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Microstructural effects on superconducting properties of sintered YBa2Cu3O7−x wires
PflYSICA
Physica C 178 (1991) 81-88 North-Holland
Microstructural effects on superconducting properties of sintered
YBazCu3Oy_x wires S.R. Su, M. O ' C o n n o r , M. L e v i n s o n a n d P.G. R o s s o n i GTE Laboratories Incorporated, 40 Sylvan Road, Waltham, MA 02254, USA
Received 2 April 1991 Revised manuscript received 20 April 1991
Microstructural effects on the superconducting properties of extruded wires were studied. The most important factors controlling microstructure are the binder system, its removal conditions, and the sintering conditions - temperature, heating rate, duration, and annealing in oxygen. The transport Jc of 1500 A/cm z has been achieved under optimized processing conditions. A new technique that uses aligned grains as seeds and the eutectic phase of BaCuO2and CuO as a low melting flux was explored as a way to achieve oriented microstructures. Properties of these wires are presented and discussed.
I. Introduction
Since discovery of the high-temperature superconductivity of YBazCu3Ov_x at 95 K, worldwide efforts have centered on fabricating wires for a wide array of potential applications [ 1 ]. Large-scale implementation will involve fabrication o f conducting rings for magnetic energy storage, windings for power generation, coils for medical diagnostics, long continuous wires for power transmission lines, and helixes for passive microwave components [2]. However, these new types of oxide superconductors are usually brittle and difficult to fabricate into useful shapes such as tapes, wires or coils. In addition, advanced processing techniques - melt-texturing [3] or zone-melting [ 4 ] - are needed if the grains are to be aligned along the anisotropic a - b conducting plane to enhance critical current densities. Practical wire applications for power generation will necessitate a critical density of the order of 100000 A / c m 2 in magnetic fields up to 5 T [ 1 ], whereas homogeneity of the bulk and surface smoothness are critical to its application in microwave circuits. Therefore, proper materials processing is imperative in order to reduce defects and to enhance mechanical and electrical properties. Several techniques that use metallurgical cladding/swaging and plastic fiber spinning/extruding have been reported to fabricate continuous wires
with promising properties [ 5,6 ]. The former method produces wires with high flexibility and workability while the latter usually yields brittle, inhomogeneous material. However, by improving processing parameters, various researchers have been able to produce sintered wires by plastic extrusion [5] or by metal cladding techniques [6] with critical current densities in the range of 1 - 2 × 103 A / c m 2 at 77 K in the earth's magnetic field. As a consequence of the anisotropic nature of the high-T~ material, superconducting properties - critical temperature, Tc and critical current density, Jc - and microwave properties are greatly influenced by their microstructures, which, in turn, are governed by processing parameters. These parameters, including particle size, morphology and purity of the powder, type of binders used in the plastic extrusion process and its removal schedule, densification and oxygenation conditions, are important variables which could lead to materials with superior superconducting properties and high reproducibility. This paper present results relating systematic studies of these parameters to the microstructure of superconducting extruded wires and to the transport critical current density Jc. As a result, we have developed a process which produces wires with grains oriented along its axis and have fabricated helical coils whose surface resistance is
0921-4534/91/$ 03.50 © 1991 Elsevier Science Publishers B.V. All rights reserved
82
S.R. Suet al. / Microstructural ~[]bcts on superconducting properties ~l.sintered YBa:( "u~O~ , wires'
one-sixth of a copper reference at a resonance frequency o f 1.0 G H z and 77 K [7].
2. Experimental procedure Two types o f YBa2Cu3OT_ ,. powders with particle sizes in the range o f 2 to 5/am, one processed by the solution method and the other by solid-state reaction, are selected for this study. The platelet-like powder, with an average size o f 3 gm, is synthesized from spray-drying a m i x e d - n i t r a t e solution, and the resultant salt is thermally d e c o m p o s e d at 700°C and annealed in oxygen at 910 °C [8]. Powders p r e p a r e d from the solid-state reaction are equiaxed in shape and were milled either by the jet-milling technique or by conventional ball-milling. The former yields powder with an average particle size o f 2.5 gm, and the ball-milled p o w d e r has a m a j o r i t y (80%) o f particles 5 g m in size. The c o m p o s i t i o n o f the p o w d e r was analyzed by inductively coupled p l a s m a ( I C P ) to ensure correct stoichiometry. Powder phases were characterized by X-ray diffraction. The particle size was measured by a M i c r o m e r i t i c s Digisorb 2600; surface area was measured by a five-point BET nitrogen a d s o r p t i o n surface area analyzer. The average surface area was 3.3 m2/g for the ball-milled powder and about 7.0 m 2 / g for the nitrate-precursor powder. The critical t e m p e r a t u r e ( T c ( o n s e t ) ) o f these powders, which is in the range o f 95 to 93 K, was measured by a magnetic p r o p e r t y m e a s u r e m e n t system ( M P M S ) m a n u f a c t u r e d by Q u a n t u m Design. The density o f the wire was measured based on Archimedes' theory, with toluene as the displacing solution. Wires o f various d i a m e t e r s are fabricated by a plastic extrusion technique. In this process, the YBa2Cu3OT_ ~ powder is c o m b i n e d with a set o f organic vehicles consisting o f a high molecular weight organic wax, whose melting point is in the range o f 77°C to 85°C, and dispersants or surfactants; the mixture is subsequently mixed mechanically at the melting t e m p e r a t u r e o f the b i n d e r to reach h o m o geneity. The mixed material is then extruded through a capillary die with a d i a m e t e r between 70 ~m and 2 ram. In o r d e r to eliminate t r a p p e d air inside the mixed material, the heated extrusion c h a m b e r is evacuated with a v a c u u m p u m p prior to extrusion.
