No Title

SESSION R3: POSTER SESSION:
TEXTURED YBCO
Chair: Martin P. Maley
Tuesday Evening, April 1, 1997
8:00 P.M.
Salon 7

R3.1
SPATIAL DISTRIBUTION OF DEGREE OF TEXTURING AND J

IN LARGE YBCO GRAINS, Wai Lo, Cambridge Univ, Cambridge, UNITED KINGDOM; C. D. Dewhurst, H. T. Leung, J. C. L. Chow, D. A. Cardwell, Cambridge Univ, IRC in Superconductivity, Cambridge, UNITED KINGDOM.

The J

of melt-processed YBCO has been repeatedly shown to be strongly correlated with the number density and average size of Y

BaCuO

(211) inclusions in melt-processed YBCO grains. The number density of 211 inclusions in large YBCO grains has also been demonstrated to increase to a saturation value as the dimension of the grains developed, which suggests the possible existence of higher J

in grains larger than a critical dimension. However, detailed study of the spatial distribution of J

in YBCO grains having 20 mm in diameter showed that the maximum J

could be found at positions about 4 mm from the center of the grains, which is to be compared with the minimum J

for positions near the edge of the grains. In addition, the degree of texturing of the YBCO grain deteriorates as the dimemsion of the grains exeeds 4-5 mm, such that high degree of crystallographic misorientation between different local regions can be frequently observed near the edge of apparently single grains with dimensions larger than 15 mm. This spatial distribution of degree of texturing is a result of the lack of stable planar growth morphology of the YBCO grains in the peritectic state. As crystallographic orientation is critical in determining J

, the degradation in J

at positions in excess of 4 mm from the center of the YBCO grains can be explained by the higher degree of crystallographic misorientation. It is therefore concluded that while 211 number density is important for achiving higher J

in melt-processed YBCO grains, the performance and maximum useful size of large YBCO grains are limited by their grown morphology in the peritectic state.

R3.2
CRYSTAL GROWTH OF THE FACETED SrTiO

SINGLE CRYSTALLINE FIBERS FOR YBa

COATED CONDUCTORS, Junichi Kawashima, ISTEC, Nagoya, JAPAN; Yasuji Yamada, ISTEC, Superconductivity Research Lab, Nagoya, JAPAN; Izumi Hirabayashi, ISTEC, Nagoya, JAPAN.

We developed the flexible yttria stabilized zirconia (YSZ) single crystalline fiber by CO

-laser heated pedestal growth (LHPG) method as a substrate for YBa

(YBCO) conductors and coated YBCO thick films by liquid phase epitaxy (LPE). We have developed the SrTiO

(STO) single crystalline fiber by LHPG as a substrate for YBCO coated conductors. Since the thermal expansion coefficient of STO takes a similar value to that of the a-b directions of YBCO, STO becomes a good candidate of the substrate for the LPE thick film avoiding crack formation. A typically obtained STO fiber was 80 mm in length and 0.55 mm in diameter. The cross sectional form of the fiber depends on growth conditions including pulling direction and shape of molten zone. We obtained the octagonally faceted STO fiber by growing the fiber downward with the condition that upper fixture pushing speed was higher than that of the lower fixture pulling speed. The facet surfaces are (100) and (110) characterized by XRD pole figures. They are suitable as a substrate for the successive oxide film depositions. The YBCO thin film on STO (100) faceted surface by pulsed laser deposition (PLD) showed the biaxially in-plane alignment, which is suitable for LPE growth. This work was supported by NEDO.

R3.3
EFFECT OF Y

BaCuO

ADDITION ON THE CRITICAL CURRENT DENSITY, FLUX PINNING AND TWINNING IN MELT-TEXTURED YBa

, Manoi Chopra, V. S. Boyko, Columbia Univ, Dept of Materials Science, New York, NY; Ruling Meng, Univ of Houston, Dept of Physics, Houston, TX; C. W. Chu, Univ of Houston, Houston, TX; Siu-Wai Chan, Columbia Univ, Dept of Materials Science, New York, NY.

