Atmospheric Breakdown Research Articles

Plasma formation during flash sintering of boron carbide – Part II: Effect on sintering and optimisation of process variables

Field assisted sintering techniques, such as Flash Sintering (FS), allow rapid heating and densification of ceramics within seconds at significantly reduced environmental temperatures. FS of oxides has been extensively researched since 2010, whereas the potential densification of non-oxides such as carbides is rarely investigated. In the first part of this investigation the atmospheric breakdown and plasma formation under FS-conditions relevant to SiC and B4C was researched. In the present work, the FS of B4C was investigated with and without plasma formation for comparison, and the influence and optimum combination of the other main FS-parameters determined. The results showed that at a furnace temperature of 1500 °C, it is possible to densify B4C by DC FS with or without an initial plasma formation. However, though plasma formation did not itself lead to densification of the material, the plasma-induced removal of the B2O3-layer on the surface of the powder led to a significant improvement in subsequent densification during FS. The effect of the Ar-plasma generated in removing the surface oxide from B4C-particles was confirmed by SEM and XPS. Other process variables such as carbon additives, thermal insulation and hold time also significantly influenced the densification of B4C by FS. With the optimised conditions, 96 % dense B4C, with a grain size of 10 μm and a hardness of 31 GPa (HV1) was produced by pressureless FS in 5 min at a furnace temperature of 1500 °C.

Numerical Modeling of Atmospheric Air Breakdown in a Weakly Nonuniform Electric Field Due to a Positive Lightning Impulse

Numerical Modeling of Atmospheric Air Breakdown in a Weakly Nonuniform Electric Field Due to a Positive Lightning Impulse

Clarifying the mechanisms of edge glow generation in CFRP laminates exposed to simulated lightning currents

This study examines edge glow mechanisms of CFRP laminates exposed to simulated lightning current by performing direct electrical conduction tests. An impulse electric current, in accordance with SAE ARP 5412B, was applied to the laminates. Numerical simulations based on finite element analysis (FEA) were performed to clarify the voltage and temperature distributions of the CFRP laminates. The analytical results were validated using the experimentally obtained I–V characteristics and temperature responses. Resultingly, edge glow was observed in quasi-isotropic but not in unidirectional and cross-ply laminates. The edge glow was suppressed more in silicone oil than in air. When the edge glow did not occur, the numerical simulation results, I–V characteristics, and temperature response, exhibited good agreement with the experimentally obtained results. Contrary to Joule heating, potential differences of several hundred volts between each layer, which cause dielectric breakdown in the testing atmosphere, are the criterion for edge glow formation.

The assessment of the atmospheric air breakdown voltage generated by the interaction between microwaves and metallic wires

The aim of this research is to understand the plasma initiation process generated by metallic wires when interacting with high energy density microwaves. Lead (Pb) and molybdenum (Mo) wires of 0.5 mm diameter were investigated in this experiment. The tip of the metallic wire was placed into the nodal point of a waveguide cavity attached to a microwave generator, where it was exposed to the high energy density of the microwave field. Following the interaction between microwaves and the metallic wire, a plasma was initiated having as effect the wire vaporization. The experiments were conducted in atmospheric air at ∼1 bar pressure. From optical emission spectroscopy investigations it was observed that electronic excitation of the plasma has high values and it is in a local thermal equilibrium. The theoretical calculation of the voltages induced in the metallic wires when exposed to high energy density microwave field are similar to those measured in air breakdown experiments. The scanning electron microscopy analysis of the tips of the metallic wires showed that the field emission process is responsible for the ignition of the metallic wires and plasma generation.

The enclaved body: Crises of personhood and the embodied geographies of urban gating

This essay analyzes embodied experiences of enclaving. It argues that by tracking revolutions in built form that gating enacts, urban geography has simultaneously tracked revolutions in urban subjectivity. It highlights three enclaved “body types” within existing literature: securitized bodies in fortressed cities, performative bodies in consumptive enclaves, and hygienic bodies in purified zones. It then offers three ethnographic scenes of gating related to new crises of personhood: metabolic illness, atmospheric breakdown, and resurgent ethno-nationalism. Attention to the psychic forces behind gating, it finally argues, can further show the gender, class, and ethnic underpinnings of what appear as generic architectural zones.

