N2 Discharge Research Articles

Influence of N2 admixture on mode transition of discharge in N2–Ar helicon plasma

Effects of N2 admixture on multiple wave modes and transitions were investigated in N2–Ar helicon plasma under fixed input power and magnetic field. The structures of helicon waves were measured by a B-dot probe to verify the different eigenmodes. The experimental results show that the plasma morphology, emission spectrum, and spatial profile change significantly during mode transitions with the N2–Ar ratio. The calculated results from the pressure balance model indicate that the densities of species N2, N+, Ar, and Ar+ will change largely during mode transition around some specific N2 percentages, which will help to improve the application of N2–Ar helicon plasma in material processing greatly.

Hysteresis in strongly magnetized N2 discharges

A semi-empirical global model for a nitrogen discharge in a strong magnetic field is developed. The model is based upon experimental data from high-resolution Doppler and extreme-ultraviolet vacuum spectroscopy, which establish the plasma composition, discharge parameters, and, most importantly, electronic transitions. This allows the number of required molecular systems and atomic/ionic states to be reduced, thereby retaining only the essential plasma chemistry reactions. The set of 35 stiff non-linear ordinary differential equations is numerically integrated using an unconditionally stable adaptive method. Simulations show the existence of two solution branches with low and high electron temperature, respectively. A distinct hysteresis is exhibited by the discharge and illustrated for three typical N2 mass flow rates. The dependencies of the plasma parameters on the applied power are presented and discussed in detail, including in the vicinity of the bifurcation points. The efficiency of operation in the opposing limits of N2 discharge behavior as either a source of plasma or light emission is examined, with special emphasis on electric propulsion capabilities.

Characterization and effectiveness of a miniaturized dielectric barrier discharge module for air disinfection

Effective inactivation of airborne pathogens is crucial for air quality control and the prevention of infectious diseases, especially in indoor human-living environments. In this work, a novel air disinfector based on surface dielectric barrier discharge (DBD) plasma was proposed. The discharge characteristics and the disinfection performance of the disinfector were investigated under different applied voltages, airflow rates, and humidity. The used DBD mainly works in the O3-dominant product mode with a discharge power of 1–3 W. The decreases in N2(C-B) emission intensity, ozone, discharge power, and discharge current with the increasing airflow humidity were observed. Humidity was found to be the most pronounced factor promoting the single-pass disinfection efficiency. The Z value reached 0.32 l/J for relative humidity of 70%–80%. Additionally, ozone and ions were found to play a minor role in plasma-mediated bacterial inactivation. The antibacterial effectiveness of the disinfector was also validated by 2-h circulating disinfection experiments. Efficiency was maintained above the disinfection level (>99.9%) after a 5000-h continuous running. Meanwhile, the ozone emission was below the standard limit requirements. The proposed air disinfector is promising for household use with the advantages of small size, bacterial inactivation capability, low ozone emission, and high perceptibility.

Nanosecond pulsed gliding arc plasma for ammonia synthesis: better insight from discharge mode and vibrational temperature

Low-temperature plasma technology is a promising technological route to achieve green and efficient ammonia synthesis at ambient temperature and pressure. In this work, a Laval nozzle type gliding arc plasma reactor was designed for the direct synthesis of ammonia from N2 and H2 discharges ignited by a high voltage nanosecond pulsed power supply to investigate the effect of different electrode gaps, pulse voltages, and V N2:V H2 on ammonia synthesis. The nanosecond pulsed plasma discharges were characterized through oscilloscope and optical emission spectroscopy (OES). The maximum rate of NH3 synthesis was 538.12 μmol·h−1 at 1.5 mm electrode gap, 16 kV peak pulse voltage, 6 kHz pulse repetition frequency, 100 ns pulse width, 100 ns pulse rising edge, 100 ns pulse falling edge, and 200 mL·min−1 total gas flow rate with V N2:V H2 = 1:1. It was demonstrated that the discharge mode of the nanosecond pulsed gliding arc plasma can transit from a unipolar state to a bipolar state determined by the duty cycle accompanied with higher discharge power and vibrational temperature. Bipolar discharge mode is beneficial to improve the efficiency of plasma ammonia synthesis because of it can strengthen the plasma discharge and increase the vibrational temperature. The ammonia synthesis rate and N2 conversion rate increased with the increase of the discharge power and vibrational temperature.

