Reversible elastocaloric effects with small hysteresis in nanocrystalline Ni-Ti microwires

Abstract

The elastocaloric effect (eCE) with high reversibility and small hysteresis associated with the B2↔R transformation was demonstrated in nanocrystalline Ni50.5Ti49.5 microwires. The microwire, with diameter of 100 μm and nanoscale grains 15-75 nm, was perpared by multi-step cold-drawing followed by low-temperature annealing. R—B2 transition occurs in the annealed microwire at ambient temperatures. Large isothermal entropy change (ΔSiso) 20.5 J/kg·K during cooling and 21.4 J/kg·K during heating under a stress of 335 MPa associated with the R—B2 transformation was confirmed. As a result, a refrigeration capacity (RC) 257.3 J/kg during cooling and 249.5 J/kg during heating over a working temperature window (ΔTFWHM) 298.5-313.6 K and 304.5-313.6 K, respectively, was achieved. Furthermore, the annealed microwire exhibited a small temperature hysteresis 5-8.5 K and very equivalent ΔSiso during forward and backward R phase transitions, giving rise to a highly reversible eCE with small hysteresis.