According to the UN statistics, 25 to 30 percent of the world’s electricity is consumed for refrigeration. Current refrigeration technology mostly involves the conventional vapour compression cycle, but the materials used in this technology are of growing environmental concern because of their large global warming potential. As a result, both the research community and industries are devoting to exploiting environment-friendly, efficient refrigeration technology. In particular, China is not the best player in the cutting-edge refrigeration technology based on the vapour compression cycle so that such an exploration might lead to Chinese 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, Profs. Li Bing, Zhidong Zhang, Weijun Ren and collaborators have 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 important, the entropy changes exceed one half of the maximum at 15.2 MPa, which is very beneficial to the 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. These two merits make plastic crystals the best barocaloric materials so far. In Fig. 1, plastic crystals reported in this study are compared to other leading caloric materials.
This research has established the microscopic scenario on colossal barocaloric effects of plastic crystals and also suggested that plastic crystals are an emerging class of caloric materials, which might benefit the design of better caloric materials and solid-state refrigeration technology in the future.
Fig. 1:QENS measurements at ambient pressure (a) and 286 MPa (b), obtained at 325 K with Ei=2.64 meV. INS measurements at ambient pressure (c) and 286 MPa (d), obtained at 325 K with Ei=23.72 meV.