The triboelectric effect is ubiquitous in our everyday life and results from two different materials coming into contact. It is generally regarded as a negative effect in industry given that the electrostatic charges induced from it can lead to ignition, dust explosions, dielectric breakdown, electronic damage, etc. From an energy point of view, those electrostatic charges constitute a capacitive energy device when the two triboelectric surfaces are separated, which led to the invention of early electrostatic generators such as the “friction machine” and Van de Graaff generator.[1] As the world is marching into the era of the internet of things (IoTs) and artificial intelligence, the most vital development for hardware is a multifunctional array of sensing systems, which forms the foundation of the fourth industrial revolution toward an intelligent world. Given the need for mobility of these multitudes of sensors, the success of the IoTs calls for distributed energy sources, which can be provided by solar, thermal, wind, and mechanical triggering/vibrations. The triboelectric nanogenerator (TENG) for mechanical energy harvesting developed by Z.L. Wang’s group is one of the best choices for this energy for the new era, since triboelectrification is a universal and ubiquitous effect with an abundant choice of materials. The development of self-powered active sensors enabled by TENGs is revolutionary compared to externally powered passive sensors, similar to the advance from wired to wireless communication. In this paper, the fundamental theory, experiments, and applications of TENGs are reviewed as a foundation of the energy for the new era with four major application fields: micro/nano power sources, self-powered sensors, large-scale blue energy, and direct high-voltage power sources. A roadmap is proposed for the research and commercialization of TENG in the next 10 years.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/aenm.201802906

Triboelectric Nanogenerator: A Foundation of the Energy for the New Era

Although the contact electrification (CE) (or usually called 'triboelectrification') effect has been known for over 2600 years, its scientific mechanism still remains debated after decades. Interest in studying CE has been recently revisited due to the invention of triboelectric nanogenerators (TENGs), which are the most effective approach for converting random, low-frequency mechanical energy (called high entropy energy) into electric power for distributed energy applications. This review is composed of three parts that are coherently linked, ranging from basic physics, through classical electrodynamics, to technological advances and engineering applications. First, the mechanisms of CE are studied for general cases involving solids, liquids and gas phases. Various physics models are presented to explain the fundamentals of CE by illustrating that electron transfer is the dominant mechanism for CE for solid-solid interfaces. Electron transfer also occurs in the CE at liquid-solid and liquid-liquid interfaces. An electron-cloud overlap model is proposed to explain CE in general. This electron transfer model is extended to liquid-solid interfaces, leading to a revision of the formation mechanism of the electric double layer at liquid-solid interfaces. Second, by adding a time-dependent polarization termPscreated by the CE-induced surface electrostatic charges in the displacement fieldD, we expand Maxwell's equations to include both the medium polarizations due to electric field (P) and mechanical aggitation and medium boundary movement induced polarization term (Ps). From these, the output power, electromagnetic (EM) behaviour and current transport equation for a TENG are systematically derived from first principles. A general solution is presented for the modified Maxwell's equations, and analytical solutions for the output potential are provided for a few cases. The displacement current arising frome∂E/∂t is responsible for EM waves, while the newly added term ∂Ps/∂t is responsible for energy and sensors. This work sets the standard theory for quantifying the performance and EM behaviour of TENGs in general. Finally, we review the applications of TENGs for harvesting all kinds of available mechanical energy that is wasted in our daily life, such as human motion, walking, vibration, mechanical triggering, rotating tires, wind, flowing water and more. A summary is provided about the applications of TENGs in energy science, environmental protection, wearable electronics, self-powered sensors, medical science, robotics and artificial intelligence.

From contact electrification to triboelectric nanogenerators.

With the speedy development of sensor network, along with the increasing demand or wearable and ubiquitous “smart” devices, a tremendous quantity of small electronic devices has been widely applied all over the world. These small electronics include sensors, wireless transmitters, and/or actuators, etc. Generally, the power requirement for each of these small electronics is around milliwatt (mW) or microwatt (μW) range, with features such as low cost, mobility, light-weight, and sustainability. A conventional and most commonly used approach to provide the power for these electronics is installing batteries. Nevertheless, batteries have limited life spans. As the amount of the electronics increases Triboelectric nanogenerators are devices that effectively convert ambient mechanical energy into electricity, which can be used as a power source or a sensor signal. Since first proposed by Dr. Zhong Lin Wang in 2012, the development of triboelectric nanogenerators has grown rapidly. Herein, the development of the triboelectric nanogenerator and its global impact is investigated by analyzing the statistical publication numbers and the geographic distribution of the publications. In addition, this article also features the main applications of triboelectric nanogenerators such as blue energy, self-powered sensor/systems, and micro-/nanoenergy, and points out its future outlook. Several challenges and fundamental physical questions are also discussed to provide a more comprehensive view of this revolutionary technology. Triboelectric Nanogenerator

The Current Development and Future Outlook of Triboelectric Nanogenerators: A Survey of Literature

We propose a framework to realize helical edge states in phononic systems using two identical lattices with interlayer couplings between them. A methodology is presented to systematically transform a quantum mechanical lattice which exhibits edge states to a phononic lattice, thereby developing a family of lattices with edge states. Parameter spaces with topological phase boundaries in the vicinity of the transformed system are illustrated to demonstrate the robustness to mechanical imperfections. A potential realization in terms of fundamental mechanical building blocks is presented for the hexagonal and Lieb lattices. The lattices are composed of passive components and the building blocks are a set of disks and linear springs. Furthermore, by varying the spring stiffness, topological phase transitions are observed, illustrating the potential for tunability of our lattices.

