Effect of film thickness on output power of a piezoelectric vibration energy harvester using hydrothermally synthesized (K,Na)NbO₃ film

The dependence of the output power of piezoelectric vibration energy harvesters on film thickness was systematically investigated using {100}_c -oriented epitaxial (K,Na)NbO₃ films prepared on single-crystal SrTiO₃ substrates by the hydrothermal deposition technique. First, we measured the vibration energy harvesting properties by a unimorph Pt/(K,Na)NbO₃//SrRuO₃//SrTiO₃ cantilever. Based on the fact that the elastic stiffness of (K,Na)NbO₃ is lower than that of SrTiO₃ substrate, it can be understood that increasing film thickness leads to decreasing Q _m, resulting in the decrease in output power as film thickness increased under the same input acceleration condition. In the next step, these cantilevers were attached to an Al plate having a much larger volume than the Pt/(K,Na)NbO₃//SrRuO₃//SrTiO₃ cantilever. The output power of this type of harvester was more than 400% greater when the film thickness was increased from 3.5 to 22.3 μm due to the almost constant Q _m value.