Tikalon Blog by Dev Gualtieri

Magnetic Refrigeration

September 3, 2014

One simple, but fundamental, experiment demonstrating the existence of atoms is the one shown in the figure. This experiment demonstrates the Einstein-de Haas effect in which a suspended cylinder of iron twists when a magnetic field is applied. The field generated by the solenoid is parallel to the axis of the cylinder; so, according to classical physics, there should be no twist. The twist arises from the angular momentum of the electrons of the iron atoms.

An experiment demonstrating the Einstein-de Haas effect

An iron cylinder hanging from a thin wire twists when a magnetic field is applied along its axis. In this case, the field is generated by a solenoid.

The twist is measured by light deflected from a mirror.

(Figure by Jasper Olbrich, modified, via Wikimedia Commons.)

One other notable coupling of a field and a mechanical property is the piezoelectric effect by which an electric field is generated by mechanical strain; or, inversely, an electric field can generate a mechanical strain. Pyroelectricity is another electric field effect present in materials called pyroelectrics , an example of which is tourmaline .[1] As I wrote in an earlier article (Pyroelectric Energy Harvesting, October 15, 2010), pyroelectrics generate a temporary voltage when heated or cooled.

Nature loves symmetry, so we're not surprised by the fact that an inverse of the pyroelectric effect exists. In the electrocaloric effect, materials show a reversible temperature change in response to an applied electric field. The supposed mechanism is a change of the system entropy as electric dipoles align themselves with the field. The piezoelectric material, PZT (lead zirconate titanate) demonstrates a cooling of more than 12 °C when a field of 480 kilovolts per centimeter is applied at 215 °C.[2]

Magnetic materials demonstrate a similar magnetocaloric effect. In 1881, German physicist Emil Warburg found that iron would cool about a degree Celsius when subjected to an applied field of 10,000 gauss. To illustrate how small this effect is, Earth's magnetic field is about half a gauss. Although many credit Warburg with the discovery of the magnetocaloric effect, some believe that credit should really go to Pierre Weiss and Auguste Piccard.[3]

It's possible to use the magnetocaloric effect in a magnetic refrigerator. The thermodynamics of the refrigeration cycle are shown in the figure, and it's essentially a way of using the entropy associated with the alignment of the magnetic moments of the atoms in a solid as a heat pump.

The thermodynamic cycle of a magnetic refrigerator.

(Click for a larger image.)

The key to a good magnetic refrigerator is the magnetic material. The magnetocaloric effect is greatest near the vicinity of a magnetic phase transition. Some alloys of gadolinium, such as Gd₅Si₂Ge₂, exhibit a "giant magnetocaloric effect" (GMCE) around room temperature.[4] Some of my former colleagues worked on similar materials in the 1990s.[5]

Magnetocaloric materials act as discontinuous heat pumps, since they must go through their thermodynamic cycle to act as a refrigerator. A group of mechanical engineers at the Massachusetts Institute of Technology (MIT) have just published a paper that describes how the magnetic quasiparticles called magnons can transport heat.[6-7]

Magnons are collective rotations of magnetic spins. The electrons responsible for the existence of these magnons are also conductors of heat, so a magnetic field gradient that moves the magnons will also transport heat.[7] By such means it would be possible to build a continuously operating refrigerator without moving parts. Says MIT graduate student, Bolin Liao, who is a coauthor of the study,

"You can pump heat from one side to the other, so you can essentially use a magnet as a refrigerator... You can envision wireless cooling where you apply a magnetic field [by] a magnet one or two meters away to, say, cool your laptop."[7]

Artist's conception of a magnon-enabled magnetic refrigerator.

The field gradient is depicted by the different size of the arrows.

(MIT illustration by Jose-Luis Olivares.)

The MIT magnon transport theory was based on the Boltzmann transport equation used to model electron transport in thermoelectrics. The modeled effect is more pronounced at cryogenic temperatures, so it would be most useful in laboratory experiments. The wireless aspect of the process would make some experiments easier to perform.[7] At this point, it appears that yttrium iron garnet would be a good material for experimental validation of the result.[6]

The work was funded by the U.S. Department of Energy and the U.S. Air Force Office of Scientific Research.[7]

Magnetic Refrigeration

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