American
Superconductor Corporation, a leading electricity solutions
company, today reported it has doubled the electric performance of its latest
high temperature superconductor (HTS) wire in strong magnetic fields at
relatively high temperatures, an achievement necessary for incorporating the
advanced wires in a range of commercial and military applications.
The company attained an electrical current of 102 Amperes per centimeter
of width (A/cm-width) of its second generation (2G) wire at 65 degrees Kelvin
(K) in a strong magnetic field of 3 Tesla (T) applied perpendicular to the
surface of the tape-shaped wire. This performance level is significant
because it is already at 88% of the level for electrical current in high
magnetic fields targeted for two years from now by the Defense Advanced
Research Projects Agency (DARPA), which supports this work. AMSC's DARPA
program goal is to reach 115 A/cm-width by the beginning of 2007, all at 65K
with a 3T magnetic field applied perpendicular to the surface of the 2G wire.
By comparison, the earth's magnetic field is approximately 0.000025 T and a
typical kitchen magnet has a field around a tenth of a Tesla.
AMSC achieved the substantial performance improvement by adding a certain
amount of the rare-earth metal holmium to the superconductor coating in its
proprietary 2G wire manufacturing process. Under the proper manufacturing
conditions, the addition of holmium leads to the creation of atomic-scale
holmium-containing particles (nanodots). The nanodots serve to immobilize
("pin") the magnetic lines of force in place in the superconductor. The
pinning allows higher amounts of electrical current to flow even in the
presence of strong magnetic fields and at relatively high temperatures of
operation.
Today's technical media advisory was accompanied by the public release by
AMSC of an updated version of its white paper on 2G HTS wire technology. This
white paper, "2G HTS Wire Technology: An Assessment," can be found in the
Learn More section of American Superconductor's home page at
http://www.amsuper.com along with other supporting materials.
Technical Background
Designers planning to incorporate the latest high temperature
superconductor (HTS) technology in their commercial and military products are
paying close attention to this progress. First generation (1G) HTS wire is
commercially available today and being applied in a variety of electric power
grid, motor, generator and magnet applications (see
http://www.amsuper.com/products/htsWire/index.cfm). 2G wire, which is designed
to be a form, fit and function replacement for 1G wire, is expected to extend
the reach of HTS technology by providing a combination of higher electrical
performance and lower cost. AMSC currently is fitting out a pre-pilot plant
for producing 2G HTS wire and expects to start shipping larger quantities of
2G wire to customers starting in the second half of calendar year 2005.
Earlier this year, AMSC achieved a new record for 2G electrical
performance of wire fabricated with its low-cost process: 380 A/cm-width at 77
K (liquid nitrogen coolant) with no applied magnetic field, well above the
commercial threshold of 300 A/cm-width for applications such as power cables
that operate in very low magnetic fields. Applications such as power cables
do not require the presence of nanodots in the superconductor. By contrast
the present nanodot-enhanced wire is optimized for coil-based applications,
which require 2G wires to maintain their electrical performance in strong
magnetic fields.
Military applications, such as very high speed generators and kinetic
energy weapons systems, require electromagnets operating at high magnetic
fields. They are expected to be made possible thanks to the electrical
properties of 2G HTS wire -- its ability to carry enormous amounts of current
with zero resistance and thus enhance the power of the electromagnetic coils
used in these applications.
The flow of current in the superconductor wire making up an
electromagnetic coil creates a magnetic field that enables the device to
operate. However, the increased magnetic field also lowers the maximum
current that can flow with zero resistance through the wire.
In recent work, AMSC has found a way to enhance the high levels of 2G
wires' electrical performance to make their use practical even in these high
magnetic field settings. The company's scientists achieved the required
electrical performance by "pinning" the magnetic lines of force (known
technically as flux lines or vortices) within the 2G wire. They accomplished
the pinning by carefully introducing a variety of defects into the
superconductor, including a dispersion of tiny foreign particles or
"nanodots." Each type of defect has a different effect on the wire's
electrical performance, with the sum result being improved current carrying
abilities under a range of temperature and magnetic field conditions.
AMSC uses a proprietary metal-organic deposition (MOD), process that
involves applying a liquid precursor of the superconductor material, an
yttrium-barium-copper oxide commonly known as "YBCO," as a film onto a
metallic base. Together, the base and the metal-organic film deposited on it
are processed into electrical wire. The company has adapted this patented
manufacturing method to include nanodots that are regularly distributed in the
YBCO film. The composition and distribution of particles is controlled
through chemical modification of the liquid precursor and by the company's
proprietary processing conditions.
In particular AMSC has been successful in doping the YBCO films with
holmium (Ho) cations creating a dispersion of (Ho,Y)2O3 nanodots throughout
the YBCO. The appropriate concentration of Ho-containing nanodots results in
a 100 percent increase in the critical current of the YBCO film at 65 degrees
Kelvin in a magnetic field of 3 Tesla (oriented perpendicular to the wire's
surface) compared to an undoped YBCO film of the same thickness. A record
critical current of 102A/cm-width (at 65K, 3T) has been achieved in a 1.4
micron-thick YBCO film doped with a certain concentration of Ho.
Achieving a critical current of over 100 A/cm-width at 65K in a 3T field
is a significant milestone in developing 2G wire with a performance level
meeting the demands for both commercial and military applications. This work
is supported by a DARPA contract, which AMSC received in June 2004, and is
aimed at military applications. It has a full program goal of 115 A/cm-width
at 65K in a 3T field for early 2007, making the present advance well ahead of
the DARPA program schedule. The company is presently initiating work to apply
these results to longer lengths of 2G wire.
