A method of charge measurement for contact electrification

A method for studying contact electrification charge between different materials was developed. Physical models for the contact electrification measurement system of metal/metal, metal/insulator and insulator/insulator were proposed, where the relationships between charge and measuring potential were developed. According to the models, an electrification charge measurement system was built. As an example of using the method, contact electrification experiment between polytetrafluoroethylene (PTFE) and carbon steel plates was conducted. Comparison of the charge results by this method and Faraday cup method was made, which suggested that the current method reduced the error resulted from the charge dissipation.     

Contact electrification between polymers and steel

Electrification occurs when metals are put into contact with polymers, and the mechanism is still not fully understood. In this paper, experimental study of contact electrification between a metal and polymers was conducted. Effects of contact cycle, load, and nominal area on electrification were investigated. Results showed that electrification charge increased with real contact area. However, experimental results showed that charge density increased with nominal area and decreased with load. The key factor that determined the charge density was contact stress. Charge density decreased with contact stress linearly. A quantitative relationship between charge density and contact stress was proposed.     

First-principles calculation of contact electrification and validation by experiment

Contact electrification is one of the most well-known phenomena in physics and examples arise in almost every industry. However, a scientific basis for contact charging remains unknown. Here, we present a theoretical study of contact electrification, supported by experiments, to calculate for the first time charge transfer between material surfaces from first principles physics. Electronic structure calculations and experiments are performed on single-crystal alumina (sapphire) and silicon oxide (quartz) surfaces, which have well-ordered structures that enable rigorous modeling. Both experiments and calculations show that sapphire charges positively and quartz charges negatively. The calculations cannot determine the magnitude of charge densities remaining on separated surfaces from first principles, as these are non-equilibrium effects, but our analysis is consistent with experimentally obtained charge densities of 10 μC/m². These results indicate the possibility of quantitatively predicting and explaining contact electrification from only the molecular structure of material surfaces.     

Contact electrification phenomena on phosphor particle surfaces

Contact electrification phenomena are reviewed to understand phosphor powder behavior. The origin of contact electrification is described in terms of the acid–base concept for solid surfaces. Electric charges produced by contact electrification affect phosphor properties. Various examples related to contact electrification are shown: particle surface modification, dispersion in liquids, adhesion strength to a substrate, electrostatic coating, interaction between a phosphor and mercury in a fluorescent lamp, and electron emission ability in a fluorescent lamp.     

First-principles study of electronic properties of interfacial atoms in metal-metal contact electrification

The mechanism of contact electrification between metals was studied using the first-principles method, taking the Ag-Fe contact as an example. Charge population, charge density difference, the orbitals and densities of states (DOS) were calculated to study the electronic properties of the contacting interfacial atoms. Based on the calculation, the amount of contact charge was obtained. The investigation revealed that the electrons near Fermi levels with higher energies transfer between the outermost orbitals (s orbitals for Ag and d orbitals for Fe). Meanwhile, polarized covalent bonds form between the d electrons in the deep energy states. These two effects together lead to an increase of charge magnitude at the interface. Also, the electrons responsible for electrification can be determined by their energies and orbitals.     

Theory of local electrification of cavitation bubbles: new approaches

The theory of local electrification of cavitation bubbles has been generalised. The major cases for a local electrification of bubbles in a cavitation field were considered; i.e., fragmentation and deformation of cavitation bubbles. The splitting of cavitation bubbles was considered taking into account surface tension, bubble perturbation, Stokes force and electrostatic forces between like charges on the wall of the collapsing neck of the fragment bubble. The problem of the uncompensated charge on the surface of the deformed cavitation bubble is solved. For this purpose radial deformations are considered in terms of the paraboloid of rotation and axial deformation approximated by one cavity hyperboloid of rotation. The maximum electric strength is accounted for. An explanation for some physical and physico-chemical effects in cavitation fields is proposed in terms of the electrical theory of the local electrification of cavitation bubbles.     

Heterogeneity of surface potential in contact electrification under ambient conditions: A comparison of pre- and post-contact states

We measured the pattern of charging by contact electrification, following contact between a polydimethylsiloxane (PDMS) stamp and a glass substrate with gold electrodes. We used scanning Kelvin probe microscopy to map the surface potential at the same regions before and after contact, allowing a point-by-point comparison. After contact, the mean surface potential of the glass shifted by 360 mV and micron-scale heterogeneity appeared with a magnitude of ∼100 mV. The gold electrodes showed charge transfer but no discernible heterogeneity. These results show that contact electrification causes heterogeneity of surface potential even on non-polymer surfaces such as glass under ambient conditions.     

Electrostatic charging behaviour of dielectric particles in a pressurized gas–solid fluidized bed

The charging behaviour of insulating particles in pressurized fluidized beds was investigated by fluidizing five polyethylene resins in a column of 150 mm inner diameter and 2.0 m height. Seven collision ball probes at different levels and radial positions monitored the electrostatic charge generation in the bed. The influences of operating pressure and superficial gas velocity on the degree of electrification were studied. For each polyethylene resin, the electrostatic charges of the particles in the upper part of the bed gained a polarity which differed from the particles in the lower part of the bed due to bipolar charging and particle segregation. The hydrodynamics in the fluidized bed significantly influenced the particle electrification. Due to increased bubble size and rise velocity, electrostatic charge generation was enhanced as the superficial gas velocity increased. However, it was difficult to predict the influence of elevated pressure on the charging behaviour of each resin as a result of the complex impacts of pressurization on the hydrodynamics and electrification.     

