Signal Modulation by Martensitic Control of Shape Memory Alloy Thin Film Actuator Architectures

Abstract

The paper reports on theoretical and experimental results related to the thermal control of microactuators based on shape memory alloy thin films. The behaviour of actuators that have one or more phase transforming films deposited on a non-transforming substrate is influenced by the thermal stress that grows in the bimorph or trimorph architecture on cooling from the deposition or annealing temperature. When a phase transition occurs or is induced in the film it leads to a corresponding change in the stress state in the film/substrate architecture and can be reflected accordingly in the actuation of a cantilever. The hysteretic characteristics of the actuation by shape memory alloy films can be controlled by appropriately selecting the chemical composition of the film, the substrate material, the film/substrate thickness ratio and in some cases the external stimuli. The deposition temperature was a factor considered for modulating the output signal of the bimorph and trimorph cantilevers, as well as the sequence(s) of deposition in case of multilayers and trimorphs. Bimorphs with bilayer and structurally graded films and trimorph architectures have been characterized based on known results for bimophs with single layer deposited on the same type of Si cantilever-type substrates. The results show how the martensitic transformation occurring in the films or in the layers or microlayers is affecting the response of the actuator to thermal stimuli. The models proposed could allow the selection of appropriate parameters in order to generate a specific type of actuation or a modulated sensorial response to thermal (for shape memory alloy) or thermal and magnetic (for ferromagnetic shape memory alloy) stimuli.