Solid Dielectric Surface Research Articles

Dynamics of Nonisotherhmal Plasma Rotating near Solid Dielectric Surfaces

Dynamics of Nonisotherhmal Plasma Rotating near Solid Dielectric Surfaces

Experimental and theoretical study of electrowetting dynamics on slippery lubricant-infused porous surfaces

The droplet electrowetting dynamic behaviors on slippery lubricant-infused porous surfaces (SLIPS) remain elusive because the soft liquid-liquid interface nature is much different from solid dielectric hydrophobic surfaces. To understand the dynamic process, the impact of the voltage applied, the oil thickness, and viscosity on dynamic electrowetting behavior was experimentally studied, respectively. Meanwhile, a numerical dynamic model was also developed for the quantitative interpretation of the droplet dynamic spreading process on SLIPS. It is found the droplet always spreads smoothly to the equilibrium state without overshooting on the SLIPS. The droplet is always over-damped with the increase of applied voltage, the settling time is proportional to the 0.9th power of the electrowetting number. Then, by changing the silicone oil viscosity, it is found the viscous dissipation of the oil-water interface becomes dominant, causing the droplet to spread slowly. By fitting the theoretical models to experimental results, it is found the friction coefficient is nearly proportional to 1/6th power of oil viscosity and rarely influenced by applied voltage and oil thickness. Finally, it is found both the initial oil thickness and the high wetting ridge have a minor influence on the electrowetting dynamic spreading. The relationship between the actual oil layer thickness and the initial oil layer thickness was estimated. This study will provide helpful information and theory support for electrowetting-on-dielectric device design, lab on a chip, and other potential applications on SLIPS.

Influence of voltage and gap distance on the dynamics of the ionization front, plasma dots, produced by nanosecond pulsed discharges at water surface

A streamer discharge is a highly reactive and dynamic non-thermal plasma. It has been used in many applications, including environmental remediation, medicine, and material processing. Although the physics of streamer discharges in gaseous media is well understood, its interaction with a solid and liquid dielectric surfaces remains under investigation, in particular when quantitative data are searched for. In this study, we investigate the influence of voltage amplitude (V a) and interelectrode air gap distance (d) on the pulsed discharge behavior at the surface of distilled water. Time resolved images show the formation and propagation of plasma dots (ionization front of streamers) at water surface. Because of its stochastical nature, a large number of discharge was performed to address the influence of V a and d on the number of plasma dots (N Dots) as well on the charge per dot (Q Dot). As expected, for a given V a, the breakdown voltage is found to increase with d. Moreover, N Dots decreases linearly with d at the rate of ∼1 dot by 200 μm of increase, while the total injected charge decays linearly with a rate of ∼8–9 nC by 200 μm of increase. Based on the measurement of the propagation velocity of the plasma dots and on the estimation of the electric field in the medium, an average mobility of plasma dots of ∼1.5 cm2 Vs−1 is evaluated. From both this value and the instantaneous measured propagation velocity, the temporal evolution of Q Dot and charge number are determined. The observations reported here are of interest for fundamental studies as well as for applications where well-controlled charge transfer to surfaces is crucial.

Microwave Resonance Method for Measuring Microliter Volumes of Free Moisture of Aviation Fuels

One of the conditions for ensuring the safety of air transport operation is the quality of aviation fuel refueled in aircraft. Fuel quality control is a multi-parameter task that includes monitoring the free moisture content. Regulatory documents establish the free water content of not more than 0.0015 wt.%. A direct electrometric microwave resonator method for controlling free moisture in aviation fuels has been developed. The method consists in measuring the resonance characteristics of a cylindrical cavity resonator. At that, on a solid dielectric surface inside the resonator, there are moisture drops that are pressed by a thin plate. As a result, dielectric losses increase dramatically. Analytical and experimental analyses of the proposed method are carried out. The monitoring range of 0.5–30 μl of the absolute moisture volume in aviation fuels with a maximum error of no more than 25% is justified. The sensitivity of the proposed method for monitoring microwave losses in free moisture drops transformed into a thin layer by pressing with a thin plate is an order of magnitude higher than the sensitivity of the method for monitoring microwave losses in moisture drops on a solid surface in a resonator. The proposed method can be used as a basis for the development of instruments for monitoring the free moisture of aviation fuels in airfi eld and laboratory conditions. The direction of development of the method is shown.

