Study of piezoelectric behaviour of sputtered KNbO3 nanocoatings for flexible energy harvesting

Novel potassium niobate (KNbO3) nanocoatings were sputtered on polyethylene naphthalate (PEN) substrates at different sputtering voltages and concentrations of sputtering gas. The relations growth time - thickness and sputtering voltage-deposition speed were established and the vacuum deposition modes were optimized. This was realized, in order to correlate the microstructure, surface morphology and the growing conditions for the nanosized KNbO3 films. Uniform coatings with variety of thicknesses were obtained without overheating degradation of the substrates at process parameters Us and PAr varying from 0.6 kV to 0.9 kV and from 1 × 10−2 Torr to 9 × 10−3 Torr, respectively. New Al/KNbO3/Al energy harvesting element was fabricated and tested on tension-compression cycles. For the samples, showing the lowest surface roughness of the KNbO3 film (3.6%), maximum piezoelectric voltage of 431 mV was measured at loading up to 3 kg. Basic electric parameters, such as capacitance, contact and bulk resistance of the piezoelectric element, involving KNbO3 film, produced at the optimum sputtering conditions, were measured as a function of the applied mass load. Output electrical power of 2.324 μW was obtained at maximum mechanical loading, which is comparable with the results for lead-containing thin film harvesters. Theoretical analysis of the novel elements was made based on their dielectric behaviour in terms of polarization type. They were connected to passive power management system for operation as an independent power supply for low-power electronics.