Effects of grain size and grain-boundary segregation on superconducting properties of dense polycrystalline La1.85Sr0.15CuO4

Physica C 152 ( 1988) 77-90 North-Holland, Amsterdam

EFFECTS OF GRAIN SIZE AND GRAIN-BOUNDARY SEGREGATION ON 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 O F D E N S E P O L Y C R Y S T A L L I N E Lal.ssSro.15CuO4 Y.-M. CHIANG, D.A. RUDMAN, D.K. LEUNG*, J.A.S. IKEDA, A. R O S H K O and B.D. FABES** Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

Received 14 January 1988

The grain size dependence of magnetic and resistive properties in a series of dense (96-> 99%) polycrystallineLa~.ssSrot5CuO4 superconductors ranging in grain size from ~ 1 to ~ l0/~m has been measured. Samples were prepared by controlled hot-pressing from a single lot of chemically-derived homogeneous powder. Critical currents have been calculated from the magnetization hysteresis assuming that supercurrents flow (l) throughout the sample as a whole, and (2) only within individual grains. It is found that intra-grain critical currents are identical regardless of grain size, indicating that grain decoupling is present at low fields ( < 20 Oe). It is furthermore observed that resistive transitions are broadened, and magnetic transitions exhibit unusual structure, in fine-grained sample but not in coarse-grained samples. The relationship of these results to an equilibrium segregation of Sr at grain boundaries, measured using in situ AES, is discussed.

1. Introduction Recent measurements in the newly discovered high-To oxide superconductors [ 1 - 6 ] have shown much lower critical current (Jc) values in polycrystalline than single crystalline materials. Whereas magnetization-derived critical currents in bulk single crystals [ 1,2] and magnetization and transport critical currents in epitaxial thin films o f YBa2CuaO7 [3] are in excess of l06 A/cm 2 in the favorable a - b plane, direct transport values in YBaECU307 polycrystals [4,5] are orders of magnitude lower. This difference cannot be attributed to the intrinsic anisotropy o f the material alone. The rapid decrease o f J~ with increasing magnetic field in polycrystaUine YBa2Cu307 [5,6], and low field magnetization results [ 7 - 1 3 ] have been interpreted in terms o f Josephson decoupling between grains at low fields ( < 20 Oe). These results suggest that Josephson effects at the grain boundaries in polycrystalline materials play a detrimental role in limiting the transport critical current. * Presently at the University of California, Santa Barbara, California, USA. ** Presently at the University of Arizona, Tuscon, Arizona, USA. 0921-4534/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

Many studies to date have been carried out in porous materials with a wide variation in grain size and unknown grain boundary characteristics (such as secondary phases and impurity segregation). The present work investigates the grain size dependence o f resistivity, magnetization, and inferred critical currents in polycrystalline Lal.asSro.~sCuO4 o f more ideal microstructure. In order to remove questions o f compositional inhomogeneity and porosity present in the previous work, a series of homogeneous and dense ( > 96% o f theoretical) materials have been prepared with systematically varying grain size from l to ~ l 0 / t m . Measurements o f resistivity versus temperature, magnetization versus temperature, and magnetization versus field have been carried out. In addition, the grain boundary composition have been characterized using scanning transmission electron microscopy (STEM) and Auger electron spectroscopy (AES). The results indicate that even in fully dense homogeneous polycrystalline materials, graindecoupling occurs at fields o f less than 20 Oe. The weak-links at grain boundaries are not associated with either distinct second phases or gross change in composition within the single phase o f LaE_xSrxCuO4, although there are minor variations in the cation

78

Y.-M. Chiang et al. /Effects of grain size on superconducting properties of La~ ~sSro.15Cu04

compositions at the boundaries, specifically an enrichment of Sr.

