed in covered alumina crucibles, and it is possible some Bi and Cu may have evaporated from these samples. The surface and cross-section microstructures were observed by SEM. To prevent edge rounding while grinding and polishing longitudinal cross-sections, a piece of Ag foil was epoxied to the superconductor before mounting the film in konductomet. The samples were ground and polished through 0.05 ~m alumina, using oil for all polishing steps except the last one, which was done with cyclohexane. The polished films were etched for 1 rain with a mixture of 1 part (by volume) 60% perchloric acid and 99 parts 2-butoxy-ethanol at 0°C to expose the grain structure. Phases were identified by X-ray diffraction and energy dispersive spectroscopy using a scanning electron microscope (SEM/EDS). 3. Results 3.1. Effect o f T~,~x on microstructure
To investigate the effect of T~,~x, we processed 2212/Ag films using Tm~ from 870-910°C. For
TIT~
821YC 600"C
21~
1 hr
Fig. 1. Typicalheat-treatmentschedulefor doctor-bladed2212/Agfilm by meltingprocess.
W. Zhang, E.E. Hellstrom /Aligned microstructure of BSCCO-2212
Tm~ < 905 ° C, the microstructure of fully-processed films (i.e., films processed using the heat treatment schedule in fig. 1 ) shown in fig. 2(a), consisted of 2212 and small second phases of (Sr, Ca) CuOx ( 1 : 1 ) and a Cu-free phase, which was identified by SEM/EDS as Bi(Sro.yCao.3)1.sOx. On the other hand, fig. 2(b) shows that for Tm~,=910°C, fully-processed films contained large grains of the Cu-free phase. We have observed identical differences in microstructure in Ag-sheathed 2212 tapes with varying Tm~ [ 7 ], which we attributed to the presence or absence of the Cu-free phase in the melt at Tm~, as discussed below. To confirm whether this was also the case for films, we studied the phase equilibria in films quenched at different temperatures during melt processing. Table 2 summarizes the results, which agree with those for tapes [ 7]. Figure 3 shows the microstructure of fdms that were quenched at different temperatures during processing. As shown in fig. 3(a), when the film was quenched from Tm~,=890°C during heating, small grains of the Cu-free phase and 1 : 1 were observed in the liquid, whereas films quenched from Tm~=910°C (fig. 3(b)) contained CaO and (Sr, C a ) 2 C u O x ( 2 : 1 ) in the liquid. On the other hand, when the film was processed with Tm~ = 910 °C and
143
quenched from 890°C during cooling, 1 : 1 and large grains of Cu-free phase were present in the liquid (fig. 3 (c)). These quenching studies indicate that the Cufree phase melts into the liquid above 905°C and precipitates from the liquid on cooling below 905 ° C. Based on these studies, Tm~ was kept below the temperature at which the Cu-free phase disappeared from the melt (Tm~<905°C for Ag), and we typically used Tm~=880 to 890°C.
3.2. Formation and alignment of the 2212 phase Figure 4 shows a series of micrographs of films quenched at various temperatures during cooling from Tm~,=890°C. It presents a back scatter electron (BSE) image of a polished cross-section and a secondary electron (SE) image of the same area after etching. At 890°C (fig. 3(a) ) and 880°C, only 1 : 1, the Cu-free phase, and liquid were present (table 2). At 870°C, figs. 4 ( a ) and 4(b) show liquid, l : 1, the Cu-free phase and a few 2212 grains, which were randomly oriented in the film. There was no evidence of preferential formation of 2212 on the l : l or Cu-free phases, on the Ag foil, or at the oxide/air interface (i.e., free surface). With continued cooling from 870-840°C (figs. 4 ( c ) - ( f ) ) , the number of
(a)
(b) Fig. 2. Back scatter image of cross-sections of fully-processed films with (a) Tm~=890°C and (b) Tm~=910°C. B= 1 : 1, C=Cu-free and D=2212.
W. Zhang, E.E. Heilstrom / Aligned microstructure of BSCCO.2212
144
Table 2 Phases observed in samples that were quenched from different temperatures during processing. Samples quenched during heating had been held at the indicated temperature for 10 min, and those quenched during cooling had been processed following the schedule in fig~ I with Tnm=910°C Quenching temperature (°C)
Heating or cooling
890 900 905 910 905 896 890 880 870 840
heating heating heating heating cooling cooling cooling cooling cooling cooling
2212
Liquid
Cu-free
1: 1
x x X X X X
x x X
x x X"
Xa x
X x
X x
X x
x
x
x
x
x
x
x
x
2:1
X X X X
CaO
X
• Only a few grains were observed.
(a)
(b)
(c)
Fig. 3. Back-scatter images of cross-sections of films that were quenched from (a) 890°C during heating, and (b) 910 °C and (c) 890 °C during cooling from Tm~ = 910 ° C. A = 2 : 1, B-- 1 : 1, Cu ffiCu-free, E ffiCaO and L ffiliquid. 2 2 1 2 grains i n c r e a s e d a n d t h e y b e c a m e m o r e highly aligned, w h i l e t h e n u m b e r o f grains a n d g r a i n size o f t h e l : I a n d C u - f r e e p h a s e s d e c r e a s e d . T h e r e was no
significant d i f f e r e n c e in the m i c r o s t r u c t u r e b e t w e e n fully-processed s a m p l e s (fig. 2 ( a ) ) and those q u e n c h e d at 8 4 0 ° C (figs. 4 ( e ) a n d ( f ) ) .
IV. Zhang, E.E. Hellstrom/ Aligned microstructureof BSCCO-2212
145
(a)
0,)
(c)
(d)
(e)
(t)
Fig. 4. Back-scatterimases and second-electronimages of polished and etched cross-sectionsof films that were quenchedfrom 8700C (a, b), 860°C (c, d) and 840°C (e, f) during cooling (Tin-- 8900C). B= 1: 1, C--Cu-free,D--2212 and L--.liquid.
3.3. Effect of film thickness on microstructure Figure 5 compares the etched microstructure of fully-processed thin (20 ttm) and thick (65 ~tm) films. In the thin film, the 2212 grains were uniformly well-aligned throughout the entire cross-section and the second-phase grains were quite small. In contrast, in the thick film, the 2212 was well-aligned within about 25 ~tm of the free surface, but beyond this, the 2212 alignment was much poorer. In fact, for thick films, fig. 5(b) shows that the alignment at the Ag interface, which is normally reported to be the most highly-aligned region in tapes [8], was poorly aligned compared to the free surface. The size of the second phases varies with position in thick films. Near the free surface where the 2212 was well aligned, the second phases were small,
whereas they were larger in the interior region where the 2212 alignment was poorer. Comparing 2212 film ( fig. 6 (a)) and tape (fig. 6 ( b ) ) with almost the same oxide thickness, that underwent the same thermal processing, even the largest 1 : 1 grains found in the interior of the film were much smaller than the 1 : 1 grains in the tape.
3.4. Effect of the free surface and gravity on alignment To study the effect the free surface has on alignment, an experiment was carried out using a film with a thickness gradient along its longitudinal direction: the free surface of this fdm was not parallel to the substrate. The film was processed on Ag with Tm~ = 890 ° C. Figures 7 (a) and 7 ( b ) show that near
146
W. Zhang, E.E. Hellstrom / Aligned microstructure of BSCCO-2212
(a)
(b)
Fig. 5. Etched cross-sections of fully-processed ( Tm~= 890 °C ) ( a ) thin film and ( b ) thick ftim. B = 1 : 1 and C = Cu-free.
