Charge Decay Research Articles

Adiabatic and nonadiabatic charge separation dynamics in graphene oxide quantum dots for overall water splitting

Chemical functionalization and heteroatom doping are two effective strategies for improving the conductivity of a graphene lattice. Nitrogen-doped graphene oxide quantum dot (GOQD) has been reported to possess both p- and n-conductivity that is induced by an oxygen functional group and nitrogen doping, respectively, and is suitable for catalyzing hydrogen and oxygen evolution reactions for complete water splitting. The experimental study shows that the hydrogen evolution reaction occurs considerably faster than the oxygen evolution reaction. However, the mechanism of this phenomenon remains unknown, which poses a challenge to the chemical modification of such classes of materials. In the present work, we perform nonadiabatic ab initio molecular dynamics to explore the charge separation dynamics in N-doped GOQD with oxygen functional groups. Our results show that there exists multiple charge decay channels governed by different mechanisms, which complicates the overall charge separation dynamics in N-doped GOQD. The intramolecular electron transfer mainly occurs through the nonadiabatic channel, whereas the intramolecular hole transfer mainly occurs through the adiabatic channel. There is an additional adiabatic decay channel in the p-domain, which accelerates electron–hole recombination in the p-domain. We further calculated the decoherence times and found that the decoherence of intramolecular electron transfer occurs considerably faster than that of hole transfer, which slows the quantum transition of intramolecular electron transfer. Finally, we simulated the carrier relaxation times and found that the electron relaxation is approximately one order of magnitude longer than that of the hole relaxation. Our calculation rationalizes the experimentally observed higher catalytic activity towards the hydrogen evolution compared to the oxygen evolution. More importantly, the calculation explains the fundamental roles of nitrogen doping and oxygen functional groups in the adiabatic and nonadiabatic charge decay mechanisms.

Search for heavy neutral leptons at the NA62 experiment at CERN

Search for heavy neutral lepton (HNL) production in charged kaon decays using the data collected by kaon experiments NA48/2 and NA62 at CERN are reported. Upper limits are established on the elements of the extended neutrino mixing matrix for heavy neutral lepton mass in the range 130–450 MeV, improving on the results from previous HNL production search.

Anatomy of b \u2192 c \u03c4 \u03bd anomalies

In recent times, one of the strongest hints of Physics Beyond the Standard Model (BSM) has been the anomaly in the ratios RD and {R}_{D^{*}} measured in the charged current decays of B-mesons. In this work, we perform a comprehensive analysis of these decay modes, first in a model independent way and subsequently, in the context of composite Higgs models. We discuss in depth as to how linearly realised SU(2)L × U(1)Y symmetry imposes severe constraint on the various scenarios because of correlations with other ΔF = 1 processes and Z τ τ and Z ν ν couplings. In the composite Higgs paradigm with partial compositeness, we show that, irrespective of the flavour structure of the composite sector, constraints from ΔF = 2 processes bring the compositeness scale down to ∼ 650 GeV which is in tension with electroweak precision observables. In the presence of composite leptoquarks, the situation improves only marginally (a factor of sqrt{2} in the compositeness scale), thus making the new states soon discoverable by direct searches at the LHC. We also comment on the possible explanation of the {R}_{K,{K}^{*}} anomalies within the composite Higgs framework.

Influence of TiO2 nanoparticle size on creeping flashover property of transformer oil-impregnated pressboards

Nanoparticles have exhibited great potential in improving insulating property of liquid and solid dielectrics. To explore their modification mechanism on transformer oil-impregnated pressboards, TiO2 nanoparticles with different diameters of 5nm, 10nm, and 15nm were synthesized and used to prepare transformer oil-based nanofluids. The creeping flashover characteristics of both pure oil-impregnated paperboards (OPs) and nanofluid-impregnated pressboards (NPs) were examined under lightning impulse and AC voltages. It was found that the creeping flashover property of NPs is significantly improved with the decreasing of nanoparticle size. Especially, it is enhanced by 2.44 times under positive lighting impulse as nanoparticle size reduced from 15 nm to 5 nm. The results of space charge decay and trap characteristics reveal the decrease of nanoparticle size can lower trap depth and greatly enhance the space charge dissipation rate, leading to more uniform electric field distribution at the interface between nanofluid and pressboards and higher creeping flashover property.

