Novel Class of Additive Manufactured NiTi-Based Hierarchically Graded Chiral Structure with Low-force Compressive Actuation for Elastocaloric Heat Pumps

Elastocaloric refrigeration is the most promising green solid-state refrigeration technology to replace conventional vapor compression refrigeration. The development direction of the elastocaloric component that acts as a key part of the elastocaloric refrigeration system contains a large elastocaloric effect, low stress hysteresis, high heat exchange performance, and small driving loads. The first two indices can be realized by material modification; however, the last two are more dependent on a novel porous structure design. However, the conventional porous structure is confronted with some critical challenges, including inhomogeneous stress, a significant hysteresis area, and deformation instability under the alternating cyclic loading. In this study, a NiTi-based elastocaloric structure model with chirality feature and gradient design as innovative elements was presented, bio-inspired by the structure of the plant tendrils. A quantitative optimization for the NiTi-based elastocaloric structure was performed using the finite element analysis (FEA) method. Strain and martensite volume fraction (MVF) fields during the loading and unloading processes were predicted and evaluated. The simulated results indicated that increasing the thickness gradient g₁ of the strip or decreasing the diameter gradient g₂ of the structure was beneficial to achieving more homogeneous strain and martensite distribution, simultaneously with higher energy storage efficiency and specific surface area. In addition, these NiTi-based chiral structures with different structural parameters were fabricated by laser powder bed fusion (LPBF). At the optimized structure parameters of g₁ = 2 and g₂ = 1.11, the LPBF-fabricated NiTi-based chiral structure could achieve an adiabatic temperature change ΔT_ad of 2.3 K, driving force of as low as 149.11 N, and |ΔT_ad/F| of as high as 15.42 K/kN at a recoverable compressive strain of 10%.