5.2 Thermal and piezoelectric actuation mechanisms

Thermal and piezoelectric actuation are key mechanisms in MEMS/NEMS devices. Thermal actuation uses heat to create motion through expansion or shape changes, while piezoelectric actuation relies on materials that deform when exposed to electric fields.

These methods offer unique advantages for different applications. Thermal actuation can produce large forces and displacements, while piezoelectric actuation provides precise, fast responses. Understanding their principles is crucial for designing effective micro and nano-scale actuators.

Thermal Actuation

Joule Heating and Thermal Expansion

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• Joule heating occurs when an electric current passes through a conductor, causing an increase in temperature due to electrical resistance
  • Also known as resistive heating or ohmic heating
  • The amount of heat generated is proportional to the square of the current and the resistance of the conductor (tungsten, nichrome)
• Thermal expansion coefficient quantifies how much a material expands or contracts with changes in temperature
  • Defined as the fractional change in length per unit change in temperature
  • Different materials have different thermal expansion coefficients (aluminum, copper, steel)
  • Mismatches in thermal expansion coefficients can be exploited to create actuation in devices like bimetallic strips
• Bimorph actuators consist of two layers of materials with different thermal expansion coefficients bonded together
  • When heated, the difference in expansion causes the actuator to bend or deform
  • Commonly used in thermostats, microvalves, and other MEMS devices requiring large displacements

Thermopneumatic Actuation and Shape Memory Effect

• Thermopneumatic actuation utilizes the expansion of a gas when heated to create pressure and actuate a diaphragm or piston
  • Often uses a sealed cavity filled with a gas that expands when heated
  • Can generate large forces and displacements but has slower response times compared to other actuation methods
• Shape memory effect is a property of certain alloys (nitinol) that allows them to "remember" and return to a pre-deformed shape when heated
  • The material can be deformed at low temperatures but will return to its original shape when heated above a certain transition temperature
  • Useful for creating actuators that can generate large strains and forces with a simple heating/cooling cycle

Piezoelectric Actuation

Piezoelectric Effect and Coefficients

• The piezoelectric effect is the ability of certain materials (quartz, PZT) to generate an electric charge in response to applied mechanical stress
  • Conversely, these materials will also deform when an electric field is applied, which is known as the inverse piezoelectric effect
  • Piezoelectric materials can be used as both sensors and actuators in MEMS devices
• Piezoelectric coefficients quantify the relationship between mechanical stress and electric field in a piezoelectric material
  • The d33 coefficient relates the strain in the same direction as the applied electric field
  • The d31 coefficient relates the strain perpendicular to the applied electric field
  • Higher piezoelectric coefficients indicate a stronger piezoelectric response

Piezoelectric Actuator Designs and Hysteresis

• Unimorph piezoelectric actuators consist of a single piezoelectric layer bonded to a passive substrate
  • When an electric field is applied, the piezoelectric layer expands or contracts, causing the actuator to bend
  • Unimorphs are simpler to fabricate but have lower displacements compared to bimorphs
• Bimorph piezoelectric actuators have two piezoelectric layers with opposite polarities bonded together
  • When an electric field is applied, one layer expands while the other contracts, causing the actuator to bend
  • Bimorphs can generate larger displacements than unimorphs but are more complex to fabricate
• Hysteresis is a non-linear behavior exhibited by piezoelectric materials where the strain-electric field relationship depends on the history of the applied field
  • Hysteresis can cause positioning inaccuracies and reduced repeatability in piezoelectric actuators
  • Compensation techniques, such as closed-loop control or charge control, can be used to mitigate the effects of hysteresis