Unraveling the transition from secondary electron emission dominated to surface charge trap dominated electronic avalanche process along the solid dielectric surface in vacuum

Electron-surface interaction plays a fundamental role in surface science, which could evolve into an electronic avalanche under a high electrical field, resulting in devastating device failures. In the present study, using a thin polymer film approach, the effective surface layer, whose thickness is estimated to be about 200 nm, where the electron-surface interaction occurs is directly probed. The morphological evolution of thin polymer films with thicknesses from nanometers to micrometers is investigated with a focus on its influence on the electron avalanche process (or flashover) under a dc electric field in vacuum. It is found that the film thickness dependence of flashover is divided into three parts, i.e., fast increase, slow increase, and saturation, each of which has a dominant microscopic mechanism. The results indicate that the secondary electron emission (SEE) yield decreases significantly even when a discontinuous polymer layer is deposited, which varies little afterward. In contrast, the shallow surface traps develop into deep ones with the film thickness. The density ratio of deep traps increases exponentially after a continuous film is formed. The clear transition from SEE dominated to surface charge trap dominated flashover and their unique dependence on film morphology provide a deeper insight into the electron-surface interaction, which can be used for theoretical modeling, surface modification, and advanced functional devices.

The structure of field-induced near-wall charged layers arising in weakly conducting liquids near the surface of solid dielectrics

The present paper investigates space charge formation that occurs near the surface of a solid dielectric in a weakly conducting liquid and takes place under the action of the strong external electric field and due to constant dissociation. The cases of 1D and a more complex 2D macro scale geometries are studied numerically using Comsol Multiphysics software. In the general case, the charge accumulates in a diffuse layer and can get in the bulk in consequence of its transport along the dielectric surface; and there is also a dissociation-recombination layer of ion deficit (same as in the electrohydrodynamic conduction pumping).

A stochastic model for contact surfaces at polymer interfaces subjected to an electrical field

Abstract Morphology of the contact area between solid insulation materials ultimately determines the short- and long-term electrical properties of the complete insulation system. The main purpose of this paper is to propose a statistical model to examine the real area of contact between solid dielectric surfaces and secondly to verify and correlate the model outputs with experiments. The model computes real area of contact, number of contact spots and average cavity size at the interface as a function of elasticity, contact force and surface roughness. Then, using the average cavity size and the Paschen's law, the discharge inception field of the cavity (CDIE) is estimated. AC breakdown strength (BDS) testing of solid-solid interfaces was carried out, where cross-linked polyethylene (XLPE) samples with four different surface roughnesses were tested at various contact pressures. Following the increased contact force, the calculated average cavity size decreased by a factor of 4.08−4.82 from the roughest to the smoothest surface, corresponding to increased CDIEs by a factor of 2.01−2.56. Likewise, the experimentally obtained BDS values augmented by a factor of 1.4−1.7 when the contact pressure was elevated from 0.5 MPa to 1.16 MPa. A linear correlation between the CDIE and BDS was assumed, yielding a correlation coefficient varying within 0.8−1.3. When the 90% confidence intervals were considered, the range reduced to 0.86−1.05. This correlation suggests that interfacial breakdown phenomenon is strongly related to the interfacial cavity discharge. Hence, the proposed model is verified with experiments.

Inversion Algorithm to Calculate Charge Density on Solid Dielectric Surface Based on Surface Potential Measurement

Charge accumulation on a solid dielectric surface is one of the critical concerns for the design and optimization of the insulation system in a high-voltage power equipment, since it will lead to the overstress of electrical insulation. Therefore, it is important to obtain the charge density distribution on a solid dielectric surface with high accuracy. The acquisition of surface charge for insulators requires multipoint potential measurements to establish the inverse calculation for the determination of an unknown charge distribution. Up to now, extensive studies have been conducted on this subject; nevertheless, the methods are either too complicated and time consuming, or only applicable for specific arrangements, or with poor accuracy. In this paper, the problem is divided into two categories, i.e., shift-variant system and shift-invariant system, and the basic principle of an improved inversion algorithm is interpreted to solve the problem. The 2-D Fourier transform and Wiener filter techniques are employed in the algorithm for shift-invariant system thus the relationship between potential and charge density can be processed in spatial frequency domain, which tremendously simplifies the conventional procedure. The accuracy and resolution of the algorithm are discussed in detail with the aid of numerical examples. In the end, experiments are conducted and the effectiveness of the algorithm is verified.

