Nonlinear change of ion-induced secondary electron emission in theκ-Al_(2)O_(3) surface charging from first-principle modelling

Secondary electron emission(SEE)induced by the positive ion is an essential physical process to influence the dynamics of gas discharge which relies on the specific surface material.Surface charging has a significant impact on the material properties,thereby affecting the SEE in the plasma-surface interactions.However,it does not attract enough attention in the previous studies.In this paper,SEE dependent on the charged surface of specific materials is described with the computational method combining a density functional theory(DFT)model from the first-principle theory and the theory of Auger neutralization.The effect ofκ-Al2O3 surface charge,as an example,on the ion-induced secondary electron emission coefficient(SEEC)is investigated by analyzing the defect energy level and band structure on the charged surface.Simulation results indicate that,with the surface charge from negative to positive,the SEEC of a part of low ionization energy ions(such as Ei=12.6 eV)increases first and then decreases,exhibiting a nonlinear changing trend.This is quite different from the monotonic decreasing tendency observed in the previous model which simplifies the electronic structure.This irregular increase of the SEEC can be attributed to the lower escaped probability of orbital energy.The results further illustrate that the excessive charge could cause the bottom of the conduction band close to the valence band,thus leading to the decrease of the orbital energy occupied by the excited electrons.The nonlinear change of SEEC demonstrates a more realistic situation of how the electronic structure of material surface influences the SEE process.This work provides an accurate method of calculating SEEC from specific materials,which is urgent in widespread physical scenarios sensitive to surface materials,such as increasingly growing practical applications concerning plasma-surface interactions.

Surface charging activated mechanism change: A computational study of O, CO, and CO2 interactions on Ag electrodes

Electrocatalytic and plasma-activated processes receive increasing attention in catalysis. Density functional theory(DFT) calculations are state-of-the-art tools for the fundamental study of reaction mechanisms and predicting the performance of catalytic materials. Proper application of DFT-based methods is crucial when investigating charge-doped electrode surfaces during electrocatalytic and plasma-activated reactions. Here, as a model electrode for plasma-activated CO2 splitting, we studied the interactions of O, CO, and CO2 with the neutral and progressively charged Ag(111) metal surfaces. We show that the application of correction procedures is necessary to obtain accurate adsorption energy profiles of O atoms,CO and CO2 molecules on Ag surfaces that are under the influence of additional electrons. Interestingly,the oxidation of CO is found to shift from a Langmuir–Hinshelwood mechanism on a neutral electrode to an Eley–Rideal mechanism on charged electrodes. Furthermore, we show that the surface charging of Ag(111) electrodes increase their CO2 reduction performance by enhancing the adsorption of O atoms and desorption of CO molecules. A further increase in the absolute charge-state of the electrode surface is expected to waive the thermodynamic barriers for the CO2 splitting reaction.

Surface Charging Phenomena on HVDC Spacers for Compressed SF_(6) Insulation and Charge Tailoring Strategies

Surface flashover of spacers is a key factor limiting the application of HVDC GIS/GIL,while the charge accumula-tion on the surface of the spacer could have a potential adverse effect on the surface flashover voltage.This paper discusses the laws regarding distribution patterns of surface charges and the related mechanisms.The field-dependent property is discussed in detail to comprehensively illustrate the charge transport mecha-nism and explain the research differences regarding different surface charge patterns obtained by previous researchers.In addition,the main surface charge control methods for epoxy resin are summarized and discussed.The potential research directions of charge control methods and key points in manufacturing of spacers used in HVDC GIS/GIL are also explored.

Recent advances in fabricating high-performance triboelectric nanogenerators via modulating surface charge density

Triboelectric nanogenerators(TENGs),a type of promising micro/nano energy source,have been arousing tremendous research interest since their inception and have been the subject of many striking developments,including defining the fundamental physical mechanisms,expanding applications in mechanical to electric power conversion and self-powered sensors,etc.TENGs with a superior surface charge density at the interfaces of the electrodes and dielectrics are found to be crucial to the enhancement of the performance of the devices.Here,an overview of recent advances,including material optimization,circuit design,and strategy conjunction,in developing TENGs through surface charge enhancement is presented.In these topics,different strategies are retrospected in terms of charge transport and trapping mechanisms,technical merits,and limitations.Additionally,the current challenges in high-performance TENG research and the perspectives in this field are discussed.

