Thermo-Mechanical Ni 50 Ti 50 /Si Composite Thin Film Switch
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ABSTRACT The mechanical properties of Ni50Ti50 deposited on Si and 3C-SiC substrates were studied focussing on the interaction of the film and substrate. This interaction determines the transformation characteristics through interface accommodation and mechanical constraints exerted by the substrate stiffness. Substrate stiffness, controlled by the film/substrate thickness ratio, was found to have a substantial influence on the output energy of the film/substrate composite. A switch type composite based on this knowledge was fabricated and tested.
INTRODUCTION The demand for mechanical actuators and sensors of micron dimensions has increased considerably in recent years. The new trend is to engineer materials into micron size dimensions and integrate material science and electronic engineering into one functional chip. Shape memory alloys (SMAs) possess the unique property of a large output strain. As a candidate material for micro-actuation SMAs attracted renewed interest in the last few years1 . However, SMAs suffer from a speed limit due to their slow cooling rate. If SMAs are made into thin films on a substrate with good thermal conductivity, such as Si, the frequency of operation can be increased to a few kilo Hertz2 . Although first successes of micron sized SMA actuators have been achieved , SMA film/substrate composites remain to be a challenging problem 5 . From an application point of view, the enhanced operation speed relies on the composite; from a material science point of view the SMA transformation under substrate constraints may involve new problems in contrast to bulk transformations. An SMA film on a substrate is subjected to two types of constraints: 1. continuity at the interface and 2. substrate stiffness. The former is a structural constraint, systematically studied previously6 , and the latter can be viewed as a mechanical constraint. This paper presents results aimed at making a thermal switch type SMA film/substrate composite as well as the consequence of mechanical constraints which affect the switching efficiency. For cantilever film/substrate composite the SMA transformation exhibits itself as additional bending through transformation induced stress. The output of the composite is therefore a stress driven displacement. Another interesting configuration of the film/substrate composite is a membrane, as shown in Fig. 1. As transformation occurs, the large strain generated by the SMA is expected to tune the membrane resonance frequency through the change of the composite stress. Both configuration will be studied.
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Mat. Res. Soc. Symp. Proc. Vol. 360 01995 Materials Research Society
EXPERIMENT Details of the film deposition process was reported in reference 5. The as deposited films are amorphous and were crystallized at 480 'C. All films were treated at 600 'C for grain growth which establishes well defined transformation properties. An epitaxial SiC membrane grown on Si was chosen for making the composite membrane. The superior mechanical properties of SiC, compared to Si, render
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