(b)
N Fig. 6. Back-scatter image of fully-processed ( Tm~= 890 ° C ) (a) i'tim and (b) tape that were processed identicallyusing the schedule in ~
fig. I. B= 1 : 1, C-- Cu-free and D=2212. the free surface the 2212 was well aligned parallel to the free surface, whereas in the interior o f the film, the alignment was low, a n d there were m o r e second phases a n d pores a n d their sizes were larger than near the free surface. Figure 7 ( b ) also shows a pore with 2212 grains aligned parallel to the surface o f the pore. T h e pore a p p a r e n t l y acted as a free surface for 2212 alignment.
To study whether 2212 alignment at the free surface is assisted b y b u o y a n c y effects in the melt, further e x p e r i m e n t s were done processing samples with different orientations in the gravitational field. Figure 8 shows the processing orientations a n d resulting microstructures. The 2212 grains were aligned at the free surface for all three orientations. In films processed with the free surface in the up a n d vertical po-
147
W. Zhang, E.E. Hellstrora / Aligned microstructureof BSCCO-2212
(a)
fo)
Fig. 7. Etchedcross-sectionof fully-processedfilm (T=~ = 890"C) that had a thicknessgradientalongits longitudinaldirection. sitions (see figs. 8 (c) and ( d ) ) , there were more 1:1 grains and pores near the Ag interface than near the free surface. The film processed with the free surface down had the most porosity, and it too was concentrated near the Ag interface.
processed 2212/Au and 2212/MGO films with Tm,~= 905 ° C.
3.5. Effect of the substrate on alignment
4.1. Phase formation
To prepare for this study, the melting points of 2212 powder, a mixture of 2212 and MgO powder, and 2212 film on Ag and Au foil were measured by D T A in flowing air. The onset of melting in the 2212/ Ag film was 862 ° C, whereas in 2212 powder, 2212 plus MgO powder, and the 2212/Au film it was 873°C, 873°C, and 874°C, respectively. These resuits show that 2212/Au and 2212/MGO films require higher processing temperature than 2212/Ag films. Figure 9 shows highly-aligned 2212 in fully-
One of the primary goals of melt processing is to form a homogeneous, highly-aligned 2212 microstructure. Since 2212 melts peritectieally, second phases are necessarily present in the melt as 2212 begins to form on cooling, and the liquid and crystalline phases must ultimately react to form 2212. To achieve a homogeneous microstructure, the grain size of the second phases should be as small as possible and they should be uniformly distributed throughout the film when 2212 begins to form. Large grains of
4. Discussion
148
W. Zhang, E.E. Heltstrom / Aligned microstructure of BSCCO-2212
(a)
n
B
(c)
Fpa •"
(d)
(e)
(f)
Fig. 8. IPolished and etched cross-sections of fully-processed films (Tm~= 890 °C) processed with the t'flm down (a, b), up (c, d) and sideways (e, f).
nonsuperconducting second phases are detrimental since they tie up cations, which hinders complete conversion to 2212, they block the growth of 2212, and they disturb the local 2212 alignment, which is discussed below. Thus, the microstructure in the melt when 2212 begins to form on cooling (about 870°C) is crucial to forming the desired microstructure in the fully-processed film.
Our studies on f'flm and tape [ 7 ] show that the size of the Cu-free phase varies depending on T ~ used for processing. When the 2212 melts, it forms the Cu-free phase, 1 : 1, and liquid, and the grains of Cu-free phase are rather small. However, with higher Tm~, ( > 905°C), the Cu-free phase is no longer present in the melt, and on cooling the Cu-free phase nucleates and grows to large size (fig. 3(c) ), compared
W. Zhang, E.E. Hellstrom ~Aligned microstructure of BSCCO-2212
149
(a)
(b) Fig. 9. Etchedcross-sectionsof fully-processedfilms ( Tm~= 905°C ) on ( a ) MgO singlecrystaland (b ) Au foil substrates. to its size before it melted onto the liquid (fig. 3(a) ). For Tm~=905°C, fully-processed 2212/Ag films contained large grains of the Cu-free phase, whereas with 2212/MGO and 2212/Au films they were not present. However for Tm~ = 910 ° C, fully-processed 2212/Au and 2212/MgO film contained large grains of the Cu-free phase. In analogy with the 2212/Ag film, this suggests that in 2212/Au and 2212/MGO films, the Cu-free phase melts between 905 and 910 ° C. To minimize the size of the Cu-free phase in the melt we limited Tmax~ 900°C on Ag and used 905°C for Au and MgO substrates. As we have discussed for Ag-sheathed 2212 tapes, we believe the fall off in Jc with increasing Tm~ reported by Shimoyama et al. [4] for doctor-bladed 2212 on Ag and by Shibutani et al. [9 ] for Ag-sheathed 2212 tapes, was due to T~.~ exceeding the stability range for the Cu-free phase and the presence of large grains of the Cu-free phase in the melt when 2212 began forming from the liquid leading to a microstructure with a lower Jc. 2212 melts peritectically, so on cooling it is expected to undergo the standard peritectic reaction where 2212 forms around the crystalline phases in the liquid. However, fig. 4(a) shows that the 2212 does not form around the second phases, but rather it appears to form and grow directly from the liquid. This is consistent with the direct observation of 2212 formation using high-temperature optical microscopy [10], with our previous work on 2212 tapes [11 ], and observations on 2212 fibers [ 12]. From fig. 4, we observed that the number and size of second-phase particles decreased while the amount of
2212 increased during cooling. This shows that 1 : 1 and the Cu-free phase reacted with the liquid to form 2212. They must dissolve into the liquid to provide the cations needed to continue growing 2212. This is similar to the formation mechanism proposed for YBa2Cu3Ox [ 13, 14 ]. Heterogeneous nucleation of 2212 could occur at the Ag interface, the free surface, at the large 1 : 1 or Cu-free second phases, or at other, as yet unobserved, minute second phases. Homogeneous nucleation could also occur in the liquid. We speculate 2212 nucleates heterogeneously, but the microstructures of films quenched during the initial stages of forming 2212 (fig. 4) do not suggest preferential nucleation at the Ag interface, free surface, or on the grains of 1 : 1 or the Cu-free phase. This remains an open issue for melt processing 2212 conductors. The ideal 2: 2: 1 : 2 composition used in this study has been reported to be both inside and outside the one-phase region for the 2212 phase. Sinclair et al. [ 15 ] reported that the 2: 2: 1 : 2 composition forms phase-pure 2212. In contrast, Miiller et al. [ 16 ] reported that the 2: 2: 1 : 2 composition is in equilibrium with a Cu-free phase of composition Bi3Sr4Ca3Ox at 830°C in air with no Ag present, and Majewski et al. [ 17 ] reported that the 2: 2: 1 : 2 composition lies within a four-phase region consisting of 2212, 2: l, 14: 24, and Bi2Sr3_xCaxOy. We have been unable to prepare the 2 : 2 : 1 : 2 composition phase pure, and agree that this composition is not in the single-phase region. None of the phase studies [ 1821 ] shows 2212 to coexist with 1 : 1 in the solid state, so 1 : 1, which is a stable phase in the melt, is a meta-
150
W. Zhang, E,E. Hellstrom / Aligned microstructureofBSCCO-2212
stable phase in fuily-proccssed film. We have seen that as we have improved the melt processing of 2212, the amount of second phases present in fullyprocessed films and tapes decreased. Ideally one wants a microstructure with no second phases present, except those that are small enough to pin flux. This suggests that in the future we should use a composition that is within the one,phase 2212 region. Figure 5 shows that the thin film contained fewer and smaller grains of 1 : 1 per unit volume than the thick film. In the thick films, the 1 : 1 grains are not uniformly distributed throughout the film, but are more numerous and larger farther from the free surface. However, the size of even the largest 1 : 1 grains in thick doctor-bladed fdms is much smaller than in Ag-sheathed 2212 tape processed identically, as shown in fig. 6. These differences in size of the 1 : 1 in thick and thin films, and tapes suggest that oxygen plays an important role in forming 2212. When the powder in the tube melts in air, it loses oxygen that must be added during cooling. Oxygen is needed for the second phases to react with the liquid to form 2212. We suggest that initially on cooling oxygen diffusion through the liquid is rapid, but as 2212 forms in the liquid, it blocks the diffusion paths, so oxygen diffusion becomes slower. Near the free surface, diffusion can supply enough oxygen that nearly complete conversion to 2212 can occur and the second phases are consumed. However, less oxygen diffuses into the interior of the film, so the reaction to form 2212 there does not go to completion, leaving large grains of second phase. We also suggest that oxygen flux through the Ag foil in films and the Ag sheath in tapes is not high enough to supply all the oxygen needed to form 2212 under normal ( 5 - 1 0 ° C / h ) cooling, so the second phases remain large in the intcrior of film and tapes. Matheis et al. [ 22 ] reported the important role oxygen plays in forming 2212 when crystallizing quenched glass ceramics. They reported enhanced oxygen absorption resulting in more 2212 phase formation in powders than bulk glass because o f the powder's much larger surface to volume ratio. This is consistent with our speculations. The quenched films in figs. 3 and 4 appear to have more 1 : 1 near the free surface than in the interior of the film. Since these films were processed without the protective 2212 atmosphere, this may be due to Bi loss, which Shimoyama et al. [4] have reported
