Maxwell Displacement Research Articles

Flexoelectricity, Triboelectricity, and Free Interfacial Charges.

Triboelectricity has been a topic of some confusion for many years, probably because it is very diverse and some of the fundamental science has not been clear. This is now starting to change. A few years ago, the importance of flexoelectricity at asperities is pointed out. That paper exploited the established physics of compensation of bound surface or interfacial charges without going into detail. The purpose of this paper is to expand further on this, mapping from the established physics of electrostatics with contact potentials and Maxwell's displacement field to the underlying fundamentals of charge transfer in triboelectricity. Examples from the published literature are used to illustrate this. In the discussion, some of the open questions and challenges to the community are mentioned.

Positive impact of surface defects on Maxwell's displacement current-driven nano-LEDs: The application of TENG technology

As the core component of a nanopixel light-emitting display, the GaN-based nanoscale light-emitting diode (nLED) faces the problem of low electroluminescence efficiency resulting from the introduction of surface defects when its lateral size is reduced to the nanometer scale. Thus, reducing the surface defect density is an important direction in nLED-related research. This study, with the triboelectric nanogenerator-driven LED as its inspiration, reveals that surface defects have a positive impact on the performance of nLEDs driven by Maxwell’s displacement current, and we call the related driving mode the noncarrier injection mode. Through finite element simulations, we studied the dynamic variations of the carrier concentration, the energy band, and the light emission rate to analyze the impact of the behavior of surface defect excitation on device performance. We found that surface defects can act as electron pumps under the combined effect of the reverse electric field and the built-in electric field and can generate carriers through surface defect excitation to increase the intensity of noncarrier injection luminescence, which is completely different from the traditional understanding of surface defects. In addition, we propose a tapered structure to further increase the light emission rate by regulating the behaviors of radiation recombination and surface defect excitation. The results of this work open a new perspective on the impacts of surface defects on nLEDs and provide significant information for additional applications of Maxwell's displacement current.

The coupled-motion enhanced wireless signal transmission with long distance based on Maxwell’s displacement current

Wireless signal transmission plays an increasingly imperative role in numerous aspects of modern society, but it still remains challenging to achieve it with low-cost and efficient way. Herein, we demonstrate a long-distance wireless signal transmission system, which mainly includes an electrodeless triboelectric nanogenerator coupled with linear motion and rotational motion (LR-TENG) as a transmitter and a receiver located at a distance. Based on the varying electric field originated from Maxwell's displacement current, the maximum transmission distance of LR-TENG can reach 86 cm under the external excitation of 1 Hz, creating the highest record among the relevant researches. In addition, the influence of obstacle type, thickness, size and position on signal transmission has been systematically investigated for the first time, and the distribution of time-varying electric field in space is also analyzed qualitatively. Furthermore, a Labview interface is developed for accurate positioning in complex scenes relying on the received signals from multiple receivers at different positions. This work illustrates a simple and feasible design method for extending the distance of wireless signal transmission based on TENG, and promotes its application in the field of wireless communication.

Momentum Transfer in Triboelectric Nanogenerators

AbstractThe triboelectric nanogenerator (TENG) reflects a prospering field, that uses Maxwell's displacement current as the driving force to transform mechanical energy into electricity. Based on the law of energy conservation, many theoretical models of TENGs are proposed to provide a detailed insight into how energy flows and transformation in the energy harvesting system, while ignoring a hidden but extremely important point about TENG's momentum transfer and conservation. Here a series of analysis is presented for the momentum transfer and conservation in TENGs based on Maxwell equations and stress tensor. Using a time‐dependent 3D mathematical model, it is elaborated that how the time‐ and spatial‐dependent momentum current is influenced by the field and the dielectric materials, demonstrating that momentum is overall conserved for a TENG. In other words, the TENG device can not only convert mechanical energy into electricity, but it is also able to transfer momentum. Momentum transfer is another important characteristic of TENGs, and finally, the essential differences and similarities among the momentum transfer, energy transfer, and energy transformation in TENGs are systematically discussed. This study will certainly serve as a new starting point for exploring momentum transfer and conservation in the TENG momentum transfer system.