Green wires or helical coils with great flexibility and uniform microstructure arc fabricated in lengths o f over 100 cm. The microstructure of all samples is e x a m i n e d by an A M R A Y 1200B scanning electron microscope ( S E M ) . The binder removal process is carried out in air at 500=C with a slow heating schedule, 1 - C / m i n , to avoid crack or void formation. The b i n d e r - r e m o v e d green wires are then sintered in air between 920 C and 965°C to densify, followed by annealing in oxygen at 500°C for 24 to 48 h, The zero resistance (;'~) o f these wires occurs at 90 to 91 K. The transport critical current density m e a s u r e m e n t is performed at liquid nitrogen temperature. Silver contact is evaporated on the top surface o f the sample in a four-point configuration with a 4 m m interval; gold wires used as leads are attached to the contact using silver paste. The transport J~ with a criterion o f I JuV/cm is used. Ba-rich and Cu-rich powder with a nominal composition Yl.oBa2.3Cu3 ~O7 , used for texturing is prepared from the nitrate precursors as m e n t i o n e d earlier. The melt-growth process is performed in air at or above the incongruent temperature of YBa2Cu307 , between 980°C and 1050°C, cooled at a m o d e r a t e rate, 0 . 2 ° C / m i n , to 950°C, and annealed in oxygen at 500°C. The c o m p o s i t i o n o f these sintered and textured wires is analyzed by energydispersive spectrum analysis ( E D S ) .
3. Results and discussion 3.1. Fabrication o f extruded wires
The rheological property o f the binder system used in the plastic extrusion process is an i m p o r t a n t factor which controls green density o f wires by deflocculating the p o w d e r to provide a homogeneous microstructure, alleviating processing defects. In this work, a b i n d e r system containing high molecular weight organic p o l y m e r attached with carbonyl (-C=O) functionality is selected. The morphological properties o f the two types o f powders - nitrate-precursor and solid-state - vary slightly due to differences in the shapes o f the particle - platelet-like versus equiaxed, surface areas - 7.0 m Z / g versus 3.3 m2/g, and m e t h o d s o f processing. However, by modifying the b i n d e r c o m p o s i t i o n with the a d d i t i o n o f
S.R. Su et al. / Microstructural effects on superconducting properties of sintered YBa2Cu307 ~wires
surfactant, we are able to keep the solids loading in the extruded mixture at 64.0 v/o. The solids loading was measured by thermogravimetric analysis (TGA). A typical TGA result is shown in fig. 1, where the binder starts to decompose at 200°C and is completely removed at 500 ° C; 92.15 w / o of solids, corresponding to 64.8 v/o, remained. (The heating rate of this run is 10°C/min with oxygen flow rate at 100 cc/min). The slight weight gain at 700°C (initiates at 500°C) and weight loss at 900°C is believed to relate to oxygen absorption and desorption of YBa2Cu3Ov_x. Since this temperature for oxygen absorption is slightly higher, a combination of CO2 uptake cannot be ruled out. However, when the thermogram is run under argon, the weight gain starting at ~ 500°C is no longer observed. The dispersibility of the powder in the binder system, examined by a scanning electron microscope (SEM), reveals a homogeneous microstructure; each particle is uniformly embedded in the organic binder matrix. As a result of this high dispersibility, green wires with uniform porous structure formed after a binder removal process. An interesting observation is that despite the shape of the powder particle, some alignment of elongated grains is evident on the as-sintered surface of wires. This result may be attributed to the fact that because of high shear stresses induced during the extrusion process, particles were aligned in some degree toward the orifice of the die. The SEM micrographs of these sintered wires and a typical
83
fracture cross section are shown in figs. 2 (a) and (b). 3.2. Densification process 3.2.1. Effect o f green density on sintered density
As expected, the higher than powder concentration, the higher the green density of the wire after the binder removal process. Samples with 91.5 w/o powder fraction yield green density of 70.7% (based on 6.38 g/cm3). In contrast, samples loaded with 89.5 w / o of powder produce wires with 64.0% of green
102 100
~
258"9°C
98
~ ~,~
i
. 45.5oc(I)
96
94
/ ~
92.1 W e i g h t %
.
424.3°C
~
92-90
i 0
[ 200
i
I
400
l
[ 600
950.0~C 90.6% I l 800
% i
1000
Temperature(~C)
Fig. 1. Thermogravimetricanalysis of powder/binder mixture, indicating high powder concentration.
Fig. 2. SEM micrographsof extruded sintered wire, (a) as-sintered surface, indicating some alignmentof elongated grains; (b) cross section of the sintered wire, indicatingno processingdefects.
84
S.R. S u e t a/. /Microstructural e{l~cts on superconducting properties ~/ smtered YBa:( "usO- ~ wire.s
density. Under identical sintering conditions - sintering temperature and duration of sintering - wires with higher green density produce samples with higher sintered density. This relationship is demonstrated in fig. 3: a sample with a green density of 70.7% produces 94.0% dense material, whereas a sample with 64.0% green density yields wire with 88.0% theoretical density, both after sintering at 950°C. Microstructural evaluation of a series of green wires with different green densities and of their corresponding sintered materials further supports this conclusion. Figures 4 (a) to (d) show that denser wires with closely joined grains are obtained from wires with denser green microstructure. This densification process is most likely assisted by the presence of a liquid phase [9], BaCuO2, resulting from the impurity of the powder. This liquid phase sintering can lead to complete densification if the volume of liquid present is sufficient to fill in the interstices completely [ I0]. When particles are more effectively packed, less spaces are to be filled. As a consequence, denser sintered compacts result.
3. 2. 2. Effect o f particle size on sintering temperature As for all ceramic systems, particle size in the green compact affects reactivity in ceramic processing, the smaller the particle size, the higher the reactivity, the lower the sintering temperature to achieve a dense compact. However, wires with diameters below 250
100
o ~ -¢ '~
90 I 80 7O
62
l
i
i
64
Green Density
66
68 70 72 (% of TheoreticalDensity)
Fig. 3. Relationship of the sintered density vs. the green density of the wire.
Jam can be densified at 9 2 0 C in 15 rain regardless of particle size. This observation may be partly instrumental. Because of the size of the sample, the method of density measurement is not sensitive enough to detect the difference. When the diameter of the wire exceeds 600 ~tm or larger, a longer time is required to densify the samples prepared from ballmilled solid state powder with 5.0 ~tm in size. For example, the sintering temperature for the wire made of 2.5 ~am size powder is 950°C for 30 minutes to reach 94% of the theoretical density, whereas holding at 965~C for one hour is needed to achieve the same results for wires made of 5.0 lain size powder. Since the optimal sintering condition varies with particle size of the powder, consistent powder morphology is essential to yield highly reproducible material. We also found that with the same particle size, namely 2.5-3.0 ~tm, the nitrates precursor powder is more reactive than the powder made from the solidstate method. As a result, denser wires formed under identical sintering conditions. This can be attributed to the fact that chemical processing leads to the precursor powder with sized crystals of the various phases in intimate contact with each other. After the calcination step, this chemical processing produces powder not only with small, uniform size but also with homogeneous composition at the microscopic level as compared to powders prepared by conventional solid state method.
