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 trans-formation, 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 sys-tems 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 experi-mentally 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 constitu-tive model within the finite strain framework and considering the thermo-mechanical coupling effect is devel-oped. 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.

Automated summary

This study experimentally investigates the elastocaloric effect of NiTi shape memory alloy helical springs with different geometric parameters and applied loads. It is found that the elastocaloric performance can be tailored by adjusting the spring's geometric parameters and the applied load. A three-dimensional thermodynamic-consistent constitutive model is developed, and simplified analytical relations are derived to predict the cooling temperature and elastocaloric strength of the springs. The experimental data and predictions from both the finite element analysis and analytical relations are in good agreement, demonstrating the potential of developing cooling technology with high elastocaloric strength and low driving force.