| Author: | Fan, Lei |
| Title: | Multi-band phononic topological insulators for localizing and guiding elastic waves : design, experimental validation and applications |
| Advisors: | Su, Zhongqing (ME) |
| Degree: | Ph.D. |
| Year: | 2024 |
| Subject: | Topological insulators Topological insulators -- Acoustic properties Elastic waves Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Mechanical Engineering |
| Pages: | xxxvii, 219 pages : color illustrations |
| Language: | English |
| Abstract: | Phononic structures represent a category of artificially engineered materials constructed to exhibit exotic wave dynamics absent in nature, thereby bringing about numerous unconventional phenomena and cutting-edge applications of elastic waves. In recent years, great advances in phononic structures have been driven by their integration with topological physics, enabling remarkable topological effects for elastic waves. In this burgeoning realm, this thesis mainly concerns the development of multi-band phononic topological insulators (PTIs) for manipulating elastic waves at different frequencies, starting from forward designs of a generic type of topological phononic plates, through intelligent inverse design methodology, and finally to a practical application aspect. The research aims to advance the study of topological mechanical devices and explore their potential applications in controlling multi-frequency elastic waves. This thesis starts with the designs and fabrications of a type of perforated phononic plates towards realizing general topological effects of flexural waves, including quantum spin Hall effect (QSHE) and quantum valley Hall effect (QVHE). The key scheme is perforating a class of split-ring resonators (SRRs) in elastic plates. The multimodal resonances of SRRs enable multiple omnidirectional bandgaps of flexural waves, with the ease of implementation and fabrication. When these SRRs are endowed with prescribed crystalline symmetries for desired topological effects, multi-band topological bandgaps are easily formed to host topological edge states. Specifically, glide-symmetric SRRs based on a square lattice are used to enable dual-band QSHE for flexural waves while C3-symmetric SRRs are utilized for realizing tri-band QVHE for flexural waves. Besides the first-order topological edge states, this thesis further extends into the realization of higher-order topology corner states for flexural waves. A perforated phononic plate with C4v-symmetric SRRs is constructed to emulate a higher-order topological model, that is, two-dimensional (2D) Su–Schrieffer–Heeger (SSH) model. Topologically nontrivial and trivial unit cells are selected by lattice transformation in the plate, which carry different topological corner charges within multiple bandgaps. Assisted by a laser Doppler vibrometer, the involved multi-frequency topological edge and corner states of flexural waves on the developed phononic plates are experimentally demonstrated. Different from the previous forward design approaches that rely on trial and error, inverse optimization algorithms can guide high-performance PTI designs in an intelligent manner. This thesis also focuses on the inverse design methodology of multi-band second-order PTIs (SPTIs). Particularly, the advanced machine learning (ML) and deep learning (DL) technologies are utilized to empower the entire inverse design processes. As an illustrative example, scatterers centered in two-dimensional (2D) phononic crystals (PnCs) are suitably shaped to enable on-demand multi-band SPTI, which is accomplished via evolutionary algorithms accelerated by artificial neural networks (ANNs). Besides, this thesis further considers pixelated 2D PnCs and aims to construct a large design space encompassing much richer multi-band SPTIs with the required symmetry and fabrication feasibility, empowered by a generative DL model. Multi-frequency topological corner states in the artificial intelligence-driven multi-band SPTIs are numerically verified by finite element (FE) simulations. The above studies mainly concern the design approaches of PTIs and demonstrations for topological phenomena of interest. The final chapter of the thesis attempts to bring these intriguing topological elastic waves into a practical application, that is, vibration energy harvesting. This thesis reports an elaborately designed topological phononic device, in which the edge-corner coupling effect is achieved, aimed at trapping elastic waves emitted from the edge channel at different corner sites in terms of frequencies. Vibration energy harvesting is experimentally demonstrated by taking advantage of piezoelectric materials, which convert mechanical strains amplified by trapped elastic waves into electric powers. In comparison to conventional phononic devices for energy harvesting, the superiority of the developed topological phononic device is underscored by its exceptional robustness rooted by nontrivial topological physics, by which the vibration energy harvesting performance is experimentally confirmed to be immune to structural perturbations. |
| Rights: | All rights reserved |
| Access: | open access |
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