High-performance elastocaloric materials for the engineering of bulk- and micro-cooling devices
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Introduction Refrigeration and cooling account for a significant fraction of global energy usage. The need for reduction of energy consumption and greenhouse gas emissions has motivated research in new solid-state cooling concepts (ferroic cooling).1 These include magneto-, electro-, or elastocaloric effects. Pseudoelastic NiTi-based shape-memory alloys (SMAs) are a very promising class of candidate materials for elastocaloric cooling.2,3 While NiTi SMAs are better known for being able to restore their initial geometry after large deformations,4–6 these materials have recently also been identified as a promising solid-state alternative to well-established vapor compression cooling systems.7 The elastocaloric effect (i.e., pseudoelasticity, a mechanical shape-memory behavior) relies on a reversible martensitic phase transformation,4–6 a specific type of solid-state transformation. It represents a shear process in the crystal lattice,8 and can be triggered by cooling or mechanical loading. Figure 1 shows a schematic elastocaloric cooling cycle starting at the high-temperature austenite state, stage 0 in Figure 1a. During mechanical loading, a stress/strain plateau is reached where the parent phase, austenite, transforms into stress-induced martensite, stage 1 to stage 2. The reverse transformation,
stage 3 to stage 4, is characterized by an unloading plateau at lower stresses. Both transformation events are associated with large latent heat ΔH, and thus cause remarkable temperature changes in the SMA. The area, W, in Figure 1a represents the mechanical work input (i.e., force and distance) required to drive an ideal cooling cycle. It is common to characterize the cooling efficiency by the coefficient of performance (COP), which represents the ratio between the cooling energy output ΔH and the required work input, W.9 Figure 1b shows different material stages of the cooling cycle. The forward phase transformation that occurs during loading between stages 1 and 2 results in the release of latent heat and thus in an increase in the SMA temperature. This heat amount needs to be absorbed by a heat sink, such that a lower temperature is again approached in the material. During unloading, the reverse transformation causes a significant temperature drop between stages 3 and 4. This drop, which corresponds to values between 15°C and 30°C for NiTi,9–11 determines the lowest achievable temperature in a ferroic cooling process. Present research on elastocaloric cooling follows two main directions. First, basic aspects related to the material behavior, such as heat effects, functional stability, and alloy design, are
Jan Frenzel, Ruhr University Bochum, Germany; [email protected] Gunther Eggeler, Ruhr University Bochum, Germany; [email protected] Eckhard Quandt, Kiel University, Germany; [email protected] Stefan Seelecke, Saarland University, Germany; [email protected] Manfred Kohl, Institute of Microstructure Technology, Karlsruhe Institute of Technology, Germany; [email protected] doi:10.1557/mrs
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