2. Experimental procedure 2. I. Sample preparation

A citrate liquid-mix process was used to prepare a single lot of powder, nominally of La~ 85Sro ~5CuO4 composition, which was used for all samples throughout the study. This is a versatile synthesis route which has previously been used for the preparation of a variety of multicomponent ceramics such as the titanates, zirconates and niobates [ 14,15 ]. Its primary advantages are excellent compositional homogeneity, accurate control of cation stoichiometry, and often, decreased firing temperature for powder synthesis compared to conventional ceramic techniques. Individual stock solutions containing La and Cu were prepared by dissolving La203 and CuO in a mixture of citric acid and ethylene glycol. These solutions were assayed by weight after pyrolysing to the oxide, and then mixed in the desired proportions with Sr dissolved as SrCO3. The liquid solution was thoroughly mixed for homogeneity, and then heated to evolve water (which condenses upon polyesterification of the citrate), yielding first a viscous polymer and with further heating, a glassy resin. Upon subsequent pyrolysis to 850-900°C, a fine powder (equiaxed, 0.1-0.2 #m particles) of the K2NiF4 structure results. The X-ray diffractogram in fig. 1 shows this powder to be single-phase with the exception of a small amount of La203. The presence of free La203 in a nearly cation-stoichiometric composition (analysed to be slightly La-deficient, LaL822Sro ~52CuO4, using inductively-coupled-plasma (ICP) emission spectroscopy [ 16 ] demonstrate that the single phase composition is in fact La-deficient, as was previously known for La2_xCOO4 [ 17] and recently confirmed for undoped La2_xCuO4 [18]. In order to prepare dense polycrystalline specimens in which: (1) the grain size distribution is narrow; (2) the average grain size varies over a broad range; and (3) full thermal and atmospheric equilibration is achieved, the following steps were taken. Calcined powder was first annealed in flowing 02 at

500°C for 4 hours. Then, pellets were uniaxially pressed at ~ 200 MPa, wrapped tightly in platinum foil, and hot-pressed at 900°C for 1-3 hours in graphite dies at 60-70 MPa, using ZrO2 powder as a protective barrier layer between the pellets and the die. Subsequent chemical analysis via electron microprobe showed no Pt contamination of the measurement samples sectioned from hot-pressed pellets. Different hot-pressing times yielded samples with a wide range of grain sizes. In order to obtain still larger grain sizes, some samples were annealed for prolonged times at 1000 ° C. Specimens for magnetization and resistivity measurements were than sectioned from these samples in desired geometries. For SQUID magnetometer meaurements, the dense polycrystalline samples were core-drilled into 3 m m discs 0.5 m m thick, and polished to < 1/lm surface finish. Rectangular bars of ~ 15 m m length and ~ t m m 2 cross-sectional area were used for some resistivity measurements. All specimens in their final dimensional were annealed in flowing 02 at 700 °C for more than 200 hours to ensure complete oxidation regardless of grain size. Systematic study of the stabilization of properties with increasing anneal time, combined with calculations based on the oxygen diffusion coefficients reported by Routbort et al. [ 19 ], showed that these times and temperatures were necessary to fully equilibrate the samples. The slow equilibration due to restricted transport in dense materials has not been fully appreciated in much of the earlier work on porous materials. Thin sections for scanning transmission electron microscopy were prepared by ion-milling with 6 keV argon ions at room temperature. Samples for Auger spectroscopic analysis of in situ fractured grain boundaries were sectioned from bulk samples as thin square rods ( ~ 10 m m X 0.5 m m × 0.5 m m ) and annealed along with the other samples. 2.2. Properties measurements

Four-point resistivity measurements were carried out in the van der Pauw geometry for disc-shaped samples, and with the usual linear contact array for bar-shaped samples. Both AC (490 Hz) and DC measurements were conducted, using currents of 10-3-10 - ~ A/cm 2. No variation in results with cur-

Y.-M. Chiang et al./ Effects of grain size on superconducting properties of La, ssSro lsCu04

