Meetings & Events

Publishing Alliance

1997 MRS Spring Meeting & Exhibit

March 31 - April 4, 1997 | San Francisco
Meeting Chairs: Linda G. Griffith-Cima, David J. Eaglesham, Alexander H. King

Symposium R—Materials Issues Related to Development of Textured High-Temperature Superconducting Conductors

Chairs

Paul Grant
EPRI
3412 Hillview Ave
Palo Alto, CA 94303-0813
415-855-2234

Yasuhiro Iijima
Materials Research Lab
Fujikura, Ltd.
Tokyo, 135 Japan
81-3-5606-1066

D. Kroeger
Solid State Div
Oak Ridge National Laboratory
M/S 6116
Oak Ridge, TN 37831-6116

Martin Maley
Superconductivity Tech Ctr
Los Alamos National Laboratory
MS-K763/MST-STC
Los Alamos, NM 87545
505-665-0189

Symposium Support

*EPRI
*Los Alamos National Laboratory
*Oak Ridge National Laboratory
*Southwire Company
*University of Tennessee Space Institute

In sessions below "*" indicates an invited paper.

SESSION R1: BIAXIALLY TEXTURED YBCO CONDUCTORS
Chairs: D. M. Kroeger and Martin P. Maley
Tuesday Morning, April 1, 1997
Salon 15

8:30 AM *R1.1
DEVELOPMENT OF HIGH J Y-123 TAPES WITH TEXTURED BUFFER LAYERS BY IBAD METHOD, Yasuhiro Iijima, Fujikura, Ltd., Materials Research Lab, Tokyo, JAPAN; Mariko Hosaka, Nobuo Tanabe, Nobuyuki Sadakata, Takashi Saitoh, Osamu Kohno, Kaoru Takeda, Fujikura, Ltd., Tokyo, JAPAN.

Ion-Beam-Assisted Deposition (IBAD) is a method to introduce biaxially aligned structure on practical polycrystalline substrates, which is one of the effective ways to eliminate intergranular weak links of Y-123 tape shaped conductors. This paper presents our experimental results on development of IBAD process, transporting properties of bitextured Y-123 films, and long tape samples by continuous deposition. The angular dependence on the incident ion beam and this thickness dependence of the crystalline alignment of YSZ films agreed with a selective resputtering model based on ion-channeling. On the contrary, larger ion radius and lower ion energy were preferred in low temperature growth, which indicates homoepitaxial crystallization would be so sensitive to ion bombarding conditions. The properties of strongly coupled current paths in bitextured Y-123 films were evaluated by Jc measurements in applied field of B//I. The higher J samples near 1 MA/cm (77 K, TO) had more straight current paths not disturbed by weak links. 1 m-order tape shaped YBCO conductors with J of 2.0 x 10 A/cm were fabricated by continuous depositions. The efforts to enhance overall Jc values (thicker Y-123 films and thinner substrate tapes) would also be presented.

9:00 AM *R1.2
STATUS OF THE RABiTS APPROACH TO FABRICATE BIAXIALLY ALIGNED, HIGH-J SUPERCONDUCTORS, Amit Goyal, Oak Ridge National Laboratory, Metals & Ceramics Div, Oak Ridge, TN; David P. Norton, Oak Ridge National Laboratory, Oak Ridge, TN; David K. Christen, D. M. Kroeger, E. D. Specht, Oak Ridge National Laboratory, Solid State Div, Oak Ridge, TN; Mariappan Paranthaman, Oak Ridge National Laboratory, Dept of Chemical & Analytical Science, Oak Ridge, TN; John D. Budai, Qing He, F. A. List, Oak Ridge National Laboratory, Solid State Div, Oak Ridge, TN; Dominic F. Lee, Oak Ridge National Laboratory, Metals & Ceramics Div, Oak Ridge, TN; B. Saffian, Oak Ridge National Laboratory, Solid State Div, Oak Ridge, TN; J. Matthis, E. Hatfield, C. E. Clabunde, Oak Ridge National Laboratory, Oak Ridge, TN; Shara S. Shoup, Oak Ridge National Laboratory, Chemistry & Analytical Sciences Div, Oak Ridge, TN; P. M. Martin, Oak Ridge National Laboratory, Oak Ridge, TN.

Fabrication of epitaxial, high J, biaxially aligned YBCO thick films on Rolling-assisted-biaxially-textured-substrates (RABiTS) will be reviewed. The RABiTS technique utilizes standard thermomechanical processing to obtain long lengths of flexible, biaxially oriented substrates with smooth surfaces (rms50 nm). The strong biaxial texture of the metal (in-plane 6-7 FWHM) is conferred to the superconductor by deposition of intermediate metal and/or oxide layers which serve both as a chemical as well as a structural buffer. Conductors prepared using this process have critical current densities at zero applied field approaching 10 A/cm at 77 K. The field dependence of J for H//c is comparable to those of epitaxial films on single crystal substrates. The transport properties of these conductors is significantly superior to the best Bi-2223/Ag multifilamentary tapes and Ti-1223 single crystal films. Moreover, the J (8 T) at 40 K is comparable to the best NbSn at 4.2 K, 8 T. Progress made in addressing key issues in further development of this process for practical applications including 1) deposition on thinner metals and thinner buffer layers to enhance J, 2) fabrication of alternative textured substrates with reduced magnetism followed by deposition of epitaxial buffer layers, and 3) deposition of buffer layers and YBCO by faster deposition methods including sol-gel, e beam, sputtering and MOCVD, will be presented.

9:30 AM *R1.3
PROGRESS ON BIAXIALLY ALIGNED SUBSTRATES FOR YBCO COATED CONDUCTORS, Paul N. Arendt, Stephen R. Foltyn, Los Alamos National Laboratory, Superconductivity Tech Ctr, Los Alamos, NM; Paul C. Dowden, J. Randy Groves, Los Alamos National Laboratory, Materials Science & Technology Div, Los Alamos, NM.

We have used ion-assisted ion-beam deposition techniques to prepare biaxially aligned template films on nickel alloy substrates. These template films are then overcoated with intermediate buffer layer films using either pulsed laser deposition or magnetron sputter deposition techniques. The final YBCO film is then applied using pulsed laser deposition. There is a complex interaction of deposition parameters which affect the growing ion-assisted films. We will discuss optimization of several of these parameters, e.g., sputter gas species, sputter beam energy, incident vapor to substrate angle, assist ion-beam to substrate angle, etc. We will also discuss current efforts and progress toward reducing the deposition time needed to obtain good quality template films.

10:30 AM *R1.4
HIGH-CURRENT-DENSITY PLD-YBaCuO FILMS ON PLANAR AND CURVED SUBSTRATES: PROCESSING AND PERSPECTIVES, H. C. Freyhardt, J. Dzick, K. Heinemann, J. Hoffmann, A. Isaev, Univ of Goettingen, Inst fur Metallphysik, Goettingen, GERMANY; J. Wiesmann, Univ Goettingen, Inst fur Metallphysik, Geottingen, GERMANY; F. Garcia-Moreno, S. Sievers, A. Usoskin, Univ of Goettingen, Zentrum fur Funktionswerkstoffe, Goettingen, GERMANY.

YBaCuO tapes are considered to represent HTS conductors of the second generation. To achieve high critical current densities, YBaCuO has to be deposited on well biaxially aligned substrate surfaces. In this contribution, we report on recent results of the IBAD processing of bi-YSZ buffer layers on planar and cylindrical technical substrates. The bi-YSZ is highly textured with optimum FWHM , for thick planar buffer films. A sophisticated pulsed-laser-deposition (PLD) technique allows the preparation of YBaCuO films with maximum current densities, J, up to 2 MA/cm (77 K, self field) and with well reproducible J values, which are about a factor of 2 smaller. Technically relevant properties and the perspectives of these conductors, in particular for applications in power engineering, will be discussed.

11:00 AM *R1.5
IN-PLANE ALIGNED YBCO THIN FILM TAPE FABRICATED BY ALL PULSED LASER DEPOSITION, Katsuya Hasegawa, N. Yoshida, K. Fujino, H. Mukai, Sumitomo Electric Industries Ltd, Osaka Research Labs, Osaka, JAPAN; Kazuhiko Hayashi, Ken-ichi Sato, Sumitomo Electric Industries Ltd, Osaka Research Lab, Osaka, Japan; Shoichi Honjo, Tokyo Electric Power Co Inc, Power Engineering R&D, Yokohama Kanagawa, JAPAN; Y. Sato, Tokyo Electric Power Co Inc, Power Eng R&D Center, Yokohama, JAPAN; T. Ohkuma, Tokyo Electric Power Co Inc, Power Engr R&R Center, Yokohama, JAPAN; H. Ishii, Y. Iwata, Tokyo Electric Power Co Inc, Power Engr R&D Center, Yokohama, JAPAN; T. Hara, Tokyo Electric Power Co Inc, Power Engr R&R Center, Yokohama, JAPAN.

