Device physics: the heat is on--and off.


T he discovery of the equivalence between energy and heat was one of the most important achievements in the history of science, and was also of immense practical importance as it underpinned the industrial revolution that started in the late eighteenth century. Th e discovery of electricity in the following century was equally important, and its impact is still being felt today. Devices that allow electric charges to fl ow in only one direction, such as electrical rectifi ers and semiconductor diodes, play a central role in modern technology, so one might wonder why similar devices have not been developed to control the fl ow of heat. Writing in Science, Alex Zettl and colleagues1 at the University of California at Berkeley and the Lawrence Berkeley National Laboratory report the fi rst experimental thermal rectifi er, which is based on ideas put forward by myself and co-workers just four years ago2. Heat always fl ows from hot to cold but, analogous to the electric diode, a thermal rectifi er only allows heat to fl ow in one direction. Research in this fi eld could eventually lead to entirely new types of devices that can control the heat fl ow in various applications in electronics and biotechnology. One of the basic principles of thermodynamics, the Fourier law of heat conduction, states that the heat current is proportional to the temperature gradient. However this law is empirical in nature and relies on statistical assumptions. In the 1930s Pierre Debye pointed out that nonlinear eff ects play a crucial role, and his work was continued in the 1950s by Enrico Fermi and other great physicists, but it was only in the second part of the last century that physicists fi rst attempted to provide a rigorous justifi cation of the Fourier law— a task that some of their colleagues are still working on. As Rudolph Peierls said in the 1950s: “It seems there is no problem in modern physics for which there are on record as many false starts and as many theories which overlook some essential features as in the problem of the thermal conductivity of non-conducting crystals”. It is known that there are several cases in which heat fl ows much faster or slower than predicted by the Fourier law. In 2002 my co-workers and I realized that by taking advantage of materials with diff erent conduction properties, it is possible to propose a device that allows the heat current to fl ow mainly in one direction and only weakly in the opposite one2. Although at fi rst sight this might seem to violate one of the basic principles of thermodynamics, if we are clever enough, it is possible to control the fl ow of heat in non-equilibrium systems without doing so. How can we do this? First we recall that thermal energy or heat is a manifestation of the vibration of atoms. When atoms are joined together in molecules or in a crystal, they vibrate back and forth. Whereas the amplitude of the vibrations increases with the temperature, their frequency depends on By coating a nanotube with a molecular layer that is thicker on one end than the other, it is possible to make a thermal rectifi er that allows heat to fl ow easily along the tube in one direction, but not so easily in the opposite direction. DEVICE PHYSICS

DOI: 10.1038/nnano.2006.191

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Cite this paper

@article{Casati2007DevicePT, title={Device physics: the heat is on--and off.}, author={Giulio Casati}, journal={Nature nanotechnology}, year={2007}, volume={2 1}, pages={23-4} }