Power transfer efficiency in an air-breathing radio frequency ion thruster

Abstract Due to a series of challenges such as low-orbit maintenance of satellites, the air-breathing electric propulsion has got widespread attention. Commonly, the radio frequency ion thruster is favored by low-orbit missions due to its high specific impulse and efficiency. In this paper, the power transfer efficiency of the radio frequency ion thruster with different gas composition is studied experimentally, which is obtained by measuring the radio frequency power and current of the antenna coil with and without discharge operation. The results show that increasing the turns of antenna coils can effectively improve the radio frequency power transfer efficiency, which is due to the improvement of Q factor. In pure N2 discharge, with the increase of radio frequency power, the radio frequency power transfer efficiency first rises rapidly and then exhibits a less steep increasing trend. The radio frequency power transfer efficiency increases with the increase of gas pressure at relatively high power, while declines rapidly at relatively low power. In N2/O2 discharge, increasing the N2 content at high power can improve the radio frequency power transfer efficiency, but the opposite was observed at low power. In order to give a better understanding of these trends, an analytic solution in limit cases is utilized, and a Langmuir probe was employed to measure the electron density. It is found that the evolution of radio frequency power transfer efficiency can be well explained by the variation of plasma resistance, which is related to the electron density and the effective electron collision frequency.

Statistical analysis on branching characteristics of positive streamer discharges in N2–O2 mixtures

Streamers are fast-propagating ionization channels that can usually branch and form complex tree-like structures in dielectric media. In this paper, we perform experiments on positive streamers in different N2–O2 mixtures under varying conditions including voltage, pressure, and electrode geometry, with at least 125 discharge images captured for each condition. We present a statistical analysis on streamer branching characteristics from 3D models that are reconstructed by stereoscopic stroboscopic images and our dedicated semi-automatic 3D reconstruction method.We found that by varying the concentration of O2, the morphology and branching characteristics are greatly changed. Specifically, the average branching angle decrease significantly from 90∘ in air to 66∘ in 1% O2, suggesting that photoionization plays an important role in streamer branching. The branching angles in our work are generally larger than previously reported results due to the resolved 3D structures of discharges by our method. A linear relation between the streamer diameter ratio and the branching direction difference of two daughter branches is found, which intersects the vertical axis almost at unity. It is also found that the average branching angles, streamer velocities and diameters increase as the voltage increases. This is again attributed to stronger photoionization effect under higher voltages. The velocities and diameters are similar at different pressures but at the same reduced electric field. The average branching angle decreases from 90∘ at 133 mbar to 79∘ at 200 mbar. This suggests that stochastic fluctuations become dominant over photoionization effect at higher pressures.

Tuning the Pt-N coordination in Pt/MOFs nanosheets under N2 plasma for enhanced oxygen reduction reaction

Regulation of the local coordination of metal sites can tune the electronic structure of catalysts, thereby improving electrocatalytic activity and reaction selectivity. This study proposes a strategy for designing highly active catalysts by regulating Pt-N coordination through N2 plasma treatment. Zn-BDC (1,4-benzenedicarboxylic acid) nanosheets embedded with Pt nanoparticles (Pt/Zn-BDC) were successfully fabricated. Subsequently, radio frequency (RF) discharge N2 plasma was applied to tune the Pt sites of Pt/Zn-BDC at room temperature. The plasma-treated Pt/Zn-BDC showed enhanced catalytic activity for the oxygen reduction reaction (ORR) with a high half-wave potential (0.835 V), superior to those of pristine Pt/Zn-BDC and 20 wt% Pt/C. Notably, rechargeable Zn-air batteries based on plasma-treated catalysts were demonstrated to exhibit high efficiency and stability. The enhanced catalytic performance originated from the optimized coordination environment and electronic structure of the Pt sites under N2 plasma treatment. This study provides a facile, green method of regulating promising ORR electrocatalysts via low-temperature plasma treatment.