Much effort has recently been undertaken on the fabrication of YBCO thin film tapes with high J values for power application. It is necessary to develop a method to form in-plane alignment of YBCO with high deposition rate. We have tried to do this using pulsed laser deposition (PLD) which has an advantage of high deposition rate. We investigated deposition conditions of YSZ buffer layer on flexible Ni based alloy substrates using PLD. We have discovered that in-plane alignment of YSZ is possible by inclining the substrate to the plasma plume. Highly in-plane aligned YSZ films were realized with high deposition rate of 0.5 m/min. The YBCO films deposited on the YSZ were also highly in-plane aligned. Long and high Jc YBCO tapes were successfully fabricated. The J value of 1.5 x 10 A/cm (77.3 K, OT) was obtained over the whole length of 1 meter. The best J value of 4.3 x 10 A/cm (77.3 K, OT) was achieved for a short sample, and FWHM of YBCO(103) poles was 12.8. An overall critical current density including substrate was improved up to 4000 A/cm at 100 mm length. A magnetic shielding type fault current limiter using YBCO tape showed good current-limiting properties.

11:30 AM *R1.6
NEW SCHEME FOR CONTINUOUS DEPOSITION OF YBCO FILMS ON A 20CMx20CM SQUARE WITHOUT SAMPLE ROTATION, Helmut Kinder, Susanne Rieder-Zecha, Bernd Utz, Technische Univ Muenchen, Dept of Physics, Garching, GERMANY.

For the fabrication of YBCO coated conductors the production rate will be all important. It scales with the growth rate and the deposition area. The former is intrinsically limited to some tens of nm/min, so latter must be increased as much as possible. A square area can be profitably used for tape production by placing many tapes in parallel, or by arranging a single tape in loops. Other examples where square deposition areas are required are resistive fault current limiters on large technical substrates. In last years we were developing a deposition scheme for YBCO thin films on very large areas by thermal reactive co-evaporation. To allow for ballistic propagation of the metal atoms in a fairly good vacuum, and yet to achieve homogeneous oxidation conditions, we used a heater with a rotating sample holder moving the substrates in and out of an oxygen pocket during deposition. This rotary setup is not suitable for tapes or for large rectangular substrates. Therefore we have developed a new scheme where the substrate has a fixed position, and the oxygen pocket is set in linear motion. To achieve a continuous production process, we have also developed an in-situ refill mechanism for the thermal boats using vibrator conveyors, and a new method of rate control using compensated atomic absorption spectroscopy (AAS). First results will be reported.

SESSION R2: PROCESSING ISSUES FOR COATED CONDUCTORS
Chairs: Paul N. Arendt and Amit Goyal
Tuesday Afternoon, April 1, 1997
Salon 15

1:30 PM *R2.1
FABRICATION OF LONG Tl-1223 SUPERCONDUCTING TAPES ON BIAXIALLY TEXTURED SILVER SUBSTRATES, Toyotaka Yuasa, Hitachi Ltd, Ibaraki, JAPAN; Naomi Inoue-Sugiyama, Hiroyuki Akata, Kazutoshi Higashiyama, Hitachi Ltd, Hitachi Research Lab, Ibaraki, JAPAN; Kei Kimura, Super-GM, Dept of Engineering Research, Osaka, JAPAN.

In fabricating a Tl-1223 superconductor, crystal alignment is needed to obtain a high transport Jc. For this purpose, we developed biaxially textured silver substrates using a hot-rolling technique. Over 80 of grains of silver substrates were aligned in a <001> direction. The FWHM in the x-ray scan at = 54.7 was about 8 degrees. These substrates do not require to use of a buffer layer; thus, we were able to fabricate a Tl-1223 superconductor directly. We deposited precursor oxide (Ba/Sr-Ca-Cu-O) films on the substrate ( length: 15 mm) using the spray pyrolysis method. Then, the precursor films were fired in the controlled thallium oxide vapor atmosphere. Although a certain degree of the Tl-1223 grains were aligned biaxially, we observed 3 different orientational angles of 0, 26 and 45 in the film. These orientational angles agree well with theoretical values of Near Coincidence Site Lattices (NCSL). With biaxial crystal orientation, the Tl 1223 superconducting film showed Jcs of 100,000 A/cm at 77 K, OT and 12,000 A/cm at 77K, 1T. ln the fabrication of long tape, we developed a continuous fabrication process of Tl-1223 superconducting tape. First, we deposited the precursor film on the silver tape (40 ) using the continuous spray pyrolysis method. The 2 thick precursor film was fabricated on a 10 m length of silver tape traveling at the rate of 1.0 m/h. The precursor film had the stoichiometric composition of a Tl-1223 superconductor. Second, the tapes were continuously heat treated by passing them through a furnace in a controlled flow of Tl-oxide vapor to convert the precursor to a Tl-1223 superconductor. This work was carried out as a part of RD on superconducting technology for electric power apparatuses under the New Sunshine Project of AISI, MITI being consigned by NEDO.

2:00 PM *R2.2
RAPID THICK FILM YBCO GROWTH FOR WIRES AND TAPES BY PHOTO-ASSISTED MOCVD, A. Ignatiev, Q. Zhong, P. C. Chou, X. Zhang, Y. M. Chen, Univ of Houston, Texas Ctr for Superconductivity, Houston, TX.

The development of YBCO-based wires and tapes requires a technology for the formation of high quality superconducting YBCO thick films on top of metallic substrates. Metal organic chemical vapor deposition (MOCVD) has been shown in the past as a technique for the growth of YBCO; however, moderate film quality and slow growth rates have been detractors for the technology. A modification of the MOCVD technique, however, has been developed that overcomes these drawbacks: photo-assisted MOCVD. YBCO films have been grown in both horizontal and vertical quartz reactors where the only source of energy for the reaction has been photo irradiation from a bank of tungsten halogen lamps. YBCO films grown with this technology have shown T 91 K, and J> 1 x 10 A/cm. In addition, and just as critical, growth rates greater than 1 m/min have been realized with photo-assisted MOCVD. These growth rates support rapid thick film growth of YBCO, and films of up to 8 m have been grown with, however, a measured decrease of J with increasing thickness. This effect has been shown to be due to a lack of pinning centers in the thick films and has been rectified through the injection of point defects into the thick films by 1 MeV proton bombardment. An approach is underway to degrade the crystalline quality of the thick YBCO films during growth in an attempt to generate additional pinning centers. The high growth rates exhibited by photo-assisted MOCVD, however, fully support the commercial needs of the technology for high yield YBCO wire and tape fabrication. The application of the photo-assisted MOCVD technique to YBCO growth on metallic substrates has been initiated, and has shown preliminary positive results for the fabrication of YBCO on patterned silver and nickel substrates.

2:30 PM R2.3
A MOCVD APPROACH TO BIAXIALLY-TEXTURED THICK FILM Y-Ba-Cu-O SUPERCONDUCTOR FABRICATION ON METALLIC SUBSTRATES, Venkat Selvamanickam, Michael Walker, Pradeep Haldar, Intermagnetics General Corp, Latham, NY; Ana Ivanova, SUNY-Albany, Albany, NY; Alain E. Kaloyeros, SUNY-Albany, Dept of Physics, Albany, NY; David B. Fenner, Fenner Engineering Assocs, Simsbury, CT; Michael T. Lanagan, Argonne National Laboratory, Ceramic Section, Argonne, IL; Gregory Kozlowski, Wright State Univ, Dept of Physics, Dayton, OH; Charles E. Oberly, Air Force Wright Laboratory, WL/POOX, Dayton, OH; Siu-Wai Chan, Columbia Univ, Dept of Materials Science, New York, NY.

Metal Organic Chemical Vapor Deposition (MOCVD) has been successfully used to fabricate biaxially textured Y-Ba-Cu-O superconductors on metallic substrates. A high degree of biaxial texture was achieved in long lengths of metallic substrates by a metal working process. Metal and ceramic buffer layers were deposited on the textured substrates by a variety of techniques such as laser ablation, sputtering, e-beam evaporation, and Metal Organic Deposition (MOD). Biaxially-textured, stoichiometric, and dense YBCO films have boon deposited by MOCVD on the buffered metallic substrates. The growth process of buffer layers and the superconducting layer on the textured substrates has been examined by cross-sectional Transmission Electron Microscopy. Superconducting properties of YBCO thick films deposited under identical conditions on biaxially textured metal substrates and single crystal ceramic substrates have been compared. The in-plane texturing of metal substrates and YBCO films have been examined by Electron Back Scattered Diffraction. A continuous MOCVD reactor has been developed for long length deposition of biaxially-textured superconducting tapes on buffered metallic substrates. Our results show that MOCVD is a viable deposition approach for biaxially textured, superconducting thick film conductors.

3:15 PM *R2.4
USE OF A MODIFIED BIAS SPUTTERING TECHNIQUE TO FABRICATE IN-PLANE TEXTURED BUFFER LAYERS FOR YBCO FILM GROWTH, Masao Fukutomi, Nat Research Inst for Metals, Ibaraki, JAPAN; Masakazu Saitoh, Mitsuba Corp, Gumma, JAPAN; Kazunori Komori, Kazumasa Togano, Nat Research Inst for Metals, Ibaraki, JAPAN.

