Image courtesy of John Cumings
Artist's rendering of remote Joule heating. Silver blocks are palladium plates. Carbon nanotube is shown in dark blue.
If you had to summarize the biggest challenge confronting the field of electronics in a single word today, you might well say, "heat." With the extreme miniaturization of electronic components, and the packing of millions upon millions of transistors into today's microchips, heat management in electronic devices has become a growing problem—and is likely to become even more daunting in the future.
The key concern is with resistive heating, also known as "Joule heating," which occurs as a result of electrons encountering resistance as they travel through wires and other conductors. Joule heating (named for the famous nineteenth-century British physicist James Prescott Joule) is a fancy name for a phenomenon that is in fact familiar and pervasive. It's what happens when you turn on an electric range or switch on an electric iron. It's the source of heat in countless devices such coffee machines, hair driers, and space heaters, and for that matter in millions of electrically heated homes. And it's why the bottom of your laptop is often warm to the touch.
Industry has pursued a number of strategies to manage Joule heating in electronic devices—a major one being the development of more and more sophisticated "heat sinks," devices designed to absorb heat, often combined with a fan to dissipate the heat after it is absorbed. But scientists are also looking over the horizon at more radical potential solutions to the problem. These speculative new approaches are for the most part in the very early stages of theoretical and experimental development. Among them are "photonic crystals," semiconductors that work on cooler light rather than hot electricity, and "topological insulators," exotic materials that conduct electricity virtually without resistance on their surface.
Still other scientists are studying novel materials such as graphene and carbon nanotubes, which have special electronic and thermal properties not found in metals. Recently, a team of researchers at the University of Maryland (UMD) has demonstrated that a carbon nanotube can conduct electricity without heating up, while it simultaneously heats up the material on which it is mounted. The result is puzzling: the nanotube "wire" stays (mostly) cool, but the material on which the wire is mounted grows hot, somehow absorbing heat energy from the electricity that is flowing through the wire, without the wire itself heating up.
Probably the closest analogy to this effect in everyday life is the operation of a microwave oven, whose walls stay cool even as your dinner warms up. (Induction cooking, which operates on magnetic fields rather than heat transfer, is another example, though somewhat less familiar.) But the physical principles at work in the UMD experiments turn out to be quite different from those involved in microwave or induction cooking, which use alternating current to deliberately generate electromagnetic fields.
The researchers call the new phenomenon "remote Joule heating." It's importance—apart from the pure scientific interest of the discovery—is that it might eventually point toward a means of segregating electrical and thermal behavior in an electronic system, thereby laying the groundwork for a wholly new approach to heat management. The research, led by UMD Associate Professor of Materials Science John Cumings and published in the journal Nature Nanotechnology, was supported by DOE's Office of Science.
This phenomenon of remote Joule heating was predicted (though not so named) back in 2009. Cumings's group, however, is the first to demonstrate the phenomenon experimentally.
The researchers argue that a better understanding of the effect could potentially lead to new approaches to heat management in electronics.”
To do so, the researchers relied on a novel method of microscopic thermal imaging that Cumings and an earlier team had developed in the middle of the last decade. The method, called "electron thermal microscopy," is designed to overcome the limitations of traditional thermal microscopy, which uses infrared light. Hot objects generate infrared radiation, and detecting this infrared light is usually a great method for detecting heat. However, when you're working at the nanoscale, the problem with infrared light is that its wavelength is too long. The shortest wavelength in the infrared portion of the spectrum, at .75 micrometers, is too large for imaging small nanometer-scale objects.
Cumings's method instead uses a standard transmission electron microscope. The researchers take a thin membrane or "substrate" of silicon nitride and speckle the back of it with myriad nanoscale particles made of the metal indium. These indium particles have a melting point of 156.6°C, and when they melt they have the handy property of not changing shape. So they can melt and "freeze" reversibly while remaining attached to the silicon nitride. When these particles do melt, they change contrast inside the electron microscope from dark to light. The result is that the heated area of the silicon nitride membrane (which is transparent to the electrons emitted by the microscope) shows up as a lighter or brighter area in the electron microscope image. Where heat is concentrated, in short, you see a bright spot.
