PORTLAN, 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 nearest 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. By putting a thousand
voltage potential between the electrodes, air molecules become charged and an ionic wind is generated between them across the surface of the chip. Ordinarily the "no-slip" effect in air flow keeps the air molecules closest to a surface increasingly stationary, thereby inhibiting thermal transfer. However, if ionic wind engines could be integrated in arrays on the backside of chips, then normal cooling fans would become more than double their efficiency because air near the surface of chips would no longer be stationary.
"We get a 250 percent improvement in heat-transfer coefficient; however, the challenge is to achieve the cooling enhancement we have shown at lower operating voltages," said Suresh Garimella, 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, on either side of a chip, and thousands of volts were applied to them. The positive electrode was a wire that ran across the anode side of the chip, while several negatively charged cathode electrodes emitted electrons to charge the air from the other side of the chip. In these tests, a chip cooled to 140 degrees Fahrenheit by a traditional fan was further cooled to 95 degrees Fahrenheit with the use of the ionic wind engine.
To eliminate the need for high voltages, the researchers hope to reduce the anode and cathode separation from millimeters to microns, and accordingly reduce the voltages required, compensating by using arrays with stepped voltage gradients, instead of just a single widely separated voltage potential.
"Our current work is toward reducing the dimensional scales of the device as well as the operational voltage," said Garimella. "Our intention is for an array of these engines to be used to cover any desired area."
Also contributing to the project were Purdue professor Timothy Fisher, Intel research engineer Rajiv Mongia, and Purdue engineering doctoral candidate David Go. Undergraduate engineering student Raul Maturana also worked on the project by virtue of a fellowship from the National Science Foundation.
Purdue has spent several years developing the ionic-wind technique with funding from the National Science Foundation. The researchers estimate that within two years they will produce a new prototype with lower operating voltages and a ruggedized electrode array design.