The magnetocaloric effect (Scientific American, May 1998, p. 44) relies on the ability of a ferromagnetic material to heat up in the presence of a magnetic field and then cool down once the field is removed. When a ferromagnet is placed in a magnetic field, the magnetic moments of its atoms become aligned, making the material more ordered. But the amount of entropy must be conserved, so the atoms vibrate more rapidly, raising the material's temperature.
When the material is taken out of the field, the material cools. Water, or some other heat transfer fluid, can be cooled down by running it through the ferromagnetic material as it cools. (a) The magnitude of the magnetocaloric effect reaches a maximum at the Curie temperature of the ferromagnet. Based upon the above description of the application and the Curie temperature, which of the following three materials will make the best ferromagnetic material for a typical household refrigeration unit? Justify your selection.
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Saturation
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Magnetization° at
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Curie Temperature
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Material
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Room Temperature
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(K)
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Melting Point (K)
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Iron
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1707
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1043
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1811
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Gadolinium
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1090
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289
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1585
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Nickel
|
485
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631
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1728
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° Maximum possible magnetization that results when all magnetic dipoles am aligned with the external field.
(b) The amount of refrigeration is also determined by the strength of the applied magnetic field. Superconducting magnets are used to generate massive magnetic fields that cause large magnetizations in the ferromagnetic inductor. Below are three candidate materials for the superconducting magnet. Based on what you know about superconductors, which of these three materials will make the best superconducting magnet for this application? Justify your selection.
| Material |
Tc, K |
Hc, tesla |
| Bi2Sr2Ca2Cu3O10 |
110 |
100 |
| Gadolinium |
5.9 |
0.0001 |
| NbTi |
9.5 |
10 |