Editor’s Note: This is a guest post by Mark Losego, a research assistant professor of chemical and biomolecular engineering at NC State. Losego recently co-authored a News and Views article about nanoscale heat flow in Nature Materials with David Cahill of the University of Illinois.
The basics of heat flow have long been overlooked, but now, as highlighted in a recent News and Views article I co-authored in Nature Materials, it’s seeing a resurgence of scientific interest.
Consider your laptop computer: a billion transistors control electrical flow at the nanoscale; microscopic liquid crystals provide optical resolution finer than the human eye can distinguish; but our standard technology for cooling the entire unit is still a run-of-the-mill fan. Engineering of heat flow remains rather rudimentary because, unlike the flow of electricity and the propagation of light, heat flow across interfaces is poorly understood. This lack of understanding limits the ability of scientists and engineers to predictively design materials with optimized thermal properties.
Heat travels through materials via collective atomic vibrations called phonons. Scientists largely understand how phonons travel through a single material, but our understanding becomes a bit fuzzy when phonons reach an interface between two materials. As depicted in the above cartoon, scientists believe they know what phonons are “thinking” about when they reach an interface between two materials—vibrational frequency, acoustic velocity, interfacial bond stiffness, etc.—but it is still unclear as to which of these considerations is most important in determining whether or not a phonon will “hop the gap.”
Scientifically understanding which of these factors is most important to the rate of interfacial heat transfer remains a critical challenge to the thermal sciences community—one that must be solved if we hope to build material systems with rationally designed thermal properties.
However, research to improve our understanding of heat flow at interfaces is reaching an exciting new era. Key advances in measurement technologies to accurately probe heat flow at the nanoscale, combined with atomic-level precision in materials synthesis and high-performance computing simulations, are beginning to reveal answers to this question.
Achieving nanoscale control of heat flow would further advance the microelectronics industry by reducing cooling requirements. It would also enable new renewable energy harvesting technologies, including thermoelectric devices that generate useful electricity from waste heat sources such as car exhausts or power plant cooling towers.
Ultimately, we would like to understand basic thermal physics well enough that we could predictively compute heat flow rates between any two materials. This is largely possible for electrical current flow and has directly led to our microelectronics revolution. Designing heat flow with the same level of precision could open up a number of other exciting technological opportunities—so it is no wonder that the search for answers in interfacial thermal science is heating up!