Atmospheric breakdown chemistry of the new “green” solvent 2,2,5,5-tetramethyloxolane via gas-phase reactions with OH and Cl radicals

Abstract. The atmospheric chemistry of 2,2,5,5-tetramethyloxolane (TMO), a promising “green” solvent replacement for toluene, was investigated in laboratory-based experiments and computational calculations. Results from both absolute and relative rate studies demonstrated that the reaction OH + TMO (Reaction R1) proceeds with a rate coefficient k1(296 K) = (3.1±0.4) ×10-12 cm3 molecule−1 s−1, a factor of 3 smaller than predicted by recent structure–activity relationships. Quantum chemical calculations (CBS-QB3 and G4) demonstrated that the reaction pathway via the lowest-energy transition state was characterised by a hydrogen-bonded pre-reaction complex, leading to thermodynamically less favoured products. Steric hindrance from the four methyl substituents in TMO prevents formation of such H-bonded complexes on the pathways to thermodynamically favoured products, a likely explanation for the anomalous slow rate of Reaction (R1). Further evidence for a complex mechanism was provided by k1(294–502 K), characterised by a local minimum at around T=340 K. An estimated atmospheric lifetime of τ1≈3 d was calculated for TMO, approximately 50 % longer than toluene, indicating that any air pollution impacts from TMO emission would be less localised. An estimated photochemical ozone creation potential (POCPE) of 18 was calculated for TMO in north-western Europe conditions, less than half the equivalent value for toluene. Relative rate experiments were used to determine a rate coefficient of k2(296 K) = (1.2±0.1) ×10-10 cm3 molecule−1 s−1 for Cl + TMO (Reaction R2); together with Reaction (R1), which is slow, this may indicate an additional contribution to TMO removal in regions impacted by high levels of atmospheric chlorine. All results from this work indicate that TMO is a less problematic volatile organic compound (VOC) than toluene.

Nanoscale plasma-activated aerosol generation for in situ surface pathogen disinfection

Plasma treatment constitutes an efficient method for chemical-free disinfection. A spray-based system for dispensing plasma-activated aerosols onto surfaces would facilitate disinfection of complex and/or hidden surfaces inaccessible to direct line-of-sight (for example, UV) methods. The complexity and size of current plasma generators (for example, plasma jet and cometary plasma systems)—which prohibit portable operation, together with the short plasma lifetimes, necessitate a miniaturized in situ technique in which a source can be simultaneously activated and administered on-demand onto surfaces. Here, we demonstrate this possibility by combining two nanoscale technologies for plasma and aerosol generation into an integrated device that is sufficiently small and lightweight. Plasma is generated on a carpet of zinc oxide nanorods comprising a nanoneedle ensemble, which when raised to a high electric potential, constitutes a massive point charge array with near-singular electric fields to effect atmospheric breakdown. The plasma is then used to activate water transported through an underlying capillary wick, that is subsequently aerosolized under MHz-order surface acoustic waves. We show that the system, besides being amenable to miniaturization and hence integration into a chipscale device, leads to a considerable improvement in plasma-activation over its macroscale cometary discharge predecessor, with up to 20% and 127% higher hydrogen peroxide and nitrite ion concentrations that are respectively generated in the plasma-activated aerosols. This, in turn, leads to a 67% reduction in the disinfection time to achieve 95% bacterial load reduction, therefore demonstrating the potential of the technology as an efficient portable platform for on-demand field-use surface disinfection.

Revolutionizing Spaceflight: A Study on Electric Propulsion and Air-Breathing MPDs

Modern liquid-fuel rocket propulsion harbors a number of great limitations. Among those is the weight of fuel, which makes up more than 90% of the mass of the SpaceX Falcon 9 (NASA, 2018). Electric propulsion has been used for decades as an alternative to liquid-fuel rockets due to low propellant requirements and high specific impulse. Although electric thrusters have strictly been used in non-atmospheric conditions, recent innovations attempt to expand its use to airspace. This quasi-experimental study focuses on the creation of an air-breathing magnetoplasmadynamic (MPD) thruster, with attempts being made to maximize the efficiency of the engine. Immense safety concerns prevented testing from occurring after the engine was built. However, the estimated performance of the built MPD is compared to a multitude of existing forms of electric propulsion, from Hall-effect thrusters to electrodynamic tethers. The concluding evidence suggests that air-breathing MPDs are not currently viable, high-power photon thrusters being of greater use in atmospheric conditions. Further research focusing on decreasing atmospheric breakdown voltage and increasing mirror reflectance of photon thrusters is suggested.

