Action Of Strong Electric Field Research Articles

Investigation of Field Emission Properties of Carbon Nanotube Arrays of Different Morphologies.

This article presents, for the first time, a comparative analysis of the emission characteristics of large-area field-effect cathodes (LAFE) based on carbon nanotubes (CNTs) of various morphologies according to key parameters using a unique computerized technique. The work presents a description of a technology for creating various CNT arrays and their comprehensive structure characterization. All CNT arrays synthesized by the catalytic PECVD method on a silicon substrate showed a high degree of chemical purity under the presented technological conditions. In some cases, nanoisland films of Fe were used as a catalyst; in others, thin films of NiO were used, which were deposited on a silicon wafer by chemical vapor deposition (CVD) and atomic layer deposition (ALD), respectively. As a result of these studies, it turned out that an array with a thick CNT coating has good resistance to the action of strong electric fields, fairly good uniformity of distribution of emission centers, a fairly high selection current (2.88 mA/cm2 at 4.53 V/μm), and compliance with the normal current mode according to the "orthodox" test, which makes the morphology of such structures the most promising for further technological optimization of CNT-based cathodes for various practical applications.

Study on Surface Discharge Characteristics of GO-Doped Epoxy Resin-LN2 Composite Insulation.

Superconducting power lead equipment for epoxy insulation, such as high-temperature superconducting DC power or liquefied natural gas energy pipelines, as well as high-temperature superconducting cables, has long been used in extreme environments, from liquid nitrogen temperatures to normal temperatures. It is easy to induce surface discharge and flashover under the action of strong electric field, which accelerates the insulation failure of current leads. In this paper, two-dimensional nano-material GO was used to control the electrical properties of epoxy resins. The DC surface discharge and flashover characteristics of the prepared epoxy resin–GO composite insulation materials were tested at room temperature with liquid nitrogen. The surface discharge mechanism of the epoxy resin–GO composite insulation materials was analyzed. The experimental results show that the insulation properties of epoxy composites doped with GO changed. Among them, the surface flashover voltage of 0.05 wt% material is the best, which can inhibit the discharge phenomenon and improve its insulation properties in extreme environments, from room temperature to liquid nitrogen temperature. It is found that the development process of surface discharge of composite insulating materials under liquid nitrogen is quite different from that under room temperature. Before critical flashover, the repetition rate and amplitude of surface discharge remain at a low level until critical flashover. Furthermore, the voltage of the first flashover is significantly higher than that of the subsequent flashover under the action of the desorption gas on the surface of the composite insulating material and the gasification layer produced by the discharge. Given that the surface flashover voltage of 0.05 wt% epoxy composite is the best, the research and analysis of 0.05 wt% composite is emphasized. In the future design of superconducting power lead insulation, the modification method of adding GO to epoxy resin can be considered in order to improve its insulation performance.

Chiral Magnetic Effect of Hot Electrons.

We propose a way to observe the chiral magnetic effect in noncentrosymmetric Weyl semimetals under the action of a strong electric field, via the nonlinear part of their I-V characteristic that is odd in the external magnetic field, or odd-in-magnetic field voltages in electrically open circuits. This effect relies on valley-selective heating in such materials, which, in general, leads to nonequilibrium valley population imbalances. In the presence of an external magnetic field, such a valley-imbalanced Weyl semimetal will, in general, develop an electric current along the direction of the magnetic field-the chiral magnetic effect. We also discuss a specific experimental setup to observe the chiral magnetic effect of hot electrons.

A Molecular-Level Picture of Electrospinning

Electrospinning is a modern and versatile method of producing nanofibers from polymer solutions or melts by the action of strong electric fields. The complex, multiscale nature of the process hinders its theoretical understanding, especially at the molecular level. The present article aims to contribute to the fundamental picture of the process by the molecular modeling of its nanoscale analogue and complements the picture by laboratory experiments at macroscale. Special attention is given to how the process is influenced by ions. Molecular dynamics (MD) is employed to model the time evolution of a nanodroplet of aqueous poly(ethylene glycol) (PEG) solution on a solid surface in a strong electric field. Two molecular weights of PEG are used, each in 12 aqueous solutions differing by the weight fraction of the polymer and the concentration of added NaCl. Various structural and dynamic quantities are monitored in production trajectories to characterize important features of the process and the effect of ions on it. Complementary experiments are carried out with macroscopic droplets of compositions similar to those used in MD. The behavior of droplets in a strong electric field is monitored using an oscilloscopic method and high-speed camera recording. Oscilloscopic records of voltage and current are used to determine the characteristic onset times of the instability of the meniscus as the times of the first discharge. The results of simulations indicate that, at the molecular level, the process is primarily driven by polarization forces and the role of ionic charge is only minor. Ions enhance the evaporation of solvent and the transport of polymer into the jet. Experimentally measured instability onset times weakly decrease with increasing ionic concentration in solutions with low polymer content. High-speed photography coupled with oscilloscopic measurement shows that the measured instability onset corresponds to the formation of a sharp tip of the Taylor cone. Molecular-scale and macroscopic views of the process are confronted, and challenges for their reconciliation are presented as a route to a true understanding of electrospinning.

