Manipulating Phase Transitions in Barocaloric Materials

Solid-state phase transitions are central to the design of advanced responsive materials needed to address a wide range of pressing global challenges. This is particularly relevant in the context of barocaloric effects—thermal changes in a material induced by applied hydrostatic pressure—that are strongest near first-order phase transitions. Barocaloric effects in solids can be used to drive energy-efficient heating and cooling cycles, offering a promising alternative to traditional vapor-compression technologies that rely on potent greenhouse gases. Although critical to realizing the full potential of barocaloric effects, it remains difficult to manipulate the thermodynamics of phase transitions in the solid state, and the microscopic mechanisms responsible for barocaloric effects are not well understood.<br/><br/>In this talk, I will describe recent examples of how barocaloric effects in phase-change materials can be leveraged for sustainable refrigeration. First, I will discuss the new classes of barocaloric materials recently discovered in our laboratory—including two-dimensional hybrid perovskites [1], molecular spin-crossover complexes [2], and dialkylammonium halide salts [3]. I will highlight our efforts to elucidate key molecular factors that govern entropy change, pressure sensitivity, and hysteresis of their barocaloric phase transitions. Second, I will describe a new mechanism for driving barocaloric effects that renders solid-state phase transitions extremely sensitive to pressures. This mechanism leverages the thermodynamic effects of a pressure-transmitting medium on hydrocarbon order–disorder transitions. Specifically, I will show how this mechanism enables barocaloric solids to operate over a wide temperature window at record-low pressures. This approach substantially reduces the cost and power consumption required for operating barocaloric cooling cycles, unlocking the use of solid refrigerants in practical devices. Finally, I will highlight our recent efforts to evaluate barocaloric materials at the system level. I will discuss how direct measurements of cooling performance in a cooling prototype allows us to directly probe the impact of hysteresis on device-level efficiency, cooling power, and cyclability under a variety of driving conditions. Bridging the gap between materials discovery and prototype development, this work represents a crucial step toward the development of barocaloric refrigeration system and provides fundamental insights into thermo-mechanical coupling and hysteresis phenomena in phase-change materials.<br/><br/><b>References</b><br/>[1] Seo, J.; McGillicuddy, R. M.; Slavney, A. H.; Zhang, S.; Ukani, R.; Yakovenko, A. A.; Zheng, S.-L.; Mason, J. A. “Colossal Barocaloric Effects with Ultralow Hysteresis in Two-Dimensional Metal–Halide Perovskites” <i>Nature Communications</i> <b>2022</b>, <i>13</i>, 2536.<br/>[2] Seo, J.; Braun, J. D.; Dev, V. M.; Mason, J. A. “Driving Barocaloric Effects in a Molecular Spin-Crossover Complex at Low Pressures” <i>J. Am. Chem. Soc</i><i>. </i><b>2022</b>,<i> 144</i>, 6493.<br/>[3] Seo, J.; Ukani, R.; Zheng, J.; Braun, J. D.; Wang, S.; Chen, F. E.; Kim, H. K.; Zhang, S.; Thai, C.; McGillicuddy, R. M.; Yan, H.; Vlassak, J.; Mason, J. A. “Barocaloric Effects in Dialkylammonium Halide Salts” <i>J. Am. Chem. Soc</i><i>. </i><b>2024</b>,<i> 146, </i>2736.