Electromagnetic coupling character of [001] twist boundaries in sintered Bi2Sr2CaCu2O8+x bicrystals

PhysicaC 230 (1994) 189-198

ELSEVIER

Electromagnetic coupling character of [ 001 ] twist boundaries in sintered Bi2Sr2CaCu2Os+x bicrystals J y h - L i h W a n g a,b, X . Y . C a i ", R . J . K e l l e y a,c, M . D . V a u d i n d, S.E. B a b c o c k a,b, D . C . L a r b a l e s t i e r a'~'* aApplied Superconductivity Center, Universityof Wisconsin, 1500 Johnson Drive, Madison, W153706, USA b Materials Science and Engineering, Universityof Wisconsin, 1500Johnson Drive, Madison, W153706, USA c Department of Physics, Universityof Wisconsin, 1500 Johnson Drive, Madison, W153706, USA a Ceramics Division, National Institutes of Standards and Technology, Gaithersburg,MD 20899, USA

Received9 June 1994

Abstract The electromagnetic characteristics of [ 001 ] twist grain boundaries in Bi2Sr2CaCu2Os+x (BSCCO-2212) have been deduced from measurements of the resistive transitions, voltage-current (V-I) characteristics, and field-dependent transport critical current densities (Jet(H)) of single- and bicrystalline samples. The bicrystals were prepared by solid-state sintering of (001) faces of freshly-cleaved,bulk-scale single crystals placed at a pre-determined misorientation angle, 0. A low-angle 2 ° bicrystal was strongly coupled with V-I and Jet(H) characteristics that were indistinguishable from those of the single crystals, indicating that the sintering process itself does not produce weak coupling. 30 ° and 36 ° [001 ] bicrystais were weakly coupled. 23 ° and 88 ° twist bicrystals were strongly coupled, even though their boundary Tc values were reduced. This behavior contrasts starkly with that of [001 ] tilt boundaries in YBa2CuaO7_&where a reduced boundary Tc is always associated with weak coupling.

1. Introduction

Weak electromagnetic coupling at high-angle grain boundaries is a major impediment to the development of high critical current densities in polycrystalline high-To superconductors. Although this weak-link problem is general to all high-To materials, systematic grain-boundary studies have focused on [001] tilt boundaries in YBa2Cu3OT_6 [ 1-3 ]. The conclusions of these studies often have been assumed to pertain to all types of boundaries in all high-temperature superconductors. Investigations of thin-film [001 ] tilt bicrystals of other high-T~ superconductors provide some support for a general extrapolation * Correspondingauthor.

of the YBa2Cu30~_~ data [4-8 ]. However, the complete generality of the [ 001 ] tilt YBa2Cu307_~ thinfilm results is brought into question by observations of strong coupling in high-angle bulk-scale bicrystals [9-11] and 90 ° thin-film boundaries [12,13] of YBa2Cu307_6. Furthermore, a direct application of the conclusions of the YBa2Cu3OT_~ thin-film bicrystal studies [ 1-3 ] would not predict the large highfield transport critical current density (Jet) values (e.g., 10-100 kA/cm 2) that have been obtained in silver-sheathed Bi2Cu2CaCu2Os +x (BSCCO-2212) and Bi2Cu2Ca2Cu3Olo+x (BSCCO-2223) tapes [ 1418 ]. The achievement of such high J~t values in polycrystalline tapes with strong c-axis texture underlines the importance of studying the electromagnetic charactcristics and (micro) structure of individual

0921-4534/94/$07.00 © 1994 ElsevierScienceB.V. All rights reserved SSDI0921-4534(94)00427-7

190

J.-L. Wang et al. /Physica C 230 (1994) 189-198

grain boundaries in the Bi-Sr-Ca-Cu-O (BSCCO) system, particularly in bulk-scale samples. Currently, two models for the microstructural and electromagnetic character of the active current path in BSCCO dominate the discussion. Both models are built upon the common assumption that J~t is limited by transport across grain boundaries. However, they diverge in their hypotheses about the types of boundaries in the supercurrent path and about the nature of the superconducting coupling across them. The microstructural basis of the brick-wall model developed by Bulaevskii et al. [ 19,20] is that BSCCO grains tend to grow as plates which align themselves with mutually parallel c-axes such that neighboring plates overlap each other, as in a brick wall. The essential features of this microstructure which lead to appreciable high-field supercurrents are: ( 1 ) that the length-to-thickness ratio of the grains is large, so that current can pass around weakly-coupled tilt boundaries by crossing large-area [001 ] twist boundaries in the c direction; (2) that the electrically layered structure of insulating (I) Bi-O and superconducting (S) CuO2 sheets makes the compound a series of weakly coupled SIS junctions such that the c-axis current is a Josephson current; (3) that the CuO2-CuO2 plane spacing across [ 001 ] twist boundaries which form the broad "brick" faces is no greater than that within the grains, because the boundary forms within the double Bi-O insulator layer, and (4) that the near-atomic scale of the SIS sheets and spatial non-uniformity in the plane of the [001 ] twist boundary subdivides the boundary into an array of parallel Josephson j unctions. Such a parallel j unction array was hypothesized to carry significant supercurrents in fields of many tesla, even though the grain boundaries are weakly coupled. An alternative picture of the current-limiting mechanism and boundary type is suggested by the railway-switch model. Hensel et al. [ 21 ] argued that t h e Jet(H) properties of Bi2Sr2Ca2Cu3Olo+x (BSCCO-222.3) tapes are not consistent with a weakly-coupled current path, nor is the real grain structure as well aligned as was suggested in the brickwall model. They proposed that the active current path instead includes a network of strongly-coupled grain boundaries, many of whose grain intersections

