Functionally graded lead-free piezoelectric composites for enhanced ultrasound energy harvesting

Abstract

Potassium Sodium Niobate (KNN)-based lead-free piezoelectric ceramics have emerged as a promising candidate for next-generation ultrasonic electronics, owing to their biocompatibility and exceptional piezoelectric properties. However, conventional dense KNN ceramics still face challenges in ultrasonic reception applications, including high acoustic impedance and insufficient receiving sensitivity. Here, we propose a functionally graded lead-free composite (referred to as HP-1-3) with a hierarchical porosity-induced acoustic impedance gradient by controlling the porosity and the 1-3 composite parameters within the KNN matrix. The introduced porous and 1-3 composite structure not only features variable acoustic impedance, thereby improving impedance matching and minimizing reflection losses at the interface, but also lowers the dielectric constant, thereby enhancing the ultrasonic receiving sensitivity. Under ultrasonic excitation, the electrical output of the HP-1-3 composite was significantly higher than that of the dense KNN ceramic, reaching 2.2 times the value of the latter. The composite was further fabricated into a 2×4 flexible ultrasonic array, achieving an ultrasound-induced output of 35.6 Vpp and an average charging power of 61.25 μW, while maintaining functionality on curved surfaces and enabling cross-tissue power transmission. This work provides a feasible structural optimization pathway for developing high-performance lead-free piezoelectric materials and devices.