can lead to the formation of 1 : 1 at the free surface. An alternate explanation is that in these tapes, which were processed in the down orientation ( fig. 8), the 1 : 1 phase, which is less dense than the liquid, floated upward. Buoyancy of 1 : 1 may also explain the larger amount of 1 : 1 in the interior near the Ag interface in the film processed in the down orientation (fig. 8(a) ) than in the interior of the film processed in the up orientation (fig. 8(b) ). 4.2. Alignment of 2212
The micrographs in fig. 4 show the growth and alignment of 2212 during cooling. At 870°C, 2212 has begun to form from the liquid. There is no apparent preferred nucleation site, and only liquid is present in some regions at the Ag interface and at the free surface. The 2212 grains are randomly oriented throughout the films at 870°C, but on cooling to 840°C, the 2212 grains have become very highly aligned. We have reported the same alignment mechanism in Ag-sheathed 2212 tapes [23], and have proposed a constrained growth mechanism to explain it. We think this same constrained growth mechanism is valid for films, and we suspect that films have an additional alignment mechanism that occurs at their free surface. In the constrained growth mechanism, here extended to films, the 2212 grains initially form at random orientations in the film. Those aligned parallel to the substrate (Ag foil) can grow to large size before they impinge on the substrate. The micrographs in fig. 4 strongly suggest that during cooling the 2212 grains grew and underwent dynamic processes that increased their alignment. The exact mechanism by which the alignment increases in film during cooling due to the constrained growth mechanism is not yet known, but two possibilities are that ( 1 ) large, favorably-oriented grains grow at the expense of small, misoriented grains during cooling, or (2) as the favorably oriented grains grow they rotate smaller, misoriented grains into favorable alignment. At present we cannot chose between these. In thick films, we observed that the 2212 at the free surface is more highly aligned than in the interior (fig. 5 ). Kase et al. [3] reported that in fullyprocessed thick films ( > 30 pm thick) the region near the surface was well aligned, and the 2212 at the Ag/
IF.. Zhang, E.E. Hellstrom / Aligned microstructure of BSCCO- 2212
oxide interface appeared to be better aligned than in the interior of the ftim. They suggested that the alignment begins at these two interfaces and proceeds into the films during processing. However, the torn films they used to examine the microstructure of the film did not allow them to see the fine details of the microstructure, and the slow air quench they used did not preserve the high-temperature microstructure. The polished cross-section of our samples that were oil-quenched during cooling did not show preferential alignment at the free surface or the Ag/ oxide interface (fig. 4), which does not support the suggestion of Kase et al. [ 3 ] suggestion that alignment begins at the free surface or Ag/oxide interface and proceeds inward. In another study, Hasebe et al. [ 10] observed 2212 floating on the surface of a 2212 melt using hightemperature optical microscopy. This suggests that buoyancy effects may induce alignment at the free surface. We had earlier concluded that in Ag-sheathed tapes buoyancy did not affect the alignment [24 ], but repeated these buoyancy studies with films, with which we can achieve a more homogeneous microstructure than in tapes. Figure 8 shows that the 2212 is aligned at the free surface in thick films processed in all three orientations. Again, fig. 4 does not show high 2212 alignment anywhere in the film during the initial stages of 2212 growth. This strongly suggests that the buoyancy of 2212 is not a major factor inducing alignment at the free surface. We think that the alignment at the free surface is due to surface energy effects. Specifically, we suspect that the surface energy of the 2212/gas interface is lower than for the 2212/liquid or 2212/Ag interface. During cooling, the 2212 grains can move in the liquid, so some 2212 grains are expected to contact the free surface and rotate so their a-b planes are paraUel to the free surface, which reduces the overall surface energy. As these 2212 grains rotate, they sweep out a volume in the melt below the surface. 2212 grains within (or partially within) this volume can be rotated into alignment as the 2212 grains rotate into alignment with the free surface. For thick films, 2212 grains can be very misoriented and still grow to large size, resulting in poor alignment. However, we have observed that alignment at the free surface of thick films extends as deep as 15-25 pro. Thin films up to about 15-25 }lm thick
151
could be processed with high alignment throughout the entire film. Figure 5 shows the alignment in the thin film was higher than in the surface region of the thick film. Thus we believe that in the thin film, the constrained growth mechanism dominates and some alignment from the surface energy effect may occur, whereas in the thick films, the alignment near the surface is due to the surface energy alignment mechanism. In both the constrained growth mechanism and the surface energy alignment mechanisms the 2212 alignment is independent of the substrate. Our resuits on Au foil and single-crystal MgO confirm this and show that aligned 2212 films can be made on a variety of substrates by properly adjusting the processing temperature. Kase et al. [ 3 ] reported that for Tm,~=890°C 2212/Au films had little alignment. Our studies showed that for T , ~ = 9 0 0 ° C , both 2212/Au and 2212/MgO films were well aligned. The Tm~ needed with Au and MgO substrates is higher than that needed for Ag. A critical factor for Ag may be its interaction with the Bi-Sr-Ca-Cu-O system, depressing the melting point of 2212, while not being incorporated into the 2212 structure. Although buoyancy is not directly responsible for 2212 alignment at the free surface, it may have a significant on processing and alignment. The best alignment at the free surface appears to be in the film processed with the film down (fig. 8 (a)). Here some of the 1 : 1 may have floated up away from the free surface, making it easier for the 2212 to rotate and align near the free surface. Film processed in the up and vertical orientations had little residual porosity compared to the film processed in the down orientation. We speculate that the pores near the free surface in films processed in the up orientation migrate upward to the film surface where they are consumed. But in film processed in the down orientation, the pores migrate upward where the Ag interface traps them. Pores are also present in the interior of very thick films processed in the up orientation (fig. 7(b) ). This may be due to the longer migration distance for pores to reach the free surface in thicker films. Pores are detrimental as they degrade Jc by blocking the current path and by disrupting the local 2212 alignment. Films, with their free surface that can consume pores, can have an inherent processing
152
W. Zhang, E.E. HeUstrom /Aligned microstructure ofBSCC0-2212
advantage over Ag-sheathed wires and tapes where the sheath can trap the pores.
