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Clothing-integrated piezoelectric sensors possess great potential for future wearable electronics. In this paper, we reported a phase-separation approach to fabricate flexible piezoelectric sensors based on poly(vinylidene fluoride) (PVDF)/graphene composite coating on commercially available fabrics (PVDF/graphene@F). The structural units of -CH- and -CF- of PVDF chains were arranged directionally due to the structural induction of graphene and water during phase separation, which is the key for electroactive phase enrichment. In optimized case, integrating into fabric substrates endows the phase-out PVDF/graphene composite coating 4 times higher voltage output than its film counterpart. Piezoelectric sensor based on PVDF/graphene@F exhibits a sensitivity of 34 V N, which is higher than many reports. It also shows low detecting threshold (0.6 mN), which can be applied to distinguish the voices or monitor the motion of body. This simple and effective approach toward PVDF/graphene@F with excellent flexibility provides a promising route toward the development of wearable piezoelectric sensors.

This article aims to provide a comparative study on the functions of piezoelectric energy harvesting, dissipation, and their effects on the structural damping of vibrating structures. Energy flow in piezoelectric devices is discussed. Detailed modeling of piezoelectric materials and devices are provided to serve as a common base for both analyses of energy harvesting and dissipation. Based on these foundations, two applications of standard energy harvesting (SEH) and resistive shunt damping (RSD) are investigated and compared. Furthermore, in the application of synchronized switch harvesting on inductor (SSHI), it is shown that the two functions of energy harvesting and dissipation are coexistent. Both of them bring out structural damping. Further analyses and optimization for the SSHI technique are performed.

This paper performs an analysis of maximum power output of piezoelectric energy harvesters. It has been observed that there exists an overall power limit that can be obtained by tuning energy harvesting circuits, including both linear and nonlinear. The significance of the power limit is that it represents the maximum possible power output or capacity of an energy harvester. In other words, the harvested power is always capped by this limit regardless of the type and tuning of the energy harvesting circuit interface. The power limit and the optimal generalized electrical load or impedance to reach this power limit are first obtained directly by using the electromechanically coupled equations of the system, and then obtained by using the equivalent circuit analysis and impedance matching approach. Both are commonly used methods in energy harvesting research. This paper presents an effort to unify them but also offer insights on the power limit from two different perspectives. In the second part of this paper, the power limit and impedance matching results are applied to a linear energy harvesting circuit interface, i.e., resistive energy harvesting (REH) circuit, and a nonlinear circuit interface, i.e., standard AC–DC energy harvesting (SEH) circuit, to study their physical constraints on the impedance matching and clearly explain their power behaviors such as the maximum power and the effect of electromechanical coupling on the power. In addition, closed-form expressions, a relationship between the mechanical damping and the effective electromechanical coupling coefficient, to define the three types of coupling, i.e., weak, critical, strong, are obtained. It is found that the SEH harvesters require about 1.5 times of minimum electromechanical coupling of that of REH harvesters to reach the power limit, and the frequency bandwidth between the two power limit frequencies of a SEH harvester is narrower than that of a REH harvester given the same level of strong electromechanical coupling.