Achieving great comprehensive elastocaloric cooling performances of superelastic NiTi by grain size engineering

Elastocaloric materials, as cutting-edge alternatives to hazardous refrigerants utilized in vapor-compression systems, are promising for eco-friendly solid-state cooling, but their various refrigeration metrics still lack reconciliation. Here, we investigate the grain size (GS) dependence of elastocaloric effect of NiTi with average GSs of 11, 22, 30, 45, and 70 nm and eventually find a suitable GS yielding great comprehensive cooling properties. Adiabatic temperature variation (|ΔTad|), isothermal entropy change (ΔS), and energy dissipation (ΔW) increase monotonically with GS, while cyclic stability and working temperature span (Tspan) exhibit an opposite trend, showing inevitable compromises among these criteria. The closer to 11 or 70 nm the GS is, the more intense conflicts are, due to the extreme natures of second-order-like and first-order transformation. The conflicts are significantly relaxed in the 30 nm-GS specimen with robust mechanical properties and mild martensitic transformation characteristics. The reconciled large |ΔTad| and low ΔW, benefiting from appreciable transformation strain and small hysteresis, make its coefficient of performance 3.7 and 2.32 times that in 11 and 70 nm-GS counterparts, respectively. High repeatability manifested as negligible degradation by 1.2% in the magnitude of |ΔTad| after 200 tensile cycles is also achieved owing to its high strength and good crystallographic compatibility, which contrasts sharply to that of the 70 nm-GS one decayed by 15%. Furthermore, a superior combination of wide Tspan (64 K) and large ΔS (31 J kg−1 K−1) results in a high refrigeration capacity of 1.7 kJ kg−1. The work demonstrates a concrete strategy for designing efficient and reliable elastocaloric refrigerants.