Quantitative microscopy studies have been conducted on large grained YBa

(Y123) samples containing various volume fractions of Y

BaCuO

(211). Analysis of the electrical properties shows that the Jc of the Y123 matrix increases and achieves a maximum for a measured Y123 volume of

. Further increase in the 211 content leads to a reduction in the conducting pathways in the Y123 matrix. However, an analysis of the flux pinning forces reveals an improved Y123 matrix. Transmission electron microscopy (TEM) done on the (001) plane shows the presence of twin structures in the Y123 matrix. It is observed that the mean twin spacing (Tw) decreases linearly with the square root of the local interparticle spacing (S

). Furthermore, measurements of the local twin spacing at the Y123/211 interface show that the twin spacing decreases as one approaches the interface, showing a nonuniform distribution of strain energy around the 211 particles in the matrix (produced during the t

o transformation). Using Bitter decoration, the effect of a novel defect structure and its relative importance in the flux pinning properties is determined. A change in the density of this defect structure is obtained by the process of oxygen annealing. Leading to a nearly 5 times increase in the trapped magnetic field at 77 K. Also the effect of additions such as Pt and CeO

on the twin microstructure shall be discussed.

R3.4
LAYER BY LAYER GROWTH OF HIGH-TEMPERATURE SUPERCONDUCTORS, Dave H.A. Blank, Guus J.H.M. Rijnderss, Gertjan Koster, Horst Rogalla, Univ of Twente, Dept of Applied Physics, Enschede, NETHERLANDS.

The typical layered structure of high-Tc cuprate superconductors, copper oxide layers separated other metal oxide layers, has been an increasing topic of research. In search for higher Tc values, or well defined insulators, structures are being made with different separations between the copper oxide planes. Pulsed laser deposition has been used to fabricate these structures. We have developed a RHEED-system, for use at PLD conditions, for in-situ growth studies at high pressures. Using a two stage differential pumping system we are able to increase the deposition pressure up to 50 Pa without losing clear RHEED patterns. We are able to monitor the homo-epitaxial growth of SrTiO3 and hetero-epitaxial growth of YBa2Cu3O7 on SrTiO3 substrates. Optimising the growth parameters, layer by layer growth is realised, as indicated by RHEED oscillations. E.g., in the case of SrTiO3 intensity oscillations up to 200 unit cells have been observed.

R3.5
FORMATION OF TEXTURED Y-123 CERAMICS FROM THE MELT, Vladimir Vashook, Inst of General & Inorganic Chemistry, Minsk, BYELARUS.

The formation process of textured ceramics YBa

on a polycrystalline samples surface Y

BaCuO

and Y

placed in BaCuO

and CuO melt at temperatures 960-1040

C was investigated. It was shown, that only a YBa

layer is formed on a Y

BaCuO

sample at cooling of the melt with a rate 1

C/hour in the all temperature interval, whereas the one-phase product on Y

is formed only at cooling of the melt from 1000

C. The formation of a ceramic textured layer Y

BaCuO

was observed alongside with a YBa

phase in a more high-temperature zone. No transition of yttrium in the melt lower 1020

C took place, and no more than 1

yttrium quantity located near to solid phases Y

BaCuO

and Y

in a temperature interval 1020-1040

C in the melt was found. Optimal conditions of textured YBa

ceramics formation on the dense Y

BaCuO

samples were determined. They are depending on the melt composition, initial crystallization temperature and the cooling rate. The rate of textured YBa

layer formation is limited by diffusion processes. The model of textured ceramic layers formation is offered in this work.

R3.6
HRTEM STUDY OF DEFECTS IN EPITAXIAL YBa

FILMS IRRADIATED BY 100 keV OXYGEN IONS, N. D. Zakharov, Dietrich Hesse, Max-Planck-Inst, Dept of Microstructure Physics, Halle/Saale, GERMANY; J. Hollkot, Shike Hu, C. Becker, Inst fur Halbleitertechnik II, Aachen, GERMANY; Juergen Auge, Bernd Spangenberg, Inst for Semiconductor Electronics, Aachen, GERMANY; Heinrich Kurz, Rheinisch-Westfalische Technische Hochschule, Inst of Semiconductor Electronics II, Aachen, GERMANY.

