Positive streamers: inception and propagation along mineral-oil/solid interfaces

Electronegative gas components and gas pressure significantly change residual charge dynamics, which are critical for pulse-periodic streamer discharge behaviors. Evolutions of repetitively pulsed positive streamer discharge and the streamer-to-spark transitions were investigated at high pressures and compared between typical weak (O2) and strong (SF6) electronegative gas mixtures. Pulse-sequence resolved electrical and optical diagnostics were implemented to capture discharge evolutions in long pulse trains. We observe that streamer inception and propagation under subsequent pulses in N2 and N2–O2 mixtures are similar, including the earlier inception of the primary streamer and the accelerated propagation of the secondary streamer. The repetitively pulsed breakdown is extended to the low pulse repetition frequency region with the addition of O2. Discharge evolutions are unexpectedly different in N2–SF6 mixtures. Subsequent discharge channels prefer to propagate around the periphery of the inception cloud region with large radial deviations. Another difference is the precursor channel identified besides multiple streamer channels. Effects of electronegative gas on streamer evolutions under positive repetitive pulses have been qualitatively analyzed. Collisional electron detachment and photo-ionization are crucial in N2–O2 mixtures. With the presence of strong electronegative gas (SF6), the leader formation is probably induced by the earlier corona inception and longer voltage stressing period under following pulses in a pulse train, which are caused by the strong electronegativity of SF6 and the small ion mobility.

Physical mechanisms for streamer initiation and electrical breakdown in water are discussed. The focus is on various processes that could lead to the inception and propagation of streamers in water. Physical details associated with observed disparities in breakdown at the anode versus the cathode, differences in luminosity, and variations in streamer propagation speeds are discussed. Creation of a localized low‐density region near the site of a field enhancement is shown to be essential for streamer initiation. Finally, the use of a trigatron‐type electrically triggered switch with a triple‐point geometry is shown to be a low‐cost, compact alternative to laser (or acoustic) switching. Such controlled breakdown could be used for various applications including water purification and bio‐decontamination.magnified image

Streamer propagation physics is investigated in 2D and 3D pin-to-plane geometries at atmospheric pressures using a PIC-DSMC code with an air model comprised of standard Townsend breakdown (electron-neutral elastic, excitation, ionization, attachment, and detachment collision chemistry and secondary electron emission) and streamer mechanisms (photoionization and ion-neutral collisions)1 via tracking excited state neutrals which can then either quench through collisions or spontaneously emit based on transition-specific Einstein-Acoefficients2,3. Here we model a 100 $\mu \mathrm{m}$ hemispherical pin electrode (at 10 kV) 1.5 mm from a planar cathode at ground. The gap is filled with air at 760 Torr and a 0.5 eV, 1020m-3 plasma is used to seed the simulation domain. Prior studies have shown that the reduced electric field (E/n) can have a significant impact on streamer evolution4,5 and here we investigate how the approximate inverse (2D) and inverse-square (3D) falloff in field effects streamer evolution. To reduce computational costs, the 3D computational domains are wedges. We sweep the wedge angle from 10 to 45 degrees to examine the influence of wedge angle on streamer branching and propagation. Other parameters of interest include streamer velocity and inception cloud formation.

Time of flight-secondary ion mass spectrometry analysis of protein adsorption on a polyvinylidene difluoride surface modified by ion irradiation

The accumulation of plastics in the soil ecosystem poses an increasing environmental concern worldwide. However, little is known about the effect of plastic concentrations on soil properties and soil biota. In this study, we investigated the effect of low‐density polyethylene (LDPE) microplastics (MPs) on the chemical and microbial properties of agricultural soil using a set of microcosm experiments. The soil was incubated for 100 days with LDPE at concentrations of 0%, 0.1%, 1%, 3%, 5%, and 7% at 25°C with 70% water‐holding capacity. Along with soil chemical analysis, we conducted an analysis of soil microbial properties on the first day and again after 100 days of incubation. LDPE concentrations of ≥1% significantly ( p < .05) decreased the pH but increased the electrical conductivity of the soil in comparison with the control (0% LDPE at 100 days). Increasing the LDPE concentration did not affect the soil exchangeable cation content or the available Pb concentration. Firmicutes were the most abundant phyla in the soil on the first day, whereas Proteobacteria, Firmicutes and Actinobacteria became dominant in all treatments after 100 days. An increasing LDPE concentration increased the abundance of Actinobacteria and decreased Proteobacteria. Principal component analysis demonstrated that only 7% LDPE was positively correlated with Actinobacteria, indicating that higher concentrations of LDPE contributed to the growth of this phylum. The findings of this study imply that MP contamination could affect soil chemical properties and microbial activity and that these effects primarily depend on MP concentrations in soil.

