Elastocaloric effect vs fatigue life: Exploring the durability limits of Ni-Ti plates under pre-strain conditions for elastocaloric cooling

Abstract

Structural fatigue is the major obstacle that prevents practical applications of the elastocaloric effect (eCE) in cooling or heat-pumping devices. Here, the eCE and fatigue behaviour of Ni-Ti sheets are systematically investigated in order to define the material's fatigue strain limit and the associated eCE. Initially, the eCE was evaluated by measuring adiabatic temperature changes at different strain amplitudes and different mean strains along the loading and unloading transformation plateaus. By comparing the eCE with and without pre-strain conditions, the advantages of cycling an elastocaloric material at the mean strain around the middle of the transformation plateau were demonstrated. In the second part of this work, we evaluated the fatigue life at the mean strain of 2.25% within the loading plateau and at the unloading plateau after initial pre-straining up to 6% and 10%, respectively. It is shown that on polished samples, durable operation of 10 5 cycles can be reached with a strain amplitude of 0.50% at the loading plateau, which corresponds to adiabatic temperature changes of approximately 5 K. At the unloading plateau (after initial pre-strain of 10%), durable 1 operation was reached at a strain amplitude of 1.00%, corresponding to adiabatic temperature changes of approximately 8 K. The functional fatigue was analysed after the cycling and it is shown that once the sample has been stabilized there is no further degradation of the eCE, even after 10 5 cycles. These results present guidelines for the design and operation of efficient and durable elastocaloric devices in the future.

Future prospects for elastocaloric devices

“…Fatigue life is an important challenge that must also be addressed before the technology can be considered for commercialization. I have listed over 1 million cycles as a goal because there are indications that eCMs that can withstand even 100 000 cycles may have infinite fatigue life [33]. If we consider a 20 000 h lifetime operating at 1 Hz, the required lifetime will be 72 000 000 cycles.…”

“…The potential benefits of operating in compression are discussed in [34] and several reported devices are currently operating in compression, but their fatigue life has not yet been reported. Other methods to improve fatigue life include implementing advanced materials [16] and applying the strain in tension at the middle of the superelastic region rather than at a fully relaxed state [33]. Another solution to the fatigue challenge is to develop new, more resilient materials or to improve fatigue properties by controlling hysteresis [35].…”

Novel mechanocaloric materials for solid-state cooling applications

“…Consequently, the temperature of the EC element increases up to T2 = T1 + ΔT, where the size and sign of ΔT depends on the material and applied stress [see Eq. (9)]. The EC material, which still remains elastically strained, then is put into contact with a thermal sink so that heat is ejected from the system.…”

“…SMA alloys present two main drawbacks in the context of EC cooling [9,10]. One is related to the unavoidable hysteresis effects associated with the first-order nature of the martensitic phase transition.…”

“…Likewise, the large latent heat accompanying the first-order martensitic phase transition in archetypal EC compounds renders excellent cooling performances [8]. Nevertheless, conventional EC materials also present some drawbacks, mainly associated with their structural stability, mechanical fatigue, and phase-transition hysteresis, which lead to durability and irreversibility issues that may limit severely their cooling performance and applicability [9]. Finding new MC materials with improved mechanical and phase switching properties is highly desirable for the deployment of sustainable, environmentally friendly, and highly scalable solid-state cooling applications.In this review article, we survey a large number of MC materials ranging from archetypal shape-memory alloys to less renowned EC compounds like polymers, polar oxides, organometal halide perovskites, molecular crystals, multiferroics, and fast-ion conductors.…”