The Elastocaloric Effect: A Way to Cool Efficiently

Abstract

up to 20-times larger cooling powers per mass compared to gadolinium (Gd), a benchmark magnetocaloric material, with comparable coeffi cient-of-performance (COP) values (≈5).These results can open up a new way of making cooling devices with much more compact systems and with the possibility of avoiding expensive rare-earth materials.The martensitic transformation is a fi rst-order solid-tosolid diffusionless structural transformation responsible for the shape memory effect and superelasticity. [ 23 ] When a shape memory alloy in the austenitic (cubic) phase is axially stressed, an exothermic austenitic-martensitic transformation occurs, which under adiabatic conditions causes the material to heat up. This heated material then rejects heat to the surroundings and cools down to the ambient temperature. When the stress is removed, the crystal structure transforms back to the austenitic phase, the material cools down and is now able to absorb heat from the surroundings. This process (as a cooling cycle) is schematically shown in Figure 1 a. In general, the main groups of superelastic alloys are Ni-Ti-based (doped with Cu, Co, Pd, etc.); Cu-based (doped with Al, Ni, Zn, Mn, etc.), and Fe-based (doped with Pd, Rh, Mn, Si, Ni, etc.) alloys. [ 23 ] All of those alloys can also be considered as potential elastocaloric materials when they undergo stress-induced transformation and its transfor-

Combined caloric effects in a multiferroic Ni–Mn–Ga alloy with broad refrigeration temperature region

“…As a result, an eC effect is promising to be achieved in NMG due to the latent heat associated with the MT. 21,23 Therefore, eC in the ferromagnetic austenitic state could be utilized as a refrigeration method in the aforementioned broken region. …”

“…Since the eC effect stems from the stress-induced structural transformation, 21,23 it is necessary to investigate the detailed transformation process between martensite and austenite with the application of stress on NMG. Here, the in situ neutron diffraction patterns for NMG are measured at 303 K with the stress loading and unloading, which gives a direct evidence for the stress-induced reversible MT.…”

“…2(c), suggesting that a reversible eC effect could be achieved in NMG. 21,23 Before investigating the eC effect, the isothermal stress-strain curves for NMG were measured along the SD up to a compressed stress of 100 MPa at different temperatures. As show in Fig.…”

Giant Mechanocaloric Effects in Fluorite-Structured Superionic Materials

“…We note that the represented cooling sequence works in "inverse" order to usual refrigeration cycles based on ferroelectrics and shape-memory alloys. [11][12][13][14] The reason for this is that the state of maximum entropy is accessed through the switching-on of the external field. Nonetheless, the normal order can be recovered by setting T > T s (0) operation conditions and applying compressive stresses.…”

“…We note that in ferroelectric-paraelectric or austenite-martensite transformations, the resulting adiabatic temperature changes inevitably start decreasing beyond a certain threshold value of the external field due to saturation of the involved degrees of freedom. 12,13 In fact, the critical temperature in superionic materials can be reduced significantly by means of nanopatterning and chemical substitution strategies. 2,23 Aimed at alleviating the technical shortcomings found in CaF 2 , we investigated the same class of superionic and mechanocaloric phenomena in PbF 2 , a related fluorite-structured FIC with a much lower transition temperature of T s (0) ∼ 700 K. 24 (We note that by minimally doping PbF 2 with potassium ions it is possible to reduce the corresponding critical temperature practically down to ambient.…”