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.
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. 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.
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.
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 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. Some of my former colleagues worked on similar materials in the 1990s.
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. 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."
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. At this point, it appears that yttrium iron garnet would be a good material for experimental validation of the result.
The work was funded by the U.S. Department of Energy and the U.S. Air Force Office of Scientific Research.
- Sidney B. Lang, "Pyroelectricity: From Ancient Curiosity to Modern Imaging Tool, Physics Today, August 2005, pp. 31-36.
- A. Mischenko, Q. Zhang, J.F. Scott, R.W. Whatmore, and N.D. Mathur, "Giant electrocaloric effect in thin film Pb Zr_0.95 Ti_0.05 O_3," arXiv Preprint Server, November 19, 2005. Also appears as A. Mischenko, Q. Zhang, J.F. Scott, R.W. Whatmore, and N.D. Mathur, "Giant Electrocaloric Effect in Thin-Film PbZr0.95Ti0.05O3," Science, vol.311, no. 5765 (March 3, 2006) pp. 1270-1271.
- Anders Smith, "Who discovered the magnetocaloric effect?" The European Physical Journal H, vol. 38, no 4 (September,2013), pp 507-517.
- V. K. Pecharsky and K. A. Gschneidner, Jr., "Giant Magnetocaloric Effect in Gd5(Si2Ge2)," Phys. Rev. Lett., vol. 78 (June 9, 1997), Document No. 4494, DOI: http://dx.doi.org/10.1103/PhysRevLett.78.4494.
- J. M. Elbicki, L. Y. Zhang, R. T. Obermeyer, and W. E. Wallace, "Magnetic studies of (Gd1−x M x )5Si4 alloys (M=La or Y)," J. Appl. Phys., vol. 69, no. 8 (April 15, 1991), pp. 5571ff.
- Bolin Liao, Jiawei Zhou, and Gang Chen, "Generalized Two-Temperature Model for Coupled Phonon-Magnon Diffusion," Phys. Rev. Lett., vol. 113, Document No. 025902 (July 10, 2014), DOI: http://dx.doi.org/10.1103/PhysRevLett.113.025902.
- Jennifer Chu, "Refrigerator magnets," MIT Press Release, July 28, 2014.
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Linked Keywords: Experiment; atom; Einstein-de Haas effect; cylinder; iron; magnetic field; solenoid; parallel; classical physics; angular momentum; electron; iron; wire; magnetic field; light; mirror; Wikimedia Commons; field; mechanical property; piezoelectricity; piezoelectric effect; electric field; mechanical strain; pyroelectricity; material; pyroelectric; tourmaline; voltage; heat; Nature; symmetry; inverse; electrocaloric effect; reversible process; temperature; entropy; electric dipole moment; electric dipole; lead zirconate titanate; celsius; kilovolt; centimeter; magnet; magnetic material; magnetic refrigeration; magnetocaloric effect; Germany; German; physicist; Emil Warburg; gauss; Earth's magnetic field; Pierre Weiss; Auguste Piccard; thermodynamics; refrigeration cycle; entropy; heat pump; thermodynamic cycle; phase transition; gadolinium; Gd; silicon; Si; germanium; Ge; room temperature; colleague; 1990s; mechanical engineering; mechanical engineer; Massachusetts Institute of Technology; scientific literature; publish; academic publishing; paper; quasiparticle; magnon; heat; thermal conductivity; conductor of heat; gradient; graduate student; Bolin Liao; coauthor; artist's conception; Jose-Luis Olivares; Boltzmann transport equation; mathematical model; thermoelectric material; thermoelectric; cryogenic temperature; laboratory; yttrium iron garnet; U.S. Department of Energy; U.S. Air Force Office of Scientific Research.
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