Thermoelectric devices, which can harness temperature differences to produce electricity, might be made more efficient thanks to new research on heat propagation（传播，繁殖） through structures called superlattices. The new findings show, unexpectedly, that heat can travel like waves, rather than particles, through these nanostructures: materials made up of layers only a few billionths of a meter in thickness. Heat -- the vibration¹ of atoms and molecules² in a material -- usually travels in a "random⁴ walk," which is difficult to control. The new observations show a very different pattern, called coherent（连贯的） flow, which is more like ripples⁵ that move across a pond in an orderly way.

 

This opens the possibility of new materials in which the flow of heat could be precisely⁶ tailored -- materials that could have important applications. For example, such research might lead to new ways of shedding the heat generated by electronic devices and semiconductor⁷ lasers, which hampers⁸ performance and can even destroy the devices.

 

The new work, by graduate student Maria Luckyanova, postdoc Jivtesh Garg and professor Gang Chen, all of MIT's Department of Mechanical Engineering -- along with other students and professors at MIT, Boston University, the California Institute of Technology and Boston College -- is reported this week in the journal Science.

 

The study involves a nanostructured material called a superlattice: in this case, a stack of alternating thin layers of gallium arsenide（砷化镓） and aluminum⁹ arsenide, each deposited in turn through a process called metal-organic chemical vapor¹⁰ deposition¹¹. Chemicals containing these elements are vaporized in a vacuum, and then deposited on a surface, their thicknesses precisely controlled through the duration of the deposition process. The resulting layers were just 12 nanometers thick -- about the thickness of a DNA¹² molecule³ -- and the entire structures ranged in thickness from 24 to 216 nanometers.

 

Researchers had previously¹³ believed that even though such layers could be atomically perfect, there would still be enough roughness at the interfaces¹⁴ between the layers to scatter¹⁵ heat-transporting quasi-particles, called phonons, as they moved through the superlattice. In a material with many layers, such scattering¹⁶ effects would accumulate, it was thought, and "destroy the wave effect" of the phonons, says Chen, the Carl Richard Soderberg Professor of Power Engineering. But this assumption had never been proved, so he and his colleagues decided¹⁷ to re-examine the process, he says.

 

Indeed, experiments by Luckyanova and computer simulations by Garg showed that while such phase-randomizing scattering takes place among high-frequency phonons, wave effects were preserved among low-frequency phonons. Chen says he was very surprised when Luckyanova came back with the first experimental data to show "that coherent conduction of heat is really happening."

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