Smart piezoelectric phononic crystals and metamaterials: State-of-the-art review and outlook

As artificially designed novel periodic materials and structures, phononic crystals (PCs) and acoustic/elastic metamaterials (AMs) have many unique and extraordinary wave propagation characteristics, which provide a novel research pathway and a promising application opportunity for the efficient vibration control and precise elastic wave manipulation. However, the conventional PCs/AMs after their design and fabrication can be hardly modified with respect to their geometrical and material parameters in order to meet the actual demands, which significantly restricts their practical applications. Smart piezoelectric PCs/AMs based on the piezoelectric or electro-mechanical coupling effect can be utilized to manipulate their vibration or elastic wave propagation properties on demand by controlling the electrical field. This feature can significantly broaden the practical application ranges of the conventional PCs/AMs. This paper first roughly classifies the smart piezoelectric PCs/AMs into three types according to different combination forms of the piezoelectric and elastic materials or structures. The first type is the unitary or monotype piezoelectric PCs/AMs, which only contain a single piezoelectric material without or with electrodes. The second type is the embedded or infill piezoelectric PCs/AMs in which the piezoelectric scatterers are embedded into an elastic matrix or vice versa. The third type is the externally bonded piezoelectric composite PCs/AMs which consist of the piezoelectric patches attached to the surfaces of an elastic base structure (rod, beam, plate. etc.). Then, based on this classification and according to the dimensional periodicity as well as the tuning method and the tuning target, the state-of-the-art research topics on the different smart piezoelectric PCs/AMs and their potential applications are briefly reviewed. Previous research demonstrates that compared to the conventional elastic wave tuning methods, the essential advantages of the smart piezoelectric PCs/AMs based on the piezoelectric or electro-mechanical coupling effect are characterized by their quick responses and the unnecessary changes of the original structural configuration and composition. The tuning method only based on the piezoelectric effect but without an external electric circuit or active control has a rather limited tuning impact on the mechanical vibration and elastic waves, and hence a relatively low efficiency. However. the utilization of the external electric circuit or active control can broaden the Bragg scattering band-gaps, generate novel local resonance band-gaps, merge the Bragg scattering and the local resonance bandgaps, and efficiently achieve the abovementioned tuning objectives of the novel elastic wave characteristics. Finally, based on the findings of the previous research, some conclusions on the state-of-the-art of the research are drawn, several still existing problems are mentioned. and an outlook on the future research demands, expectations and development trend is also given.