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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    ABSTRACT: We fabricated polymer piezoelectric energy harvesters (PEHs) that can generate electric power at wind speed of less than 4.7 m/s due to their high sensitivity to wind. In order to optimize their operating conditions, we evaluated three distinct PEH operation modes under the boundary conditions of single-side clamping. We found that a PEH connected to an external load of 120 kΩ shows the largest output power of 0.98 μW at 3.9 m/s, with wind incident on its side (mode I). We attribute this result to large bending and torsion involved in this operation mode.
    Applied Physics Letters 12/2013; 104(1). · 3.52 Impact Factor
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    ABSTRACT: We report a substantial improvement of piezoelectricity for poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) copolymer films by introducing carbon black (CB) into the PVDF-HFP to form PVDF-HFP/CB composite films. The optimized output voltage of the composite film at an optimal CB content of 0.5 wt% is found to be 204% of the pristine PVDF-HFP film. Its harvested electrical power density is 464% and 561% of the pristine PVDF-HFP film by using ac and dc circuits, respectively. Through Fourier transform infrared spectroscopy analysis, differential scanning calorimetry analysis, and polarized optical microscopy observations, we clarify the enhancement mechanism of piezoelectricity for the PVDF-HFP/CB composite films. We find that the added CB acts as nucleating agent during the initial formation of crystals, but imposes an insignificant effect on the α─β phase transformation during stretching. We also demonstrate that the addition of optimal CB reduces crystal size yet increases the number of crystals in the composite films. This is beneficial for the formation of elongated, oriented and fibrillar crystalline morphology during stretching and consequently results in a highly efficient poling process. The addition of overdosed CB leads to the formation of undersized crystals, lowered crystallinity, and hence reduced piezoelectric performance of the PVDF-HFP/CB composite films.
    Journal of Physics D Applied Physics 03/2014; 47(13). · 2.52 Impact Factor
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    ABSTRACT: In order to improve the piezoelectric properties of poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), we added various amounts of silver nanowires (AgNWs) into PVDF-HFP and N,N-dimethylformamide (DMF) solution to prepare composite films. Stretching and poling were applied to the films to induce the formation of the polar β-phase and reorientation of the dipole moment. The crystal structure of the films was investigated by polarized optical microscopy (POM), Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). The obtained results showed that the phase transformation mainly occurred in stretching and the reorientation of the dipole moment was attributed to poling. It was also concluded that the AgNWs played a role as nucleate agents in the β-phase formation and the phase transformation. The piezoelectric properties were also evaluated by the output voltage and harvested power density. It was found that the open circuit voltage of 0.1 wt% AgNWs containing films was 52% higher than that of pure PVDF-HFP films. Furthermore, the harvested power density was increased by 159%. Overdose of AgNWs resulted in crystal defects and the lower degree of crystallinity, leading to worse piezoelectric properties.
    RSC Advances 08/2014; 4(68). · 3.71 Impact Factor


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