5. Conclusion A l i g n e d 2212 o n Ag a n d A u foil a n d M g O single crystal were p r e p a r e d b y m e l t processing doctor-
bladed films. The phase assemblage was studied as a function of temperature. In 2212/Ag film, the Cufree phase melted about 905 ° C, and in 2212/Au and 2212/MGO films between 905 and 910°C. When cooling from Tm~ above the melting point of the Cufree phase, this phase precipitated from the liquid phase and grew to large size compared with its size before melting. Therefore, Tm~ should not exceed 900°C when melt processing 2212/Ag film. For 2212/Au and 2212/MGO films, it should not exceed 905°C. On cooling, the 2212 phase nucleated around 870°C, and there is no evidence that it preferentially nucleated at the oxide/substrate interface or on the second phases in the melt. The 2212 phase grew directly from the liquid without preferential orientation. The free surface played an important role in aligning 2212 grains. In the thick film, the 2212 near the free surface was well aligned and this region extended ~ 25 ttm into the film with our processing conditions. On the other hand, near the oxide/Ag interface, the alignment was poorer, and this region contained more and larger second phases. Two alignment mechanisms, constrained growth and alignment due to a free surface effect were presented.
Acknowledgements We would like to thank our collaborators at the University of Wisconsin-Madison and in particular Roger D. Ray II for helpful discussions. This work was supported by the Defense Advanced Research Projects Agency contract N000 14-90-J-4115 and the Electric Power Research Institute contract RP800905.
References [ 1 ] J. Kase, IC Togano, H. Kumakura, D.R. DictdErich, N. Irisawa, T. Morimoto and H. Maeda, Jpn. J. Appl. Phys. 29 (1990) L1096. [ 2 ] Y. KOikE,T. Nakanomyo and T. Fukase, Jpn. J. Appl. Phys. 27 (1988) L841. [3]J. Kase, T. Norimoto, IC Tngano, H. Kumakura, D.R. Dictderich and H. Maeda, IEEE Trans. Mag. 27 (1991) 1254. [4] J. Shimoyama, J. Kase, T. Morimoto, H. Kitaguchi, H. Kumakura, IC Togano and H. Maeda, Jpn. J. Appl. Phys. 31 (1992) L1167. [ 5 ] T. Sara, K. Sakai and S. Tashiro, J. Am. Cer. Soc. 75 (1992) 805. [ 6 ] R.D. Ray II and E.E. Hellstrom, Physica C 175 ( 1991) 255. [7]E.E. HEllstrom, R.D. Ray II and Wei Zhan8, Appl. Supercond. 1 (1993) 1535. [8 ] lC Nomura, M. Seido, H. Irdtasuchi, H. Kumakura, T. Tngano and H. Maeda, Appl. Phys. Lett. 62 (1993) 2131. [ 9 ] K. Shibutani, T. Egi, S. Hayashi, R. Ogawa and Y. Kawate, Jpn. J. Appl. Phys. 30 ( 1991 ) 3371. [ 10 ] T. HasEbe, Y. Tanaka, T. YanagJya, T. Asano, M. Fukutomi and H. Maeda, Jpn. J. Appl. Phys. 31 (1992) L21. [ 11 ] Y. Feng, ILE. Hautanen, Y.E. High, D.C. Larbalestier, R.D. Ray H, E.E. Hellstrom and S.E. Babcock, Physica C 192 (1992) 293. [ 12 ] M.J. Cima, X.P. Jiang, H.M. Chow, J.S. Haggerty and M.C. Fleminp, J. Mater. RES. 5 (1990) 1834. [ 13 ] T. Izumi, Y. Nakamura and Y. Shiohara, J. MatEr. Res. 7 (1992) 1621. [ 14] M.J. Cima, M.C. Flemings, A.M. Figueredo, M. Nakade, H. Ishii, H.D. Brody and J.S. Ha88erty, J. Appl. Phys. 72 (1992) 179. [ 15 ] D.C. Sinclair, J.T.S. Irvine and A.R. West, J. Mater. ChEm. 2 (1992) 579. [ 16 ] R. Miiller, T. Schweizer, P. Bohac, R.O. Suzuki and L.J. Gauckier, Physica C 203 (1992) 299. [ 17 ] P. Majewski, H.L. Su and B. Hettich, Adv. MatEr. 4 (1992) 508. [ 18 ] R.O. Suzuki, S. Kambara, H. Tsuchida, IC Shimizu and K. Ono, Advances in Superconductivity II, Proc. 2nd Int, Symp. on Superconductivity, Tskuba, Japan, Nov. 1989, eds. T. Ishiguro and IC Kajimura (SpringEr, Tokyo, 1989) p. 235. [19]B. Hong, J. Hahn and T.O. Mason, J. Am. Set. Soc. 73 (1990) 1965. [20] K. Schulze, P. Majewski, B. Hettich and G. PEtzow, Z. Metailkd. 81 (1990) 836. [21 ] R. Milller, T. Schweizer, P. Bohac, R.O. Suzuki and L.J. Ganckier, Physica C 203 (1992) 299. [ 22 ] D.P. Matheis, S.T. Misture and R.L. SnydEr, Physica C 207 (1993) 134. [ 23 ] E.E. Hellstrom, JOM 44 (1992) 48. [24] R.D. Ray II and E.E. HEllstrom, Appl. Phys. Lett. 57 (1990) 2984.
The development of an aligned microstructure during melt processing of BiESrECaCuEOx(BSCCO-2212) doctor-bladed films Wei Z h a n g a n d Eric E. H e l l s t r o m Applied Superconductivity Center and Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, WI53706, USA
Received 28 July 1993 Revised manuscript received 13 September 1993
Bi2Sr2CaCu2Ox(2212) thick films have been made by the doctor-blade technique and melt processed on Ag, Au, and MgO substmtes. The growth and alignment of the 2212 phase have been investigatedusing quenched samples. The 2212 grains appear to grow directly from the liquid with random orientation, not preferentiallyat the substrate/oxide interface or at second phases. The free surface of the film plays an important role in 2212 alignment. Within about 25 tan of the free surface the 2212 grains were found to be well aligned with the free surface and the second-phaseparticles were small. Beyondabout 25 ttm from the free surface, the 2212 was not as well aligned and the second phases were larger. Well-aligned 2212 films (<25 ttm thick) were obtained by melt processingon Ag, Au, and MgO substrates.
1. Introduction
The Bi oxide-based superconductors have been of great interest since they were first reported. In oxide superconductors, the microstructure plays an important role in determining the critical current density (Jc) of the materials. It has been reported that an aligned microstructure is required to attain high J~ in Bi-based superconductors [1]. In Bi2Sr2Ca2Cu3Ox(2223) it is difficult to highly align the microstructure because of the complicated 2223 formation mechanism, whereas with Bi2Sr2CaCu2Ox (2212), alignment can be more easily obtained by partial melting, followed by slow cooling. It has been reported that 2212 not only has a relatively high transition temperature (about 70 to 90 K), but also an extremely high critical magnetic field [ 2 ]. Meltprocessed, doctor-bladed 2212 films can attain a J~ of more than l0 s A / c m 2 at 4.2 K in magnetic fields as high as 25 T [1, 3]. Since no conventional metallic superconductor can be used at such high magnetic field, the 2212 phase is a promising material for high magnetic field applications. Much work has been done on 2212 superconducting wires, tapes, and films using melt processing.
Wires and tapes are generally made by the oxidepowder-in-tube ( O P I T ) technique, which involves a multistep process of filling a silver tube with powder, sealing the ends, cold drawing, rolling to make tape, followed by thermal processing. In contrast, the doctor-blade technique is a much simpler process that does not require mechanical deformation. Even though the melt processing procedures used for developing high-Jc wires, tapes, and films are similar, it appears that the alignment and microstructure are best in the films. It is therefore useful to study the doctor-bladed films in detail to determine what factors control the microstructure and superconducting properties, and to compare these to wires and tapes. This paper presents a systematic study of phase formation and the development of alignment during melt processing of doctor-bladed films using samples that were fully processed or quenched at various points during processing. The effects of the maxim u m processing temperature (Tm~), film thickness, the free surface, and substrates on phase formation and alignment have been studied.
