7.2 Unimorph and bimorph structures

Unimorph and bimorph structures are key designs in piezoelectric energy harvesting. Unimorphs use one piezo layer on a substrate, while bimorphs have two layers. These setups affect how the device bends and generates power.

The neutral axis and bending moment play crucial roles in these structures. By tweaking these elements, engineers can boost energy output. Electrical connections also matter – series for higher voltage, parallel for more current.

Unimorph and Bimorph Structures

Structural Configurations and Components

• Unimorph structures consist of a single piezoelectric layer bonded to a non-piezoelectric substrate (aluminum, steel)
• Bimorph structures incorporate two piezoelectric layers
  • Symmetric bimorph contains two identical piezoelectric layers
  • Asymmetric bimorph uses piezoelectric layers of different thicknesses or materials
• Neutral axis represents the line within a beam where no longitudinal stress or strain occurs during bending
  • Located at the center of the beam for symmetrical structures
  • Shifts towards the stiffer material in asymmetrical configurations
• Bending moment causes curvature in the beam structure
  • Generates compressive stress on one side of the neutral axis
  • Produces tensile stress on the opposite side

Performance Characteristics and Applications

• Unimorph devices offer simplicity and cost-effectiveness
  • Suitable for low-power applications (vibration sensors)
• Bimorph configurations provide higher power output and sensitivity
  • Used in energy harvesting systems (footstep energy harvesters)
• Neutral axis position affects stress distribution and energy conversion efficiency
  • Optimizing neutral axis location can enhance device performance
• Bending moment magnitude influences the generated electrical output
  • Larger bending moments typically result in higher voltage and power generation

Electrical Connections

Series and Parallel Configurations

• Series connection involves linking piezoelectric elements end-to-end
  • Increases overall voltage output
  • Maintains constant current through all elements
• Parallel connection joins elements side-by-side
  • Boosts total current output
  • Keeps voltage consistent across all elements
• Output voltage depends on the chosen connection type
  • Series connection: $V_{total} = V_1 + V_2 + ... + V_n$
  • Parallel connection: $V_{total} = V_1 = V_2 = ... = V_n$

Electrical Performance and Design Considerations

• Series connections suit high-voltage, low-current applications (voltage sensors)
• Parallel configurations benefit high-current, low-voltage systems (current sources)
• Impedance matching between the piezoelectric device and load optimizes power transfer
  • Series connection increases output impedance
  • Parallel connection decreases output impedance
• Hybrid configurations combine series and parallel connections
  • Allow customization of voltage and current outputs
  • Enable fine-tuning of device characteristics for specific applications

Piezoelectric Properties

Piezoelectric Coupling and Energy Conversion

• Piezoelectric coupling quantifies the efficiency of mechanical-to-electrical energy conversion
  • Represented by the electromechanical coupling coefficient (k)
  • Higher k values indicate more efficient energy conversion
• Coupling coefficient depends on material properties and device geometry
  • Ranges from 0 to 1, with typical values between 0.3 and 0.7 for common piezoelectric materials
• Piezoelectric coupling affects device performance metrics
  • Determines maximum achievable power output
  • Influences bandwidth and resonance characteristics

Material Selection and Device Optimization

• Common piezoelectric materials include PZT, PVDF, and PMN-PT
  • PZT offers high coupling coefficients (ceramic)
  • PVDF provides flexibility and biocompatibility (polymer)
• Coupling coefficient can be improved through various methods
  • Doping piezoelectric materials to enhance properties
  • Optimizing device geometry and electrode placement
• Trade-offs exist between coupling coefficient and other material properties
  • High coupling materials may have lower temperature stability
  • Flexible materials often exhibit lower coupling coefficients but better durability
• Applications consider coupling coefficient alongside other factors
  • High-power applications prioritize materials with strong coupling (ultrasonic transducers)
  • Sensing applications may focus on other properties like sensitivity or linearity (pressure sensors)