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rent was observed in this range, and AC and DC measurements yielded identical results. Contracts were made with either silver paint or I n - G a alloy; contact resistances were typically 2.5-10 Ddmm 2. Two-point contact resistances were measured for all lead combinations, at low temperature as well as at room temperature, to ensure that results were not to spurious contact effects. Magnetization measurements were conducted using an SHE SQUID Magnetometer at the Frances Bitter National Magnet Laboratory. Magnetization versus temperature was measured by first cooling the sample to 6 K in zero field ( + ,,, 2 0 e ) , applying a field of 20 Oe, warming to 50 K (shielding magnetization), and then cooling in the same field (Meissner effect). Magnetization versus field measurements were conducted by cooling to 6 K in zero field, and then increasing field in increments up to 50 kOe, followed by reducing field in the same increments, yielding a hysteretic magnetization curve.

3. Results

3. I. Microstructural characterization of samples Figure 2a shows a fracture surface of the finestgrained microstructure used in this study (hotpressed for 1 h), in which the average grain size is slightly less than 1 /tm. The density of this sample (as determined by the Archimedes method) was 96%. A 3 h hot-pressing cycle at the same temperature (900°C) resulted in the fully dense ( > 99%) microstructure shown in fig. 2b, where the averaged grain size is 3 #m. Finally, hot-pressing for 3h at 900°C, followed by annealing for grain growth at 1000°C for 168 h, resulted in a fully dense microstructure with the largest grains. Whereas the two finer-grained samples have a narrow grain size distribution, this sample had undergone considerable secondary grain growth such that the grain size distribution is large (fig. 2c), and the average grain size is ~ l0/gin. A thorough search for compositional inhomogeneities and second phases was conducted in these samples. X-ray diffraction, electron microprobe and

80

E-M. Chiang et al. /Effects of grain size on superconducting properties of Lal ssSro lsCu04

this system to be La-deficient at equilibrium, as was previously known for the La2_xCoO4 system [ 17] and recently demonstrated for undoped La2_xCuO~ as well [ 18]. That is, the equilibrium single-phase composition is Lal.85_xSro ~sCuO4_y. The composition of these samples is therefore considered to be pinned at the La-rich phase boundary. For confirmation, samples with compositions deliberately prepared to be La-deficient show an absence of the second phase [20]. Importantly, both STEM imaging at high magnifications and AES grain boundary compositional analyses (described below) revealed no evidence for continuous grain boundary phases, La-rich or otherwise. The carbon content of some ashot-pressed samples were also analyzed [21] and found to be less than 0.1 wt% in all cases. No Pt contamination ( < 0.2%) from the encapsulation foil was detected in either SEM (energy dispersive X-ray analysis), electron microprobe microanalysis, or STEM analyses of samples used for properties measurements. 3.2. Grain boundary segregation

Fig. 2. Secondaryelectronimagesof the samplesused in this work. (a) ~ 1 /~m grain size; (b) ~3/lm grain size; and (c) ~ 10/tm grain size. STEM investigations revealed a small fraction of La203 present as isolated precipitates. It is believed that these precipitates exist due to the propensity for

The STEM results showed no detectable segregation at grain boundaries in these samples (fig. 3), corroborating STEM observations by Larbalestier et al. [22] on the same system. However, since the STEM microanalysis technique has an effective spatial resolution of ~ 5 nm for typical metal oxides due to electron beam broadening, approximately the first 5 atomic planes to each side of the grain boundary are sampled by the probe [23,24]. Changes in composition over narrower spatial extent, as is observed for a variety of solutes in both metal and ceramic systems [25-27], are diluted by the adjacent sampled material and will go undetected if the concentration change is sufficiently small. Auger electron spectroscopy of in situ fractured grain boundaries offers finer depth resolution (limited to the escape depth of the Auger electrons of interest), usually I - 2 atomic layers. We examined surface of samples fractured in situ in an Auger spectrometer [ 28 ], on which there were usually a combination of transgranular and intergranular fracture features, to search for grain boundary compositional changes. Here the transgranular

81

Y.-M. Chiang et al. /Effects of grain size on superconducting properties of Lat.ssSro.15CuO4