0921-4534/93/$06.00 © 1993 ElsevierSciencePublishers B.V. All fights reserved.
142
W. Zhang, E.E. Hellstrom / Aligned microstructure of BSCCO-2212
2. Experimental
Powder of nominal 2: 2: 1 : 2 (Bi: Sr: Ca: Cu ) composition was prepared from Bi203, SrCO3, CaCO3, and CuO. The powders were mixed, calcined in air at 780 to 800°C for 24 h then at 850°C for 200 h with several grindings at each stage. The final powder was jet milled, yielding powder with < 3 Ixm particles. The powder was added to an organic formulation consisting of solvent, binder, and dispersant (table 1 ) and mixed with a magnetic stirrer for 1.5 h. The slurry was then doctor-bladed onto glass. Samples were prepared by cutting 1 X20 mm 2 pieces of film and placing them on Ag or Au foil, or single-crystal MgO ( 100 face). Ag foil that was < 50 )xm thick bowed due to the difference in expansion between Ag and 2212. Ag foil > 100 ~tm thick or samples with 2212 on both sides of the foil were used to prevent bowing. Thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) were used to determine the proper temperature to burn out the organics and the processing temperature for the different substrates. Figure 1 shows a schematic heat treatment schedule. The composites were slowly heated to 600°C and held there to remove the organics, then heated to the maximum processing temperature Tma~ (875-890 ° C for Ag, 900-910°C for Au and MgO) and held there Table 1 Compositionof organicmixtureused for the doctor-bladedfdm Component oxide powder solvent binder dispersant
Material
Fraction (wt.%)
2212 trichlorethylene polyvinylbutyral sorbitan trioleate
22 68 6 4
for 5 to 10 min, cooled to 820°C at 10°C/h, held at 820°C for 2 h to eliminate the remaining liquid, then cooled to room temperature at 300°C/h. This processing was done in air. To reduce evaporation of Bi and Cu species during melt processing [4, 5 ], all films, except those used for quench experiments (see below), were heat treated in a covered alumina crucible that contained 2212 in a Ag crucible. The surface area of this 2212 was much larger than that of the films. Samples were processed with the film in the upright orientation, unless otherwise noted (see
fig. 8). To study the development of the microstructure, samples were oil-quenched at various points during processing [ 6 ]. These samples were not processed in covered alumina crucibles, and it is possible some Bi and Cu may have evaporated from these samples. The surface and cross-section microstructures were observed by SEM. To prevent edge rounding while grinding and polishing longitudinal cross-sections, a piece of Ag foil was epoxied to the superconductor before mounting the film in konductomet. The samples were ground and polished through 0.05 ~m alumina, using oil for all polishing steps except the last one, which was done with cyclohexane. The polished films were etched for 1 rain with a mixture of 1 part (by volume) 60% perchloric acid and 99 parts 2-butoxy-ethanol at 0°C to expose the grain structure. Phases were identified by X-ray diffraction and energy dispersive spectroscopy using a scanning electron microscope (SEM/EDS). 3. Results 3.1. Effect o f T~,~x on microstructure
To investigate the effect of T~,~x, we processed 2212/Ag films using Tm~ from 870-910°C. For
TIT~
821YC 600"C
21~
1 hr
Fig. 1. Typicalheat-treatmentschedulefor doctor-bladed2212/Agfilm by meltingprocess.
W. Zhang, E.E. Hellstrom /Aligned microstructure of BSCCO-2212
Tm~ < 905 ° C, the microstructure of fully-processed films (i.e., films processed using the heat treatment schedule in fig. 1 ) shown in fig. 2(a), consisted of 2212 and small second phases of (Sr, Ca) CuOx ( 1 : 1 ) and a Cu-free phase, which was identified by SEM/EDS as Bi(Sro.yCao.3)1.sOx. On the other hand, fig. 2(b) shows that for Tm~,=910°C, fully-processed films contained large grains of the Cu-free phase. We have observed identical differences in microstructure in Ag-sheathed 2212 tapes with varying Tm~ [ 7 ], which we attributed to the presence or absence of the Cu-free phase in the melt at Tm~, as discussed below. To confirm whether this was also the case for films, we studied the phase equilibria in films quenched at different temperatures during melt processing. Table 2 summarizes the results, which agree with those for tapes [ 7]. Figure 3 shows the microstructure of fdms that were quenched at different temperatures during processing. As shown in fig. 3(a), when the film was quenched from Tm~,=890°C during heating, small grains of the Cu-free phase and 1 : 1 were observed in the liquid, whereas films quenched from Tm~=910°C (fig. 3(b)) contained CaO and (Sr, C a ) 2 C u O x ( 2 : 1 ) in the liquid. On the other hand, when the film was processed with Tm~ = 910 °C and
143
quenched from 890°C during cooling, 1 : 1 and large grains of Cu-free phase were present in the liquid (fig. 3 (c)). These quenching studies indicate that the Cufree phase melts into the liquid above 905°C and precipitates from the liquid on cooling below 905 ° C. Based on these studies, Tm~ was kept below the temperature at which the Cu-free phase disappeared from the melt (Tm~<905°C for Ag), and we typically used Tm~=880 to 890°C.
3.2. Formation and alignment of the 2212 phase Figure 4 shows a series of micrographs of films quenched at various temperatures during cooling from Tm~,=890°C. It presents a back scatter electron (BSE) image of a polished cross-section and a secondary electron (SE) image of the same area after etching. At 890°C (fig. 3(a) ) and 880°C, only 1 : 1, the Cu-free phase, and liquid were present (table 2). At 870°C, figs. 4 ( a ) and 4(b) show liquid, l : 1, the Cu-free phase and a few 2212 grains, which were randomly oriented in the film. There was no evidence of preferential formation of 2212 on the l : l or Cu-free phases, on the Ag foil, or at the oxide/air interface (i.e., free surface). With continued cooling from 870-840°C (figs. 4 ( c ) - ( f ) ) , the number of
(a)
(b) Fig. 2. Back scatter image of cross-sections of fully-processed films with (a) Tm~=890°C and (b) Tm~=910°C. B= 1 : 1, C=Cu-free and D=2212.
W. Zhang, E.E. Heilstrom / Aligned microstructure of BSCCO.2212
144
Table 2 Phases observed in samples that were quenched from different temperatures during processing. Samples quenched during heating had been held at the indicated temperature for 10 min, and those quenched during cooling had been processed following the schedule in fig~ I with Tnm=910°C Quenching temperature (°C)
Heating or cooling
890 900 905 910 905 896 890 880 870 840
heating heating heating heating cooling cooling cooling cooling cooling cooling
2212
Liquid
Cu-free
1: 1
x x X X X X
x x X
x x X"
Xa x
X x
X x
X x
x
x
x
x
x
x
x
x
2:1
X X X X
CaO
X
• Only a few grains were observed.
(a)
(b)
(c)
Fig. 3. Back-scatter images of cross-sections of films that were quenched from (a) 890°C during heating, and (b) 910 °C and (c) 890 °C during cooling from Tm~ = 910 ° C. A = 2 : 1, B-- 1 : 1, Cu ffiCu-free, E ffiCaO and L ffiliquid. 2 2 1 2 grains i n c r e a s e d a n d t h e y b e c a m e m o r e highly aligned, w h i l e t h e n u m b e r o f grains a n d g r a i n size o f t h e l : I a n d C u - f r e e p h a s e s d e c r e a s e d . T h e r e was no
significant d i f f e r e n c e in the m i c r o s t r u c t u r e b e t w e e n fully-processed s a m p l e s (fig. 2 ( a ) ) and those q u e n c h e d at 8 4 0 ° C (figs. 4 ( e ) a n d ( f ) ) .
