Structure and Electrical Properties of Lead-Free (Na_0.5K_0.5)NbO₃-BaTiO₃ Ceramics

(Na_0.5K_0.5)NbO₃ with 0–20 mol% BaTiO₃ has been prepared following the conventional mixed oxide process. High-density samples were obtained through the addition of BaTiO₃ into (Na_0.5K_0.5)NbO₃ because of an increase in grain size. X-ray diffraction analysis revealed that, during sintering, all of the BaTiO₃ diffuses into the lattice of (Na_0.5K_0.5)NbO₃ to form a solid solution, in which orthorhombic symmetry changes to tetragonal symmetry at x≈0.06, and tetragonal symmetry changes to cubic symmetry at x=0.20 or higher. It was found that the samples with a low content of BaTiO₃ exhibit relatively good ferroelectric and piezoelectric properties. For 0.98(Na_0.5K_0.5)NbO₃-0.02 BaTiO₃ ceramics, the remanent polarization (P_r) is 7.5 µC/cm² with a coercive field (E_c) of 12 kV/cm. The piezoelectric constant d₃₃ reaches 104 pC/N. The electromechanical coupling coefficients of the planar mode k_p and the thickness mode k_t reach ∼29% and ∼38%, respectively. Our results show that (Na_0.5K_0.5)NbO₃-BaTiO₃ is a good lead-free piezoelectric ceramic.