they likened to railway switches. On the basis of YBa2Cu307_6 results [ 1 ], Hensel et al. postulated that boundaries need to be of low angle for them to remain superconducting in strong magnetic fields. The experimental evidence to date neither confirms nor denies either model. Virtually all experimental work has shown the importance of transport across grain boundaries, but polycrystalline data have not allowed a clear determination of the coupling character ( i.e. strong or weak) of BSCCO grain boundaries. In previous bulk-scale bicrystal studies by Tomita et at. [22,23]~ sintered high-angle [001 ] twist boundaries of BSCCO-2212 all showed weaklink behavior. However, the grain boundary J~ values did vary with the misorientation angle (0) and 27 (the reciprocal coincidence site density as defined in the coincidence site lattice (CSL) model [24]). These results support two postulates of the brick-wall model, namely that supercurrents can pass in the c direction across high-angle [001 ] twist boundaries and that these boundaries are weakly coupled. Umezawa et at. [25,26 ] have presented electromagnetic and microstructural evidence that the active current path in polycrystalline BSCCO-2223 tapes includes c-axis transport and crosses [001 ] twist boundaries. However, many of these boundaries also contain BSCCO-2212 intergrowths. The Umezawa et al. resuits suggest that there are two types of current path. At low fields the proximity effect couples 2212 intergrowths to the 2223 matrix and the supercurrent crosses all [001] twist boundaries. At higher fields those grains and grain boundaries that contain BSCCO-2212 intergrowths become de-coupled and are eliminated from the active current path, perhaps only leaving low angle boundaries in the current paths. Thus, the coupling characteristics of the supercurrent paths are different in these two field regimes. The railway-switch model appears more relevant to the higher fields. Central to both the brick-wall and the railwayswitch models are conjectures about the coupling character of the boundaries that lie in the supercurrent path. The brick-wall model assumes Josephson junction weak coupling and the railway-switch model assumes single-crystal-like strong coupling. This paper describes direct studies of the coupling character of individual synthetic [001 ] twist bicrystals of BSCCO-2212 formed by sintering two freshly-cleaved

J.-L. Wanget al. /Physica C230(1994) 189-198 single crystals together in a way that permits both the inter- and intra-granular electromagnetic properties to be measured. In view of the fundamental differences in the postulates of the brick-wall and railwayswitch models, the study emphasizes the coupling character of [001 ] twist grain boundaries. In contrast to the prior results [22,23 ], both strongly and weakly coupled [ 001 ] twist boundaries were found. Some bicrystals exhibited a decreased Tc in the grain boundary, but still showed strong coupling characteristics. Conversely, weak-coupling characteristics were observed for boundaries having T¢ values almost equal to those of the grain interiors. This behavior markedly contrasts that of YBa2Cu307_6 bicrystals, for which a reduced T¢ at the grain boundary invariably implies weak coupling across the interface [ 10,27 ]. All of these observations add to the diverse electromagnetic behavior exhibited by high-T¢ superconductor grain boundaries [ l - 13 ].

2. Experimental details 2.1. Single crystals 2212 single crystals were prepared by a directional solidification method [ 28,29 ]. Bi203 (99.9% cation purity), SrCO3 (99%), CaCOa (99.9%), and CuO (99%) were mixed in the 2212 cation stoichiometry, calcined at 600°C, reground, and reacted at 800°C to form a powder. This powder was melted at 950°C for 2 h in a covered alumina crucible in a vertical furnace with a temperature gradient of 0.5 °C/mm. The melt was then cooled to 900 ° C in 2 h and cooled further to 820 ° C at 0.5 ° C / h before being removed from the furnace and allowed to cool to room temperature. All processing was carded out in air.

