Energy harvesting by using ferroelectric switching

The aim of the current work is to develop a novel energy harvesting method by using ferroelectric/ferroelastic switching and to establish a practical prototype device with stability and robustness. The results are expected to provide a deep mechanistic understanding of the polarization state in ferroelectrics and assist in the improvement of transducers' performance in practical applications. Ferroelectric/ferroelastic switching, which can generate greater energies and charge flows than those of piezoelectrics has attracted increased interest to improve transducers' performance in recent years. However, the challenges of nonlinearity, fatigue degradation and the difficulty of driving an electrical cycle by stress have limited ferroelectrics' use in energy harvesting applications. In the current work, by using a simple model of switching, a working cycle that could generate electrical energy from a harmonically varying source of stress is explored. The cycle uses depolarization by stress, followed by repolarization with combined electromechanical loading. A harvesting electric field and bias electric field are imposed to ensure a stable repeatable working cycle during the depolarization process and repolarization process, respectively. The bias electric field, which is much less than the coercive field, provides a preferred direction of repolarization. The conversion efficiency of this cycle is estimated and improvements to the cycle are explored by adjusting the electrical and mechanical field amplitudes. The current work then explores a novel and robust ferroelectric energy harvester based on partial ferroelectric switching. The device is of simple construction and achieves a per-cycle energy density of about 1mJ/cm³, orders of magnitude greater than that of typical piezoelectrics. Instead of using an external electrical bias field, the internal residual electric fields, generated during a partial depoling of an electroceramic, are adopted to provide 'memory' of a preferred direction for repolarizing it. A crucial feature is the control of residual stress by poling the active material in stages, first in a mechanically unconstrained state, then adhered to a substrate before completing the poling process. Then the method works by partially depolarizing a poled ferroelectric using a single application of tensile stress, and then using cycles of compression to vary the polarization state. It is shown that only periodic compressive stress is needed to induce the energy harvesting cycles, yielding unique and promising mechanical attributes that limit fatigue or fracture during cyclic loading. The results show this prototype device operating stably over 0.5×10⁶ cycles at low frequency. The explorations of various polarization states and resistive electrical loads are also carried out to improve the performance for further practical applications.