Rational surface charge engineering of haloalkane dehalogenase for boosting the enzymatic performance in organic solvent solutions

Biocatalysis in organic solvents(OSs)has numerous important applications,but native enzymes in OSs often exhibit limited catalytic performance.Herein,we proposed a computation-aided surface charge engineering strategy to improve the catalytic performance of haloalkane dehalogenase DhaA in OSs based on the energetic analysis of substrate binding to the DhaA surface.Several variants with enhanced OS resistance were obtained by replacing negative charged residues on the surface with positive charged residue(Arg).Particularly,a four-substitution variant E16R/E93R/E121R/E257R exhibited the best catalytic performance(five-fold improvement in OS resistance and seven-fold half-life increase in 40%(vol)dimethylsulfoxide).As a result,the overall catalytic performance of the variant could be at least 26 times higher than the wild-type DhaA.Fluorescence spectroscopy and molecular dynamics simulation studies revealed that the residue substitution mainly enhanced OS resistance from four aspects:(a)improved the overall structural stability,(b)increased the hydrophobicity of the local microenvironment around the catalytic triad,(c)enriched the hydrophobic substrate around the enzyme molecule,and(d)lowered the contact frequency between OS molecules and the catalytic triad.Our findings validate that computationaided surface charge engineering is an effective and ingenious rational strategy for tailoring enzyme performance in OSs.

Surface charge characteristics in a three-electrode surface dielectric barrier discharge

The surface charge characteristics in a three-electrode surface dielectric barrier discharge(SDBD)are experimentally investigated based on the Pockels effect of an electro-optical crystal. The actuator is based on the most commonly used SDBD structure for airflow control, with an exposed electrode supplied with sinusoidal AC high voltage, a grounded encapsulated electrode and an additional exposed electrode downstream supplied with DC voltage. The ionic wind velocity and thrust can be significantly improved by increasing DC voltage although the plasma discharge characteristics are virtually unaffected. It is found that the negative charges generated by the discharge of the three-electrode structure accumulate on the dielectric surface significantly further downstream in an AC period compared to the actuator with a two-electrode structure. The negative charges in the downstream region increase as the DC voltage increases.In addition, the DC voltage affects the time required for the positive charge filaments to decay.The positive DC voltage expands the ionic acceleration zone downstream to produce a greater EHD force. The amplitude of the DC voltage affects the electric field on the dielectric surface and is therefore a key factor in the formation of the EHD force. Further research on the surface charge characteristics of a three-electrode structure has been conducted using a pulse power to drive the discharge, and the same conclusions are drawn. This work demonstrates a link between surface charge characteristics and EHD performance of a three-electrode SDBD actuator.

Mechanisms of surface charge development of serpentine mineral

The electrokinetic behavior and surface dissolution of serpentine mineral were studied through Zeta potential measurements, dissolution experiments and X-ray photoelectron spectroscopy. The results show that serpentine has an iso-electric point （IEP） of 11.9, which is higher than that of other phyllosilicate minerals. Dissolution experiments show that the hydroxyl is easy to dissolve with respect to the magnesium cations in the magnesium oxide octahedral sheet. As a result of hydroxyl dissolution, the magnesium ions are left on serpentine surface, which is responsible for serpentine surface charge. The removal of magnesium ions from serpentine surface by acid leaching results in a decrease of serpentine IEP. Therefore, it has been clearly established that the surface charge developed at the serpentine/aqueous electrical interface is a function of the serpentine surface incongruent dissolution.

Speciation and fractionation of heavy metals in soil experimentally contaminated with Pb, Cd, Cu and Zn together and effects on soil negative surface charge

Speciation and fractionation of heavy metals in soil subsamples experimentally loaded with Pb, Cd, Cu and Zn in orthogonal design was investigated by sequential extraction, and operationally defined as water soluble and exchangeable(SE), weakly specific adsorbed(WSA), Fe and Mn oxides bound(OX) and organic bound(ORG). The results showed that fractions of heavy metals in the soil subsamples depended on their speciation. About 90% of Cd and 75% of Zn existed in soil subsamples in the SE fraction. Lead and Cu existed in soil subsamples as SE, WSA and OX fractions simultaneously, although SE was still the major fraction. Organic bound heavy metals were not clearly apparent in all the soil subsamples. The concentration of some heavy metal fractions in soil subsamples showed the good correlation with ionic impulsion of soil, especially for the SE fraction. Continuous saturation of soil subsamples with 0.20 mol/L NH 4Cl, which is the first step for determination of the negative surface charge of soil by the ion retention method, resulted in desorption of certain heavy metals from the soil. It was found that the percentage desorption of heavy metals from soil subsamples depended greatly on pH, the composition and original heavy metal content of the soil subsamples. However, most of the heavy metals in the soil subsamples were still be retained after multiple saturation. Compared with the parent soil, the negative surface charge of soil subsamples loaded with heavy metals did not show difference significantly from that of the parent one by statistical analysis. Heavy metals existed in the soil subsamples mainly as exchangeable and precipitated simultaneously.

