M.Sc Thesis


M.Sc StudentLevinton Julian
SubjectModeling and Experimental Investigation of Piezoelectric
Actuators Made from PMN-PT
DepartmentDepartment of Mechanical Engineering
Supervisor PROF. David Elata
Full Thesis textFull thesis text - English Version


Abstract

Piezoelectric ceramic materials are gaining increasing attention as an alternative to electrostatic-based, electromagnetic-based, and thermal-based actuation devices. The large transduction efficiency of piezoelectric structures results from the strong coupling between the mechanical and electrical domains.

Piezoelectric unimorph actuators are typical examples, which are employed in many applications as converters of electric input to mechanical motion, or vice versa. Examples include loudspeakers, precise positioning motors, micromirror devices, etc.

The piezoelectric response is determined by the orientation of poling in the material. In most cases, this coupling can be considered as linear. Many piezoelectric actuators are made from PZT, which exhibits extremely strong piezoelectric coupling.

Here, we investigate the possibilities of using PMN-PT for bending and twisting micro-scale actuators. The coupling coefficients of PMN-PT are more than twice as large as those of PZT. We show experimentally the benefits of PMN-PT actuators over PZT actuators, driven at similar operating voltages.

However, PMN-PT has two unique properties that make it challenging for actuators. First, PMN-PT exhibits some electrostriction response, where the deformation is related to the square of the electrostatic field, rather than a linear relation in piezoelectricity. The deformation in electrostriction is independent of the field polarity. Second, PMN-PT has a very low coercive field, which is the field at which the orientation of poling can be flipped.

By modeling and experimenting, we investigate three different types of response of PMN-PT bending unimorphs, driven at different levels of harmonic field. We find that the response to low levels of fields is piezoelectric and linear. For higher fields the response is strongly affected by electrostriction. For fields that are stronger still, which exceeds the coercive field, the PMN-PT actuators poling seemed to have flip.

We perform various dynamic experiments on PMN-PT and PZT actuators, comparing the results to reveal the uniqueness of PMN-PT. In these experiments, we observe two principal harmonic components of the response of both actuators. The first harmonic component, for both actuators, is at the driving frequency. However, the second harmonic components are at twice and three-times the driving frequency, for PMN-PT and PZT, respectively. For moderate driving fields, the considerable response at double the driving frequency is attributed to electrostriction. However, for excessive driving fields we attribute the second harmonic component of the PMN-PT actuator to both electrostriction and poling flipping. In contrast, the considerable response at three-times the driving frequency, observed in the PZT actuator, is attributed to non-linear softening.

Moreover, we investigate the re-poling mechanisms using FEA and show that for specific driving conditions, a run-away transition zipping response occurs. In these simulations, we found three types of response, that relate to the three types of response identified experimentally.

In this work, we also consider PMN-PT for a twisting-beam actuator. We describe the poling and actuation schemes, and examine the electromechanical response of the actuator using FEA. Also, we designed and fabricated an electrodes pattern for pure twisting deformation, and experimental testing will be performed in the near future by our research group.