2.2. Bicrystal fabrication Plate-shaped crystals of 2212 were cleaved across their (001 ) planes to produce flakes that were several tens of microns thick and 0.5-2 mm long in the a-b plane directions. Two freshly cleaved crystals were placed face-to-face at the desired misorientation angle and put into a furnace in which gas (7.5 vol.% oxygen mixed with argon) at a pressure of 1 atm was flowing. The furnace was heated rapidly from

191

room temperature to 790°C and then to 850°C at 1 ° C/min, a temperature still well below the melting point ( ~ 8 7 5 ° C ) of the 2212 phase [30]. Samples were held at the sintering temperature for 24 to 100 h. The mechanical integrity of the bonded interfaces indicated that 24 h was sufficient for substantial sintering. The samples were furnace cooled to room temperature at a rate of approximately 100°C/h. This method is different from that of Tomita et al. [22,23 ] in which the processing temperature was above the melting temperature of the BSCCO-2212 phase. One of the sintered bicrystals is shown in Fig. 1.

Z 3. Misorientation-angle determination The misorientation angles of the sintered twist boundaries can be specified by the rotation 0 about [001 ] and the angle ~ formed by the [ 001 ] axes of the two crystals. They were determined in two ways: by study of light micrographs, and by analysis of backscattered electron K.ikuchi diffraction patterns (BEKP's) [31 ]. Surface steps which were easily visible by light microscopy formed on the exposed (001) surfaces of the crystals during sintering. Transmission electron microscopy confirmed that the edges of these steps lie along [ 100 ] and not along [010 ] [ 32 ]. Thus, the angle between the steps on the two crystals measured from light micrographs of the (001 ) surfaces is the twist misorientation angle 0, as is illustrated in the inset to Fig. 1. The angle ~ was determined by BEKP analysis, and is shown in Table 1 for three of the bicrystals (the other two broke before BEKP measurement could be performed). The average 0 values obtained by BEKP agreed with those determined by light microscopy to within 1 o for the 23 ° and 88 ° bicrystals. The variation of the 0 and values observed across individual crystals substantially exceeded the orientational precision of the BEKP technique ( _+0.5 ° ) [31 ], indicating, not surprisingly, that there is mosaic structure in the single crystals. This mosaic accounts both for the point-topoint variation in measured BEKP 0 values and for the difference between the BEKP and light-microscopy misorientation for the 2 ° sample. The BEKP showed that the c-axes misalignments were small, lying in the range 0-4 °. Although these crystals are nominally tetragonal, boundaries with 0 between 45 ° and 90 ° are distin-

J.-L. Wang et al. / Physica C 230 (1994) 189-198

192

bl

Fig. 1. Plan-view light micrograph o f the sintered 36* [001 ] twist bicrystal. The misorientation angle can be measured from surface steps which are visible on each crystal surface.

Table 1 Misorientation relationships of the bicrystals determined by light microscopy ( L M ) and backscattered electron Kikucbi patterns (BEKP)

0by LM

0 about [001 ] by BEKP

~ by BEKP (c-axis misalignment)

23 °

1.6 ° 4.5 ° 1.9° 22.3 °

0.2 ° 2.4° 1.5° 3.9 °

88 °

23.0 ° 22.6 ° 87.4 °

4.1 ° 1.0 ° 2.5 °

2°

guished from their equivalent values between 0 ° and 45 ° because of the incommensurate modulation in the b direction [33]. Such a distinction may be particularly important because [001 ] twist boundaries lie at the Bi-O planes that force the modulation. It therefore seems wise to distinguish the a and b directions until more is known about the role that the modulation plays in the boundary structure and properties.

2.4. Electromagnetic measurements Standard four-terminal techniques were used to characterize the electromagnetic properties of the bicrystals. The lead wires (gold or silver, with diameters ranging from 12.5 ~tm to 50 ~tm) were attached to the bicrystals with silver-loaded epoxy. In order to minimize the contact resistance, the samples were then annealed in 1 atm of flowing pure oxygen for 2 h at 400 ° C. In all cases, the room-temperature contact resistance between any two leads was 3 fl or less. Six leads were attached to the bicrystal, as denoted in the insets to Figs. 2, 4-7. For all but the 2 ° bicrystal, current (I) and voltage (V) leads were placed on the top (leads 1 and 2 ) and bottom (leads 3 and 4) (001 ) surfaces of one crystal and on the top (001 ) surface of the other crystal (leads 5 and 6). When current is injected from lead 1 to lead 6 and voltage is measured between leads 2 and 5, the lead arrangement is denoted as GBSC (I1-1/2- V5-I6 ). This arrangement measures the combined effects of c-axis transport in the upper crystal, transport across the boundary and a-b plane transport in the lower grain. Lead arrangement SC (I1- V2- V3-I4) measures the c-axis transport in the upper single crystal, while lead arrangement GB (I3- I,'4- V5-I6) characterizes transport from the