工业中粉体颗粒的荷电机理及数值模拟方法

工业过程中粉体颗粒不可避免地会相互摩擦碰撞而荷电. 荷电颗粒的存在可能会危害正常的工业生产过程, 也可能对工业过程起促进作用. 因此, 荷电粉体颗粒及其特性受到了广泛的关注, 但目前对粉体颗粒的荷电机理依然缺乏透彻的了解, 尤其是在气固两相流动中的粉体颗粒荷电现象. 事实上, 工业中存在的粉体颗粒的运动都受到流体的影响, 是典型的气固两相流系统, 流体对粉体颗粒的作用使粉体颗粒接触的荷电现象变得更为复杂, 因此从两相流动的观点来研究粉体颗粒荷电的物理本质就显得越来越重要. 本文介绍了工业过程中的几种不同类型的粉体颗粒荷电行为, 回顾了颗粒的荷电机理与描述颗粒荷电的数学模型. 对于工业过程中颗粒的荷电现象及颗粒在多相流体中的动力学行为, 介绍了研究颗粒受流体影响时荷电特性的数值模拟方法. 本文旨在对粉体颗粒的荷电机理、应用以及研究方法进行梳理与探讨, 为正确认识工业过程中粉体颗粒的荷电现象并加以控制利用提供理论借鉴.     

The chemical hardness of molecules and the band gap of solids within charge equilibration formalisms

This work finds that different charge equilibration methods lead to qualitatively different responses of molecules and solids to an excess charge. The investigated approaches are the regular charge equilibration (QE), the atom-atom-charge transfer (AACT), and the split-charge equilibration (SQE) method. In QE, the hardness of molecules and the band gap of solids approaches zero at large particle numbers, affirming the claim that QE induces metallic behavior. AACT suffers from producing negative values of the hardness; moreover valence and conduction bands of solids cross. In contrast to these methods, SQE can reproduce the generic behavior of dielectric molecules or solids. Moreover, first quantitative results for the NaCl molecule are promising. The results derived in this work may have beneficial implications for the modeling of redox reactions. They reveal that by introducing formal oxidation states into force field-based simulations it will become possible to simulate redox reactions including non-equilibrium contact electrification, voltage-driven charging of galvanic cells, and the formation of zwitterionic molecules.     

Contact electrification of polymers due to electron transfer among mechano anions,mechano cations and mechano radicals

Many studies have been reported for contact electrification based on the electron transfer from donors to acceptors. However, the chemical structures of donors and acceptors have not been identified. Here we calculated the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of model structures of mechano anions, mechano cations and mechano radicals which were produced by the heterogeneous and homogeneous scissions of covalent bonds comprising polymer main chain in vacuum at 77 K. We identified the donors are mechano anions(HOMO) and mechano radicals(HOMO), and the acceptors are mechano cations(LUMO) and mechano radicals(LUMO). The contact electrification is due to the electron transfer from the donors to the acceptors during contacting on the friction surface, and produces mosaic nano-scopic domains with opposite sign. The sign of the net charge of polymer was deduced from the number of paths of electron acceptance reaction. The relative sign of charge and position on the triboelectric series were deduced from their chemical structure.     

Contact charging of oxidized metal powders

Samples of oxidized metal powders (Ag, Al, Cr, La, Nb, Ni, Ti) were repeatedly contacted with a gold plate under high vacuum conditions. The charge transfer was measured on both the samples and the gold plate. We observed charge saturation after one contact in most cases. Positive and negative polarities were detected. The charge densities were calculated and correlated with work functions of oxidized metals. Contact electrification of aluminum shows exceptional behavior with repeated contacts.     

Contact electrification

If two materials are brought into contact and then separated they are found to be charged; this is the phenomenon of ‘contact electrification’. The subject falls naturally into three divisions—electrification of metals by metals; of insulators by metals; and of insulators by insulators. The first of these is well understood; charge transfer between metals is such as to bring the two Fermi levels into coincidence. The second division, electrification of insulators by metals, has been much studied recently and takes up the main part of our review; our understanding remains imperfect, chiefly because of lack of knowledge about the relevant electron states in insulators. Electrification of insulators by insulators has not been studied so extensively, but there is evidence that an understanding of the metal/insulator case will lead to an understanding of the insulator/insulator case as well.     