IV. Zhang, E.E. Hellstrom/ Aligned microstructureof BSCCO-2212
145
(a)
0,)
(c)
(d)
(e)
(t)
Fig. 4. Back-scatterimases and second-electronimages of polished and etched cross-sectionsof films that were quenchedfrom 8700C (a, b), 860°C (c, d) and 840°C (e, f) during cooling (Tin-- 8900C). B= 1: 1, C--Cu-free,D--2212 and L--.liquid.
3.3. Effect of film thickness on microstructure Figure 5 compares the etched microstructure of fully-processed thin (20 ttm) and thick (65 ~tm) films. In the thin film, the 2212 grains were uniformly well-aligned throughout the entire cross-section and the second-phase grains were quite small. In contrast, in the thick film, the 2212 was well-aligned within about 25 ~tm of the free surface, but beyond this, the 2212 alignment was much poorer. In fact, for thick films, fig. 5(b) shows that the alignment at the Ag interface, which is normally reported to be the most highly-aligned region in tapes [8], was poorly aligned compared to the free surface. The size of the second phases varies with position in thick films. Near the free surface where the 2212 was well aligned, the second phases were small,
whereas they were larger in the interior region where the 2212 alignment was poorer. Comparing 2212 film ( fig. 6 (a)) and tape (fig. 6 ( b ) ) with almost the same oxide thickness, that underwent the same thermal processing, even the largest 1 : 1 grains found in the interior of the film were much smaller than the 1 : 1 grains in the tape.
3.4. Effect of the free surface and gravity on alignment To study the effect the free surface has on alignment, an experiment was carried out using a film with a thickness gradient along its longitudinal direction: the free surface of this fdm was not parallel to the substrate. The film was processed on Ag with Tm~ = 890 ° C. Figures 7 (a) and 7 ( b ) show that near
146
W. Zhang, E.E. Hellstrom / Aligned microstructure of BSCCO-2212
(a)
(b)
Fig. 5. Etched cross-sections of fully-processed ( Tm~= 890 °C ) ( a ) thin film and ( b ) thick ftim. B = 1 : 1 and C = Cu-free.
(b)
N Fig. 6. Back-scatter image of fully-processed ( Tm~= 890 ° C ) (a) i'tim and (b) tape that were processed identicallyusing the schedule in ~
fig. I. B= 1 : 1, C-- Cu-free and D=2212. the free surface the 2212 was well aligned parallel to the free surface, whereas in the interior o f the film, the alignment was low, a n d there were m o r e second phases a n d pores a n d their sizes were larger than near the free surface. Figure 7 ( b ) also shows a pore with 2212 grains aligned parallel to the surface o f the pore. T h e pore a p p a r e n t l y acted as a free surface for 2212 alignment.
To study whether 2212 alignment at the free surface is assisted b y b u o y a n c y effects in the melt, further e x p e r i m e n t s were done processing samples with different orientations in the gravitational field. Figure 8 shows the processing orientations a n d resulting microstructures. The 2212 grains were aligned at the free surface for all three orientations. In films processed with the free surface in the up a n d vertical po-
147
W. Zhang, E.E. Hellstrora / Aligned microstructureof BSCCO-2212
(a)
fo)
Fig. 7. Etchedcross-sectionof fully-processedfilm (T=~ = 890"C) that had a thicknessgradientalongits longitudinaldirection. sitions (see figs. 8 (c) and ( d ) ) , there were more 1:1 grains and pores near the Ag interface than near the free surface. The film processed with the free surface down had the most porosity, and it too was concentrated near the Ag interface.
processed 2212/Au and 2212/MGO films with Tm,~= 905 ° C.
3.5. Effect of the substrate on alignment
4.1. Phase formation
To prepare for this study, the melting points of 2212 powder, a mixture of 2212 and MgO powder, and 2212 film on Ag and Au foil were measured by D T A in flowing air. The onset of melting in the 2212/ Ag film was 862 ° C, whereas in 2212 powder, 2212 plus MgO powder, and the 2212/Au film it was 873°C, 873°C, and 874°C, respectively. These resuits show that 2212/Au and 2212/MGO films require higher processing temperature than 2212/Ag films. Figure 9 shows highly-aligned 2212 in fully-
One of the primary goals of melt processing is to form a homogeneous, highly-aligned 2212 microstructure. Since 2212 melts peritectieally, second phases are necessarily present in the melt as 2212 begins to form on cooling, and the liquid and crystalline phases must ultimately react to form 2212. To achieve a homogeneous microstructure, the grain size of the second phases should be as small as possible and they should be uniformly distributed throughout the film when 2212 begins to form. Large grains of
4. Discussion
148
W. Zhang, E.E. Heltstrom / Aligned microstructure of BSCCO-2212
(a)
n
B
(c)
Fpa •"
(d)
(e)
(f)
Fig. 8. IPolished and etched cross-sections of fully-processed films (Tm~= 890 °C) processed with the t'flm down (a, b), up (c, d) and sideways (e, f).
nonsuperconducting second phases are detrimental since they tie up cations, which hinders complete conversion to 2212, they block the growth of 2212, and they disturb the local 2212 alignment, which is discussed below. Thus, the microstructure in the melt when 2212 begins to form on cooling (about 870°C) is crucial to forming the desired microstructure in the fully-processed film.
