Highly efficient cascaded elastocaloric cooling device enabled by functionally graded SMA tubes

Elastocaloric refrigeration has attracted significant interest in recent years as an efficient and environmentally benign alternative to traditional vapor-compression systems. Numerous devices have demonstrated the potential of elastocaloric cooling using both NiTi and NiTiCu materials, achieving temperature lifts of up to 75 K. However, to ensure the feasibility and scalability of these systems in competing with propane-based ones, further optimization of these material systems is necessary. In this context, functionally graded materials (FGMs) can play a key role in maximizing performance by enabling longitudinal control of material properties. This study investigates the fabrication and integration of functionally graded NiTiCu shape memory alloy (SMA) tubes for elastocaloric refrigeration.

To achieve this, an induction heating method was employed to selectively heat treat specific regions of the NiTiCu tubes. This technique allowed for precise control over the thermal exposure and consequently altered the transformation temperature of the material. By varying the process parameters such as heating duration, and power level, the austenite finish temperature (A_f) was successfully tuned across a broad range, from approximately 40 to 55 °C, enhancing heat actuation and cooling performance. These functionally graded tubes are integrated into a SMA actuator that are specifically designed to utilize the tuned temperatures, demonstrating sufficient force to drive elastocaloric cooling of 100 W.