Magneto-Mechanical Measurements for High Current Applications
Goals
Project staff members in the laboratory for
superconductor
strain measurements.
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This project specializes in measurements of the
effect of mechanical strain on materials for highcurrent
applications. Recent research has produced
the first electromechanical data for the new class
of high-temperature coated superconductors, one
of the few new technologies expected to have an
impact on the U.S. electrical power grid and the
large electric power industry. The project’s research
has also lead to the first four patents on contacts for
high-temperature superconductors.
Recent research also includes extending the highmagnetic-
field limits of electromechanical measurements
for development of nuclear-magneticresonance
(NMR) spectrometers operating at 23.5
teslas and 1 gigahertz, and the next generation of
accelerators for high-energy physics.
The Strain Scaling Law, previously developed by
the project for predicting the axial-strain response
of low-temperature superconductors in high magnetic
fields, is now being extended to high compressive
strains for use in finite-element design of
magnet structures.
Customer Needs
The recent success of the “second generation” of
high-temperature superconductors has brought with
it new measurement problems in handling these
brittle conductors. We have the expertise and equipment
to address these electromechanical problems.
Stress and strain management is one of the key
technology areas needed to move the second-generation
high-temperature coated conductors to the
market place. The project utilizes the expertise and
unique electromechanical measurement facilities at
NIST to provide performance feedback and engineering
data to companies and national laboratories
fabricating these conductors in order to guide their
decisions at this critical scale-up phase of coatedconductor
development.
The project serves industry primarily in two areas.
First is the need to develop a reliable measurement
capability in the severe environment of superconductor
applications: low temperature, high
magnetic field, and high stress. The data are being
used, for example, in the design of magnets for
the magnetic-resonance-imaging (MRI) industry,
which provides invaluable medical data for health
care, and contributes $2 billion per year to the U.S.
economy.
The second area is to provide data and feedback to
industry for the development of high-performance
superconductors. This is especially exciting because
of the large effort being devoted to develop
superconductors for grid reliability and enhanced
power-transmission capability. We receive numerous
requests, from both industry and government
agencies, for accurate electromechanical data to
help guide their efforts in research and development
in this decisive growth period.
Technical Strategy
Our project has a long history of unique measurement
service in the specialized area of electromechanical
metrology. Significant emphasis is placed
on an integrated approach. We provide industry
with first measurements of new materials in areas
where there is significant research potential.
Electromechanical Measurements of Superconductors — We have developed an array of specialized measurement
systems to test the effects of mechanical
stresses on the electrical performance of superconducting
materials. Extensive, advanced measurement
facilities are available, including high-field
(18.5 teslas) and split-pair magnets, servohydraulic
mechanical testing systems, and state-of-the-art
measurement probes. These probes are used for
research on the effects of axial tensile strain and
transverse compressive strain on critical current,
measurement of cryogenic stress-strain characteristics,
composite magnetic coil testing, and variabletemperature
magnetoresistance measurements. Our electromechanical test capability for superconductors
is one of only a few in the world.
Collaboration with Other
Government Agencies — These measurements are an important element of
our ongoing work with the U.S. Department of
Energy (DOE). The DOE Office of High Energy
Physics sponsors our research on electromechanical
properties of candidate superconductors for particle-
accelerator magnets. These materials include
low-temperature superconductors (Nb3Sn, Nb3Al,
and MgB2), and high-temperature superconductors
— Bi-Sr-Ca-Cu-O (BSCCO) and Y-Ba-Cu-O
(YBCO) — including conductors made on rollingassisted,
biaxially textured substrates (RABiTS)
and conductors made by ion-beam-assisted deposition
(IBAD). Our research is also sponsored
by the DOE Office of Electric Transmission and
Distribution. Here, we focus on high-temperature
superconductors for power applications, including
power-conditioning systems, motors and generators,
transformers, magnetic energy storage, and
transmission lines. In all these applications, the
electromechanical properties of these inherently
brittle materials play an important role in determining
their successful utilization.
Characterization of
Superconductors For Electric
Power Grid Reliability — Improved superconductors are being developed
by U.S. companies and demonstrated for power
transmission. Superconductors’ greater current
carrying capability is advantageous for upgrading
real-estate-limited transmission lines in cities. Superconductors
are also being developed for use in
superconductor magnetic energy storage (SMES).
Our work on characterizing superconducting properties
at high stresses and high strains, and over
variable temperatures is critical for the development
of these superconductors. This work is also
supported by DOE.