Physical, chemical, and surface charge properties of bauxite residue derived from a combined process

A detailed understanding of the composition,buffering capacity,surface charge property,and metals leaching behavior of bauxite residue is the key to improved management,both in reducing the environmental impact and using the material as an industrial by-product for other applications.In this study,physical,chemical,and surface charge properties of bauxite residue derived from a combined process were investigated.Results indicated that the main alkaline solids in bauxite residue were katoite,sodalite,and calcite.These minerals also lead to a higher acid neutralizing capacity of bauxite residue.Acid neutralizing capacity(ANC)to pH 7.0 of this residue is about 0.9 mol H^+/kg solid.Meanwhile,the Fe-,Al-,and Si-containing minerals in bauxite residue resulted in an active surface;The isoelectric point(IEP)and point of zero charge(PZC)were 7.88 and 7.65,respectively.This also leads to a fact that most of the metals in bauxite residue were adsorbed by these surface charged solids,which makes the metals not readily move under natural or even moderately acidic conditions.The leaching behavior of metals as a function of pH indicated that the metals in bauxite residue present low release concentrations(pH>3).

Calibration Algorithm of Surface Charge Density on Insulating Materials Measured by Pockels Technique

Surface charges greatly affect the discharge/flashover development process across an insulator. The relationship between surface charge distribution on insulating materials and measurement data based on Pockels technique is discussed, and an improved algorithm is built to calculate the real surface charge density from original data. In this algorithm, two-dimensional Fourier transform technique and Wiener filter are employed to reduce the amount of numerical calculation and improve the stability of computation, Moreover, this algorithm considers not only the influence of sample＇s thickness and permittivity, but also the impact of charges at different positions. The achievement of this calibration algorithm is demonstrated in details. Compared with traditional algorithms, the improved one supplies a better solution in the calibration of surface charge distribution on different samples with different thickness.

Chemical composition and surface charge properties of montmorillonite

The effects of the cell parameter and chemical composition on the surface charge properties of five kinds of different colour montmorillonites were studied. The results indicate that the surface isoelectric point（IEP） of the montmorillonite shows positive correlation with the mass fractions of Fe203 and K20, but it has little relation to the mass fractions of other chemical compositions. At around pH=6.8, the surface zeta potential of the montmorillonite shows the negative relationship with the mass fractions of Fe203 and MgO, but it does not linearly correlate to the mass fractions of other chemical compositions. Cell parameter（bo） of the montmorillonite expresses negative linear relationship with mass fractions of K2O and Na2O, so does cosinβ with mass fractions of SlOE and Fe2O3. And there is no specific relationship between bo and IEP of different montmorillonites, but there is positive correlation between cosinβ and IEP of different montmorillonite samples.

Correlation between surface charge and hydration on mineral surfaces in aqueous solutions: A critical review

Surface charges and hydration are predominant properties of colloidal particles that govern colloidal stability in aqueous suspensions.These properties usually coexist and interact with each other.The correlation between the surface charge and hydration of minerals is summarized on the basis of innovative experimental,theoretical,and molecular dynamics simulation studies.The factors affecting the adsorption behavior of ions and water molecules,such as ion concentration,ion hydration radius and valence,and surface properties,are discussed.For example,the hydration and adsorption states completely differ between monovalent and divalent ions.For ions of the same valence,the effect of surface charge on the hydration force follows the Hofmeister adsorption series.Electrolyte concentration exerts a significant effect on the hydration force at high ion concentrations.Meanwhile,the ion correlations in high-concentration electrolyte systems become long range.The interfacial water structure largely depends on surface chemistry.The hydration layer between different surfaces shows large qualitative differences.