193

J.-L Wang et al. / Physica C 230 (1994) 189-198

lower face of the upper crystal across the boundary and out of the upper face of the lower crystal. It occurs largely through the a - b planes of the two crystals. The placement of the crystals in the 2 ° bicrystal precluded attachment of leads in positions 3 and 4 on the bottom of the upper crystal (see schematic inset in Fig. 2). Therefore, a set of leads was attached to the bottom of the lower crystal (positions 7 and 8) to allow equivalent characterization to be made of this bicrystal. Resistance vs. temperature ( R - T ) data between room temperature and 4.2 K were obtained using a 1 mA, 43 Hz AC current. Voltage-current ( V - I ) measurements of the critical current (I~) transition were performed at various temperatures with external fields ( H ) applied parallel to the grain-boundary plane and perpendicular to the nominal current direction through the [001 ] twist boundary. The critical current density was defined at a criterion of I ttV by dividing Ic by the overlap area between the two crystals. Because the bicrystals are not completely sintered in the overlap area, this method underestimates the Jet value. However, numerical uncertainty in Jet does not hinder determination of the character of the coupling across the boundary, as is discussed later.

3. Results on the "zero-degree" sintered bicrystal: the "proof of principle" experiment A nominally 0 ° twist bicrystal (actual 0 = 2 ° ) was manufactured in order to test whether or not the bicrystal fabrication process influences the electromagnetic properties of the grain boundary. This sample was produced by cleaving a single crystal, then repositioning the cleaved surfaces together as close as possible to their original orientation. The electromagnetic properties of this "resynthesized single crystal" should be single-crystal-like if the fabrication process is benign. Fig. 2 shows the c-axis R - T characteristics across both crystals and the grain boundary obtained from the 2 ° [001 ] bicrystal using lead arrangement GBSC (I1- F2- V7-18). The normal-state resistance was almost metallic, exhibiting only a small upturn near Tc. The superconducting transition started at 91 K, but residual resistance existed down to 67 K, as

0.4

6 11 °'f"'

I [

cr'/sta/I crystal2

Iz

--+H

0.3

I~

s

I I

]

sl

A

n- 0.2 0.02 t 0.1

"

0.01 [-

"; /

[ .Tc=67 K/ o oo/.1 ~ 0.0

0

50

100

,

65

70 ,

75,

80

150 T (K)

200

250

300

,

Fig. 2. Resistance vs. temperature curve for the 2 = [00l ] twist bicrystal with lead arrangement GBSC(II-V2-V7-/8). The applied AC current is 1 m_A.The boundary Tc is 67 K. The inset showsan enlargedscale.

shown in the inset. The zero-resistance Tc of the upper single crystal in all of the other bicrystal samples (lead arrangement SC(II-V2-V3-14), as in Fig. 5), and of the bottom single crystal of this bicrystal SC (•5-/16-V7-18), measured after bicrystal fabrication, was 85-87 K. Thus, it appears that the highertemperature transition isdue to the singlecrystaland that the sintering process caused a local Tc reduction at the newly-bonded interface. Fig. 3 shows V-I characteristicsfor this bicrystalat 4.2 K and at 61 K, 6 K below the boundary To. At neither temperature was there field sensitivity of the transitions, and the shape of the V-I traces was characteristicof flux flow in the strong coupling state.The J~ (0 mT, 4.2 K) values were approximately 50 A/ cm 2. T w o key points were established with this bicrystal: ( I )the sintcring process did not cause weak electromagnetic coupling at the boundary, and (2) strong coupling was maintained, even though the Tc of the boundary region was significantly reduced below the single crystal value. 4. High-angle [001] twist boundary results A selection of the V-/curves GB (13- V4- V5-I6) for the 23 ° [ 001 ] twist bicrystal, for which the boundary

194

J.-L. Wang et aL /Physica C 230 (1994) 189-198

40

i

T=4.2 K

T=61 K

60

40

2O

m T I~0 . . ~ . ~ . ~ >

>

>

20

Ore'. "fly

,,.

(a) '

-20

=

=

=

=

-10

0

10

20

(b) "20

I

I

-300 -200 -100

l(mA)

I

I

I

I

0

100

200

300

I(mA)

Fig. 3. V-I curves for the 2 ° [001 ] twist boundary at (a) T=61 K, and (b) T=4.2 K. Magnetic fields were applied parallel to the boundary plane. The lead arrangement is the same as in Fig. 2. 80

i

[ 60

i

i

i

ax's5 6 crystal1 I I3 4I I =yst=2 I ---)H

100

,

Z--3

1

K

2

c-

I

80

J '1' L crystal1

axis 5

6

II

I

T=4.2K

6O

,o

> -~ 40 > 20

0

, C~

-20

-6

-4

,

-2

,

0

,

2

,

4

(b)

-20

(a)

6

l(mA)

I

I

I

I

I

-8

-4

0

4

8

I(mA)

Fig. 4. V-I curves for the 23 ° [001 ] twist boundary at (a) T=35 K, and (b) T=4.2 K. Magnetic fields were applied parallel to the boundary plane.

