Overcoming fatigue through compression for advanced elastocaloric cooling

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Introduction Solid-state cooling with caloric materials is a viable alternative to conventional vapor compression, and has the potential to take on a sizable share of the worldwide market in heating, ventilation, and air conditioning technologies.1–3 Caloric materials are categorized as magnetocaloric, electrocaloric, or mechanocaloric when the cooling effect is driven by a magnetic, an electric, or a mechanical field, respectively. Mechanocaloric materials are further divided into elastocaloric materials, wherein uniaxial mechanical stress is used, and barocaloric materials, where isotropic stress such as hydrostatic pressure is applied to the material.3,4 The first observation of the elastocaloric effect was probably on a piece of rubber dating back to 1859;3 however, the systematic demonstration of elastocaloric cooling using shape-memory materials began only recently in 2012.5,6 The elastocaloric effect exhibits remarkable potential as manifested in the directly measured cooling ΔT of −17°C in Ni-Ti,5 a level not easily achievable by other caloric materials. The exploration of elastocaloric materials has now been expanded from Ni–Ti-based alloys to Cu-based alloys

and various magnetic alloys.7 At the same time, elastocaloric cooling system prototypes have been demonstrated based on tensile,8 compressive,9 and bending modes.10 Elastocaloric cooling is based on shape-memory alloys (SMAs). An SMA is able to “remember” its original shape when heated above its martensitic transformation temperature.11 SMAs in the austenite phase exhibit superelasticity where stress-induced phase transformation leads to a large reversible strain. Elastocaloric cooling takes place when the latent heat is absorbed back in the material during the reverse transformation from martensite to austenite in the unloading part of superelasticity.5,7 Continuous operation of elastocaloric cooling requires repetitive application of stress to SMAs, and the large number of mechanical loading cycles can potentially lead to onset of fatigue. Fatigue causes deterioration of mechanical properties of a material and eventual failure under cyclic application of mechanical loads.12 In this article, we highlight recent efforts in overcoming fatigue associated with SMAs used in elastocaloric cooling, from fundamental mechanisms to the design of cooling devices and systems. The microstructural origins of fatigue

Huilong Hou, Department of Materials Science and Engineering, University of Maryland, USA; [email protected] Jun Cui, Iowa State University, and Ames Laboratory, USA; [email protected] Suxin Qian, Department of Refrigeration and Cryogenic Engineering, Xi’an Jiaotong University, China; [email protected] David Catalini, University of Maryland, USA; [email protected] Yunho Hwang, Center for Environmental Energy Engineering, University of Maryland, USA; [email protected] Reinhard Radermacher, Center for Environmental Energy Engineering, University of Maryland, USA; [email protected] Ichiro Takeuchi, University of Maryland, USA; [email protected] doi:10.155