High Klason lignin content (23 wt%) cellulose nanofibrils (LCNF) were successfully isolated from eucalyptus pulp through catalyzed chemical oxidation, followed by high-pressure homogenization. LCNFs had a diameter of ca. 13 nm according to AFM evaluation. Dense films were obtained through vacuum filtration (nanopaper) and subjected to different drying methods. When drying under heat and mild vacuum (93 °C, 95 kPa) a higher water contact angle, lower roughness and oxygen transmission rate were observed, compared to those drying at room temperature under compression conditions. DSC experiments showed difference in signals associated to Tg of LCNF compared to CNF produced from spruce bleached pulp through enzymatic pre-treatment. The LCNF-based nanopaper showed mechanical properties slightly lower than for those made from cellulose nanofibrils, yet with increased hydrophobicity. In summary, the high-lignin content cellulose nanofibrils proved to be a suitable material for the production of low oxygen permeability nanopaper, with chemical composition close to native wood.

Residual charges residing at gas volume and trapped at the solid dielectric surface are crucial memory effect agents for evolutions of repetitively pulsed surface streamer discharges. The low-photon-energy laser irradiation can intentionally release and then exploit volume and surface residual charges to regulate discharge characteristics, enabling more flexibilities than conventional voltage waveform modulation methods. However, important parameters and regulation mechanisms of the laser irradiation on evolutions of repetitively pulsed surface streamers are still ambiguous. Evolutions of repetitively pulsed surface streamers in atmospheric pressure air were investigated after low-photon-energy pulsed visible (532 nm) laser irradiations. The effects of the laser irradiation modes and pulse repetition frequency (PRF) dielectric material on streamer inception and propagation features were determined by pulse-sequence and temporally resolved diagnostics. The laser irradiation and increased PRF have similar effects on the evolution of streamer light emission. PRF had little effect on the streamer velocity. When the relative permittivity of the dielectric is smaller, the number of streamer branches increases, which is dominated by the photoionization. Interestingly, the morphology and propagation velocity of the streamer vary greatly depending on the laser irradiation mode. Streamer characteristics exhibit noticeable changes only when the laser perpendicularly irradiates the solid surface rather than the laser beam is tangent to (slightly above) the surface. The effect of laser-induced surface trapped electron desorption on streamer characteristics is analyzed, focusing on the distribution morphology and density of desorption electrons.

Deals with prebreakdown and breakdown behavior of composite liquid-solid-insulation with multiple solid-barriers in liquid gaps submitted to impulse voltages. Experiments have been carried out in strong and weakly inhomogeneous fields with gap distances from 8 to 34 mm, respectively, using impulse voltages up to 700 kV. Configuration was close to transformer insulation with up to three pressboard layers of 2 mm thickness each in the liquid gap. Comparisons between streamer behavior in single and multiple barrier insulation are presented. Streamer inception and propagation was visualized via highspeed shadowgraph photography. Compared to pure liquid insulation an increase of the insulation ability of 470% of the pure liquid gap was observed, most likely due to field homogenisation through solid barriers. Streamer inception field strengths differ little from phenomena in pure oil gaps. Microdischarges appear at board surfaces closest to electrodes, leading to Lichtenberg-type figures on the board surface which cause degradations of the board. Investigations were made with pre-stressed and new pressboard.

Although the removal of lignin and hemicelluloses from cellulose pulp to produce fully bleached cellulose nanofibrils (B-CNF) is the most common practice, the presence of residual lignin and hemicelluloses in raw materials for the production of lignin containing cellulose nanofibrils (LCNFs) holds several advantages. In this work, we investigated the effect of residual lignin in Eucalyptus globulus cellulose fibers on the properties of the resulting LCNFs. The stability of the colloidal suspensions was assessed by zeta-potential values and charge density analyses. Morphology of the CNFs was studied using scanning electron microscopy and atomic force microscopy. Fibril diameter and diameter distributions for CNFs with different levels of residual lignin showed a decrease on fiber diameter as the lignin content increases. Differences in the chemical composition of the CNFs was evidenced as indicated in the Fourier-transform infrared spectroscopy spectra, particularly in fingerprint region. Thermal behavior of the CNFs was not altered by the presence of lignin, as indicated by thermogravimetric analysis. Finally, the rheological behavior of the samples was evaluated observing a gel-like behavior as well as an increase of the viscosity in LCNFs with higher lignin contents.

