Researchers are proposing a new technology that might control the flow of heat the way electronic devices control electrical current, an advance that could have applications in a diverse range of fields from electronics to textiles.
The concept uses tiny triangular structures to control "phonons," quantum-mechanical phenomena that describe how vibrations travel through a material's crystal structure.
Findings in research using advanced simulations show the triangular or T-shaped structures - if small enough in width - are capable of "thermal rectification," or permitting a greater flow of heat in one direction than in the opposite direction, said
Rectification has made possible transistors, diodes and memory circuits central to the semiconductor industry. The new devices are thermal rectifiers that might perform the same function, but with phonons instead of electrical current.
"In most systems, heat flow is equal in both directions, so there are no thermal devices like electrical diodes. However, if we are able to control heat flow like we control electricity using diodes then we can enable a lot of new and exciting thermal devices including thermal switches, thermal transistors, logic gates and memory," said Ruan, whose research group collaborated with a group led by
Findings are detailed in a research paper that has appeared online in the journal
The researchers used an advanced simulation method called molecular dynamics to demonstrate thermal rectification in structures called "asymmetric graphene nanoribbons." Molecular dynamics simulations can simulate the vibrations of atoms and predict the heat flow in a material.
Hu, Ruan, and Chen also published a paper four years ago in the journal
"We demonstrate that other asymmetric materials, such as asymmetric nanowires, thin ?lms, and quantum dots of a single material can also be high-performance thermal recti?ers, as long as you have lateral confinement," Ruan said. "This really broadens the potential of this rectification to a much wider spectrum of applications."
Thermal rectification is not seen in larger triangular-shape structures because they lack lateral confinement. In order for lateral confinement to be produced, the cross section of the structure must be much smaller than the "mean free path" of a phonon, or only a few to hundreds of nanometers depending on the material, Wang said.
"This is the average distance a phonon can travel before it collides with another phonon," he said.
However, although the devices must be tiny, they could be linked in series to produce larger structures and better rectification performance.
The concept could find uses in "thermal management" applications for computers and electronics, buildings and even clothing.
"For example, on a winter night you don't want a building to lose heat quickly to the outside, while during the day you want the building to be warmed up by the sun, so it would be good to have building materials that permit the flow of heat in one direction, but not the other," Ruan said.
A potential, although speculative, future application could be thermal transistors. Unlike conventional transistors, thermal transistors would not require the use of silicon, are based on phonons rather than electrons and might make use of the large amount of waste heat that is already generated in most practical electronics, said Chen.
The research was funded by the
Note to Journalists: An electronic copy of the research paper is available from
Phonon Lateral Con?nement Enables Thermal Recti?cation in Asymmetric Single-Material Nanostructures
Yan Wang,dagger,double dagger Ajit Vallabhaneni,dagger,double dagger Jiuning Hu,double dagger,section Bo Qiu,dagger,double dagger
double dagger Birck Nanotechnology Center,
?Department of Physics,
We show that thermal recti?cation (TR) in asymmetric graphene nanoribbons (GNRs) is originated from phonon con?nement in the lateral dimension, which is a fundamentally new mechanism different from that in macroscopic heterojunctions. Our molecular dynamics simulations reveal that, though TR is significant in nanosized asymmetric GNRs, it diminishes at larger width. By solving the heat diffusion equation, we prove that TR is indeed absent in both the total heat transfer rate and local heat ?ux for bulk-size asymmetric single materials, regardless of the device geometry or the anisotropy of the thermal conductivity. For a deeper understanding of why lateral con?nement is needed, we have performed phonon spectra analysis and shown that phonon lateral con?nement can enable three possible mechanisms for TR: phonon spectra overlap, inseparable dependence of thermal conductivity on temperature and space, and phonon edge localization, which are essentially related to each other in a complicated manner. Under such guidance, we demonstrate that other asymmetric nanostructures, such as asymmetric nanowires, thin ?lms, and quantum dots, of a single material are potentially high-performance thermal rectifiers.
TNS 30TagarumaMar-140128-4615537 30TagarumaMar
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