Corona discharge of a vibrated insulating box with granular medium

Corona discharges are luminous signs of strong local electric fields allowing a continuous discharge into the surrounding atmosphere. They commonly occur at the ends of conductors at high voltage. Here we report the observation of a faint glow surrounding an insulating cm-sized box filled with mm-sized basalt beads. At an ambient pressure in the mbar range two light bands occur as soon as and only if the box is vibrated and only if it is filled with a granular medium. In addition, a glow also occurs at the inside of the box. We measured periodic electric fields at the outside of the box with spatial peaks at the positions of the light bands. The period correlates to the vibration frequency. These observations imply strong alternating fields beyond atmospheric breakdown, which are generated inside and also emerge at the outside of the insulating box. The observations can be explained by tribocharging and periodic displacement of charges between grains and the inside walls of the box.

Will the Mars Helicopter Induce Local Martian Atmospheric Breakdown?

Abstract Any rotorcraft on Mars will fly in a low-pressure and dusty environment. It is well known that helicopters on Earth become highly charged due, in part, to triboelectric effects when flying in sandy conditions. We consider the possibility that the Mars Helicopter Scout (MHS), called Ingenuity, flying at Mars as part of the Mars 2020 Perseverance mission, will also become charged due to grain-rotor triboelectric interactions. Given the low Martian atmospheric pressure of ∼5 Torr, the tribocharge on the blade could become intense enough to stimulate gas breakdown near the surface of the rotorcraft. We modeled the grain–blade interaction as a line of current that forms along the blade edge in the region where grain–blade contacts are the greatest. This current then spreads throughout the connected quasi-conductive regions of the rotorcraft. Charge builds up on the craft, and the dissipative pathway to remove charge is back into the atmosphere. We find that for blade tribocharging currents that form in an ambient atmospheric dust load, system current balance and charge dissipation can be accomplished via the nominal atmospheric conductive currents. However, at takeoff and landing, the rotorcraft could be in a rotor-created particulate cloud, leading to local atmospheric electrical breakdown near the rotorcraft. We especially note that the atmospheric currents in the breakdown are not large enough to create any hazard to Ingenuity itself, but Ingenuity operations can be considered a unique experiment that provides a test of the electrical properties of the Martian near-surface atmosphere.

A study on the macroscopic self-organized structure of high-power millimeter-wave breakdown plasma

Air breakdown plasma induced by a strong electromagnetic (EM) wave field with a millimeter wavelength has a self-formed filamentary structure. The scale of the breakdown plasma is much larger than the wavelength scale, and the power density is lower than the self-ignition threshold of atmospheric breakdown. We observed a breakdown plasma that consisted of small plasma spots, and their trajectory was identical to that of the filamentary structure observed macroscopically. The numerical analysis showed that the interaction of the ionization front, EM field, and neutral-gas heating determines the self-organized structure of the branching plasma. The plasma trajectory identified in the analysis was identical to that of the experimental results. This novel physical process was entirely different from the discrete filament formation observed in the breakdown plasma formed under a focused beam, and the physical process of the breakdown strongly depended on the scale and power density.

