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

Abstract

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.

Thermal stability of self-polarization in a (K,Na)NbO3 film prepared by the hydrothermal method

“…These data obviously shows that the as-prepared film showed almost the same piezoelectric constant after the poling treatment as already reported. 20) Figures 2(a)-2(e) summarize the room-temperature σ-E butterfly curves measured at 55. 3 Hz for the Pt/(K 0.88 Na 0.12 )NbO 3 //SrRuO 3 //SrTiO 3 cantilever heattreated at various temperatures.…”

“…On the other hand, those of heat-treated film at 600 °C are ∼560 and 0.06, respectively. 20) Consequently, taking account of the relative dielectric constant and dielectric loss mentioned above, hydrothermal (K 0.88 Na 0.12 )NbO 3 films have the potential to be good candidates for altering Pb-based materials as energy harvesting devices using environmentally vibrations.…”

“…We selected (K x Na 1−x )NbO 3 with x = 0.88 composition because it has almost a perfectly selfpolarized state without applying an electric field as a poling treatment and shows the highest FOM among the entire composition range of (K,Na)NbO 3 films. 20) Epitaxial SrRuO 3 layers of 50 nm thickness were deposited on (100)SrTiO 3 substrates at 550 °C by RF magnetron sputtering method and were used for the bottom electrode. Here a pseudocubic index designated as (hkl) c was used for SrRuO 3 .…”

“…26) Details of the measurement in inverse piezoelectric properties are described in previous studies. 20,26,27)…”

“…Also, these films show self-polarization characteristics that reveal piezoelectricity without any poling treatment. 20,21) In contrast, a poling treatment to align the polarization direction is generally necessary for ferroelectric materials to obtain the piezoelectric response. The self-polarized state could avoid electrical breakdown, which often occurs under the poling treatment with high voltage.…”