REPOSITORY > RESULTS

Abstract

Autonomous electronic devices require a long-term, sustainable power supply. One possible solution is the transduction of mechanical energy, available in the sensor’s environment, into electrical energy by piezoelectric layers integrated on a flexible substrate. A-few-tens-of-micrometres-thick piezoelectric layers can be processed on a substrate using electrophoretic deposition (EPD). EPD is based on the mobility of charged particles in a solvent and their deposition on a conductive substrate upon the application of an electric field. After the EPD, the layers are sintered. Most piezoelectric energy harvesters (PEHs) integrate lead-based piezoelectric materials. However, due to governmental regulations, alternative lead-free materials have been investigated. Thus, a lead-free piezoelectric based on potassium sodium niobate ((K0.5Na0.5)0.99Sr0.005NbO3, KNNSr) was selected for this work.
In the first part we studied the stabilisation of KNNSr particles in ethanol using poly(acrylic acid-co-maleic acid) (PAM) and an organic base: n-butylamine (BA). By optimising the PAM amount and the PAM/BA ratio, we stabilized the particles, with a zeta-potential (ZP) of around -45 mV and tailored the conductivity of the suspensions in the range 9–30 𝜇S/cm. The deposition of KNNSr particles on a 640-𝜇m-thick platinised alumina substrate (RAO), demonstrated that the conductivity of the suspensions influences the deposited masses, the homogeneity and the thickness uniformity of the layers. The thickness uniformity of the layers depends on the geometry of the deposition setup. Using a finite-element analysis (FEA) we predicted and optimised the electric field strength in the setup by varying the interelectrode distance and the counter electrode diameter. The FEA results were well correlated with the experimental thickness profiles of the layers.
In the second part the thick films deposited on the RAO substrate were sintered. The thick films sintered at 1075 °C were porous, whereas the ones sintered at 1120 °C delaminated from the substrate. To avoid the delamination, we first deposited a ~10-𝜇m-thick layer and fired it at 1000 °C, followed by the deposition of a second, thicker layer. We sintered the layers at 1090, 1100 and 1110 °C, both in air and oxygen, to understand the microstructure evolution of the KNN thick films. The microstructure, X-ray-diffraction patterns and local ferroelectric domain structure were correlated with the electromechanical properties. The KNNSr thick films sintered at 1100 °C for 2 hours had a homogenous microstructure, a relative density of ~ 81 % and a thickness-coupling factor of ~ 40 %.
In the third part we focus on the processing of a bimorph structure on flexible, 110-𝜇m-thick platinised alumina (DFAO) substrates. For this purpose we modified the EPD setup. The sintering of KNNSr layers on one side of the DFAO substrate results in the bending of the structure. To minimise this spurious effect, the ~ 65-𝜇m-thick layers were deposited on both sides of the DFAO substrate. After sintering at 1100 °C for 2 h in oxygen we obtained a thick film with a homogeneous microstructure, a thickness of ~ 40 𝜇m and a relative density of ~85 %. We theoretically estimated and showed the influence of the porosity on the figure of merit (FOM) for a KNNSr-based PEH operating in the 31-mode. We found that the FOM at resonance was more impacted by the porosity than the FOM in off-resonance conditions. This work demonstrated that EPD is a suitable method for the preparation of bimorph cantilevers made of lead-free thick films for PEHs.