Multi-Length Engineering of (K, Na)NbO₃ Films for Lead-Free Piezoelectric Acoustic Sensors with High Sensitivity

With increasing concerns about noise pollution, the pursuit of highly dependable piezoelectric acoustic sensors for real-time noise monitoring has come to the forefront of scientific research. Lead-based perovskite piezoelectric films, exemplified by lead zirconate titanate Pb(Zr,Ti)O-3 (PZT), surpass traditional piezoelectric materials such as ZnO and AlN in their piezoelectric properties, promising substantial advancements in next-generation acoustic sensor technologies. However, the toxic nature of lead in PZT materials poses formidable environmental and human health risks. In an unprecedented breakthrough, it presents the pioneering development of an environmentally benign lead-free piezoelectric Micro-Electro-Mechanical System (MEMS) acoustic sensor based on potassium sodium niobate (K,Na)NbO3 (KNN) film. High-quality <001> textured 3 mu m-thick KNN film is successfully integrated into commercially used Si substrate, rendering exceptional piezoelectricity (transverse piezoelectric coefficients e(31)* of approximate to 8.5 C m(-2)) with satisfactory thermal stability. The atomic-scale Z-contrast imaging and piezoresponse force microscopy characterizations reveal that the outstanding piezoresponse originates from the local coexistence of multiple phases and the enhancement of extrinsic piezoelectric contributions from in-plane polarization anisotropy. Finite element simulation is employed to design the triangular cantilever structure and annular diaphragm structure, each corresponding to different operating bandwidths. The resultant MEMS acoustic sensors stand out with outstanding acoustic performance (the high sensitivity and expansive receiving field of view), which are attributed to the microstructural engineering at multi-length scales for the excellent piezoelectric properties of KNN film. These features enable sensitive acoustic monitoring in various environments, including large-scale power grids and urban traffic.