Reorientation dynamics of nematics encapsulated in microscopic volumes in a strong electric field

We theoretically describe a new regime of reorientation of the director field \(\widehat n\) and velocity v of a nematic liquid crystal (LC) encapsulated in a rectangular cell under the action of strong electric field E directed at angle α (~π/2) to the horizontal surfaces bounding the LC cell. The numerical calculations in the framework of nonlinear generalization of the classical Eriksen–Leslie theory showed that at certain relations between the torques and momenta affecting the unit LC volume and E ≫ Eth, transition periodic structures can arise during reorientation of \(\widehat n\), if the corresponding distortion mode has the fastest response and, thus, suppresses all the rest of the modes, including uniform ones. The position of sites of these periodic structures is affected by the value of field E, angle α, and the character of anchoring of LC molecules to the bounding surfaces. The calculations performed for the nematic formed by 4-n-penthyl-4’-cyanobiphenyl showed that several vortexes can form in an LC cell under the action of reorientation of the nematic field; the boundaries of these vortexes are determined by the positions of periodic structure sites.

Intense deep-blue electroluminescence from ITO/Y₂O₃/Ag structure.

ITO/Y₂O₃/Ag devices were fabricated using Y₂O₃ films as insulator. Four intense and sharp lines with half-peak width of 4 nm were observed for the 293.78 nm InI, 316.10 nm InI, 444.82 nm InII and 403.07 nm InIII transitions. Luminescence mechanism was illustrated by cross-section of the devices based on the analysis of surface morphology. Under the action of strong electric field, the loss of K-shell electrons led to the occurrence of characteristic radiation of indium ions. In addition, the device with turn-on voltage of 10V demonstrates typical I-V diode characteristics. Moreover, Y₂O₃/In₂O₃ multiple films as the insulation layer instead of single Y₂O₃ films was found to improve the device performance with excellent CIE (x, y) coordinates (0.16, 0.03).

Three-dimensional LBE simulations of a decay of liquid dielectrics with a solute gas into the system of gas–vapor channels under the action of strong electric fields

The three-dimensional simulations of an anisotropic decay of binary mixtures of a dielectric liquid with solute gas in a strong electric field are carried out. The Lattice Boltzmann Equation method (LBE) is exploited for computer simulations of the evolution of such systems with the newly arising interfaces between vapor and liquid phases. The parallel implementation of the LBE algorithm is realized on a large number of cores in the GPU. For the GPU programming, the CUDA technology is used.It is important that new regions of the low-density phase appear as thin quasi-cylindrical gas–vapor channels oriented along the electric field. The gas–vapor channels expand because of the diffusion of the solute gas from the mixture, evaporation of liquid into the channels and also due to the coalescence of channels with each other. The critical values of electric field necessary for such decay of a binary mixture are considerably lower than the critical electric field for pure dielectric liquids. Hence, if we take into account a solute gas, the electric fields for which the anisotropic mechanism of streamer channels generation and growth is operated, become considerably lower.Thus, at a breakdown of dielectric liquids in a strong electric field, the anisotropic instability is possibly the key mechanism of the generation of a gas phase, inception of conducting streamer structures, their fast growth in the form of thin filamentary channels, as well as branching of streamer structures during propagation.

Correlation between the catalytic activity of polyoxometallates and the special features of their tunnel and optical spectra

The tunnel spectra of phosphomolybdic acid, a classic heteropoly acid with a Keggin anion, were measured in ultrahigh-vacuum experiments with the use of scanning tunnel microscopy. The dependences of the resonance characteristics of the spectra, “negative differential resistances,” on the vacuum gap, material of contacts, and field polarity were studied. An earlier unknown mechanism of the formation of these characteristics was described. The mechanism included the action of strong electric fields in scanning tunnel microscope nanocontacts and a low degree of the delocalization of Keggin anion peripheral electrons. Strong electric fields (∼107 V/cm) characteristic of spectroscopic measurements with the use of scanning tunnel microscopes could break exchange bonds in heteropoly acids and their derivatives. This produced spectroscopic effects of interest for catalysis and nanoelectronics.

