Hinnerk Ossmer

Energy-efficient miniature-scale heat pumping based on shape memory alloys

Cooling and thermal management comprise a major part of global energy consumption. The by far most widespread cooling technology today is vapor compression, reaching rather high efficiencies, but promoting global warming due to the use of environmentally harmful refrigerants. For widespread emerging applications using microelectronics and micro-electro-mechanical systems, thermoelectrics is the most advanced technology, which however hardly reaches coefficients of performance (COP) above 2.0. Here, we introduce a new approach for energy-efficient heat pumping using the elastocaloric effect in shape memory alloys. This development is mainly targeted at applications on miniature scales, while larger scales are envisioned by massive parallelization. Base materials are cold-rolled textured Ti49.1Ni50.5Fe0.4 foils of 30 μm thickness showing an adiabatic temperature change of +20/−16 K upon superelastic loading/unloading. Different demonstrator layouts consisting of mechanically coupled bridge structures with large surface-to-volume ratios are developed allowing for control by a single actuator as well as work recovery. Heat transfer times are in the order of 1 s, being orders of magnitude faster than for bulk geometries. Thus, first demonstrators achieve values of specific heating and cooling power of 4.5 and 2.9 W g−1, respectively. A maximum temperature difference of 9.4 K between heat source and sink is reached within 2 min. Corresponding COP on the device level are 4.9 (heating) and 3.1 (cooling).

Elastocaloric heat pumping using a shape memory alloy foil device

This paper explores, develops and demonstrates a miniature heat pump based on the elastocaloric effect in shape memory alloys (SMAs). The active material is a Ti50.5Ni49.1Fe0.4 foil of 30 μm thickness showing a maximum temperature change during pseudoelastic loading/unloading up to +20/-16 K. This effect is implemented in a heat pump design consisting of SMA bridge, heat source and sink. First-of-its-kind demonstrators reveal a temperature difference between source and sink of about 7 K after 100 s for a ratio of heat capacities of SMA bridge and heat source of 1:85. The coefficient of performance (COP) of the device for cooling is 2.8 whereas the theoretical COP of the material is 10.5.

Elastocaloric cooling using shape memory alloy films

The elastocaloric effect in magnetron-sputtered Ni50.4Ti49.6 films of 20 μm thickness is studied by means of uniaxial tensile tests and infrared thermography. For the investigated films, the usable quantity of latent heat is about 7.2 J/g. When relieving the stress after tensile loading and subsequent temperature equalization at strain rates larger than de/dt = 0.2 s−1, a maximum temperature change of ΔT = −16 K is observed as expected for adiabatic conditions. Compared to bulk specimens, the heat transfer times are reduced to about 850 ms due to the larger surface-to-volume ratio, which is attractive for rapid cooling.

TiNi-based films for elastocaloric microcooling— Fatigue life and device performance

The global trend of miniaturization and concomitant increase of functionality in microelectronics, microoptics, and various other fields in microtechnology leads to an emerging demand for temperature control at small scales. In this realm, elastocaloric cooling is an interesting alternative to thermoelectrics due to the large latent heat and good down-scaling behavior. Here, we investigate the elastocaloric effect due to a stress-induced phase transformation in binary TiNi and quaternary TiNiCuCo films of 20 μm thickness produced by DC magnetron sputtering. The mesoscale mechanical and thermal performance, as well as the fatigue behavior are studied by uniaxial tensile tests combined with infrared thermography and digital image correlation measurements. Binary films exhibit strong features of fatigue, involving a transition from Luders-like to homogeneous transformation behavior within three superelastic cycles. Quaternary films, in contrast, show stable Luders-like transformation without any signs of degradation. The elastocaloric temperature change under adiabatic conditions is −15 K and −12 K for TiNi and TiNiCuCo films, respectively. First-of-its-kind heat pump demonstrators are developed that make use of out-of-plane deflection of film bridges. Owing to their large surface-to-volume ratio, the demonstrators reveal rapid heat transfer. The TiNiCuCo-based devices, for instance, generate a temperature difference of 3.5 K within 13 s. The coefficients of performance of the demonstrators are about 3.

Film and Foil-Based Shape Memory Alloy Microactuators for Fluid Handling

In this contribution, the potential of film and foil-based shape memory alloy (SMA) microactuators for fluid handling applications is explored. SMAs provide a high work density and allow for compact and robust actuator designs based on the one-way shape memory effect. Compared to more commonly used wires, actuators fabricated from thin film or foil material allow for more complex designs having several degrees of freedom and enable shorter switching times in the range of 10 ms. In order to commercialize such actuators, the “memetis GmbH” was founded as a high-tech spin-off of the Karlsruhe Institute of Technology (KIT) in Germany. Memetis is focused on miniature fluid handling products and combines rapid prototyping and rapid manufacturing techniques such as 3D printing, laser cutting and CNC milling of polymer housings for customer-specific device development. A normally open microvalve is presented here as an example, which is actuated by a novel fatigue-free TiNiCu film actuator.