Study On Ionic Liquid Based Microfluidic Vibrational Energy Harvesting

In recent years, with the development of the micro-electromechanical system and precision machining technologies, the miniature and portable microelectronic devices and systems have gained the more and more widespread applications. At this stage, the chemical batteries are often used for powering, but the chemical batteries have the large volume and short lifetime, need charging or replacement regularly, the cost will be high and they can pollute the environment easily, so they limit the applications of some microelectronic products, In surrounding environments, everywhere is the vibrational energy and it is a kind of clean energy, which can be converted to electricity continuously by applying energy harvest technologies. This paper introduces the microfluidic technology to electrostatic energy harvesting, and proposes three kinds of micro-liquid bridge based vibrational energy harvester (VEH) for converting the low frequency and low amplitude vibrations to electricity. For the microfluidics based VEH, the liquid medium will affect the properties of the VEH. Up to now, the proposed microfluidics based VEHs usually used water or liquid metal mercury as liquid medium, however, the water is easy to evaporate and has bad thermal stability, narrow liquid temperature range, while the mercury has the toxicity, so the suitable liquid materials for microfluidics based VEHs should be desired. As a novel class of soft functional materials, ionic liquids (ILs) have many advantages, such as non-volatility, wide liquid temperature range, good stability, strong conductivity, green and environmental friendly etc. In this paper, ILs have been adopted to the three kinds of microfluidics based VEH and find that ILs using microfluidics based VEH can work well continuously in the natural condition without airtight space, and can adapt wide operating temperature range, particularly, it has the better performance at high temperatures.The first kind of VEH is the electrical double layers (EDLs) based micro IL bridge VEH. This VEH was composed of two planar electrodes and the micro IL bridge between them, the top planar electrode was covered with hydrophobic dielectric layer. When the micro IL bridge between two electrodes is periodically squeezed driven by vibrations, the two EDL capacitors formed on the liquid-solid overlap areas are continuously charged and discharged at different phases, and then the alternating current is generated. The equivalent electric circuit model was established and the expression of output voltage was derived under the sinusoidal vibration. Applying the theoretical model, the relationship between charge density in the top liquid-solid overlap area, bottom overlap area, the thickness and the permittivity of the dielectric layer on the top electrode, and the generated power were discussed respectively. The effects of vibrational mode, amplitude, frequency, BLs type, operating temperature, water content of IL, working time and load resistance on output power were studied experimentally. The results imply, within certain limits, the output effective power of the VEH increases with vibrational amplitude. With increasing frequency, the effective power increases firstly, starts to saturate and remains constant, or decreases slightly finally. In the same conditions, the VEH using imidazolium ILs with short alkyl chain and low viscosity can generate more power. Moreover, the output power improves with increasing the operating temperature. We obtained two main factors which determine the output power, i.e., the capacitance variation quantity of the EDL capacitor formed on the top liquid-solid overlap area and the charge density.The electret based micro IL bridge VEH is the second kind of VEH in this paper. This VEH also comprised two planar electrodes and the micro IL bridge, the differences between this VEH and the first kind of VEH are, the top electrode was coated with electret layers and the micro IL bridge was the conductive electrode in this VEH. Biased by the electret, the variation of the top liquid-solid overlap area which results from periodically compressing micro IL bridge make the variable parallel plate capacitor in VEH charge and discharge periodically, thus generating electrical energy. Based on the established equivalent electric circuit model, the electrical output equation of this VEH was obtained. The effects of vibrational mode, amplitude, frequency, ILs type, load resistance, electrets’ parameters, thickness of insulation layer for charging and working temperature on electrical output were revealed. That is, the IL which can obtain larger variation magnitude of the top contact area during vibration will output more power. Meanwhile, the output power improves drastically with the surface charge density (surface potential) of the electret. The two most important parameters of this VEH were acquired and they are, the variation magnitude of top liquid-solid overlap area and the surface charge density (surface potential) of the electret layer. By far, the output power has been improved about 99 times compared with the first kind of microfluidics based VEH.Combined the micro liquid bridge based VEH and mass-spring vibrational system, the micro liquid bridge based inertial VEH was proposed. The structure of this VEH was simple and the VEH was easy to install, additionally, the vibrational amplitude for energy converting could be amplified based on the resonance phenomenon and the generated power would be enlarged. The relationships between the natural resonance frequency of the VEH and it’s structural parameters were investigated theoretically, moreover, the natural resonance frequency of the VEH was shifted to low frequency range by tunning the relative parameters. According to the obtained structural parameters, the micro liquid bridge based inertial VEH was designed and fabricated, the natural resonance frequency was measured and this experimental data was in agreement with theoretical predicition. The effects of vibrational frequency, amplitude, load resistance and working time on electrical output were studied. We find that the output power will reach to the maximum when the frequency of vibration source approaches to the natural resonance frequency of the VEH. The generated bipolar electrical pulse of the VEH was rectified by using the diode bridge rectifier and then stored in a capacitor, the obtained electrical energy lighted a red LED.The overall results of this dissertation illustrate that this paper presented microfluidics based VEH can efficiently harvest low frequency and low amplitude mechanical vibrations, compensate for the shortcomings of the majority of VEH which the high energy harvesting efficiency can be achieved only at high frequency and large amplitude. Adopting the ILs as liquid medium, the microfluidics based VEH can work well continuously at temperature with extreme changing and in the open air space, particularly they can endure high temperatures, which extends the scope of application. The microfluidics VEH proposed in this paper will be much valueable for the area of vibrational energy harvesting.