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Title: Flexible electric generator for wearable electronics applications
Authors: Chen, Song
Degree: Ph.D.
Issue Date: 2016
Abstract: Wearable electronics and portable electronics for entertainment, sports, healthcare, medical and industrial application have been brought to public's attention and been considerable improvement in the last two decades. The demand of power source for wearable electronics implies huge increase in long-term monitoring, while the traditional power source is still battery with limited lifetime and capacity. Therefore, extending the lifetime of batteries for portable and wearable electronics or replacing batteries by self-powered systems becomes the new trend and an arduous task in the development of energy harvesting technology. Now it is likely to investigate and fabricate the generators from human motion or ambient to power the devices and extend the capacity for applications (such as health monitoring system, detection system or nanorobotics), since wearable electronics have been developed with emphasis on miniaturization, larger data handling capacity, data interaction and synchronization with much lower power consumption. To increase the power output of generators, the project is aimed to investigate the mechanisms of piezoelectric and hybrid generators based on piezoelectric and triboelectric effects, to optimize the key parameters to improve the power output, and to design harvesting circuits for piezoelectric and hybrid generators to charge capacitor, batteries so as to drive wearable electronics. In this thesis, a theoretical analysis was conducted for piezoelectric generators working in d31 and d33 mode, with a view to increase the power output of mechanical generator to a target of an average power of 1mW for wearable electronics. This is in light of the fact that at present, the generated power by most mechanical generators is not sufficient even for wearable systems, not to mention replacing batteries. The key factors of the power output under consideration are strain, strain rate and loading pattern. As such, numeric simulations were conducted and the model was evaluated by experimental results. The electro-mechanic action was divided into three stages: the first compression stage where the positive charges were generated and transferred, the second holding stage where the discharge occurred, and the third releasing stage where the negative charges were transferred. The duration of the holding stage was found to be critical to the power output of the generator. The duration of the holding stage should be three times more than the effective time constant of the equivalent circuit. If the duration of the holding stage was too short, the negative charges would offset the positive charges, resulting in the reduction of power output. Moreover, it's demonstrated in both the simulation and experimental results of a piezoelectric generator with PVDF-TrFE film in arc-shaped structure that more power out was generated under d31 mode. A second theoretical analysis was attempted for hybrid generator with resistance type of external load based on both piezoelectric and contact electrification effects. The power output can be increased by a high average charge density of contact surface and high relative dielectric constant of piezoelectric and triboelectric layers with lower dielectric loss. This treatment can only deal with the peak values of powers instead of power output over a period of time. Thus an extension was made to include a rectifier circuit and a capacitor. The extended analysis can be applied to optimize the generator together with harvesting circuit according to the generated charge, charge transfer and extract circuit.
Prototypes of generators were then fabricated and investigated experimentally, based on the theoretical analysis. Firstly, piezoelectric generators were made from PVDF-TrFE nanofiber/silver nanowires. 18wt% PVDF-TrFE solution with 2.0wt% silver nanowires were used for electro-spinning. The diameter of PVDF-TrFE nanofiber ranged from 200nm to 700nm with the silver nanowires of about 120nm. The peak open-circuit voltage and the peak current under 5.1MΩ were 33.97V and 4A under 1500N peak compression force, respectively. Secondly, PVDF-TrFE/silver nanowires nanofiber and PDMS film with graphite nanoparticles were used to fabricate hybrid generator. When the optimized proportion of silver nanowires in PVDF-TrFE nanofibers was 2.0wt% and the proportion of graphite nanoparticles in PDMS film was 3.0wt%, the hybrid generator generated most power output which could charge the 100V/100F capacitor to 11.57V after 1220 times and generate total average power of 16.46 µW. The hybrid device could drive more than 53 LEDs. The output performance improvement was attributed to the conductive filler which raised the relative dielectric constant with low dielectric loss. When the proportion of silver nanowires was more than 2.0wt% and the proportion of graphite nanoparticles was more than 3.0wt%,the dielectric loss increased rapidly because of the formation of conductive network in local area which deteriorated the performance of the devices. Also, the mechanisms of hybrid generators were analyzed in detail. There were two separate superimposable contributions to the total power output: the piezoelectric contribution of the piezoelectric layer, the triboelectric effects from the triboelectric layer. Differing from the previous treatments where the piezoelectric contribution can be enhanced by tribo-electricity, the piezoelectric effect was not increased by triboelectricity in the present study. Finally, another new type of piezoelectric generators was made from composites of lead-free Bi₀.₅Na₀.₅TiO₃(BNT) particles and silver nanowires in PDMS, with conductive fabric electrodes. Silver nanowires were utilized firstly in piezoelectric generator to increase the power output. The optimized composite with 12wt% BNT particles and 1.0wt% silver nanowires based piezoelectric generator could generate peak open-circuit voltage of 14.6V and peak current of 2.476 µA under 1MΩ resistance load which was larger than the power output in the previous work. Besides the piezoelectric generator, the composite was used to fabricate new type of hybrid generator, and the piezoelectric layer plays two roles: piezoelectric and triboelectric. When force was applied on the top electrode, the hybrid generator could generate a peak open-circuit voltage of 76.739V and a peak current of 8.948A under 1MΩ resistance load. The generator could charge 100V/100µF capacitor to 1.36V under 1000 times and the average power output was about 0.2774 µW which was lower than the power output of triboelectric generator with PDMS and carbon fillers. The generated negative triboelectric charges were decreased by the generated positive piezoelectric ones when the top electrode contacted with the surface of composite, which showed that this type of hybrid generator is not preferred.
Subjects: Electric generators.
Energy harvesting.
Hong Kong Polytechnic University -- Dissertations
Pages: xxix, 183 pages : color illustrations
Appears in Collections:Thesis

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