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NIST
metallurgist Robert Shull loads a sample into an ultracold
chamber to measure its promise as a magnetic refrigerant.
By adding
small amounts of iron to a gadolinium-germanium-silicon
alloy, NIST
researchers found they could boost how efficiently the
material first
increases and then decreases in temperature as an external
magnentic
field was cycled on and off.
Photo by: Kathie Koenig |
A pinch
of iron dramatically boosts the cooling performance of a material
considered key to the development of magnetic refrigerators,
report researchers at the Commerce Department’s National
Institute of Standards and Technology (NIST) in tomorrow’s
issue of Nature. The achievement might move the promising
technology closer to market, opening th e way to substantial
energy and cost savings for homes and businesses.
By adding
a small amount of iron (about 1 percent by volume), the NIST
team enhanced the effective cooling capacity of the so-called
“giant magnetocaloric effect” material by 15 to
30 percent. The result, writes materials scientist Virgil
Provenzano and his NIST colleagues, “is a much-improved
magnetic refrigerant for near-room-temperature applications.”
The original
material—a gadolinium-germanium-silicon alloy—already
is considered an attractive candidate for a room-temperature
magnetic refrigerant. However, its cooling potential is undercut
by significant energy costs exacted during the on-and-off
cycling of an applied magnetic field, the process that drives
the refrigeration device. These costs—called hysteresis
losses—translate into commensurate losses of energy
available for cooling.
Hysteresis,
which results in magnetized remnants that persist after an
applied magnetic field is relaxed, can be technologically
useful. For example, it enables bits of data to be stored
on magnetic disks and tapes. In the case of the gadolinium-germanium-silicon
alloy, however, it penalizes cooling performance.
The iron
supplement overcomes this disadvantage. It nearly eliminates
hysteresis and the associated energy cost, permitting the
material to perform near the peak of its potential.
Independently
suggested by two scientists in the 1920s, earning one the
Nobel Prize in 1949, magnetic refrigerators offer sizable
prospective advantages over the century-old technology of
today’s vapor-compression cooling systems. Potential
pluses include substantial gains in energy efficiency, lower
cost of operation, elimination of environmentally damaging
coolants, and nearly noise- and vibration-free operation.
With
recent progress in materials science and engineering, magnetic
refrigeration technology is edging into contention for specialized
uses, such as cooling sensors in spacecraft and liquefying
gases. Further advances, like the one reported by the NIST
team, are necessary if the technology is to replace household
refrigerators, freezers, dehumidifiers and air conditioners,
which account for about 25 percent of residential energy usage.
When
exposed to a magnetic field, the gadolinium alloy and other
magnetocaloric-effect materials heat up as their spinning
electrons align with the field, thereby magnetizing the materials
and raising their temperature. When the external field is
removed, the materials demagnetize—the electrons revert
to a disordered magnetic-spin state—and their temperature
drops. The two-stage process forms the magnetic refrigeration
cycle.
In the
case of the iron-free gadolinium alloy, however, the electrons
become disorganized at field strengths different from those
required to align them. Consequently, energy is required to
cycle the field, diminishing the material’s effective
capacity for cooling.
Adding
the iron supplement largely suppresses a rearrangement of
atoms that occurs as the applied magnetic field increases
(or temperature decreases) in the original gadolinium alloy.
In turn, stifling the shift in atomic structure all but eliminates
the hysteresis loss, the NIST team found. This substantially
increased the efficiency of the refrigerant by reducing the
energy cost corresponding to the application, and then, removal
of a magnetic field.
As important,
the iron broadened the range of temperatures over which the
material achieves acceptable cooling performance, peaking
at 32 degrees Celsius (89 degrees Fahrenheit), compared with
0 degrees Celsius (32 degrees Fahrenheit) for the iron-free
alloy.
NIST
magnetics researcher Robert Shull, one of the NIST inventors
of the new nanocomposite material, notes that the modest addition
of iron results in the formation of nanometer-sized magnetic
clusters in the gadolinium alloy. In earlier research, Shull
and his colleagues demonstrated that dispersing what are essentially
tiny magnets throughout a material can greatly enhance the
size of the temperature increase resulting from the application
of an external magnetic field.
"How
such a nano-magnetic structure developed is unknown,”
Shull explains. “It’s existence is certain, but
its cause is very subtle.”
As a
non-regulatory agency of the U.S. Department of Commerce’s
Technology Administration, NIST develops and romotes measurement,
standards and technology to enhance productivity, facilitate
trade and improve the quality of life.
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