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 (Jc). 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 Jc. 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
- 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 (Ic) vs applied magnetic field has been measured for Bi2Sr2CaCu2Ox (Bi-2212)
and (Tl0.5Pb0.5)(Sr0.8Ba0.2)2Ca2Cu3Ox (Tl-1223) wires for a range of temperatures. Ic 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 Ic 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
Jc, 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
- Superconducting properties were measured for TlBa2Ca2Cu3O9y (Tl-1223) thick films grown on Ag
substrates. Thallium-free precursor films of the type Ba2Ca2Cu3Ag0.37O7 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 Jcs 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 Jc 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 Ic for the undamaged section of the cable was
900 A, exceeding the original goal of 500 A.
- We report superconducting properties of TlBa2Ca2Cu3O9y (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 Ba2Ca2Cu3Ag0.37O7 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 Jcs 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 Ic 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
- 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:
- 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-Ic 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 Jcs, 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.