Sensors and actuators play a critical role in modern life, where they are seamlessly integrated into systems such as automobiles, homes, cameras, sonar, printers, medical imaging and diagnostics, smart wearable devices, electronic system controls, and so on. The dominant piezoelectric material system used for most of these applications is the lead zirconate titanate (PZT) system, due to the high electromechanical conversion efficiencies that can be achieved. However, the toxicity of lead (Pb), however, has led to global efforts to identify a replacement system. This project takes a novel approach to actuator materials, design and fabrication, and provides a demonstration of high strain lead-free actuators with robust device structures. It will provide fundamental understanding of the lead-free transducer materials operating at high strains. The project also demonstrates that greater functionality can be built into flexible electronics for wearable devices and mobile applications. The project will be a demanding research training vehicle, as it demands both research depth and breadth, and covers multidisciplinary topics ranging from fundamental properties of environmentally friendly ferroelectric and piezoelectric materials, processing, device structure and integration, and design elements of flexible electronic systems.
Traditionally a small-signal piezoelectric coefficient (d33) value has been used to compare different piezoelectric materials. However, for the actuator applications the achievable strain Smax at the applied electric field Emax is the key figure of merit and their ratio Smax/Emax (normalized strain, large-signal d33, or d33*) is the quantity of interest. The difference between the two is very important and has to be carefully addressed; in traditional "soft" PZT small signal and large signal strain responses can differ by a factor of >2 due to different contributions to the response. The basic understanding of the piezoelectric behavior of the PZT materials is relatively well developed. However, it is becoming increasingly clear that the non-lead based materials display rather different behavior in terms of microscopic contributions to polarization and strain, switching mechanisms, domain behavior, extrinsic contributions, etc. In addition, the high-field high-strain properties of the lead-free piezoelectric materials, critical for high strain actuators, are only at very early stages of research, as most of the research has been focused on small signal properties of the lead-free materials. The Xerox-Brown collaborative project for high-field-high-strain properties and fundamental investigation of their origin on these materials will provide critical support and understanding for the collaboration that is focused on developing high strain lead-free actuators. The collaboration will build upon Brown University innovations in processing and devices and Xerox experience in high strain piezoelectric actuators and modeling capability. This project lies in the use of the emerging collective understanding of the behavior of environmentally-benign non-lead based piezoelectric materials and applying this understanding to a new process method that we are developing for multi-functional high strain actuators using flexible metal foil substrates. In doing so, we will develop an improved understanding of the property-control parameters of the piezoelectric actuators, and an understanding of the mechanisms of high-field high-strain effects and the possible performance-limiting degradation of high quality piezoelectric devices, including but not limited to high strain sensors and actuators. The research will provide guidance and a platform for the innovative processing of emerging environmentally benign piezoelectric materials and devices with high performance, and will deliver design concepts for a new class of flexible piezoelectric sensors and actuators that could be integrated with other flexible electronics.
