A class of materials called plastic crystals could be effective solid-state coolants, two independent studies have found. This is the first time that a large pressure-dependent temperature change has been observed in these materials. The work could have important implications for environmentally friendly refrigeration technologies.

Current cooling devices rely on gaseous coolants that are compressed to liquefy them in a repeated cycle. These systems are bulky and energy-intensive besides using coolants such as hydrofluorocarbons that are greenhouse gases. Cooling technologies based on solid-state materials could be more earth-friendly, efficient, and compact.

Solid-state coolants made so far absorb or release heat when triggered by an electric or magnetic field. They include bismuth-based thermoelectric materials, which are notoriously energy-inefficient, or magneto-caloric materials, which are expensive. More recently researchers have looked at materials that display an electrocaloric effect—where applying an electric field increases a material’s temperature and removing it cools the material—but no one has made a practical solid-state cooling system yet using these materials.

The two new advances point to another exciting alternative, the barocaloric effect, in which temperature changes are achieved by applying pressure. The best caloric materials so far have shown entropy changes ranging in dozens of Joules per kilogram per Kelvin, too small to be harnessed for practical cooling. Now, two research groups, one based in China and the other in Europe, have observed much larger changes in plastic crystals when a pressure is applied at room temperature.

Plastic crystals are materials whose molecules are regularly spaced as in other crystals but can be oriented randomly. At low pressures, the molecules are far apart but under pressure, these molecules come closer together and go from a disordered to an ordered state, while releasing heat in the process.

Both groups used the plastic crystal neopentylglycol for their studies. Bing Li at the Chinese Academy of Sciences and his colleagues observed an entropy change of 389 J kg^-1 K^-1 by applying a pressure of up to 100 MPa.

The group in Europe, meanwhile, used a larger pressure of 520 MPa and measured an entropy change of 510 J kg^-1 K^-1, comparable with hydrofluorocarbon fluids used in commercial refrigerators. The research team, led by Josep-Lluis Tamarit of the Polytechnic University of Catalonia in Barcelona and Xavier Moya of the University of Cambridge, secured a patent in 2017 for their work on the barocaloric effect in plastic crystals.

While the Chinese group saw hysteresis at the applied pressure of 100 MPa, the Europeans’ use of a much higher pressure permits a reversible barocaloric effect without hysteresis, Moya says. This reversible effect is critical for making a practical solid-state cooling device since refrigerators operate under cyclic application and removal of pressure.

There is a long road ahead before the availability of practical refrigerators based on plastic crystals though. While neopentylglycol plastic crystals are commercially available, inexpensive, easy to produce, non-toxic, and flexible, they also have low melting points and are highly deformable, which means they could degrade quickly during refrigeration cycles. “Many engineering issues have to be addressed,” Li says, “such as enhancing thermal conductivity and reducing thermal hysteresis.” He and his colleagues are now working toward practical applications by optimizing the overall performance of neopentylglycol and other plastic crystals.

While plastic crystals have been considered as materials for heat storage in the past, their use for barocaloric effects is new, says Ekkes Brück, materials and energy researcher at the Delft University of Technology in the Netherlands. The large barocaloric effects in these cheap, readily available materials sound promising for solid-state cooling “but the high pressure needed and the difficulty for fast cycling pose serious challenges,” he says.

Read the Moya, Tamarit, et al. article in Nature Communications and the abstract for the Li et al. article in Nature.