3.2.3. E{fect :~/sintering conditions on critical current density J~ It is well known that the critical current density of bulk high-T~, superconductors is limited [ 11 ] by the presence of high-angle grain boundaries; the presence of the secondary phases such as BaCuO2, CuO_~ and Y2BaCuO~; porosity; localized oxygen frequency; and microcracks. All of these limitations are more or less governed by processing conditions, especially the sintering temperature, heating rate, holding time, and annealing condition in oxygen. Based on the phase diagram of YBa2Cu3OT_, [9] the superconducting phase begins to form Y2BaCuO~, BaCuO2 and CuO impurities at 980°C. Samples need to be sintered below this temperature in order to achieve single-phase material. The heating rate and duration of sintering are also important for controlling the microstructure of the sintered wire. Inade-
S.R. Suet al. /Microstructural effects on superconducting properties o f sintered YBa 2Cu jO T_x wires
85
Fig. 4. SEM micrographscomparinggreen wires with sintered wires. quate heating rate and holder time can cause microcracks due to stress induced by grain growth. A transport Jc of 1500 A / c m 2 at 77 K in the earth's magnetic field is achieved when wires with diameter in the range of 250 to 600 ~tm were sintered at 950°C at a 2°C/rain heating rate for ½ h and oxygenated at 500°C for 24 h. The microstructure of this sample, examined by a scanning electron microscope as shown in fig. 5(a), exhibits densely packed elongated grains with an average length of 10 ~tm and width of 2.5 ktm. However, when the sample is sintered to 1000°C at 5°C/min for a short period and slowly cooled to 950°C at 0.2-0.5 °C/rain, large-sized grains with faceted platelet growth are clearly evident from the microstructure of the sample, illustrated in fig. 5 (b). Besides high-angle grain boundaries, secondary phases identified as Y2BaCuOs, CuO
by X-ray diffraction are also present. As a result, the transport Jc of this sample drops to 235 A / c m 2. Porosity is also an important factor and can be detrimental to the critical current density. However, samples with interconnected pores with 95-97% of the theoretical density ( T D ) usually have higher Jc than the denser samples with above 98% TD. In contrast, samples with scattered porosity, as illustrated in fig. 5 (c), usually have lower Jc. The transport Jc of this sample is about 820 A / c m 2. In fact, samples with interconnected pores (examined by SEM) are more easily oxygenated during phase transformation from tetragonal to orthorhombic than those of nearly full density materials. Our results indicate that samples with 92 to 95% of the theoretical densities can achieve a Tc of 90 K after an anneal in oxygen at 500°C for 24 h, while an annealing for 36 h is required for sam-
86
S.R. S u e t al. /Microstructural eflects on superconducting properttes (~fsintered YBa e('u+O, , wires
,+a .
~,,, + +
+,
¢" +
+
,,+
t" ~ + .++ +++
++ ~
"~
ples with 96-97% of TD. On the other hand, scattered porosity suppresses the current flow, leading to lower Jc- Our observation agrees with that of AIford's early report [ 12 ].
3.3. Method of grain aligment
Fig. 5. SEM micrographs of as-sintered wires (a) sintered at 950°C, indicating uniform elongated grains; (b) sintered at 1000°C. indicating faceted grain growth; and (c) sintered at 920 ° C, indicating scattered porosity.
The low Jc of the sintered Y B a 2 C U 3 O T _ ~ material is related to Josephson junction weak-link effects [ 13 ]. Two major problems associated with this have been identified: small coherence length, causing grain boundaries to act as Josephson junctions; and a high degree of anisotropy, severely suppressing tunneling currents at high-angle grain boundaries. The melttextured growth technique and a liquid-phase processing method have been widely used to achieve a highly textured microstructure with elongated grains aligned in the a-b plane through which the transport currents are coupled [4,11]. However, these methods require heating the sample above the solidus temperature, 1050- t 200 ° C, to dissolve the impurity phases and an extremely slow cooling rate to align grains along the current conducting plane. We have explored an alternate way to process wires with oriented microstructure at a lower temperature and a faster cooling rate. Results show that wires with highly oriented microstructure can be achieved by sintering the sample slightly above the incongruencc temperature of YBa2Cu307 ,., 980-1050°C, and by cooling it in a moderate rate at 0.2 +C/min to 950°C. In this process, several weight percent of previously aligned clusters in size below 210 lam were doped into the YBa2Cu3OT_, powder as nuclei prior to wire fabrication. These grains were partially aligned along the long axis of the wire the extrusion process. It was hoped that during the liquid sintering process, the individually aligned clusters would act as nucleation centers, leading to elongated grains growing along the wire axis. The superconducting powder prepared from thermal decomposition of nitrates with nominal composition of Y i . 0 B a 2 . 3 C u 3 . 3 0 7 _ v was to test the idea. It was further hoped that this Ba-rich and Curich composition would lead to low-temperature sintering by using the eutectic phase of BaCuO2 and CuO as a low melting flux. A zero resistance Tc at 90 K is obtained with a narrow AT transition. A transport J~ of 900 A / c m 2 is achieved at 77 K in the earth's magnetic field. The compositional analyses on the
S.R. Suet al. /Microstructural effects on superconducting properties of sintered
as-sintered surface and the bulk performed by EDS indicate CuO and Ba/Cu rich region. Because of the shape of the sample, we could not detect whether these phases are on the surface or between grain boundaries. When the sample is melt-textured at 1050 ° C, elongated grains oriented along the fiber axis are clearly evident as shown in the micrograph in fig. 6 (a). In fig. 6 (b), we show the fractured cross section of the sample, indicating the platelet growth steps throughout the entire cross section. The transport Jc of this sample is ~ 1000 A/cm 2 with a zero resistance at 90 K. The presence of secondary phases, CuO
YBa2Cu307_ x
wires
87
and BaCuO2, were confirmed by X-ray diffraction. The Jc values are not as high as expected for the melttextured samples. However, this growth morphology seems to support the mechanism proposed by Hepp et al. [ 14 ] that step growth of grain along the c-axis is controlled by the surface nucleation rate, and platelet growth along the a - b plane is limited by the mass-transpor( rate. In this process, the presence of aligned clusters and low melting flux seems to promote the elongated grain-growth with growth steps along the c-axis as well as the platelet growth along the a - b plane. As a result, the processing temperature can be lowered and the cooling rate can be accelerated to 12°C/h. In a comparative experiment, we applied the aforementioned sintering schedule to wires extruded from a nominal composition of YBa2Cu307_x powder prepared by the solution method. We observed no grain alignment and only random platelet growth. More work is required to optimize processing parameters and to minimize the amount of second phase material. We believe that this process offers us a potential opportunity to process long continuous wires with highly oriented microstructures at substantially lower temperature than those methods currently in use. For example it is important to operate at a lower temperature for grain alignment of helices to maintain the integrity of the configuration.