Tc value was 45 K, is shown in Fig. 4. Fig. 4 ( a ) shows that the Ic at 35 K, only 10 K below T¢, exhibited little sensitivity to applied fields o f up to 25 m T and showed no sign o f any oscillation with field. More-

over, the shape o f the V - I curves was characteristic o f flux flow. These characteristics are quite different from those observed for YBa2Cu3OT_a tilt [ 1,10,27 ] and BSCCO twist boundaries [22,23 ] that are weakly

Z-L. Wang et aL /Physica C 230 (1994) 189-198

coupled (see also Fig. 6 ). At 4.2 K the characteristics were field insensitive up to at least 30 mT (Fig. 4(b) ). This result is intriguing, considering the rather low Tc of 45 K measured for the boundary. Fig. 5 shows R - T c u r v e s for the 36 ° [001] twist bicrystal. The three experimental curves are shown by solid lines. The resistance for lead arrangement OB (13-V4-VS-I6) had a positive temperature coefficient from 180 K to room temperature and a small upturn at lower temperatures. Curve SC (I1- V2-1/314) shows that the temperature dependence for resistance in the c direction in the upper crystal had a negative temperature coefficient for all temperatures above To. The room-temperature resistivity along the c direction (Pc) of this crystal was 4 f~ cm, consistent with other reports [28,36]. The behavior of curve GBSC(II-V2-VS-I6) was similar to curve GB at higher temperatures, where it showed a metallic behavior. The rather good agreement between experimental curve GBSC and the sum of curves SC and GB (Fig. 5 ) suggests that the actual current path can be treated as the sum of the components described

above. The onset of the superconducting transition was at about 91 K for all three resistive transitions (GB, SC, GBSC). The zero resistance Tc was about 84 K for curves GBSC and GB and 86 K for curve SC. Fig. 6 shows the V-I characteristics of the 36 ° 80 1

60

1 I

2.5

I

2

c - axis

I

1'

3

crystal 2

4

'~>H 8 mT

40

/

A

> >

0.8 mT

20

:~.

/

0 mT

(a)

I

-40

-20

,

I

I

0

20

40

60

150

I

1

--->H

2.0

n,-

4

T=77K

6

II

II

I

c - axis

I(mA)

S

crystal1

2

I I I" 5 6 c~ I I I I I I I o~,t=2 I 3

-20 -60 3.0

195

1.5

Sum of GB and SC

120'

GBSC (11-V2-V5-16)

9O

2

T= 4.2 K

¢ - axis

I

I t 56 I e~=all I I I I

II

cry==2

3

4

"---~H 8mT I

60 1.0 >

SC (11-V2-V3-14)

3o

0.5

0 mT

/

0 0,0

0

=

s

i

m

I

50

100

150 T (K)

200

250

300

Fig. 5. Resistance vs. temperature curves from the 36 ° [001 ] twist bicrystal for three different lead arrangements (curve GB (•3V4-V5-16), curve SC(13-V2-V3-14) and curve GBSC (II-V2-V516) ). The applied AC current is I mA for all three curves. The sum of the resistances from curves GB and SC is shown as the dotted line. Note the good agreement between the sum of GB and SC and curve GBSC.

-30 (b) -60 ' ' ' -200 -150 -100 -50

I

I

0 50 I(mA)

I

I

100 150 200

Fig. 6. V-/curves of the 36 ° [001] twist boundary for lead arrangement GB(13-V4-VS-16) in magnetic fields parallel to the boundary plane at (a) T=77 K, and (b)Tffi4.2 K.

J.-L. Wang et al. /Physica C 230 (1994) 189-198

196

[ 001 ] twist boundary at 77 K and 4.2 K for lead configuration GB (13- F4- V5-I6). Ic was very sensitive to small applied fields at 77 K and decreased sharply as the field was increased to only 8 mT (Fig. 6(a) ). A similar field sensitivity was observed for lead arrangement GBSC (I1-1/2-V5-I6). At 4.2 K (Fig. 6 ( b ) ) , the field dependence was less pronounced than that observed at 77 K, but Ic still dropped from 130 mA at zero field to about 110 mA at 8 mT. In contrast, field insensitivity of I~ was demonstrated by the upper single crystal for fields up to 8 roT. Thus, the single crystal and the bicrystal had different characteristics, the single crystal being strongly coupled and 120

i

1

!

c - axis

I

I"

