Elastocaloric TiNi-based Films and Devices

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This proposal represents the renewal of the joint project 'Elastocaloric Ti-Ni based Films and Devices' aiming at solid-state cooling using TiNi-based shape memory films. During the first funding period we demonstrate that SMA film devices show large temperature changes up to -16 K, high cycling frequencies faster than 1 Hz as well as ultra-high fatigue resistance (> 10 million cycles). These properties will be further explored in innovative cooling devices using the potential of thin film and MEMS technology. The underlying mechanisms for long term stability are investigated by focusing on the influence of precipitates, austenite/martensite crystallographic compatibility and grain size (E. Quandt). The results are then transferred to other material systems by investigating elastocaloric effect size and efficiency to identify well suited materials for small-scale elastocaloric cooling applications. Two approaches in demonstrator development are pursued based either on solid/solid heat transfer or on heat transfer in a liquid medium (M. Kohl). Solid/solid heat transfer will be improved by adapting the shape and force of contacting surfaces as well as by investigating the influence of thermally conductive layers and surface topography due to micro/nano-machining. Heat transfer in a liquid medium will be investigated for a regeneration-based elastocaloric device. Finite element simulations are performed on the device level taking into account the thermo-mechanical coupling of SMA film device and its environment. Demonstrator systems (M. Kohl and E. Quandt) will be fabricated and evaluated with respect to cooling power and long-term operation. The potential for microcooling applications will be assessed for the case of a lab-on-chip system.