Boosting low-pressure reversible barocaloric effect in plastic crystal KPF6 through controlled particle size

Abstract

Plastic crystals have large entropy change during first-order phase transitions due to strong molecular orientation disorder and volume changes. This feature has revived interest in plastic crystals, as they have great potential in solid-state refrigeration applications induced by external pressure. However, the reversible barocaloric effect in most plastic crystals does not compare favorably with their isothermal entropy change. Here, we demonstrate that controlling the particle size of the plastic crystal KPF6 can help achieve a compromise between the phase transition resistance at grain boundaries and the phase transition correlation within grains and induce a giant reversible barocaloric effect at low pressure. In particular, the 75–150 μm plastic crystal KPF6 displays the lowest thermal hysteresis, with a giant reversible barocaloric entropy change of 50.0 J · kg−1 · K−1 under a pressure change of 40 MPa. The reversible barocaloric effect can be more than doubled by simply optimizing the microstructures of the plastic crystal KPF6. Our work provides an effective way to enhance the reversible barocaloric effect in plastic crystal materials. We also give insight into the physical mechanism that enhances the reversible barocaloric effect through transmission electron microscopy analysis.