A ferroelectric/ferroelastic energy harvester: Load impedance and frequency effects
Graphical abstract
Introduction
In recent years, the development of electronics has promoted advances in batteries to meet increasing power demands. However, frequent recharging or replacement of batteries is still required to satisfy the demands of portable electronics which now commonly include continuous network access. Hence, energy harvesting technologies, which are eco-friendly and cost-effective, provide an opportunity to replace or support batteries [1], [2]. In the last decade, various kinds of energy harvesting methods, including piezoelectrics [3], [4], pyroelectrics [5] and triboelectrics [6] have been investigated. The energy harvesters scavenge mechanical energy [7], acoustic energy [4], [8], thermal energy [9], [10] or solar energy [11] from the ambient environment. Among these, vibrational energy is clean and sustainable, being available from many sources, including human motion, machines, and fluid flows [12], [13].
The piezoelectric effect has been applied to scavenge mechanical or vibrational energy via electromechanical coupling [14], [15], [16]. Compared with other methods, piezoelectrics provide a simple, solid state generator, without the need for additional components [17]. They can provide high output voltages with very low electrical currents, reducing dissipation. However, the magnitude of loads and displacements is limited in piezoelectric transducers in order to maintain linearity. Consequently, their power density is low. Existing research shows that Piezoelectric energy harvesters usually take the forms of bimorph or unimorph straight cantilevers, with top and bottom electrodes in mode and interdigital electrodes (IDEs) in mode [18], [19], [20]. By optimizing the IDEs design, the mode can produce greater energy output and voltage than the mode, which differs from the common studies [21], [22]. Apart from this, lowering the resonant frequency through lowering the stiffness of the structures can generate more significant strain, like using circular diagrams or zigzag beams, thereby improving the energy output of piezoelectric energy harvesters [23], [24]. As for multi-direction energy harvesting, while some researchers investigated multi-degree-of-freedom (multi DOF) piezoelectric energy harvesters [25], [26], [27], [28], others studied two or three-dimensional energy harvesters, broadening the bandwidth and energy output [29], [30], [31], [32]. With the assistance of magnetic force, Mono-stable nonlinear [33], [34] and bi-stable nonlinear mechanisms [35], [36] are also introduced to the pie piezoelectric energy harvesters technology to adapt to the low ambient vibration resonant frequency, leading to similar effects by using frequency up-conversion mechanism [37], [38]. Furthermore, triboelectric generators [39] provide a novel transduction mechanism that exploits the separation of electrostatic charge for harvesting mechanical energy, with greater power density than piezoelectrics and diverse, scalable architecture [40], [41]. In 2017, Wang et al. [42] exploited the high dielectric permittivity of a ferroelectric ceramic layer in a triboelectric nanogenerator to enhance its performance. However, the full ferroelectric hysteresis was not used.
Ferroelectric materials possess spontaneous polarization that can be reoriented through domain wall motion. This can be induced by an external electric field, and is known as ferroelectric switching. Alternatively, stress can induce switching, known as ferroelastic switching [43], [44]. The resulting non-linear changes in shape and polarization can be used to generate electrical work, with greater output than conventional piezoelectric transducers. However, a problem is that, although stress can depolarize an electroceramic, a depolarized electroceramic cannot normally be repolarized by stress. This generally prevents a working cycle in which a cyclic stress could give rise to a cyclic change of polarization. Some attempts at theoretical energy harvesting designs using combinations of ferroelectric and ferroelastic switching have been developed [45], [46], [47], [48], [49], [50], [51], [52]. Patel et al. [47] proposed a working cycle, wherein a compressive stress depolarizes an antiferroelectric material and repolarization can be carried out by an external electric field. This demonstrated the potential for giant mechanical energy density in electroceramic energy harvesters. Later, Balakrishna and Huber [48] exploited the idea of an engineered domain structure in a nano-layer to design a ferroelectric energy harvester, which uses periodic tensile and compressive stress to drive a charge flow onto and off electrodes. Wang et al. [49], [50] used a phase-field model to improve the design, introducing a bias field to stabilize the working cycles. Kang and Huber [51] subsequently developed an energy harvesting cycle based on bulk polycrystalline ferroelectrics, using tensile stress to depolarize the ferroelectric and electromechanical loading to restore the polarization, which was further analysed and improved by Behlen et al. [52].
