COMPACT fridges that chill using magnets are on every physicist鈥檚 wish list, but now they have moved a step closer to our kitchens, as two teams unveiled the smallest magnetic refrigerators so far. The chillers don鈥檛 need to use coolants that damage the environment. And they promise to be more versatile and efficient than conventional fridges, leading to potential energy savings worth $10 billion every year in the US alone.
Magnetic refrigerators were first suggested in 1926. They exploit 鈥渕agnetocaloric鈥� metals, in which the atoms act like tiny bar magnets pointing in random directions. When they are placed in a strong magnetic field, the bar magnets quickly line up. This transition reduces the randomness, or entropy, of the atoms, and according to the laws of thermodynamics, entropy has to increase in some other way. In fact the atoms compensate by vibrating more vigorously, heating up the metal. Physicists soon realised that the process could be reversed: switching off the magnetic field would cool the metal.
To work as a refrigerator, the magnetocaloric material must be placed in a magnetic field while a coolant such as water carries its heat away. Removing the material from the field then chills the circulating coolant. Repeat the cycle fast enough and the temperature drops.
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But building a practical magnetic refrigerator has been dogged by problems because the cooling effect is quite weak in gadolinium 鈥� the simplest magnetocaloric material. In the past researchers got round this problem by using huge magnetic fields that can only be generated by bulky superconducting magnets several metres high.
Now Naoki Hirano鈥檚 team at Chubu Electric in Nagoya, Japan, and a team led by Carl Zimm at Astronautics in Madison, Wisconsin, have independently come up with a new design that makes magnetic refrigerators more practical. Their idea is to whirl a tray filled with balls made of gadolinium or an alloy called gadolinium dysprosium in and out of a magnetic field hundreds of times every second, while water is pumped through the spheres ever few seconds to remove the heat. Such high speeds mean that you can achieve the same effect with much weaker magnetic fields, so you can use much cheaper and smaller permanent magnets. Chubu鈥檚 fridge is just 75 centimetres long and so far can chill objects to ten degrees below room temperature.