Photocatalytic Nanoparticulate ZrxTi1‐xO2 Coatings with Controlled Homogeneity of Elemental Composition

AbstractThe mixed oxide ZrxTi1‐xO2 nanoparticulate photocatalysts of different compositions (0≤x≤1) were prepared via sol‐gel method in a rapid micromixing reactor followed by chemical colloid deposition on borosilicate glass beads, drying and heat treatment at temperatures between 400 to 650 °C. The point‐like reaction conditions in the regime of low Damköhler numbers resulted in nucleation of size‐selected reactive zirconium‐titanium oxo‐alkoxy (ZTOA) nanoparticles from zirconium (IV) propoxide and titanium (IV) isopropoxide precursors, which enable strong covalent bonds with hydrophilic supports. The ZTOA nanoparticles with compositions 0≤x≤0.2 convert after a heat treatment to nanoporous monocrystals in distorted anatase phase. The photocatalytic activity of the prepared nanoparticulate coatings on gaseous ethylene decomposition attained maximum at x=0.043 after heat treatment at 500 °C, which was two times higher than that of pure anatase TiO2. A net correlation between the material photocatalytic activity and amplitude of the slow power decay of photoinduced charges was observed. A model is proposed explaining the power law decay by a thermal release of the localised charges from shallow surface Urbach states. The number density of these states is connected to the specific surface area of the photocatalyst.

A novel approach to investigating frictional electrification and charge decay on woven textile fabrics treated with ionic antistatic and hydrophilic surface finishes

The results of a study of charge generation and decay for range of woven fabrics from ring spun cotton yarns, ring spun polyester yarns, and flat continuous filament polyester yarns treated with topical finishes are reported. The topical finishes include commercially available ionic antistatic finishes, one commercially available hydrophilic finish, and a new experimental moisture management finish. The fabrics were rubbed against surface of steel and Polytetrafluoroethylene (PTFE; Teflon®) and their frictional electrification and charge dissipation were monitored. It is shown that fabrics from cotton have a more rapid inherent charge decay than fabrics from polyester. Ionic antistatic finishes are shown to be effective in reducing charge generation and promoting charge decay and this is particularly true for the fabrics made from filament polyester. When commercially available and experimental hydrophilic finishes were applied on fabrics made from filament polyester, less charge was generated and the generated charge decayed quickly compared to the control untreated fabrics. In an attempt to understand the mechanism of charge decay, two probes were used with one placed on the top of the rubbed area and the other placed some distance from the rubbed area. It was found that the generated charge migrated from the rubbed areas to other parts of the sample. This behavior is more pronounced for fabrics treated with hydrophilic and moisture management finishes, which cause the charge to decay exponentially.

Measurement of the form factors of charged kaon semileptonic decays

A measurement of the form factors of charged kaon semileptonic decays is presented, based on 4.4 × 106K± → π0e±νe (Ke3±) and 2.3 × 106K± → π0μ±νμ (Kμ3±) decays collected in 2004 by the NA48/2 experiment. The results are obtained with improved precision as compared to earlier measurements. The combination of measurements in the Ke3± and Kμ3± modes is also presented.

Contact electrification and charge decay on polyester fibres: A KPFM study

Surface contact electrification experiments have been performed on insulating polyester fibres in contact with a conductive fibre using biased atomic force microscopy tips. With positive tip bias, charge spots have been deposited in a reproducible manner. The density of deposited charges on the fibre surface was observed by Kelvin probe force microscopy as a function of time, charging position and relative humidity. Two main charge dissipation mechanisms were identified with different importance depending on the relative humidity value.

Nanocomposite polyester fabric based on graphene/titanium dioxide for conducting and UV protection functionality

An efficient process for fabrication of electroconductive, antistatic, ultraviolet rays protective polyester fabric using a graphene/titanium dioxide (graphene/TiO2) nanocomposite is prepared. Polyester fabric was treated with graphene oxide (GO) using a simple exhaust method. The GO-treated polyester fabric was then immersed in TiCl3 aqueous solution which acted as reducing agent to yield a fabric coated with graphene/TiO2 nanocomposite. The treated fabric was characterised by X-ray diffraction and Fourier transform infrared spectroscopy. Surface resistivity, volume resistivity, static charge decay time and ultraviolet protection factor of treated fabric were also assessed. The electrical conductivity of the graphene/TiO2 nanocomposite-coated fabrics was improved significantly by the presence of graphene on the surface of polyester fabrics. Nanocomposite fabric also showed excellent mechanical properties and hydrophobicity. It also exhibited protection from UV radiations as shown by higher UPF due to the presence of titanium on the fabric surface.