We have proposed a new bias sputtering technique to grow in-plane textured yttria-stabilized zirconia (YSZ) buffer layers on polycrystalline metallic and alumina substrates for deposition of YBaCu(YBCO) films. The principle of developing an in plane texturing by this technique is basically the same as that of ion beam assisted deposition (IBAD); an in-plane texturing occurs by off-normal ion beam bombardment because of the higher sputtering yields of all orientations other than the channeling direction. In our process, however, a flux of energetic particles impinging on the growing film is generated using specially devised dc and/or rf biased electrodes installed in a magnetron sputtering system instead of a separate ion-source in IBAD. So far, a x-ray phi-scan width of 17 (FWHM) was attained for about 0.5 m thick YSZ films on Hastelloy tapes. Epitaxial YBCO films grown on these buffer layers using pulsed laser deposition showed the Jcs exceeding 10 A/cm (77 K, OT). In this paper, we will also present variations of the bias sputtering technique to obtain the textured films on both sides of the tape substrates and on large area substrates. Although the proposed process offers a very convenient method to grow textured films, growth rates of the films must be increased ( currently 0.1 nm/s) for large scale application of YBCO films.

3:45 PM *R2.5
THE MECHANISM FOR TEXTURE DEVELOPMENT IN FILMS PREPARED BY ION BEAM ASSISTED DEPOSITION, Michael J. Cima, MIT, Dept of MS&E, Cambridge, MA; Kevin G Ressler, Neville Sonnenberg, MIT, Ceramics Processing Res Lab, Cambridge, MA.

The mechanism of biaxial alignment is examined for yttria-stabilized zirconia (YSZ) films deposited by ion beam assisted deposition (IBAD). (200) biaxial aligned films have been deposited at both high and low temperatures and using e-beam and ion beam as the deposition sources. Model experiments on different orientations or single crystal YSZ were undertaken to elucidate mechanism. Two temperature dependent IBAD growth modes are observed. Biaxial alignment does not occur due to ion channeling or due to different crystallographic orientations etching at different rates. IBAD biaxial alignment occurs so the damage tolerant planes face the assisting ion beam. The growth direction of misaligned grains changes in discrete units corresponding to the introduction of low angle grain boundaries and special boundaries in the crystal. Low angle grain boundaries are introduced to grains with larger amounts of ion induced damage. Thus, a growth direction change occurs with highest frequency for misaligned grains and with lowest frequency for grains with damage tolerant planes exposed to the ion beam.

4:15 PM R2.6
YBCO SUPERCONDUCTOR TEXTURE CONTROL USING ION-ASSISTED PULSED-LASER DEPOSITION OF INTERMEDIATE LAYERS, Ronald P. Reade, Paul Berdahl, Richard E. Russo, Lawrence Berkeley National Laboratory, Berkeley, CA.

The development of biaxially-textured YSZ intermediate layers for YBCO superconductor growth on difficult substrates (randomly-textured polycrystalline or amorphous materials) was introduced by Iijima et al. using ion-assisted sputtering and Reade et al. using ion-assisted pulsed-laser deposition more than five years ago. Since that time, significant improvements in the texturing have been demonstrated to provide YBCO films on flexible metal tapes with critical current densities greater than 1 MA/cm2. However, it is clear that the fundamental mechanisms of ion-assisted texturing are still not established. A better understanding is needed for purposes of process control and for improving critical parameters such as deposition rates, minimum intermediate layer thickness, etc. We will present recent studies of process parameters related to ion beam divergence and thus texture control, texturing as a function of layer thickness, and methods to induce greater texture control in thinner layers. Recent results for YBCO deposition on flexible metal substrates and YBCO multilayer structures for hybrid electronic devices will be discussed. We will summarize the advantages, particularly for multilayer and hybrid devices, and disadvantages of ion-assisted processing versus other methods for texturing.

4:30 PM R2.7
YBCO GROWTH ON TEXTURED BUFFER LAYERS PREPARED BY ION-BEAM-ASSISTED DEPOSITION, Connie Pin-Chin Wang, Stanford Univ, Stanford, CA; Khiem Do, Stanford Univ, Ginzton Lab, Stanford, CA; Malcolm R. Beasley, Theodore H. Geballe, Robert H. Hammond, Stanford Univ, Dept of Applied Physics, Stanford, CA.

Ion beam assisted deposition (IBAD) has been demonstrated to be potentially a good scale-up scheme for large area YBa applications. An in-plane textured buffer layer (MgO, YSZ, etc.) can be deposited on an amorphous (Si, SiO, etc.) or a polycrystalline metallic substrate. Consequently, YBCO growth can be in plane textured with high J. However, the contribution of ion beam to the microstructure evolution still remains to be understand. We have deposited biaxially textured MgO on Si and Ni substrates. We used RHEED to characterize the IBAD process in real time, and planar view and cross-section TEM, AFM and XRD to examine the substrate/buffer, buffer/ YBCO interface as a function of growth parameters, such as substrate type, substrate temperature, pressure, ion/atom flux ratio, film thickness, and ion incidence angle. In this paper, we will present the recent result and analysis of the ion induced-texture experiment. We will also discuss the potential candidates for the second buffer layer (NiO, CeO, metal or metal oxide) and the substrate, and report the quality of YBCO films attained using some of these buffer layers.

4:45 PM R2.8
LARGE-AREA DEPOSITION OF YBCO FILMS AND TEXTURED BUFFER LAYERS ON CERAMIC SUBSTRATES, Wolfgang Schmidt, H. -W. Neumuller, H. -K. Kramer, Siemens AG, Dept ZT EN4, Erlangen, GERMANY; Helmut Kinder, Technische Univ Muenchen, Dept of Physics, Garching, GERMANY; M. Bauer, R. Semerad, Technische Univ Muenchen, Garching, GERMANY; H. C. Freyhardt, K. Heinemann, Univ of Goettingen, Inst fur Metallphysik, Goettingen, GERMANY; J. Wiesmann, Univ Goettingen, Inst fur Metallphysik, Geottingen, GERMANY.

Thick films of YBCO have been prepared by thermal co-evaporation on polycrystalline zirconia for the development of a resistive fault current limiter. To achieve critical current densities above 10, buffer layers of biaxially textured cubic zirconia were prepared by IBAD using a dual-beam ion-gun sputtering system. Within a joint project a 100 kVA function model of a resistive limiter is planned, which requires a YBCO film thickness of about 2 m and a film area of about 400 cm. To achieve this requirement, an upscaling of the deposition processes to areas of 20x20 cm is performed. Due to the beam size of the ion-guns in the IBAD system, the homogeneous area is presently limited to sample sizes of 10x10 cm within which half-widths of the in-plane orientation of about 14 are routinely achieved in YSZ films with thicknesses of 0.9 m. The YBCO films deposited on those buffer layers with thermal co-evaporation have thicknesses of 0.7 m and 1.4 m, respectively, and critical current densities in the range of 0.5 . . . 1x10. Meander shaped strip lines were etched from the 10x10 cm samples with a width of 1 cm carrying critical currents of up to 90 over a length of about 40 cm.

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 YBaCuO (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 YBaCuO 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 YBaCuO (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 YBaCuO ADDITION ON THE CRITICAL CURRENT DENSITY, FLUX PINNING AND TWINNING IN MELT-TEXTURED YBaCuO, 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 YBaCuO (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 YBaCuO on a polycrystalline samples surface YBaCuO and YCuO placed in BaCuO and CuO melt at temperatures 960-1040C was investigated. It was shown, that only a YBaCuO layer is formed on a YBaCuO sample at cooling of the melt with a rate 1C/hour in the all temperature interval, whereas the one-phase product on YCuO is formed only at cooling of the melt from 1000C. The formation of a ceramic textured layer YBaCuO was observed alongside with a YBaCuO phase in a more high-temperature zone. No transition of yttrium in the melt lower 1020C took place, and no more than 1 yttrium quantity located near to solid phases YBaCuO and YCuO in a temperature interval 1020-1040C in the melt was found. Optimal conditions of textured YBaCuO ceramics formation on the dense YBaCuO samples were determined. They are depending on the melt composition, initial crystallization temperature and the cooling rate. The rate of textured YBaCuO 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 YBaCuO 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 YBaCuO (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/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 YO 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.

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.

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.

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 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.

SESSION R4: CRITICAL CURRENTS, PINNING AND GRAIN BOUNDARIES
Chairs: Paul Grant and Yasuhiro Iijima
Wednesday Morning, April 2, 1997
Salon 15

8:30 AM *R4.1
IMPLICATIONS OF d-WAVE SYMMETRY FOR GRAIN BOUNDARY CRITICAL CURRENTS, Jochen Mannhart, H. Hilgenkamp, Univ of Augsburg, Dept of Experimentalphysik VI, Augsburg, GERMANY.

Various experiments have shown that the order parameter in most high T cuprates has a predominant -symmetry component. In the presentation, the consequences of the -wave symmetry on the transport properties of grain boundaries in high T superconductors will be analyzed. It will be shown that a boundary region with a depressed order parameter is at grain boundaries in high T superconductors, so that for fundamental reasons the boundary is a weak link or a Josephson junction with properties controlled by the misalignment angle.

9:00 AM *R4.2
"HARD" AND "EASY" DIRECTIONS OF VORTEX MOTION THROUGH SPLAYED COLUMNAR DEFECTS, Lia Krusin-Elbaum, D. Lopez, A. D. Marwick, J. Z. Sun, H. Safar, V. M. Vinokur, IBM T.J. Watson Research Ctr, Yorktown Heights, NY.

