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Nanowick supercooling system for 'power electronics'

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July 22, 2010

Test facility for nanowicks (Image: Purdue University School of Mechanical Engineering)

Test facility for nanowicks (Image: Purdue University School of Mechanical Engineering)

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An advanced cooling technology being developed for high-power electronics in military and automotive systems is capable of handling roughly 10 times the heat generated by conventional computer chips. The new type of cooling system can be used to prevent overheating of devices called insulated gate bipolar transistors, high-power switching transistors used in hybrid and electric vehicles. The chips are required to drive electric motors, switching large amounts of power from the battery pack to electrical coils needed to accelerate a vehicle from zero to 60 mph in 10 seconds or less.

The miniature, lightweight device uses tiny copper spheres and carbon nanotubes to passively wick a coolant toward hot electronics, said Suresh V. Garimella, the R. Eugene and Susie E. Goodson Distinguished Professor of Mechanical Engineering at Purdue University. The researchers say this wicking technology represents the heart of a new ultrathin "thermal ground plane," a flat, hollow plate containing water.

Similar "heat pipes" have been in use for more than two decades and are found in laptop computers. However, they are limited to cooling about 50 watts per square centimeter, which is good enough for standard computer chips but not for "power electronics" in military weapons systems and hybrid and electric vehicles, Garimella said.

A heat pipe used for cooling in electronics and computers (Image: School of Mechanical Eng...

Inside the cooling system, water circulates as it is heated, boils and turns into a vapor in a component called the evaporator. The water then turns back to a liquid in another part of the heat pipe called the condenser. The wick eliminates the need for a pump because it draws away fluid from the condenser side and transports it to the evaporator side of the flat device, Garimella said.

Allowing a liquid to boil dramatically increases how much heat can be removed compared to simply heating a liquid to temperatures below its boiling point. Understanding precisely how fluid boils in tiny pores and channels is helping the engineers improve such cooling systems.

Smaller pores for greater drawing power

The wicking part of the heat pipe is created by sintering, or fusing together tiny copper spheres with heat. Liquid is drawn sponge-like through spaces, or pores, between the copper particles by a phenomenon called capillary wicking. The smaller the pores, the greater the drawing power of the material, Garimella said.

Such sintered materials are used in commercial heat pipes, but the researchers are improving them by creating smaller pores and also by adding the carbon nanotubes.

"For high drawing power, you need small pores," Garimella said. "The problem is that if you make the pores very fine and densely spaced, the liquid faces a lot of frictional resistance and doesn't want to flow. So the permeability of the wick is also important."

The researchers are creating smaller pores by "nanostructuring" the material with carbon nanotubes, which have a diameter of about 50 nanometers, or billionths of a meter. However, carbon nanotubes are naturally hydrophobic, hindering their wicking ability, so they were coated with copper using a device called an electron beam evaporator.

Absorbing ten times the heat

The researchers’ findings indicate the wicking system that makes the technology possible absorbs more than 550 watts per square centimeter, or about 10 times the heat generated by conventional chips, which is more than enough cooling capacity for the power-electronics applications.

The team is working to create heat pipes about one-fifth the thickness of commercial heat pipes and covering a larger area than the conventional devices, allowing them to provide far greater heat dissipation.

"We know the wicking part of the system is working well, so we now need to make sure the rest of the system works," said Mark North, a co-author of a paper detailing the research findings and an engineer with Thermacore, a producer of commercial heat pipes.

Potential military applications include advanced systems such as radar, lasers and electronics in aircraft and vehicles. The chips used in the automotive and military applications generate 300 watts per square centimeter or more.

"We have made great progress in understanding and designing the wick structures for this application and measuring their performance," said Garimella. He said that once ongoing efforts at packaging the new wicks into heat pipe systems that serve as the thermal ground plane are complete, devices based on the research could be in commercial use within a few years.

The findings are detailed in a research paper appearing online this month in the International Journal of Heat and Mass Transfer and will be published in the journal's September issue.

About the Author
Darren Quick Darren's love of technology started in primary school with a Nintendo Game & Watch Donkey Kong (still functioning) and a Commodore VIC 20 computer (not still functioning). In high school he upgraded to a 286 PC, and he's been following Moore's law ever since. This love of technology continued through a number of university courses and crappy jobs until 2008, when his interests found a home at Gizmag.   All articles by Darren Quick
1 Comment

As far as I can tell, the IJHMT paper says nothing about nanowicks and the literature review mentions other work that has demonstrated similar heat flux capabilities in the 500 W/cm^2 range. That's not to take away from the qualitity of the work - Suresh and the others have published a really nice paper that adds a lot of insight to understanding the physics in heat pipe wicks through their careful measurements and flow visualizations. But it isn't the technology breakthrough indicated in this article... This seems like another case of good science being drowned out by nanotechnology hype. Or maybe you are confusing this work with a couple other papers that have come out of this same group this summer. They do talk about the use of carbon nanotubes, but the actual measurements don't seem to have demonstrated any significant thermal improvement beyond conventional sintered powder wicks.

rkwilcoxon
7th August, 2010 @ 10:31 am PDT
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