Bernhard Winzek

Shape memory microvalves based on thin films or rolled sheets

Abstract This paper reports on recent developments of gas microvalves, which are actuated by microfabricated shape memory alloy (SMA) thin films or sheets with stress-optimised shapes. TiNi, TiNiCu and TiNiPd thin films of 10 μm thickness have been fabricated by sputter deposition. TiNi sheets of 30–100 μm thickness have been made by hot-forging and cold-rolling. The main fabrication technologies of the valves have been laser cutting or electrolytic photoetching and subsequent hybrid integration. By variation of the chemical composition of the thin films, the valve operation temperature was adjusted between 30 and 120°C. The thin film devices have been operated in tension mode allowing the control of maximum pressure differences of 3000 hPa (3 bar). The corresponding maximum gas flow is about 360 Standard ccm. The maximum control power for TiNiCu and TiNi microvalves is 110 mW, for TiNiPd microvalves 220 mW. SMA sheet microdevices have been operated in bending mode, which allows large strokes and thus large gas flows.

Shape memory effect and magnetostriction of sputtered NiMnGa thin films

NiMnGa thin films have been deposited by magnetron sputtering on Mo substrates using a Ni50Mn30Ga20 powder metallurgical target. Independent from variation of substrate temperature during the sputtering process the deposited films are found to be polycrystalline. X-ray diffraction patterns show a decreasing peak width and a shift to slightly higher Bragg angles with increasing substrate temperature during sputtering, which is even amplified when subsequent rapid thermal annealing is applied. Annealing temperatures above 500°C lead to a remarkable enhancement of the shape memory effect as well as of the magnetostriction. Temperature induced martensitic transformations have been measured by a cantilever deflection technique and a cantilever resonance method. Martensitic start temperatures (MS) range between 50 and 90°C depending on composition and annealing temperature. Stress relief upon the martensitic transformation ranges between 200 and 300 MPa whereas the magnetostrictive coupling constant b is about 2 MPa. Magnetization measurements and Curie temperature determination reveal ferromagnetic behavior within the temperature range of the martensitic transformation.

Bistable shape memory thin film actuators

Shape memory alloys (SMA) are able to provide high work output when they undergo the martensitic transformation. Therefore, they present a favorable actuation mechanism for microsystems, e.g. for microvalves, switches or microgrippers. Sputter deposited thin SMA films are already in use as free-standing films or as composites in combination with a substrate. In the case of a composite, the substrate works as a bias spring and enables the SMA actuator to show a two way behavior. To enlarge the potentialities of shape memory based actuators a bistable principle is presented. This is realized by the combination with a polymer exhibiting a glass transition temperature (Tg) between the hysteresis loop of the shape memory composite. The fabrication of this composite is described with a special emphasis on the development of suitable polymer samples.

Bistable thin film composites with TiHfNi-shape memory alloys

The energy consumption of thermal driven shape memory actuators can be drastically reduced by the use of bistable actuators. This work demonstrates how a bistable composite can be realized using shape memory alloy thin films in combination with a metallic substrate and additional polymer layers. Due to their suitable hysteresis properties and transformation temperatures, shape memory composites with Ti-Hf-Ni films were investigated. The results show, that the composites provide large work output since the film stress is reduced by more than 400 MPa within the martensitic transformation.

SMA-thin film composites providing traveling waves

Actuators providign traveling waves are attractive for several industrial applications, like active skins for turbulent drag reduction or transport devices for assembling processes. Traveling waves require a flexible structure in contrast to standing waves which contain knots without vertical motion. Therefore, different concepts to realize these waves have been developed. This work presents the functional principle of wave generation by means of shape memory allow (SMA) thin film composites and the conditions that have to be considered for the performance of traveling waves with continuous wave flow. Devices using temperature inhomogeneities, an arrangement of separately addressed SMA composites as well as structures using different SMAs have been investigated and their feasibilty is discussed.

Smart motion control by phase-coupled shape memory composites

This contribution describes a new functional principle for the motion of walking robots by means of shape memory composites. It illustrates the phase-coupled motion of actuator components with different shape memory alloys allowing wave-like or inch-worm behavior, respectively. The thermo-mechanical properties of the corresponding thin film composites with Ti(NiCu) and (TiHf)Ni alloys are discussed with the focus on their hysteresis. The shape memory effect of the actuators is characterized and samples are performed to prove their capabilities. The contribution also presents a bistable shape memory composite in which a PMMA polymer fixes the bending states of the metallic composite to reduce the energy consumption.

Shape Memory and Magnetostrictive Materials for Mems

Ferroic materials are ideally suited for actuators in MEMS applications. Ferro- elastic materials, i.e. shape memory alloys (SMAs), produce a significant, o. o. 10 4 microstrain, phase transformation induced thermoelastic eigenstrain. Rare earth based ferromagnetic materials possess large, o.o. 10 3 microstrain, saturation magnetostriction which can be developed at fields of the order of mT readily achieved in MEMS geometries. This paper discusses the essentials of the thermoelastic stress evolution in SMA/Substrate bimorphs and of the development of rare earth based ferromagnetic multilayers. SMA actuationunder the constraint of a substrate and the optimization of the multilayers are emphazized.