Measurable surface d charge of Pd as a descriptor for the selective hydrogenation activity of quinoline

Au Pd nanoalloys with tunable Pd concentrations have been synthesized and used as model catalysts. They have been directly imaged by high-angle annular dark-field scanning transmission electron microscopy and investigated by thorough analyses of their extended X-ray absorption fine structure, X-ray absorption near-edge structure, X-ray diffraction and X-ray photoelectron spectroscopy measurements. The bimetallic nanoparticles are embedded in a carbonaceous matrix and have almost an identical structure at the atomic level and the same electronic properties as Au Pd bulk alloys with the same compositions. The d-electron increase at surface Pd sites is determined by the Pd concentration of the alloy. Similarly, their activation entropy and catalytic activity for the hydrogenation of quinoline is related to the Pd concentration, with Au50 Pd50 the most active of the alloys investigated. An almost 11 times higher activity was achieved compared to a pure Pd catalyst. The experimentally measurable surface d charge at the Pd sites in the Au Pd was found to linearly correlate with the activation entropy and catalytic activity for the hydrogenation of quinoline. The alloy structure is stable, showing negligible metal segregation, dissolution-redeposition and aggregation during the hydrogenation process which involves strong adsorption.

Relationship Between Iron Oxides and Surface Charge Characteristics in Soils

The relationship between iron oxides and surface charge characteristics in variable charge soils ( latosol and red earth ) was studied in following three ways. ( 1 ) Remove free iron oxides ( Fed ) and amorphous iron oxides ( Feo ) from the soils with sodium dithionite and acid ammonium oxalate solution respectively. ( 2 ) Add 2% glucose ( on the basis of air-dry soil weight ) to soils and incubate under submerged condition to activate iron oxides, and then the mixtures are dehydrated and air-dried to age iron oxides. ( 3 ) Precipitate various crystalline forms of iron oxides onto kaolinite. The results showed that free iron oxides ( Fed ) were the chief carrier of variable positive charges. Of which crystalline iron oxides ( Fed-Feo ) presented mainly as discrete particles in the soils and could only play a role of the carrier of positive charges, and did little influence on negative charges. Whereas the amorphous iron oxides ( Feo ) , which presented mainly as a coating with a large specific surface area, not only had positive charges, but also blocked the negative charge sites in soils. Submerged incubation activated iron oxides in the soils, and increased the amount of amorphous iron oxides and the degree of activation of iron oxide, which resulted in the increase of positive and negative charges of soils. Dehydration and air-dry aged iron oxides in soils and decreased the amount of amorphous iron oxides and the degree of activation of iron oxide, and also led to the decrease of positive and negative charges. Both the submerged incubation and the dehydration and air-dry had no significant influence on net charges. Precipitation of iron oxides onto kaolinite markedly increased positive charges and decreased negative charges. Amorphous iron oxide having a larger surface area contributed more positive charge sites and blocked more negative charge sites in kaolinite than crystalline goethite.

Effect of plasma step gradient modification on surface electrical properties of epoxy resin

In this paper,plasma fluorination is combined with plasma silicon deposition to achieve step gradient modification on an epoxy resin surface.The physicochemical characteristics of samples are investigated and the electrical performances measured.The obtained results show that compared with untreated and single treated samples,the samples treated by step gradient modification significantly improve the flashover performance.According to experiment and simulation,the mechanism explanations are summarized as follows.First,it is found that the step gradient conductivity can effectively optimize the electric field distribution of a needle-needle electrode.Then,step gradient modification suppresses the accumulation of surface charge at the triple junction and makes the charge distribution more uniform.Furthermore,it can accelerate the surface dissipation on a high electrical field region and control the dissipation rate on a low electrical field region.All these results can restrain surface discharge and increase the flashover voltage.The step gradient modification method proposed in this paper provides a new idea for improving the surface insulation performance.

Deposition of SiCxHyOz thin film on epoxy resin by nanosecond pulsed APPJ for improving the surface insulating performance

