Researchers at the University of Illinois at Chicago, the University of Massachusetts-Amherst and Boise State University explored the effects of boundaries between grains of graphene on its heat conductivity by developing a technique to measure heat transfer across a single grain boundary.
The surprising results revealed an order of magnitude (about 10 times) lower than the theoretically predicted value. The scientists then devised computer models that can explain the surprising observations from the atomic level to the device level. The team developed an experimental system that lays down a graphene film onto a silicon-nitrate membrane around four-millionths of an inch thick and can measure the transfer of heat from one single graphene crystal to another. The system is sensitive to even the tiniest changes, like nano-scale grain boundary. When two crystals are neatly lined up, heat transfer occurs as predicted by theory. When two crystals have mis-aligned edges, though, the heat transfer that occurs is 10 times less.
To understand this considerable difference, the team designed a computer simulation of heat transfer between grain boundaries at the atomic level. The scientists found that when the computer examined grain boundaries with different mismatch angles, the grain boundary was not just a line but rather a region of disordered atoms. This region significantly affected the heat transfer rate in the computer model and may explain the experimental values. The researchers state that they are now able to "...explain several factors - the shape and size of the grain boundaries, and the effect of the substrate."
Understanding this relationship may bring developers a step closer to being able to engineer films at a scale useful for cooling microelectronic devices and many other applications as virtually every device needs a cooling system to optimize its function.