Highlights for Fiscal Year 1996

• Effects of friction and strain rate on the deformation of multifilament BSCCO-2223 powder-in-tube (PIT) tapes have been investigated. In contrast to monofilament tapes, multifilament tapes undergo inhomogeneous deformation when subjected to increased friction and reduction per pass with no noticeable benefits in critical current density (J_c). The different effects of friction on the properties of mono- and multifilament tapes are believed to be caused by the development of "dead zones." Consequently, although the high temperature superconducting (HTS) core of monofilament tape is densified and can carry high current, only a portion of the filaments in multifilament tape are compacted. That is, a substantial amount of the filaments in tapes deformed under high-friction conditions are weak conductors and do not contribute significantly to J_c. The identification of dead zones in PIT tapes indicates that modifications in deformation conditions and/or tape geometry can be carried out to achieve more homogeneous deformation and densification of the HTS cores.



• A preliminary design review for the HTS transformer project team was held at Waukesha Electric Systems. Recent work has involved calculations and design work for the 1-MVA prototype transformer and supporting tests for alternating current (ac) losses and conductor coatings. Measurements of losses in ac fields can be performed by controlling field magnitude and direction; however, it may be difficult to relate the results to the actual fields seen by the conductor in the transformer. For this reason, a scalable measurement that closely simulates transformer conditions was suggested by Intermagnetics General Corporation (IGC). Oak Ridge National Laboratory (ORNL) staff conducted the measurements at ORNL with the test configuration supplied by IGC. In addition, a practical, commercially available coating for protection of the Bi-2212 tapes was successfully tested.



• A new cooperative research and development agreement (CRADA) was initiated with Southwire Company (Carrollton, Georgia) in October 1995. The funds-in CRADA provides for the development and demonstration of the technology needed for HTS underground transmission cables. Southwire delivered the first bare prototype cable in late October 1995. Four test cables are planned for Phase I of what could be a 5-year, three-phase program. The Phase I project is heavily leveraged by private-sector funding, with Southwire contributing at least $750,000 of the anticipated $850,000 total cost.



• The critical current (I_c) vs applied magnetic field has been measured for Bi₂Sr₂CaCu₂O_x (Bi-2212) and (Tl_0.5Pb_0.5)(Sr_0.8Ba_0.2)₂Ca₂Cu₃O_x (Tl-1223) wires for a range of temperatures. I_c is not a single-valued function and can depend on magnetic and thermal history. For the Bi-2212, there was essentially no difference in the I_c for the zero-field-cooled and field-cooled samples, for either decreasing or increasing applied fields. In contrast, the Tl-1223 results exhibit significant hysteresis that may be attributed to behavior of weak-linked grain boundaries that trap magnetic flux. This property of the weak-linked thallium-based conductors may be significant if such conductors are to be used in ac applications.



• The calorimetric technique was used to measure ac losses in a series of high-quality, twisted multifilamentary BSCCO 2223/Ag superconducting tapes produced by American Superconductor Corporation (ASC). The ac losses were correlated to various superconducting properties such as the J_c, the tape aspect ratios, and other parameters to guide ASC toward reducing the ac losses in the next generation of tapes. The ac losses are highly anisotropic with the applied field orientation. This anisotropy leads to a critical twist-pitch relation that depends on the filament aspect ratio rather than simply on the filament diameter. Therefore, most of the ac-coupling losses will be determined by the perpendicular field component exposed to the tapes. Also, the hysteretic losses depend primarily on the degree of bridging between the individual filaments. We find that the losses are described very well by the critical-state model and that the coupling losses are shown to decrease in wires with twisted filaments.



• Superconducting properties were measured for TlBa₂Ca₂Cu₃O_9­y (Tl-1223) thick films grown on Ag substrates. Thallium-free precursor films of the type Ba₂Ca₂Cu₃Ag_0.37O₇ were deposited on both smooth and rough Ag substrates using a spray pyrolysis technique. The film thickness varied between 1 and 3 µm. The surface microstructure of the Ag substrate dictates the growth behavior of Tl-1223 films. The films grown on smooth Ag substrates had local biaxial alignment extending in the plane of the film up to 5 mm. The measured transport J_cs for Tl-1223 films grown on smooth and rough Ag substrates were ~65,000 A/cm² and ~37,000 A/cm², respectively, at 77 K and zero magnetic field. The films grown on smooth Ag substrates were strongly linked and had good in-field properties with a J_c value of ~10⁴ A/cm² at 77 K and 0.5 T. This result was obtained with the magnetic field applied perpendicular to the substrate.



• An electrical measurement with direct current (dc) has been completed on the first HTS cable fabricated by the Southwire Company. The cable contained 73 separate tapes in four layers. A copper pipe served as the former. One section of the cable was visibly damaged during fabrication and handling. When tested in liquid nitrogen, the maximum I_c for the undamaged section of the cable was 900 A, exceeding the original goal of 500 A.



