One-way heat valve possible

By Kimberly Patch, Technology Research News

Learning to control the flow of electrons enabled every device that uses a battery or outlet. Controlling the way heat flows may also be useful, but has proved more elusive.

Researchers from Universities in France and Italy have come up with a scheme to control the flow of heat in a solid. The scheme takes advantage of the different ways unlike materials conduct heat at different temperatures.

The scheme shows that it is possible to construct a thermal rectifier, which could eventually lead to entirely new types of devices that take advantage of heat flow. Electrical rectifiers, often in the form of semiconductor diodes, are a critical component of electronics because they allow electrical current to flow in only one direction.

While there are hints that some biological molecules can control the flow of heat, at first glance the possibility seems out of step with a basic rule of physics, the principle of thermodynamics. "While electrical current is under control in all electronic devices, heat current was still considered as something that one cannot control except... by mechanically removing a connection or putting in an insulating material," said Marcello Terraneo, a researcher who collaborated on the work when he was at the Insubria University at Como, Italy and is now at P. Sabatier University in France.

The basic law of thermodynamics says that heat cannot flow from a cool place to a hot place. This seems like it runs counter to the concept of shunting heat around in a solid, but the researchers work does not violate thermodynamics simply because the principle refers to closed systems. The researchers' scheme, in contrast, is an open system -- heat can be moved in and out of it. Because "we're working out of equilibrium... it does not violate thermodynamics," said Terraneo.

The researchers' simulations show that it is possible to guide the flow of heat in a solid device that has certain nonlinear characteristics. The difference between linear and nonlinear systems is that the output of a linear system is directly proportional to its input.

Thermal energy, or heat, is a manifestation of the vibration of atoms. Near absolute zero, atoms vibrate much more slowly than they do at room temperature, for instance.

In a linear system, heat vibrations spread and thus conduct heat if their frequencies are included in a certain range -- the phonon bands. In nonlinear systems heat is transmitted less efficiently. The phonon bands that conduct heat well still exist, but are dependent on temperature. By controlling temperature, it is possible to cause a system to change from "a situation with a large heat influx to an almost-insulating behavior," said Terraneo.

The researchers worked out that it is possible to control heat this way using a one-dimensional chain of material with three distinct segments. "We have three different phonon bands for the three parts of the chain, [and] their temperature dependence is different as well," said Terraneo.

The middle segment is strongly nonlinear, and therefore its heat conductance changes considerably depending on the temperature. When the middle material is hot, it more easily conducts large heat vibrations, but when it is cold it more easily conducts lower heat vibrations. The photon bands of the two outer segments conduct at different frequencies that do not change much when the temperature of the material changes. The left side conducts large, higher-temperature vibrations, and the right lower-temperature vibrations.

When the left segment is hot and the right segment cold, heat easily travels from the hot left segment to the warm left end of the middle segment where both materials conduct large heat vibrations, then from the cooler right end of the middle segment to the cold right segment, where both materials conduct lower heat vibrations, said Terraneo.

If the temperature gradient is reversed, however, making the left segment cold and the right segment hot, the phonon bands of the central segment do not match well with the phonon bands of the end materials, and heat does not travel well through the system.

In addition, at low temperatures, the system lets less heat through, but as the temperature of the system as a whole rises, it conducts heat more efficiently. "This is possible since... the three bands overlap at large temperatures, while they are separated at small temperatures," Terraneo said. In order to propagate through the system a heat vibration must have a frequency that is present in the bands of all three regions, he said.

Although the researchers' proof-of-concept used a one-dimensional chain of material, the same principles should apply in the three-dimensional real world, said Terraneo.

In principle, a thermal rectifier, which would allow heat to flow in one direction, could be built from large molecules like DNA, Terraneo said. A thermal rectifier could also be built from a very weakly conducting substrate like glass layered with molecules that easily transfer heat to their neighbors, according to Terraneo. "By controlling this molecular layer, one could in principle get a rectifier. The choice of the appropriate molecules is, however, something we have not thought of yet," he said.

The big challenge in actually making a device would be to properly control the effective phonon bands, said Terraneo.

A working prototype could possibly be developed within five to ten years, he said. "But the effort will not be undertaken if one does not start thinking of possible applications for a thermal rectifier. The concept is too new to have generated ideas of applications," yet, he added.

One possible practical application is a device to reroute excess heat in microchips, said Terraneo. The work itself is also useful in better understanding energy transfer in biological molecules, he said.

It is known, for example, that the energy stored in muscles in the biological energy currency, ATP is released, then used elsewhere after some delay, said Terraneo. "Thus there should be some mechanism by which energy is locally stored and then released," he said. The current work is not a clear explanation of this, but "we have shown that playing with nonlinearities can give some results on heat control," he said. Strong nonlinearities exist in biological molecules and these could even be tuned by the molecules changing shape, according to Terraneo.

The work is a new look at a difficult problem, said Daniel Lesnic, a senior lecturer at the University of Leeds in England. "Energy transport in dynamical systems [is] one of the outstanding unsolved problems of modern physics," he said.

The researchers' work investigates the possibility of controlling the energy flow inside a nonlinear, one-dimensional chain connecting two thermostats at different temperatures, said Lesnic. "It offers a model for thermal rectifiers," he said.

Terraneo's research colleagues were M. Peyrard of Ecole Normale Supérieure in Lyon, France and G. Casatil of the Italian National Institute for the Physics of Matter (INFM) and the Italian National Institute for Nuclear Physics (INFN). They published the research in the March 4, 2002 issue of Physical Review Letters. The research was funded by the European Union and the Italian Research and University Ministry (MURST).

Timeline:   5-10 years, > 10 years
Funding:   Government, University
TRN Categories:  Materials Science and Engineering; Nanotechnology
Story Type:   News
Related Elements:  Technical paper, "Controlling the Energy Flow in Nonlinear Lattices: A Model for Thermal Rectifier," Physical Review Letters, March 4, 2002.


June 12/19, 2002

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One-way heat valve possible


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