Significant progress in second-generation superconductors
was reported in 2005-2006. These thin,
highly textured YBCO films are deposited with
mainly non-vacuum techniques on fl exible metal
substrates. They are now available in lengths of
over 300 meters, carrying very high currents of
over 2.5 to 3.0 mega-amperes per square centimeter
at 77 kelvins. These superconductors have
the potential to replace and improve parts of the
ageing power grid in the United States. However,
with the first coils fabricated from second-generation
conductors, manufacturers learned that the
layered architecture of the conductor may pose
a problem: delamination of the ceramic layers
under transverse tensile stress. This is important
for rotating machinery, because of the centrifugal
forces on the conductors, and more generally,
because differential contraction in coil structures
can place the conductors under severe transverse
tensile stresses.
Scaling Laws for Magnet Design — In the area of low-temperature superconductors,
we are generalizing the Strain Scaling Law (SSL),
a magnet design relationship we discovered two
decades ago. Since then, the SSL has been used in
the structural design of most large magnets, based
on superconductors with the A-15 crystal structure.
However, this relationship is a one-dimensional
law. We are developing a measurement system to
carefully determine the three-dimensional strain
effects in A-15 superconductors. The importance
of these measurements for very large accelerator
magnets is considerable. The SSL is also being
developed for high-temperature superconductors,
since we recently discovered that practical
high-temperature superconductors also exhibit an
intrinsic axial-strain effect.
Accomplishments
- New Apparatus Developed to Measure
Delamination in YBCO Coated Conductors — Over the past two years we have measured
the delamination strength of second-generation
YBCO superconductors. This required the design
and construction of a new test apparatus. The test
fixture head of the apparatus consists of two anvils
made of Ni-5at.%W, which is the same material as
the substrate of most coated conductors. The bottom
anvil is soldered to the substrate side of the
sample, while the top anvil is soldered to the silver
cap layer on top of the ceramic YBCO layer. The
choice of fabricating both anvils from the same
material as the substrate of the superconductor
ensures the absence of thermal shear stress between
the sample and the anvils when they are cooled
to 77 kelvins. The transverse tensile strength of
the conductor is measured in two steps. First, the
internal strength of the ceramic layers is measured,
without edge effects. Second, the overall strength
is measured, including edge effects. Edge damage
may arise from, for instance, slitting of the conductor
to smaller widths (a common procedure in the
manufacturing process), and we anticipated that it
may play a role in initiating delamination. Indeed, successful testing with the new apparatus
showed that slit conductors have a relatively low
transverse tensile strength. The overall strength of
the conductor is reduced significantly by the slitting
process. One company developed a structure to reinforce
the slit conductor by soldering copper strips
around the conductor (three-ply structure). The
added solder joints at the edges of the conductor
help restore the overall strength. We are currently collaborating with another company to find other
novel solutions to avoid edge damage caused by
slitting. In particular, we are pursuing solutions
that produce a strong conductor without the need
for extra reinforcement that lowers the conductor’s
overall critical current density.
Test fixture head of a new apparatus to measure the
delamination strength of second-generation high-
temperature
superconductors. The photo shows
a YBCO
coated conductor tape mounted between
the two anvils
of the apparatus.
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Conductor slitting reduces the overall transverse tensile
strength to
an average of 17.3 megapascals (numbers
in the bars indicate the
average value), which
is far below the average internal strength of
26.5 megapascals. Reinforcing the slit sample by soldering
copper
strips (three-ply structure) raised the average
transverse tensile
strength to 24.8 megapascals. The
solid parts of the bars give an
indication of the spread
in strength among samples.
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- Discovery of Large Universal Effect of
Axial Strain on the Critical Current of High-
Temperature Superconductors — Although
remarkable technical advances have been achieved
during the past several years in the development of
high-temperature superconductors (HTS) for use
in large-scale applications, these have occurred
without a clear understanding of the underlying
mechanism of superconductivity in these materials.
One of the main areas not fully understood is the
change of the superconducting current density with
applied strain. We have discovered a very large universal,
reversible change in the Jc of YBCO coated
conductors, which is symmetric under both high
compressive and high tensile strain. We anticipate
these findings will initiate detailed research on the
effect of strain on the underlying mechanisms of
superconductivity in practical HTS. For instance,
strain fields at grain boundaries in HTS may be the
main limiting mechanism for supercurrents.
