Exceptionally Large Thickness-Independent Longitudinal Strain in Lead-free Piezoceramics via Defect Dipole Engineering

The development of large-strain piezoceramics is critical for the progress of ultrasonic transducers and actuators. Recent investigations on piezoceramics have shown that defect dipole engineering can induce significant strains. However, these strains are in many cases dominated by bending deformation and decline rapidly with increasing thickness. Here, a breakthrough in the lead-free BiFeO₃-BaTiO₃ system is reported, where a specific defect-dipole alignment process induces thickness-independent large electro-strains across thicknesses ranging from micrometers to millimeters. Through phase-field simulations and specifically designed testing modes, it is confirmed that the dominant strain mechanism is defect-dipole-regulated longitudinal strain, which is distinct from bending-induced deformation. Under a low electric field of 20 kV cm⁻¹, the material exhibits an outstanding piezoelectric strain coefficient of 1933 pm V⁻¹ at 80 °C, demonstrating superior low-field driving efficiency. Ultrasonic transducers fabricated from this material demonstrate remarkable performance, characterized by a wide −6 dB bandwidth of 58.3%, an excellent electromechanical coupling coefficient of 39.5%, and a stable transmitting sensitivity of 0.93 kPa V⁻¹, comparable to the current commercialized lead-based products. These findings not only deepen the understanding of electro-strain mechanisms in piezoceramics but also lay the foundation for high-performance ultrasonic devices.