Laser absorption spectroscopy for plasma-assisted thermochemical treatment. Part II.: Impact of the carbon and water contaminants on a low-pressure N2–H2 discharge

The density evolution of , , and molecules over time was monitored by laser absorption spectroscopy in a low pressure DC pulsed discharge in N2–H2 gas mixtures with addition of CH4 or O2. The discharge was maintained in an industrial-scale, active screen (AS) plasma nitrocarburizing (ASPNC) reactor with a steel AS. The measured species densities were analysed using a simplified kinetic model that includes three characteristic times for chemical processes in the ASPNC reactor. The shortest time (1–4 min) was associated with the gas residence time in the reactor, the middle one (about 20 min) was assigned to surface reactions on the AS and workload, whereas the largest one (about 3–5 h) was assigned to surface reactions on the cold reactor walls. The work highlights the importance of monitoring the gas composition during plasma nitrocarburizing processes in order to maintain defined treatment conditions and compensate for continuously changing chemical kinetics at the internal reactor surfaces.

Study of the efficiency of ozone synthesis in a point‐to‐plain discharge in N2–O2 mixtures

AbstractThe experimental results of the ozone synthesis and electrodynamic characteristics of the point‐to‐plane DC voltage corona discharge in N2–O2 mixtures at atmospheric pressure are presented. Four different modes of DC corona discharge were investigated: stationary mode of positive point‐to‐plane discharge, two non‐stationary (streamers) modes of positive point‐to‐plane discharge, and mode of negative point‐to‐plane discharge. It was shown that ozone synthesis highly depends on discharge modes and correlates with discharge conditions appropriated to dissociative attachment processes. In the case of positive point‐to‐plane discharge, ozone synthesis behavior is similar to that of the pulse component of the discharge current. It was shown that the mode with secondary streamers propagation is the most effective for ozone production among the four different modes of the point‐to‐plane DC corona discharge.

Unraveling Exclusive In-Plasma Initiated Oxidation Processes Occurring at Polymeric Surfaces upon O2 Admixtures to Medium Pressure Ar and N2 DBD Treatments.

Polymeric surfaces have been increasingly plasma-activated to adopt adequate chemistries, enabling their use in different applications. An unavoidable surface oxygen insertion upon exposure to non-oxygen-containing plasmas was always observed and mainly attributed to in-plasma oxidation stemming from O2 impurities in plasma reactors. Therefore, this work investigates exclusive in-plasma oxidation processes occurring on polyethylene surfaces by purposely admixing different O2 concentrations to medium-pressure Ar and N2 dielectric barrier discharges (base pressure: 10-7 kPa). Hence, distinctive optical emission spectroscopy and in-situ X-ray photoelectron spectroscopy (XPS) data were carefully correlated. Pure N2 discharge triggered an unprecedented surface incorporation of large nitrogen (29%) and low oxygen (3%) amounts. A steep rise in the O-content (10%) at the expense of nitrogen (15%) was detected upon the addition of 6.2 × 10-3% of O2 to the feed gas. When the added O2 exceeded 1%, the N content was completely quenched. Around 8% of surface oxygen was detected in Ar plasma due to high-energy Ar metastables creating more surface radicals that reacted with O2 impurities. When adding only 6.2 × 10-3% of O2 to Ar, the surface O content considerably increased to 12%. Overall, in-plasma oxidation caused by O2 impurities can strikingly change the surface chemistry of N2 and Ar plasma-treated polymers.

Phenomenology of High-Current Discharge in N2 and He at Medium Pressures between Two Electrodes of Rail Geometry

Phenomenology of High-Current Discharge in N2 and He at Medium Pressures between Two Electrodes of Rail Geometry

Benchmarking and validation of a hybrid model for electropositive and electronegative capacitively coupled plasmas

In this work, a fluid/Monte Carlo collision (fluid/MCC) hybrid model is developed based on the framework of multi-physics analysis of plasma sources. This hybrid model could be highly accurate in predicting the nonequilibrium phenomena in capacitively coupled plasmas and meanwhile avoid the limitation caused by the computational cost. Benchmarking against the well-established particle-in-cell/MCC (PIC/MCC) method and comparison with experimental data have been presented both in electropositive N2 discharges and electronegative O2 discharges. The results indicate that in N2 discharges, the ion density evolves from a uniform distribution to an edge-high profile as power increases. Besides, the electron energy distribution function (EEDF) at the bulk center exhibits a ‘hole’ at about 3 eV, and the ‘hole’ becomes less obvious at the radial edge, because more low energy electrons are generated there. In O2 discharges, the EEDF exhibits a Druyvesteyn-like distribution in the bulk region, and it evolves to a Maxwellian distribution in the sheath, indicating the dominant influence of the electric field heating there. The results obtained by the hybrid model agree well with those calculated by the PIC/MCC method, as well as those measured by double probe, except for a slight discrepancy in absolute values. The qualitative agreement achieved in this work validates the potential of this hybrid model as an effective tool in the deep understanding of plasma properties, as well as in the improvement of plasma processing.

