Figure. Comparison of the maximum entropy changes of leading caloric materials. MCE: magnetoclaoric effect; ECE: electrocaloric effect; eCE: elastocaloric effect; BCE: barocaloric effect. The plastic crystals identified in the present work are neopentylglycol (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).

        A new discovery funded by the NSFC grants (Nos. 51671192, 51531008, 11804346) reveals colossal barocaloric effects in plastic crystals and indicates a new direction for emergent solid-state refrigeration technologies. Professor Li Bing, Zhang Zhidong, and Ren Weijun of the Institute of Metal Research, Chinese Academy of Sciences in cooperation with researchers from Japan, United States, and Australia reveal the new discovery together. The result has been published on Nature on March 28th, 2019. The link is http://www.nature.com/articles/s41586-019-1042-5.

According to the UN statistics, 25 to 30 percent of the world’s electricity is consumed annually for various cooling applications. The technology of current cooling applications relies on traditional vapor compression cycle, which is harmful to the environment and human body. As a result, both research community and industries are devoted to exploiting environment-friendly, efficient refrigeration technology. In particular, China is not the best player in the cutting-edge refrigeration technology，therefore, exploring new vapor compression cycle technology might lead China to own next-generation refrigeration technology. As a promising alternative, refrigeration technologies based on solid-state caloric effects have been attracting attention in recent decades. These effects are described by the isothermal entropy changes (∆S) and the caloric effects of current leading materials are characteristic of entropy changes of dozens of joules per kilogram per kelvin. In addition, unpractically large driving fields are also required. These limited performances are the obstacle to the application.

Recently, an international research team led by Professor Li Bing performed pressure-dependent differential scanning calorimetric measurements, high-resolution neutron scattering, and synchrotron X-ray diffraction on neopentyl glycol (NPG) as the prototype material.  It was found that this material exhibited the maximum entropy changes of 389 J kg-1K-1, achieved at applied pressure of 45.0 MPa. This value is one order of magnitude larger than those of current leading caloric materials, as shown in Figure. More importantly, the entropy changes exceed one half of the maximum at 15.2 MPa, which is very beneficial to practical application. Accessing large-scale facilities in Japan (J-PARC and SPring-8) and Australia (ANSTO) to utilize neutron scattering and synchrotron X-ray diffraction techniques, the team revealed that the constituent molecules of NPG are extensively orientationally disordered on the lattices and these materials are intrinsically very deformable. As a result, a tiny amount of pressure is able to suppress the extensive orientational disorder to induce the phase transitions to the ordered state, and thus huge pressure-induced entropy changes are obtained. So far, these two merits make plastic crystals the best barocaloric materials.

This research establishes the microscopic scenario on colossal barocaloric effects of plastic crystals and suggests that plastic crystals are an emerging class of caloric materials. The finding is likely to benefit the design of better caloric materials and solid-state refrigeration technology in the future.