Good piezoelectricity of self-polarized thick epitaxial (K,Na)NbO₃ films grown below the Curie temperature (240 °C) using a hydrothermal method

Using a hydrothermal method, (K_0.88Na_0.12)NbO₃ films were deposited at 240 °C on (100) _cSrRuO₃//(100)SrTiO₃ substrates. Moreover, without any poling treatment, direct and inverse transverse piezoelectric coefficients, e_31,f, near 0 kV/cm were approximately −5.0 C/m² for the as-deposited film. This value was nearly unchanged following the application of an electric field and poling treatment, suggesting that as-deposited films are almost in a fully self-polarized state without the application of an electric field. As-deposited films with a thickness of up to 22 μm showed constant piezoelectricity without any poling treatment. The films did not crack or peel because of substrates due to the small thermal strain originating from the low deposition temperature. The figures of merit (FOM) for the vibration energy harvester [FOM = e_31,f²/(ε₀ε_r)] and sensor [FOM = e_31,f/(ε₀ε_r)] were estimated to be good at 32.8 GPa and –5.9 GV/m, respectively, primarily because of the low relative dielectric constant of ∼110. Furthermore, the piezoelectric voltage coefficient g₃₁ [= d₃₁/(ε₀ε_r)] was estimated and demonstrated a high value of 0.073 Vm/N.