Defect structure and composition variations of epitaxially c-oriented YBa

(YBCO) thin films grown by DC-sputtering on (OOl)SrTiO

substrates and irradiated by lOO keV oxygen ions were investigated by high-resolution transmission electron microscopy (HRTEM) and EDX. In the irradiated YBCO films, holes about 8.5 nm in diameter, amorphized regions and antiphase boundaries were observed in plan view samples. According to EDX microanalysis the amorphized regions are strongly Cu deficient (C

). It follows that the oxygen implantation results in a depletion of the YBCO films by Cu and possibly by Ba, because these elements have much higher vapor pressures compared with Y. This is also the reason why the formation of Y

platelet precipitates was observed. Obviously the film is locally melted at the place of oxygen ion impact resulting predominantly in Cu and Ba evaporation. Dislocation loops of vacancy type were observed near the place of oxygen ion impact, which points to a strain field of vacancy type. The structure of antiphase boundaries observed was studied by HRTEM and computer simulation. They form an extraction of one CuO (100) plane. Electrical characteristics of SNS Josephson junctions fabricated by O

implantation were determined.

This work was supported by BMBF Nos. 13N6402 and 13N6451/1.

R3.7
STUDIES ON AC SUSCEPTIBILITY OF HIGH J TEXTURED YBCO, Dao-Le Yin, K. X. Chen, G. Lu, L. X. Xue, X. P. Bai, C. D. Wei, Peking Univ, Dept of Physics, Beijing, CHINA; H. T. Ren, L. Xiao, Q. He, General Research Inst, Dept of Non-Ferrous Metals, Beijing, CHINA.

We studied the complex susceptibility - of textured bulk YBCO with very high critical current density Jc The observed peak position Tp is related to the ac-field amplitude and frequency and can be described a general relationship as h [Tp(h)-T^3/2 with T^Tp(h = 0) about 88 K close to T. These results are compared with reports on other high T superconductors and discussed in the framework of a new material equation for type-II superconductors.

R3.8
FLUX PINNING IS CAUSED BY DISLOCATION MOTION AND MULTIPLICATION IN FLUX-LINE LATTICES, Valery P. Kisel, Inst of Solid State Physics, Dept of Crystal Growth, Chernogolovka, RUSSIA.

This work shows that so-called uniform (continuous) flow (or creep) in flux-line lattices (FLL) is similar to the analogous stage of deformation in crystals which is always preceded by a jump-like plasticity (in fine scale of observation). The local, well-resolved little jumps in FLL [1] are identical to the sequential unpinning of a greater number of dislocations in crystalline lattice with increasing stress up to the yield stress. Above this critical value the simultaneous unpinning of mobile dislocations looks like uniform or ''viscous'' flow in any crystalline or FL lattices under different types of loadings [1, 2]. The product of critical current, J, and magnetic field plays the role of the yield stress for FLL. The numerous features in the details of micro- and macroplasticity of crystals and FLL are common: the effect of time, temperature (non monotonous or athermal behavior of nearby 0K or ), (peak-effects), stress (), stress rate (field sweep rate, ), concentration and state of pin-defects, state of the surface, size- and softening effects, etc., on sequential unpinning and abrupt motion of dislocations between pinning defects. Again, the same scaling relations between the parameters of microplasticity and FLL deformation at atomic, mesoscopic, and macroscopic length scales confirm the identity of dislocation mechanisms in different lattice structures.
[1] U. Yaron et al., Nature, v. 376, No 6543, 753 (1995).
[2] V. P. Kisel et al., Phil. Mag. A, v. 67, 343 (1993).

R3.9
THE EFFECT OF THE ADSORBED OXYGEN, NITROGEN AND ARGON ON SUPERCONDUCTIVITY IN GADOLINIUM HTSC FILMS, Lev Lvovich Makarshin, Dmitry V. Andreev, Boreskov Inst of Catalysis, Lab of Solar Energy Conversion, Novosibirsk, RUSSIA; Oleg M. Tuhto, Inst of Thermal Physics, Novosibirsk, RUSSIA; Valentin N. Parmon, Boreskov Inst of Catalysis, Lab of Solar Energy Conversion, Novosibirsk, RUSSIA.

