There is a pressing need to discover and understand the behavior of environmentally benign, non-toxic, and sustainable replacements for lead-based materials in all applications (as already has been done for lead-based paints and solders). Suitable replacements have yet to be realized for piezoelectric materials (those that change shape with applied electric field and vice versa) that are important for a large range of devices and applications, such as actuators, accelerometers, filters, vibration control, fuel injection, ink jet printing, ultrasound generation and sensing, and resonators. One class of materials that shows great promise as replacements is based on bismuth sodium titanate (BNT). In thin films for microelectronic applications, the fundamental piezoelectric effect in these new materials is not yet understood well enough to allow devices to be reliably implemented. Accordingly, the objective of this project is to explore the mechanisms associated with piezoelectricity in BNT-based piezoelectric thin films using novel characterization methods available at Professor Hiroshi Funakubo's laboratory at the Tokyo Institute of Technology. Professor Funakubo's characterization tools allow for the measurement of atomic structure while applying electric fields, giving direct insight into the piezoelectric strain mechanisms of these exceptional materials.
In bulk form, these BNT-based materials belong to a class of materials called relaxor ferroelectrics, which show enticing properties with regards to electric field induced displacement mechanisms that are just beginning to be understood. Many BNT-based compositions show the strain and polarization behavior that is characteristic of a reversible field-induced relaxor-to-ferroelectric phase transition; these are known as ergodic relaxors. However, BNT-based materials in thin film embodiments do not exhibit the enhanced properties that are found in the bulk. In particular, the ergodic relaxor compositions that have been studied in thin films do not show the signs of a reversible field-induced relaxor-to-ferroelectric phase transition, but rather behave similar to normal ferroelectrics and have comparatively low displacements. This differing response between bulk and thin films needs to be understood for this promising material system to be viable for real-world applications. Therefore, novel in situ microstructural characterization methods (2D X-ray diffraction and Raman spectroscopy), available at the host institution, will be applied to elucidate the electric field induced strain mechanisms in BNT-based piezoelectric thin films.
This award under the East Asia and Pacific Summer Institutes program supports summer research by a U.S. graduate student and is jointly funded by NSF and the Japan Society for the Promotion of Science.