• We report superconducting properties of TlBa₂Ca₂Cu₃O_9­y (Tl-1223) thick films grown on high-strength substrates. The high-strength substrate was made up of Ag bonded to a suitable metal alloy. Thallium-free precursor films of the type Ba₂Ca₂Cu₃Ag_0.37O₇ were deposited on these high-strength substrates using a spray pyrolysis technique. The as-deposited precursor films were heat treated and thallinated in a two-zone furnace. The film thickness was 3 µm. X-ray diffraction (XRD) results showed the presence of c-axis-textured Tl-1223 films and some degree of local in-plane texture (colony microstructure). The measured transport J_cs for Tl-1223 films were ~52,000 A/cm² at 77 K and zero magnetic field. The microstructures of Tl-1223 films were quite similar to those observed previously for films on polycrystalline yttria-stabilized zirconia (YSZ) substrates.



• A special test coil, fabricated by IGC of Bi-2212 surface-coated wires, was delivered to ORNL in January 1996. The coil was used to help determine the expected ac loss power that will be generated in the full-scale, 1-MVA prototype transformer. ORNL verified that the I_c of the coil was 9.5 A at 77 K. Calorimetric ac loss tests with ac transport current were carried out at several different temperatures, ranging from 4.2 to 50 K. The conductor consists of a 6-mm-wide silver tape surface-coated with BSSCO-2212 and is quite similar to the conductor to be used in the 1-MVA prototype transformer. At the rated operating current of 72 A rms, losses were somewhat above calculated values. We expect that the losses can be reduced considerably with modifications to the coil geometry.



• Epitaxial LaAlO₃ films were grown successfully on SrTiO₃ (100) single crystal substrates. A LaAlO₃ precursor solution was prepared via an all alkoxide sol-gel route. Synthesis of buffer layers and superconductors via sol-gel routes may represent a low-cost, convenient (nonvacuum) way of coating textured metallic substrates. Complete hydrolysis of the solution formed a gel that yielded well-crystallized LaAlO₃ powders when fired in air at 800°C. Epitaxial films of LaAlO₃ were formed during pyrolysis of a solution that was spin-coated on SrTiO₃ (100) single crystal substrates in a rapid thermal annealing furnace. The films were strongly c-axis-oriented and had good in-plane texture shown by a -scan of the (202) plane with full width half maximum (FWHM) = 1.07°.



• The "Teacher's Guide to Superconductivity for High School Students" has been placed on the World Wide Web. The guide may be accessed by going to the ORNL Superconductivity home page:



http://www.ornl.gov/HTSC/htsc.html


• The use of a biaxially textured (001) Ni tape, formed by recrystallization of cold-rolled pure Ni, as an in-plane-aligned substrate material for subsequent HTS film deposition is a possible approach for achieving in-plane-aligned, high-I_c YBa₂Cu₃O₇ films on long-length substrates. The epitaxial growth of a (001)-oriented oxide buffer layer architecture that maintains the sharp crystallographic cube texture of the metal substrate while providing a barrier to chemical interaction of the Ni with the HTS film results in a structure referred to as a Rolling-Assisted, Biaxially Textured Substrate (RABiTS). We have grown in-plane-aligned, c-axis-oriented YBa₂Cu₃O₇ films on thermomechanically rolled textured (001) Ni tapes using pulsed laser deposition with superconducting J_cs, as high as 700,000 A/cm² at 77 K. The superconducting properties of films deposited on the biaxially textured Ni tapes are comparable with those realized for epitaxial YBa₂Cu₃O₇ on single crystal oxide substrates.



• Electrical measurements with both dc and ac currents have been completed on the second HTS cable made by Southwire. This cable is different from the first one in that the tapes on each layer were separated from each other and Kapton tape was used between layers for insulation. Alternating current loss measurements of the first cable and a thermal cycle test were also performed during this period. Analysis of the loss data indicated that the measured loss is governed by the power law behavior of the HTS tape in the resistive transition.



• Two buffer layer architectures for coated conductors were developed on rolled Ni substrates using an electron beam evaporation technique. The first buffer layer architecture consists of an epitaxial laminate of CeO₂/Pd/Ni. The second alternative buffer layer consists of an epitaxial laminate of YSZ/CeO₂/Ni. The CeO₂ films were grown epitaxially on both Pd-buffered Ni and pure Ni substrates. The YSZ films were grown epitaxially on CeO₂-buffered Ni substrates. The crystallographic orientation of the Pd, CeO₂, and YSZ films was (100). We also studied the effect of the CeO₂ layer thickness on rolled-Ni substrates. The CeO₂ layer thickness was found to be critical. The presence of YSZ layers on top of CeO₂ layers seems to alleviate the cracks that are formed underneath. Our scanning electron microscopy (SEM) studies showed that both CeO₂ and YSZ layers were smooth and continuous. The buffer layer architectures that we have developed are useful for the subsequent growth of superconductors such as YBa₂Cu₃O₇ and other high-critical-temperature materials that are chemically and epitaxially compatible with CeO₂ and YSZ buffer layers.



• Electrical measurements with both dc and ac currents have been performed on the third HTS cable made by Southwire. This cable is rated for 2 kA and has been successfully tested to and above the rated current. Two papers were written on the design and test of the cables made during this phase of work and were presented at the 1996 Applied Superconductivity Conference.