The effect of the adsorbed gas layer on the superconducting properties of HTSC material was determined by studying the low-temperature adsorption of nitrogen, oxygen and argon at the thin film of GdBa2Cu3O7 composition. The film was made by laser sputtering onto monocrystal sapphire sublayer in the oxygen atmosphere at 1055 K. Magnetic susceptibility of a sample was registered at AC magnetometer. The temperature of superconducting transition decreases almost linearly with the surface concentration of the nitrogen, argon and oxygen molecules. Charging the film surface by physical gas adsorption seems to be one of the possible mechanisms responsible for the observed phenomenon. Nitrogen and argon adsorption causes the linear growth of the transport critical current in the film. Formation of additional pinning centers due to adsorption and charging the surface via adsorption, is the possible mechanism responsible for this phenomenon. The oxygen adsorption gives rise to cubic decline of transport critical current of the film. This phenomenon is related to the effect of paramagnetic momentum of the oxygen molecule on the conductivity of intercrystalline contacts in the film. The observed influence effect of low-temperature adsorption of nitrogen, argon and oxygen gases at the surface of thin HTSC film is reversible and was reproduced repeatedly without decrease in the film superconducting properties.
REFERENCES 1. Xi X.X., Doughty C., Walkenhorst A. Et al. Phys. Rev. Lett., 1991, v.68, N 8, p.1240-1243. 2. L.L. Makarshin, A.L. Dautov and D.V. Andreev, Physica C, 235-240 (1994) 1583.

R3.10
CRITICAL CURRENT OF Na DOPED YBCO TEXTURED THIN FILMS, Jacob Azoulay, Igor Lapsker, Armen Verdyan, Center Technological Education, Dept of Physics, Holon, ISRAEL.

Superconducting YBCO and Na doped thin films were deposited on MgO substrate using a resistive evaporation technique. In situ heat treatment has been carried out at low oxygen partial pressure. Scanning electron microscopy and X-ray diffractometry were used for structure analysis. The X-ray diffraction pattern analysis shows that the YBCO with and without Na have the orthorhombic structure and texture. The electrical properties of the films were determined using astandard DC four point probe method. Improved transport properties are observed in polycrystalline YBCO films doped by Na. The critical current dencity was measured to be higher than that of the undoped YBCO thin film prepared by the same technique and conditions.

R3.11
DIRECTIONAL SOLIDIFICATION IN AIR OF NdBaCuO SUPERCONDUCTORS WITH HIGH T AND J , S. Pinol, B. Martinez, F. Sandiumenge, F. Vilalta, Xavier Obradors, R. Yu, CSIC, Inst de Ciencia de Matls de Barcelona, Bellaterra, SPAIN.

Single domain NdBaCuO (Nd123) with addition of NdBaCuO (Nd422) superconducting bars (150.0 mm in length and 8.0 mm in diameter) have been directionally solidified by a modified Bridgman method in air. The additions of Nd422 and CeO were found to reduce the loss of liquid phase during melt-texturing, as well as to refine the Nd422 precipitates. From AC and DC magnetic measurements, we demonstrate that Nd123-Nd422 superconducting composites directionally solidified in air exhibit T 95.0 K, which was slightly changed with the addition of the Nd422 precursors. This is a completely new result which demonstrates that low oxygen partial pressure (Po) is not necessary to fabricate NdBCO superconductors with high T and J, indicating that the Ba-Nd substitution is minimized during the air directional solidification process. Microstructural characterizations by means of polarized optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) have been performed in order to clarify the change of the superconducting properties with the composition and heat treatment. The critical current densities deduced from SQUID measurements show that the values of J can be enhanced by further refinement of the entrapped Nd422 particles through the inclusions of finer Nd422 precursors and CeO. The effect of post solidification thermal treatment on the anomalous field dependence of J has been investigated in detail. Additionally, c-axis oriented Nd123 + 15.0 Nd422 thick films were grown on MgO(001) single crystalline substrates by organic evaporation method. In a similar way, the microstructure of the NdBCO thick films was characterized and correlated with the superconducting properties.

