英文摘要 | Flexible Wearable/implantable electronics have a wide range of applications in human physiological parameter monitoring, disease treatment and animal behavior research et al. Conventional batteries remain the most commonly used energy source for these devices, but their low energy density, large size, rigidity, risk of leakage and need for surgical replacement limit the life-span of wearable/implantable electronics. Various flexible energy harvesters can convert the ambient mechanical and thermal energy into electrical energy and continuously power electronics. Flexible pyroelectric energy harvesters (PyEH) can harvest energy from temperature fluctuations, and flexible piezoelectric energy harvesters (FPEH) can collect electrical energy from the movement of human and animals. The both energy harvesters have received a great deal of attention in recent years. In this thesis, the performance evaluation standard and structural design of high-performance PyEH and FPEH are investigated as follows:
The output voltage is an important criterion for evaluating the performance of various flexible energy harvesters. Taking PyEH as an example, many literatures usually use the output voltage under a certain temperature fluctuation to characterize the energy harvesting performance, and some of them express the result as “open-circuit voltage”. However, these experimental voltage curves usually show alternating positive and negative waveforms even under positive temperature fluctuation, which contradicts the constant positive voltage waveform predicted by the open-circuit voltage theory. Based on this problem, we systematically analyze the output voltage measurement of PyEH through experiments and theoretical analyses, with the following specific contents: (1) We found that the resistance and capacitance of the voltmeter used in the measurement have a non-negligible influence on both the amplitude and waveform of the output voltage curve. When the resistance of the voltmeter is low and high, the amplitude of the output voltage is correlated with the rate and the amplitude of temperature change, respectively. (2) The output voltage theory considering the resistance and capacitance of both the voltmeter and PyEH was established. The intrinsic voltage was proposed as a new criterion for the pyroelectric performance.
In addition to the development of high-performance functional materials, electrodes, and energy storage circuits, structural design is also important for improving the overall performance of flexible energy harvesters. Laminated structures based on piezoelectric films and substrates are the most widely used structural designs for FPEHs. Thick and stiff substrate can make FPEH have higher piezoelectric energy output from piezoelectric film during bending, but this will also significantly increase the mechanical stiffness and reduce the energy efficiency. To address this contradiction, (1) we established a theoretical model of FPEH with soft substrate sandwich structure considering substrate shear deformation and found that when the FPEH is long enough, a soft substrate with elastic modulus much lower than that of piezoelectric layer can significantly reduce the mechanical stiffness without affecting the piezoelectric energy output, and an optimal value exists to maximize the energy efficiency. (2) We designed and fabricated an optimal soft-substrate sandwich FPEH which has high areal energy output density, low mechanical stiffness and high energy efficiency simultaneously. (3) We explored the application of the FPEH to harvest energy from heartbeat, breathing, fish swimming and bird flying.
The theory of output voltage and performance evaluation criteria can be further extended to other energy harvesters such as piezo- and tribo-electric energy harvesters. The proposed structure optimization method advances the practical application of FPEHs and can be further extended to various laminated structure FPEHs and other energy harvesters such as triboelectric energy harvesters. |
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