Magnetic field assisted high performance triboelectric nanogenerators based on P(VDF–HFP)/NiFe2O4 nanofiber composite

Triboelectric nanogenerators (TENGs), which are powered by Maxwell's displacement current, have a wide range of applications, including energy harvesting and self-powered sensors. However, very little attention has been paid to the effect of the ferromagnetic medium on TENG displacement current. Herein, flexible ferrimagnetic poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF–HFP))/nickel ferrite (NiFe2O4) fiber composite film based TENGs were fabricated and their performances were analysed. Highly crystalline, ferrimagnetic NiFe2O4 nanofibers with high aspect ratio were prepared using electrospinning technique by systematic optimization of spinning parameters. Thereafter, the fibers were reinforced into the polymeric matrix to improve the β–phase and dielectric constant which in turn significantly enhanced the TENG performance. The β–phase enhancement in the composite films was confirmed by XRD and FTIR analysis; 77 % of the β–phase was achieved with the incorporation of NiFe2O4 fiber. Morphological features were examined using SEM and AFM, which revealed more β–phase spherulite formation in the composite films. A notable increase in the dielectric properties was evidenced with the addition of NiFe2O4 fibers and a maximum dielectric constant of∼26 was attained at 100 Hz. The TENG was developed using P(VDF-HFP)/NiFe2O4 and nylon which generated an output voltage (Vpp) and current (Ipp) values of 584 V and 25 μA, respectively. Furthermore, the performance of TENG under an external magnetic field was examined, and a notable improvement in performance was observed, paving the way for the application of NiFe2O4 fibers in hybrid energy harvesters and self-powered sensors.

Optimization of polarization behavior enabled by multiscale heterostructure design for excellent energy storage performance in Na0.5Bi0.5TiO3-based relaxor ferroelectric thin film

Dielectric capacitors are promising candidates in the field of pulse power systems due to their ultrafast charge/discharge speed and high power density. A major challenge, however, is to improve the recoverable energy storage density (Wrec) and energy storage efficiency (η). In this work, Na0.5Bi0.5(Ti,Fe)O3 (NBFT)-based system is selected and the equivalent circuit of the corresponding capacitors is studied according to the Maxwell's displacement current hypothesis, providing a way of optimizing the polarization behavior by multiscale heterostructure design via adjusting the specific arrangement of layers and interface within the NBFT-based capacitors. By introducing heterogeneous components of (Ba,Sr)TiO3 (BST) and Na0.5Bi0.5(Ti,Zr)O3 (NBZT), abundant heterogeneous interfaces are obtained for NBFT/BSxT and NBFT/NBZT heterostructure thin films. Both the thin films possess superior breakdown strength (EBD) and polarization response (Pm–Pr) than that of NBFT, which is highly associated with the interfacial resistance, improvement of local component heterogeneity, changes in domain wall motion and Von Mises stress/strain distribution. Notably, the energy storage performance is modulated for NBFT/NBZT with enhanced relaxor ferroelectricity as the heterogeneous interface is increased with Wrec and η of 66.57 J/cm3 and 64.33%, respectively. These findings prove effective ways of optimizing NBT-based relaxor ferroelectric thin film for energy storage devices.

Nickel Metal‐Organic Framework/PVDF Composite Nanofibers‐based Self‐Powered Wireless Sensor for Pulse Monitoring of Underwater Divers via Triboelectrically Generated Maxwell's Displacement Current

AbstractWith the increasing need for underwater exploration to monitor environment, collect data or recreation, underwater diving and its associated technologies have drawn significant attention. So, a single‐electrode triboelectric nanogenerator (STENG) as self‐powered wireless sensor based on Maxwell displacement current concept for underwater usages to monitor arterial pulse and body movements of diver is demonstrated. The STENG is fabricated with an optimized 7.5 wt.% of hydrothermally synthesized Ni‐MOF and PVDF based composite nanofibers (Ni‐MOF/PVDF CNF) on copper foil as electronegative tribo‐material and electrospun nylon 66 nanofibers as electropositive tribo‐material. The PDMS encapsulation of STENG exhibits water‐proof properties with Voc, Isc, and charge output at 45 V, 0.77 µA, and 0.169 µC/cycle, respectively under single‐finger tapping. As pulse sensor, STENG monitors arterial pulse and wirelessly transmits information underwater (without external power), wherein STENG generated current creates polarized field in water to get transmitted within a certain distance. Additionally, STENG is demonstrated as a strain sensor to monitor joint movements like ‘elbow, shoulder, knee, etc. Detailed reliability studies reveal that the device is robust against multiple cycles of bending, retains its performance in saline/muddy water, and in the presence of obstacles in the transmission path, which confirms its potential in real life conditions for pulse monitoring of underwater divers.

