Researchers at Cornell University have developed a new method of precisely measuring the subtle movement of heat in nanostructures.
Heat flows at the nanoscale differently than at larger scales, the researchers say, and understanding how certain surfaces affect the transport of heat at the phonon level could have implications in everything from thermoelectric materials to microelectronic cooling devices.
The Cornell researchers used a phonon spectrometer, which is capable of measurements 10 times more sensitive than standard methods, to measure the frequency and surface scattering of phonons in thin silicon nanosheets. According to the researchers, the scattering of phonons on surfaces influences how well heat flows through a structure, similar to how light bounces off a smooth or rough structure.
“If waters are calm you see a reflection, but in choppy waters you see diffuse scattering,” Jared Hertzberg, a former postdoctoral associate and the paper’s first author, said. “This diffuse scattering slows down the transmission of phonons. This decrease in phonon transport becomes particularly important in nanoscale materials where surfaces play a larger role in the heat flow.”
Researchers found that total diffusive scattering of phonons occurred at much lower frequencies than had been previously predicted by a 50-year-old theory known as the Casimir-Ziman theory, which determines the probability of phonon scattering based on surface roughness and phonon wavelength. Their research is published in the journal Nano Letters.
According to the researchers, “since diffusive scattering effectively lowers phonon transmission, high phonon scattering rates have implications for thermal conductivity in nanostructures: The actual thermal conductance will be much lower than predicted using the standard Casimir-Ziman theory.”
“If we can precisely understand how this process works, then we can begin to engineer heat flow at the nanoscale, which can lead to more efficient alternate energy applications, such as thermoelectrics, or advanced phononic heat-logic circuits,” Robinson said.
