Investigation of Precipitate-rich TiNi-based Thin Films for Elastocaloric Cooling Devices

Ferroic cooling exploits the transformation enthalpy of reversible solid state phase transitions driven by external fields. The stress induced elastocaloric effect in NiTi based alloys is associated with high transformation enthalpies and neither high magnetic or electrical fields are required. However, elastocaloric cooling is still in basic research. One major obstacle is the mandatory cyclic stability since the material has to undergo more than 10⁸cycles during the lifetime of a device.

It will be shown that Ti-rich TiNiCu films are able to withstand more than 10⁷full superelastic transformation cycles without any degradation. Measurements of the adiabatic temperature change have been conducted by IR-thermography. Despite a large volume amount of non-transforming precipitates, the elastocaloric cooling capability of ΔT > 7K is still suitable. The realization of a feasible actuation mechanism remains however an open question. In this regard thin films have the advantage of low transformation forces. Additionally the transformation strain in TiNiCu films is decreasd to 1-1.5% which simplifies actuator demands compared to the large strains of 5-7% in binary NiTi.

The authors acknowledge funding by the DFG SPP 1599 program