4. Conclusions
Fig. 6. Microstructures of the aligned wire: (a) as-sintered surface, indicatingelongatedgrains oriented alongthe wire axis; (b) fractured cross-section, indicating the step-growth of platelet grains.
Effects of microstructure on the superconducting properties of Y B a 2 C u 3 0 7 _ x extruded wires were studied. Results of these studies suggest that the important factors controlling the microstructure are the binder system and the sintering conditions - temperature, rate, duration and annealing in oxygen. In fact, these parameters directly affect critical current densities of the superconducting wire. Under the best processing conditions, the transport Jc of 1.5× 103 A/cm 2 is achieved. This result is higher than those of Jc values reported for sintered bulk materials (typical transport Jc values at 77 K are ~ 500 A/ cm2). These improved results may be attributed to sintered wires with partially aligned Y B a z C u 3 0 7 _ x platelet crystals, resulting from the extrusion process. By seeding the Ba-rich and Cu-rich powder with
88
,%R S u et al. /Mlcroslructural ~(['ects on superconducting properties qFsintered ) Ba:('u.~O: , wires
oriented grains, wires with highly aligned micros t r u c t u r e are a c h i e v e d b y a l i q u i d s i n t e r i n g t e c h n i q u e at 9 8 0 - 1 0 5 0 ° C . T h e t r a n s p o r t Jc's o f ~ 1000 A / c m : w i t h a z e r o r e s i s t a n c e at 90 K a r e m e a s u r e d for t h e s e s a m p l e s . T h e s e l o w e r t h a n e x p e c t e d J~ values m a y b e d u e to t h e p r e s e n c e o f s e c o n d a r y p h a s e s , m a i n l y B a C u O 2 a n d C u O . W e b e l i e v e t h e g r a i n text u r i n g m e c h a n i s m is r e l a t e d to t h e fact t h a t w i t h t h e a s s i s t a n c e o f t h e low m e l t i n g flux, p l a t e l e t g r o w t h a l o n g t h e s e e d i n g m a t e r i a l occurs, r e s u l t i n g in w i r e with oriented microstructure. Process optimization to m i n i m i z e t h e a m o u n t o f s e c o n d a r y p h a s e s is u n d e r way to e n h a n c e t h e i r s u p e r c o n d u c t i n g p r o p e r t i e s .
Acknowledgements T h e a u t h o r s a c k n o w l e d g e E. G u t m a n , R. H a m m o n d a n d M. D o w n e y for e l e m e n t a l , t h e r m a l , a n d X - r a y d i f f r a c t i o n a n a l y s e s . W e also t h a n k Dr. B. Ditc h e k for his v a l u a b l e c o m m e n t s a n d s u g g e s t i o n s .
References [I] (a) T.D. Schlabach, Am. Soc. of Mech. Eng., Materials Division MD. vol. 29 (1988): (b) M.T. Lanagan, R.B. Poeppel, J.P. Singh, D.I. DosSantos, J.K. Lumpp, U. Balachandran, J.T. Dusek and K.C. Goretta, J. Less-Common Metals, 149 (1989) 305:
(c) T. Hughes, Power Transmission Design (November. 1990) pp. 45-47. [2] B. Hammond, Supercurrents 66 (1989). [3] S. Jim T.H. Tiefel, R.B. van Dover, M.E. Davis, G.W. Kammlott and R.A. Fastnacht, Phys. Rev. 37 ( 1988 ) 7850. [4] D. Shi, H. Krishnan, J.M. Hong, D. Miller, P.J. McGinn el al., J. Appl. Phys. 68 (1990) 228. [ 5 ] J.R. Gaines Jr., Cer. Bul. 68 ( 1989 ) 857. [6] (a) S. Jin et al., App[. Phys. Len. 51 (1987) 203; (b) N. Sadakata ctal., High-Temperature Superconductors, vol. 99 (Mater. Res. Soc., Pittsburg) (1988) 293: (c) Y. Yamada et al.. Jpn. J. Appl, Phys. 26 ( 1987 ) L865. [7]S.R. Su et al., in: Correlation of Microwave and Superconducting Properties of YBa2Cu307 • Wires and Helical Coils, to be presented at the MRS Spring Meeting, 1991. [8] S.R. Su, M. O'Connor and M. Levinson, J. Mater. Res. ( 1991 ) 244. [9] T. Aselage and K. Keefer, J. Mater. Res. 3 (1988) 1279. [10]W.D. Kingery, H.K. Bowen and D.R. Uhlmann, eds.. Introduction to Ceramics, 2nd ed. (John Wiley & Sons, New York, 1976) p. 448. [ 11 ] (a) R.L. Meng, C. Kinalidis, Y.Y. Sun, L. Gao, Y.K. Tao, P.H. Hor and C.W. Chu, Nature 345 (1990) 326; (b) K. Salama, V. Selvamanickam, L. Gao and K. Sun, App|. Phys. Lett. 54 (1989) 2352. [ 12] N.McN. Alford, J.D. Birchall, W.J. Clegg, M.A. Harmer and K. Kendall, J. Mater. Sci. 23 (1988) 761. [ 13 ] T. Worthington, W. Gallagher and T. Dinger, Phys. Rev. Len. 59 (1987) 1160. [14]A.F. Hepp, J.R. Gaier, G.A. Landis and S.G. Bailey, Research Update, 1988, Ceramic Superconductors II, ed. Man F. Yam (1988) pp. 356-366.
