Working Principle, Simulation, Fabrication and Drive

In-plane Poled Piezoelectric Diaphragm

Double-sided spiral electrodes can generate an in-plane poling scheme. The diaphragm utilizes d₃₃ effect to generate the in-plane strain (thus a d₃₃ mode transducer), which is converted to an out-of-plane displacement due to the constrained boundary condition.

Due to the fact that d₃₃ is ~2 times the magnitude of d₃₁ in piezoelectric ceramics, and that the electrode spacing s can be much larger than the piezoelectric film thickness t, the sensitivity of the d₃₃ mode transducer can be much larger than the sensitivity of a d₃₁ mode transducer (sandwich structure).

The polarization has a distribution in the diaphragm due to the patterned electrode. In the uniform field modeling, the 3-D diaphragm is simplified as a two dimensional model, which is then subdivided into several areas. Polarization in each area is supposed to be uniform. Areas under electrodes are unpoled and areas between electrodes are uniformly poled radially outward or inward.

In the distributed material model (a), the poling electric field was simulated (b). The electric field strength in each element was compared with the coercive field of the material. The polarization in each element was adjusted to align with the electric field by executing a program code (c)(d).

Fabrication process flow: (a) PZT wafer was coated with photoresist. (b) Deposition of Cr/Au. (c) Patterning of top-side electrode by lift-off process. (d) Patterning of bottom-side electrode by lift-off process. (e) PMMA substrate was drilled with through holes using a laser. (f) PDMS soft mold was molded. (g) Epoxy precursor was tape casted. (h) Soft mold was demolded. (i) PMMA and PZT were bonded. (j) Another PMMA part was bonded on the top.

A diaphragm bearing double-sided electrode can be driven in several configurations. The quasi-static displacement of the diaphragm center was characterized by a laser Doppler vibrometer. A configuration (d)(4) was found to be most efficient in generating displacement. In this configuration, the stress on the top and on the bottom of the diaphragm have different polarization. A single layer device behaves as a bimorph.

The impedance spectra was characterized by a impedance analyzer. Coupling coefficient and quality factor were calculated based on some characteristic frequencies on the impedance spectra. The resonances of the diaphragm follows the circular plate vibrational model. Different driving configurations can effectively excite different resonances. A single bell rings with two pitches.

For more information, please refer to:

[1] Zhiyuan Shen, Mohamad Olfatnia, Jianmin Miao, and Zhihong Wang, “Displacement and resonance behaviors of a piezoelectric diaphragm driven by a double-sided spiral electrode”, Smart Materials and Structures, 21, (2012), pp. 055001.