Feasibility study on substitution of SF6 insulated current transformer with environment-friendly natural gas

Many SF6 gas-insulated current transformers (CT) are used in the power grid. The global warming potential (GWP) of SF6 is 23,900 times that of CO2, and the leakage of SF6 will aggravate the global warming trend. It is a hot issue to carry out the study of environment-friendly gas substitution in the power grid. This paper systematically studied the voltage withstand, partial discharge, and temperature rise characteristics of dry air, N2, and CO2 gas-insulated CTs. Through the study, the permissible critical pressures in environment-friendly gas-insulated CTs were determined to be 0.9 MPa for dry air, 0.7 MPa for N2, and 0.8 MPa for CO2, respectively. The partial discharge intensity was less than 5 pC for environment-friendly gas-insulated CTs at 0.3 MPa∼0.8 MPa pressure. The experimental results of temperature rise characteristics showed that under the condition of a 600A rated operating current, the maximum temperature rise of the CTs was about 80 °C, and the maximum temperature rise happened at the conductive rod. The experimental results verified that environment-friendly gases could meet the requirements of insulation and temperature rise. Due to the increase of pressure in CTs, the action pressure of the explosion-proof membrane should be changed to meet the mechanical strength requirements. It is recommended that when using 0.9 MPa dry air, the action pressure should be designed as 1.8 MPa; when using 0.7 MPa N2, the pressure should be designed as 1.4 MPa; when using 0.8 MPa CO2, the pressure should be designed as 1.6 MPa. The results of this paper can provide a reference for development of environment-friendly gas-insulated CTs.

Characterization of a DC glow discharge in N2–H2 with electrical measurements and neutral and ion mass spectrometry

The addition of small amounts of H2 were investigated in a DC glow discharge in N2, at low pressure (∼1 mbar) and low power (0.05–0.2 W cm−3). We quantified the electric field, the electron density, the ammonia production and the formation of positive ions for amounts of H2 varying between 0 and 5%, pressure values between 0.5 and 4 mbar, and currents between 10 and 40 mA. The addition of less than 1% H2 has a strong effect on the N2 plasma discharges. Hydrogen quenches the (higher) vibrational levels of N2 and some of its highly energetic metastable states. This leads to the increase of the discharge electric field and consequently of the average electron energy. As a result, higher quantities of radical and excited species are suspected to be produced. The addition of hydrogen also leads to the formation of new species. In particular, ammonia and hydrogen-bearing ions have been observed: N2H+ and NH4 + being the major ones, and also H3 +, NH+, NH2 +, NH3 +, N3H+ and N3H3 +. The comparison to a radiofrequency capacitively coupled plasma discharge in similar experimental conditions shows that both discharges led to similar observations. The study of N2–H2 discharges in the laboratory in the adequate ionization conditions then gives some insights on which plasma species made of nitrogen and hydrogen could be present in the ionosphere of Titan. Here, we identified some protonated ions, which are reactive species that could participate to the erosion of organic aerosols on Titan.

Investigation of cyclohexane catalytic degradation driven by N atoms from N2 discharges

The effect of N2 discharge products on cyclohexane degradation over a MnO2/γ-Al2O3 catalyst has been evaluated by feeding N2 discharge products to the catalyst using a specially designed dielectric barrier discharge reactor. At a reaction temperature of 100 °C, the cyclohexane conversion increased from 2.46% (without N2 discharge products) to 26.3% (with N2 discharge products). N- and O-containing by-product (3, 4-dehydroproline) was found on the catalyst surface using gas chromatograph-mass spectrometry identification, in which C=N–C and C=N–H bonds were also confirmed from x-ray photoelectron spectroscopy analysis results. Operando analysis results using diffuse reflectance infrared Fourier transform spectroscopy revealed that N atoms can react with surface H2O possibly to NH and OH reactive species that have reactivities to promote CO oxidation to CO2. The mechanism of N-atom-driven cyclohexane degradation to CO and CO2 is proposed.