Research on Dielectric Focusing Lens Antenna for HPM Atmospheric Breakdown Experiments

To distinguish the dielectric surface breakdown from the atmospheric breakdown, one method for atmospheric breakdown experiment by loading dielectric focusing lens is proposed. The focusing characteristics of dielectric focusing lens are investigated for the microwave frequency on the order of 1.3GHz to 9.3GHz, the lens focal length on the order of 0.3m to 0.9m and the dielectric constant on the order of 0.3 to 0.9. Simulations show that higher frequency, shorter focal length, and fitter dielectric constant result in better focus effect that the rising and falling edges of electric field strength change faster near the focus which is closer to the theoretical value. A dielectric focusing lens for the S-band is optimally designed with the polytetrafluoroethylene of 0.32m thickness, 0.6m diameter, and 0.4m focal length. The focus indicator of dielectric lens is measured and the atmospheric breakdown experiment is carried out. The experimental values of atmospheric breakdown are consistent with the theoretical values. The electric field strength with the lens at the focus is 5-6 times greater than that without focusing lens. The image of atmospheric breakdown by the dielectric lens focusing method experiment is clearer than that by the direct radiation method.

Effect of high power microwave injection on ozone balance

High power microwave (HPM) injection into the troposphere to block the chain of chlorofluorocarbons (CFCs) or injection into the stratosphere to promote ozone generation are two feasible methods to artificially affect the ozone number concentration. Based on Maxwell’s equations and current density control equations, this study adopts the finite difference time domain (FDTD) method to quantitatively analyze the process of HPM injection into different heights of atmosphere. By taking consideration of the main atmospheric components and the major chemical reactions in the atmospheric, the decomposition of CFCs and the change of the ozone number density is also quantitatively simulated in response to the injection of single pulse microwaves. The numerical simulation results show that under the injection of high power microwaves, the number density of electrons increased exponentially with time, reaching a maximum value of 1017cm-3, and it increased considerably with the pulse time of injected high power microwaves and finally saturates when the state of atmospheric breakdown occurs. The decomposition of CFCs mainly occurs in the electron relaxation process, and the decomposition rate can reach 6% according to the simulation result. Before the atmosphere breaks down occurs, increase of microwave intensity or pulse width facilitates the production of more free electrons, which can consequently lead to a considerable increase in the ozone number density in ozone layer for a prolonged period.

Contribution of Hydrofluorocarbons (HFCs) and Hydrofluoro-Olefins (HFOs) Atmospheric Breakdown Products to Acidification (“Acid Rain”) in the EU at Present and in the Future

While hydrogen fluoride (HF) and hydrogen chloride (HCl) are not considered main air-pollutants in the EU, they have the potential to contribute to acidification. Hydrofluorocarbons (HFCs), hydrofluoro-olefins (HFOs) and hydrochlorofluoro-olefins (HCFOs) are used as refrigerants and for other applications. They break down in the atmosphere to produce HF and HCl (for HCFOs) and some of these fluorocarbons also break down to produce trifluoroacetic acid (TFA). For the emissions of these fluorocarbons in the EU, a worst-case scenario estimates their theoretical potential contribution to acidification and compares it to the acidification potential for the main air pollutants contributing to acidification, which are nitrous oxides (NOx), sulphur oxides (mainly SO2), and ammonia (NH3). The Acidification Potential from these fluorocarbons in 2016 is estimated at 2, NOx, NH3, and it can be concluded that this is insignificant in the context of the main acidification air-pollutants. Assuming that the EU targets for emissions of SO2, NOx and NH3 by 2030 are achieved, the Acidification Potential from HFCs, HFOs and HCFOs in 2030 is also estimated at 2, NOx, NH3 and will remain insignificant.

Model predictions for atmospheric air breakdown by radio-frequency excitation in large gaps

The behavior of the breakdown electric field versus frequency (DC to 100 MHz) for different gap lengths has been studied numerically at atmospheric pressure. Unlike previous reports, the focus here is on much larger gap lengths in the 1–5 cm range. A numerical analysis, with transport coefficients obtained from Monte Carlo calculations, is used to ascertain the electric field thresholds at which the growth and extinction of the electron population over time are balanced. Our analysis is indicative of a U-shaped frequency dependence, lower breakdown fields with increasing gap lengths, and trends qualitatively similar to the frequency-dependent field behavior for microgaps. The low frequency value of ∼34 kV/cm for a 1 cm gap approaches the reported DC Paschen limit.