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Fig. 3. STEM generatedcompositionalprofiles across a grain boundary in a sample hot-pressed at 900°C, plotted as X-ray intensity ratios against distance from the boundary. fracture sites were used as the reference bulk composition. Figure 4a shows a secondary electron image of an in situ fracture surface, in which points labeled 1-5 are grain boundary fractures, whereas points 6 and 7 are the transgranular sites. Figure 4b shows the relative Sr concentration at each site, where the grain boundaries appear to be slightly Sr-rich compared to the grain interiors. Other more detailed results [ 29 ] have confirmed that grain boundaries in this system are Sr-rich at equilibrium relative to the bulk, in some cases by greater than a factor of 2, and that the segregation increases with decreasing anneal temperature. On the other hand, neither impurity segregation nor oxygen stoichiometry changes were detected at the grain boundary. It is significant that the magnitude of the concentration changes do not suggest the presence of continuous grain boundary phases, consistent with high resolution STEM imaging resuits. That is, the concentration changes observed are insufficient to form even a 1 nm layer of a different bulk crystalline phase. (Note that since the occasional LazO3 precipitates described above are non-

wetting, they would not have an impact on grain boundary segregation once thermodynamic equilibrium between the phases, as well as between the grain boundary and the bulk, has been reached.) 3. 3. Resistivity m e a s u r e m e n t s

Room temperature resistivities and residual resistivity ratios (R300K/P45K)a r e give in table I for dense samples of varying grain size. The room temperature resistivities (1.43-3.27 m~2 cm) are of the same order as the lowest values reported in the literature for this composition, which is expected from the high sample density. The somewhat higher value for the finest grained sample may reflect a combination of slightly greater porosity and a high density of grain boundaries. For comparison, a porous pressed-andsintered sample (from the same powder and annealed identically) shows a room temperature resistivity more than an order of magnitude greater (51.2 mr2 cm). The residual resistivity ratios of the dense samples (1.49-2.14) are all slightly lower than literature values (typically 3-5) [ 30-35 ]. These val-

82

Y.-M. Chiang et al. /Effects of grain size on superconducting properties of Lal 85Sro lsCuO~

b

grained samples (figs. 2b,c) were alike, each exhibiting a Tc midpoint at ~ 36 K and a transition width (90% to 10%) of ~ 2 K. This midpoint Tc is slightly lower than the maximum reported literature values of 38-40 K for the same x = 0 . 1 5 nominal composition [ 30,32], but is otherwise typical of literature results. In contrast to these samples, the ~ 1 #m grain-sized sample showed a "foot" which broadened the transition to lower temperatures (fig. 3a), reaching zero (within the resolution of the measurement) at ~ 25 K. The appearance and variation of this "foot" as a function of current density and magnetic field is being examined, and will be discussed in a later report.

3.4. Magnetization versus temperature and field

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Fig. 4. (a) Secondaryelectron imagetaken in the scanningAuger spectrometer of an in situ fractured surface, showingintergranular fracture surfaces (1-5) and transgranular fracture surfaces (6,7). (b) Sr composition (x in La2_xSrxCuO4) at each of the points indicated in (a), computed using the transgranular fracture points 6 and 7 as compositional standards representing the bulk compositionofLa~ s22Sro~52CuO4. ues appear to scale with decreasing grain size, suggesting that they may be related to grain boundary effects. The resistive transitions of samples corresponding to the microstructure shown in figs. 2a-c are shown in figs. 5a-c, respectively. The behavior of the largerTable I Room temperature resistivity and resistivity ratios in LaLasSr0.~5CUO4as a function of grain size Grain size 1/zm 3/~m 10/~m Pressed and sintered

R.T. resistivity

Resistivityratio

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(P300K/P45 K )

