An Investigation into the Cyclic Electric Fatigue of Ferroelectric Ceramics as Actuators: High Temperature and Low Pressure

This investigation aims to characterize the fatigue of ferroelectric ceramic actuators as a result of cyclic electric loading in a variety of environmental conditions. Actuators are used in myriad everyday applications, some of which require small displacements at high frequencies in small devices such as MEMs, a bill that ferroelectric ceramics fit nicely. However, the use of these materials in high temperature applications, as is common in the aerospace industry, limits their effectiveness due to the low Curie Temperature of most compositions. Another limitation of these materials is their susceptibility to electromechanical fatigue. In this work, fatigue is defined as the systematic change (usually detrimental) in material properties as a function of the number of cycles applied during the application of an alternating electric load. The material properties most often recorded in fatigue studies are polarization and strain measured in hysteresis loops using a cycle much slower than the frequencies used to drive most actuators. This work follows this convention while measuring many more characteristics, and also records strain and amplifier current in-situ during electric cycling at frequencies common in actuator applications. While PZT is an oft-studied material in fatigue studies, most compositions are ‘soft,’ with low Curie Temperatures and large piezoelectric coefficients. However, for high temperatures, these materials are limited in their use for actuator applications.In this study, a modified PZT composition with a substantially higher Curie Temperature than most is driven with electric loads up to 200C in a vacuum chamber at pressures down to 5x10-7 Torr, or 5E-7 Torr. The vacuum environment is one in which very little fatigue research has been done on these materials, while interest in these materials for use as actuators in space continues to build in the aerospace industry. Thus a large portion of the time spent on this work was involved in assembling and testing a measurement apparatus which is able to produce very low pressures, very high temperatures, allow attachment of external measurement devices, and simultaneously electrically drive a sample and measure its displacement. The description of this system and the data recorded are provided herein, which may possibly be of use to designers interested in using this material in future applications involving similar environmental conditions.