R3.12
PEAK AND FISH-TAIL EFFECT IN YBaCuO SINGLE CRYSTALS, Krzysztof Rogacki, Bogdan M. Dabrowski, Northern Illinois Univ, Dept of Physics, DeKalb, IL; Ulrich Welp, George W. Crabtree, Argonne National Laboratory, Matls Science Div, Argonne, IL.

The peak effect, defined as a sharp increase of the critical current just below the irreversibility line, and the fishtail effect, defined as a second peak in magnetization measured as a function of magnetic Field, have been recently studied extensively in low and high temperature superconductors. Several pinning mechanisms have been proposed to understand these features but no satisfactory explanation has been found. As reported in many papers, the peak effect was usually observed for B// c-axis and vanished for B// (a,b)-plane. However, in high quality YBa single crystals, the peak effect was demonstrated to exist for the both magnetic field orientations [K. Rogacki et al., Proc. LT21 Con., Prague'96, 1615 (1996)]. This observation eliminates some explanations of the peak effect proposed in the literature and demonstrates the complicated nature of the phenomenon. In this paper we study the vortex behavior below the irreversibility line in Y%Ba single crystals using the ac complex susceptibility (first and higher harmonics) and dc magnetization techniques. We report ac and dc results for the irreversibility line, the peak effect and the fishtail, comparing their experimental manifestations in each measurement. The work was supported by the National Science Foundation-Science and Technology Center for Superconductivity, under contract DMR 91 20000 (KR, BD) and U.S. Department Of Energy, Basic Energy Sciences- Materials Science, under contract W-3 1-109-ENG-38 (UW, GWC).

R3.13
SPECTRAL COMPONENT MONITORING OF THE PLUMES GENERATED DURING DEPOSITION OF RE(Y,Nd)BaCuO FILMS BY PULSED LASER ABLATION, Rand Biggers, Air Force Wright Laboratory, WL/MLPO, Wright Patterson AFB, OH; C. Varanasi, Univ of Dayton, Research Inst, Dayton, OH; I. Maartense, Univ of Dayton, Dayton, OH; E. K. Moser, Air Force Wright Laboratory, WL/MLPO, Wright Patterson AFB, OH; D. Dempsey, Univ of Dayton, Dayton, OH; Tim L. Peterson, Air Force Wright Laboratory, Wright Patterson AFB, OH; Charles E. Oberly, Air Force Wright Laboratory, WL/POOX, Dayton, OH.

YBa (Y123) and NdBa (Nd123) films were deposited by Pulsed Laser Ablation (PLD) using a 248 nm wavelength KrF excimer laser. The time-resolved spectral components of the plumes generated from Y123, Nd123 targets were compared at similar deposition conditions. It was noticed that in the case of Nd 123, the Cu, Ba species move faster as compared to Y123 at similar laser fluence and other deposition conditions (150 mT oxygen pressure, 760C substrate temperature, etc.). High quality Y123 films (T 91 K, J > 10 amps/cm) were grown reproducibly by standardizing the deposition conditions by spectral component monitoring. The differences seen in Y123 and Nd123 time-resolved spectral components suggests that deposition conditions need to be changed to obtain good quality Nd123 films. Thick (>1 m) Y123 and Nd123 films were processed on LaAlO substrates and the depositions were monitored by comparing the spectral components to observe target aging effects. Microstructure and superconducting properties of the thick Y123 and Nd123 films will be presented.

R3.14
PULSED LASER DEPOSITION OF YBaCuO THICK FILMS WITH Ag ADDITIONS, C. Varanasi, Univ of Dayton, Research Inst, Dayton, OH; Rand Biggers, Air Force Wright Laboratory, WL/MLPO, Wright Patterson AFB, OH; I. Maartense, Univ of Dayton, Dayton, OH; E. K. Moser, Air Force Wright Laboratory, WL/MLPO, Wright Patterson AFB, OH; D. Dempsey, Univ of Dayton, Dayton, OH; D. Liptak, J. Busbee, Air Force Wright Laboratory, Wright Patterson AFB, OH; G. Kozolwski, Wright State Univ, Dayton, OH; R. Nekkanti, UES Inc, Dayton, OH; Tim L. Peterson, Air Force Wright Laboratory, Wright Patterson AFB, OH; Charles E. Oberly, Air Force Wright Laboratory, WL/POOX, Dayton, OH.