Non-thermal plasma surface modification for epoxy resin（EP）to improve the insulation properties has wide application prospects in gas insulated switchgear and gas insulatedtransmission line.In this paper,a pulsed Ar dual dielectrics atmospheric-pressure plasma jet（APPJ）was used for Si CxHyOzthin film deposition on EP samples.The film deposition was optimized by varying the treatment time while other parameters were kept at constants（treatment distance：10 mm,precursor flow rate：0.6 l min-（-1）,maximum instantaneous power：3.08 k W and single pulse energy：0.18 m J）.It was found that the maximum value of flashover voltages for negative and positive voltage were improved by 18%and 13%when the deposition time was3 min,respectively.The flashover voltage reduced as treatment time increased.Moreover,all the surface conductivity,surface charge dissipation rate and surface trap level distribution reached an optimal value when thin film deposition time was 3 min.Other measurements,such as atomic force microscopy and scanning electron microscope for EP surface morphology,Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy for EP surface compositions,optical emission spectra for APPJ deposition process were carried out to better understand the deposition processes and mechanisms.The results indicated that the original organic groups（C–H,C–C,C=O,C=C）were gradually replaced by the Si containing inorganic groups（Si–O–Si and Si–OH）.The reduction of C=O in ester group and C=C in p-substituted benzene of the EP samples might be responsible for shallowing the trap level and then enhancing the flashover voltage.However,when the plasma treatment time was longer than 3 min,the significant increase of the surface roughness might increase the trap level depth and then deteriorate the flashover performance.

Measurement of surface charges on the dielectric film based on field mills under the HVDC corona wire

The ion flow field on the ground is one of the significant parameters used to evaluate the electromagnetic environment of high voltage direct current（HVDC） power lines.HVDC lines may cross the greenhouses due to the restricted transmission corridors.Under the condition of ion flow field,the dielectric films on the greenhouses will be charged,and the electric fields in the greenhouses may exceed the limit value.Field mills are widely used to measure the groundlevel direct current electric fields under the HVDC power lines.In this paper,the charge inversion method is applied to calculate the surface charges on the dielectric film according to the measured ground-level electric fields.The advantages of hiding the field mill probes in the ground are studied.The charge inversion algorithm is optimized in order to decrease the impact of measurement errors.Based on the experimental results,the surface charge distribution on a piece of quadrate dielectric film under a HVDC corona wire is studied.The enhanced effect of dielectric film on ground-level electric field is obviously weakened with the increase of film height.Compared with the total electric field strengths,the normal components of film-free electric fields at the corresponding film-placed positions have a higher effect on surface charge accumulation.

Decay characters of charges on an insulator surface after different types of discharge

In an insulating system including solid and gas dielectrics, discharge type has a strong impact on charge accumulation at the interface between two dielectrics, and hence charge decay. In order to clarify the influence, a surface charge measurement system was constructed, and three types of discharge, i.e. surface discharge, and low intensity and high intensity coronas, were introduced to cause surface charge accumulation. The decay behavior of surface charges after different types of discharge was obtained at various temperatures. It was found that total surface charges monotonically decreased with time, and the decay rate became larger as temperature increased. However, after a surface discharge or a high intensity corona, surface charge density in the local area appeared to fluctuate during the decay process. Compared with this, the fluctuation of surface charge density was not observed after a low intensity corona. The mechanisms of surface charge accumulation and decay were analysed. Moreover, a microscopic physical model involving charge production, accumulation, and decay was proposed so that the experimental results could be explained.

Surface-charge-governed electrolyte transport in carbon nanotubes

The transport behavior of pressure-driven aqueous electrolyte solution through charged carbon nanotubes(CNTs) is studied by using molecular dynamics simulations. The results reveal that the presence of charges around the nanotube can remarkably reduce the flow velocity as well as the slip length of the aqueous solution, and the decreasing of magnitude depends on the number of surface charges and distribution. With 1-M KCl solution inside the carbon nanotube, the slip length decreases from 110 nm to only 14 nm when the number of surface charges increases from 0 to 12 e. This phenomenon is attributed to the increase of the solid–liquid friction force due to the electrostatic interaction between the charges and the electrolyte particles, which can impede the transports of water molecules and electrolyte ions. With the simulation results,we estimate the energy conversion efficiency of nanofluidic battery based on CNTs, and find that the highest efficiency is only around 30% but not 60% as expected in previous work.

Effects of Size and Surface Charge of Polymeric Nanoparticles on in Vitro and in Vivo Applications

Biodegradable polymeric materials are the most common carriers for use in drug delivery systems. With this trend, newer drug delivery systems using targeted and controlled release polymeric nanoparticles (NPs) are being developed to manipulate their navigation in complex in vivo environment. However, a clear understanding of the interactions between biological systems and these nanoparticulates is still unexplored. Different studies have been performed to correlate the physicochemical properties of polymeric NPs with the biological responses. Size and surface charge are the two fundamental physicochemical properties that provide a key direction to design an effective NP formulation. In this critical review, our goal is to provide a brief overview on the influences of size and surface charge of different polymeric NPs in vitro and to highlight the challenges involved with in vivo trials.