I

I 90

|

2

c~tall I I I 3

s

I

6

i

T=4.2K

I I I c,~stal 2 I

4

the bicrystal weakly coupled. The boundary Jet was 40 A/cm 2 at 4.2 K in zero applied field. Analogous results were obtained for a 30 ° high-angle twist bicrystal. The boundary Tc for this sample was 84 K. At 77 K the Jet was ~ 5 A/cm 2, and it was strongly field dependent. The bicrystal broke before it could be tested at 4.2 K. Fig. 7 shows the V-I curves obtained at 4.2 K for the 88 ° [001] bicrystal. The boundary T~, determined using lead arrangement GB (I3-V4-V5-I6), was 50 K. The bicrystal measured at 4.2 K using the same lead arrangement, showed no field dependence in the range of 0 to 25 mT. The shape of the curves and the J¢ values ( 10 A/cm 2 at 4.2 K) were very similar to those of the 2 ° boundary and, again, indicated strong coupling across the boundary. The electromagnetic properties of these synthetic sintered bicrystals are summarized in Table 2.

[

--')H

/

5. Discussion

313

"30

-90

'

'

'

'

'

-60

-30

0

30

60

90

I (mA)

Fig. 7. V-/curves of the 88° [001 ] twist boundaryat 4.2 K. Magnetic fieldswereapplied parallelto the boundaryplane. The lead arrangementis GB(I3-V4-VS-I6). Table 2 0

Intragranular Tc (K)

Grain boundary Tc (K)

J= (A/cm 2) (0 rat, 4.2 K)

Coupling character

2° 23 ° 30 ° 36* 88*

86 85 87 86 87

67 45 84 84 50

50 1 5" 40 10

Strong Strong Weak Weak Strong

• At 77 K, 0mT.

The present experiments show that it is possible to sinter crystals of BSCCO-2212 together at temperatures somc 25°C below the onset of melting ( ~ 8 7 5 ° C ) to form [001] twist boundaries and retain superconductivity across the grain boundary. In contrast to thin-film deposition onto SrTiO3 bicrystal substrates, the method of bicrystal fabrication which has been employed principally so far, sintering permits the fabrication of [001 ] twist boundaries, which cannot be fabricated by the thin-film process. Study of [001 ] twist boundaries is needed because these boundaries dominate the spectrum of grainboundary types found in Ag sheathed tapes of BSCCO-2212 and BSCCO-2223 [ 34 ]. Although all of the sintered boundaries had a nonzero Jet value after synthesis, they did exhibit diverse electromagnetic properties. Both strongly coupled and weakly coupled boundaries were obtained. The coupling character of the boundaries and of the grains themselves was assessed on the basis of the field dependence of J~ and the shape of the V-/curves. These characteristics are good guides for the presence of strong and weak coupling in thin-film and bulk-scale YBa2Cu307_ 6 bicrystals [ l, 10,27 ]. For the BSCCO2212 bicrystals studied here, a pronounced low-field dependence of Jet was observed for the 30 ° and 36 °

J.-L. Wang et al. /Physica C230(1994) 189-198

grain boundaries, while essentially no dependence was exhibited for the 2 ° , 23 ° and 88 ° boundaries. Furthermore, field independence was observed for c-axis transport within the grains in all bicrystals. The weaklinked bicrystals also exhibited sharper V - I transitions. The V - I transition differences were not as distinctive as those in YBa2Cu307_~, however, probably because the Jet values were low. Even in the single crystals, the c-axis act values were only of order 100 A/cm 2 at 4.2 K. This is consistent with substantial fractions of the interface regions being bonded. Underlying the electromagnetic differences between [ 001 ] twist and [ 001 ] tilt boundaries may be structural differences related to the layered structure of the phase. The Bi-O double layer has very special properties: it is easily cleaved and takes up oxygen readily, compensating for the varying Bi-O bond length by adjusting the incommensurate modulation period. On the basis of these characteristics Lay [ 35 ] proposed that the boundary disorder for [001] boundaries is confined within the Bi-O double layer, which has the same thickness at a grain boundary as within the grains. No such natural confinement exists for other boundary types, for which disorder associated with high-angle misorientations will tend to be longer in range. The special nature of [001 ] twist boundaries is further emphasized by the curious conjunction of a reduced boundary T~ without destruction of strong coupling. Conceivably this behavior is linked to the reduction in c-lattice parameter that accompanies oxygen overdoping. In single crystals, oxygen overdoping also results in a reduced To, but increased caxis coupling that leads to metallic behavior [ 28,37 ]. Similar effects at the boundary would increase its coupling strength, consistent with the present observations. Whatever the truth of these speculations, it is clear that the properties of synthetic [ 001 ] twist bicrystals differ significantly from those of [ 001 ] tilt boundaries in YBa2Cu307_6 thin films and bulk bicrystals. The need for further study of [ 001 ] twist boundaries in bulk samples is reinforced by the paradox that those boundaries for which the Tc decrease was greatest (more than 40 K for the 23 ° boundary) were strongly coupled, whereas those for which the Tc decrease was small (only 2 K for the 36 ° boundary) were weakly coupled. Understanding the grain-boundary micro-

