5.3 Magnetic and shape memory alloy actuators

Magnetic and shape memory alloy actuators are game-changers in MEMS/NEMS devices. They use electromagnetic forces and special materials to create motion and force at tiny scales. These actuators enable precise control in applications like microvalves and microgrippers.

Magnetic actuators harness electromagnetic and Lorentz forces, while shape memory alloys use temperature-induced phase changes. Both types offer unique advantages in size, force, and control, making them essential tools for miniature device design and operation.

Magnetic Actuators

Electromagnetic Force and Lorentz Force

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• Electromagnetic force is the force exerted on a current-carrying conductor in a magnetic field
• Lorentz force describes the force experienced by a charged particle moving through an electromagnetic field
  • Calculated using the equation $F = q(E + v \times B)$, where $F$ is the force, $q$ is the charge, $E$ is the electric field, $v$ is the velocity of the particle, and $B$ is the magnetic field
• Magnetic actuators harness electromagnetic and Lorentz forces to generate motion or displacement
• Commonly used in MEMS/NEMS devices for precise positioning, switching, and actuation (microrelays, microvalves)

Magnetostrictive Materials and Magnetic Reluctance

• Magnetostrictive materials exhibit a change in shape or dimensions when exposed to a magnetic field
  • Positive magnetostriction materials elongate when magnetized (Terfenol-D)
  • Negative magnetostriction materials contract when magnetized (nickel)
• Magnetostriction can be used to create actuators that convert magnetic energy into mechanical energy
• Magnetic reluctance is the resistance to magnetic flux in a magnetic circuit
  • Analogous to electrical resistance in an electrical circuit
• Reluctance-based actuators utilize the change in magnetic reluctance to generate motion or force
  • Commonly used in MEMS/NEMS devices for linear and rotary actuation (microstepper motors)

Shape Memory Alloy Actuators

Shape Memory Alloys (SMAs) and Martensitic Transformation

• Shape memory alloys (SMAs) are materials that can return to their pre-deformed shape when heated above a certain temperature
• SMAs undergo a solid-state phase transformation known as martensitic transformation
  • Martensitic transformation is a diffusionless, reversible phase change between high-temperature austenite and low-temperature martensite phases
• Austenite phase has a cubic crystal structure and is stable at high temperatures
• Martensite phase has a monoclinic crystal structure and is stable at low temperatures
• SMAs can be deformed in the martensite phase and will return to their original shape when heated to the austenite phase

One-Way and Two-Way Shape Memory Effects

• One-way shape memory effect occurs when an SMA is deformed in the martensite phase and returns to its original shape upon heating to the austenite phase
  • Cooling back to the martensite phase does not cause the material to revert to the deformed shape
• Two-way shape memory effect allows an SMA to remember both its high-temperature and low-temperature shapes
  • Material can alternate between two different shapes by heating and cooling
• Two-way shape memory effect requires specialized training or conditioning of the SMA
• Shape memory effect can be used to create actuators that generate motion or force (microgrippers, microvalves)

Nitinol

• Nitinol is a commonly used shape memory alloy composed of nickel and titanium
• Exhibits excellent shape memory properties, biocompatibility, and corrosion resistance
• Widely used in MEMS/NEMS devices for actuation and sensing applications (microfluidic valves, micropumps)
• Nitinol actuators can generate high forces and displacements relative to their size
• Actuation is typically triggered by resistive heating (Joule heating) of the Nitinol element