A lot of engineering revolves around pumping it in to make a reaction happen, like glass melting, for example. A lot of engineering revolves around sucking it out, like quenching, for example. Plenty of engineers make good livings managing heat.
But, for all the time and effort given to heat transfer and thermal management, how much is really known about the mechanics of how heat transfers?
Two new papers look at heat transfer from different perspectives—interfaces and surfaces—and provide some interesting insights.
A team at the University of Illinois at Urbana-Champaign studied the atomic-scale mechanism of heat transfer between two interfacing surfaces. Interfaces between materials disrupt heat flow, so understanding the nature of heat transfer at interfaces is important, for example, for coatings and bonded joints.
Heat propagates via phonons, which are lattice vibrations that move in a wavelike fashion through the material. But, the mechanism by which this happens is not well understood, and thus, neither are the factors that influence it. David Cahill, UIUC materials science professor and coauthor of the paper, says in a press release, “Compared to our knowledge of how electricity and light travel through materials, scientists’ knowledge of heat flow is rather rudimentary.”
Part of the reason is that accurately measuring temperatures at very small time and length scales has been very difficult. Cahill’s group has developed a technique for studying heat flow using laser pulses of a trillionth of a second with nanometer-depth resolution. Cahill’s group teamed up with UIUC MSE professor, Paul Braun, to study how atomic-scale features affect heat transport.
Using a quartz substrate, the team created a “molecular sandwich” comprising the quartz, a self-assembled monolayer and a very thin topping of gold. The gold layer was zapped with a heat pulse and they measured how the heat travelled through the SAM to the quartz. The effect of bond strength on heat transfer was studied by modifying the SAM layer’s end group chemistry and, therefore, its bond strength with the gold layer.
The key, they found, was the strength of the bond between the SAM layer and the gold layer. Stronger bonds led to more heat flow, up to twice as much. Specifically, SAM-gold interfaces with van der Waals bonds were twice as resistant to heat as covalently bonded interfaces.
“We’ve basically shown that changing even a single layer of atoms at the interface between two materials significantly impacts heat flow across that interface,” says Mark Losego in the release. Losego is lead author of the paper, which is based on his postdoctoral work. He is now a research professor at North Carolina State University. He says, too, “If the vibrational modes for the two solids were more similar, we could expect changes of up to a factor of 10 or more.”
That, is, the interface can be engineered for heat transfer efficiency. They verified that this could be done by “dialing in” a heat flow value by systematically adjusting the SAM-gold interface chemistry.
This work is consistent with some of the ideas batted around last spring at the Grand Challenges in Ceramic Science workshop, where several of the grand challenges identified are based on new abilities to control, engineer and characterize interfaces and their properties.
The UIUC paper is “Effects of chemical bonding on heat transport across interfaces,” Nature Materials (doi : 10.1038/nmat3303)
An MIT group also just published a paper on heat transfer, but their focus was on how surface roughness influenced heat transfer in cooling systems based on liquids that change phases, for example, water boiling on a surface.
In an elegantly simple experiment, the researchers found that microscale texturing of a surface can dramatically increase the heat transfer and that, according to the press release, “increasing roughness led to a proportional increase in heat-dissipation capability, regardless of the dimensions of the surface-roughening features.” The study found that heat dissipation could be more than doubled by roughening the surface.
The team, led by Evelyn Wang, associate professor of mechanical engineering, explains in the paper that surface roughness enhances the capillary action at the surface, which keeps a layer of vapor bubbles “pinned” to the heat transfer surface. Because the vapor bubbles are pinned, the formation of a vapor layer around the surface, which has an insulating effect that suppresses cooling, is delayed.
The MIT work could improve the heat management of systems like server farms, concentrating solar power plants, desalination plants or other large systems that depend on liquid phase change cooling systems.
The MIT paper is “Structured surfaces for enhanced pool boiling heat transfer,” Applied Physics Letters (doi:10.1063/1.4724190).