Research On Phononic Crystal Resonators And Their Acoustic Energy Harvesters

Phononic crystals(PCs) have extensive application prospect in vibration attenuation, noise cancellation, sound focusing/imaging, acoustic cloak and many other fields. Especially, the PC with a point defect(phononic crystal resonator, PCR) can trap acoustic waves and amplify acoustic pressure because of the acoustic wave localization effect induced by the point defect. Hence, PCRs can be used for acoustic energy harvesting and or so. However, due to the small pressure amplification and low resonance Q value of previous PCRs, the acoustic energy harvester exploiting PCR shows the low harvesting efficiency and then its wide application gets into trouble.The use of acoustic energy harvesting technology provides a power source for the low-power electronic device and eliminates ambient noise. But, originating from the ambient noise characterized by low energy density and multidirectional propagation, the study on the PCR with great resonance pressure amplification and high resonance Q value has been the difficulty and keystone in the field of multidirectional and high-effective acoustic energy harvesting. Therefore, funded by the National High Technology Research and Development Program of China(863 Program, Grant 2012AA040602), the State Key Program of National Natural Science of China(Grant 61374217), this thesis investigates theoretically and experimentally the PCR. In addition, the thesis researches the potential applications of the PCR on the acoustic energy devices. The main work of this dissertation is summarized as follows:1) The dual-coupled PCR structure created by placing a smaller two-dimensional square PCR into the cavity center of another larger one is presented. Due to the acoustic resonance coupling between these two PCRs, the dual-coupled PCR exhibits enhanced wave localization effect and enlarged pressure amplification. The differences of band structures, wave localization effects and pressure amplification spectra between single and dual-coupled PCRs have been studied. Besides, a measurement system is designed to measure the acoustic pressure amplification spectra of single and dual-coupled PCR structure. The experimental results show that the dual-coupled PCR structure owns a stronger wave localization effect and 2.1~3.3 times larger resonance pressure amplification than the single PCR structure. The experimental data agree well with the theoretical results.2) The multiple-coupled PCR structure consisting of the concentric Matryoshka system of multiple PCRs(sub-PCRs) is presented. Due to the cross-coupling effect between the band gaps and defect bands of individual PCRs, a few flatter bands related to the strong wave localization mode occur in the band structures of the multiple-coupled PCR. Consequently, the multiple-coupled PCR has stronger wave localization effect and larger pressure amplification. The effects of the sub-PCR number on the band gap, pressure amplification and resonance Q value of the multiple-coupled PCR structure have been studied. The results show that the pressure amplification of the multiple-coupled PCR is in direct proportion to the sub-PCR number, and the average enhancement ratio of resonance pressure amplification to sub-PCR number is about 2.73. Moreover, the five-coupled PCR structure exhibits 33.9 times larger resonance pressure amplification and 36.7 times higher resonance Q value than a single PCR structure with the same size.3) The high-Q and great-pressure-amplification phononic crystal resonator(Cr-PCR) consisting of an array of cross plates in air is proposed. Due to the strongly directional wave scattering effect of the cross-plate corners, the intense constructive interference between scattered waves from the cross-plate corners and sequentially the strong confinement of acoustic waves emerge. Consequently, this proposed Cr-PCR structure exhibits multiple concentration points with high Q value and large acoustic pressure amplification. The effects of the cross-plate thickness on the band gap, resonance frequency, pressure amplification and resonance Q value of the Cr-PCR structure have been studied. The pressure amplification spectra and resonance Q values of the proposed Cr-PCR and the traditional phononic crystal resonator(Cy-PCR) with rods are measured and compared. The experimental results show that the Cr-PCR structure owns the Q value of 2020~3535 and the maximum pressure amplification of 14.3~16.6(in multiple concentration points), which are 202~353.5 times higher and 5.3~6.1 times larger than that of the traditional Cy-PCR, respectively. The experimental data agree well with the theoretical results.4) The multistage-local resonance PCR structure characterized by great pressure amplification in the sub-wavelength operation is presented. Each scattering unit of the proposed PCR is composed of multiple concentric Helmholtz resonators(HRs). Because of the enhanced constructive interference between amplified scattering-waves by each scattering unit, strong confinement of acoustic waves and large magnification of acoustic pressure emerge. The effects of the HR number and orientation(in each scattering unit) on the band gap, resonance frequency, pressure amplification and resonance Q value of the proposed PCR structure have been studied. The results demonstrate that under the asymmetric and reversely symmetric orientations of HRs, the proposed PCR structure owns 4.2 and 5.9 times larger maximum pressure amplification than the traditional PCR with rigid scatterers, respectively. On the other hands, the multistage-local resonance PCR structure under asymmetric orientations of HRs has ultra-low resonance frequency corresponding with a wavelength 4.6 times larger than the lattice constant.5) The effective approach for tuning the property of the phononic crystal device is proposed by applying a small static magnetic field(Hdc). Due to the magnetic torque effect, the Nd Fe B magnets generate huge bending moments under Hdc, and so the large bending deformations occur on the scatterers. Consequently, remarkable changes in the lattice constant and the band structures are achieved, which leads to the large changes in the the property(i.e., transmission) of the phononic crystal. Experimental results show that the property of the phononic crystal device can be the significantly altered be by applying the external static magnetic field with the amplitude of 130 Oe, which is ~1/100 as large as that in the previous study.6) The high-effective acoustic energy harvester(AEH) using the phononic crystal and electromechanical Helmholtz resonators(EMHR) is presented. Due to the strong acoustic coupling between PCR and EMHR, the enhanced confinement of acoustic waves and sequentially the improved harvesting efficiency occur. Besides, a measurement system is designed to measure the output voltage and power spectra of AEH devices under different directions of incident waves. Furthermore, in combination with a management circuit, the feasibility of the AEH used as a supply of the wireless sensor node is tested. The experimental AEH devices include two models, one using an EMHR structure and a traditional Cy-PCR structure, and the other using an EMHR structure and a proposed high-Q and great-pressure-amplification Cr-PCR structure. The experimental results indicate that the proposed AEH device using Cy-PCR and EMHR owns 23 and 262 times greater maximum harvesting efficiency than the traditional AEH using single Cy-PCR and EMHR, respectively. When incident waves from multiple directions, the AEH using Cr-PCR and EMHR has 13.6~22 times larger maximum output power volume density than the AEH using Cy-PCR and EMHR.7) The acoustic-energy-powered wireless sensor node(WSN) consisting of an AEH, a power management circuit and a traditional WSN is created. The charging/ discharging characteristics for the management circuit are measured experimentally, and the management circuit is also used to power a WSN experimentally. The experimental results demonstrate that when the combination of an AEH and a management circuit works under input acoustic waves with the pressure amplitude of 100 d B, the WSN with a maximum power consumption of 78 m W can be driven to acquire and transmit data over a period of 620 ms.