Physica C 178 (1991) 81-88 North-Holland
Microstructural effects on superconducting properties of sintered
YBazCu3Oy_x wires S.R. Su, M. O ' C o n n o r , M. L e v i n s o n a n d P.G. R o s s o n i GTE Laboratories Incorporated, 40 Sylvan Road, Waltham, MA 02254, USA
Received 2 April 1991 Revised manuscript received 20 April 1991
Microstructural effects on the superconducting properties of extruded wires were studied. The most important factors controlling microstructure are the binder system, its removal conditions, and the sintering conditions - temperature, heating rate, duration, and annealing in oxygen. The transport Jc of 1500 A/cm z has been achieved under optimized processing conditions. A new technique that uses aligned grains as seeds and the eutectic phase of BaCuO2and CuO as a low melting flux was explored as a way to achieve oriented microstructures. Properties of these wires are presented and discussed.
I. Introduction
Since discovery of the high-temperature superconductivity of YBazCu3Ov_x at 95 K, worldwide efforts have centered on fabricating wires for a wide array of potential applications [ 1 ]. Large-scale implementation will involve fabrication o f conducting rings for magnetic energy storage, windings for power generation, coils for medical diagnostics, long continuous wires for power transmission lines, and helixes for passive microwave components [2]. However, these new types of oxide superconductors are usually brittle and difficult to fabricate into useful shapes such as tapes, wires or coils. In addition, advanced processing techniques - melt-texturing [3] or zone-melting [ 4 ] - are needed if the grains are to be aligned along the anisotropic a - b conducting plane to enhance critical current densities. Practical wire applications for power generation will necessitate a critical density of the order of 100000 A / c m 2 in magnetic fields up to 5 T [ 1 ], whereas homogeneity of the bulk and surface smoothness are critical to its application in microwave circuits. Therefore, proper materials processing is imperative in order to reduce defects and to enhance mechanical and electrical properties. Several techniques that use metallurgical cladding/swaging and plastic fiber spinning/extruding have been reported to fabricate continuous wires
with promising properties [ 5,6 ]. The former method produces wires with high flexibility and workability while the latter usually yields brittle, inhomogeneous material. However, by improving processing parameters, various researchers have been able to produce sintered wires by plastic extrusion [5] or by metal cladding techniques [6] with critical current densities in the range of 1 - 2 × 103 A / c m 2 at 77 K in the earth's magnetic field. As a consequence of the anisotropic nature of the high-T~ material, superconducting properties - critical temperature, Tc and critical current density, Jc - and microwave properties are greatly influenced by their microstructures, which, in turn, are governed by processing parameters. These parameters, including particle size, morphology and purity of the powder, type of binders used in the plastic extrusion process and its removal schedule, densification and oxygenation conditions, are important variables which could lead to materials with superior superconducting properties and high reproducibility. This paper present results relating systematic studies of these parameters to the microstructure of superconducting extruded wires and to the transport critical current density Jc. As a result, we have developed a process which produces wires with grains oriented along its axis and have fabricated helical coils whose surface resistance is
0921-4534/91/$ 03.50 © 1991 Elsevier Science Publishers B.V. All rights reserved
82
S.R. Suet al. / Microstructural ~[]bcts on superconducting properties ~l.sintered YBa:( "u~O~ , wires'
one-sixth of a copper reference at a resonance frequency o f 1.0 G H z and 77 K [7].
2. Experimental procedure Two types o f YBa2Cu3OT_ ,. powders with particle sizes in the range o f 2 to 5/am, one processed by the solution method and the other by solid-state reaction, are selected for this study. The platelet-like powder, with an average size o f 3 gm, is synthesized from spray-drying a m i x e d - n i t r a t e solution, and the resultant salt is thermally d e c o m p o s e d at 700°C and annealed in oxygen at 910 °C [8]. Powders p r e p a r e d from the solid-state reaction are equiaxed in shape and were milled either by the jet-milling technique or by conventional ball-milling. The former yields powder with an average particle size o f 2.5 gm, and the ball-milled p o w d e r has a m a j o r i t y (80%) o f particles 5 g m in size. The c o m p o s i t i o n o f the p o w d e r was analyzed by inductively coupled p l a s m a ( I C P ) to ensure correct stoichiometry. Powder phases were characterized by X-ray diffraction. The particle size was measured by a M i c r o m e r i t i c s Digisorb 2600; surface area was measured by a five-point BET nitrogen a d s o r p t i o n surface area analyzer. The average surface area was 3.3 m2/g for the ball-milled powder and about 7.0 m 2 / g for the nitrate-precursor powder. The critical t e m p e r a t u r e ( T c ( o n s e t ) ) o f these powders, which is in the range o f 95 to 93 K, was measured by a magnetic p r o p e r t y m e a s u r e m e n t system ( M P M S ) m a n u f a c t u r e d by Q u a n t u m Design. The density o f the wire was measured based on Archimedes' theory, with toluene as the displacing solution. Wires o f various d i a m e t e r s are fabricated by a plastic extrusion technique. In this process, the YBa2Cu3OT_ ~ powder is c o m b i n e d with a set o f organic vehicles consisting o f a high molecular weight organic wax, whose melting point is in the range o f 77°C to 85°C, and dispersants or surfactants; the mixture is subsequently mixed mechanically at the melting t e m p e r a t u r e o f the b i n d e r to reach h o m o geneity. The mixed material is then extruded through a capillary die with a d i a m e t e r between 70 ~m and 2 ram. In o r d e r to eliminate t r a p p e d air inside the mixed material, the heated extrusion c h a m b e r is evacuated with a v a c u u m p u m p prior to extrusion.
