The Characterization of TiNi Shape-Memory Actuated Microvalves
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The Characterization of TiNi Shape-Memory Actuated Microvalves B.-K. Lai, G. Hahm*, L. You*, C.-L. Shih, H. Kahn, S. M. Phillips* and A. H. Heuer Dept of Materials Science and Engineering, *Dept of Electrical Engineering and Computer Science Case Western Reserve University Cleveland, OH 44106-7204 ABSTRACT Co-sputtering has been used to fabricate equiatomic thin films of TiNi, a shape memory alloy, which form the basis of microactuators with many applications in MEMS. The stress evolution of TiNi films will be described. The performance of the TiNi actuators has been characterized, with regards to actuation force, recoverable strain, time response, and fatigue resistance. The performance of microvalves using these actuators will also be presented.
INTRODUCTION Shape Memory Alloys (SMAs) are a promising material for MEMS microfluidic applications, such as micropumps and microvalves. The physical basis of the shape memory effect is the reversible martensitic transformation. In TiNi, the material transforms reversibly on heating from the low temperature ductile martensite phase with the B19’ structure to the stiffer, high temperature austenite phase with the B2 structure. The high recoverable strain and high actuation work density of SMAs are responsible for the significant interest in device applications [1]. The high recoverable strain allows large strokes, while the high actuation work density generates large output force per unit volume for microfluidic devices. Among the many alloy systems known to produce SMAs, TiNi is the most widely studied. Bulk SMAs based on equiatomic TiNi have been known for over 35 years [2], and in thin film form appropriate for MEMS for about a decade [3]. It is well known that equiatomic TiNi has the highest transformation temperatures. Departures from exact TiNi stoichiometry in either direction cause a decrease in the transformation temperatures. TiNi films appropriate for MEMS applications have been batch fabricated successfully by our group [4]. Good thickness and compositional uniformity across a large area were observed. The TiNi films also have high recoverable stresses and reproducible transformation temperatures. Since the performance of microactuators is strongly related to the recoverable stress, the stress evolution of TiNi films during fabrication and thermal cycling is important and will be described here. It has been previously demonstrated that SMAs display good fatigue resistances; lifetimes of a few million cycles are expected at 3% strain (in the martensite phase) [1]. However, most of the previous fatigue studies focused on TiNi in bulk form. In this paper, the fatigue of co-sputtered TiNi in thin film form and the performance of TiNi actuated MEMS microvalves will be described.
EE8.3.1
Microscope with z-direction stage
7.5 mm
7.5 mm
Needle-tipped glass tubing
Precision
Figure 1. TiNi microactuator
Figure 2. Setup of fatigue test
EXPERIMENTAL Deposition procedure The sputtering system used is a commercial Denton Vacuum Inc. Discovery 18 apparatus, which ha
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