Unlocking large compressive strains in thin active elastocaloric layers

Abstract

• • Durable composites enabling compression of elastocaloric layers are demonstrated. • • Elastocaloric materials can be in any thin-layer geometry, e.g., strip, plate, foil. • • Composites achieve 8.1 K temperature change at 2.5% strain. • • Mechanical and thermal properties are stable after 104 loading cycles at 2% strain. Elastocaloric cooling attracts broad interest and rapidly growing attention due to its potential for high efficiency and low environmental impact. While it is common knowledge that triggering reversible entropy and temperature changes with stress applied in compression prevents rapid failures of materials, realizing this regime in elastocaloric systems is highly challenging because nearly all geometries suited for efficient heat transfer are prone to buckling even under modest loads. This work describes a concept of a novel composite, where an active NiTi layer is embedded into a polymer support structure such that the elastocaloric material is entirely in compression when the assembly is subjected to bending. The active layer achieves 8.1 K temperature change at 2.5% compressive strain without buckling. After 10,000 cycles at 2% compressive strain, the composite maintains mechanical integrity without degradation of the elastocaloric effect. The results show that NiTi and, potentially, other elastocalorically active materials in geometries previously thought impossible can be successfully implemented in regenerative cooling systems operating in compression.