Chapter 8: Piezoelectric Sensing and Actuation

1. Chapter 8: Piezoelectric Sensing and Actuation

2. Outline • Introduction to piezoelectric sensing and actuation • Properties of piezoelectric materials • Applications of piezoelectric sensing and actuation.

3. Introduction of Piezoelectric Piezoelectric phenomenon was discovered in the late 19th century. Piezoelectricity means “pressing” electricity which is the link between electrical and mechanical phenomena. The piezoelectric thin films convert electrical energy to mechanical energy and vice versa depending on the design and application.

4. Introduction of Piezoelectric • Thin film piezoelectric materials has been explored for use as: • on-chip acoustic transducers • pumps and valves for liquid and particles • accelerometers • speaker and microphones • mirrors • chemical sensors • Important properties of piezoelectric materials stem from its crystalline structures.

5. Introduction of Piezoelectric Direct effect of piezoelectric: materials generate an electric charge/voltage when it is under a mechanical stress. Inverse/Converse effect of piezoelectric: materials would be able to produce a mechanical deformation/force when an electric field is applied to it.

6. Introduction of Piezoelectric Piezoelectric Effect on Piezoelectric Materials If an external stress (F) is applied in the piezoelectric material, a voltage appears between the electrodes due to deformation of the dipole. If the stress changes from compressive to tensile, the voltage on the electrodes changes to the opposite polarity. A converse piezoelectric effect occurs when an electric field is applied to the material, an internal strain and deformation results if its boundaries are free to move. If a voltage with opposite polarity is applied to the electrodes, the piezoelectric material will contract. If the polarity of the applied voltage is the same as the voltage pole, the piezoelectric materials will expand. If alternating voltage is applied, the piezoelectric material will alternately contract and expand.

7. Piezoelectric Constitutive Equations and Constant • Piezoelectric materials are crystals. • Piezoelectric effects are strongly orientation dependent. • Normal stress is denoted by subscripts 1,2,3. • Shear stress and Strain Stress are denoted by subscripts 4,5,6. Schematic illustration of piezoelectric crystal in a rectangular system. D=dT + εE In a piezoelectric crystal, the constitutive equation that relates electrical polarization (D) and applied mechanical stress (T) is: d=piezoelectric coefficient matrix ε=electrical permittivity matrix E=electrical field

8. Piezoelectric Constitutive Equations and Constant

9. Piezoelectric Constitutive Equations and Constant s=ST + dE • The inverse effect of piezoelectricity can be similarly described by a matrix-form constitutive equation. • The total strain is related to both the applied electric field and any mechanical stress as s=strain vector S=compliance matrix • Thefull matrix form can be expanded as:

10. Piezoelectric Constitutive Equations and Constant If there is no mechanical stress present (Ti,i=1,6=0), the strain is related to the electrical field by dij components connecting the strain and the applied field in the inverse effect is identical to the dij components connecting the polarization and the stress in the direct effect.

11. Electromechanical Coupling Coefficient

12. Cantilever Piezoelectric Actuator Model Bending of Piezoelectric Bimorph • The deflection of a two-layer piezoelectric structure can be described by compact formula. • A compact model for calculating the curvature of bending has been made under the following assumptions: • The induced stress and stress are along axis-1 or longitudinal axis of the cantilever. • Cross sections of the beam originally plane and perpendicular to the beam axis remain plane and perpendicular to the resulting curved axis. • The beam maintains a constant curvature throughout the beam. • Shear effect are negligible. • Beam curvature due to intrinsic stress may be ignored. • The beam thickness is much less than the piezoelectric –induced curvature. • The beam thickness is much less than the piezoelectric-induced curvature. • Second order effects such as the influence of d33 and electrostriction are ignored. • Poisson’s ratio is isotropic for all films.

13. Cantilever Piezoelectric Actuator Model

14. Properties of Piezoelectric Materials Elemental semiconductors such as silicon and germanium show centrosymmetric crystal structure and do not exhibit piezoelectric behavior. III-V compound and II-VI compound such as GaAs and CdS are held together by covalent and ionic bonding. They show acentric crystal symmetry and are thus piezoelectric. Drawback high cost and low piezoelectric coefficients.

15. Properties of Piezoelectric Materials Commonly employed piezoelectric materials and their representative properties

16. Properties of Piezoelectric Materials • Other materials of piezoelectric are: • Aluminum nitride (AlN) • Lithium niobate (LiNbO3)

17. Example of Piezoelectric Applications Active middle-ear implant based on piezoelectric thin-film cantilevers

18. Applications of Piezoelectric Sensors and Actuators Inertia Sensors Acoustic Sensors Tactile Sensor Flow Sensors Surface Elastic Waves

19. Inertia Sensors Cantilever Piezoelectric Accelerometer

20. Inertia Sensors Membrane Piezoelectric Accelerometer

21. Acoustic Sensors -used as underwater acoustic imager. -imager is akin to a CCD imager for optic. Schematic Diagram of a Piezoelectric Microphone Fabrication Process PZT Piezoelectric Acoustic Sensor

22. Acoustic Sensors PZT Piezoelectric Microphone

23. Tactile Sensors Tactile sensor array Polymer Piezoelectric Tactile Sensor

24. Flow Sensors Piezoelectric Flow-Rate Sensor

25. Surface Elastic Waves Two most commonly encountered elastic waves are: * Surface Acoustic Wave (SAW) - occurs on sample of appreciable depth. * Flexural Plate Wave (Lamb Wave) - occurs in thin plates of materials.

26. Surface Elastic Waves The principles of launching a SAW on a bulk piezoelectric material. Launch of SAW wave using comb drive electrodes

27. EXAMPLE Explain direct effect of piezoelectricity and inverse effect of piezoelectricity. A 500µm long cantilever-type actuator is made of two layers, a ZnO layer and a polysilicon layer. The width, thickness and material properties are given in table. Evaluate the amount of vertical displacement at the end of cantilever and the transverse force at the end when the applied voltage is 10 V.