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Highly piezoelectric lead-free thin films of (K0.5Na0.5)NbO3 + x mol% Na(I) and K(I) (KNN, x = 0–50 mol%) were fabricated using a sol-gel method on Pt (111)/Ti/SiO2/Si(100) substrates and the effects of additives on microstructure, oxygen vacancies, and electrical properties of the proposed samples were investigated. Excess quantities of Na(I) and K(I) were shown to decrease the formation of secondary phases and promote the effective growing of the grains. The addition of these elements was also shown to inhibit the formation of alkaline ion vacancies and decrease the leakage current density. Our findings indicate that the crystallinity and microstructure of the samples have a stronger effect than the oxygen vacancies on the piezoelectric properties of KNN films. Non-stoichiometric KNN films showed the highest remnant polarization (Pr = 11.2 μC/cm2), piezoelectric coefficient (d33 = 40.23 pm/V), voltage coefficient (g33 = 7.9 mm V/N), and lowest leakage current (∼3.46 × 10−7 A/cm2) when the excess ratio was 40 mol% following annealing at 700°C. Our results also demonstrate that the transport mechanism of the films is governed by Ohmic behavior under low electric fields and the effects of Poole-Frenkel emission under a strong electric field.

In this study, composition of (Na0.5K0.5)NbO3 + x mol% CuO ceramics (NKNCx, where x = 0–1 mol%) were separately prepared using the B‐site oxide precursor method (BO method) and the conventional mixed oxide method (MO method). The diffracted angles of the NKNCx ceramics decreased as the concentrations of CuO dopants increased, because Nb5+ ions were replaced by Cu2+ ions, leading to the formation of oxygen vacancies. The microstructures of NKNCx ceramics prepared using the BO method were more homogeneous microstructures than that of ceramics prepared using the MO method. The mechanical quality factor (Qm) and dielectric constant (ε33T/ε0) of NKNCx ceramics prepared using the BO method were therefore by 21% and 25% better, respectively. The activation energy and internal bias field were measured and calculated to confirm the presence of oxygen vacancies. A low activation energy corresponds to a high Qm value. NKN ceramics with 0.75 mol% CuO doping prepared using the BO method had an activation energy and internal bias field of 0.95 eV and 3.5 kV/cm, respectively, at x = 0.75 mol%. The bulk density, kp, Qm, ε33T/ε0, and dielectric loss of NKNCx ceramics with this composition were 4.488 g/cm3, 41.5%, 2100, 280, and 0.15%, respectively.

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