One of the principal factors limiting current conduction in high-T superconductors is easy motion of magnetic vortices in the bulk material. The most efficient way to pin vortices is with columnar defect structure, which can be installed either by irradiation with swift (1 GeV) heavy ions (e.g., Au) or by fission of heavy nuclei (such as Bi) within a superconductor by energetic (0.8 GeV) protons. The former results in a nearly aligned columnar tracks of amorphous damage, while the latter produces splayed tracks of similar diameter. We have shown that uniformly splayed fission tracks boost current conduction by orders of magnitude and enlarge the useful field-temperature range of highly anisotropic Bi- and Hg-based cuprates. In a less anisotropic YBCO, there is a splay angle (10) for which the performance is optimal. In this talk we discuss results of the experiment designed to probe the anisotropy of vortex motion through a forest of splayed columnar pins. C axis oriented epitaxial YBCO films, with two crisscrossing families of parallel columnar tracks, were patterned in a geometry allowing control of the relative orientation between the external current and the splay planes in the same sample. This permits us to establish ''hard'' and ''easy'' directions of vortex motion. Our results indicate that vortex motion is favored in the direction normal to the splay plane, in contradiction with present theoretical models. The dynamics of vortices in the presence of small-angle splay, tilt, and isotropic splay will be compared.

9:30 AM *R4.3
EDGE DISLOCATIONS AND CRITICAL CURRENT DENSITY IN EPITAXIAL YBaCu0, Vladimir M. Pan, V. L. Svetchnikov, Inst for Metal Physics, Kiev, UKRAINE; A. I. Kasatkin, Inst for Metal Physics, Superconductivity, Kiev, UKRAINE; H. C. Freyhardt, Univ of Goettingen, Inst fur Metallphysik, Goettingen, GERMANY.

The most effective pinning is ensured by linear defect like a nonsuperconducting core of edge dislocation with cross-size 2 if dislocation line is parallel to vortex and dislocation density equals or exceeds matching field. Indeed, the highest j ( A/cm at 77 K) among any high-T superconducting materials is observed in the highly biaxially-oriented YBaCuO (YBCO) thin films, where dense edge dislocation structure (up to 2 10 cm) revealed by TEM study. TEM of YBCO films (2 15 nm thick, prepared by magnetron sputtering or PLD on MGO and SrTiO substrates) is done on plane-view specimens and demonstrated very high-density of edge dislocations having their lines perpendicular to substrate. Due to a use of Moire pattern technique, the images of a single dislocation and edge dislocation arrays are clearly observed. The power spectrum (optical diffraction pattern) over the Moire fringes picture is exhibited two reflections due to regularity in the fringes sequence; the angular splitting of reflections (about 15) being recalculated to real misorientation angle ( ) confirms a presence of low-angle tilt boundaries (LAB) in YBCO films. Rows (''walls'') of edge dislocations arranged in domain sub-boundaries are responsible for the azimuthal ' 'mosaic''spread in YBCO film which is detectable by X-ray diffraction analysis. A consistent model of vortex depinning from linear defect and its motion along the equidistant array of parallel dislocation lines in the LAB is developed to evaluate j and its dependence upon the misorientation of adjacent domains. It is taking into account (for the thin film case) the surface potential interaction with a depinned fraction of vortex which is deviated from a linear defect along the surface under the Lorentz force action. The critical current should be observed when this depinned vortex ''tongue'' aligned to a surface reaches the adjacent dislocation with a current increase. In the framework of this model it is easy to explain quantitatively the dependence of j on the misorientation angle of the adjacent domains in -plane which is the intrinsic feature of vortex behavior in such a LAB, assuming nothing as to the misorientation angle effect on its quasiparticle transparency and Josephson properties.

10:30 AM *R4.4
SUPERCONDUCTING PROPERTIES OF HIGH-J, BIAXIALLY ALIGNED YBaCuO THICK FILMS ON METALLIC TAPE SUBSTRATES, David K. Christen, David P. Norton, Oak Ridge National Laboratory, Solid State Div, Oak Ridge, TN; Amit Goyal, Oak Ridge National Laboratory, Metals & Ceramics Div, Oak Ridge, TN; John D. Budai, Oak Ridge National Laboratory, Solid State Div, Oak Ridge, TN; Qing He, Univ of Tennessee, Knoxville, TN; C. E. Klabunde, D. M. Kroeger, Oak Ridge National Laboratory, Solid State Div, Oak Ridge, TN ; Dominic F. Lee, Oak Ridge National Laboratory, Metals & Ceramics Div, Oak Ridge, TN; F. A. List, Oak Ridge National Laboratory, Solid State Div, Oak Ridge, TN; Mariappan Paranthaman, Oak Ridge National Laboratory, Dept of Chemical & Analytical Science, Oak Ridge, TN; B. Saffian, Univ of Tennessee, Knoxville, TN; E. D. Specht, Matthew F. Chisholm, Oak Ridge National Laboratory, Solid State Div, Oak Ridge, TN.

Thick films of biaxially-aligned YBCO have been deposited on short segments of sharply textured nickel tapes produced by thermomechanical processing. The nickel tapes are coated with appropriate epitaxial oxide buffer layers using vapor deposition techniques, including evaporation, sputtering, and pulsed laser ablation. To date, bilayer buffers of CeO and YSZ have served to provide both chemical isolation from the nickel and conferred biaxial texture in YBCO films grown to thicknesses of 1 to 3 m. X-ray diffraction shows typical in-plane textures of 7 10 FWHM, with -perpendicular alignment to 1 FWHM. The resulting films typically have T(R = 0) 88 K, with zero-field J values in the range 5-7 x 10 A/cm at 77 K. In magnetic fields, J(Hc) and irreversibility lines are comparable to, or exceed, those of epitaxial films on single crystal oxides. Comparative measurements confirm the strongly coupled nature of electrical transport, and reveal properties that are characteristic of strong pinning by extended defects. Atomic resolution, Z-contrast TEM shows that the YBCO low-angle grain boundaries are free of impurities, with periodicity in the disorder along the boundary.

11:00 AM R4.5
CRITICAL CURRENTS AND FLUX PINNING IN BIAXIALLY TEXTURED YBCO THICK FILM CONDUCTORS, Jeffrey O. Willis, Martin P. Maley, Los Alamos National Laboratory, Los Alamos, NM; J. Yates Coulter, Paul N. Arendt, Stephen R. Foltyn, Los Alamos National Laboratory, Superconductivity Tech Ctr, Los Alamos, NM; Hugo F. Safar, Los Alamos National Laboratory, Superconductivity Technology Center, Los Alamos, NM.

Recently, we have produced high J, biaxially textured YBCO thick films by means of the Ion-Beam-Assisted-Deposition technology. Critical current densities, Js, in excess of 1.0 MA/cm at 75 K can be achieved in film thicknesses in excess of 2.0 microns and remain above 0.1 MA/cm in magnetic fields above 4.0 T. The dependence of J on magnetic field angle shows a prominent peak with the field aligned with the c-axis and other evidence for strong pinning defects extending through the film thickness along the film normal. Patterned bridge studies show strong in-plane anisotropy of J correlated with current flow along or diagonal to the prevailing twin-plane mosaic orientation, and indicating a substantial contribution from twin boundary pinning. We report results of critical current measurements as a function of temperature magnetic field and field orientation on films of various thickness and discuss pinning mechanisms.

11:15 AM R4.6
TEM MICROSTRUCTURAL ANALYSIS OF YBaCuO (001) FACETED ASYMMETRIC AND SYMMETRIC BOUNDARIES GROWN USING LASER ABLATION TECHNIQUE AND CORRELATION WITH ITS ELECTRICAL PROPERTIES, Jack W.H. Tsai, Columbia Univ, Dept of Materials Science, New York, NY; Steven C. Tidrow, U.S. Army Research Laboratory, Dept of Physical Sciences, Ft. Monmouth, NJ; Siu-Wai Chan, Columbia Univ, Dept of Materials Science, New York, NY.

The studies of the various grain boundary structures in the high T superconductor has long been an interest both from understanding the fundamental properties of superconductivity and electron devices application perspective. Given the short coherence length of its order parameter for the HTS materials, the grain boundary behaves as SNS junction in which the electrons tunnel across the weak links as quasiparticles. Recent findings attributing the depression of J across the boundary is due to combination boundary faceting and the wave symmetry of the pairing state [1]. It was also observed that a macroscopically straight boundary plane can contain many microscopic facets [2]. The faceted boundary plane can be thermodynamic stable if the energy sum of the individual facet planes is less than the energy of the straight boundary plane. Here, we study the faceting mechanism of [001] tilt boundaries of the YBa bicrystal thin film grown on the CeO bicrystal templates using the laser ablation technique. We use the Transmission Electron Microscope to study the faceted plane and correlate the boundary structure to its electrical properties.

11:30 AM R4.7
GENERALIZED RANDOM DISLOCATION THEORY OF INTERGRAIN BOUNDARY CRITICAL CURRENT, Evgenii Z. Meilikhov, R. M. Farzetdinova, RRC Kurchatov Inst, Moscow, RUSSIA.

Dislocation model of the weak link at the boundary between two superconducting grains is based on the assumption of order parameter suppression in the regions subjected to strong stresses. It was shown [1] that a critical current theory based on calculating the stresses for a system of periodical edge dislocations (have been considered up to not) is not consistent with experiments. The theory taking into account randomization of dislocations at the intergrain boundary (e.g., due to thermal fluctuations or incommensurability of dislocation period with the lattice constant) is developed. At small misorientation angles produced stress field is weakly sensitive to random displacements of dislocations. With the rise of the angle, the mutual compensation of individual dislocations' stresses ceases that results in the fast lowering of the intergrain critical current ic. Corresponding angular dependence for 5 may be approximated as follows: 1/sin (/2).
We conclude that the adequate description of intergrain boundary superconductive features could only be obtained in the framework of the randomly distributed dislocations model.

