Textured Lead-Free Piezoelectric Ceramics for Flexible Energy Harvesters

A lead-free (K,Na)NbO₃-based piezoelectric ceramic is textured along the (001) direction using the NaNbO₃ (NN) seeds. The composition 0.96(K_0.5Na_0.5)(Nb_0.965Sb_0.035)O₃-0.01CaZrO₃-0.03(Bi_0.5K_0.5)HfO₃ (KNN) is found to provide an excellent combination of electromechanical coefficients at room temperature. The textured composition with 5 wt % NN template (KNN-5NN) exhibits considerably improved electromechanical coefficients, d₃₃ ∼ 590 pC/N, k₃₁ ∼ 0.46, and d₃₁ ∼ 215 ×10^-12 C/N, at room temperature. A flexible piezoelectric energy harvester (F-PEH) is fabricated using the textured KNN-5NN ceramic and tested under cyclic force. F-PEH exhibits enhanced output voltage (V_oc ∼ 25 V), current (I ∼ 0.4 μA), and power density (P_D ∼ 5.5 mW/m²) (R_L of 10 MΩ) in the off-resonance frequency regime. In comparison to the random ceramic KNN-0NN-based F-PEH (V_oc ∼ 8 V and I ∼ 0.1 μA), the textured F-PEH significantly outperformed energy harvesting capability due to the large figure-of-merit value (d₃₁ × g₃₁) ∼ 3354 ×10^-15 m³/J. This work provides a methodology for texturing lead-free materials and further implementing them in flexible energy harvesting devices and sensors.