197

structure and chemistry responsible for a decreased Tc that does not produce weak coupling may be very fruitful for exploring how the grain-boundary properties depend on the processing conditions. A final point to note is the properties of the grains themselves. Unlike optimally oxygen-doped BSCCO2212 [ 36 ], the c-axis resistance had a small negative temperature coefficient which appeared only at T< 150 K. This temperature dependence is consistent with the oxygen heat treatment which tends to overdope the superconductor, reducing Tc below the maximum value of ,,, 92 K [ 37 ]. The consequences for the c-axis supercurrent appear to be considerable. Unlike the results of Kleiner at al. [ 36 ], there was no sign of weak coupling within the grains. Evidently, overdoping the compound markedly affects the c-axis coupling, presumably by making the double Bi-O layer metallic [ 28 ] and thus removing the SIS behavior postulated in the brick-wall model. This change of character with doping within the grains should influence the grain boundary properties too, but it has not yet been studied explicitly. In view of the complex range of points raised above, the results neither confirm or deny either the brickwall or the railway-switch model. The results do suggest the possibility that some high-angle [001 ] twist boundaries may be included in the strongly coupled path postulated by the railway-switch model. They also confirm the possibility of supercurrent transport across [ 001 ] twist boundaries, as postulated by the brick-wall model, however, the c-axis transport within the grains appears to lack a weak-link signature. The results also imply that the grain-boundary properties of high-temperature superconductors are not all similar, and that both of the above models may oversimplify the transport of supercurrent in BSCCO-2212 and, presumably, BSCCO-2223 as well.

6. Conclusions

BSCCO (2212) [001 ] twist bicrystals have been fabricated by solid-state sintering of two cleaved single crystals. Weak coupling behavior was observed for 30 ° and 36 ° twist bicrystals, while 2 °, 23 °, and 88 ° twist bicrystals were found to be strongly coupled. The observation of strong coupling across the 2 o, 23 °, and 88 o bicrystals proves that the sintering process alone

198

J.-L. Wang et al. / Physica C 230 (1994) 189-198

does not produce weak-link character by introducing an artificial barrier into the sample. I n contrast to the general behavior o f YBa2Cu307_6 grain boundaries, BSCCO [001 ] twist b o u n d a r i e s can have a locally reduced Tc a n d r e m a i n strongly coupled, showing that the m e c h a n i s m s which produce weak links in high temperature superconductor grain boundaries are still far from being understood.

Acknowledgements We are grateful to W.S. Starch for m u c h experimental assistance. This work was sponsored by the Advanced Research Projects Agency (contract N00014-90-J-4115) a n d the N S F Materials Research G r o u p program (grant DMR-9214707 ).

References [ 1] D. Dimos, P. Chaudhari and J. Mannhart, Phys. Rev. B 41 (1990) 4038. [2] Z.G. Ivanov, P.A. Nilsson, D. Winlder, J.A. Alarco, T. Claeson, E.A. Stepantsovand A.Y. Tzalenchuk,Appl. Phys. Lett. 59 (1991) 3030. [ 3 ] R. Gross, P. Chaudhari, M. Kawasaki, M.B. Ketchen and A. Gupta, Appl. Phys. Lett. 57 (1990) 727. [4] B. Mayer, L. Alff, T. Trauble, R. Gross, P. Wagner and H. Adrian, Appl. Phys. Lett. 63 (1993) 996. [ 5 ] T. Amrein, M. Seitz, D. Uhl, L. Schultzand IC Urban, Appl. Phys. Lett. 63 (1993) 1978. [6] A.H. Cardona, H. Suzuki, T. Yamashita, ICH. Young and L C. Bourne, Appl. Phys. Lett. 62 (1993) 411. [7]T. Takami, IC Kurodo, K. Kojima, M. Kataoka, J. Tanimura, O. Wada and T. Ogama, Jpn. J. Appl. Phys. 32 (1993) L583. [8] M. Kawasaki, E. Sarnelli, P. Chaudhari, A. Gupta, A. Kussmaul,J. Laceyand W. Lee, Appl. Phys. Lett. 62 (1993) 417. [9] S.E. Babcock, X.Y. Cai, D.L. Kaiser and D.C. Larbalestier, Nature (London) 347 (1990) 167. [ 10] D.C. Larbalestier, S.E. Babcock, X.Y. Cai, M.B. Field, Y. Gao, N.F. Heinig, D.L. Kaiser, IC Merkle, L.K. Williams andN. Zhang, Physica C 185-189 (1991) 315. [ 11 ] A.S. Parikh, B. Meyer and K. Salama, Supercond. Sci.Tech. (1994), to be published. [ 12 ] C.B. Eom, A.F. Marshall,Y. Suzuki,B. Boyer, R.F.W. Pease and T.H. Geballe, Nature (London) 353 (1991) 544. [ 13] C.B. Eom, J.M. Phillips, A.F. Marshall and T.H. GebaUe, Interface Sci. 1 (1994) 267. [ 14] IC Sato, T. Hikata, H. Mukai and M. Ueyama, IEEE Trans. Mag. 27 (1991) 1231.