Our studies on f'flm and tape [ 7 ] show that the size of the Cu-free phase varies depending on T ~ used for processing. When the 2212 melts, it forms the Cu-free phase, 1 : 1, and liquid, and the grains of Cu-free phase are rather small. However, with higher Tm~, ( > 905°C), the Cu-free phase is no longer present in the melt, and on cooling the Cu-free phase nucleates and grows to large size (fig. 3(c) ), compared
W. Zhang, E.E. Hellstrom ~Aligned microstructure of BSCCO-2212
149
(a)
(b) Fig. 9. Etchedcross-sectionsof fully-processedfilms ( Tm~= 905°C ) on ( a ) MgO singlecrystaland (b ) Au foil substrates. to its size before it melted onto the liquid (fig. 3(a) ). For Tm~=905°C, fully-processed 2212/Ag films contained large grains of the Cu-free phase, whereas with 2212/MGO and 2212/Au films they were not present. However for Tm~ = 910 ° C, fully-processed 2212/Au and 2212/MgO film contained large grains of the Cu-free phase. In analogy with the 2212/Ag film, this suggests that in 2212/Au and 2212/MGO films, the Cu-free phase melts between 905 and 910 ° C. To minimize the size of the Cu-free phase in the melt we limited Tmax~ 900°C on Ag and used 905°C for Au and MgO substrates. As we have discussed for Ag-sheathed 2212 tapes, we believe the fall off in Jc with increasing Tm~ reported by Shimoyama et al. [4] for doctor-bladed 2212 on Ag and by Shibutani et al. [9 ] for Ag-sheathed 2212 tapes, was due to T~.~ exceeding the stability range for the Cu-free phase and the presence of large grains of the Cu-free phase in the melt when 2212 began forming from the liquid leading to a microstructure with a lower Jc. 2212 melts peritectically, so on cooling it is expected to undergo the standard peritectic reaction where 2212 forms around the crystalline phases in the liquid. However, fig. 4(a) shows that the 2212 does not form around the second phases, but rather it appears to form and grow directly from the liquid. This is consistent with the direct observation of 2212 formation using high-temperature optical microscopy [10], with our previous work on 2212 tapes [11 ], and observations on 2212 fibers [ 12]. From fig. 4, we observed that the number and size of second-phase particles decreased while the amount of
2212 increased during cooling. This shows that 1 : 1 and the Cu-free phase reacted with the liquid to form 2212. They must dissolve into the liquid to provide the cations needed to continue growing 2212. This is similar to the formation mechanism proposed for YBa2Cu3Ox [ 13, 14 ]. Heterogeneous nucleation of 2212 could occur at the Ag interface, the free surface, at the large 1 : 1 or Cu-free second phases, or at other, as yet unobserved, minute second phases. Homogeneous nucleation could also occur in the liquid. We speculate 2212 nucleates heterogeneously, but the microstructures of films quenched during the initial stages of forming 2212 (fig. 4) do not suggest preferential nucleation at the Ag interface, free surface, or on the grains of 1 : 1 or the Cu-free phase. This remains an open issue for melt processing 2212 conductors. The ideal 2: 2: 1 : 2 composition used in this study has been reported to be both inside and outside the one-phase region for the 2212 phase. Sinclair et al. [ 15 ] reported that the 2: 2: 1 : 2 composition forms phase-pure 2212. In contrast, Miiller et al. [ 16 ] reported that the 2: 2: 1 : 2 composition is in equilibrium with a Cu-free phase of composition Bi3Sr4Ca3Ox at 830°C in air with no Ag present, and Majewski et al. [ 17 ] reported that the 2: 2: 1 : 2 composition lies within a four-phase region consisting of 2212, 2: l, 14: 24, and Bi2Sr3_xCaxOy. We have been unable to prepare the 2 : 2 : 1 : 2 composition phase pure, and agree that this composition is not in the single-phase region. None of the phase studies [ 1821 ] shows 2212 to coexist with 1 : 1 in the solid state, so 1 : 1, which is a stable phase in the melt, is a meta-
150
W. Zhang, E,E. Hellstrom / Aligned microstructureofBSCCO-2212
stable phase in fuily-proccssed film. We have seen that as we have improved the melt processing of 2212, the amount of second phases present in fullyprocessed films and tapes decreased. Ideally one wants a microstructure with no second phases present, except those that are small enough to pin flux. This suggests that in the future we should use a composition that is within the one,phase 2212 region. Figure 5 shows that the thin film contained fewer and smaller grains of 1 : 1 per unit volume than the thick film. In the thick films, the 1 : 1 grains are not uniformly distributed throughout the film, but are more numerous and larger farther from the free surface. However, the size of even the largest 1 : 1 grains in thick doctor-bladed fdms is much smaller than in Ag-sheathed 2212 tape processed identically, as shown in fig. 6. These differences in size of the 1 : 1 in thick and thin films, and tapes suggest that oxygen plays an important role in forming 2212. When the powder in the tube melts in air, it loses oxygen that must be added during cooling. Oxygen is needed for the second phases to react with the liquid to form 2212. We suggest that initially on cooling oxygen diffusion through the liquid is rapid, but as 2212 forms in the liquid, it blocks the diffusion paths, so oxygen diffusion becomes slower. Near the free surface, diffusion can supply enough oxygen that nearly complete conversion to 2212 can occur and the second phases are consumed. However, less oxygen diffuses into the interior of the film, so the reaction to form 2212 there does not go to completion, leaving large grains of second phase. We also suggest that oxygen flux through the Ag foil in films and the Ag sheath in tapes is not high enough to supply all the oxygen needed to form 2212 under normal ( 5 - 1 0 ° C / h ) cooling, so the second phases remain large in the intcrior of film and tapes. Matheis et al. [ 22 ] reported the important role oxygen plays in forming 2212 when crystallizing quenched glass ceramics. They reported enhanced oxygen absorption resulting in more 2212 phase formation in powders than bulk glass because o f the powder's much larger surface to volume ratio. This is consistent with our speculations. The quenched films in figs. 3 and 4 appear to have more 1 : 1 near the free surface than in the interior of the film. Since these films were processed without the protective 2212 atmosphere, this may be due to Bi loss, which Shimoyama et al. [4] have reported
can lead to the formation of 1 : 1 at the free surface. An alternate explanation is that in these tapes, which were processed in the down orientation ( fig. 8), the 1 : 1 phase, which is less dense than the liquid, floated upward. Buoyancy of 1 : 1 may also explain the larger amount of 1 : 1 in the interior near the Ag interface in the film processed in the down orientation (fig. 8(a) ) than in the interior of the film processed in the up orientation (fig. 8(b) ). 4.2. Alignment of 2212
The micrographs in fig. 4 show the growth and alignment of 2212 during cooling. At 870°C, 2212 has begun to form from the liquid. There is no apparent preferred nucleation site, and only liquid is present in some regions at the Ag interface and at the free surface. The 2212 grains are randomly oriented throughout the films at 870°C, but on cooling to 840°C, the 2212 grains have become very highly aligned. We have reported the same alignment mechanism in Ag-sheathed 2212 tapes [23], and have proposed a constrained growth mechanism to explain it. We think this same constrained growth mechanism is valid for films, and we suspect that films have an additional alignment mechanism that occurs at their free surface. In the constrained growth mechanism, here extended to films, the 2212 grains initially form at random orientations in the film. Those aligned parallel to the substrate (Ag foil) can grow to large size before they impinge on the substrate. The micrographs in fig. 4 strongly suggest that during cooling the 2212 grains grew and underwent dynamic processes that increased their alignment. The exact mechanism by which the alignment increases in film during cooling due to the constrained growth mechanism is not yet known, but two possibilities are that ( 1 ) large, favorably-oriented grains grow at the expense of small, misoriented grains during cooling, or (2) as the favorably oriented grains grow they rotate smaller, misoriented grains into favorable alignment. At present we cannot chose between these. In thick films, we observed that the 2212 at the free surface is more highly aligned than in the interior (fig. 5 ). Kase et al. [3] reported that in fullyprocessed thick films ( > 30 pm thick) the region near the surface was well aligned, and the 2212 at the Ag/
IF.. Zhang, E.E. Hellstrom / Aligned microstructure of BSCCO- 2212