Green wires or helical coils with great flexibility and uniform microstructure arc fabricated in lengths o f over 100 cm. The microstructure of all samples is e x a m i n e d by an A M R A Y 1200B scanning electron microscope ( S E M ) . The binder removal process is carried out in air at 500=C with a slow heating schedule, 1 - C / m i n , to avoid crack or void formation. The b i n d e r - r e m o v e d green wires are then sintered in air between 920 C and 965°C to densify, followed by annealing in oxygen at 500°C for 24 to 48 h, The zero resistance (;'~) o f these wires occurs at 90 to 91 K. The transport critical current density m e a s u r e m e n t is performed at liquid nitrogen temperature. Silver contact is evaporated on the top surface o f the sample in a four-point configuration with a 4 m m interval; gold wires used as leads are attached to the contact using silver paste. The transport J~ with a criterion o f I JuV/cm is used. Ba-rich and Cu-rich powder with a nominal composition Yl.oBa2.3Cu3 ~O7 , used for texturing is prepared from the nitrate precursors as m e n t i o n e d earlier. The melt-growth process is performed in air at or above the incongruent temperature of YBa2Cu307 , between 980°C and 1050°C, cooled at a m o d e r a t e rate, 0 . 2 ° C / m i n , to 950°C, and annealed in oxygen at 500°C. The c o m p o s i t i o n o f these sintered and textured wires is analyzed by energydispersive spectrum analysis ( E D S ) .
3. Results and discussion 3.1. Fabrication o f extruded wires
The rheological property o f the binder system used in the plastic extrusion process is an i m p o r t a n t factor which controls green density o f wires by deflocculating the p o w d e r to provide a homogeneous microstructure, alleviating processing defects. In this work, a b i n d e r system containing high molecular weight organic p o l y m e r attached with carbonyl (-C=O) functionality is selected. The morphological properties o f the two types o f powders - nitrate-precursor and solid-state - vary slightly due to differences in the shapes o f the particle - platelet-like versus equiaxed, surface areas - 7.0 m Z / g versus 3.3 m2/g, and m e t h o d s o f processing. However, by modifying the b i n d e r c o m p o s i t i o n with the a d d i t i o n o f
S.R. Su et al. / Microstructural effects on superconducting properties of sintered YBa2Cu307 ~wires
surfactant, we are able to keep the solids loading in the extruded mixture at 64.0 v/o. The solids loading was measured by thermogravimetric analysis (TGA). A typical TGA result is shown in fig. 1, where the binder starts to decompose at 200°C and is completely removed at 500 ° C; 92.15 w / o of solids, corresponding to 64.8 v/o, remained. (The heating rate of this run is 10°C/min with oxygen flow rate at 100 cc/min). The slight weight gain at 700°C (initiates at 500°C) and weight loss at 900°C is believed to relate to oxygen absorption and desorption of YBa2Cu3Ov_x. Since this temperature for oxygen absorption is slightly higher, a combination of CO2 uptake cannot be ruled out. However, when the thermogram is run under argon, the weight gain starting at ~ 500°C is no longer observed. The dispersibility of the powder in the binder system, examined by a scanning electron microscope (SEM), reveals a homogeneous microstructure; each particle is uniformly embedded in the organic binder matrix. As a result of this high dispersibility, green wires with uniform porous structure formed after a binder removal process. An interesting observation is that despite the shape of the powder particle, some alignment of elongated grains is evident on the as-sintered surface of wires. This result may be attributed to the fact that because of high shear stresses induced during the extrusion process, particles were aligned in some degree toward the orifice of the die. The SEM micrographs of these sintered wires and a typical
83
fracture cross section are shown in figs. 2 (a) and (b). 3.2. Densification process 3.2.1. Effect o f green density on sintered density
As expected, the higher than powder concentration, the higher the green density of the wire after the binder removal process. Samples with 91.5 w/o powder fraction yield green density of 70.7% (based on 6.38 g/cm3). In contrast, samples loaded with 89.5 w / o of powder produce wires with 64.0% of green
102 100
~
258"9°C
98
~ ~,~
i
. 45.5oc(I)
96
94
/ ~
92.1 W e i g h t %
.
424.3°C
~
92-90
i 0
[ 200
i
I
400
l
[ 600
950.0~C 90.6% I l 800
% i
1000
Temperature(~C)
Fig. 1. Thermogravimetricanalysis of powder/binder mixture, indicating high powder concentration.
Fig. 2. SEM micrographsof extruded sintered wire, (a) as-sintered surface, indicating some alignmentof elongated grains; (b) cross section of the sintered wire, indicatingno processingdefects.
84
S.R. S u e t a/. /Microstructural e{l~cts on superconducting properties ~/ smtered YBa:( "usO- ~ wire.s
density. Under identical sintering conditions - sintering temperature and duration of sintering - wires with higher green density produce samples with higher sintered density. This relationship is demonstrated in fig. 3: a sample with a green density of 70.7% produces 94.0% dense material, whereas a sample with 64.0% green density yields wire with 88.0% theoretical density, both after sintering at 950°C. Microstructural evaluation of a series of green wires with different green densities and of their corresponding sintered materials further supports this conclusion. Figures 4 (a) to (d) show that denser wires with closely joined grains are obtained from wires with denser green microstructure. This densification process is most likely assisted by the presence of a liquid phase [9], BaCuO2, resulting from the impurity of the powder. This liquid phase sintering can lead to complete densification if the volume of liquid present is sufficient to fill in the interstices completely [ I0]. When particles are more effectively packed, less spaces are to be filled. As a consequence, denser sintered compacts result.
3. 2. 2. Effect o f particle size on sintering temperature As for all ceramic systems, particle size in the green compact affects reactivity in ceramic processing, the smaller the particle size, the higher the reactivity, the lower the sintering temperature to achieve a dense compact. However, wires with diameters below 250
100
o ~ -¢ '~
90 I 80 7O
62
l
i
i
64
Green Density
66
68 70 72 (% of TheoreticalDensity)
Fig. 3. Relationship of the sintered density vs. the green density of the wire.
