Utilizing Order-Disorder Phase Transitions for Solid-State Cooling: From Thermodynamics to Devices

Vapor-compression air conditioning is a technology which is essential to modern life. However, the established hydrofluorocarbon refrigerants central to this technology are potent greenhouse gases—one to five thousand times more effective than CO₂. The unintentional release of these refrigerants to the atmosphere during air conditioner installation, maintenance, and disposal is currently responsible for ca. 4% of planetwide global warming and is expected to rise to 10% of all warming by 2050. To eliminate this source of atmospheric emissions, we are developing solid-state barocaloric materials which can serve as direct replacements for hydrofluorocarbons in air conditioners and other heat-pump applications. We have recently discovered a promising new family of barocalorics: layered halide perovskites with long alkyl ammonium tails. These materials undergo solid-solid, order-disorder transitions within the alkyl sublattice which are analogous to the melting of simple <i>n</i>-alkanes, albeit confined to two dimensions by the layered perovskite structure. Layered perovskite transitions occur near ambient temperature and with high transition entropies making them able to adsorb and release significant amounts of heat in the temperature range relevant for indoor cooling. Additionally, these phase transitions show low hysteresis, and the transition temperatures are highly sensitive to pressure. This combination of properties enables layered perovskites to realize efficient barocaloric cooling with a pressure swing of 200 bar or less—a range achievable with standard hydraulic systems—whereas prior barocaloric systems require pressures above 1000 bar to realize measurable cooling. To demonstrate this in practice, we have designed and constructed a custom barocaloric prototype device and achieved efficient barocaloric cooling at pressures at and below 200 bar for the first time. I will discuss our current progress, ongoing challenges, and future directions of this work.