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A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 2 0 4 0 FEATURE To that end, an improved elastocaloric cooling material was developed by blending nickel and titanium metal pow- ders together, forged using a 3D printer (see Fig. 2). The new material is more efficient than current SMA technology and is also a significantly more “green” cooling technology than the commercial technologies available today. Moreover, it can be quickly scaled up for use in larger devices. FATIGUE-RESISTANT MICROSTRUCTURES Comparatively speaking, there are three classes of caloric cooling technology–magnetocaloric, electrocaloric, and elas- tocaloric–all of which are “green” and vapor-less [4-6] . Magneto- caloric, the oldest of the three, has been under development for 40 years and is just now on the verge of being commercial- ized. Some magnetic SMAs are also the leading elastocaloric materials. However, to date, their lifetimes in cooling appli- cations have been limited to several thousand cycles at best (Fig. 1a). The breakthrough in the ability for the 3D printed NiTi to sustain more than a million elastocaloric cycles extends the potential life of a refrigerator out to ten years or more, leading to commercial viability. Moreover, the 3D printed NiTi material showed performances that were robust to functional fatigue, which often plagues the adoption of SMA technologies. Func- tional fatigue describes the degradation of the performance of the material, even though the material itself remains struc- turally intact. For elastocaloric materials, functional fatigue is exhibited in two ways: 1) the mechanical stress-strain, or the “elasto” part of the performance, changes with cycling, and 2) the heat exchange time-temperature, or “caloric” part of the performance, degrades with cycling. As shown in Fig. 1b-c, the 3D printed NiTi material resisted both types of degradation. The reason the 3Dprintedmaterials performbetter is that a secondary non-SMA phase, specifically a Ni 3 Ti intermetallic phase, forms in well dispersed, wavy nanostructures amongst the NiTi SMA phase during 3D printing, as shown in Fig. 3a-b. This nanocomposite, two-phase microstructure strengthens the alloys to degradation while also improving thermal effi- ciencies during elastocaloric cycles. The local processing of material in a 3D printing process allows these special struc- tures to form. While the same phase can form in conventional- ly processed SMAs, these structures have not been previously observed. In fact, when these same phases are formed via con- ventional processing, where an entire ingot of material is ther- momechanicallyprocessedat once, the structures are typically one to tens of micron sized inclusions that are detrimental to the performances of conventionally processed SMAs (Fig. 3c- d [7] ). In the 3Dprinting process, the rapid, localized heating and cooling of moltenmaterial limits these phases fromcoalescing and growing into the conventionally attained larger, detrimen- tal structures. In addition to the metallurgical advancement, the 3D- printing process also provides opportunities for transforming the mechanical engineering of solid-state cooling technolo- gies. In elastocaloric cooling, the SMA material functions as both the refrigerant and the heat exchanger. 3D printing pro- vides the ability to manufacture the material into advanced, topology optimized heat exchanger geometries that are other- wise too expensive or even impossible tomakewith traditional formingandmachiningprocesses, asdemonstrated inFig. 2f-g. Hence, in addition to the improved material performance, this new technology also provides for better cooling performan- ces via new and improved designs of the heat exchange de- vices. ~SMST For more information: Aaron Stebner, associate profes- sor, Georgia Institute of Technology, 771 Ferst Dr. Atlanta, GA 30332, 404.894.5167, aaron.stebner@gatech.edu. Acknowledgment The research for Huilong Hou was supported by the Na- tional Natural Science Foundation of China (NSFC) under grant No.12002013. References : 1. H. Hou, et al., Fatigue-Resistant High-Performance Elas- tocaloric Materials Made by Additive Manufacturing, Science, 366, p 1116-1121, 2019, DOI:10.1126/science.aax7616. 2. D. Coulomb, J.-L. Dupont, and A. Pichard, The Role of Re- frigeration in the Global Economy, International Institute of Refrigeration, Paris, France, 2015. 3. W. Goetzler, et al., Energy Savings Potential and RD&D Op- portunities for Non-Vapor-Compression HVAC Technologies, Navigant Consulting, Inc., (prepared for U.S. Department of Energy), Washington, D.C., 2014. 4. X. Moya, S. Kar-Narayan, and N. D. Mathur, Caloric Materi- als Near Ferroic Phase Transitions, Nat. Mater., 13 , p 439-450, 2014. 5. L. Mañosa, A. Planes, and M. Acet, Advanced Materials for Solid-State Refrigeration, J. Mater. Chem. A, 1 , p 4925-4936, 2013. 6. I. Takeuchi and K. Sandeman, Solid-State Cooling with Ca- loric Materials, Phys. Today, 68 , p 48-54, 2015. 7. O. Benafan, et al., Deformation Characteristics of the Inter- metallic Alloy 60NiTi, Intermetallics 82 , p 40-52, 2017. Advertise in SMST NewsWire in April 2021! For information about advertising in SMST NewsWire: Kelly Johanns, kelly.johanns@asminternational.org Current rate card may be viewed online at asminternational.org/mediakit. 1 0