Lignin containing micro and nano-fibrillated cellulose obtained by steam explosion: Comparative study between different processes

Cellulose nanofibrils ( NF ) have several advantages such as biodegradability and safety toward human health. Zein is a biodegradable polymer with potential use in food packaging applications. It appears that polymer nanocomposites are one of the most promising applications of zein films. Cellulose NF were prepared from starting material Microcrystalline cellulose (MCC) by an application of a high-pressure homogenizer at 20,000 psi and treatment consisting of 15 passes. Methods such as atomic force microscopy were used for confirmation of nanoscale size production of cellulose. The average diameter 45 nm were observed. Zeincellulose NF nanocomposite films were prepared by casting ethanol suspensions of Zein with different amounts of cellulose NF in the 0% to 5%wt. The nanocomposites were characterized by using Fourier transform infrared spectroscopy ( FTIR ), Atomic force microscopy ( AFM ) and X-ray diffraction ( XRD ) analysis. From the FTIR spectra the various groups present in the Zein blend were monitored. The homogeneity, morphology and crystallinity of the blends were ascertained from the AFM and XRD data, respectively. The thermal resistant of the zein nanocomposite films improved as the nanocellulose content increased. These obtained materials are transparent, flexible and present significantly better physical properties than the corresponding unfilled Zein films.

The formation and propagation of pseudo-filamentary dielectric barrier discharge in atmospheric air are investigated through a 2D fluid model. The discharge development can be divided into three stages: the volume streamer stage, the surface streamer stage, and the reverse discharge stage. The simulations show that the streamer head becomes wider and the electron density of the volume streamer head increases six times when the volume streamer interacts with the dielectric, and the volume streamer transforms into the surface streamer after the interaction. Compared with volume streamers, surface streamers have a smaller radius, a higher electric field, and a higher electron density. Furthermore, the parameters that may influence the discharge characteristics are also studied. It is found that a larger dielectric permittivity, a thinner dielectric, or a shorter voltage rise time leads to earlier inception of volume streamers, faster propagation of surface streamers, and higher current density. It is observed that the velocity of the surface streamer increases first, and then, decreases with the accumulated charges on the surface.

The effect of dc prestress on positive streamer initiation and propagation was investigated at atmospheric conditions in a small liquid-filled point-plane gap, representing a model for the insulation in power electronic components. Shadowgraphic imaging and photomultipliers to record emitted light were used. The applied voltages were step voltages (100 ns rise time) with or without a dc bias. The results obtained under step voltage show that many features of positive streamers (shape, velocity, initiation voltage) in Midel 7131 fluid are the same as those previously observed in hydrocarbon and mineral oils in similar test conditions. The presence of space charges in the vicinity of electrodes was demonstrated. The superposition of a same polarity dc prestress on the applied step voltage results in the increase of streamer inception voltage and the decrease of propagation lengths. For a polarity reversal for positive steps the initiation voltages were reduced and propagation lengths increased.

In this study, a biodegradable and sustainable piezoelectric–triboelectric hybrid nanogenerator (HENG) was designed and fabricated using cellulose nanofibrils (CNFs) as a nucleating agent and phycocyanin (PC), an algae-derived protein from Spirulina platensis, as the tribo-positive layer paired with poly(vinylidene fluoride) (PVDF) films. The investigation was conducted in two parts to investigate the effect of CNF incorporation into different layers. In the first part, CNFs were incorporated into the PVDF layer, whereas in second part incorporated into the PC layer. Electromechanical performance was characterized under periodic contact–separation motion. Even in the absence of nanofillers, the PVDF–PC pair exhibited efficient electromechanical behavior, generating an open-circuit voltage (V oc ) of 84 V and a short-circuit current (I sc ) of 87 µA. Upon CNF incorporation, the output was significantly enhanced. The highest performance was observed when CNFs were added to the PVDF layer (at 20 wt.% CNFs loading) with V oc and I sc values of 228 V and 199 µA, respectively. On the other hand, when CNFs were incorporated into the PC layer and paired with neat PVDF, the maximum output reached 128 V and 126 µA (at 5 wt.% CNFs). This outcome highlights that CNFs contribute more effectively to performance improvement in PVDF than in PC.

When studying the properties of cellulose nanofibrils (CNF) enriched fiber products, it is essential to be able to determine the retention and the spatial distribution of the CNF inside the end-product. That is, to determine how much and where the CNF has been attached. As the CNF and cellulose fibers share the same density and chemical composition, labeling of the CNF is required to distinguish them from each other. In this work, we have applied iron and iron-cobalt -labeling. Labeling with iron is more desirable because of the carcinogenic and toxic properties of cobalt chloride. The benefits of our labeling method are the possibility to determine the retention of the labeled nanocellulose using inductively coupled plasma optical emission spectroscopy (ICP-OES), and to define the spatial distribution using X-ray micro-computed tomographic (XμCT). With XμCT we were able to measure fairly large samples (2 cm × 5 cm × 5 cm). Our study found that the retention of iron-labeled CNF was about 95 % and that of iron-cobalt labeled CNF was 84–94 %. Labeling of CNF improves the contrast of X-ray images. Labeled CNF is attached to fiber network also in the inner structures of the sample. Furthermore, when making thick porous structures using cationic starch, there might be agglomerates in the sample that cannot be visually detected by looking the sample.