Effect of Temperature on Space Charge Distribution of Oil–Paper Insulation under DC and Polarity Reversal Voltage

The electric field distortion caused by space charge is an important factor affecting the operation reliability of oil–paper insulation in a converter transformer. To study the accumulation and decay characteristics of the space charge within oil-impregnated pressboard under DC and polarity reversal voltage, and consider the possible operating conditions of the converter transformer, the space charge behavior of oil-impregnated pressboard was measured by the pulsed electro-acoustic (PEA) method in the temperature range from −20 °C to 60 °C. The effect of temperature on the accumulation and decay characteristics of space charge is also analyzed. The space charge accumulated within the pressboard at low temperature is mainly homocharge injected by the electrode, while heterocharge formed by ion dissociation counteracts some of the homocharge at high temperature. Thus, the space charge of pressboard first increases, then decreases, with an increase in temperature. However, slow decay of the space charge causes severe distortion of the electric field distribution in the pressboard during voltage polarity reversal.

Surface charges on silicon rubber (SiR) insulator and their effect on potential and electric field distribution

This paper presents experimental results of the surface charges and charge decay measurement on the surface of polymeric insulator. In this research, the effect of surface charges on electric field and potential distribution has been investigated. Therefore, using measured electric charge and charge decay, the electric field and surface potential on insulator are studied by computer simulations. The accumulation of electric charge under DC electric field makes a significant difference compared to AC electric field, and this increases the rate of electrical failure and corrosion in HVDC insulators. So, a thorough understanding of the surface charge effect on insulation performance is necessary under various stresses. The experimental results revealed that the amount of charges increases linearly with the rising corona voltage and the charging time for both polarities. It was also observed that the amount of surface charges deposited by positive corona charging is less than negative corona charging. The simulation results illustrate that surface charges are important in potential and electric field variations.

Understanding the water droplet initiated discharges on gamma irradiated silicone rubber insulation

Corona inception voltage (CIV) due to water droplet on gamma‐irradiated silicone rubber indicates that its value reduces with increase of conductivity and with number of droplet in the electrode gap along the axis of the electrode. Increase in dosage of gamma irradiation to silicone rubber reduces the CIV. The UHF signal generated during corona inception due to water droplet has frequency bandwidth in the range of 0.3–1.2 GHz. The discharges occur in the phase range of 30°–130° in the positive half cycle and about 210°–270° in the negative half cycle. The water droplet shape gets altered on application of voltage. Surface charge accumulation studies indicate characteristic change with gamma irradiated specimen. In the presence of water droplet, the charge decay occurs rapidly with a single droplet and with three droplets. Thermal analysis of gamma radiation dosed samples reveals the destabilization of silicone material and the formation of higher order cyclic siloxane oligomers and oxygenated organics, which are expected to act as potential charge trapping sites. POLYM. ENG. SCI., 59:182–191, 2019. © 2018 Society of Plastics Engineers

Novel HVDC Spacers by Adaptively Controlling Surface Charges – Part I: Charge Transport and Control Strategy

A spacer is a challenging part of high voltage direct current (HVDC) gas-insulated substations and gas-insulated transmission lines. This paper proposes a novel design of HVDC spacers by self-adaptively controlling the surface charge. The paper briefly reviews dielectric modification methods that aim to limit surface charge accumulation. A novel design of HVDC spacer is introduced that is based on the concept of adaptively controlling surface charges using nonlinear materials, and its surface charge control and regulation abilities are investigated. The surface charge accumulation and decay properties of the novel spacer are compared to a downsized spacer. The results show that the proposed model spacer can restrict surface charge accumulation in the charge adaptive control region of the spacer. Doping the nonlinear material can facilitate surface charge decay in the charge adaptive control region when the electric field exceeds a threshold. These positive results represent a new idea of designing a suitable spacer to avoid surface flashover in HVDC SF 6 apparatus.

