Although Dr. Ronggui Yang's research on nanoscale transport phenomena focuses on a physically small space, the applications are wide-ranging — from increasing efficiency of automotives and solar-thermal utilization to thermal management of electronic devices to electrochemical energy storage for mobile systems.
Yang, the ASME NEES Steering Committee Vice Chair, said nanostructures can both slow down and speed up the heat transfer processes. When slowing down the thermal conduction, nanostructures can be utilized as better thermal insulation and protection systems or even to enhance the solid-state thermoelectric energy conversion. Nanostructures also can enhance the efficiency of evaporation or condensation. Such liquid-vapor phase-change processes depend highly on the surface it contacts, and can be found ubiquitously in coal, gas, or nuclear power plants and air conditioners. Nanostructures also could find application in safer nuclear power plants or more compact air conditioners.
Overheating is becoming a threat in electronics. Yang and his collaborators addressed a common issue that engineers encounter in electronic cooling. Pumping liquid through small, connected parallel microchannels, with a size similar to a hair, allow heat to be removed from an electrical device. The heat boils in these hair-sized microchannels. But vapor buildup can block the channels, rendering ineffective cooling and sometimes electronics device failure. To remedy the problem, they grew silicon nanowires directly in the hair-sized microchannels. The nanowires draw liquid to the channel surface through "capillary wetting." The agglomeration of nanowires creates many micron-sized nucleation sites so that many tiny bubbles are generated more uniformly inside channel avoiding the blockage.
In another study, published in Nano Letters, Yang and his colleagues used three-dimensional nanowire networks to increase the energy capacity and power density of lithium ion batteries. The trussed nanowires are built in a one-step electrodeposition process in which lithium enters and leaves the nanowires during the charging and discharging process.
In addition to exploring the cutting edge in nanotechnology and heat transfer, another rewarding aspect of his research is partnering with scientists in different fields, including physics, chemistry, materials science, and other areas of engineering, Yang said. "By working with researchers in other disciplines, I've been able to expand my toolbox in solving new problems and can approach those problems from different angles," he added.
One instance Yang recalled was his collaboration with physicists to quantitatively measure the transition from diffusive to ballistic thermal transport from a nanoscale hotspot that is becoming increasingly important in modern electronics. Their finding demonstrated a significant decrease in energy transport away from the nanoscale heat source that was three times greater than the decrease initially anticipated by Fourier law. Their work also stimulates the measurement of a phonon mean free path — that is, how far an energy carrier can travel in a material.
Yang, the Founding Vice Chair of the K-9 Nanoscale Thermal Transport Technical Committee of the ASME Heat Transfer Division, is an associate professor of mechanical engineering directing the Nano-Enabled Energy Conversion, Storage, and Thermal Management Systems (NEXT) Group and a faculty fellow in the materials science and engineering program at University of Colorado at Boulder. He earned his doctorate in mechanical engineering from MIT in 2006 under the tutelage of ASME Fellow Gang Chen and MIT Institute Professor Mildred Dresselhaus, a pioneer in thermoelectrics and a leader in research on carbon of all forms including nanotubes and graphene. (Both Chen and Dresselhaus are founding members of the ASME Nanotechnology Institute.) In 2008, Yang was awarded MIT's "Top Innovators Under 35" honor for his work in developing nanotechnologies that derive electricity from waste heat. In 2010, he was awarded the ASME Bergles-Rohsenow Young Investigator Award in Heat Transfer for his inspiring and original work in nanoscale heat transfer.
Jun Zhou, Ronggui Yang, Gang Chen and Mildred S. Dresselhaus, Optimal Bandwidth for High Efficiency Thermoelectrics, Physical Review Letters, Vol. 107, Art # 226601 (5 pages), 2011
Dan Li, Gensheng Wu, Wei Wang, Yunda Wang, Dong Liu, Dacheng Zhang, Yunfei Chen, G.P. Peterson, and Ronggui Yang, Enhancing Flow Boiling Heat Transfer in Microchannels for Thermal Management with Monolithically-Integrated Silicon Nanowires, Nano Letters, Vol. 12, pp 3385–3390, 2012, Web Link.
Wei Wang, Miao Tian, Aziz Abdulagatov, Steven George, Yung-Cheng Lee and Ronggui Yang, Three-Dimensional Ni/TiO2 Nanowire Network for High Areal-Capacity Lithium-Ion Microbattery Applications, Nano Letters, Vol. 12, pp 655–660, 2012, Web Link.
Mark Siemens, Qing Li, Ronggui Yang, Keith A. Nelson, Eric Anderson, Margaret Murnane, and Henry Kapteyn, Quasi-ballistic thermal transport from nanoscale interfaces observed using ultrafast coherent soft X-ray beams, Nature Materials, Vol. 9, pp. 26-30, 2010.