Engineered 'sand' may help cool electronic devices

Scientists have engineered a kind of 'sand' - consisting of silicon dioxide nanoparticles coated with a polymer - that can inexpensively cool power-hungry electronic devices.
The unique surface properties of the coated nanoscale silicon dioxide conducts the heat at potentially higher efficiency than existing heat sink materials, researchers said.
The theoretical physics behind the phenomenon is complicated, involving nanoscale electromagnetic effects created on the surface of the tiny silicon dioxide particles acting together.
The bottom line could be a potentially new class of high thermal conductivity materials useful for heat dissipation from power electronics, LEDs and other applications with high heat fluxes.

"We have shown for the first time that you can take a packed nanoparticle bed that would typically act as an insulator, and by causing light to couple strongly into the material by engineering a high dielectric constant medium like water or ethylene glycol at the surfaces, you can turn the nanoparticle bed into a conductor," said Baratunde Cola, an associate professor at the Georgia Institute of Technology.
"Using the collective surface electromagnetic effect of the nanoparticles, the thermal conductivity can increase 20-fold, allowing it to dissipate heat," Cola said.

The researchers decided to experiment by first using water to coat the nanoparticles and turn the silicon dioxide nanoparticle bed into a conductor.

However, the water coating was not robust, so the researchers switched to ethylene glycol, a fluid commonly used in vehicle antifreeze.
The new combination increased the heat transfer by a factor of 20 to about one watt per metre-kelvin, which is higher than the value ethylene glycol or silicon dioxide nanoparticles could produce alone, and competitive with expensive polymer composites used for heat dissipation.
"You could basically take an electronic device, pack these ethylene glycol-coated nanoparticles in the air space, and it would be useful as a heat dissipation material that at the same time, won't conduct electricity," said Cola.

"The material has the potential to be very inexpensive and easy to work with," he said.
Though the ethylene glycol works well, it will eventually evaporate. For that reason, Cola plans to identify polymeric materials that could be adsorbed to the silicon dioxide nanoparticles to provide a more stable coating with a reasonable product lifetime.
The effect depends on the collective action of the silicon dioxide nanoparticles.
"We are basically showing a macroscopic translation of a nanoscale effect," Cola said.
The study was published in the journal Materials Horizons.