Single nanodimensional emitting protrusion on the surface of a tungsten carbide field emitter

Variations in the shape of a tungsten carbide emitter under the simultaneous action of strong electric fields and high temperatures have been studied by field emission microscopy techniques. Using controlled decrease in the applied electric field strength at a certain temperature of the emitter, it is possible to grow a single nanodimensional surface protrusion capable of emitting charged particles with stability comparable to that of carbon-based materials. The values of emission currents, current densities, emission angles, and reduced brightnesses are comparable to those known for emitters based on carbon nanotubes. Advantages of single nanoprotrusions as emitters are their complete reproducibility and the ability to emit both electrons and ions

Specifics of structural transformations in poly(vinylidene fluoride)-based ferroelectric polymers in high electric fields

The behavior of polymorphic transformations in the solid phase under the action of strong electric fields in crystallizable ferroelectric polymers based on PVDF was analyzed. The occurrence of two transitions, α → αp and α → β, was revealed by means of IR spectroscopy in homopolymer films crystallized as a mixture of the nonpolar α phase and the polar β phase. If such films were crystallized into the ferroelectric β phase, the action of the field reduced to the processes of reorientation of chain segments in the all-trans (planar zigzag) conformation. Reversible and irreversible changes in the crystallinity are possible in this case. A shift in the frequency of some skeletal-vibration bands indicates a change in the mechanical stress on atomic bonds. For vinylidene fluoride copolymers in polar crystals, the field can initiate molecular rearrangements, which lead to an increase in the interchain density of chain packing in the lattice. It was assumed that the delayed kinetics of field-induced solid-state transformations is controlled by two factors. On the one hand, the nucleation of the new phase follows the fluctuation mechanism of appearance of conformation defects of the kink-link type in the initial crystal. An important role is played in such processes by the dynamics of amorphous-phase tie chains bordering the crystals, when a decrease in their activation energy in micro-Brownian motion processes increases the probability of a conformational defect appearing in the crystal. On the other hand, the role of space charge (including the charge due to carrier injection from the electrode metal) in the formation of a local electric field is substantial. Allowance for both factors may provide a qualitative explanation of the specifics of the kinetics of these structural transformations in an electric field.

Structural changes in ferroelectric polymers under the action of strong electric fields by the example of polyvinylidene fluoride

The regularities of solid-phase polymorphic transformations under the action of strong electric fields in polyvinylidene fluoride (PVDF) films have been analyzed. For the homopolymer films crystallized in a mixture of nonpolar α and polar β phases, two transitions are revealed: α → αp and α → β. In contrast to predictions, these transitions occur simultaneously. For the isotropic PVDF films crystallized in the α phase, the critical fields corresponding to their transition to the ferroelectric β phase may differ by almost an order of magnitude. It is suggested that nucleation of a new phase occurs through the fluctuation mechanism of formation of conformational defects (of the kink-bond type) in the initial crystals. Such processes are controlled to a great extent by the dynamics of through chains in the amorphous-phase regions adjacent to crystals when a decrease in their activation energy in the processes of micro-Brownian motion increases the probability of forming conformational defects in a crystal. If PVDF is crystallized in the ferroelectric phase, the action of strong fields leads to two consequences. First, according to the X-ray diffraction data, rotation of the polar b axes of a crystal through the mechanism of 60° reorientations is observed. Second, the degree of crystallinity can increase (partially irreversibly) through an increase in the crystal size.

Temperature-and field-induced changes in the form of the Mo-Hf alloy

Simultaneous action of strong electric fields and high temperatures on point field emitters made of the Mo-15%Hf alloy is studied by field emission methods. Such alloys enriched with an emission-active component and containing the Mo2Hf intermetallide exhibit basically the same stages of temperature-and field-induced changes in their form as for pure metals, although a number of peculiarities associated with the surface segregation of Hf also exist. Thermal-field processing of emitters was accompanied with high-temperature field evaporation and emission of predominantly Hf (both atomic and cluster-type) ions. Thermal-field processing also enhances emission localization, but to a smaller extent as compared to that in alloys with a low Hf concentration.

Field emission energy distributions of electrons from tungsten tip emitters coated with diamond-like film prepared by ion-beam deposition

The energy distributions of electrons emitted from the surfaces of diamond-like tip cathodes under an action of strong electric field (10 7 V/cm) are presented. The diamond-like coatings were prepared on very thin tungsten tips with the curvature radii not more than 1 μm by means of ion-beam deposition under ultrahigh vacuum conditions (10 −7 Torr). The structure of the carbon films on tungsten tips was investigated by means of field emission microscopy. Also, the results on temporal stability of the field emission current of the cathodes and Fowler–Nordheim plots have been obtained. The work function of the diamond-like surface was estimated to approximately 2 eV. On the basis of experimental results the model of tungsten field emission cathode with relatively thin diamond-like film explaining the field emission energy distributions was proposed.

