Assessing and tackling the barocaloric fatigue for applicable solid-state refrigeration

Barocaloric materials with pressure-driven first-order transitions are promising for solid-state refrigeration but often suffer from severe cycling fatigue. Here, we select NH4I as a model material, which exhibits a giant barocaloric effect and is ultrasensitive to pressure. Its barocaloric response is directly assessed under loading-unloading cycles with stress up to 100 MPa, with the adiabatic temperature change (ΔTad) decaying from 15 K to approximately 0.8 K after 100 cycles. In situ x-ray diffraction, Raman spectroscopy, and neutron diffraction concertedly reveal the existence of the residual high-pressure phase even after the first unloading, whose fraction is rapidly increased to 90% at the 100th cycle. A phenomenological model is developed to elucidate this fatigue behavior, and an excellent agreement with experimental data has been achieved. To relieve the intergranular stress and enhance the mobility of grains, we encapsulate the composite of NH4I particles and silicone oil into a 3D-printed polymer shell. Such an architectural tailoring has markedly improved the cyclic stability of the barocaloric effect with ΔTad rising to ∼4.6 K after 100 cycles. Our results establish a fundamental understanding of the barocaloric fatigue behavior and pave a feasible route to applicable barocaloric cooling technology.