Portland, Ore. -- Just as ionic rain can irrigate a forest of nanotubes, ionic winds can cool the surface of chips.
Harnessing ionic winds to accelerate charged air between high-voltage electrodes can enhance a chip's heat-transfer coefficient by 250 percent, according to Purdue University (West Lafayette, Ind.). Its chip-sized ionic wind engine prototype, funded by Intel Corp., works by overcoming the "no slip" effect that ordinarily keeps the air molecules closest to the chip surface relatively stationary.
The ionic wind engine prototype consists of two high-voltage electrodes positioned on either side of a chip's backside. Putting a thousand-voltage potential between the electrodes charges the air molecules and generates an ionic wind across the chip's surface.
Ordinarily, the no-slip effect in airflow keeps the air molecules closest to a surface increasingly stationary, thereby inhibiting thermal transfer. But if ionic wind engines could be integrated in arrays on the backside of chips, then normal cooling fans would more than double their efficiency, because air near the surface would no longer be stationary.
"We get a 250 percent improvement in heat-transfer coefficient; however, the challenge is to achieve the cooling en- hancement we have shown at lower operating voltages," said Suresh Gari- mella, an engineering professor at Purdue. "The key challenges ahead are ensuring that we can operate at lower voltages and making sure we have a robust electrode design."
For the prototype demonstration, tiny electrodes were placed 10 millimeters apart, and thousands of volts were applied to them. The positive electrode was a wire that ran across the anode side, while several negatively charged cathode electrodes emitted electrons to charge the air from the other side of the chip. In those tests, a chip cooled to 140°F by a traditional fan was further cooled to 95°F with an ionic wind engine.
The researchers hope to reduce the anode and cathode separation from millimeters to microns to obviate high voltages. Arrays with stepped voltage gradi- ents would be used instead of a single, widely separated voltage potential.
"Our intention is for an array of these engines to be used to cover any desired area," said Garimella.
Purdue has spent several years developing the ionic wind technique with National Science Foundation funding. A new prototype with lower operating voltages and a ruggedized electrode array design is expected within two years, the researchers estimate.
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