Analysis of the interaction of cathode microprotrusions with low-temperature plasmas

On real cathode surfaces which are in contact with plasmas, micrometre-sized protrusions are abundant. A two-dimensional numerical model has been developed to investigate the interaction of such a cathodic micro-tip with a low-temperature plasma. The model allows integration and analysis of the various physical effects and an evaluation of their specific role in the heating, emission and erosion of cathode surfaces. In particular, the influence of heating due to the intense ion flux from the plasma sheath onto the cathode surface has been investigated in detail. Special attention was paid to the simultaneous action of strong electric fields in the cathode fall region. The plasma-cathode interaction is analysed for a range of parameters typical for high-current-density, low-pressure gas discharges and vacuum arcs. The temporal evolution of the thermophysical properties of the micro-tip, its interaction with the resulting metal vapour and its shape are modelled on a nanosecond time scale. Special cases of the evolution of microemitters have been identified, such as quasi-stationary emission on a microsecond time scale and extremely fast, explosive-like behaviour with emitter lifetimes on a nanosecond time scale. It is found that ion bombardment heating plays an essential part in the inception of emissive and erosive behaviour for a wide range of parameters.

The behaviour of [Pt(η3-allyl)XP(C6H5)3] complexes in electrospray ionization conditions compared with those achieved by other ionization methods

Three Pt(II) η3-allylic compounds, [Pt(η3-allyl)XP(C6H5)3] (X = Cl, Br, I) have been studied under electrospray ionization (ESI) conditions. Unexpected behaviour, such as the complete lack of either [M]+. or [MH]+ ions, the formation of complexes with the solvent and bis-triphenylphosphino-containing species, was observed. To rationalize such results, a series of parallel measurements on the same compounds have been performed under electron impact, fast-atom bombardment and field ionization conditions. The data so obtained suggest that molecular species are not produced by ESI, probably due to the action of strong electric fields and collisionally induced decompositions occurring in different regions of the instrument. Complexes with the solvent originate through gas-phase ion–molecule reactions, while the bis-triphenylphosphino-containing species are formed by reaction of ions with triphenylphosphine formed by thermal decomposition inside the heated capillary line, just after the electrospray ionization source. © 1997 John Wiley & Sons, Ltd.

On the role of auroral electric fields in the formation of low altitude sporadic- E and sudden sodium layers

The characteristics of metallic and molecular ion sporadic- E ( E s ) layers, formed by the action of strong electric fields at auroral latitudes, are examined using computer simulations. It is found that, for electric fields directed between northward and westward (northern hemisphere), thin metallic ion layers (<2 km thick) can be formed above about 105 km altitude. For electric fields directed from westward, through southward, to south-eastward, slightly thicker (4–6 km thick) metallic ion layers can form between 90 and 105 km altitudes. Thin layers of molecular ions can be formed by electric fields directed between north and west if the ion density is low. Examples of E s layers observed by the EISCAT radar, together with simultaneous observations of electric fields and ion drifts are presented which show good agreement with the simulations. The relationship between the lower-altitude E s layers and sudden sodium layers (SSLs) is discussed leading to an explanation of some of the characteristics of SSLs at high latitude. A possible involvement of smoke particles in the formation of both E s layers and SSLs is proposed.

The action of strong electric fields on the current from a thermionic cathode

It is well known that the “Saturation Current” from a thermionic source rises continuously as the applied voltage is raised and in reality never reaches a saturation value. W. Schottky predicted this rise in the saturation current on theoretical grounds and obtained a rough experimental confirmation of his theoretical formula. The present investigation was undertaken to find what connection it has with the emission of electrons from cold metals under the action of intense electric fields. If the truth of Schottky’s theory be assumed it is possible to deduce the value of the electronic charge e from observations of the rise of the saturation current with the applied field. It was not the primary intention of the author to determine the value of e in this way, nevertheless the results obtained show that the method might be developed into one of some accuracy, and thus to give a value of this important constant in a way differing completely from that of Millikan (oil drop method), Perrin (Brownian movement) or of Hull and Williams (shot effect).

The Action of Strong Electric Fields on the Current from a Thermionic Cathode

The Action of Strong Electric Fields on the Current from a Thermionic Cathode