Using this technique, the Cumings group staged two experiments in sequence.
First, they performed an initial experiment to show that the pattern of heating in and around the carbon nanotube was different from what would be expected if Joule heating were really at work. Second, they developed an even more ingenious experiment to "take the temperature" of the carbon nanotube itself.
In the first experiment, the carbon nanotube was mounted on a silicon nitride membrane (with the required indium islands on the reverse side). The ends of the nanotube were covered by plates that were made of palladium metal and that were connected to the positive and negative terminals of a power source. Electrical current at various voltages was run through the nanotube via the two palladium plates, and a "map" of the resulting heated area of the silicon nitride was developed.
The map turned out to be at odds with what one would expect if Joule heating were taking place.
If Joule heating were at work, the electrical current would cause the carbon nanotube to heat up. The carbon nanotube would in turn transmit heat to its surroundings. However, the palladium metal plates make a better thermal contact to the nanotube, and you would therefore expect to see a pattern of maximum heat at the two ends of the carbon nanotube wire. This expectation was confirmed through mathematical modeling of the system. Contrary to this expectation, the experiments showed that the heat peaked in the area of the silicon nitride in the middle of the electric circuit.
So it appeared that Joule heating was not the appropriate model for understanding the behavior of the system.
Photo courtesy of John Cumings.
University of Maryland Associate Professor John Cumings led the research.
Then came the second experiment, designed to demonstrate that unlike an ordinary wire, the carbon nanotube was not really being heated very much at all by the electrical current flowing through it. Here again, the researchers created a circuit using two plates of palladium, one connected to the positive, the other to the negative terminal of a power source. The circuit itself was only about a hundred nanometers across. But the researchers extended the nanotube wire beyond the circuit out to a third, small, isolated plate of palladium, over one thousand nanometers away.
The reasoning was as follows. It is known from many previous experiments that carbon nanotubes are excellent heat conductors, and if the electrical current was heating the nanotube wire, you would expect it to transmit heat readily to the third small palladium plate. Therefore, you would expect the heat to emerge first in the silicon nitride region around the third palladium plate. Again, mathematical models confirmed this expectation.
In reality, the bright or heated zone observed in experiments remained centered on the area of silicon nitride directly in the middle of the electrical circuit. The distant palladium plate showed little to no evidence of having been heated. The implication was that the nanotube wire itself was not heating up as a result of the electrical current passing through it, even though the silicon nitride substrate directly beneath the circuit was heating up.
The researchers performed further mathematical simulations to confirm their conclusions. When Joule heating was assumed, the simulation showed heat emerging, as would be expected, around the third palladium plate. Then the researchers added a new term to the simulation representing remote Joule heating. The term—which the researchers called "beta"—had a value of 0 if there was no remote Joule heating and a value of 1 if all heat was transmitted to the substrate via remote Joule heating. The researchers found that with a beta value in the range of 0.84 to 1.0, the simulation closely matched the experimental results. The result implied that at least 84 percent of the heat energy generated by the flow of electrons through the carbon nanotube was being transferred to the silicon nitride substrate via remote Joule heating.
The effect, though not fully understood, seems to result from the interaction of electromagnetic fields generated by electrons flowing through the carbon nanotube with vibrations of surface atoms in the silicon nitride substrate. It's a surprising effect considering that the nanotube is conducting direct current rather than alternating current. The researchers argue that a better understanding of the effect could potentially lead to new approaches to heat management in electronics. In any event, our deepening knowledge of such exotic effects at the nanoscale is likely to play an important role in shaping the electronics of the future.
—Patrick Glynn, DOE Office of Science, Patrick.Glynn@science.doe.gov
Kamal H. Baloch, Norvik Voskanian, Merijntje Bronsgeest, and John Cumings, "Remote Joule heating by a carbon nanotube," Nature Nanotechnology 7, 316 (2012).
DOE Office of Science, Office of Basic Energy Sciences (Research Support)
Nuclear Regulatory Commission (Faculty Development Grant)
University of Maryland, A. James Clark School of Engineering, Department of Materials Science and Engineering
DOE Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division