Article

A spring-type piezoelectric energy harvester3

RSC Advances (Impact Factor: 3.71). 01/2013; 3:3194. DOI: 10.1039/c2ra22554a

ABSTRACT We developed a three-dimensional spring-type piezoelectric energy harvester using a dip-coating method and multi-direc-tional electrode deposition. The energy harvester consists of a bi-layered structure composed of a surface electrode and a ferro-electric polymer, on a conventional spring which has two roles – the core electrode and the mechanical substrate for the ferro-electric polymer. The energy harvester generated an output voltage of up to 88 mV as a function of cycling compression stress, which leads to a piezoelectric constant of 28.55 pC N 21 for unpoled P(VDF-TrFE) films. Since the spring structure significantly decreases the resonance frequency of the harvester, the spring-type energy harvester can effectively generate electricity using low-frequency vibration energy abundant in the nature. Vibration-based energy harvesting (VEH) devices have attracted great interest for use as sustainable and clean electric power supplies for wireless sensor networks that enable health monitor-ing of important infrastructures such as power plants, bridges and remote power grids. 1–3 Since there are abundant vibration sources with a low frequency (between 1 and 200 Hz) in nature, 4 low frequency vibrations are of high interest and are targeted in VEH device design for a wide range of potential applications. 5–10 Several approaches exist to convert vibrations to electrical power including electromagnetic, electrostatic and piezoelectric conversion, among which piezoelectric energy harvesting systems (PEHSs) have received the most attention. This is because they directly convert applied mechanical energy into electricity, leading to a simpler device design in comparison to other mechanisms, which require complex geometries and numerous additional components. 1 However, PEHSs are facing challenges such as low output power and high resonance frequency. 8 As the resonant frequency is usually higher than the vibration frequency with the highest amplitude in the environment when PEHSs are scaled down to micron size, they suffer from low output power because energy harvesters generate the maximum power at the resonance frequency. 11 The most commonly adopted ways to reduce the resonance frequency of the harvester are 1) to add a mass to the harvester 12 or 2) to use a spring structure or equivalent that can decrease the overall system stiffness. We came up with the idea of a spring-type structure, which can significantly decrease the resonance frequency toward 1 kHz or less as compared with a beam-type structure with the same weight. Furthermore, our idea can be applied to existing spring structures in automobiles, bridges or even in mattress, which enables us to convert otherwise wasted volume and energy into useful ones. However, in fabricating a spring-type piezoelectric energy harvester, there are processing challenges such as conformal coating of piezoelectric material onto a substrate with a complex geometry, and uniform electrode deposition to ensure maximum contact with the deposited piezoelectric materials. Here, we report the way we addressed the processing challenges of spring-type piezoelectric energy harvesters, namely a combination of dip-coating method and multi-directional electrode deposition, and measured the output voltage as a function of cycling compression stress without an external poling process. The method and preparation for fabricating the spring-type energy harvesters are described as follows. Firstly, poly(vinylidene fluoride trifluoroethylene) (P(VDF-TrFE)) solution, a spring and a low speed motor were prepared. The spring was commercially available (Seoul Spring, Inc.), with a wire diameter of 0.97 mm, an outer spring diameter of 11.55 mm and a length of 40 mm. The spring had 10 turns. The spring constant was measured by placing a mass of 500 g at the end of the spring. The precursor solution was prepared by dissolving 15 wt% P(VDF-TrFE) (VDF:CH 2 –CF 2 / TrFE:CHF–CF 2 , 75/25) in methyl ethyl ketone solvent (MEK). 13 The iron spring was immersed in the solution for 30 s. The spring was then withdrawn from the solution at a speed of 0.3 mm s 21 . The coated P(VDF-TrFE) was left to dry for 1 h in vacuo (less than 1 kPa). To obtain the optimum thickness, we repeated the above

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