Building on these developments, a novel prototype device was proposed by Kang et al. [53], [54] using internal fields instead of externally applied bias fields to direct the polarization changes. The resulting energy harvesters demonstrate high values of energy output and have the advantage of simple structure and operation, similar to established piezoelectric devices. By controlling the polarization state and intrinsic residual tensile stress, only cyclic compressive loading was needed to drive the energy harvester, while partial ferroelectric/ferroelastic switching occurred in each cycle. The fabrication process requires a special step in which partial prepoling of an electroceramic is carried out before it is bonded to a stiff substrate. Afterwards, a strong electric field is applied to complete the poling, and then tensile stress is introduced to partially depolarize the electroceramic. This leaves the ceramic in an electrically and mechanically biased state from which small perturbations can produce significant reversible polarization changes. From this state, compressive loading cycles drive a stable electrical energy output.
The present work extends and develops the energy harvesting method of Kang et al. [53], [54] focusing on the effects of varying electrical load impedance and working frequency. In the previous study [53], [54], off-substrate 30% prepoled devices have shown the best performance among 30%, 50%, 75% and 100% prepoled devices, but the optimal energy output of the devices are still needed to be further explored. Therefore, this work mainly focuses on performance improvement of the 30% prepoled devices. Vibrational tests are reported and a frequency dependent optimum resistive load for maximum energy output is identified. A simplified model based on a piezoelectric energy harvester coupled with an external circuit is presented and used for the interpretation of results. This enables a comparison between the present energy harvester and piezoelectric devices. Subsequently, fatigue tests are reported, demonstrating that the optimum energy cycle can be operated without substantial fatigue degradation for more than cycles.
Section snippets
Design and test arrangement
The energy harvester developed by Kang et al. [53], [54] introduces a state of engineered polarization and residual stress to guide the repolarization of a ferroelectric ceramic, resulting in a stable cycle for energy harvesting. The material preparation and working cycle is illustrated in Fig. 1. Initially the ferroelectric ceramic is in an unpolarized, as-sintered, state (A in Fig. 1). In the first step, the electroceramic is partially polarized to state B using electric field. At this stage
Piezoelectric energy harvester analogue
Modelling of the full ferroelectric and ferroelastic response with non-linear cycles in a partially poled state is challenging. However, a relatively simple piezoelectric model can provide insight into the operating performance of the energy harvester, as shown in Fig. 3. The model adopts the same structure and loading pattern as the ferroelectric energy harvester, with a uniform bending moment applied along the length of the composite beam.
Euler–Bernoulli beam theory, coupled with the
Results and discussion
Initial quasi-static tests were carried out to establish a practical range of external impedance values for subsequent testing. Additionally, dynamic tests were used to confirm that the energy harvester was behaving as a non-linear ferroelectric/ferroelastic device — see supplementary material. In practical applications, the impedance of the external circuit will result in a voltage on the ferroelectric layer during energy harvesting. Therefore, the effect of varying the total external
Conclusion
This paper reported the effect of resistive load and operating frequency on compressive stress-induced energy harvesting cycles using ferroelectric/ ferroelastic switching. By varying the operating frequency in the range 1–20 Hz and changing the electrical load, optimized energy output could be identified, with superior performance compared to typical piezoelectric transducers operating at low frequency. The results show that the peak voltage increases with the rise of frequency, and stabilizes
CRediT authorship contribution statement
Wenbin Kang: Provided the design for the experiments, Implementation steps, Experimental set-up establishment, Fabricated the devices, Measured the output performance, Analysed data, Wrote the manuscript. Cameron Cain: Fabricated the devices, Measured the output performance, Analysed data. Robert Paynter: Experimental set-up establishment, Fabricated the devices, Measured the output performance, Analysed data. John E. Huber: Provided the design for the experiments, Implementation steps, Edited
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The authors gratefully acknowledge support for Wenbin Kang from the Jardine Foundation .
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Cited by (0)
- 1
The two corresponding authors have the same contribution to this study.
- 2
All authors contributed to the manuscript.