New insights into Maxwell’s equations based on new experimental discoveries

No theory is absolute as scientific exploration is the product of human ingenuity, which will continue to evolve with the development of new discoveries and technology.It has been established that the electric field is produced by friction, and the changing electric field can be then generated by constant friction. After the friction stops, this electrostatic field will exist for a designated period of time or even longer. Accordingly, a series of experiments were designed to test the proposed Collision Electro–Magnetic Theory (CEMT) that the electromagnetic energy can be generated by the collision. The 1-W bulb can be lighted by collecting the changing electric field energy when a polymethyl methacrylate (PMMA) board was rubbed with polytetrafluoroethylene (PTFE) film by hand. When a closed coil was placed in the changing electric field, an induced current and an induced electromotive force were generated in the coil, which also magnetized the scissors in the coil. The 1-W bulb can produce a current by the generated changing electric field, when one end of the plastic rods was rubbed by fur.In addition, we discovered that shaking the plastic rod with an electrostatic field generated by the friction can also generate a changing electric field, which not only wirelessly lighted the 28-W fluorescent tube lamp, but also produced an induced current and an induced electromotive force in the closed coil nearby the lamp.Based on the above experiments, we have the following speculation: just as the unified theory of electromagnetism suggests, the changing electric field produces a magnetic field, and the varying magnetic field produces an electric field. Stated plainly, the electric field is in fact a magnetic field and the magnetic field is also an electric field, so they can be both be regarded as energy fields in essence. The coil and conductor can be regarded as the energy collector that can collect energy from the energy field, as long as there is a constantly changing energy field in the collector. The conduction current is transmitted in the collector, while part of the energy field that is not collected in free space, which corresponds to the displacement current calculated in the fourth equation of Maxwell's equations.The essence of Maxwell's equations and related laws are two different phenomena resulting from an energy field in the closed-loop self-circulating state or open-loop acyclic state. The first equation describes an open-loop static energy field, and the second equation describes a closed-loop static energy field. The third equation describes how the induced conduction current can be generated in the closed-loop energy field, and the fourth equation describes how the induced conduction current can be generated in the open-loop energy field.Based on the new experimental discoveries, this work interprets Faraday's law of electromagnetic induction, Maxwell's displacement current and Maxwell's equations from a new perspective, which is important to understand the nature of Maxwell's equations systematically.

Collecting the space-distributed Maxwell's displacement current for ultrahigh electrical density of TENG through a 3D fractal structure design

A three-dimensional fractal structured nanogenerator (FSNG), which can efficiently collect the space-distributed Maxwell's displacement current and significantly improve the electrical energy density of TENG, has been developed.

Self-powered sensors driven by Maxwell's displacement current wirelessly provided by TENG

Electrodeless triboelectric nanogenerator (EL-TENG) is a kind of novel TENG that has no metal electrodes and the power is transmitted via Maxwell's displacement current. For a rotation EL-TENG, the power is collected by an induction coil at a vertical distance of 20 cm. A self-powered sensor is demonstrated based on Maxwell's displacement current supplied wirelessly by EL-TENG. Such a design can use one EL-TENG to drive multiple wireless senses for the purposes such as control switches for desk lamps and alarm system. This kind of wireless and self-powered sensor has broad applications in the fields of Internet of Things, big data and artificial intelligence.

Piezoelectric Nanogenerators Derived Self‐Powered Sensors for Multifunctional Applications and Artificial Intelligence

AbstractWith the arrival of the Internet of Things (IoTs) era, there is a growing requirement for systems with many sensor nodes in a variety of fields of applications. The demands for wireless, sustainable and independent operation are becoming more and more important for large‐scale sensor networks and systems. For these purposes, a self‐powered sensory system that can utilize the self‐harvested energy from its surroundings to drive the sensors and directly sense external stimuli has attracted great attention. The invention and rapid development of piezoelectric generators (PENGs), which take Maxwell's displacement current as the driving force, has been pushing forward research on self‐powered active mechanical sensors, electronic skins, and human‐robotic interaction. Here, this review starts with a brief introduction of piezoelectric materials, fabrication, and performance improvement. Then, the energy harvesters used for self‐power systems based on recent progress are reviewed. After that, PENGs applications toward recent self‐powered active sensors are divided into four aspects and highlighted, respectively. Moreover, some challenges and future directions for the self‐powered multifunctional sensors are put forward. It is believed that through the continuous investigations into PENG‐based self‐powered active sensors, they will soon be used in touch screens, electronic skins, health care, environmental monitoring, and intelligence systems.

