This PhD thesis deals with electret and piezoelectret transducers with focus on their ability to measure structure-borne sound and energy harvesting.
Chapter 1 begins with an introduction to the topic of the thesis and then the objectives of the research work are explained. In Chapter 2, the physical principles of the materials and trans-ducers are briefly presented and the literature on electrets, piezoelectrets, accelerometers, and energy harvesting is analyzed. The method of making electrets and piezoelectrets as well as some of their main applications is described in some more detail. Then follows an introduction to the theoretical background, the various transducing methods, the main applications, and the methods of characterizing accelerometers. An analysis of the topic of energy harvesting con-cludes the chapter: the different energy harvesting methods are presented, the issue of power conditioning is introduced, some methods of characterizing energy harvesters are proposed, and finally various figures of merit to evaluate and compare energy harvesters are analyzed. Chapter 3 deals with piezoelectret and electret accelerometers. First, an analytical model of pi-ezoelectret accelerometers with one or several piezoelectret stacks is derived from a simple equation of motion. Two corresponding practical applications of piezoelectret accelerometers, namely sensors integrated in circuit boards and modal sensors, are presented. The chapter ends with the derivation of another analytical model of electret accelerometers, also from their equa-tion of motion. Chapter 4 is dedicated to piezoelectret and electret energy harvesters. For each harvester, an analytical model is again developed similarly to the accelerometers’ models and experimentally verified. This verification includes for all harvesters the dependence of the res-onance frequency and the maximum power on the seismic mass. In the case of piezoelectret transducers with d33 effect, the dependence of the maximum power on the properties of the piezoelectret stacks are particularly studied and for the piezoelectret transducers with d31 effect the influence of the different stress orientation on the power is analyzed. In the case of electret transducers, the effect of the mechanical properties of the spacer between the seismic mass and the ground electrode and of the airgap dimensions on the resonance frequency and maximum power are investigated. At the end of the chapter, the power generated by the harvesters are evaluated by the above figures of merit and compared to the power generated by piezoelectric-ceramic harvesters. Finally, the main results of the thesis are summarized in Chapter 5, followed by an outlook on future investigations and possible practical applications of the transducers. | English |