3.27 1.43 1.59 51.2

1.42 1.82 2.12 -

Magnetization results showed far more dramatic effects of grain size. Figures 6a-c show magnetization versus temperature results corresponding to the microstructures in figs. 2a-c respectively. These are plotted on different vertical scales for clarity, but are plotted together on the same scale in fig. 6d for comparison. The samples used in these measurements were fiat discs with an aspect ratio of 6 : 1. The demagnetization coefficient for this geometry is significant ( ~ 0.75) and in principle must be considered in interpreting the magnetization data near and below Hcl. (For fields much greater than Hcl, sample shape becomes unimportant.) However, as will be discussed below, the samples behave more as an aggregate of isolated particles than as a continuous body, and thus the actual geometric effects on the magnetization are known accurately. Since the primary purpose of this work is to compare the properties of samples with different grain sizes, no corrections will be made for demagnetization effects. In the coarsest grained sample (fig. 6c, 10 pm grain size), results were comparable to those usually reported for well-annealed, long-fired samples. Flux exclusion (zero-field-cooled shielding) at 6 K and 20 Oe corresponded to 44% of ideal diamagnetism (fig. 6c). The flux expulsion (Meissner effect) corresponded to ~ 22% of ideal diamagnetism under the same conditions. The transitions were considerably

Y.-M. Chiang et al. /Effects of grain size on superconducting properties of La+.s~Sro lsCuO+

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curves suggesting multiple transitions, one at about the resistive Tc and another at ~ 20 K. Recent results suggest that in these fine-grained materials the structure can be removed by equilibrating at higher temperatures where grain boundary segregation is diminished [29]. Magnetization versus field results for all three grain sizes are shown in figs. 7a and 7b for high and low field ranges, respectively. Here again the total magnetization as well as the magnitude of the hysteresis clearly scales with grain size.

84

Y.-M. Chiang et al. /Effects of grain size on superconducting properties of Lal ssSro tsCuO+

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4. Discussion

The magnetization data given above are consistent with grain-decoupling at fields of less than 20 Oe, such that the polycrystal behaves magnetically more like an aggregate of isolated powder particles than a dense continuum. From the hysteresis of the magnetization shown in fig. 7b, a critical state model for a cyclindrical geometry of radius R: Jc = 15 JMp/R. For extreme type II materials and applied fields much greater than Hc~, demagnetization effects are negligible, and Bean's model can be applied to our sample geometry. Table II lists the m a x i m u m inferred Jc calculated under two different assumptions for the sam-

pie radius: ( 1 ) R -- R (sample), the disc radius (0.15 cm) corresponding to shielding currents flowing in the entire sample; and (2) R = R ( g r a i n ) , the average grain radius, corresponding to shielding currents flowing only in effectively isolated grains. (Note: for the second calculation we neglect variations due to the distribution of grain sizes in each sample.) It is apparent that when the characteristic sample dimension is taken to be the grain size, the calculated critical currents are the same within a factor of 2 in all samples. Given the simplifications of the model, we feel that for all practical purposes these values are identical. On the other hand, the values computed using the actual sample size scale systematically with average grain size, but are of question-

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able significance at the time. A comparison between transport J¢ and these values would clearly be of interest, and is underway. The magnitudes of the diamagnetism observed in fine versus coarse grained samples are also consistent with this view. For decoupled grains of 1 #m diameter, the majority of the grain volume is occupied by the London penetration depth (2), leading to a greatly reduced diamagnetic signal. The volume fraction of a spherical grain not penetrated is (R-2)3/R 3, and for 2~ 300 nm in this system [33], ranges from 16% for 1 btm diameter grains to 64% and 88% for 3 and 10 #m grains, respectively. The results in fig. 6 follow this general trend of increasing diamagnetism with grain size, although the complex

structure of the transitions precludes application of any simple scaling law. The structure seen below Tc in the magnetization results in fig. 6 for the fine-grained samples, and the "foot" seen in the resistive transition of the finest grained sample (fig. 5a), are also consistent with the assumption of weakly coupled grains. The observed Sr enrichment at grain boundaries would correspond, in bulk form, to a composition of reduced Tc [30,32]. In a dense polycrystal, such regions of weakened superconductivity could enhance grain decoupling and allow penetration of field along grain boundaries upon warming, resulting in behavior like that in figs. 6a and 6b. These results support the proposals of Finnemore