Jam can be densified at 9 2 0 C in 15 rain regardless of particle size. This observation may be partly instrumental. Because of the size of the sample, the method of density measurement is not sensitive enough to detect the difference. When the diameter of the wire exceeds 600 ~tm or larger, a longer time is required to densify the samples prepared from ballmilled solid state powder with 5.0 ~tm in size. For example, the sintering temperature for the wire made of 2.5 ~am size powder is 950°C for 30 minutes to reach 94% of the theoretical density, whereas holding at 965~C for one hour is needed to achieve the same results for wires made of 5.0 lain size powder. Since the optimal sintering condition varies with particle size of the powder, consistent powder morphology is essential to yield highly reproducible material. We also found that with the same particle size, namely 2.5-3.0 ~tm, the nitrates precursor powder is more reactive than the powder made from the solidstate method. As a result, denser wires formed under identical sintering conditions. This can be attributed to the fact that chemical processing leads to the precursor powder with sized crystals of the various phases in intimate contact with each other. After the calcination step, this chemical processing produces powder not only with small, uniform size but also with homogeneous composition at the microscopic level as compared to powders prepared by conventional solid state method.
3.2.3. E{fect :~/sintering conditions on critical current density J~ It is well known that the critical current density of bulk high-T~, superconductors is limited [ 11 ] by the presence of high-angle grain boundaries; the presence of the secondary phases such as BaCuO2, CuO_~ and Y2BaCuO~; porosity; localized oxygen frequency; and microcracks. All of these limitations are more or less governed by processing conditions, especially the sintering temperature, heating rate, holding time, and annealing condition in oxygen. Based on the phase diagram of YBa2Cu3OT_, [9] the superconducting phase begins to form Y2BaCuO~, BaCuO2 and CuO impurities at 980°C. Samples need to be sintered below this temperature in order to achieve single-phase material. The heating rate and duration of sintering are also important for controlling the microstructure of the sintered wire. Inade-
S.R. Suet al. /Microstructural effects on superconducting properties o f sintered YBa 2Cu jO T_x wires
85
Fig. 4. SEM micrographscomparinggreen wires with sintered wires. quate heating rate and holder time can cause microcracks due to stress induced by grain growth. A transport Jc of 1500 A / c m 2 at 77 K in the earth's magnetic field is achieved when wires with diameter in the range of 250 to 600 ~tm were sintered at 950°C at a 2°C/rain heating rate for ½ h and oxygenated at 500°C for 24 h. The microstructure of this sample, examined by a scanning electron microscope as shown in fig. 5(a), exhibits densely packed elongated grains with an average length of 10 ~tm and width of 2.5 ktm. However, when the sample is sintered to 1000°C at 5°C/min for a short period and slowly cooled to 950°C at 0.2-0.5 °C/rain, large-sized grains with faceted platelet growth are clearly evident from the microstructure of the sample, illustrated in fig. 5 (b). Besides high-angle grain boundaries, secondary phases identified as Y2BaCuOs, CuO
by X-ray diffraction are also present. As a result, the transport Jc of this sample drops to 235 A / c m 2. Porosity is also an important factor and can be detrimental to the critical current density. However, samples with interconnected pores with 95-97% of the theoretical density ( T D ) usually have higher Jc than the denser samples with above 98% TD. In contrast, samples with scattered porosity, as illustrated in fig. 5 (c), usually have lower Jc. The transport Jc of this sample is about 820 A / c m 2. In fact, samples with interconnected pores (examined by SEM) are more easily oxygenated during phase transformation from tetragonal to orthorhombic than those of nearly full density materials. Our results indicate that samples with 92 to 95% of the theoretical densities can achieve a Tc of 90 K after an anneal in oxygen at 500°C for 24 h, while an annealing for 36 h is required for sam-
86
S.R. S u e t al. /Microstructural eflects on superconducting properttes (~fsintered YBa e('u+O, , wires
,+a .
~,,, + +
+,
¢" +
+
,,+
t" ~ + .++ +++
++ ~
"~
ples with 96-97% of TD. On the other hand, scattered porosity suppresses the current flow, leading to lower Jc- Our observation agrees with that of AIford's early report [ 12 ].
3.3. Method of grain aligment
Fig. 5. SEM micrographs of as-sintered wires (a) sintered at 950°C, indicating uniform elongated grains; (b) sintered at 1000°C. indicating faceted grain growth; and (c) sintered at 920 ° C, indicating scattered porosity.
The low Jc of the sintered Y B a 2 C U 3 O T _ ~ material is related to Josephson junction weak-link effects [ 13 ]. Two major problems associated with this have been identified: small coherence length, causing grain boundaries to act as Josephson junctions; and a high degree of anisotropy, severely suppressing tunneling currents at high-angle grain boundaries. The melttextured growth technique and a liquid-phase processing method have been widely used to achieve a highly textured microstructure with elongated grains aligned in the a-b plane through which the transport currents are coupled [4,11]. However, these methods require heating the sample above the solidus temperature, 1050- t 200 ° C, to dissolve the impurity phases and an extremely slow cooling rate to align grains along the current conducting plane. We have explored an alternate way to process wires with oriented microstructure at a lower temperature and a faster cooling rate. Results show that wires with highly oriented microstructure can be achieved by sintering the sample slightly above the incongruencc temperature of YBa2Cu307 ,., 980-1050°C, and by cooling it in a moderate rate at 0.2 +C/min to 950°C. In this process, several weight percent of previously aligned clusters in size below 210 lam were doped into the YBa2Cu3OT_, powder as nuclei prior to wire fabrication. These grains were partially aligned along the long axis of the wire the extrusion process. It was hoped that during the liquid sintering process, the individually aligned clusters would act as nucleation centers, leading to elongated grains growing along the wire axis. The superconducting powder prepared from thermal decomposition of nitrates with nominal composition of Y i . 0 B a 2 . 3 C u 3 . 3 0 7 _ v was to test the idea. It was further hoped that this Ba-rich and Curich composition would lead to low-temperature sintering by using the eutectic phase of BaCuO2 and CuO as a low melting flux. A zero resistance Tc at 90 K is obtained with a narrow AT transition. A transport J~ of 900 A / c m 2 is achieved at 77 K in the earth's magnetic field. The compositional analyses on the
S.R. Suet al. /Microstructural effects on superconducting properties of sintered
as-sintered surface and the bulk performed by EDS indicate CuO and Ba/Cu rich region. Because of the shape of the sample, we could not detect whether these phases are on the surface or between grain boundaries. When the sample is melt-textured at 1050 ° C, elongated grains oriented along the fiber axis are clearly evident as shown in the micrograph in fig. 6 (a). In fig. 6 (b), we show the fractured cross section of the sample, indicating the platelet growth steps throughout the entire cross section. The transport Jc of this sample is ~ 1000 A/cm 2 with a zero resistance at 90 K. The presence of secondary phases, CuO
YBa2Cu307_ x
wires
87
and BaCuO2, were confirmed by X-ray diffraction. The Jc values are not as high as expected for the melttextured samples. However, this growth morphology seems to support the mechanism proposed by Hepp et al. [ 14 ] that step growth of grain along the c-axis is controlled by the surface nucleation rate, and platelet growth along the a - b plane is limited by the mass-transpor( rate. In this process, the presence of aligned clusters and low melting flux seems to promote the elongated grain-growth with growth steps along the c-axis as well as the platelet growth along the a - b plane. As a result, the processing temperature can be lowered and the cooling rate can be accelerated to 12°C/h. In a comparative experiment, we applied the aforementioned sintering schedule to wires extruded from a nominal composition of YBa2Cu307_x powder prepared by the solution method. We observed no grain alignment and only random platelet growth. More work is required to optimize processing parameters and to minimize the amount of second phase material. We believe that this process offers us a potential opportunity to process long continuous wires with highly oriented microstructures at substantially lower temperature than those methods currently in use. For example it is important to operate at a lower temperature for grain alignment of helices to maintain the integrity of the configuration.
