October_AMP_Digital

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 3 8 ABREAKTHROUGH INSOLID-STATE COOLING ABILITIESOF SHAPEMEMORY ALLOYS ENABLEDBY 3D PRINTING Laser melting of elastocaloric metals can create fatigue-resistant microstructures, which enables solid-state cooling technologies. Aaron P. Stebner,* Georgia Institute of Technology, Atlanta Huilong Hou, Beihang University, Beijing, P.R. China Ichiro Takeuchi, University of Maryland, College Park A collaborative teamof researchers led by Ichiro Takeuchi and Huilong Hou of the University of Maryland (Hou is now an assistant professor at Beihang University, Bei- jing, China), together with colleagues from Iowa State Univer- sity, Ames National Laboratory, and Colorado School of Mines, developed and demonstrated that 3D printed nickel-titanium (NiTi, trade name Nitinol) shape memory alloys (SMAs) can attain better solid state cooling performances than the best performances known of traditionally manufactured SMAs. Specifically, the 3D printed materials showed 20 to 30 % less normalized energy lost without failing to over 1 million cycles relative to energy that is lost by conventionally processed NiTi materials in 100,000 cycles (Fig. 1a). The initial study was pub- lished in Science [1] . *Member of ASM International ELASTOCALORIC COOLING Cooling technology, used in refrigeration and HVAC sys- tems around the globe, is a multi-billion dollar business [2] . Vapor compression cooling, which has dominated the market for over 150 years, has not only plateaued where efficiency is concerned, but also uses chemical refrigerants with high glob- al-warming potential (GWP). Solid-state elastocaloric cooling, where stress is applied to materials to release and absorb (la- tent) heat, has been under development for the last decade and is a frontrunner among the so-called “alternative” cooling technologies [3] . Shape-memory alloys are found to display a significant elastocaloric cooling effect; however, presence of hysteresis—work lost in each cycle and cause of functional fa- tigue and eventual failure—remains an open challenge. FEATURE Fig. 1— (a) 3D printed NiTi/Ni 3 Ti nanocompositematerials (red) showed an order of magnitude improvement in lifetime together with greater energy efficient (lower energy lost (∆ E ) normalized by the total energy ( E )) during elastocaloric cycling, relative to the best performing elastoca- loric and magnetocaloric SMAs made using traditional processing methods. The 3D printed materials were also highly resistant to functional fatigue that often plagues SMAs in applications, as demonstrated by (b) the mechanical response of cycle 1,000,000 being nearly identical to the mechanical response of cycle 1 for the greatest cyclic loading used in the study, and (c) the elastocaloric performances of cycle 1,000,000 being nearly identical to cycle 1 for several different temperature intervals (∆ T ). Reproduced fromHou et al. [1] with permission. (b) (a) (c) 8