[en] A piezoelectric energy converter is presented, whose resonance frequency can be tuned by applying mechanical stress to its structure. The converter consists of a piezo-polymer cantilever beam with two additional thin arms, which are used to apply an axial preload to the tip of the beam. The compressive or tensile prestress applied through the arms leads to a shift of the beam's resonance frequency. Experiments with this structure indicate a high potential: the resonance frequency of a harvester to which a compressive preload was applied could be altered from 380 Hz to 292 Hz. In another experiment, a harvester with stiffened arms was tuned from 440 Hz to 460 Hz by applying a tensile preload. In combination with automatic control of the applied force, this type of structure could be used to enhance the performance of energy harvesters in vibrating environments with occasional shifts of the vibrational frequency

[en] For high electrical loads, the electromechanical characteristics of PZT actuators are impacted by nonlinearities. This work presents a procedure, derived from nonlinear analysis, to assess separately the electrostriction and the elastostriction parameters of bulk PZT ceramics. A detailed investigation has shown that the electrostriction is dominant in actuator applications, while the elastostriction affects sensor applications. Further, the relationship between electrostriction and polarization in PZT material is discussed

[en] This paper presents an electromagnetic vibration scavenger that exhibits a tunable eigenfrequency. By applying a static electrical field the eigenfrequency can be shifted. This feature is originated from exploiting the elastostriction of the utilized piezoelectric bimorph suspension. It is demonstrated that in the tuning operation mode more than 50 µW are scavenged continuously across the feasible frequency range of 20 Hz.

[en] In this work we propose analytical expressions for the jump-up and jump-down point of a nonlinear piezoelectric energy harvester. In addition, analytical expressions for the maximum power output at optimal resistive load and the 3 dB-bandwidth are derived. So far, only numerical models have been used to describe the physics of a piezoelectric energy harvester. However, this approach is not suitable to quickly evaluate different geometrical designs or piezoelectric materials in the harvester design process. In addition, the analytical expressions could be used to predict the jump-frequencies of a harvester during operation. In combination with a tuning mechanism, this would allow the design of an efficient control algorithm to ensure that the harvester is always working on the oscillator's high energy attractor. (paper)

[en] Piezoelectric cantilevers often form a crucial part in actuating and sensing devices. Many dynamic applications require them to operate at the basic resonance. To ensure optimal performance, the eigenfrequency has to be adjustable to match varying application demands. Within this publication, it is demonstrated that the resonance frequency of a piezoelectric bimorph cantilever can be tuned about 20% by an applied static electrical field. This beneficial feature is explained by an extended analytical model including nonlinear properties of piezoelectric materials

[en] Nonlinear piezoelectric energy harvesting generators can provide a large bandwidth combined with a good resonant power output. However, the frequency response is characterized by a strong hysteresis making a technical use difficult if the hysteresis cannot be compensated. We propose a tuning mechanism that allows both, a compensation of the hysteresis as well as maintaining the optimal work point. The compensation algorithm can reduce the hysteresis to a minimum of only 1.5 Hz and maintain a high energy oscillation in a large frequency window between 53.3 Hz and 74.5 Hz

[en] Nonlinear piezoelectric energy harvesting generators can provide a large bandwidth combined with a good resonant power output. In an experimental study, the influence of the piezoceramic material on these two parameters is investigated. The results prove hard piezoceramics to be better suited as converting element compared to soft piezoceramics. Their improved mechanical quality compensates for their low piezo-mechanical coupling leading to both, a larger bandwidth and a higher power output of the generator

[en] The focus of this work is on the development of a simple and variable process chain for the integration of flexible silicone material into silicon-based microfluidic devices. A normally-closed microvalve is chosen as a demonstrator device, as it combines features that are not easily obtained from silicon devices alone, especially, a high leak tightness of up to 1 bar pressure difference in the closed state and a high forward flow of several mL min^-1 in the open state. For this purpose, a photopatternable silicone is used as a deformable circular valve lip between a piezoelectrically actuated membrane and a valve seat, similar to a micro O-ring with a width of 50 µm. The microvalve is piezo actuated by monolayer piezo actuators with a peak-to-peak driving voltage of V_p-p = 200 V. The micro O-ring is pre-deformed by 2.8 µm during the valve fabrication process to yield the normally-closed behavior. A dry film resist lamination technology is developed for this critical process step to mate the two silicon wafers with the actuation membrane, the valve seat and the silicone O-ring in between at a well-defined distance. The dry film resist is used in a multifunctional way, not only to pre-deform the valve lip, but also to define the geometry of the valve chamber and to ensure a leak-tight connection of both wafers. Altogether, a peak value for the on- to off-ratio of the normally-closed microvalve higher than 30 000 is measured. This opens a wide range of potential applications, e.g. in micro-dosing, drug delivery, μ-TAS and microfluidics for biological or chemical applications in general.

[en] Micro heat engines have been proposed as an alternative to thermoelectric generators, to harvest thermal energy. Recently, we proposed a micro heat engine fabricated using a buckled bimetallic beam. In this paper, we present a finite element as well as an analytical model to predict the static behavior of such a heat engine. A lumped parameter model to predict the dynamic behavior of the engine has been discussed as well. Finally, the engine has been fabricated and tested to validate all models presented. The analytical model corroborates the finite element model accurately. In addition, a good correlation has been observed between the simulations and measurements. The engine is capable of operating at up to 10.4 Hz from a temperature difference of 44 K. (paper)

[en] This study introduces a novel experimental set-up to measure the pyroelectric coefficient of materials at variable frequencies of temperature change. In this method, temperature changes are periodically applied through a mechanical set-up moving the sample between a hot and cold thermal reservoir in order to measure the current obtained from the sample’s pyroelectric conversion effect. For low frequencies of temperature change, an exponential equation is suggested for the temperature change in the sample based on a unidirectional heat flux through the sample. The pyroelectric coefficient can be determined from this model by fitting an exponential decay function to the pyroelectric current obtained from a single heating cycle. In comparison to other approaches for the measurement of the pyroelectric effect, the described method exhibits some advantages concerning flexibility, accuracy and simplicity, e.g. an almost deliberate adjustment of the rate of temperature change, the avoidance of electrical noise induced by continuously temperature-modulated heat stages and furnaces, a high accuracy of temperature control and the acceptance of samples made from different materials or with different sizes. The method is verified by pyroelectric coefficient measurements on a commercial PZT ceramic and on triglycine sulfate (TGS) single crystals, which have been grown by a temperature-lowering technique in our laboratory. Pyroelectric measurements were conducted at different temperature differences for samples with different thicknesses and contact areas. The measurement results for PZT M202 (427 ± 1 μC m^–2 K^–1) are close to the datasheet value, which is 430 μC m^–2 K^–1. An average pyroelectric coefficient of 306 ± 2 μC m^–2 K^–1) is measured for the single crystal of TGS over multiple trials. The measurement results depend on the quality of the crystal and the process for the preparation of the sample. The results show internal consistency between the measurements, which are also in agreement with literature values. (paper)