Exploring Heat Transport at the Microscopic Scale
Fall 2014 - Written by Resnick Postdoctoral Scholar Moureen Kemei
Smaller devices that are faster, more portable, and compact are the state-of-the art in the electronics industry today. These miniature devices require less power and function at high speeds. However, research efforts have so far mainly examined the electronic performance of these systems largely overlooking the effects of heat transport that currently limit the their performance. For example, “In light emitting diodes, operating temperatures as high as 80 °C can lead to the degradation of these devices”, says Navaneetha Ravichandran, a graduate student in Professor Austin Minnich’s laboratory here at Caltech.
Miniaturized micro-electronic devices are ubiquitous today, and the absence of a microscopic understanding of heat transport inspired Professor Austin Minnich and his group at Caltech to develop a new experimental technique called mean free path spectroscopy (MFPS), to study heat transport at the microscopic scale. MFPS looks at how different lattice vibrations, or phonons, contribute to heat transport. Phonons carry heat in materials and the further they can travel in a material without encountering any obstacles the further heat can be transported. Collisions of phonons with other phonons or with defects in a material give rise to phonon scattering events, which impede heat transport. In analogy, consider running in an open field where you travel further in a given time frame compared to running through a crowded street where you have to stop and change your trajectory every time someone is in your way. The average distance a phonon travels in a material before being scattered is known as the mean free path. At room temperature, silicon has an average mean free path of 300 nanometers. However, the new technique MFPS is providing new insights. It shows that phonons can propagate without scattering for a few microns in silicon at room temperature although the average mean free path is only 300 nanometers. This finding suggests that engineering phonon scattering on the micron scale can be used to modify thermal conductivity in silicon.
Advances in the understanding of microscopic heat transport are providing exciting opportunities. In electronic devices, where heat accumulation during operation leads to material degradation, Navaneetha is studying how to effectively transport heat away from hot spots by examining phonon scattering at material boundaries and interfaces.
In a recent related study, Professor Minnich and collaborators investigated heat transport in ultra-sensitive low temperature electronic devices that are used in Radio astronomy. These devices probe very weak electromagnetic signals from space and low device noise is critical. The team has shown that there is a new mode of phonon transport called “radiation”, which becomes important in these systems at low temperatures. Phonon radiation determines the minimum achievable noise levels in these devices. This finding published in Nature Materials on November 10th 2014 will guide the design of ultralow-noise electronics.
Microscopic heat transport is also important in thermoelectric devices, where a heat difference is used to generate electric current. Here, it is important to increase phonon scattering to obtain high electrical power. We can envision the design of thermoelectric devices with high figures of merit by understanding how the entire phonon spectrum participates in heat transport and thereby engineering structures that are efficient in scattering heat.
Mean free path spectroscopy and the complementary computation techniques developed here at Caltech paint a bright future for the control of the universal processes that rely on thermal transport.