Helical edge states and topological phase transitions in phononic systems using bi-layered lattices

Flexible resonant acoustic sensors have attracted substantial attention as an essential component for intuitive human-machine interaction (HMI) in the future voice user interface (VUI). Several researches have been reported by mimicking the basilar membrane but still have dimensional drawback due to limitation of controlling a multifrequency band and broadening resonant spectrum for full-cover phonetic frequencies. Here, highly sensitive piezoelectric mobile acoustic sensor (PMAS) is demonstrated by exploiting an ultrathin membrane for biomimetic frequency band control. Simulation results prove that resonant bandwidth of a piezoelectric film can be broadened by adopting a lead-zirconate-titanate (PZT) membrane on the ultrathin polymer to cover the entire voice spectrum. Machine learning-based biometric authentication is demonstrated by the integrated acoustic sensor module with an algorithm processor and customized Android app. Last, exceptional error rate reduction in speaker identification is achieved by a PMAS module with a small amount of training data, compared to a conventional microelectromechanical system microphone.

https://advances.sciencemag.org/content/advances/7/7/eabe5683.full.pdf

Biomimetic and flexible piezoelectric mobile acoustic sensors with multiresonant ultrathin structures for machine learning biometrics.

We demonstrate the design, fabrication, evaluation, and use of a self-powered microphone that is thin, flexible, and easily manufactured. Our technology is referred to as a Self-powered Audio Triboelectric Ultra-thin Rollable Nanogenerator (SATURN) microphone. This acoustic sensor takes advantage of the triboelectric nanogenerator (TENG) to transform vibrations into an electric signal without applying an external power source. The sound quality of the SATURN mic, in terms of acoustic sensitivity, frequency response, and directivity, is affected by a set of design parameters that we explore based on both theoretical simulation and empirical evaluation. The major advantage of this audio material sensor is that it can be manufactured simply and deployed easily to convert every-day objects and physical surfaces into microphones which can sense audio. We explore the space of potential applications for such a material as part of a self-sustainable interactive system.

SATURN: A Thin and Flexible Self-powered Microphone Leveraging Triboelectric Nanogenerator

Quasiperiodic metastrucures are characterized by edge localized modes of topological nature, which can be of significant technological interest. We here investigate such topological modes for stiffened and sandwich beams, which can be employed as structural members with inherent vibration localization capabilities. Quasiperiodicity is achieved by altering the geometric properties and material properties of the beams. Specifically, in the stiffened beams, the geometric location of stiffeners is modulated to quasiperiodic patterns, while, in the sandwich beams, the core’s material properties are varied in a step-wise manner to generate such patterns. The families of periodic and quasiperiodic beams for both stiffened and sandwich-type are obtained by varying a projection parameter that governs the location of the center of the stiffener or the alternating core, respectively. The dynamics of stiffened quasiperiodic beams is investigated through 3-D finite element simulations, which leads to the observation of the fractal nature of the bulk spectrum and the illustration of topological edge modes that populate bulk spectral bandgaps. The frequency spectrum is further elucidated by employing polarization factors that distinguish multiple contributing modes. The frequency response of the finite stiffened cantilever beams confirms the presence of modes in the non-trivial bandgaps and further demonstrates that those modes are localized at the free edge. A similar analysis is conducted for the analysis of sandwich composite beams, for which computations rely on a dynamic stiffness matrix approach. This work motivates the use of quasiperiodic beams in the design of stiffened and sandwich structures as structural members in applications where vibration isolation is combined with load-carrying functions.

Dynamics of Quasiperiodic Beams

Modeling of initially curved and twisted smart beams using intrinsic equations

As the power requirements for microelectronics continue decreasing, environmental energy sources can begin to replace batteries in certain wearable subsystems. In this paper we report on a shoe power generator based on a micro structured piezoelectric polymer transducer. The properties of the transducer match regular shoe filling and it is designed to harness energy from human walk without affecting the user's gait. The generated power is regulated with a self-powered dc/dc conversion circuit that achieves a high 65% efficiency for converting the high piezo electrically generated voltage (130 V) to a low voltage (4 V) suitable for portable electronic devices.

Mohit Gupta

Papers

SATURN: A Thin and Flexible Self-powered Microphone Leveraging Triboelectric Nanogenerator

Dynamics of Quasiperiodic Beams

Modeling of initially curved and twisted smart beams using intrinsic equations

Mobile charger via walk

Modeling Thin-Walled Beams using VAM