Quantification of fluorine traces in solid samples using CaF molecular emission bands in atmospheric air Laser-Induced Breakdown Spectroscopy

Direct solid determination of trace amounts of fluorine using Laser-Induced Breakdown Spectroscopy (LIBS) is a challenging task due to the low excitation efficiency of this element. Several strategies have been developed to improve the detection capabilities, including the use of LIBS in a He atmosphere to enhance the signal to background ratios of F atomic emission lines. An alternative method is based on the detection of the molecular compounds that are formed with fluorine in the LIBS plasma. In this work, the detection of CaF molecular emission bands is investigated to improve the analytical capabilities of atmospheric air LIBS for the determination of fluorine traces in solid samples. In particular, Cu matrix samples containing different fluorine concentration (between 50 and 600μg/g), and variable amounts of Ca, are used to demonstrate the linear relationships between CaF emission signal and F concentration. Limits of detection for fluorine are improved by more than 1 order of magnitude using CaF emission bands versus F atomic lines, in atmospheric-air LIBS. Furthermore, a toothpaste powder sample is used to validate this analytical method. Good agreement is observed between the nominal and the predicted fluorine mass-content.

Optimized LWIR enhancement of nanosecond and femtosecond LIBS uranium emission

A carbon dioxide (CO2) transverse electrical breakdown in atmosphere (TEA), pulsed laser was used to enhance the laser-induced breakdown spectroscopy (LIBS) spectral signatures of uranium under nanosecond (ns) and femtosecond (fs) ablation. The peak areas of both ionic and neutral species increased by one order of magnitude for ns-ablation and two orders of magnitude for fs-ablation over LIBS when the CO2 TEA laser was used with samples of dried solutions of uranyl nitrate hexahydrate (UO2(NO3)2·6H2O) on silicon wafers. Electron temperature and density measurements show that the spectral emission improvement from using the TEA laser comes from plasma reheating.

An atmospheric photochemical source of the persistent greenhouse gas CF4

AbstractA previously uncharacterized atmospheric source of the persistent greenhouse gas tetrafluoromethane, CF4, has been identified in the UV photolysis of trifluoroacetyl fluoride, CF3C(O)F, which is a degradation product of several halocarbons currently present in the atmosphere. CF4 quantum yields in the photolysis of CF3C(O)F were measured at 193, 214, 228, and 248 nm, wavelengths relevant to stratospheric photolysis, to be (75.3 ± 1) × 10−4, (23.7 ± 0.4) × 10−4, (6.6 ± 0.2) × 10−4, and ≤0.4 × 10−4, respectively. A 2‐D atmospheric model was used to estimate the contribution of the photochemical source to the global CF4 budget. The atmospheric photochemical production of CF4 from CF3CH2F (HFC‐134a), CF3CHFCl (HCFC‐124), and CF3CCl2F (CFC‐114a) per molecule emitted was calculated to be (1–2.5) × 10−5, 1.0 × 10−4, and 2.8 × 10−3, respectively. Although CF4 photochemical production was found to be relatively minor at the present time, the identified mechanism demonstrates that long‐lived products with potential climate impacts can be formed from the atmospheric breakdown of shorter‐lived source gases.

2014 WSGC Elijah High-Altitude Balloon Payload Project

The purpose of this paper is to discuss the development and findings of the experiments performed on the high altitude balloon payload. After some bonding time in the beginning of the summer, the team got together and decided on the four following experiments: Atmospheric Electric Field, Breakdown Voltage, IR Imaging, and Solar Efficiency/Payload Spin. Subsequently, research and fabrication began which involved learning new skills such as Arduino programming, 3-D modeling, foam cutting, and more. The launch was successful, and the results retrieved were more or less what were expected. This internship opportunity proved to be an amazing learning experience for the team which will be valued for each of their future career experiences.

Frequency response of atmospheric pressure gas breakdown in micro/nanogaps

In this paper, we study gas breakdown in micro/nanogaps at atmospheric pressure from low RF to high millimeter band. For gaps larger than about 10 μm, the breakdown voltage agrees with macroscale vacuum experiments, exhibiting a sharp decrease at a critical frequency, due to transition between the boundary- and diffusion-controlled regimes, and a gradual increase at very high frequencies as a result of inefficient energy transfer by field. For sub-micron gaps, a much lower breakdown is obtained almost independent of frequency because of the dominance of field emission.