Jul 13 2016
Researcher Baratunde Cola wants to add “sand” into computers in order to cool them. Not beach sand, but silicon dioxide nanoparticles coated with a high dielectric constant polymer, which would provide improved cooling at a low cost for high-energy electronic devices.
Researchers have shown that silicon dioxide nanoparticles coated with a high dielectric constant polymer might inexpensively provide improved cooling for electronic devices. Shown (l-r) are Professor James Hammonds from Howard University, Associate Professor Baratunde Cola from Georgia Tech, and Georgia Tech Graduate Student Eric Tervo. (Photo Credit: Rob Felt, Georgia Tech)
The cooling is not performed by the silicon dioxide, but by the distinctive surface properties of the coated nanoscale material that conduct the heat. This coated nanoscale material does a much better job than current heat sink materials.
The theoretical physics of this occurrence is complex, and entails nanoscale electromagnetic effects formed on the surface of the minuscule silicon dioxide particles acting collectively.
The end result could potentially lead to a whole new range of high thermal conductivity materials applicable for heat dissipation from LEDs power electronics, 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. Using the collective surface electromagnetic effect of the nanoparticles, the thermal conductivity can increase 20-fold, allowing it to dissipate heat.
Baratunde Cola, Associate Professor, Georgia Institute of Technology
The research consisted of both theory and experiment. It has been reported in the July issue of the Materials Horizons journal, and showcased in the July 8 issue of the Science journal. The U.S. Air Force and the Air Force Research Laboratory supported this work. The co-authors of the research include Professor James Hammonds at Howard University, and graduate students Eric Tervo from Georgia Tech and Olalekan Adewuyi from Howard University.
For quite a few years, the ability of surface phonon polaritons to optimize thermal conduction in nanomaterials manufactured from polar materials such as silicon dioxide has been predicted in theoretical papers. Polaritons are quantum quasiparticles created by intense coupling of electromagnetic waves with a magnetic or electric dipole-carrying excitation.
With surface phonon polaritons, the electromagnetic waves are coupled to a specific frequency and polarization of vibrating atoms present in the material known as optical phonons.
When the size of a material is decreased to below 100 nm, the material’s surface properties tend to the control the bulk properties, allowing phonons of heat to surge from particle to particle in the tightly packed bed with support from the coupled electromagnetic waves.
Despite the fact that researchers could not measure heat flow from surface phonon polaritons due to the problems faced during experimental stage in the past, they have managed to study its wave propagation when light reaches the surface of a nanostructure material, thus signifying a probable role in heat dissipation.
Additionally, Cola and the team also discovered that the effect could take place when thermal energy hits a tightly packed bed of nanoparticles.
What we are also showing for the first time is that when you have nanoparticles of the right type in a packed bed, that you don't have to shine light on them. You can just heat up the nanoparticles and the thermal self-emission activates the effect. You create an electrical field around the nanoparticles from this thermal radiation.
Baratunde Cola, Associate Professor, Georgia Institute of Technology
The researchers planned to work on those exceptional properties, first by coating water on the nanoparticles and turning the bed of silicon dioxide nanoparticles into a conductor. However the coating of water was not strong, so the team opted to use ethylene glycol, a fluid generally used in vehicle antifreeze.
The new mixture enhanced the transfer of heat by a factor of 20 to around one watt per meter-kelvin, which is more than the value silicon dioxide nanoparticles or ethylene glycol could generate alone, and viable than pricey 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."
Silicon dioxide was selected as its crystalline lattice can produce resonant optical phonons - required for the effect - at close to room temperature. Although other materials could be used, the silicon dioxide nanoparticles offer an excellent advantage of both cost and properties.
The resonance frequency, converted into the thermal radiation temperature for silicon dioxide, is around 50 degrees Celsius. With this material, we can turn on this effect at a temperature range that a microelectronic device is likely to see.
Baratunde Cola, Associate Professor, Georgia Institute of Technology
Though the ethylene glycol functions ideally, it will gradually evaporate. Therefore Cola plans to locate polymeric materials that could be adsorbed to the silicon dioxide nanoparticles so as to offer better coating stability with a longer product lifetime.
The effect relies on the combined action of the silicon dioxide nanoparticles.
"We are basically showing a macroscopic translation of a nanoscale effect," Cola said. "Even though the nanoparticle bed is a bulk assembly, it is a bulk assembly that has a lot of internal surface area. The internal surface area is the gateway by which it interacts with the electromagnetic field - the light and the heat."
Up to now, the effect has been witnessed in small quantities of silicon dioxide nanoparticles. The next step would be to scale up the research to show that heat can be exported over longer distances in larger volumes of the material, Cola said.
The rate at which the thermal energy goes from one side of the particle to the other side of the particle is constant throughout the nanoparticle bed, so it shouldn't matter how thick the nanoparticle bed is. When these particles are close enough together, their modes are coupled, which allows the energy to transport.
Baratunde Cola, Associate Professor, Georgia Institute of Technology
Additional testing would be required to guarantee the long-term efficiency and to verify that there are no impacts on the consistency of the electronic gadgets cooled using the method, Cola said.