Electrostatic force of dust deposition originating from contact between particles and photovoltaic glass

Charged photovoltaic glass produces an electrostatic field. The electrostatic field exerts an electrostatic force on dust particles, thus making more dust particles deposited on the glass. In this paper, the contact electrification between the deposited dust particles and the photovoltaic glass is studied. Meanwhile, the surface charge density model of the photovoltaic glass and the electrostatic force of charged particles are analyzed. The results show that with the increasing of the particle impact speed and the inclination angle of the photovoltaic panel, the charges on particles increase to different degrees.Under a given condition, the electrostatic forces acting on the charged particles at different positions above the glass plate form a bell-shaped distribution at a macro level, and present a maximum value in the center of the plate. As the distance between the particle and the charged glass decreases, the electrostatic force exerted on the particle increases significantly and fluctuates greatly. However, its mean value is still higher than the force caused by gravity and the adhesion force,reported by some studies. Therefore, we suggest that photovoltaic glass panels used in the severe wind-sand environment should be made of an anti-static transparent material, which can lessen the dust particles accumulated on the panels.     

Numerical simulation of particle size effects on contact electrification in granular systems

Based on the contact charge transfer model between two particles due to a single collision proposed by Apodaca, the contact charges carried on a particle is derived due to multiple collisions, including the repeat collisions between two particles and the collisions with different particles, in mixed-size granular system of identical material. The effect of the particle size on the charges carried on the particle is simulated. The results indicate that for a mixed-size granular system, due to multiple collisions among particles, there exists a threshold particle radius, the particles with radius higher than which and the particles with radius lower than which carry opposite charges. The threshold particle radius is equal to mean value of particle size in the mixed-size granular system. Basically, the polarity of the charges carried on the largest particle is same as the polarity of the transfer charge carrier, and in case of the positive charge transferred, the largest particle will be positively charged and the smallest particle will be negatively charged, and vice versa. In the same size region, the more dispersive the particle size is, the more the net charges can be produced. In normal-distributed granular system, the magnitude of contact charge is determined mainly by the particle size distribution, size region, total particle number and the relative impact velocity.     

Mechanoelectrical transformations upon the elastic impact excitation of composite dielectric materials

The elastic impact excitation of dielectric materials leads to an ac electromagnetic field that is a superposition of the following several fields: the field appearing when a striker approaches a surface with an excess surface charge, a field induced by electrification processes when a striker is in contact with a material surface, the field created by a shift in the acoustic wave of charges at internal inhomogeneities, the field appearing upon acoustic surface-charge oscillations, and the field that is induced in piezoactive heterogeneous materials by the piezoelectric effect.     

An investigation on the static electrification of dimethyl ether

Static electrification characteristics of dimethyl ether (DME) were studied considering accidents during unloading (discharging) operations. Producing mist flow, static electrification by the mist was studied as a first step. Influences of flow velocity on the charge generation, produced potential, charge generation ratio, electrostatic energy, etc. were investigated experimentally. Although the produced charge is not too big, it will be requested to pay full attention during the handling process, considering the igniting characteristics of DME.     

Modelling of ions for seeding technique to electrify the atmosphere

There are number of ways in which weak electrification can affect the microphysics of clouds, with consequences for cloud lifetime, radiative properties, and precipitation efficiency. Kauffman [2011] suggested ions produced by direct current generators will add to and enhance the catalysing effects that cosmic ray ions are now known to produce in among other things, lowering nucleation barriers, stimulating charged particle growth and stability and increasing the scavenging rate in clouds. Thus to electrify the atmosphere ions can be generated artificially in abundance along with large electric field.Ions can be generated by the corona effect using Atmospheric electrifiers (a device used to generate negative ions) which makes use of corona discharge phenomenon to charge the air particles. Exact assessment of electric field and charge density distributions and the flow dynamics inside the electrifiers is essential to understand the particle behaviour inside the electrifiers.In this paper, a novel model of governing equations to evaluate the space charge density, electric field intensity and velocity of ionized airflow is suggested as a function of applied voltage. The Poisson and charge conservation equations are derived and hence can be used to estimate the electric field and charge density distributions. Navier stokes equation can be used to get the velocity of ionized airflow because of electric force on the air. Simulation is carried out to validate the proposed model and verify that velocity is function of input voltage and is proportional to it.     

Event-driven simulations of insulating grains in an external electric field

In this work we employ event-driven particle dynamics simulations for a system of spherical insulating grains interacting with an external electric field. This system resembles the electrostatic particle separation present on some industrial processes. Here, the particles collide inelastically with each other and with the container walls, for a constant normal and tangential restitution coefficients. During the collisions, the grains can acquire electric charge due to triboelectric contact charging, since two different species of insulating particles are mixed. Particle-particle electric interactions are not considered. Grains are also subjected to the gravitational field and rotation, and are confined in a cubic box with thermal walls in order to prevent the static equilibrium state. We calculate the mass and charge density profile, and the particle charge distribution for different values of the electric field and temperature of the walls. The particle charge distribution and the effect of particle sizes on the separation process were also investigated.     

雷暴云带电的理论研究

本文对雷暴云的起电机制进行了研究，提出了雷暴云起电的对流碰撞理论，建立了雷暴云带的简化模型。由该模型给出了雷暴云中电荷的分布函数以及雷暴云轴线上电场强度的计算公式。并对雷暴云闪电过程中电荷的交换情况进行了分析。