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ECM: Magnetoclaoric effect; ECE: electrocaloric effect; eCE: elastocaloric effect; ECB: barcaloric effect. The plastic crystals identified in this work are neopentyl glycol (NPG), pentaglycerin (PG), pentaerythritol (PE), 2-amino-2-methyl-1,3-propanediol (AMP), tris (hydroxymethyl ) aminomethane (TRIS), 2-methyl-2-nitro-1-propanol (MNP), 2-nitro-2-methyl-1,3-propanediol (NMP). Credit: Huang Chengyu
Phase transitions occur when heat (that is, entropy) is exchanged between materials and the environment. When such processes are driven by pressure, the induced cooling effect is called the barcaloric effect, which is a promising alternative to the conventional vapor compression cycle.
For real applications, it is desirable that a material exhibits greater entropy changes induced by lower pressure. Recently, an international research team led by Professor Li Bing of the Institute of Metals Research of the Chinese Academy of Sciences discovered that a class of disordered materials called plastic crystals presents some record barcaloric effects under very low pressure. The typical entropy variations are about several hundred joules per kilogram per kelvin, which is 10 times higher than the previous materials.
Using large-scale facilities in Japan and Australia, the team found that the constitutive molecules of these materials exhibited many orientation disorders on the networks and that these materials were intrinsically highly deformable. As a result, a minute amount of pressure is able to suppress the extended orientation disorder. As a result, pressure induced entropy changes are obtained. These two merits make plastic crystals the best barcaloric material to date.
This research is the first report that entropy changes can exceed 100 joules per kilogram per Kelvin. It represents the best results among all caloric materials (barcaloric effect and its similar effects such as magnetocaloric, electrocaloric and elastocaloric effects) and is considered a significant event.
(a). The pressure-induced entropy changes as a function of temperature under varying pressures. (B). X-ray diffraction diagrams showing a pressure-induced phase transition. (CD). Neutron scattering spectrum, showing that the pressure suppresses the quasi-elastic signal from the molecular orientation disorder. (e, f) Structural snapshots of molecular dynamics simulations, where the pressure aligns the molecules. Credit: Huang Chengyu
The microscopic physical scenario established with the help of the neutron scattering technique is useful for designing even better materials in the future.
With regard to refrigeration applications, the plastic crystals reported here are very promising because they are abundant, ecological, easy to drive and efficient.
These works point to a new direction for emerging solid-state refrigeration technologies.
Diagram of the refrigeration cycle based on barocaloric effects. Credit: Huang Chengyu
The study is published in Nature.
Explore further:
Press to eliminate heat: élastocaloriques materials allow more efficient cooling and "green"
More information:
Colossal barcaloric effects in plastic crystals, Nature (2019). DOI: 10.1038 / s41586-019-1042-5, https://www.nature.com/articles/s41586-019-1042-5
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