Actively tunable acoustic metamaterials have attracted ever increasing attention. However, their tunable frequency range is quite narrow (tens of Hz) even under ultrahigh applied voltage (about 1,000 V). Here, we report a superbroad-band actively tunable acoustic metamaterials with the bandwidth over 400 Hz under a low voltage. In the actively tunable acoustic metamaterials, the acoustic membrane is a laminated nanocomposite consisting of a poly (ethylene terephthalate) (PET) and super-aligned carbon nanotube (CNT) drawn from CNT forest array. The laminated nanocomposite membrane exhibits adjustable acoustic properties, whose modulus can be adjusted by applying external electric field. The maximum frequency bandwidth of PET/CNT nanocomposite membrane reaches 419 Hz when applying an external DC voltage of 60 V. Our actively tunable acoustic metamaterials with superbroad-band and lightweight show very promising foreground in noise reduction applications.

Energy storage devices with high energy and power densities are highly attractive for various applications ranging from portable electronics to electric vehicles and grid-level energy storage, such as rechargeable batteries and supercapacitors. One limiting factor in power density is the ion transport in electrolyte, particularly in tortuous electrode materials with low porosity. A viable approach to enhance ion transport in electrolyte is to create vertically aligned structures and thus reduce electrode tortuosity. In the past decades, various methods have been explored to develop vertically aligned structures. This review summarizes battery kinetics to illustrate the importance of low tortuosity in electrodes, and then introduces various methods to create vertically aligned nanostructures, such as direct growth, templating and microfabrications. The electrochemical performance of electrodes or electrolytes created by each method is presented. At the end, this paper discusses challenges with these structures and the directions these technologies can be taken in the future.