Piezoelectric coefficients of ceramic thin films

Abstract

This thesis describes the newly established techniques for measuring the effective longitudinal piezoelectric coefficient d'33 and effective transverse piezoelectric coefficient e31f of sol-gel derived lead zirconate titanate (PZT) ceramic films. The effects of excess lead, annealing temperature, poling field and poling time on the piezoelectric properties of the PZT films have been studied. The depolarization phenomenon of films has been investigated and explained based on the concept of the Preisach model. Sol-gel derived PZT films of thickness 1 um and 2 um were deposited on Pt/Ti/SiO2/Si substrates by the multiple-spin-coating technique. The films were dense, crack-free, and well crystallized having single perovskite phase and nontextured polycrystalline structure. The newly established techniques for measuring d'33 and e31f were based on the converse and direct piezoelectric effects, respectively. For the d'33 measurement, the single beam laser interferometry technique was employed, and the substrate bending was effectively suppressed by gluing the substrate on a large and rigid platform with mounting wax. For the e31f measurement, a new cantilever deflection technique was applied. A rectangular sample (film/substrate) was bent dynamically in such a way that a longitudinal strain was generated along the length of the sample and an ac current was induced on the surface. Both the d'33 and e3lf measurements were verified to be free from systematic errors, and the corresponding uncertainties were about 7% and 10%, respectively. The effects of excess lead, annealing temperature, poling field and poling time on the observed d'33 and e31f values of PZT films were then investigated. The optimum amount of excess lead and optimum annealing temperature for preparing PZT films with the best piezoelectric properties were 10% and 650oC, respectively. The poling time required for switching the domain was very fast, less than 20 seconds, and was almost independent of the poling field. The PZT films of different thicknesses had almost the same (saturated) values of d'33 and e31f, but the required poling field for the 1-um film was larger than that for the 2-um film (16 MV/m vs 10 MV/m). A fully polarized PZT film with 10% excess lead and annealed at 650oC for 1 hour had the observed d'33 and e31f values of 90 pm/V and 8.8 C/m2, respectively. A polarized PZT film was completely depolarized by the application of ac fields of diminishing amplitude. The amplitude of the ac fields was decreased from 19.5 MV/m to 0.2 MV/m in 8 steps. The observed d'33 and e31f values of the film decreased after each step, and ended up with almost the same values as an as-deposited film. This depolarization phenomenon was explained based on the concept of the Preisach model. The results also revealed that there existed a distribution of coercive fields in the Preisach dipolar units (micro-domains), and that, because of the interaction field between the dipolar units, the magnitudes of the switch-up and switch-down fields of each dipolar unit were not necessarily the same.