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This paper presents a study of the mechanical and thermal properties of SMA microvalves, which are driven by a microactuator of a NiTi foil or a Ni 2 MnGa thin film. By coupled finite element simulations, von Mises stress profiles and resulting actuation forces are determined, which are compared with experimental force-displacement characteristics. The relatively high phase transformation temperatures of the used Ni 2 MnGa specimens M f / A f of 92 / 125 °C, give rise to cooling times of about 25 ms compared to 90 ms for NiTi. Thus, Ni 2 MnGa microvalves show a superior switching performance. However, due to brittleness and buckling instability, Ni 2 MnGa microvalves only allow a maximum controllable pressure difference of 70 kPa, which is more than a factor of 7 smaller compared to NiTi microvalves.

For finite element simulation of shape memory actuators, a two-phase macromodel combined with a large deflection procedure is implemented in a FEM program for 3D mechanical analysis. The program is part of a tool for coupled electrical, thermal and mechanical problems. Details of the solution procedure are discussed. The model is used to investigate the performance of a shape memory alloy (SMA) microgripper. The microgripper is made of a monolithic SMA device of 2 x 5.8 x 0.1 mm 3 size comprising various integrated functional units like two microactuators for active closing and opening the gripping jaws and an optical slit. The simulated results are in quantitative agreement with experimentally determined data.

An optical scanner of 9 x 3 x 5 mm 3 size is presented, which is driven by a microactuator of Ni 2 MnGa. The microactuator is fabricated by magnetron sputtering of a Ni 2 MnGa thin film and subsequent photochemical micromachining. For operation of the scanner, a novel mechanism is proposed, which is based on the antagonism of magnetic and shape recovery forces. Thus, large bending forces in both actuation directions and low biasing forces can be generated simultaneously in a single microdevice. This mechanism is used to realize a large scanning angle of 50 deg. The dynamics of motion is characterized by heat-transfer times. Typical heating and cooling time constants are 2 and 16 ms, respectively. Below a critical frequency of about 55 Hz, the scanning angle is independent of the actuation frequency.

A SMA microgripper system with an integrated optical position sensor is presented. The microgripper consists of two actuation units and an optical slit fabricated from one piece of SMA material with overall dimensions of 2 x 5.8 x 0.23 mm3. The gripper is sandwiched by two substrates with an aligned photodiode and a photosensor, respectively. This transmission setup allows position sensing with a spatial resolution of 1 um per 2.5% optical signal change. By feedback of the position signal to a PI controller a position error of 0.3% is achieved. The typical time constant for reaching stationary positions is about 140 ms.

The mechanical and electrical properties of beam-cantilever microdevices fabricated from cold-rolled NiTi sheets and sputter-deposited NiTi thin films have been investigated by beam-bending experiments and electrical resistance measurements, respectively. In order to study possible size effects on the martensitic transformation, different device thicknesses have been realized down to grain dimensions of the NiTi sheets of about 20 μm, revealing enhanced transformation hystereses. For actuation control by resistive heating, the achievable displacements and work outputs were determined by the cross-section-dependent heat transfer to the device regions of maximum strain. Optimum heating/cooling performance and hysteresis control have been achieved for NiTi thin film devices with R-phase transformation above room-temperature. In order to demonstrate the potential of the microdevices for actuator applications, prototypes of membrane microvalves have been realised and tested.

The coupled magnetic and mechanical properties of single crystalline Ni-Mn-Ga foil actuators are investigated for decreasing thicknesses in the range from 200 oPGRZQWR� 6��oPThe foils exhibit a narrow martensitic phase transformation above room temperature below the Curie temperature of 369 K. Single crystalline Ni-Mn-Ga beam actuators are fabricated by laser micromachining. As-prepared specimens show enlarged twinning stress and thus no measurable magneto strain. Therefore, several training methods are adapted to the quasi-two-dimensional geometry of the foils. As compression training can no longer be applied in lateral direction of thin foils, more sophisticated magneto-thermo-mechanical treatments are required. We show that the low twinning stress of the bulk reference specimens can be recovered. Magnetization experiments and the direct observation of magneto strain on trained foil actuators show different degrees of reorientation up to 1% depending on the training method. The observed partial reorientation effect is discussed in terms of the foil geometry and training parameters.