Optimizing sputtered NiTi-based shape memory alloys for elastocaloric cooling systemsTuesday (18.06.2019) 12:30 - 12:45
Cooling systems based on caloric materials can reach a higher efficiency and do not rely on coolants with a high global warming potential, as they are found in environmentally harmful vapor compression or inefficient thermoelectric technology. The elastocaloric effect, associated with reversible thermal changes in dependence of an alternating mechanical stress field, shows especially large entropy and temperature changes in the vicinity of first order phase transformation, as they occur in shape memory alloys (SMA). Adiabatic temperature changes larger than 15 K are reached in binary NiTi, a kind of benchmark material for elastocaloric cooling. Main drawbacks are a poor lifetime and dissipative superelastic hysteresis losses, resulting in rather low efficiencies. Just recently it was shown that dc-magnetron sputtered TiNiCuCo films are able to reach a sufficient lifetime of more than 10 million superelastic cycles without noticeable functional fatigue and at the same time maintaining a high effect size larger than 10 K for the reverse martensitic transformation [1, 2, 3]. By varying the Co content the transformation temperatures can be adjusted well below 273 K, as needed for active regeneration and cascaded systems [3, 4]. First single stage cooling devices using one SMA film show temperature spans of 14 K and a cooling power of 19 W g-1 . This work presents an outline how to optimize TiNiCu based films for elastocaloric cooling and the corresponding performance in elastocaloric microcooling heat pumps.
Acknowledgements: Funding by the DFG priority program SPP1599 Ferroic Cooling is gratefully acknowledged.
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