Co-gradient evolution in sputtered TiNiCuCo thin films for elastocaloric coolingWednesday (07.06.2017) 20:24 - 20:27 Förde I Part of:
Since cooling applications based on conventional vapor compression, using climate damaging gases as cooling agents are responsible for a large amount of the electrical energy consumption, there is a large demand for highly effective environmentally friendly alternatives. These requirements are fulfilled by solid state cooling devices using caloric materials utilizing the transformation enthalpy of field-induced solid state transitions enabling high efficiency, environment-friendly solid state cooling. One example is the elastocaloric effect, which is based on the strain induced martensite-austenite transformation resulting under adiabatic conditions in a temperature change. High structural and functional fatigue resistance, as well as a small hysteresis and low transformation strains are necessary and matched by freestanding dc-magnetron sputtered Ti-rich TiNiCu thin films . The intrinsic coefficient of performance of these films is 15 compared to 7.7 in binary TiNi thin films. Since the adiabatic temperature change is about 9 K for TiNiCu thin films, a cascaded cooling process based on active regeneration is needed to fulfill the demands for temperature spans dT>30 K. Chluba et al.  showed that Cobalt reduces the transformation temperature by 22 K/at% without influencing the functional stability, enabling the use of stacked materials for an elastocaloric cooling device. To enhance the cooling power of a device, a Co-gradient along the films is needed which adopts to the transformations temperature along the flow channel of the cooling liquid. The samples are fabricated by multilayer-sputtering using 8” TiNiCu and Co targets with a subsequent annealing step. The Co-gradient is realized by the movement of the sample out of the plasma while Co sputtering takes place. By adjusting the sputtering time and the speed of the substrate holder, a transformation gradient > 3 K/cm can be established. The influence on the mechanical properties and the elastocaloric properties will be discussed using tensile tests, DSC-measurements and IR-measurements.