Solid-state cooling with high elastocaloric strength and low driving force via NiTi shape memory alloy helical springs: Experiment and theoretical model

High efficiency and environment-friendly elastocaloric solid-state cooling has emerged as a promising alternative to traditional vapor-compression refrigeration. Owing to the high entropy change during martensite transformation, NiTi shape memory alloy (SMA) is a competitive candidate for the core components of solid-state refrigeration systems. However, main bottlenecks for the development of NiTi SMA based refrigeration systems are their relatively low elastocaloric strength and the requirement of high driving force. In this work, the elastocaloric effect of NiTi SMA helical springs with three geometrical configurations are investigated experimentally at first. It is found that the elastocaloric performance can be tailored by changing the spring geometrical parameters and the magnitude of applied load. Giant cooling temperature of 12.5K and elastocaloric strength of 0.31K/MPa are observed for the helical spring with a spring index of 7.7 under an ultra-low tensile driving force of 71 N. The elastocaloric strength reported in this work is ten times larger than that observed in the NiTi wires, rods, tubes, sheets and films subjected to a tensile or compressive loading, and twice larger than that in the NiTi wires under a bending deformation mode. Then, a three-dimensional (3D) thermodynamic-consistent constitutive model within the finite strain framework and considering the thermo-mechanical coupling effect is developed. The proposed model is further implemented into the finite element program ABAQUS by writing a user-defined material subroutine (UMAT). Finally, simplified analytical relations among the cooling temperature, maximum driving force, geometrical parameters of the springs and applied displacement are derived. Comparing the predictions with the experimental data, it is found that the influences of geometrical parameters and loading level on the elastocaloric strength of NiTi SMA helical spring can be well captured by both the finite element analysis and proposed analytical relation. This work shows a potential for developing the cooling technology with high elastocaloric strength and low driving force, and provides a theoretical guidance to design and assess the cooling device manufactured by SMA spring.