YBa, (Y123) films with Ag additions are deposited by pulsed laser ablation on single crystal (100) LaAlO and CeO buffered (100) YSZ substrates using a Y123 target doped win 5wt Ag. The thickness of the film is varied from 30 nm to a micrometer and the superconducting properties are compared with undoped Y123 films with similar thicknesses deposited under similar deposition conditions. Y123 films with silver additions were observed to have improved properties (high T and sharper transition widths) as determined by AC susceptibility measurements in applied magnetic field up to 2.2 Oe. The films were found to be smooth (5-7 nm rms roughness) without outgrowths, as analyzed by Atomic Force Microscopy. Raman microprobe analysis on these films showed very good homogeneity of the film composition over the film surface. Efforts are now in progress to deposit Y123 films over 2 micrometers thick. Microstructures and superconducting properties of the films will also be presented.

R3.15
CRITICAL CURRENT DENSITY DEPENDENCE ON ANGLE OF HEAVY ION IRRADIATION FOR YBCO COATED CONDUCTORS, J. Yates Coulter, Martin P. Maley, Paul N. Arendt, Stephen R. Foltyn, Los Alamos National Laboratory, Superconductivity Tech Ctr, Los Alamos, NM; H. Safar, Univ of Illinois-Chicago, Dept of Physics, Chicago, IL; Kenneth E. Gray, David G. Steel, Argonne National Laboratory, Matls Science Div, Argonne, IL.

We have prepared YBCO films deposited on polycrystalline Ni alloy substrates with an intermediate YSZ buffer layer produced by ion Beam Assisted Deposition (IBAD) with critical current densities (J) in excess of 1 MA/cm. These coated conductors show a strong dependence of J on the angle of the applied magnetic field, , to the crystallographic axes. There is a large peak when the field is in the plane and a smaller peak when the field is along the axis. The magnitude of the latter peak is strongly dependent on the magnitude of and appears to vary with material properties, i.e., film thickness, preparation, substrate material, etc. We have previously identified the -axis peak as caused by extended defects in that direction, specifically twin boundaries. In the present work, we have irradiated eight YBCO films with high energy (1.3 GeV) U ions at 0, 15, and 45 with respect to the axis. We find that the parallel columnar defects created by the heavy ions produce enhancements of J for fields parallel to the ion direction, and strongly modify the overall angular dependence. We discuss the details of the angle and field dependence of J and the implications of pinning by extended defects in YBCO films on polycrystalline substrates.

R3.16
NON-VACUUM CHEMICAL DEPOSITION OF YBCO FILMS ON ROLLING-ASSISTED BIAXIALLY TEXTURED SUBSTRATES, Dominic F. Lee, Oak Ridge National Laboratory, Metals & Ceramics Div, Oak Ridge, TN; Mariappan Paranthaman, Oak Ridge National Laboratory, Dept of Chemical & Analytical Science, Oak Ridge, TN; F. A. List, E. D. Specht, Oak Ridge National Laboratory, Solid State Div, Oak Ridge, TN; Amit Goyal, Oak Ridge National Laboratory, Metals & Ceramics Div, Oak Ridge, TN; P. M. Martin, Oak Ridge National Laboratory, Oak Ridge, TN; D. M. Kroeger, Oak Ridge National Laboratory, Solid State Div, Oak Ridge, TN.

