力电耦合主动超材料及其弹性波调控

Elastic metamaterials can be used to manipulate wave propagation in many unprecedented ways. Passive metamaterials are first being widely studied and have been used to prohibit wave propagation, realize negative refraction, focus waves or achieve cloaking effects. Such abilities make them very useful in applications like low-frequency vibration and noise reduction, structural health monitoring and wave energy harvesting. However, passive metamaterials are difficult to be modified after being fabricated, which restricts their practical applications in various engineering situations. Electromechanical metamaterials have the abilities to overcome such limitation. Unlike passive metamaterials, the unconventional properties of which mainly come from the components and micro-structures, the properties of electromechanical metamaterials can also be modified via external electric fields. On one hand, one can obtain extraordinary wave propagation effects in electromechanical metamaterials with properly designed coupling effects between the metamaterial and electric field. On the other hand, the properties of such metamaterials can be tuned by modifying the external electric field. In this review, the basic conceptions of electromechanical metamaterials are introduced first. According to the features of the applied external electric fields, electromechanical metamaterials are then divided into two different types. Research history and trend of them are introduced, respectively. This review mainly clarifies three basic issues concerning each type of electromechanical metamaterials: What is the mechanism behind the unprecedented properties realized with electric field? How to design the unit cell for the applied electric field on the metamaterial? What kind of wave controls can be achieved? The first type of electromechanical metamaterials is realized by periodically bonding piezoelectric patches on the surfaces of structures, like beams and plates. The patches are shunted with external circuits. When inductors are used in the shunts, there will be low-frequency bandgaps in the metamaterials, which are results of the resonances of the shunts. The patches can also be shunted with negative capacitances to design metamaterials with tunable properties. Also, the shunts can be interconnected to form electrical networks. In such metamaterials, energy can be transported in the mechanical structures in the form of elastic waves; it can also be spread in the form of electromagnetic waves, which propagate in the electric network. Waves in the mechanical part and electrical part interact with each other, leading to peculiar wave propagation phenomena, such as coupled bandgap, nonreciprocal wave propagation. The second type of electromechanical metamaterials is designed using active control strategies. In a basic active control loop, there are sensor, actuator and controller. The sensor measures a particular signal, which is used as the input of the controller. The controller generates output signals according to the input signal and the implemented control law, and such output is applied on the actuator to control the properties of the metamaterial. By precisely designing the measured and controlled physical quantities, as well as the control law, one can design programmable metamaterials, whose properties can be modified in real-time. Also, one can realize programmable meta-layers to control the elastic wave propagation. Even the momentum or potential energy conservations of materials can be violated with active designs to obtain odd micropolar elasticity. Finally, it is suggested that, the future studies on electromechanical metamaterials should be focused on multi-functional and adaptive materials, exploring more unconventional wave propagation effects, which are difficult to be realized using passive metamaterials. Also, to overcome the drawbacks of current electromechanically metamaterials, efforts should be devoted to design metamaterials with broadband and low-frequency wave control properties. Dynamic homogenization methods and efficient numerical tools to predict wave propagation properties should also be developed to facilitate the development of electromechanical metamaterials.