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Applications of Shape Memory Alloys to MEMS
MAE 268
Greg Jarmer and Garrett Uyema
Outline of Presentation
• Shape Memory Alloys (SMA)• Advantages of SMA’s for actuation of
MEMS Devices• Example of an Application: Microgripper• Recommended Improvements of
Microgripper• Future Applications of SMA in MEMS
Shape Memory Alloys
• Principles of Shape Memory Alloys– Shape Memory Alloys (SMA) are alloys that
exhibit the shape memory effect. – The shape memory effect is the process of
restoring a deformed material back to an initial shape through a thermally induced crystalline transformation
– The crystalline transformation occurs between a low temperature ductile martensitic phase and a high temperature high strength austenitic phase.
Shape Memory Alloys
Advantages of SMA’s in MEMS
• The main advantages of SMA’s for micro-actuation are:– SMA’s are capable of producing a large
actuation force– SMA’s are capable of producing large
displacements– SMA’s are activated through thermal heating
Disadvantages of SMA’s in MEMS
• The main disadvantages of SMA’s are:– Sensitivity of material properties in fabrication– Residual Stress’s developed in thin films– Nonlinearity of actuation force– Lower maximum frequency compared to other
microactuator devices
Verification of Activation Force with FEA
MicrogripperTheory of Operation• 2 Main designs of microgrippers
• Location of TiNi thin film• SMA actuation
• Transformation from martensite to austenite
heating
SMA thin film on the inside
Microgripper
• Fabrication of the Microgripper– 2 identical cantilevers and a silicon spacer– Processes used:
• Deposit and etch• Thin film sputtering• Annealing• Eutectic bonding
Recommended Improvements of Microgripper
• Be able to control hysteresis temperature range– Change composition of
SMA• Can shift hysteresis curve
left or right– Alloy TiNi with another
element such as Cu
Recommended Improvements of Microgripper
• Reduce residual stress in the thin film– Need to reduce
thermal mismatch between NiTi and Si substrate
– Add a layer of tungsten (W)
• Difference in the coefficients of thermal expansion is reduced
Recommended Improvements of Microgripper
• Increase the maximum frequency– Decrease the time it
takes for phase transformation to occur
– Increase surface area to volume ratio
• Heat can be dissipated faster
Future Applications of Microgrippers
• Grab tiny foreign objects for removal from the body– Facilitates access to
intricate regions of the body
• Microassembly for MEMS devices
• Intravascular Therapy
Future Applications of SMA in MEMS
• MEMS and bioMEMS applications– Eliminate vibrations of read/write heads in
hard disk drives– Microstents
• Promote flow in tubular passages• Reinforce weak blood vessels
– Microsurgery• Cardiovascular applications• Orthopedic applications
Future Applications of SMA in MEMS
Simon filter
Microstents
SMA basket
Microwrapper