11:45 AM R4.8
HRTEM INVESTIGATIONS OF THE ATOMIC STRUCTURE OF INTERPHASE GRAIN BOUNDARIES IN Bi-Sr-Ca-Cu-O-TYPE CERAMICS, N. D. Zakharov, D. Hesse, Max-Planck-Inst, Dept of Microstructure Physics, Halle/Saale, GERMANY; H. Frank, R. Stollmann, G. Guntherodt, RWTH Aachen, Aachen, GERMANY.

The atomic structure of grain boundaries between different BSCCO-type phases in BSCCO ceramics is investigated by high-resolution transmission electron microscopy (HRTEM). Interphase boundaries between BiSrCaCuO (2212) and BiSrCuO (2201) frequently occur. An atomic model of the 13.5 low-angle 2212/2201 tilt boundary is proposed. The interphase boundary consists of an array of edge dislocations which, as a rule, are separated by three or four planes of type a/2(100). Remarkably, the Bi layers cross the grain boundary almost without any disturbances, whereas each central Cu layer of the 2201 phase splits into two within the 2212 phase. Low-angle grain boundaries between 2212 and 1212 phases are also frequently observed. An atomic model of the 2212/1212 low-angle grain boundary is proposed. It is formed by inserting an extra Bi plane into the 1212 phase. This procedure is equivalent to the introduction of a partial dislocation at the end of each extra Bi plane with a Burgers vector IbI 0.27 nm and the formation of an extended stacking fault in the Ca plane between these partial dislocations. Single lamellae of the 2212 phase incorporated into the 1212 matrix also occur. They obviously form by the insertion of an extra Bi-O plane into the 1212 matrix.

SESSION R5: PROCESSING OF DEPOSITS, BSCCO, AND NEW MATERIALS
Chairs: David K. Christen and Dean E. Peterson
Wednesday Afternoon, April 2, 1997
Salon 15

1:30 PM R5.1
HIGH J Ag/Bi-2223 COMPOSITE CONDUCTORS: THE IMPORTANCE OF REACTION-INDUCED TEXTURE, Nazarali Merchant, Argonne National Laboratory, Chemical Tech Division, Argonne, IL; Stephen E. Dorris, Argonne National Laboratory, Dept of Energy Technology, Argonne, IL; Victor A. Maroni, Albert K. Fischer, Argonne National Laboratory, Chemical Technology Div, Argonne, IL.

It is well known that strong preferential c-axis texturing of Bi-2223 grains in Ag-clad Bi-2223 composite conductors plays a dominant role in the attainment of high J at 77 K and zero field. The mechanisms producing this texture are still not fully elucidated. Some researchers claim that deformation induced texture (DIT) is a key contributor, while others have shown that reaction induced texture (RIT) is the dominant mechanism. In this paper we intend to present an assessment of existing literature on this subject which suggests that both mechanisms may be important in obtaining good texture. Our results indicate that the dominant mechanism of texturing is also a function of the initial phase assemblage in the powder that goes into the silver tube before deformation processing. Tapes that exhibit appreciable RIT usually have the highest J's because the liquid phase generated during heat treatment enhances grain growth and heals cracks that form during deformation processing.

1:45 PM R5.2
TEXTURE AND PHASE DEVELOPMENT IN Ag-SHEATHED (Bi,Pb)SrCaCuO TAPES, Yuan L. Wang, Lijun Wu, Yimei Zhu, M. Suonaga, Brookhaven National Laboratory, Dept of Applied Science, Upton, NY; T. R. Thurston, Brookhaven National Laboratory, Dept of Physics, Upton, NY.

Texture and phase development for Bi(2212) to Bi(2223) conversion in powder-in-tube processed Bi(2223) tapes were investigated utilizing in situ transmission x-ray diffraction and ex-situ TEM techniques. It was found that Bi(2212) begins to develop texture at temperatures as low as 750C. Bi(2212) developed texture rapidly for several hours, accompanied by rapid grain growth, upon raising the temperature to typical processing temperatures, and then slowed down dramatically, with much of the texture developing before the formation of Bi(2223). The degree of texture attained after long heating times was found to be temperature dependent, with higher temperatures generally producing better texture but not necessarily better transport properties. Briefly heating a sample near the Bi(2212) melting point can dramatically increase the texture, but such heating usually results in less Bi(2223) content. Finally, cooling rate was found to be related to the phase content and transport properties of a sample. These results and their implications for improving processing conditions are discussed.

2:00 PM R5.3
THE USE OF OXYGEN GRADIENTS AS A TEXTURING MECHANISM DURING ISOTHERMAL MELT PROCESSING OF BI-2212 SUPERCONDUCTORS., Terry G. Holesinger, John F. Bingert, Los Alamos National Laboratory, MST-6, Los Alamos, NM; J. Yates Coulter, Los Alamos National Laboratory, Superconductivity Tech Ctr, Los Alamos, NM; Jeffrey O. Willis, Los Alamos National Laboratory, MST-STC, Los Alamos, NM.

Isothermal melt processing (IMP) has shown itself to be a successful technique for processing Bi-2212 superconductors with high critical current densities. One of the interesting aspects of IMP is the oxygen gradient that occurs within the sample during the oxidation step. It is shown that this oxygen gradient can be used as a texturing mechanism to form uniaxial texture in large scale Bi-2212 conductors. In terms of bulk Bi-2212 conductors, this has lead to improved self-field critical current values and improved performance in applied fields. With the appropriate conductor design, the oxygen gradient can be used to help texture oxide powder-in-tube (OPIT) conductors.

2:15 PM R5.4
OXYGEN EXCHANGE IN Ag-SHEATHED Bi-2223 TAPES, Mario Quilitz, Wilfried Goldacker, Forschungszentrum Karlsruhe, Inst fur Technische Physik, Karlsruhe, GERMANY.

The exchange of oxygen in the Bi-2223 phase has a great influence on the conditions of phase formation and on the superconducting properties of the high temperature superconductors. It is well known that the oxygen stoichiometry has a distinct effect on the detect chemistry and therefore via the defect electron concentration on the electrical properties of the 2223 phase. It is as well known that the oxygen stoichiometry has a big influence on the kinetics of HTSc. The effect of oxygen exchange on the phase formation and the critical currents in Ag-sheathed Bi-2223 tapes was investigated via thermogravimetric analysis. The study of the oxygen uptake and the release were performed via a chemical relaxation process during which the change in weight was monitored with time after a fast change in atmosphere. For both pore Ag and a Ag-alloy with 2 Mg the oxygen exchange was studied at various temperatures. The oxygen uptake is different with regard to whether the oxygen exchanges through the open ends of the tape where the superconductor is in direct contact with the surrounding gas-phase or whether the exchange takes place via the lateral faces of the tape which means through the sheath material. Methods for a separated analysis of both processes are discussed. The different exchange properties with different sheath material are discussed as well as its effect on the phase formation and the final critical currents in the tapes.

2:30 PM R5.5
STRUCTURE AND PROPERTIES OF [001] TWIST BOUNDARIES IN Bi/2212 BICRYSTALS, Yimei Zhu, Qiang Li, Y. N. Tsay, Masaki Suenaga, Brookhaven National Laboratory, Dept of Applied Science, Upton, NY.

[001] twist boundaries account for80 of total grain boundaries in c-axis aligned polycrystalline Bi/2212 tapes, which are one of the most promising high temperature superconductors for large scale applications to date. Systematic studies of these important [001] twist boundaries were fabricated with a wide range of twist angles (0-180). The resistance and current-voltage characteristics were measured simultaneously at various temperatures and magnetic fields. The boundary structures were then characterized with TEM, using high-resolution imaging and image simulation incorporating with charge distribution, nano-probe electron energy-loss spectroscopy and energy dispersive x-ray spectroscopy, and strain-field analysis. Contrary to the conventional wisdom, all these twist boundaries, regardless of their twist angle, carried the same critical current as their constituent single crystals at magnetic field up to 9 tesla at low temperatures. The absence of the weak-link behavior of the [001] twist boundaries can be attributed to the variation of elastic properties of the (001) layers (the soft-bonding of the BiO bilayer at the boundaries and the rigid interlayer-bonding of the CuO layers across the boundaries), to the local Bi/2223 structure, and to stoichiometric chemical composition including oxygen/hole concentration of the grain boundaries. Our recent work on the structure and properties of various tilt boundaries in Bi/2212 bicrystals will also be presented.

2:45 PM R5.6
SIGNIFICANCE OF COOLING RATE ON MICROSTRUCTURE AND TRANSPORT PROPERTIES OF Ag-SHEATHED BSCCO(2223) TAPES, N. Vasanthamohan, J. P. Singh, Argonne National Laboratory, Dept of Energy Technology, Argonne, IL.