Influence of nano layer structure of polyimide film on space charge behavior and trap levels

Measurements of space charge density and trap levels are informative ways to predict failure conditions of insulation materials. The charges are injected from electrodes and penetrate from surface towards deep inside of the insulation material. In order to resist the charge injection, trapping, migration and increase charge recombination, detrapping and extraction rate, layer (nanocomposite-PI-nanocomposite) sandwich structure samples are used. Two techniques Pulse Electro Acoustic (PEA) and Thermal Stimulated Current (TSC) are combined to characterize space charge behavior and trap levels. Lab-view based program is used to detect the charge density distribution and total charge decay, different peaks of current using TSC technique are used to get activation energy and trap level density. The effects of structural interface (single, double, triple layers) and types of nano (Al 2 O 3 , SiO 2 ) fillers are analyzed to influence charge accumulation polarity and trap levels. Trap levels are calculated using TSC data as well as total charge decay data. The trap values calculated from both data match each other. Al 2 O 3 -PI-Al 2 O 3 sandwich structure showed remarkable improvements to resist accumulation of space charge for longer time. Mostly charges are shallow traps with 1.126 eV trap level and recombination rate is higher. In double layer structure in which interface is made up of SiO 2 and Al 2 O 3 , deep trap level of 1.26 eV and higher trap density is observed. The charges decay is faster in three-layer structure than double and single layer structure. It is concluded that the interface formed with different type of nano and structure of insulation, both are an important parameter for manufacturing industry to suppress space charges accumulation and traps.

A critical re-examination on conduction processes in gas-insulated DC devices at low electric fields

The established theories behind low-field ion-drift currents assume that the charge generation from natural ionization is the main source of the charge-carriers, and that it defines the current flowing through the insulation gas in gas-insulated direct current (DC) devices. This charge generation is assumed to be independent of the applied electric field in the cases where the electric field is sufficiently below the onset of microdischarges at the interfaces. However, the results of a few previous studies have suggested the presence of a field-dependent current contribution from some conduction processes, which might significantly accelerate the decay of the surface charges distributed on the insulator surfaces. In this paper, an extensive study on low-field charge transport is presented, and the influences of the insulation volume, electric field, and gas pressure on low-field charge transport are discussed. In accordance with the common structure employed in gas-insulated devices, Al 2 O 3 -filled epoxy resin is used in the solid parts of the device and sulfur hexafluoride (SF 6 ) is used for gaseous insulation. At electric fields of approximately 1800 V/m, a gas current that is up to 30 times higher, when compared to the conduction currents caused by the charge generation from natural ionization, is observed. This enhanced current reveals a further conduction process acting in a limited range of low electric fields and is characterized to be independent of the applied voltage polarity. With an increase in the gas pressure, the current amplitude is found to increase linearly and higher electric field is required to reach its maximum. At electric fields below 200 V/m, the conduction current reverses its pressure dependency and increases with decreasing gas pressure. The reported findings strongly indicate that the enhanced low-field charge transport in gas-insulated devices may be owing to electrophoretic conduction.

Flavourful axion phenomenology

We present a comprehensive discussion of the phenomenology of flavourful axions, including both standard Peccei-Quinn (PQ) axions, associated with the solution to the strong CP problem, and non-standard axion-like particles (ALPs). We give the flavourful axion-fermion and axion-photon couplings and calculate the branching ratios of heavy meson (K, D, B) decays involving a flavourful axion. We also calculate the mixing between axions and heavy mesons K0, D0, B0 and Bs0, which affects the meson oscillation probability and mass difference. Mixing also contributes to meson decays into axions and axion decays into two photons, and may be relevant for ALPs. We discuss charged lepton flavour-violating decays involving final state axions of the form ℓ1 → ℓ2a(γ), as well as μ → eee and μ − e conversion. Finally we describe the phenomenology of a particular “A to Z” Pati-Salam model, in which PQ symmetry arises accidentally due to discrete flavour symmetry. Here all axion couplings are fixed by a fit to flavour data, leading to sharp predictions and correlations between flavour-dependent observables.

