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May 2013

New Ways of Thinking About Heat Flow

Study reveals wavelike behavior of phonons, which carry heat in materials

Dennis M. Callahan

By designing a periodic structure called a superlattice, researchers have identified the wavelike behavior of heat-carrying phonons for the first time. Photo credit: Adam Jandl and Maria Luckyanova.

Controlling the flow of heat and energy is an important part of improving energy conversion efficiency and reducing the generation of waste heat in everything from solar cells to consumer electronics. A team of researchers based at the Solid-State Solar Thermal Energy Conversion Center, S3TEC, took a major step towards manipulating the flow of heat at will by measuring the microscopic way in which it flows through a cleverly designed nanostructure, a thousand times thinner than a sheet of paper.

Heat is carried through a material by quasi-particles called phonons. Similar to how light travels, phonons can be considered both a particle and a wave. Wavelike phenomena, such as interference, are caused by a property called coherence, which is a measure of how uninterrupted the flow of a wave is. Heat, however, is usually observed to travel more like particles. These particles are thought to be constantly crashing into each other and into defects in the material, randomizing their direction and causing the waves to lose their coherence. The team showed for the first time that many of the phonons in a material could travel coherently with uninterrupted wavelike behavior.

To prove this, the team fabricated what is called a superlattice, or a structure containing alternating layers of two different materials. Usually, phonons scatter at the boundaries between two different materials, randomizing the phonons and destroying their coherence. What the researchers observed, however, was that as they added more layers to the superlattice—and more interfaces to scatter the phonons, the rate of heat transfer did not decrease. This result, as well as detailed computer simulations, suggested that the interfaces between layers were not randomizing the heat-carrying phonons, but instead preserving their coherence. This occurs because the periodicity of the superlattice alters the way the phonon travels because of repeated constructive or deconstructive interference events, much like the way an electron can act like a wave and travel through a crystal of atoms.

Understanding and predicting phonon behavior opens up new possibilities for controlling heat and energy flow on the nanoscale. “Once you can predict something, you can also design for it,” says Maria Luckyanova, a lead author on the article.

For example, thermoelectric energy conversion devices, which convert temperature differences to electrical power, could become much more efficient if new methods of decreasing heat flow are developed. Alternately, if new methods of increasing heat flow are developed, waste heat could be more efficiently removed in many consumer electronics devices such as laptop computers and cell phones. This new understanding could, remarkably, allow for both of these avenues to be pursued, simply because of the versatility of exploiting the coherence of heat waves.

Many other centers are engaged in research on thermoelectrics and thermal management, which could benefit greatly as a whole from this study’s findings. “Our current directions are definitely influenced by this result,” says Austin Minnich, now a professor at the California Institute of Technology who was also involved with the study. He, like many, believes that there is a bright future in new fields of research such as thermal metamaterials, enabled by the concept of coherent phononics.

More Information

Luckyanova MN, J Garg, K Esfarjani, A Jandl, MT Bulsara, AJ Schmidt, AJ Minnich, S Chen, MS Dresselhaus, Z Ren, EA Fitzgerald, and G Chen. 2012. “Coherent Phonon Heat Conduction in Superlattices.” Science 338(6109):936-939. DOI: 10.1126/science.1225549

Acknowledgments

This work was funded by the Solid-State Solar Thermal Energy Conversion Center, an Energy Frontier Research Center, funded by the Department of Energy, Office of Science, Office of Basic Energy Sciences. Maria Luckyanova was partially supported by a fellowship from the National Science Foundation. The work also featured contributions from researchers currently at the California Institute of Technology, Boston University and Boston College.

About the author(s):

  • Dennis M. Callahan is a Ph.D. candidate at the California Institute of Technology. He is a member of the Light-Materials Interactions in Energy Conversion, an Energy Frontier Research Center, under the advisement of Harry Atwater. His research focuses on design and fabrication of novel solar cells in which the electromagnetic environment has been intentionally engineered to enhance performance. This includes solar cells incorporating elements of plasmonics, photonic crystals, optical resonators and other nanophotonic elements.

Keeping It Together for Heat's Sake

Basic science unlocks behavior of heat "waves," potentially opening doors in energy technologies

By designing a superlattice, researchers have identified the wavelike behavior of heat-carrying phonons for the first time. Photo credit: Adam Jandl and Maria Luckyanova.

The performance of electronic devices, including laptop computers and cell phones, is often degraded because of the inefficient management of heat and energy. Squeezing more efficiency from these devices in the future will include better control of the flow of heat through them. In scientific circles, the flow of heat is described with quasi-particles known as phonons, which transport heat through a material and can act like particles or waves. For the first time, scientists showed that these phonons can travel coherently in uninterrupted waves even when various boundaries and interfaces are present. Using an extremely small structure with repeated layers of two materials, they found that, rather than scattering from interfaces between the materials as expected, the phonons remained in coherent waves, behaving as though the stack of different materials was one homogeneous material. This work is a major step towards manipulating heat flow at will. Understanding and controlling phonons opens the door to controlling heat and energy flow on the nanoscale and eventually the macroscale. Scientists at the Solid-State Solar Thermal Energy Conversion Center, led by the Massachusetts Institute of Technology, conducted the research.

More Information

Luckyanova MN, J Garg, K Esfarjani, A Jandl, MT Bulsara, AJ Schmidt, AJ Minnich, S Chen, MS Dresselhaus, Z Ren, EA Fitzgerald, and G Chen. 2012. “Coherent Phonon Heat Conduction in Superlattices.” Science 338(6109):936-939. DOI: 10.1126/science.1225549

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.