Thermally and Electrically Induced Ferroelectric Transitions in NaNbO₃ for Lead-Free Piezoelectric Composite Applications

Ferroelectric (FE) polymer composites have received increasing attention for potential applications in flexible piezoelectric-based devices. The macroscopic performance of these devices is closely associated with the microscopic FE properties of the embedded oxides, which remain a topic of considerable research activity. In this study, we investigate the thermally and electrically induced antiferroelectric (AFE) to FE phase transition in hydrothermally synthesized NaNbO3 (NNO) particles and demonstrate its potential for piezoelectric applications within composite devices. NNO submicron particles, initially synthesized by a hydrothermal method, exhibit a predominant AFE phase with a minor FE fraction. However, a phase transition from AFE to FE is observed after thermal annealing and tracked by detailed in situ X-ray diffraction (XRD) analysis during a complete heating–cooling cycle. The NNO particles can be incorporated into a poly(vinyl alcohol) (PVA) polymer matrix to form flexible composites. XRD analysis and piezoelectric measurements on the composites reveal a clear AFE-FE phase transition in the initial nonannealed NNO particles after electrical poling above a threshold field of 75 kV/cm, a result which is consistent with well-known AFE-FE phase transitions in NNO bulk materials. The PVA plus 50 wt % NNO composite displays a piezoelectric coefficient d33 of approximately 4.0 pC/N, outperforming composites using commercial tetragonal BaTiO3 (BTO) nanoparticles. We demonstrated that these PVA-NNO composites generated piezoelectric output voltages of 0.3 and 0.8 V when subjected to manual bending and compression, respectively. These findings indicate that NNO-based composites can be considered promising lead-free candidate materials for soft sensors and piezoelectric energy harvesting applications.