86

Y.-M. Chiang et al. /Effects of grain size on superconducting properties of Lal 85SrolsCuO4

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et al. [7,8], Farrell et al. [9], Kwak et al. [ 11 ], Senoussi et al. [ 12 ] and very recent results by McHenry et al. [ 13 ] on YBa2Cu307, that grain-decoupling occurs at low applied fields ( < 2 0 Oe). The AES resuits show that in this system the decoupling behavior is in part associated with an intrinsic grain boundary nonstoichiometry on the cation sublattice. This does not rule out the possibility that grain boundary structural disorder alone is sufficient to cause decoupling. However, discrete second phase layers or impurity segregation at the grain boundary do not play a ma-

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Y.-M. Chiang et aL /Effects of grain size on superconducting properties of Lat.s~Sro.15CuO~

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tire cooling). In the present results the segregation of Sr is an equilibrium phenomena, and is temperature dependent. Its effects can be lessened by annealing at higher temperatures [29] but probably not eliminated entirely. The limits to which the critical currents of polycrystalline oxide superconductors can be improved by control of composition cannot be established without more detailed work. It is clear, however, that improvements will require a sophisticated level of grain boundary engineering which will involve control of both chemistry and structure.

5. Conclusions

The resistive and magnetic superconducting transitions in dense polycrystalline Lat.ssSro. 15CuO4have been characterized as a function of grain size over the range < 1/2m to > 10 #m. The resistive transitions under zero field show little distinction between different grain-sized dense materials, except in the material with the greatest density of grain boundaries (smallest grain size), where a broader transi-

88

Y.-M. Chiang et al. /Effects of grain size on superconducting properties of Lal esSro isCuO+

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FIELD (De) Fig. 7. Magnetization versus field at 6 K for the sample represented in figs. 5 and 6. (a) High-field results; (b) low-field results.

tions with a "foot" extending to lower temperatures appears. Magnetic transitions of fine-grained materials are markedly different from those of coarse grained materials; there is evidence for multiple transitions in M versus T traces at 20 Oe, suggesting rapid penetration of field upon warming. The finest grained materials have very little net magnetization. These results are interpreted as representing decoupled grains under the 20 Oe field, in which a fully dense material therefore behaves magnetically more like a

powder. The low magnetization of < 1 /tm grained materials is consistent with a large reduction in the magnetization volume due to the penetration depth of ~ 300 nm. Application of Bean's critical state model to M versus H results allows the critical current within grains to be determined, assuming all grains to be decoupled. With this assumption, good agreement is obtained over all grain sizes, with Jc ~ 5 × 105 A/cm 2. The critical current obtained assuming circulating currents throughout the sample is orders of magni-

Y.-M. Chiang et al. /Effects of grain size on superconducting properties of La~.ssSro.tsCu04

Table II Magnetization-inferredcritical current (critical state model*), for volume elements corresponding to the sample size and grain size respectively Grain size

1/zm 3/tm lO/tm

Critical current Jc(A/cm2) R = sample radius

R = grain radius

128 700 1280

3.8× 105 7X 105 3.8× 105

* Jc= 15ziMp/R, where z/M is the magnetization hysteresis (emu/g), p is the density, and R the sample radius.

tude lower; while these results scale systematically with grain size, their interpretation is unclear. Careful STEM a n d AES examinations show that these results correspond to materials with clean grain boundaries free of both continuous second phases and impurity segregation. However, there is a a slight enrichment of Sr at the grain boundary. The easily decoupled weak-links which magnetization results show to be present may result from the intrinsic structural disorder of grain boundaries, or from the changes in composition f o u n d in AES results, or both. In either case, these results suggest that an especially high level of grain b o u n d a r y engineering may be necessary to overcome the limitations grain boundaries present with respect to critical current.

