Cobalt Gradient Evolution in Sputtered TiNiCuCo Films for Elastocaloric Cooling

Abstract

Elastocaloric cooling demands for temperature changes larger than 30 K to become an alternative to classical vapour compression cooling systems. The principle of active regeneration allows to exceed the materials intrinsic adiabatic temperature change. The resulting temperature gradient of the cooling demonstrator leads to a change in the thermomechanical response along the regenerator bed, according to the Clausius–Clapeyron equation. A lowered efficiency as well as an increased probability of early breakdown of the device due to functional and especially structural fatigue are the result. These changes in the thermomechanical response are especially present in NiTi and NiTiCu‐based systems with a coefficient of about 7 and 10 MPa K−1, respectively. To address these issues for future applications a change in the transformation temperature along the shape memory alloy film, adapted to the temperature gradient of the regenerator, is required. Cobalt is well known to reduce the transformation temperatures, while maintaining the functional stability of the TiNiCu‐based films. In this study multilayer dc‐magnetron sputtering is used to fabricate TiNiCuCo films with a cobalt concentration gradient along the samples, which can be precisely tuned by changing the sputter conditions. Transformation gradients of 0.3 K mm−1 are obtained, showing the same functional stability and adiabatic temperature changes of about −11 K as their counterparts without transformation gradient. Under isothermal conditions, a sloped transformation plateau is observed corresponding to a highly directed martensitic transformation.