—by Marilyn Davis

In keeping our milk fresh and our veggies crisp, refrigerators of the future may also enable us to be more energy-independent. How? By using magnets and alloys, not compressors, to chill out.

SIUC physics professors Naushad Ali and Shane Stadler have received a four-year, $620,000 grant from the U.S. Department of Energy to optimize an alloy they've developed that is far better than any others to date at magnetic cooling.

Under the influence of a magnetic field, certain alloys become magnetically "organized" and heat up. When the field is removed, they lose that magnetic organization and cool down. The phenomenon is called the magnetocaloric effect. If the heat generated by the magnetic field is drawn off—by water, for example—the material will get colder upon the removal of the field than it was to start with. Hence: solid-state refrigeration.

Scientific devices have exploited the magnetocaloric effect for years. But only now is industry taking the first steps toward commercial cooling devices that rely on magnets rather than on compressed gases like ozone-depleting Freon and its modern replacements (which may still have some ozone-depletion effects).

Why has it taken so long?

In most materials, the magnetocaloric effect kicks in at temperatures far too low for consumer applications. Materials now being used for first-generation prototypes of commercial magnet-based refrigerators do work at room temperature, but they're expensive and require the power of an electromagnet.

The research team led by Ali and Stadler may have solved both problems. They have modified a common alloy so that at room temperature, under the influence of an ordinary permanent magnet, it yields a whopping magnetocaloric effect. The cooling potential is huge, greatly exceeding that of any other known material near room temperature. The researchers reported their findings in the journal Applied Physics Letters last year.

The discovery grew out of the two physicists' work with so-called "shape-memory" alloys, which expand or contract under a magnetic field and return to their original shape when the field is removed. Such alloys are used as magnetically controlled actuators for mechanical devices. "No one had really explored their magnetocaloric effects," Stadler says.

Assisted by doctoral student Mahmud Khan and others, Ali and Stadler were trying to get this structural change to take place at room temperature in an alloy made of nickel, manganese, and gallium. They found that replacing a small amount of the manganese with copper did the trick, and it also made the transition take place in a very pronounced way.

That got them wondering about the possible magnetocaloric effect of this alloy. Materials susceptible to magnetic cooling undergo both a change in structure and a change in magnetic organization. In shape-memory alloys, these transitions usually take place at very different temperatures. But by experimenting with different proportions of copper, the research team was able to "tune" their new alloy so that the structural and magnetic transitions occur at the same temperature.

"When this happens, the refrigeration effect is the largest," Ali says.

Current prototypes of magnet-based refrigerators use alloys made with the rare-earth metal gadolinium. "It's a good material," Stadler says, "but it's very expensive, and it's more toxic than our material."

In contrast, the component elements of the SIUC team's alloy are abundant, affordable, and relatively nontoxic. Plus, the alloy is easy to make.

How would a refrigerator based on such an alloy work? Here's a likely scenario.

A rotating disk that incorporated the alloy in some of its sectors would pass through a magnetic field. As a part of the disk containing the alloy entered the field, the alloy would heat up. A heat exchanger using water or some other liquid would carry away the heat. As the alloy moved out of the magnetic field, it would cool below room temperature, chilling a nearby food compartment.

The system's advantages would be simplicity and efficiency.

"It could very well be that, in the end, making these systems commercially would be cheaper and easier than making current compressed-gas systems," Stadler says.

"The mechanism would be very simple. There would be few moving parts. There would be no compressor; you'd just need a small motor to turn your disk, and maybe a circulator. You'd have no worry about gas leaks or anything like that, so it would be better for the environment."

Running this type of cooling device would be cheaper, too—another environmental benefit, since refrigerators are among the most energy-hungry appliances that consumers own.

"It would be much more energy-efficient," Stadler says. "For the same amount of electricity going into the process, you'd get a much greater cooling effect. These materials are already at 70 percent [efficiency], and they're not even developed to the point where they're optimized." (The efficiency yield with a compressed-gas system is about 40 percent.)

SIUC is filing for a patent on the material and its potential applications. However, more work needs to be done before the alloy can be commercialized.

For example, the researchers are trying to expand the temperature range at which the magnetocaloric effect can take place. They're doing that by experimenting with variations of the alloy and of the fabrication process.

Besides Ali, Stadler, and Khan (who has done many of the measurements of the alloy's properties), the team includes master's student Arjun Pathak; visiting Canadian scientist Igor Dubenko; and three Brazilian physicists based in Rio de Janeiro.

Understanding why this particular alloy possesses such unusual properties would be an important step forward in taking the magnetocaloric effect out of the laboratory and into the marketplace, Stadler says. Therefore the researchers will study the material on the atomic level by using powerful synchrotron radiation sources at two national laboratories.

"Our plan is to find out the magnetic character of each [type of atom] as the material goes through the various transitions, and determine the difference between a material that shows a high magnetocaloric effect and one that doesn't," Stadler says.

"We're trying to figure out what's happening—how, when, and why these transitions occur. If we can do that, then we can predict what it takes to optimize the effect."