Absorbing Heat

The control of heat at the nanoscale provides interesting possibilities for moving heat from locations where it is not wanted, or for using the buildup of heat in nanoparticles to drive chemical reactions more efficiently.
The Lear group in the Department of Chemistry has a long-standing program that looks at photothermal heating of nanoparticles – nanoparticles that absorb light strongly, and then efficiently converts this light to heat. As the thermal energy builds up, it is dissipated into the local environment, where it can drive catalytic reactions, ablate cancer cells, or decompose polymers.
In the process of studying this behavior of nanoparticles, they observed that driving reactions that absorbed heat resulted in more efficient cooling of the particles.

“That got us thinking that maybe we could use those endothermic or heat absorbing reactions to function as heat sinks,” said Ben Lear,
assistant professor of chemistry. Lear wrote a proposal to explore that possibility, and the Air Force funded his group to study the possibility of using the endothermic process to cool equipment such as lasers that require absorbing huge amounts of heat on a rapid time scale. “When you think about ways to absorb heat, you usually get either things that are water cooled or air cooled,” Lear continued. “You have a computer and you use a fan to blow cold air over it. It turns out the heat absorbing power of air is pretty poor.”

Likewise, car engines are cooled by water circulating from a radiator. The thermal conductivity is better, but the heat absorbing power is still poor. Water can only absorb a limited amount of heat before it boils. The real cooling comes from a phase change, such as a liquid to gas transition as in the evaporation of liquid nitrogen.  

The amount of energy that’s necessary to undergo a phase change is significantly higher than the amount of energy needed to heat up or cool a material, Lear noted. The reason for the higher energy requirement for evaporation is the attractive force between molecules is strong in liquids. It’s what holds them together. With enough energy in the system, the molecules will finally start moving fast enough to fly apart.

The attractive force holding two atoms together in a chemical bond is 10 to100 times stronger than the attractive force between molecules. They speculated that using the force of chemical bonds to absorb heat would increase the heat absorbing power by at least 10 times. In addition, the timescales associated with breaking chemical bonds are advantageous. A laser, such as the kind the Air Force would put on a plane’s
detection systems, generates large amounts of heat as it creates laser pulses in the femtosecond timescale (one quadrillionth of a second). This creates a problem for using a liquid-to-gas absorption method, because that process takes place at a nanosecond time scale, a million times slower. Chemical bonds can break at a picosecond timescale (one trillionth of a second), so they are far closer to the speed of the laser pulses and the heat they are producing.

“What we would like to do is create a reactive cooling bath that has a lot of cooling power built into it. One where we can tune the temperature so that you can run systems at their most efficient temperature,”
said Lear.

This would be useful for industry applications, where they would like to be able to hold a machine at a given temperature for long periods. By selecting an appropriate chemical bond, the desired temperature can be maintained. Ideally, the chemical reaction would be reversible – the atomic bonds would break apart, the resulting chemical could be moved to a cooler place and the bonds reformed. It would also be ideal if the chemical was not caustic in either phase. Although it is not a strict requirement, it would also be easier to work with a chemical that is in a liquid form at the desired temperature, so it can be pumped through the cooling system.
The researchers plan to start with a set of well-known endothermic reactions known as Diels-Alder reactions. These reactions are simple to run, reversible, and can be made from oil-like molecules. “This is
something we are used to using with metal systems, and it is non-reacting. Pick the right kinds of oils and whether the bond is broken or not, it will remain an oil in either case, and oils are something that we are already comfortable using around metallic and electronic components, Lear concluded.”

Contact Dr. Lear at bul14@psu.edu.