Conduction processes in gas-insulated HVDC equipment: from saturated ion currents to micro-discharges

Charge generation in gas-insulated high-voltage direct current (HVDC) equipment has been investigated since surface charge accumulation was detected as the main challenge to insulation coordination. Over the years, researchers have made enormous progress using continuous ion current measurements or surface charging experiments with model spacers downscaled from gas-insulated components. Whereas the low sensitivity of the former method limits its measurement range, the latter method is time-consuming and demands comprehensive modeling. In this paper, more than 3,000 hours of surface charge measurements are presented, covering the full range of charge generation processes from ion currents owing to natural ionization to the inception of micro-discharges. In accordance with gas-insulated equipment, Al 2 O 3 -filled epoxy resin was used for the solids, and sulfur hexafluoride (SF 6 ) was used for the gaseous insulation. It has been demonstrated that the ion pair generation from natural ionization in the gas is significantly influenced by the surrounding solid materials, and therefore, scales with the insulation volume. From the onset of micro-discharges, sharp and locally limited charge patterns are formed, as surface conductivity does not appear to influence the investigated charge distributions. Whereas the primary charge dissipation from surface areas centered at the distribution could be explained in terms of the gas volume size, the discharge currents could not be reproduced with common ion-current models. At low electric fields, the observed charge decay indicates the presence of low-field conduction processes that cannot be explained by charge generation from natural ionization in the gas.

Correlation between surface charge and DC surface flashover of plasma treated epoxy resin

Surface charge accumulation of polymer insulating materials is harmful to the safe operation of electrical equipment, especially under DC electric field excitation. In order to ensure the safety and stable operation of HVDC power system such as Gas-insulated transmission line, it is necessary to understand the influence of the surface charge on the flashover and improve the surface charge dynamic characteristics. In this paper, Epoxy resin (EP) is deposited by SiO x thin films with different plasma treatment conditions. The flashover voltages are measured with and without surface charges. The results show that the flashover occurs at lower voltage due to the accumulated charges on the EP surface. In particular, the surface charge with the same polarity of flashover discharge can reduce the surface flashover voltage significantly. Moreover, the charge decay rate is increased up to 90% by plasma treatment of 3 min, corresponding flashover voltage is increased. In addition, the flashover voltages of samples during the surface charge dissipation are obtained. Both static impact index I and dynamic impact index S are introduced to estimate effect of surface charge on dielectric surface flashover.

Research progress on space charge measurement and space charge characteristics of nanodielectrics

Since the proposal of the notion of nanodielectric, the space charge characteristics of nanodielectrics have been widely investigated. With the addition of nanoparticles, some nanodielectrics exhibit smaller space charge accumulation amount during the polarisation, and also faster charge decay during the depolarisation, which is believed to be related to the interfacial layer between the nanoparticle and the base material. In this study, the space charge measurement methods, theoretical models, and experimental results are reviewed in detail. Space charge measurement methods have been greatly improved after more than 20 years development. The nature of the interfacial layer between the nanoparticle and the base material has been investigated in detail, and based on that, several models are proposed to explain some electrical experimental results of nanodielectrics. Furthermore, the parameters of nanoparticle (such as the type, size, amount, shape, and the surface modification), the experimental conditions, and the base material properties can strongly affect the space charge characteristics of nanodielectrics. This study will provide useful research results and conclusions for researchers, and may also be an overview for recent study and an outlook for future investigation on space charge characteristics of nanodielectrics.

Surface Potential Decay of Negative Corona Charged Epoxy/Al2O3 Nanocomposites Degraded by 7.5-MeV Electron Beam

Surface potential decay on negative corona charged epoxy/Al2O3 nanocomposites irradiated with high-energy electron beam has been investigated in this paper. 2-mm-thick laminate nanocomposite samples were irradiated with electron beam at an average energy of 7.5 MeV, and the total accumulated dose was up to 500 kGy. After the irradiation, the nanocomposites were corona charged with dc voltage at −10 kV through a pair of needle to plate electrode. Surface potential of the sample was then measured by means of an electrostatic voltmeter through which trap distribution and carrier mobility were calculated. Differential scanning calorimetry curve for the each sample was measured to understand the change in chemical and physical structures induced by the irradiation. Results obtained indicated that the presence of nanofiller in the nanocomposites played a role in limiting the charge transportation both before and after the irradiation, and the charge transportation behavior was remarkably dependent upon the total irradiation dose. It is found that when irradiated with a certain total dose of electron beam, electron migration within epoxy and its nanocomposites is restricted, which should be attributed to the high-energy irradiation-induced structural change of the material.