To maximize the current-carrying capacity of Ag-sheathed Bi superconducting tapes, a two step cooling procedure was developed. Monolithic and wire-in-tube tapes at 825C in 8 O (balance N) in two 50-h cycles, for a total of 100 h, with one intermediate pressing at 1 GPa for 5 s. Subsequently, the tapes were cooled by the two-step cooling procedure. The first cooling step was used to control the solidification of the partial melt to produce a dense microstructure with large grains, while the second cooling step maintained phase purity. The results indicate a pronounced effect of cooling rate on 2223 content during the first 50 h of sintering. At longer sintering times, the effect of cooling rate on 2223 content was minimal. On the other hand, the effect of cooling rate on critical current (I) is quite pronounced even after 100 h of sintering. This strong influence of cooling rate on I is attributed to microstructural changes that occur during cooling.

3:30 PM R5.7
ANNEALING EFFECTS AND THERMOSTABILITY OF Ba-Ca-Cu-O COMPOUNDS, Yong Cao, Univ of Houston, TCSUH, Houston, TX; Ching-Wu Chu, Univ of Houston, Dept of Physics, Houston, TX; Z. L. Du, Yanyi Sum, Li Gao, Univ of Houston, TCSUH, Houston, TX; Yuyi Xue, Univ of Houston, Dept of Physics, Houston, TX.

The high-pressure synthesized new 126K superconducting phase in Ba-Ca Cu-O is successfully identified. Although the superconducting transition temperature T of (Cu,C)-1234 is about 117K, and that of (Cu,C) 1223 can be raised from 67K to 120K, which is not far away from that of the new superconducting phase, their annealing effects and thermostability are quite different from the new phase. For example, after annealed at 300C in Ar, the T of the 126K-phase can be suppressed to 80K, while the T of (Cu,C)-1223 increases to 70K and that of (Cu,C)1234 barely changes. T variations are systematically studied under different thermo-equilibrium conditions. While no significant decomposition was observed in (Cu,C)-1223 anal (Cu,C)-I234 when annealing temperature is lower than 600C, the new superconducting phase is not even stable at room temperature, especially with moisture. It can decay into another superconducting phase with T 90K, while significant change in x-ray diffraction pattern appears at the same time.

3:45 PM R5.8
RHENIUM INFLUENCE ON THE SYNTHESIS OF SUPERCONDUCTING HgReBaCaCuO CERAMICS AND THICK FILMS, S. Pinol, CSIC, Inst de Ciencia de Matls de Barcelona, Bellaterra, SPAIN; Agustin Sin, Albert Calleja, CSIC, Inst de Ciencia de Materials de Barcelona, Bellaterra, SPAIN; Xavier Obradors, CSIC, Inst de Ciencia de Matls de Barcelona, Bellaterra, SPAIN; Josep Fontcuberta, CSIC, Inst de Ciencia de Materials de Barcelona, Bellaterra, SPAIN; Ferran Espiell, Univ de Barcelona, Dept of Engr Quinica i Metallurgia, Barcelona, SPAIN.

We have prepared high quality ceramics of the superconducting phase HgReBaCaCuO with a critical temperature onset of 138 K and zero resistance at 130 K. The superconductors were prepared using precursor powders of BaCaCuO mixed with stoichiometric additions of ReO2 and HgO. We have utilized these ceramics for the preparation of thick film superconductors by the vacuum-sealed quartz technique. Ceramic pellets were obtained by solid state reaction of the precursors inside a quartz tube at 900C. We have studied the influence of different thermal treatments in the kinetics of the reaction. Different Re compositions were explored in order to optimize the purity of the 1223 phase. Only a very narrow Re composition was found to be appropriate to obtain single phases. Cell parameters and composition were characterized using a SQUID magnetometer, and AC susceptometer and electrical resistivity. We have studied the vapor-solid reaction of the Hg-vapors with a ReBaCaCuO thick films precursor (m thickness) deposited by spray techniques on different metallic and ceramic substrates (Cr, AlO, SrTiO). We have placed the samples in a sealed quartz tube together with stoichiometric Hg-Re-Ba Ca-Cu-O pellets in order to maintain an appropriate Hg-vapor pressure. Then the tubes were introduced rapidly into a preheated furnace at 900C for a few minutes and quickly cooled at room temperature by quenching in air. Superconducting Hg-1223 grains with c-axis oriented perpendicular to the substrate were identified by Scanning Electron Microscopy and EDX analysis. Homogeneity and composition of the thick film grains depend strongly on the time thermal treatment.

4:00 PM R5.9
DEVELOPMENT OF HIGH-CURRENT YBaCuO COATED CONDUCTORS ON ROLLED-Ni SUBSTRATES, Mariappan Paranthaman, Oak Ridge National Laboratory, Dept of Chemical & Analytical Science, Oak Ridge, TN; F. A. List, Oak Ridge National Laboratory, Solid State Div, Oak Ridge, TN; David P. Norton, Oak Ridge National Laboratory, Oak Ridge, TN; Amit Goyal, Oak Ridge National Laboratory, Metals & Ceramics Div, Oak Ridge, TN; E. D. Specht, David K. Christen, D. M. Kroeger, John D. Budai, Qing He, Oak Ridge National Laboratory, Solid State Div, Oak Ridge, TN; Dominic F. Lee, Oak Ridge National Laboratory, Metals & Ceramics Div, Oak Ridge, TN; P. M. Martin, Oak Ridge National Laboratory, Oak Ridge, TN.

High Current YBa (referred to as YBCO) Coated Conductors were fabricated with a layer sequence of YBCO/YSZ/CeO/Ni. The cube (100) texture in the starting Ni substrates was obtained by cold-rolling followed by recrystallization. The CeO and YSZ films were grown epitaxially on the textured Ni substrates using electron beam evaporation and rf magnetron sputtering techniques. The YBCO films were then grown by pulsed laser deposition. The biaxial texture in the YBCO layer is over 90. A high transport critical current density of 7.8 x 105 /cm at 77 K was obtained on 0.7 m thick YBCO films in zero field. The effect of buffer layer thicknesses will be discussed. We also recently demonstrated that over 6 cm long buffered substrates with good biaxial texture can be obtained using the electron beam evaporation technique. Research was sponsored by the Division of Materials Sciences, Office of Basic Energy Sciences and Office of Energy Efficiency and Renewable Energy, Office of Utility Technology-Superconductivity Program, performed at ORNL, managed by Lockheed Martin Energy Research Corporation for the U.S. Department of Energy under contract DE-AC05-96OR22464.

4:15 PM R5.10
R.F. SPUTTERING OF BIAXIALLY-TEXTURED YSZ BUFFER LAYER ON ANGULAR ORIENTED METALLIC SUBSTRATE, Gregory Kozlowski, Cary Farlow, Jason Hundley, Wright State Univ, Dept of Physics, Dayton, OH; C. Varanasi, Univ of Dayton, Research Inst, Dayton, OH; Iman Maartense, Rama Nekkanti, E. K. Moser, Rand Biggers, Air Force Wright Laboratory, WL/MLPO, Wright Patterson AFB, OH; Charles E. Oberly, Air Force Wright Laboratory, WL/POOX, Dayton, OH; Tim L. Peterson, Air Force Wright Laboratory, Wright Patterson AFB, OH; Venkat Selvamanickam, Intermagnetics General Corp, Latham, NY; Ana Ivanova, SUNY-Albany, Albany, NY; Alain E. Kaloyeros, SUNY-Albany, Dept of Physics, Albany, NY.

Biaxially-textured YSZ buffer layers were fabricated by a modified RF sputtering technique on Hastelloy C276 substrates. Our YSZ films had the strongest (200) preferred orientation when the films were grown at an RF power of 300 W and an Ar-2O pressure of 10 Torr while they were tilted at approximately 70 from the horizontal plane and at a target-to-substrate distance of approximately 2 cm. Substrates of Hastelloy C276 were polished to a mirror finish using aluminum oxide lapping powders. To characterize the in-plane texturing of the YSZ buffer layers, x-ray polo figure measurements using the Schultz reflection method were carried out. The full width at the half-maximum (FWHM) of the YSZ (111) peak, as a measure of the alignment of the YSZ grains, wa about 25 at best. Next, YBaCuO (YBCO) films deposited on YSZ/Hastelloy C276 by laser ablation or MOCVD technique. The x-ray rocking curve FWHM for the YBCO (013) peak was 13 on a YSZ buffer layer which was considerably less well textured than the best result noted above. The superconducting properties of the YBCO films will be discussed as a function of sputtering parameters.

4:30 PM R5.11
LARGE AREA DEPOSITION OF YBCO-FILMS BY PULSED-LASER DEPOSITON ON VARIOUS SUBSTRATES, Bernd Schey, Wolfgang Biegel, Gebbhard Ostreicher, Marcus Kuhn, Univ Augsburg, Dept of Physics, Augsburg, GERMANY; Bernd Stritzker, Univ Augsburg, Inst of Physics, Augsburg, GERMANY.

Fault Current Limiters (FCL) on the basis of HTS thin film material require large areas of YBaCuO (YBCO) films with homogeneous superconducting properties. Pulsed Laser Deposition (PLD) is used to deposit YBCO on two different kinds of substrates (CeO-buffered sapphire and biaxial textured YSZ buffer on polycrystalline YSZ) with a size up to 7 x 20 cm. YBCO films with constant thickness are produced by a special PLD setup with a 8 cm line-focus on a cylindrical target and substrate scanning perpendicular to it. The spatial variation of the critical current density j (77 K) and the transition temperature T over the area of 7 x 20 cm was investigated by an inductive method. Small samples (1 x 1 cm) were used to study the influence of activated oxygen provided by an ion source during YBCO deposition.