Acknowledgements The authors are grateful to Michael J. Parker, Mary M. Mathiessen, Katharine Williams a n d Sharon L. Furcone for their experimental assistance with properties measurements, John R. M a r t i n for assistance with Auger spectroscopy, a n d Michael E. M c H e n r y for useful discussions. Support has been provided u n d e r US D e p a r t m e n t of Energy G r a n t No. DEFG02-87ER45307.

References [ 1] T.R. Dinger, T.K. Worthington, W.J. Callagher and R.L. Sandstrom, Phys. Rev. Lett. 58 (1987) 2687.

89

[2] G.W. Crabtree, J.Z. Liu, A. Umezawa, W.K. Kwok, C.H. Sowers, S.K. Malik, B.W.Veal, D.J. Lam, M.B. Brodskyand J.W. Downey, Large Anisotropic Critical Magnetization Currents in SingleCrystal YBa2Cu307_x,preprint. [3] P. Chaudhari, R.H. Koch, R.B. Laibowitz, R.R. McGuire and R.J. Gambino, Phys. Rev. Lett. 58 (1987) 2684. [4] J.W. Ekin, Adv. Ceram. Mater. 2 [3B] (1987) 586-592. [ 5 ] J.W. Ekin, A.I. Braginski,A.J. Panson, M.A. Janocko, D.W. Capone, II, N. Zaluzec,B. Flandermeyer, O.F. de Lima, M. Hong, J. Kwo and S.H. Liou, Appl. Phys., submitted. [6] M.S. Osofsky, W.W. Fuller, L.E. Toth, S.B. Qadri, S.H. Lawrence, R.A. Hein, D.U. Gubser, S.A. Wolf, C.S. Pande, A.K. Singh, E.F. Skelton and B.A. Bender, Preparation, Structure, and MagneticField Studies of High T¢Superconductors, preprint. [7] D.K. Finnemore, R.N. Shelton, J.R. Clem, R.W. McCallum, H.C. Ku, R.E. McCarley,S.C. Chen, P. Klavinsand V. Kogan, Phys. Rev. B 35 [ 10] 0987) 5319. [8] D.K. Finnemore, J.E. Ostenson, L. Ji, R.W. McCallumand J.R. Clem, Superconducting Critical Currents in YtBa2Cu3OT,ICMC, submitted (June 12, 1987). [9] D.E. Farrell, M.R. DeGuire, B.S. Chandrasekhar, S. Alterovitz, P. Aron and R. Fagaly,Phys. Rev. B 35 [ 16] (1987) 8797. [ 10] M.R. DeGuire and D.E. Farrell,Adv. Ceram. Mater. 2 [3B] (1987) 593. [ 11 ] J.F. Kwak, E.L. Venturini, D.S. Ginley and W. Fu, Grain Decoupling at Low Magnetic Fields in Ceramic YBa2CU3OT_x,preprint. [ 12] S. Senoussi, M. Oussena, M. Ribault and G. Collin, The Critical Fields and the Critical Current Density of La LssSro.~sCuO4, Phys. Rev. B, submitted. [ 13] M.E. McHenry, J. McKittrick, S. Sasayama, V. Kwapong, R.C. O'Handley and G. Kalonji, Magnetizm and Microstructure of YBa2Cu30~_x Superconductors Produced by Rapid Solidification,Phys. Rev., submitted. [ 14] M.P. Pechini,Method of Preparing Lead and AlkalineEarth Titanates and Niobates and Coatings Using the Same to Form a Capacitor, U.S. Patent No. 3 330 697, July 11, 1967. [ 15] N.E. Eror and D.M. Smyth, in: The Chemistry of Extended Defects in Non-Metallic Solids, eds. L. Eyring and M. O'Keefe (North-Holland, Amsterdam, 1970) pp. 62-74. [ 16] Union Carbide Corporation, Tarrytown, NY. [ 17] J.T. Lewandowski, R.A. Beyerlein, J.M. Longo and R.A. McGauley, J. Am. Ceram. Soc. 69 [9] (1986) 699. [l 8] P.E.D. Morgan, unpublished. [19] J.L. Routbort, S.J.f Rothman, B.K. Flandermeyer, L.J. Nowicki and J.E. Baker, Oxygen Diffusion in La2_xSr~CuO4_~,J. Mater. Res., submitted. [20] A. Roshko, D.K. Leung, J.A.S. Ikeda and Y.-M. Chiang, unpublished. [ 21 ] Leco Carbon Analyzer Model WR- 12, Laboratory Equipment Corporation, Saint Joseph, MI. [22] D.C. Larbelestier, M. Daemling, P.J. Lee, T.F. Kelly, J. Seuntjes,C. Meingast,X. Cai, J. McKinnell,R.D. Ray, R.G. Dillenburg, and E.E. Hellstrom, Microstructural and Elec-