Nonpolar Liquid Lubricant Submerged Triboelectric Nanogenerator for Current Amplification via Direct Electron Flow

AbstractTriboelectric nanogenerators (TENGs) can convert a mechanical energy input to an electric energy output through Maxwell's displacement current. By increasing the electrical output and overcoming the mechanical limitations of TENGs, such devices can be used as an auxiliary power source for portable electronics. Nevertheless, the generation mechanism and structure must be optimized to compensate for the electrical and mechanical limitations of TENGs. This paper reports on a nonpolar liquid lubricant submerged TENG (LLS‐TENG), which can overcome the existing electrical and mechanical limitations of the TENG. When a nonpolar liquid lubricant is filled in the LLS‐TENG, the air breakdown can be effectively blocked owing to the large Debye length of such lubricants. In addition, the rolling friction and lubrication present in the LLS‐TENG can significantly reduce the friction wear of the device. Consequently, the LLS‐TENG can charge a commercial capacitor and battery by generating a high voltage and current output of up to 200 V and 170 mA, respectively.

Designing Rules and Optimization of Triboelectric Nanogenerator Arrays

AbstractA triboelectric nanogenerator (TENG) is an effective means for the conversion of mechanical energy into electricity. Although Maxwell's displacement current is the underline mechanism of TENGs, the spatial and temporal variations of the electric field and electric displacement remain elusive, which prohibits an effective optimization of the energy conversion process. Here, the electric field distribution and energy dynamics of TENGs is determined using 3D mathematical modeling. The electrical energies stored in TENGs and extracted into the external circuit are calculated quantitatively whereby the ratio of the two, defined as the output efficiency, is obtained. Then, the power density and energy density of TENGs are defined. Utilizing the principle of virtual work, the minimum required external force–time relationship is evaluated. The influence of device parameters, geometry, and optimum conditions are discussed systematically so as to determine general optimization guidelines for TENGs. In addition, although the fringing electric field is practically inevitable, adjustments of the gap distance between neighboring TENG devices to assure that an optimized fringing electric field is “leaked”, is demonstrated to lead to improvement in a TENG array. Then, for the first time, this work presents universal design rules and holistic optimization strategies for the network structure of TENGs.

Recent Progress in Hybridized Nanogenerators for Energy Scavenging.

SummaryAs the world's demand for alternative energy increases, the development of green energy harvesters becomes ever more important. As a result, the creation of triboelectric (TENG), piezoelectric (PENG), and pyroelectric nanogenerators, electromagnetic generators (EMG), solar cells, and electrochemical cells is attracting interest in an effort to convert mechanical, thermal, magnetic, solar, and chemical energy into electricity. In order to take advantage of the ambient energies from our surrounding environment, the design of hybridized generator units that can simultaneously scavenge energy in a variety of forms continues to develop. These systems are being considered to satisfy the energy needs of a range of electronic devices and adapt to a variety of working environments. This review demonstrates the latest progress in hybridized nanogenerators in accordance with their structure, operating principle, and applications. These studies demonstrate new approaches to developing hybrid techniques and novel assemblies for efficiently harvesting environmental energy from a number of sources.

Triboelectric and Piezoelectric Nanogenerators for Future Soft Robots and Machines.

SummaryThe triboelectric nanogenerator (TENG) and piezoelectric nanogenerator (PENG) are two recently developed technologies for effective harvesting of ambient mechanical energy for the creation of self-powered systems. The advantages of TENGs and PENGs which include large open-circuit output voltage, low cost, ease of fabrication, and high conversion efficiency enable their application as new flexible sensors, wearable devices, soft robotics, and machines. This perspective provides an overview of the current state of the art in triboelectric and piezoelectric devices that are used as self-powered sensors and energy harvesters for soft robots and machines; hybrid approaches that combine the advantages of both mechanisms are also discussed. To improve system performance and efficiency, the potential of providing self-powered soft systems with a degree of multifunctionality is investigated. This includes optical sensing, transparency, self-healing, water resistance, photo-luminescence, or an ability to operate in hostile environments such as low temperature, high humidity, or high strain/stretch. Finally, areas for future research directions are identified.

Theoretical modeling of triboelectric nanogenerators (TENGs)

Triboelectric nanogenerators (TENGs), using Maxwell's displacement current as the driving force, can effectively convert mechanical energy into electricity. In this work, an extensive review of theoretical models of TENGs is presented. Based on Maxwell's equations, a formal physical model is established referred to as the quasi-electrostatic model of a TENG. Since a TENG is electrically neutral at any time owing to the low operation frequency, it is conveniently regarded as a lumped circuit element. Then, using the lumped parameter equivalent circuit theory, the conventional capacitive model and Norton's equivalent circuit model are derived. Optimal conditions for power, voltage, and total energy conversion efficiency can be calculated. The presented TENG models provide an effective theoretical foundation for understanding and predicting the performance of TENGs for practical applications.

