Magnetic nanoparticles could aid heat dissipation.


Cooling systems generally rely on water pumped through pipes to remove unwanted heat. Now, researchers at MIT and in Australia have found a way of enhancing heat transfer in such systems by using magnetic fields, a method that could prevent hotspots that can lead to system failures. The system could also be applied to cooling everything from electronic devices to advanced fusion reactors, they say.

The system, which relies on a slurry of tiny particles of magnetite, a form of iron oxide, is described in the International Journal of Heat and Mass Transfer, in a paper co-authored by MIT researchers Jacopo Buongiorno and Lin-Wen Hu, and four others.

Hu, associate director of MIT’s Nuclear Reactor Laboratory, says the new results are the culmination of several years of research on nanofluids—nanoparticles dissolved in water. The new work involved experiments where the magnetite nanofluid flowed through tubes and was manipulated by magnets placed on the outside of the tubes.

The magnets, Hu says, “attract the particles closer to the heated surface” of the tube, greatly enhancing the transfer of heat from the fluid, through the walls of the tube, and into the outside air. Without the magnets in place, the fluid behaves just like water, with no change in its cooling properties. But with the magnets, the  is higher, she says—in the best case, about 300 percent better than with plain water. “We were very surprised” by the magnitude of the improvement, Hu says.

Conventional methods to increase heat transfer in  employ features such as fins and grooves on the surfaces of the pipes, increasing their surface area. That provides some improvement in heat transfer, Hu says, but not nearly as much as the particles. Also, fabrication of these features can be expensive.

The explanation for the improvement in the new system, Hu says, is that the magnetic field tends to cause the particles to clump together—possibly forming a chainlike structure on the side of the tube closest to the magnet, disrupting the flow there, and increasing the local temperature gradient.

While the idea has been suggested before, it had never been proved in action, Hu says. “This is the first work we know of that demonstrates this experimentally,” she says.

Magnetic nanoparticles could aid heat dissipation

Such a system would be impractical for application to an entire cooling system, she says, but could be useful in any system where hotspots appear on the surface of cooling pipes. One way to deal with that would be to put in a magnetic fluid, and magnets outside the pipe next to the hotspot, to enhance heat transfer at that spot.

“It’s a neat way to enhance heat transfer,” says Buongiorno, an associate professor of nuclear science and engineering at MIT. “You can imagine magnets put at strategic locations,” and if those are electromagnets that can be switched on and off, “when you want to turn the cooling up, you turn up the magnets, and get a very localized cooling there.”

While  can be enhanced in other ways, such as by simply pumping the cooling fluid through the system faster, such methods use more energy and increase the pressure drop in the system, which may not be desirable in some situations.

There could be numerous applications for such a system, Buongiorno says: “You can think of other systems that require not necessarily systemwide cooling, but localized cooling.” For example, microchips and other electronic systems may have areas that are subject to strong heating. New devices such as “lab on a chip” microsystems could also benefit from such selective cooling, he says.

Going forward, Buongiorno says, this approach might even be useful for fusion reactors, where there can be “localized hotspots where the heat flux is much higher than the average.”

But these applications remain well in the future, the researchers say. “This is a basic study at the point,” Buongiorno says. “It just shows this effect happens.”

Hypersensitive Wires Feel the (Electromagnetic) Force.


The ability to pack bits of data on computer hard drives has skyrocketed more than 10,000-fold over the past 3 decades. You can now fit more than 100 Hollywood movies on the average machine. One reason has been the steady improvement in sensors used to read and write bits of data in the magnetic materials used to make the disks. Now researchers describe the most powerful such sensing material yet to work at room temperature. The discovery may open the door not just to reading out smaller data bits, but also to a wide range of improved magnetic technologies such as making cheaper touch screen displays.

At the heart of data reading and recording devices is a property called magnetoresistance (MR), in which the electrical resistance of a material changes in response to the presence of an external magnetic field. Turn on a magnetic field, and the material’s ability to carry an electric current skyrockets or plummets in response. Early MR materials changed their resistance only by a few percent at room temperature. Giant magnetoresistive materials discovered in the late 1980s pushed the number up to 110%. And researchers in Japan raised it to 600% in 2002 with the discovery of materials that carry out something called tunnel magnetoresistance. But now all those numbers pale in comparison, as a paper published online today inScience reports that molecular wires are capable of a 2000% magnetoresistance change at room temperature .

Ironically, the new molecular wires aren’t made with magnetic materials at all. Rather, their MR effect relies on the conductivity of nonmagnetic organic dye molecules called DXP, which the Italian automaker Ferrari once used to give their roadsters their trademark red color. Unlike conventional inorganic metals in which electrons zip through a crystalline lattice, in organics electrons must hop from one molecule to another, like pails of water being passed by a bucket brigade. To create a MR, material researchers need to switch off that bucket brigade in the presence of a magnetic field.

In organic materials researchers do this with a little help from quantum mechanics. A tenet of quantum mechanics called the Pauli Exclusion Principle states that no two fermions (particles in a family that includes electrons) can occupy the same quantum state. If two electrons with the same quantum state try to hop onto the same DXP, they can’t. The bucket brigade turns off and resistance skyrockets.

But over the past several years, researchers have found that thin films of DXPs or other organic conductors have an MR well below the competition. The reason for this turned out to be another quantum mechanical property. In addition to carrying a negative electric charge, electrons also carry spin, which can point up or down like a tiny bar magnet. If two electrons have the same spin, they can’t hop on the same DXP together. But if one electron’s spin flips to the opposite direction, then it’s no problem. The two can hop on one DXP together, and the bucket brigade continues.

In their work with films of DXPs and other organics, researchers found that two problems prevented the films from acting like good MR materials. First, thermal fluctuations at room temperature flipped electron spins. And second, even if electrons did share the same spin direction—and were thus blocked from hopping onto the same DXP—they just jumped to a neighbor that wasn’t blocked. “If it’s a 3D film, you can always go around the blockade,” says Markus Wohlgenannt, a physicist at the University of Iowa in Iowa City, whose team was one of the first to discover organic MR materials.

To prevent this runaround, researchers led by Wilfred van der Wiel, a physicist at the University of Twente in the Netherlands sought to arrange the DXPs in straight lines. To do so, they essentially shoved them inside the narrow pores of a zeolite, a lattice-like mineral, in which the confines were so tight the organics had no choice but to line up. They then placed their zeolite atop a conductive surface with the pores facing up and used the tip of an atomic force microscope to make contact with individual DXPs at the top end of single pores. The lineup of DXPs obviously meant that electrons could no longer hop around a blockade. But they also found that even very small magnetic fields were enough to prevent thermal fluctuations from flipping electron spins. And the result was that when electrons encountered blockages, they were unable to work around them, and the resistance of the material shot upwards.

Wohlgenannt calls the new work “a groundbreaking paper.” That said, he adds that it’s not clear if this will lead to higher capacity disc drives. For starters, researchers must first pull off the effect without the use of atomic force microscopes, which aren’t a practical addition to disk drive technology. Researchers will probably also need to figure out ways to push higher electrical currents through the molecular wires to make magnetic sensors that can compete with current technology. But even if the new materials aren’t ideal for making better disk drives, Wohngenannt and van der Wiel say the powerful MR effect might still make them useful for other electronics applications such as pen-based touch screens that are responsive to a magnetic stylus or perhaps even improved magnetic sensors in smart phones that are able to pick up the Earth’s magnetic field and use that for improved navigation.

Source: sciencemag.org