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The article presents an experimental study of a system consisting of a piezoelectric energy harvesting device, Graetz bridge rectifier, capacitor, voltage comparator and radio transmitter. In the presented experimental study, the recovered electrical energy is accumulated in the capacitor and is used to send signals by a radio transmitter. In the first part, the application of piezoelectric energy harvesting devices based on the cantilever beam in wireless monitoring systems is discussed. In the second part, the mathematical model of energy conversion in the piezoelectric energy harvesting devices is presented. In the third part, the characteristics obtained during laboratory research are presented.

Piezoelectric energy harvesting from rotating applications can be realized by the use of harvesters, the basic part of which is a piezoelectric material. Such harvesters have various structures but most of them contain a cantilever composite beam, which consists of the carrying material and the piezoelectric material. A diversification of the harvester structures is, above all, the result of the application of additional mechanical structures, the task of which is to deform the cantilever composite beam. In contrast to these approaches, this article presents the simulation and laboratory investigation of the piezoelectric energy harvesting from a rotating shaft without the use of any additional mechanical structure. Energy harvesting was realized by the use of macro fiber composite (MFC), which was directly glued onto the rotating shaft surface, instead of on a cantilever beam being a part of the additional mechanical structure. In the theoretical part of the article, a mathematical model of piezoelectric energy harvesting from the rotating shaft with the use of MFC was elaborated. This model was verified on the basis of the laboratory experiments. On the basis of the simulation and laboratory experiments it was found that both an increase in the rotation frequency of the shaft and an increase in the stress in a rotating shaft caused a linear increase in the voltage generated by the MFC and caused an exponential increase in power generated by the MFC. These characteristics are significantly different to the characteristics obtained for harvesters consisting of a cantilever beam, in which the maximal value of a voltage and a power is generated only for a resonant frequency of a cantilever beam. In spite of these differences the optimal load resistance for the specific rotation frequency of the shaft can be calculated from the formula used to calculate the optimal load resistance for a harvester based on a cantilever beam.

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In this study, the influence of the addition of rare earth oxides on the phase composition and density of KNN piezoelectric ceramics was investigated. The initial powders of Na 2 CO 3 and K 2 CO 3 were dried at 150 • C for 2 h. Then, a powder mixture for synthesis was prepared by adding a stoichiometric amount of Nb 2 O 5 and 5 and 10 wt % overabundance of Na 2 CO 3 . All powders were mixed by ball-milling for 24 h and synthesized at 950 • C. The phase composition of the reaction bed was checked by means of X-ray diffraction (XRD). It had an appearance of tetragonal and monoclinic K 0.5 Na 0.5 NbO 3 (KNN) phases. Then, 1 and 2 wt % of Er 2 O 3 and Yb 2 O 3 , were added to the mixture. Green samples of 25 mm diameter and 3 mm thickness were prepared and sintered by hot pressing at 1000 • C for 2 h under 25 MPa pressure. The final samples were investigated via scanning electron microscopy (SEM)-energy-dispersive X-ray spectroscopy (EDS), XRD, Rietveld, and ultrasonic methods. Phase analysis showed tetragonal and orthorhombic KNN phases, and a contamination of (K 2 CO 3 ·1.5H 2 O) was present. The obtained KNN polycrystals had a relative density above 95%. Texturing of the material was confirmed as a result of hot pressing.Materials 2019, 12, 4171 2 of 12 lead-based piezoelectrics. Due to the significant piezoelectric response obtained by numerous teams [5,[7][8][9], KNN-based materials have been widely researched, including in this study.In a perovskite-type ABO 3 structure, A-site cations are either alkaline earth elements or rare-earth elements, with B-site cations being transition metals. Theoretical structure consists of A cations occupying cuboctahedral coordination positions in the middle of eight corner-sharing BO 6 octahedrons. Due to the specific structure and composition, perovskites have plenty of conductive properties used in material sciences, including significant piezoelectricity. The piezoelectric effect is directly derived from the noncentrosymmetric structure of materials. Such structures are electrically neutral, but lack symmetrical arrangement. Therefore, when mechanical stress is applied, and such structure shifts, it does not maintain a neutral charge [4]. The high magnitude of electromechanical effects obtained in perovskites is derived from the ordering tendency present in those materials, mainly the shift of B cations [4]. K 0.5 Na 0.5 NbO 3 has a similar structure to BaTiO 3 , which consists of two phases, ferroelectric K 1-x NbO 3 and antiferroelectric Na x NbO 3 . Both phases have different ferroelectric transitions, crystallizing in a perovskite structure, with different symmetries. The phase transitions of KNN depend on the K/Na ratio [10]. To synthesize the basic structure of KNN, K, and Na carbonates are used in conjunction with Nb 2 O 5 [5]: K 2 CO 3 + Na 2 CO 3 + 2Nb 2 O 5 → 4K 0.5 Na 0.5 NbO 3 + 2CO ↑ 2 .