Stress-Optimised Shape Memory Devices for the Use in Microvalves

A gas valve of 6x6x2 mm3 size has been developed for high pressure applications. Stress-optimised shape memory microbeams of 100 µm thickness are used to control the deflection of a membrane above a valve chamber. The shape memory thin sheets have been fabricated by melting and rolling, which creates specific textures. Investigations by X-ray diffraction revealed major orientations of [111] and [011] in rolling direction. The corresponding maximum anisotropy of transformation strain was 20%. The microbeams have been fabricated by laser cutting. For stress-optimisation, the lateral widths of the beams are designed for homogeneous stress distributions along the beam surfaces allowing an optimised use of the shape memory effect and a minimisation of fatigue effects. For actuation, a rhombohedral phase transformation is used. This allows operation below pressure differences of 1200 hPa in designs with one valve chamber and below 4500 hPa in pressure-compensated designs with a second valve chamber above the membrane. Maximum gas flows of 1600 sccm (sccm = cm2 at standart conditions / minute) and work outputs of 35 µNm are achieved for a driving power of 210 mW. The response times for closing the valves vary between 0.5 and 1.2 s and for opening between 1 and 2 s depending on the applied pressure difference

Development of Microactuators Based on the Shape Memory Effect

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