Magneto-optical measurements of Maxwell displacement current

The concept of Maxwell's displacement current is still not fully understood. In this paper, we measured the magnetic field between the plates of a charging capacitor using a magneto-optical method, which is generally explained to be produced by Maxwell's displacement current. We observed the asymmetric radial profile of the magnetic field between them, which could not be explained by using the displacement current. In contrast, it could be explained naturally, if we assumed that the magnetic field was produced by the practically existing conduction currents, which consist of the surface currents on the plates, the current in the charging wires connected to it and the return current connected to the electric power supply.

Three-dimensional modeling of alternating current triboelectric nanogenerator in the linear sliding mode

Energy harvesting from mechanical motions has immense applications such as self-powered sensors and renewable energy sources powered by ocean waves. In this context, the triboelectric nanogenerator is the cutting-edge technology that can effectively convert ambient mechanical energy into electricity through the Maxwell's displacement current. While further improvements of the energy conversion efficiency of triboelectric nanogenerators critically depend on theoretical modeling of the energy conversion process, to date only models based on single-relative-motion processes have been explored. Here, we analyze energy harvesting of triboelectric nanogenerators using a three-dimensional model in a linear-sliding mode and demonstrate a design of triboelectric nanogenerators that have a 77.5% enhancement in the average power in comparison with previous approaches. Moreover, our model shows the existence of a DC-like bias voltage contained in the basic AC output from the energy conversion, which makes the triboelectric nanogenerators an energy source more pliable than the traditional AC power generation systems. The present work provides a framework for systematic modeling of triboelectric nanogenerators and reveals the importance of obtaining direct analytical insight in understanding the current output characteristics of the triboelectric nanogenerators. Incorporating our model analysis in future designs of triboelectric nanogenerators is beneficial for increasing the energy conversion power and may provide insights that can be used in engineering the profile of the output current of the nanogenerators.

Dynamical charge transfer model for high surface charge density triboelectric nanogenerators

Triboelectric nanogenerators have made great progress and paved the way for green energy, self-powered implantable medical devices and a large number of sensors in the Internet of Things. Maxwell's displacement current is the fundamental theoretical framework of nanogenerators. Quantum model of charge transfer between atoms have been devoted to understanding the triboelectric effect. Increasing the output power of triboelectric nanogenerators is one of the most important subjects for practical application. Here, we study the charge transfer process in triboelectric materials and propose a triboelectric material charge transfer dynamic model. Dynamic charge transfer enables triboelectric nanogenerators to achieve high surface charge density and the superior device robustness, which can significantly improve the output power and life time of triboelectric nanogenerators. The charge transfer model provides not only deeply understanding for high performance of triboelectric nanogenerator, but also a useful guide for the optimization and design of high performance triboelectric materials.

On the first principle theory of nanogenerators from Maxwell's equations

Nanogenerators (NGs) are a field that uses Maxwell's displacement current as the driving force for effectively converting mechanical energy into electric power/signal, which have broad applications in energy science, environmental protection, wearable electronics, self-powered sensors, medical science, robotics and artificial intelligence. NGs are usually based on three effects: piezoelectricity, triboelectricity (contact electrification), and pyroelectricity. In this paper, a formal theory for NGs is presented starting from Maxwell's equations. Besides the general expression for displacement vector D = εE used for deriving classical electromagnetic dynamics, we added an additional term Ps in D, which represents the polarization created by the electrostatic surface charges owing to piezoelectricity and/or triboelectricity as a result of mechanical triggering in NG. In contrast to P that is resulted from the electric field induced medium polarization and vanishes if E = 0, Ps remains even when there is no external electric field. We reformulated the Maxwell equations that include both the medium polarizations due to electric field (P) and non-electric field (such as strain) (Ps) induced polarization terms, from which, the output power, electromagnetic behavior and current transport equation for a NG are systematically derived. A general solution is presented for the modified Maxwell equations, and analytical solutions about the output potential are provided for a few cases. The displacement current arising from ε∂E/∂t is responsible for the electromagnetic waves, while the newly added term ∂Ps/∂t is the application of Maxwell's equations in energy and sensors. This work sets the first principle theory for quantifying the performance and electromagnetic behavior of a nanogenerator in general.