oxide interface appeared to be better aligned than in the interior of the ftim. They suggested that the alignment begins at these two interfaces and proceeds into the films during processing. However, the torn films they used to examine the microstructure of the film did not allow them to see the fine details of the microstructure, and the slow air quench they used did not preserve the high-temperature microstructure. The polished cross-section of our samples that were oil-quenched during cooling did not show preferential alignment at the free surface or the Ag/ oxide interface (fig. 4), which does not support the suggestion of Kase et al. [ 3 ] suggestion that alignment begins at the free surface or Ag/oxide interface and proceeds inward. In another study, Hasebe et al. [ 10] observed 2212 floating on the surface of a 2212 melt using hightemperature optical microscopy. This suggests that buoyancy effects may induce alignment at the free surface. We had earlier concluded that in Ag-sheathed tapes buoyancy did not affect the alignment [24 ], but repeated these buoyancy studies with films, with which we can achieve a more homogeneous microstructure than in tapes. Figure 8 shows that the 2212 is aligned at the free surface in thick films processed in all three orientations. Again, fig. 4 does not show high 2212 alignment anywhere in the film during the initial stages of 2212 growth. This strongly suggests that the buoyancy of 2212 is not a major factor inducing alignment at the free surface. We think that the alignment at the free surface is due to surface energy effects. Specifically, we suspect that the surface energy of the 2212/gas interface is lower than for the 2212/liquid or 2212/Ag interface. During cooling, the 2212 grains can move in the liquid, so some 2212 grains are expected to contact the free surface and rotate so their a-b planes are paraUel to the free surface, which reduces the overall surface energy. As these 2212 grains rotate, they sweep out a volume in the melt below the surface. 2212 grains within (or partially within) this volume can be rotated into alignment as the 2212 grains rotate into alignment with the free surface. For thick films, 2212 grains can be very misoriented and still grow to large size, resulting in poor alignment. However, we have observed that alignment at the free surface of thick films extends as deep as 15-25 pro. Thin films up to about 15-25 }lm thick
151
could be processed with high alignment throughout the entire film. Figure 5 shows the alignment in the thin film was higher than in the surface region of the thick film. Thus we believe that in the thin film, the constrained growth mechanism dominates and some alignment from the surface energy effect may occur, whereas in the thick films, the alignment near the surface is due to the surface energy alignment mechanism. In both the constrained growth mechanism and the surface energy alignment mechanisms the 2212 alignment is independent of the substrate. Our resuits on Au foil and single-crystal MgO confirm this and show that aligned 2212 films can be made on a variety of substrates by properly adjusting the processing temperature. Kase et al. [ 3 ] reported that for Tm,~=890°C 2212/Au films had little alignment. Our studies showed that for T , ~ = 9 0 0 ° C , both 2212/Au and 2212/MgO films were well aligned. The Tm~ needed with Au and MgO substrates is higher than that needed for Ag. A critical factor for Ag may be its interaction with the Bi-Sr-Ca-Cu-O system, depressing the melting point of 2212, while not being incorporated into the 2212 structure. Although buoyancy is not directly responsible for 2212 alignment at the free surface, it may have a significant on processing and alignment. The best alignment at the free surface appears to be in the film processed with the film down (fig. 8 (a)). Here some of the 1 : 1 may have floated up away from the free surface, making it easier for the 2212 to rotate and align near the free surface. Film processed in the up and vertical orientations had little residual porosity compared to the film processed in the down orientation. We speculate that the pores near the free surface in films processed in the up orientation migrate upward to the film surface where they are consumed. But in film processed in the down orientation, the pores migrate upward where the Ag interface traps them. Pores are also present in the interior of very thick films processed in the up orientation (fig. 7(b) ). This may be due to the longer migration distance for pores to reach the free surface in thicker films. Pores are detrimental as they degrade Jc by blocking the current path and by disrupting the local 2212 alignment. Films, with their free surface that can consume pores, can have an inherent processing
152
W. Zhang, E.E. HeUstrom /Aligned microstructure ofBSCC0-2212
advantage over Ag-sheathed wires and tapes where the sheath can trap the pores.
5. Conclusion A l i g n e d 2212 o n Ag a n d A u foil a n d M g O single crystal were p r e p a r e d b y m e l t processing doctor-
bladed films. The phase assemblage was studied as a function of temperature. In 2212/Ag film, the Cufree phase melted about 905 ° C, and in 2212/Au and 2212/MGO films between 905 and 910°C. When cooling from Tm~ above the melting point of the Cufree phase, this phase precipitated from the liquid phase and grew to large size compared with its size before melting. Therefore, Tm~ should not exceed 900°C when melt processing 2212/Ag film. For 2212/Au and 2212/MGO films, it should not exceed 905°C. On cooling, the 2212 phase nucleated around 870°C, and there is no evidence that it preferentially nucleated at the oxide/substrate interface or on the second phases in the melt. The 2212 phase grew directly from the liquid without preferential orientation. The free surface played an important role in aligning 2212 grains. In the thick film, the 2212 near the free surface was well aligned and this region extended ~ 25 ttm into the film with our processing conditions. On the other hand, near the oxide/Ag interface, the alignment was poorer, and this region contained more and larger second phases. Two alignment mechanisms, constrained growth and alignment due to a free surface effect were presented.
Acknowledgements We would like to thank our collaborators at the University of Wisconsin-Madison and in particular Roger D. Ray II for helpful discussions. This work was supported by the Defense Advanced Research Projects Agency contract N000 14-90-J-4115 and the Electric Power Research Institute contract RP800905.
References [ 1 ] J. Kase, IC Togano, H. Kumakura, D.R. DictdErich, N. Irisawa, T. Morimoto and H. Maeda, Jpn. J. Appl. Phys. 29 (1990) L1096. [ 2 ] Y. KOikE,T. Nakanomyo and T. Fukase, Jpn. J. Appl. Phys. 27 (1988) L841. [3]J. Kase, T. Norimoto, IC Tngano, H. Kumakura, D.R. Dictderich and H. Maeda, IEEE Trans. Mag. 27 (1991) 1254. [4] J. Shimoyama, J. Kase, T. Morimoto, H. Kitaguchi, H. Kumakura, IC Togano and H. Maeda, Jpn. J. Appl. Phys. 31 (1992) L1167. [ 5 ] T. Sara, K. Sakai and S. Tashiro, J. Am. Cer. Soc. 75 (1992) 805. [ 6 ] R.D. Ray II and E.E. Hellstrom, Physica C 175 ( 1991) 255. [7]E.E. HEllstrom, R.D. Ray II and Wei Zhan8, Appl. Supercond. 1 (1993) 1535. [8 ] lC Nomura, M. Seido, H. Irdtasuchi, H. Kumakura, T. Tngano and H. Maeda, Appl. Phys. Lett. 62 (1993) 2131. [ 9 ] K. Shibutani, T. Egi, S. Hayashi, R. Ogawa and Y. Kawate, Jpn. J. Appl. Phys. 30 ( 1991 ) 3371. [ 10 ] T. HasEbe, Y. Tanaka, T. YanagJya, T. Asano, M. Fukutomi and H. Maeda, Jpn. J. Appl. Phys. 31 (1992) L21. [ 11 ] Y. Feng, ILE. Hautanen, Y.E. High, D.C. Larbalestier, R.D. Ray H, E.E. Hellstrom and S.E. Babcock, Physica C 192 (1992) 293. [ 12 ] M.J. Cima, X.P. Jiang, H.M. Chow, J.S. Haggerty and M.C. Fleminp, J. Mater. RES. 5 (1990) 1834. [ 13 ] T. Izumi, Y. Nakamura and Y. Shiohara, J. MatEr. Res. 7 (1992) 1621. [ 14] M.J. Cima, M.C. Flemings, A.M. Figueredo, M. Nakade, H. Ishii, H.D. Brody and J.S. Ha88erty, J. Appl. Phys. 72 (1992) 179. [ 15 ] D.C. Sinclair, J.T.S. Irvine and A.R. West, J. Mater. ChEm. 2 (1992) 579. [ 16 ] R. Miiller, T. Schweizer, P. Bohac, R.O. Suzuki and L.J. Gauckier, Physica C 203 (1992) 299. [ 17 ] P. Majewski, H.L. Su and B. Hettich, Adv. MatEr. 4 (1992) 508. [ 18 ] R.O. Suzuki, S. Kambara, H. Tsuchida, IC Shimizu and K. Ono, Advances in Superconductivity II, Proc. 2nd Int, Symp. on Superconductivity, Tskuba, Japan, Nov. 1989, eds. T. Ishiguro and IC Kajimura (SpringEr, Tokyo, 1989) p. 235. [19]B. Hong, J. Hahn and T.O. Mason, J. Am. Set. Soc. 73 (1990) 1965. [20] K. Schulze, P. Majewski, B. Hettich and G. PEtzow, Z. Metailkd. 81 (1990) 836. [21 ] R. Milller, T. Schweizer, P. Bohac, R.O. Suzuki and L.J. Ganckier, Physica C 203 (1992) 299. [ 22 ] D.P. Matheis, S.T. Misture and R.L. SnydEr, Physica C 207 (1993) 134. [ 23 ] E.E. Hellstrom, JOM 44 (1992) 48. [24] R.D. Ray II and E.E. HEllstrom, Appl. Phys. Lett. 57 (1990) 2984.