[ 15] R. Flukiger,B. Hensel, A. Jeremie, M. Decroux, H. Kupfer, W. Jahn, E. Seibt, W. Goldacker, Y. Yamada and J.Q. Xu, Supercond. Sci. Tech. 5 (1992) $61. [ 16] Y. Yamada, M. Satou, S. Murase, T. Kitamura and Y. Kamisada, Adv. Supercond.-V, eds. Y. Bando and H. Yamauchi (Springer, Tokyo, 1993) p. 717. [17]D.C. Larbalestier, X.Y. Cai, Y. Feng, H. Edelman, A. Umezawa, G.N. Riley Jr. and W.L. Carter, Physica C 221 (1994) 299. [18] M. Yoshida and A. Endo, Jpn. J. Appl. Phys. 32 (1993) L1509. [19] L.N. Bulaevskii, J.R. Clem, L.I. Glazman and A.P. Malozemoff, Phys. Rev. B 45 (1992) 2545. [20 ] L.N. Bulaevskii,L.L. Daemen, M.P. Maleyand J.Y. Coulter, Phys. Rev. B 48 (1993) 13798. [21 ] B. Hensel, J.-C. Grivel, A. Jercmie, A. Perin, A. Pollini and R. Flukiger, Physica C 205 (1993) 329. [22] N. Tomita, Y. Takahashi and Y. Ishida, Jpn. J. Appl. Phys. 29 (1990) L30. [23] N. Tomita, Y. Takahashi, M. Mori and Y. Ishida, Jpn. J. Appl. Phys. 31 ( 1992) L942. [24] W. Bollmann, Crystal Defect and Crystalline Interfaces (Springer, Berlin, 1970). [25] A. Umezawa, Y. Feng, Y.E. High, D.C. Larbalestier, Y.S. Sung and E.E. Hellstrom, Physica C 198 (1992) 261. [26]A. Umezawa, Y. Feng, H.S. Edelman, T.C. Willis, J.A. Parrell, D.C. Larbalestier, G.N. Riley Jr. and W.L. Carter, Physica C 219 (1994) 378. [27 ] X.Y. Cal, M.B. Field, N.F. Heinig,H. Zhang,N. Zhang, S.E. Babcock, D.L Kaiser and D.C. Larbalestier, The 1993 Int. Workshop of Superconductivity,Hakodate, Japan, p. 110. [28] R. Kelley, M. Onellion, X.Y. Cai, Jyh-Lih Wang and D.C. Larbalestier, unpublished work, 1993. [29] D.B. Mitzi, L. W. Lombardo, A. Kapitulnik, S.S. Laderman and R.D. Jacowitz, Phys. Rev. B 41 (1990) 6564. [30] L.M. Rubin, T.P. Orlando, J.B. Vander Sande, G. Gorman, R. Savoy, R. Swope and R. Beyers, Physica C 217 (1993) 227. [31 ] M.D. Vaudin and W.C. Carter, Proc. 25th AnnualMeeting of the Microbeam Analysis Society, ed., D.G. Howitt (San Francisco Press, San Francisco, CA, 1991 ) p. 159. [32]Jyh-Lih Wang, S.E. Babcock and D.C. Larbalestier, unpublished work, 1993. [ 33 ] O. Eibl, Physica C 168 (1990) 239. [34] Y. Fang, ICE. Hautanen, Y.E. High, D.C. Larbalestier, R. Ray II, E.E. Hellstrom and S.E. Babcock, Physica C 192 (1992) 293. [35]K.W. Lay, Superconductivity and its Applications, AIP Conf. Proc. 219, eds. Y.H. Kao et al. (American Institute of Physics, New York 1991). [36] R. Kleiner, F. Steinmeyer, G. Kunkel and P. Muller, Phys. Rev. Lett. 68 (1992) 2394. [37] K. Kishio, J. Shimoyama, Y. Kotaka and K. Yamafuji, in: Proc. of 7th Int. Workshop on Critical Currents in Superconductors, ed. H. Weber (Word Scientific, Singapore, 1994), to be published.