Electrowetting on dielectrics on lubricating fluid-infused smooth/rough surfaces with negligible hysteresis

Low-voltage electrowetting on dielectrics on substrates with a thin layer of lubricating fluid to reduce contact angle hysteresis is reported here. On smooth and homogeneous solid surfaces, it is extremely difficult to reduce contact angle hysteresis (contact angle difference between advancing and receding drop volume cycle) and the electrowetting hysteresis (contact angle difference between increasing and decreasing voltage cycle) below 10°. On the other hand, electrowetting hysteresis on rough surfaces can be relatively large (~30°); therefore, they are not useful for most of the fluidic devices. In the present report, we demonstrate that using a thin layer of dielectric lubricating fluid on top of the solid dielectric surface reduces the contact angle hysteresis as well as electrowetting hysteresis below 2° on smooth as well as rough surfaces. Electrowetting on lubricating fluid-coated surfaces also show a threshold behavior and the threshold voltage depends on the viscosity of the lubricating fluid. Modified Lippmann equation is used to explain the electrowetting on lubricant-coated surfaces quantitatively. The experimental system can be modeled as two series capacitor, one each for dielectric lubricating fluid and solid dielectric, which jointly govern the electrowetting behavior, whereas the lubricating fluid also minimizes the contact angle hysteresis

Laboratory Measurement of Lunar Regolith Simulant Surface Charging in a Localized Plasma Wake

Laboratory experiments are carried out to measure the charging of Johnson Space Center Number OneA lunar simulant surface in a localized plasma wake similar to that at the lunar terminator. The results show that the floating potential of a regolith surface in a plasma wake may be significantly different from that of a solid dielectric surface because ion collection on a regolith surface is sensitively influenced by the surface collection area of dust grains. The results also suggest that at the same negative charging potential, an isolated dust grain charges 50 times more than a dust grain on the regolith surface.

Theoretical and simulation research on self-stabilizing secondary electron emission process across solid dielectrics in vacuum

Recently, the cathode-like luminescence phenomena are observed from the surface of solid dielectrics under steady-state high voltage excitation in vacuum, which is closely related to the self-stabilizing secondary electron emission (SSEE) occurring on insulator surface. In this paper, the theoretical analysis about surface charging based on the SSEE is made, and a one-dimensional Mont Carlo simulation on surface charge accumulation under DC voltage excitation is provided. The results reveal that surface charging process can be divided into three stages including initial accumulation, fast multiplication, and final stable stage. In the final stage, the SSEE phenomena are achieved and surface charges are invariable and reach up to a stable distribution. The simulated stable surface charge distribution is consistent with the theoretical deduction results, which confirms the existence of SSEE process.

Influence of the solid dielectric surface structure on the electric field at an air-barrier boundary

The distribution of electric field in air near a solid dielectric plate has been calculated using a model that takes into account the degree of roughness of the dielectric surface. Calculations performed for pyroceram show that the electric field strength at the vertices of juts on the dielectric surface can be 2.5IaeI3.5 times (depending on the degree of surface roughness) greater than the values in the air gap. At the same time, the field strength in hollows is about ten times weaker than that at the juts. The obtained results agree with experimentally observed [1] changes in the intensity of discharge processes at a dielectric surface with increasing degree of roughness.

On the nature of the Vorob’evs effect in pulse breakdown of solid dielectrics

The Vorob’evs effect consists in certain features of the discharge observed when a solid dielectric in contact with two rodlike electrodes is placed in a liquid dielectric medium and a voltage pulse with increasing front is applied to the electrodes. When the pulse front slope is small, the discharge develops in the liquid over the solid dielectric surface; whereas the discharge at a sufficiently large slope of the pulse front penetrates into the solid and produces its fracture with cleavage of the surface fragments. In order to explain this phenomenon, it is suggested that, at a sufficiently high voltage buildup rate, a displacement current that is related to the motion of the surface discharge plasma passes through a microprotrusion occurring on the electrode surface at the contact site and causes the electric explosion of this microprotrusion. The metal plasma jet generated as a result of this explosion penetrates into the solid dielectric and forms a discharge channel in depth of this material. The surface discharge plasma formed at a small slope of the voltage pulse front closes the electrode circuit, thus preventing the discharge penetration in depth of the solid.

Creepage discharge characteristics over solid-liquid interfaces with grounded side electrode

The properties of streamers traveling over the surface of oil-immersed solid dielectrics were experimentally studied under lightning impulse conditions. Streamer polarity and the position of a grounded side electrode significantly affected the relationship between the streamer extension length and the applied voltage. Solid surface charging also had a large effect on the streamer propagation. However, the streamer propagation properties showed a consistent dependence on the potential at the solid-liquid interfaces. In addition, the potential drop inside the streamer channel was measured as a function of normalized streamer length. The curve revealed that the potential drop increased drastically within the region of /spl sim/20% from the streamer tip. The streamer appeared to progress with a constant mean velocity.

