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The findings have been announced in the International Journal of Heat and Mass Transfer, and a patent application has been filed.
“For the configurations we investigated, this approach achieves heat transfer approaching theoretical maximums,” said Terry Hendricks, the project leader from the Pacific Northwest National Laboratory. “This is quite significant.”
The improvement in heat transfer achieved by modifying surfaces at the nanoscale has possible applications in both micro- and macro-scale systems, the researchers said. The coatings produced a heat transfer coefficient 10 times higher than uncoated surfaces.
Heat exchangers are what make air conditioners and refrigerators function, and inadequate cooling is a limiting factor for many advanced technology applications, ranging from laptop computers to advanced radar systems.
“Many electronic devices need to remove a lot of heat quickly, and that’s always been difficult to do,” said Chih-hung Chang, an associate professor in the School of Chemical, Biological and Environmental Engineering at Oregon State University. “This combination of a nanostructure on top of a microstructure has the potential for heat transfer that’s much more efficient than anything we’ve had before.”
There’s enough inefficiency in heat transfer, for instance, that for water to reach its boiling point of 100°C, the temperature of adjacent plates often has to be about 140°C. But with this new approach, through both their temperature and a nanostructure that literally encourages bubble development, water will boil when similar plates are only about 120°C, said the researchers.
To do this, heat transfer surfaces are coated with a nanostructured application of zinc oxide, which in this usage develops a multi-textured surface that looks almost like flowers, and has extra shapes and capillary forces that encourage bubble formation and rapid, efficient replenishment of active boiling sites.
In these experiments, water was used, but other liquids with different or even better cooling characteristics could be used as well, the researchers said. The coating of zinc oxide on aluminum and copper substrates is inexpensive and could affordably be applied to large areas.
Because of that, this technology has the potential not only to address cooling problems in advanced electronics, the scientists said, but also could be used in more conventional heating and cooling applications. Military electronic applications that use large amounts of power are also likely.
The research has been supported by the Army Research Laboratory. Further studies are being conducted to develop broader commercial applications, the researchers said.
Publication date: 06/14/2010