The superconducting current density in its self-field
decreases reversibly by more than 40 percent under
compressive strain in a wide range of conductors
fabricated by vastly different processes. The effect
is nearly the same for all high-current conductors
measured, including YBCO deposited by metalorganic
chemical vapor deposition (MOCVD)
(which results in a columnar YBCO grain structure),
metal-organic deposition (MOD) (which
results in a laminar YBCO grain structure), and a
hybrid conductor fabricated with a double YBCO
layer (consisting of YBCO that is doped with a
large amount of Dy particles that provide extra flux
pinning centers). The effect is symmetric under
both compressive and tensile strains.
A major reversible strain effect is a consistent
and intrinsic phenomenon of high-current YBCO
biaxially aligned coated conductors, encompassing
a number of different types fabricated with
widely varying YBCO deposition techniques. The
universality and symmetry provide evidence that
the mechanism behind the reversible strain effect
is an intrinsic feature of the YBCO grain structure
(columnar for MOCVD-IBAD and laminar for
MOD-RABiTS). The results open the door for
detailed studies into the mechanism of superconductivity
in HTS.
Normalized superconducting current density plotted as
function of intrinsic strain ε0 for three different types of
samples: MOCVD-IBAD, MOD-RABiTS, and (hybrid)
MOD-RABiTS, for both bare samples and those with
copper added for stability. The solid lines describe a
power-law function. The values of the strain-sensitivity
parameter a are included in the figure. Compressive
strain is indicated by negative values of ε0; tensile
strain
is positive.
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Illustration of method for applying large axial strains
to
superconducting sample that is soldered on top of
a
Cu-2%Be bending beam. Axial tension is applied
by
bending the beam in the direction shown in (a),
whereas
axial compression is applied by bending the
beam in the
opposite direction (b). Strain is uniform
over the
thickness of the superconducting films to
within about
1 part in 2500.
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- Slit YBCO Coated Conductors Prove
Mechanically Robust Under Fatigue Cycling
— In order to evaluate the effect of slitting, we
used fatigue cycling under transverse compressive
stress, since earlier experiments had shown this to induce crack propagation. Stress was applied to
the tape sample by means of two stainless-steel
anvils. Uniformity of stress over the pressed area
of the conductor was achieved by beveling the
edges of the top anvil, and attaching it to a biaxially
gimbaled pressure foot so that this anvil conforms
precisely to the bottom anvil and sample surfaces.
Stress was cycled between positive and negative
150 megapascals (about twice the stress level of
most applications) at a frequency of 1 hertz for
up to 20,000 fatigue cycles. Jc was measured at
76 kelvins in self-field. These tests simulate conditions
in applications such as electric generators and
industrial magnets, and evaluate whether fatigue
exacerbates cracks by propagating them into the
middle of the conductor. The measurements also
help discriminate between different slitting techniques.
The samples investigated exhibited no
significant degradation under fatigue testing, thus
demonstrating that, unlike transverse-tension testing,
slitting does not affect the sample performance
under transverse-compressive-stress cycling.
Fatigue test fixture showing the top anvil, biaxially
gimbaled
to uniformly apply pressure to the conductor.
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Effect of fatigue cycling under transverse compressive
stress
in a YBCO RABiTS sample laminated with
Cu foils on both
sides (three-ply geometry). Jc showed
no significant
degradation under fatigue testing up to
20,000 cycles at
150 megapascals.
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Book
Jack Ekin’s new textbook, Experimental Techniques
Cover of new book on cryogenic
measurement techniques.
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for Low-Temperature Measurements, was
published by Oxford University Press in October
2006. The text covers the design of cryogenic
measurement probes, and the appendix provides
cryogenic materials data for carrying out that design.
The textbook is written for beginning graduate
students, industry measurement engineers, and
materials scientists interested in learning how to
design successful low-temperature measurement
systems. Topics include heat-transfer techniques
for designing a cryogenic apparatus, selecting
materials appropriate for such apparatus, how to
make high-quality electrical contacts to a superconductor,
and techniques for reliable critical-current
measurements.
The appendix is a data handbook of materials
properties and cryostat design consisting of 70
tables compiled from over 50 years of literature.
The tables were compiled for experts in the field
of cryogenic measurements and include electrical,
thermal, magnetic, and mechanical properties of
materials for cryostat construction; properties of
cryogenic liquids; and temperature measurement
tables and thermometer properties.
Award
Superconductivity for Electric Systems Program,
Office of Electricity Delivery and Energy Reliability,
U.S. Department of Energy, recognition as top
ranked project, 2005 and 2006 (Jack Ekin, Najib
Cheggour, and Danko van der Laan).