Time-resolved measurements of energy transfer at surfaces

Developments in time-resolved measurements of energy transfer at surfaces are reviewed. Picosecond and femtosecond measurements of vibrational and electronic relaxation at surfaces are highlighted. Experimental results for vibrational relaxation of simple adsorbates on metals, semiconductors, and insulators are reviewed, and relaxation mechanisms such as electron-hole pair formation, multiphonon relaxation and image dipole damping are considered. Energy transfer involving excited electronic states of molecules on liquid and solid dielectric surfaces and the relaxation of surface electronic states on semiconductors and of image states on metal surfaces are discussed.

Analytical possibilities of elemental analysis of water solutions based on the emission spectrum of a laser spark on the surface

The possibilities of spectrochemical methods of analysis of substances using lasers for atomization were investigated in detail in [1-4]. In particular, the formation and development, in both time and space, of a laser spark on the surface of metals and solid dielectrics were studied in detail [5, 6]. The investigations showed that the emission line spectrum of the laser plasma arises after the continuous background emission pulse, which lasts for several hundreds of nanoseconds [7]. The efficiency of excitation of the atoms and ions formed and the temporal characteristics of their emission depend on the power of the laser pulse [8, 9]. In addition, the ratio of the line and continuous emission intensities depend significantly on the spatial arrangement of the emitting zone in the plasma flame. All these investigations have shown that the contrast of the emission spectrum can be significantly increased [i0, ii] by optimizing the plasma flame formation process (by changing the density of the medium into which the material vaporized by the laser radiation expands, using two irradiation pulses, and optimizing the spatial and time intervals for the measurements), and in the process the relative detection limit can reach a value of the order of 10-5% [6, 7], while the sensitivity of the direct laser spectral analysis, which is of the order of 0.01% [4], is too low.

Stochastic properties of partial-discharge phenomena

The author presents a bibliography and survey of the literature concerned with theory and measurement of the stochastic behavior of pulsating partial-discharge (PD) phenomena that can occur when insulation is subjected to electrical stress. The types of PD phenomena considered include AC and DC generated electron avalanches, pulsating positive and negative corona in gases, and PD that occur in liquid media and in the presence of solid dielectric surfaces. The basic physical mechanisms of discharge initiation, growth, and memory propagation that determine the probability distributions for pulse occurrence times and pulse amplitudes are discussed. Consideration is also given to special problems associated with the measurement and interpretation of data on the various statistical properties of PD phenomena.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Flashover Characteristics of Impulse Voltages on Solid Dielectric Surfaces under High Humidity

For the design of electrical insulating devices, it is very important to observe discharge phenomena on the needle-to-plate electrode gap under high humidity. In this study, impulse flashover phenomena in a needle-to-circular-electrode gap on solid dielectric surfaces under a high humidity condition have been discussed. As a result, it was found that impulse flashover voltages are greatly influenced by both horizontal and vertical gap lengths. When the horizontal gap length is shorter than the vertical one, the flashover characteristics approach those in air. On the other hand, if the horizontal gap length is longer than the vertical one, characteristics more closely approach those on the dielectric surface. The flashover characteristics also depend upon the polarity of the needle electrode. The characteristics for positive polarity seems to be similar to those in air, and those for negative polarity approach the creepage characteristics on the dielectric surface.

Degradation and breakdown of solid dielectric materials resulting from surface discharges in air and in insulating liquids

An effort is made to identify some of the common aspects of the behavior of PDs (partial discharges) originating in different conditions and interacting with different solid dielectrics (SDs) in media of different density. Emphasis is placed on the behavior of PDs in the SD arrangement with a view to its use as a standard model system for testing the comparative resistance of various dry and impregnated solids to PDs appearing in flat and broad slits, such as those due to delamination and other defects which occur in many kinds of insulation. Observations were made on the propagation and intensity of the discharges as well as their effect on the solid dielectric surface. Results of measurements of light output in the optical range and of magnitude of the PDs are reported, and the resulting damage patterns and the breakdown probability characteristics are described.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

AC Discharge Characteristics of a Needle-to-Semicircular-Electrode Gap with Solid Dielectric Surface under High Humidity

In this study, the discharge phenomena combined with the air and the creepage discharges have been discussed experimentally with a simplified model electrode gap. A conventional frosted glass plate was used as the solid dielectric. As a result, some noticeable phenomena have been found. The flashover voltages on the solid surface depend upon the relative humidity in air and decrease with increasing humidity. Especially, when the vertical gap length (or the height) is given to the needle electrode to increase the flashover voltages, the flashover voltages in high humidity seem to be free from the vertical gap length of the needle electrode, in which the inclined angle formed with the horizontal gap length and the vertical gap length is less than 30°.