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Using a hydrothermal method, (K0.88Na0.12)NbO3 films were deposited at 240 °C on (100) cSrRuO3//(100)SrTiO3 substrates. Moreover, without any poling treatment, direct and inverse transverse piezoelectric coefficients, e31,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 = e31,f2/(ε0εr)] and sensor [FOM = e31,f/(ε0εr)] 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/(ε0εr)] was estimated and demonstrated a high value of 0.073 Vm/N. [ABSTRACT FROM AUTHOR]

A setup and approach for simultaneous measurement of the inverse and direct transverse piezoelectric coefficients of piezoelectric films was constructed. A longitudinal vibration mode resonator with a hydrothermally synthesized (K,Na)NbO 3 film deposited on a SrRuO 3 /SrTiO 3 substrate was investigated. The fabricated vibrator consists of two upper electrodes, one for actuation and the other for sensing. A driving voltage was applied to the actuator electrode to generate mechanical vibration at its resonance frequency. The vibration velocity was measured with a laser Doppler vibrometer, and the output current from the shorted sensing electrode was also measured to simultaneously evaluate the inverse and direct piezoelectric responses. The obtained inverse piezoelectric coefficient e 31 ̅ was −3.60 C/m2, while the direct piezoelectric coefficient e 31 ̅ was almost same at −3.69 C/m2. From these values, the inverse and direct piezoelectric coefficients e 31 , f were −4.93 and −5.05 C/m2, respectively. [Display omitted] • Measurement method establishment of the inverse and direct transverse piezoelectric coefficients for piezoelectric films simultaneously. • Use longitudinal vibration mode resonator using a hydrothermally synthesized (K,Na)NbO 3 film deposited on SrRuO 3 /SrTiO 3 substrate • Almost similar inverse and direct piezoelectric coefficients ( e 31 ̅) value of −3.60 and −3.69 C/m2, respectively. [ABSTRACT FROM AUTHOR]