Recent successes in high J YBCO deposited on biaxially textured substrates have generated renewed optimism that this HTS can be fabricated in conductor form. So far, HTS as well as the required buffer layers have largely been deposited by vacuum methods. In the continuing development of these next generation conductors, we have investigated the nonvacuum chemical deposition of HTS on Rolling Assisted Biaxially Textured Substrates (RABiTS) developed at ORNL. Substrates suitable for this study were obtained by rolling high purity nickel into tapes. After appropriate heat treatment procedures, the flexible Ni was found to be highly textured with both in-plane and out-of-plane textures of 6-8 FWHM. Selected buffer layers were deposited onto the textured metal by e-beam evaporation and sputtering resulting in RABiTS of various architectures. Acetylacetonate- and trifluoroacetate YBC precursors were then deposited onto the textured substrates by methods such as spray pyrolysis, spin coating and dip coating. In this paper, effects of processing parameters such as temperature profile and atmosphere on phase and texture development, film morphology, buffer integrity as well as superconducting characteristics will be presented.
* Managed by Lockheed Martin Energy Research Corp. under contract DE AC05-96OR22464 with the U.S. Department of Energy.

R3.17
BRIDGMAN METHOD TEXTURING OF YbBaCuO, Philip T. Putman, Kamel Salama, Univ of Houston, Houston, TX.

The Bridgman method has been used to produce large samples of YBaCuO (Y123) with good grain alignment and high J. However, there are two problems with the method. The first is the high processing temperature (about 1030 � C), which is higher than the 960 �C melting temperature of silver. The second problem is the slow rate (less than 1 mm/hr) of crystal growth. Recently, progress toward solving these problems has been made by using ytterbium and neodymium in place of yttrium. YbBaCuO (Yb123), with the addition of Yb211 and/or Ag, has a melting point below 960 �C. Thus it should be possible to use the Bridgman method to texture wires made by coating Yb123 onto Ag cores. Processing rates for Nd123 as high as 50 mm/hr have been achieved. Nd doping, although having the disadvantage of increasing the melting temperature, increases the rate at which Yb123 can be textured. We have studied the processing of monolithic bars (5 X 5 X 50mm) of Yb123 as well as bars of composition (YbNd)123 + (YbNd)211 in a vertical Bridgman furnace with an axial temperature gradient of about 80 �C/cm. The dependence of maximum processing rate on x was observed. Knowledge gained from these studies will be applied to the texturing of Yb123 coated onto Ag cores.
[1] K. Salama, A. S. Parikh, and L. Woolf, Appl. Phys. Lett. 68 (14) 1993 (1996)

R3.18
MAGNETIC PROPERTIES IN TOP-SEEDED MELT GROWN RE-Ba-Cu-O CRYSTALS, Seiki Takebayashi, Sang Im Yoo, Masato Murakami, ISTEC, Superconductivity Research Lab, Tokyo, JAPAN.

A top-seeded melt growth technique using a Sm-Ba-Cu-O seed is practical for fabricating a large c-axis-oriented Y-Ba-Cu-O single-grain required for systematic applications. For light rare earth superconductors, LRE-Ba-Cu-O (LRE: Nd, Sm), however, obtaining suitable seed crystals has been a serious problem because of their higher melting points. Our previous experiments revealed a MgO single crystal promoted nucleation of a LRE-Ba-Cu-O crystal under low oxygen partial pressure but the orientation of the melt-grown LRE-Ba-Cu-O domain was beyond control. For this problem, we here selected LRE-Ba-Cu-O-Ag composite systems for the top-seeded melt growth using a melt-grown Nd-Ba-Cu-O seed crystal. The Ag-dopant decreases the melting point of LRE123 phases by 20-30 degree, which allows a temperature gap for seeding. We also tried the top-seeded melt growth of the RE-Ba-Cu-O (RE: Y, Sm) crystals without silver. The field mapping of trapped flux was measured to verify a single grain crystal. We examined transition temperatures and hysteresis curves in the RE-Ba-Cu-O systems annealed at various temperatures in flowing oxygen using a SQUID magnetometer. Magnetic properties were improved by the oxygen annealing in lower temperatures. This work was partially supported by NEDO.