4:45 PM R5.12
YBaCuO COATED CONDUCTORS BY LIQUID PHASE EPITAXY USING BaF-ADDED OXIDE SOLUTION, Yasuji Yamada, ISTEC, Superconductivity Research Lab, Nagoya, JAPAN; Junichi Kawashima, Yusuke Niiori, Izumi Hirabayashi, Shoji Tanaka, ISTEC, Nagoya, JAPAN.

YBCO-coated conductors on yttria stabilized zirconia (YSZ) and SrTiO (STO) single crystalline fibers of 0.4 mm in diameter have been formed by low temperature liquid phase epitaxy using BaF added oxide flux.

SESSION R6: POSTER SESSION: BSCCO AND TBCCO CONDUCTORS AND BULK
Chair: Dominic F. Lee
Wednesday Evening, April 2, 1997
8:00 P.M.
Salon 7

R6.1
THE INFLUENCE OF CATION STOICHIOMETRY VARIATION ON THE PHASE FORMATION AND THE CRITICAL CURRENTS IN Ag-SHEATHED Bi-2223 TAPE, Mario Quilitz, Wilfried Goldacker, Forschungszentrum Karlsruhe, Inst fur Technische Physik, Karlsruhe, GERMANY.

We have investigated the effect of different variations of the stoichiometry on the critical currents 1 and the phase formation of 2223-phase in Ag-sheathed Bi-2223 tapes. For variations of the copper-, lead-, and bismuth-content, the relative effect on the critical currents was studied. Varying the copper content between 3.0 and 3.4 in BiPbSrCaCuO, we observed a distinct effect on the critical current. A maximum critical current was observed for values at x = 3.2. This result was found for annealing temperatures lower than 822C. For higher temperatures, samples with lower Cu contents achieved higher critical currents. Different results were found for the variation of the Pb content with a plateau-like region between x = 0.25 and 0.35 for a variation of x between 0.2 and 0.4 in BiPbSrCaCuO. This turns out to be strongly temperature dependent and was modified for T < 812 C and T > 821C. In the region with T < 812C, tapes with high Pb-contents gave lower critical currents whereas in the T-region above 821C, the samples with x(Pb) = 0.4 achieved high lc values. A variation of the Bi content between 1.65 and 1.95 indicated a decrease of critical currents towards low Bi contents with an optimum at x(Bi) = 1.85. Finally a combined variation of Bi and Pb content had nearly no influence on the final critical currents which improved at T >818C with lower bismuth and higher lead contents. Furthermore the dependence of the phase formation on the Cu- Pb- and Bi- content was investigated. It is also shown that the stoichiometry variations strongly affect the kinetics of phase formation and the formation of critical transport current densities in the tapes.

R6.2
OPTIMIZATION OF THE SUPERCONDUCTING CORE IN BI-2223 TAPES, Sherif Eskander Salib, Univ of Houston, Dept of Mechanical Engr, Houston, TX; Kamel Salama, Univ of Houston, Houston, TX.

High quality Ag-clad BSCCO conductors have been fabricated by the powder-in-tube (PIT) technique. This has been achieved with only a few percent of the lens shaped superconducting core being utilized. The present study is directed towards achieving a more uniform cross sectional area in these tapes by developing a cold working method, called Modified Groove Rolling (MGR). In order to compare the effect of the method on the cross sectional area and the current density, three sets of tapes were fabricated: 1) a tape processed by the conventional PIT technique, 2) a tape processed by MGR and uniaxial pressing, and 3) a tape processed only by MGR. After cold working the tapes were sintered at 830°C for 70 hours in 7% oxygen, 93% argon. The MGR method was also optimized to prevent crack formation. The microstructure was studied by etching off the silver sheath and exposing the oxide core. After improving the method a uniform cross section was achieved. This cross section was analyzed using SEM and XRD. Also the current density was measured for the overall cross section and compared to the conventional PIT tapes. Finally the cross section was sliced to determine the degree of uniformity of the current density along the cross section.

R6.3
MICROSTRUCTURAL ANALYSIS OF COMPOSITE BPSCCO TAPES, C. Vipulanandan, Univ of Houston, Dept of Civil Engineering, Houston, TX; S. Salib, M. Mironova, Univ of Houston, Dept of Mech Engineering, Houston, TX.

In order for BPSCCO tapes to be used in commercial applications, their mechanical properties must be further improved. Several methods are being investigated to improve the irreversible strain. Some of the methods include changing the sheath material or using multifilament tapes instead of monocore tapes. Understanding the changes in the microstructure of the ceramic core is essential to further improve the behavior of the tapes. In this study, BPSCCO tapes with silver sheath were reinforced with metal powder, flakes and fibers to improve the irreversible strain of the tapes. Due to processing conditions and mismatch in the coefficient of thermal expansion of the BPSCCO core and the metal reinforcements, residual stresses developed in the core. XRD was used to evalute the residual stresses in the metal sheath and ceramic core and to determine the residual compressive and tensile stress distribution in the tapes. Results from TEM study are also discussed. TEM studies indicate that one of the major difference between the monolithic and composite tapes is the dislocation density in the center of the core.

R6.4
STUDY OF A (Ag,Mg) SUBSTRATE FOR Bi-2212 TAPES PREPARED BY ELECTROLYTIC DEPOSITION, Lelia Schmirgeld-Mignot, Julie Houzet, Fabrice Legendre, Pierre Regnier, CEA Saclay, Gif Sur Yvette, FRANCE.

The properties of 50 and 25 microns thick Ag-Mg(0.2 wt.%) substrates, to be used for the fabrication of Bi-2212 textured tapes, have been studied after different treatments. Vickers micro-hardness measurements as well as optical microscopy and electronprobe micro-analysis have been performed mainly on the surfaces but also on cross-sections of the specimens. The Bi-2212 tapes have been prepared on these substrates by sequential electrolytic deposition of the constituent metals followed by very short intermediate annealings. Their critical current densities have been determined at 77K. The modifications of the thermal behaviour of the composite tapes, with respect to those of pure Ag-Bi2212 tapes, have been followed by DTA. The influence of different parameters which affect the phase diagram has been considered and taken into account for the optimisation of the crystallisation conditions for the Bi-2212 phase.

R6.5
PROCESSING BULK BiSrCaCuO WITH NANOSCALE OXIDE ADDITIVES, Y. Xu, Mankato State Univ, Mankato, MN; Ken C. Goretta, L. R. Feng, M. Jiang, P. Kostic, Argonne National Laboratory, Energy Tech Div, Argonne, IL.

Intragranular nanoscale oxide inclusions have been shown to enhance flux pinning in Y-, Tl-, and Bi-based high-temperature superconductors. Effects of adding nearly spherical 20-30 nm AlO, TiO, SnO, or ZrO particles to BiSrCaCuO (Bi-2212) were examined. Powder compacts were blended and heat treated by a variety of methods, including solid-state sintering and partial-melt processing. Melting response, microstructural development, and superconducting properties were studied, and the processing routes most likely to be successful in enhancing flux pinning were identified.

R6.6
IN SITU HIGH-TEMPERATURE XRD STUDY OF Bi-2212 SUPERCONDUCTOR COATED ON Ag PLATE, Chang Soo Kim, Yang Koo Cho, KRISS, Matls Evaluation Ctr, Taejon, SOUTH KOREA; Takeshi Hatano, Hiroaki Kumakura, Kazumasa Togano, Nat Research Inst for Metals, Ibaraki, JAPAN.

We have investigated melting and solidification processes, and grain alignment of Bi-2212 superconductor coated on 1 mm thick Ag plate, using high-temperature x-ray diffractometry (HTXRD). Especially, we focused on melting and solidification temperatures measured by HTXRD) as a function of oxygen partial pressure, and c-axis alignment of grains at or near the melting and solidification temperatures. For the investigations, x-ray scans of 2,/2 and rocking curve were made during heating and cooling of the samples. Melting and solidification temperatures showed monotonic decrease in semi-logarithmic scale as a function of oxygen partial pressure. Melting point of the Bi-2212 on Ag plate at 100 oxygen pressure is 879C and melting point at 1 oxygen pressure 827C, and solidification temperatures are 5 -10C lower than the melting points for each oxygen pressure, indicating that melting and solidification points are strongly dependent on oxygen partial pressure. On heating we observed, for all oxygen partial pressures used, melting point gap of 5 - 6C in which Bi-2212 and liquid phases coexist, and the fact was attributed to the effect of Ag diffused from substrate. In addition, we observed the strong grain alignment of the Bi-2212 compound along c-axis at the initial stage of melting as well as strong grain alignment along c-axis during solidification of the compound. The best c-axis alignment of grains was obtained for the samples cooled down slowly under 20 oxygen partial pressure.

R6.7
CHARACTERIZATION OF HIGH TEMPERATURE SUPERCONDUCTOR COATED AG WIRES, Yuan-Kai Tao, TECO, Dept of R & D, Chung-Li City, TAIWAN; M. K. Wu, National Tsing Hua Univ, Dept of Materials Science, Hsin-Chu, TAIWAN.