4. Conclusions
Fig. 6. Microstructures of the aligned wire: (a) as-sintered surface, indicatingelongatedgrains oriented alongthe wire axis; (b) fractured cross-section, indicating the step-growth of platelet grains.
Effects of microstructure on the superconducting properties of Y B a 2 C u 3 0 7 _ x extruded wires were studied. Results of these studies suggest that the important factors controlling the microstructure are the binder system and the sintering conditions - temperature, rate, duration and annealing in oxygen. In fact, these parameters directly affect critical current densities of the superconducting wire. Under the best processing conditions, the transport Jc of 1.5× 103 A/cm 2 is achieved. This result is higher than those of Jc values reported for sintered bulk materials (typical transport Jc values at 77 K are ~ 500 A/ cm2). These improved results may be attributed to sintered wires with partially aligned Y B a z C u 3 0 7 _ x platelet crystals, resulting from the extrusion process. By seeding the Ba-rich and Cu-rich powder with
88
,%R S u et al. /Mlcroslructural ~(['ects on superconducting properties qFsintered ) Ba:('u.~O: , wires
oriented grains, wires with highly aligned micros t r u c t u r e are a c h i e v e d b y a l i q u i d s i n t e r i n g t e c h n i q u e at 9 8 0 - 1 0 5 0 ° C . T h e t r a n s p o r t Jc's o f ~ 1000 A / c m : w i t h a z e r o r e s i s t a n c e at 90 K a r e m e a s u r e d for t h e s e s a m p l e s . T h e s e l o w e r t h a n e x p e c t e d J~ values m a y b e d u e to t h e p r e s e n c e o f s e c o n d a r y p h a s e s , m a i n l y B a C u O 2 a n d C u O . W e b e l i e v e t h e g r a i n text u r i n g m e c h a n i s m is r e l a t e d to t h e fact t h a t w i t h t h e a s s i s t a n c e o f t h e low m e l t i n g flux, p l a t e l e t g r o w t h a l o n g t h e s e e d i n g m a t e r i a l occurs, r e s u l t i n g in w i r e with oriented microstructure. Process optimization to m i n i m i z e t h e a m o u n t o f s e c o n d a r y p h a s e s is u n d e r way to e n h a n c e t h e i r s u p e r c o n d u c t i n g p r o p e r t i e s .
Acknowledgements T h e a u t h o r s a c k n o w l e d g e E. G u t m a n , R. H a m m o n d a n d M. D o w n e y for e l e m e n t a l , t h e r m a l , a n d X - r a y d i f f r a c t i o n a n a l y s e s . W e also t h a n k Dr. B. Ditc h e k for his v a l u a b l e c o m m e n t s a n d s u g g e s t i o n s .
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(c) T. Hughes, Power Transmission Design (November. 1990) pp. 45-47. [2] B. Hammond, Supercurrents 66 (1989). [3] S. Jim T.H. Tiefel, R.B. van Dover, M.E. Davis, G.W. Kammlott and R.A. Fastnacht, Phys. Rev. 37 ( 1988 ) 7850. [4] D. Shi, H. Krishnan, J.M. Hong, D. Miller, P.J. McGinn el al., J. Appl. Phys. 68 (1990) 228. [ 5 ] J.R. Gaines Jr., Cer. Bul. 68 ( 1989 ) 857. [6] (a) S. Jin et al., App[. Phys. Len. 51 (1987) 203; (b) N. Sadakata ctal., High-Temperature Superconductors, vol. 99 (Mater. Res. Soc., Pittsburg) (1988) 293: (c) Y. Yamada et al.. Jpn. J. Appl, Phys. 26 ( 1987 ) L865. [7]S.R. Su et al., in: Correlation of Microwave and Superconducting Properties of YBa2Cu307 • Wires and Helical Coils, to be presented at the MRS Spring Meeting, 1991. [8] S.R. Su, M. O'Connor and M. Levinson, J. Mater. Res. ( 1991 ) 244. [9] T. Aselage and K. Keefer, J. Mater. Res. 3 (1988) 1279. [10]W.D. Kingery, H.K. Bowen and D.R. Uhlmann, eds.. Introduction to Ceramics, 2nd ed. (John Wiley & Sons, New York, 1976) p. 448. [ 11 ] (a) R.L. Meng, C. Kinalidis, Y.Y. Sun, L. Gao, Y.K. Tao, P.H. Hor and C.W. Chu, Nature 345 (1990) 326; (b) K. Salama, V. Selvamanickam, L. Gao and K. Sun, App|. Phys. Lett. 54 (1989) 2352. [ 12] N.McN. Alford, J.D. Birchall, W.J. Clegg, M.A. Harmer and K. Kendall, J. Mater. Sci. 23 (1988) 761. [ 13 ] T. Worthington, W. Gallagher and T. Dinger, Phys. Rev. Len. 59 (1987) 1160. [14]A.F. Hepp, J.R. Gaier, G.A. Landis and S.G. Bailey, Research Update, 1988, Ceramic Superconductors II, ed. Man F. Yam (1988) pp. 356-366.