R3.19
STRUCTURE MODIFICATION OF SUPERCONDUCTOR YBaCuO BY LOW-ENERGY ION IRRADIATION, Irene V. Tereshko, V. I. Khodyrev, Mechanical Engineering Institute, Mogilev, BYELARUS; Q. Zhong, Univ of Houston, Texas Ctr for Superconductivity, Houston, TX; L. V. Pletnev, D. L. Vinogradov, Mechanical Engineering Institute, Mogilev, BYELARUS.

The main object of this paper is the investigation into the structure modification of high temperature superconductor (HTSC) YBaCuO after low-energy ion irradiation. The paper presents a computer simulation of oxygen, nitrogen and hydrogen ions interaction with the surface of HTSC (ions energy E < 5 keV). The standard computer program TRIM-92, as well as programs developed by the authors for evaluating ''Coulomb explosion'' and nonlinear oscillations in crystal lattices were used for the investigation. It was shown that electronic stopping of low-energy hydrogen ions was much higher than that of nuclear stopping. Most of oxygen and nitrogen ions energy was lost when they interacted with target atoms nuclei. The atoms of the crystal lattice demonstrated small displacements therein, which led to the development of nonlinear oscillations in the lattice. As a result, new structural states were formed in the lattice. The projected ranges of low-energy ions in superconductor YBaCuO were small. Therefore, it made possible for the ions to be accumulated near the surface layer. The 'Coulomb explosion' of the accumulated ions is supposed to occur. An additional surface amorphous layer is formed in HTSC.

R3.20
THE STRONG TO WEAK COUPLING ELECTROMAGNETIC TRANSITION IN THIN FILM [001] TILT YBaCuO BICRYSTALS, Nina F. Heinig, R. D. Redwing, A. Gurevich, J. E. Nordman, David C. Larbalestier, Univ of Wisconsin-Madison, Applied Superconductivity Ctr, Madison, WI.

Nanovolt sensitivity voltage-current characteristics of thin films YBa bicrystals with misorientation angles between 3 and 20 were measured in both zero field and in magnetic fields of up to 10 Tesla. A gradual transition from strongly to weakly coupled behavior was observed as the misorientation angle increased. The results of these experiments are discussed in terms of dislocation core overlap models, which predict the closure of strongly coupled superconducting channels in the grain boundary at a misorientation angle of about 6 at 77 K. The data do not follow the specific predictions of these simple models, indicating that additional grain boundary features need to e taken into account. These detailed studies of the transport properties across low angle YBa boundaries are relevant to optimizing the properties of the recently developed highly textured superconducting conductors on flexible metal substrates.

R3.21
MAGNETO-OPTICAL IMAGING OF CURRENT FLOW IN IBAD YBa THIN FILMS, Alex Pashitiski, A. Polyanskii, Univ of Wisconsin-Madison, Applied Superconductivity Center, Madison, WI; David C. Larbalestier, Univ of Wisconsin-Madison, Applied Superconductivity Ctr, Madison, WI; Stephen R. Foltyn, Paul N. Arendt, Los Alamos National Laboratory, Superconductivity Tech Ctr, Los Alamos, NM.

Magneto-optical (MO) imaging was used to investigate flux penetration and current flow in an YBCO film grown on an IBAD buffer layer on a polycrystalline Hastelloy substrate. The IBAD film (6x0.1mm and 1 m thick) had a nearly perfect c-axis texture and showed a substantial variation of transport critical current density along the bridge length. The J(75K,OT) values measured at 1 mV/Cm were 0.36, 0.28 and 0.48 MA/cm for sequential sections. Mo images of the film taken at 15 and 77 K, both in zero field cooled and field cooled regimes, displayed a non-uniform penetration of magnetic flux into the sample. To reconstruct current flow in the film we developed a new procedure based on the inversion of the Biot-Savart law. This method enable us to reconstruct the local sheet current distributions in such a inhomogeneous thin film superconductor and to locate the principal current limiting defects in the film. The reconstructed 2D current patterns demonstrate that current meanders along the IBAD sample. Local J values vary on the scale much larger than the grain size (<1 m) of YBCO and can be several times higher (>1.5 MA/cm) than transport measurements. By eliminating the principal defects present in the film, the current-carrying capability of IBAD films can be further improved.

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