A research team at the Center for Condensed Matter Sciences has published a paper introducing a revolutionary breakthrough in the field of thermal conduction. Appearing in the June 30 issue of the prestigious journal Nature Nanotechnology , the paper details the team’s discovery of a ballistic thermal conduction phenomenon along nanowires that experiences
no dissipation of energy at room temperature. This finding not only rewrites the traditional textbook model of thermal conduction but will also have a crucial impact on the research and development of thermoelectric materials and thermal energy applications.
Team leader assistant research fellow Chih-wei Chang points out that in normal thermal conduction heat is transported by phonon waves through countless molecular collisions that create heat, which means thermal energy cannot be used for transmission like electrons or fiber optics. Chang’s team was the first to present a solution to this challenge, and it did so using common silicon-germanium (SiGe) nanowires at room temperature. This research will have a profound influence on future energy research related to super-high frequency electronic components, semiconductor manufacturing processes, thermoelectric materials, as well as our fundamental understanding of thermal conduction.
Chang says the team’s discovery went beyond their expectations. Although SiGe semiconductors are a commonly used material, they are poor thermal conductors, and almost no one would use them to search for thermal conduction by phonon waves. Most experts recommend starting with special materials that are good heat conductors, such as diamond, graphene or carbon nano tubes, and then using very expensive instruments to drop the materials to super-low temperatures for experiments.
The breakthrough of the team’s finding is that ballistic thermal conduction is observable at room temperature in SiGe semiconductors. Moreover, phonon waves persist for eight microns when heat is transmitted via SiGe nanowires. This figure is one thousand times more than the textbook figure and over ten times more than that observed in diamond and graphene.
Chang explains that the metal alloy characteristics of SiGe semiconductors prohibit highfrequency thermal energy from being conducted on nanowires and allows only the transmission of super-low frequency thermal energy. According to Chang, “It’s a little like
allowing only buses carrying 50 or more passengers to get on the freeway, meaning there are almost no vehicles on the road and no collisions.”
The significance of the team’s observation of ballistic thermal conduction is that it took place at room temperature, not at superlow temperatures, and it relied on SiGe semiconductors, which are already commonly used in the semiconductor industry. It is these features that give the team’s discovery great potential for application in the information and communications technology industry.
It is noteworthy that former NTU President Si-Chen Lee, who ended his term in June, was a member of this world-class NTU research team.