High temperature superconductor (HTS) coated Ag wires were prepared by the coating methods used in the thick film and enamel wires productions. These wires have a different structure from the Ag-sheathed HTS wires produced by the popular powder-in-tube method. Ag is the central part providing the structural support and the HTS is the outer part. Potentially, the wires could find applications in the HTS thin film device connections and the pick-up coils for the SQUID with enhanced system performance. Two types of wire have been studied - round Ag wires fully coated with HTS and flat Ag ribbon coated either one side or both sides with HTS. A variety of starting materials were tested, which include Bi-2212 precursor, Bi-2223, and Y123. The coated wires were subjected to mechanical handling such as: drawing, rolling, and pressing, to improve the packing density thereby the HTS grain connections, HTS grain orientation, and HTS/Ag adhesion. Finally, the wires were heat treated. The results show that the starting materials, coating uniformity, mechanical processing, and phase purity after heat treatment will influence the superconducting properties of the wires.

R6.8
PROCESSING OF HIGH-T BSCCO SUPERCONDUCTOR TAPES, HanJin Lim, Stanford Univ, Center for Materials Research, Stanford, CA; J. Gerald Byrne, Univ of Utah, Dept of Metallurgical Engr, Salt Lake City, UT.

High-T BSCCO superconductor tapes were prepared by the conventional powder-in-tube method.Some tapes involved partial melting and some involved solid state processing only.X-ray diffraction (XRD) showed the presence of both BSCCO 2223 (T = 105 K) and BSCCO 2212 (T = 80 K) phases in all the materials. For the mostly BSCCO 2212 phase samples, X-ray studies indicated that tape#1 which involved partial melting at 850C for 0.3 h had a higher degree of basal orientation than a tape#2 which involved partial melting at 855 C for 0.5 h. For the mostly BSCCO 2223 phase samples, however, a comparison of a tape #3 (solid state processing at 840C), tapes #5 and #6 (another solid state processing followed by repeated short annealing), and tape #4 (partial melting at 865C for 0.5h), showed that the solid state processed tape (#3) had the highest degree of basal orientation. Direct current magnetic susceptibility measurements were used to follow the transition at T. Critical current density, J, values were estimated from DC magnetic hysteresis loops for all tape samples.

R6.9
IN-SITU LORENTZ FORCE HOOP STRESS AND CYCLING DEPENDENCE OF Jc OF HTS CONDUCTORS, Yusuf S. Hascicek, Osman Dur, Steven W. Van Sciver, Florida State Univ, Natl High Magnetic Field Lab, Tallahassee, FL.

A novel in-situ Lorentz force stressing method was utilized in determining the stress dependence of Jc of PIT multifilamentary Bi-2212 tape conductors. Short tape samples were heat treated on cylindrical holder having the same diameter as the sample holder for in-field measurements. Once the Jc versus H dependence were determined in the Lorentz force compressive mode, the polarity of sample current were changed and hoop stress dependence of Jc were carried out on the same sample in-situ. Degradation in Jc was observed stresses as low as 30 MPa. SEM and optical microscopy investigations shows that cracks are responsible for degradation in Jc. SEM micrographs of the fracture surfaces indicate that failure was initiated mechanically.

R6.10
EFFECT OF BEND STRAIN ON THE J OF COMMERCIAL HTS CONDUCTORS, Yusuf S. Hascicek, Lee-Ye Xiao, Beatrice Boutemy, Steven W. Van Sciver, Florida State Univ, Natl High Magnetic Field Lab, Tallahassee, FL.

PIT, surface coated, and other HTS conductors which are becoming commercially available, have been systematically tested for their Jc versus bend strain dependence under same conditions as prelude to react and wind approach to high field magnet applications. Since half the HTS material is above the neutral plane, surface coated HTS conductors are more susceptible to bend strain than PIT conductors. Microstructural results obtained by optical and SEM from the transverse and longitudinal sections of straight and bent samples will be presented.

R6.11
INFLUENCE OF PREPARATION CONDITIONS ON TEXTURE, DENSITY AND CRITICAL CURRENT OF Bi, Pb-2223 CERAMICS, Ivan F. Kononyuk, Vladimir A. Lomonosov, Svetland Petrovna Tolochko, Yury G. Zonov, Inst of General & Inorganic Chemistry, Minsk, BYELARUS; Leonid F. Makarenko, Belarusian State Univ, Dept of App Math & Comp Sci, Minsk, BYELARUS; Nikolay M. Olekhnovich, Nikolay A. Sishonok, Inst of Solid State & Semiconductors Physics, Minsk, BYELARUS; Viktoria M. Shclekhina, Aleksandr A. Shevchonok, Powder Metallurgy Research Inst, Minsk, BYELARUS.

Influence of molding and heat treatment conditions on texture, density, and critical current of BiPbSrCaCuO (x = 0,3-0,4; y = 0-0,2) ceramics have been studied. The ceramic samples were formed from synthesized powders by an ordinary static pressing at pressures 100-1500 MPa, by a dynamic explosive compacting at pressures 1,5-4 Gpa, by a hot-press method at pressures 30-60 MPa and temperatures 800-860C with using of carbon heating elements, by high-pressure technique and technology at pressures 2-7 GPa and temperatures up to 900C in an oxidizing atmosphere and by a sedimentation from suspensions with following static pressing. The texture factor, F, was determined from relationship of x-ray intensities of reflections (115) and (0010). The texture factor in very thin surface layer of pressed ceramic samples was in 1,5-2 times higher than in their volumes. At compacting pressure about 100 MPas, its values ran already up to 97-98 in perpendicular direction of pressing and F was decreased up to 80-90 in other directions. The ceramic's density was increased up to 85-90 at increase of compacting pressures up to 1000-1500 MPa. The critical current density (j) was determined by levels of texture, density, and by conditions of heat treatment of ceramics.

R6.12
GLASS CERAMIC PROCESSING OF BiPbSrCaCuO: A KINETICALLY FASTER ROUTE TO TEXTURED BSCCO, Matthew J. Kramer, L. Margulies, Iowa State Univ, Ames Laboratory, Ames, IA; K. W. Dennis, R. W. McCallum, Iowa State Univ, Ames Lab, Ames, IA.

Earlier work at Ames showed that properly processed amorphous BiSrCaCu2O8 (Bi2212) could be rapidly converted into a highly c-axis textured tape with the application of uniaxial pressure during crystallization. Unfortunately these materials were weak-linked even though the density was high and the concentration of second phases was < 5%. Attempts to demonstrate this process route in the (BiPbSrCaCuO) Bi2223 system were stymied by the considerably higher liquidus temperature and the low vapor pressure of Pb. A new high temperature furnace with controllable atmosphere has now been constructed. We have learned how to control the loss of Pb and can now routinely produce nearly amorphous Bi2223. Unlike the amorphous Bi2212, the amorphous Bi2223 is not deficient in oxygen. Flash annealing of the amorphous Bi2223 glass in 7.5% O2 (balance N2) shows that greater than 80% of the glass converts to Bi2223 in less than 24 hrs. The presence of Ag appears to improve the kinetics. The purpose of flash annealing is to suppress the formation of intermediate phases such as CaPb by jumping over the stability field for this compound. CaPb produces a liquid phase below the Bi2223 processing temperature leading to significant second phase coarsening and long processing times. Preliminary magnetization measurements suggest that the Bi2223 is not as weak-linked as the Bi2212 was. Work is underway to crystallize the amorphous Bi2223 under uniaxial load to induce texture during crystallization.

R6.13
PULSE VELOCITY CHARACTERIZATION OF BULK BPSCCO ELEMENTS DURING PROCESSING, C. Vipulananda, Univ of Houston, Dept of Civil Engr, Houston, TX; Wanli Lu, Univ of Houston, Dept of Mechanical Engr, Houston, TX.

In order for bulk BPSCCO components to be used in commercial applications, non-destructive methods for evaluation must be developed. Several methods are being investigated to monitor the changes in the bulk material properties after each stage of treatment for quality control. In this study a method to evaluate the changes in directional properties of bulk BPSCCO components with and without silver powder was investigated. Bulk BPSCCO components were produced by multiple stage uniaxial and cold-isostatic pressing and sintering. Changes in pulse velocity with direction was studied at every stage of processing. Pulse velocities measured at 150 KHz was effective in monitoring the directional changes in the bulk BPSCCO elements. Influence of specimen size and shape was also investigated. Stress-strain-critical current relationships for the bulk materials have been developed.

R6.14
FABRICATION OF THE COMPOSITE OF Bi-2223 BULK SUPERCONDUCTOR AND MgO PLATE BY SINTER-FORGING FOR CURRENT LEADS, Norimitsu Murayama, Yasuyoshi Torii, Nat Ind Research Inst Nagoya, Nagoya, JAPAN.

Bi-2223 bulk current leads with 1kA-class critical current.have been developed by the cold isostatic pressing (CIP)-normal sintering technique. However, another processing such as hot pressing or hot forging may be necessary for increasing critical current and mechanical strength, because dense Bi-2223 bodies cannot be prepared by CIP-normal sintering technique. In addition, the reinforcement with an engineering ceramics is one of promising approaches for the production of bulk current leads with a high mechanical strength. We have developed a new sinter-forging processing in that Bi-2223 superconductors are densified, grain-oriented and joined to MgO plates simultaneously. The compact body with a nominal composition of BiPbSrCaCuO was sandwiched between two MgO plates, and was sinter-forged in air at 850 C, 1.96 MPa for 5 h in the first stage and at 850.

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