A research team at the University of Illinois at Urbana-Champaign (UIUC) has developed a novel system showing heat move by examining and measuring nanoscale thermal conductance at the interface between two materials.
The UIUC team’s system starts with a substrate base of quartz crystal, upon which the researchers place molecular chains that are 12 carbon atoms long. At the base of each chain is a chemical “cap” that covalently bonds to quartz.
The attraction of these caps to the substrate spontaneously aligns all of the carbon chains into an ordered array of molecules known as a self-assembled monolayer (SAM). At the opposite end of each carbon chain is a different kind of cap, either a thiol (sulfur and hydrogen) group that bonds strongly to metals or a methyl group (carbon and hydrogen) that bonds weakly.
At the nanoscale, thermal properties are the result of vibrations between neighboring atoms. Bonds between atoms carry these vibrations similar to an oscillating spring. The UIUC team developed a technique for studying the effects of these bonds on heat transport across an interface between two different materials.
Mark Losego, who was a post-doctoral fellow at UIUC and is now a research assistant professor in chemical and biomolecular engineering at North Carolina State University picks up the explanation, “We then make use of a viscoelastic silicone stamp to ‘transfer print’ gold layers onto the SAM surface. This process is similar to transferring a decal onto a T-shirt where the gold film is the ‘decal’ attached to the silicone stamp ‘backing’. When we slowly peel away the silicone, we leave the gold layer on top of the SAM.”
It’s at the interface between the gold film and the SAM where nanoscale heat flow is characterized. “Changing the chemical groups that are in contact with the gold layer allows us to see how different bonds affect heat transfer,” said Losego.
Combined with an ultrafast laser technique capable of monitoring temperature decay (or heat loss) with picosecond (trillionth of a second) resolution, the UIUC researchers are able to use their experimental system to evaluate heat flow at the atomic scale.
Losego takes up the explanation again, “We heat the gold layer attached to the monolayer and can monitor temperature decay with time. Concurrently, we observe oscillations in the gold film that indicate the strength of the bonds at the gold-SAM junction. Using these measurements we are able to independently verify that strong bonds [fast-decaying oscillations] have rapid heat transfer while weak bonds [slowly decaying oscillations] have slower heat transfer.”
Losego will present the team’s findings during the just opening AVS 59th International Symposium and Exhibition, held Oct. 28-Nov. 2, 2012, in Tampa, Fla.
The team’s plan is to refine their nanoscale thermal measurement system and develop theoretical calculations to better interpret the data it produces.
With further refinement, the scientists believe their advance may one day provide data for applications such as harvesting electricity from waste heat, better cooling of microelectronic devices and “heat-seeking” targeting of disease cells by hyperthermal (above normal body temperature) therapeutics.
The scientific method of research has provided a fundamental understanding of how light (via photons) and electricity (via electrons) move within and between materials at the micrometer or nanometer levels, making possible a wide variety of miniature devices such as transistors, optical sensors and micro electromechanical systems.
The knowledge about heat flow by conduction has been rudimentary so far. The UIUC work takes science much closer to enabling engineers to uprate the performance of many heat related processes.
It’s very useful and helpful work that is greatly appreciated.