90

Y.-M. Chiang et al. /Effects of grain size on superconducting properties of Lal ssSro ~5Cu04

tromagnetic Characterization of La2_~SrxCuO4, Cryogenics, Aug. (1987). [23] E.L. Hall, D. Imeson, and J.B. VanderSande, Philos. Mag. A43 [6] (1981) 1569. [ 24] J.B. VanderSande, A.J. Garrett-Reed, Y.-M. Chiang and T. Thorvaldsson, Ultramicroscopy 14 (1984) 65. [ 25 ] E.D. Hondros and M.P. Seah, Int. Met. Rev. 22 (1977) 262. [26] Y.-M. Chiang, A.F. Hendriksen, W.D. Kingery and D. Fihello, J. Am. Ceram. Soc. 64 [7] (1981) 386. [27] W.D. Kingery, Pure Appl. Chem. 56 [ 12] (1984) 1703. [28] Perkin Elmer PHI 660 Scanning Auger Multiprobe, Perkin Elmer, Eden Prairie, MN. [29] Y.-M. Chiang, B.D. Fabes, A. Roshko, J.R. Martin and J.A.S. Ikeda, unpublished. [30] J.M. Tarascon, L.H. Greene, W.R. McKinnon, G.W. Hull and T.H. Geballe, Science 235 (1987) 1373. [31 ] R.J. Cava, R.B. van Dover, B. Batlogg and E.A. Rietman, Phys. Rev. Lett. 58[4] (1987) 408. [32] R.B. van Dover, R.J. Cava, B. Batlogg and E.A. Rietman, Phys. Rev. B 35 (1987) 533. [33] A.J. Panson, G.R. Wagner, A.I. Braginski, J.R. Gavaler, M.A. Janocko, H.C. Pohl and J. Talvacchio, Appt. Phys. Lett. 50 (1987) 1104.

[ 34 ] S. Uchida, H. Takagi, K. Kishio, K. Kitazawa, K. Fueki and S. Tanaka, Jpn. J. Appl. Phys. 26 [4] (1987) L443. [35] M. Decroux, A. Junod, A. Bezinge, D. Cattani, J. Cors, J.L. Jorda, A. Stettler, M. Francois, K. Yvon, O. Fischer and J. Muller, Europhys. Lett. 3 (1987) 1035. [36] R.A. Camps, J.E. Evetts, B.A. Glowacki, S.B. Newcomb, R.E. Somekh and W.M. Stobbs, Nature 329 (1987) 229. [37] Y.-M. Chiang and W.D. Kingery, Grain Boundary Composition in MnZn Ferrites, in: Advances in Ceramics, Vol. 6, eds. M.F. Yan and A.H. Heuer (The American Ceramic Society, Columbus, OH, 1983) pp. 300-311. [38] Y.-M. Chiang, Grain Boundary Mobility and Segregation in Non-Stochiometric Solid Solutions of Magnesium Aluminate Spinel, Sc. D. Thesis (Massachusetts Institute of Technology, Cambridge, MA. 1985). [39] Y.-M. Chiang and C.J. Peng, Grain Boundary Nonstoichiometry in Spinels and Titanates, in: Advances in Ceramics, Vol. 23, eds. W. Mackrodt and C.R.A. Catlow, to appear.