Surface charge distribution of piezoelectrics at thickness and planar piezoresonance
This thesis looks at piezoelectric materials surface charge profiles at the piezoresonant and anti-resonant states. Piezoelectric materials experience a resonant state at certain frequencies depending on a multitude of factors and at these frequencies the admittance is at a maximum while the admittance reaches a minimum at the antiresonant state. Two PZT samples were observed at this state in regards to its surface charge density and profile. A completely electroded sample is measured using an 11x11 matrix overlay and a sample with an 11x11 matrix engraved into its electrode forming 121 isolated islands is used. The samples were measured at the planar mode resonance, antiresonance and the thickness mode resonance. A theoretical simulation is conducted at the planar resonant and antiresonant frequencies as well as the thickness mode resonant frequency. The established surface pattern is determined to be developed independently of metallization and free charge carriers and is experimentally verified to be a bound charge contribution coming from the bulk dielectric. Comparisons are drawn between the experimental potential difference distribution figures and the simulated plots of surface displacement where it was seen to have comparable similarities, verifying theoretically what was observed experimentally. The surface charge density, by Gauss's Law is proportional to the distance derivative of the potential difference. Both samples were observed to develop a convex profile with peaking values at the sample center at and near the planar resonant and antiresonant frequencies. The thickness mode resonant frequency developed a profile that had some of the lowest surface charge values at the sample center and peaking values at the rows in between the edge and the center. The admittance values were determined to be directly proportional to the potential difference magnitude as well, verifying the work of Imai, Tanaka and Yong (2001). This work demonstrates the correlation between the resonant frequencies and the surface charge profiles of piezoelectrics as well as the correspondence to surface displacement magnitudes shown in the finite element simulation. This work provides one of the first experimental evidences that bulk electromechanical strain is coupled with and may be imaged by surface charges, on an insulating or through conductive surface layers. Such insights are important in furthering research on dynamically tunable surface wave devices for a wide range of potential applications.