ADVANCED MATERIALS & PROCESSES | APRIL 2024 50 This device was designed to address the limitation of a single material’s narrow operating temperature range (~20 K) by cascading several materials with different phase transition temperatures. By matching the working temperature distribution of three NiTi units with their austenite finishing temperature, the operating temperature window was successfully extended to 100 K, with a substantial temperature lift of 75 K achieved on the water side. The system used an applied stress of 900 MPa and an operating frequency of 1/3 Hz[6,7]. FEATURE 4 In addition to the innovative system design, Sun’s group has been working on heat exchange fluid. Their results showed the thermal conductivity of graphene nanofluids to be 50% higher than that of distilled water. The combination of a large surface-area-to-volume ratio of the working material and an enhanced heat exchange fluid makes it possible for the system to operate at a higher frequency. Using 104 g of NiTi, the new device achieved a cooling power of 1.3 kW on the water side, a temperature lift of 32 K, and survived more than two million compressive cycles. This Fig. 2 — Photo and performance graph (75 K water side temperature lift) of the cascade device built at The Hong Kong University of Science and Technology. Fig. 3 — Evolution of elastocaloric devices and systems developed at the University of Maryland. (a) (b) (c) (d)
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