Repetitive pulsed gas-liquid discharge in different atmospheres: from discharge characteristics to plasma-liquid interactions.

Gas-liquid discharges enable efficient plasma-liquid interactions and thus have promising applications, but many concerns remain unanswered regarding working gas, an important influencing factor. Investigations of a pulsed needle-to-water gas-liquid discharge in Ar, He, and N2 are carried out in this study. Voltage-current waveforms and intensified charge-coupled device (ICCD) images were combined to explore discharge modes and evolution processes. Experimental results demonstrate that the Ar tip spark discharge develops all the way to liquid compared with the He and N2 discharges concentrated in the tube, considerably increasing the plasma-liquid interaction efficiency. According to the optical emission spectra (OES), plasma properties are found to be strongly influenced by interactions between gas particles and interfering particles (H2O, etc.). For instance, excess water vapor causes quenching of excited species, resulting in OH (A-X) in Ar and NO (A-X) in N2 concentrated in the middle region with weak discharge. By adding dimethyl sulfoxide, it is demonstrated that OH dominates H2O2 production in Ar and contributes to both NO2- and NO3- generation in Ar and He, whereas in N2 it only affects NO3- generation. This study advances the "bridge building" between practical application and gas-liquid discharge employing different working gases.

Spatially averaged ion temperature model for low-temperature plasma sources

Although the ion temperature can considerably affect bulk plasma parameters and ion angle distribution functions arriving at the substrate, limited experiments have been conducted on this phenomenon because ion temperature measurement is difficult. Furthermore, only a few simulations have been performed under specific conditions. Therefore, in this study, a novel method was developed to calculate the ion temperature using a global model. The spatially averaged ion temperature equation was derived by integrating the ion temperature over space. Continuity and electron temperature equations were used in a self-consistent manner to solve the equation. We investigated radio-frequency inductively coupled Ar, N2, and Cl2 discharges in the pressure range of 5–20 mTorr and power range of 200–1000 W. The obtained results were highly consistent with experimental and two-dimensional simulation results.

Spectrally filtered ps–ns emission dynamics of atmospheric-pressure nanosecond pulsed plasmas

The temporal dynamics of atmospheric-pressure nanosecond pulsed plasma discharges in a pin-to-pin electrode configuration are studied using streak-camera line imaging of the interelectrode gap with a time resolution as short as ∼25 ps. Discharge emission initiates homogeneously throughout the interelectrode gap with no detectable streamer propagation and then temporally decays in two distinct phases. Plasma emission bands attributed to various electronic transitions are tracked for single discharges in air and N2. Spectral filtering of the excited molecular states reveals that the N2(C–B) and N2(B–A) emission bands evolve in distinct early and late phases, respectively, with a time separation of ∼15–20 ns. Furthermore, significant differences in the temporal dynamics of plasma discharges in air and N2 are observed. High levels of excited-state atomic oxygen and NO appear after the initial decay of the N2(C) state and coincide primarily with the latter phases of plasma evolution in air environments. From temporal traces of discharge emission, the formation and relaxation timescales of the electronically excited states of N2 are quantified in pure N2 and air environments with sub-nanosecond resolution. The streak-OES (optical emission spectroscopy) technique enables quantitative time-resolved studies of key chemical species for model validation in ultra-short-pulsed plasmas.

Effect of Downstream Plasma Exposure on Schottky Diodes Fabricated on β-Ga2O3

The effects of downstream plasma exposure on Ga2O3 Schottky diode forward and reverse IV characteristics were investigated. Ultra-wide bandgap semiconductor β-Ga2O3 has attracted increasing attention because of the prospects for use in next generation high-power electronics. However, the Ga2O3 surface has been reported to be particularly sensitive to plasma-induced damage, which could lead to device performance degradation[1]. In our study, the plasma treatments were performed using a PIE Scientific Tergeo Plasma Cleaner with O2, N2 or CF4 discharges. The system can be operated either in immersion mode or downstream mode. The samples were exposed to fluxes of electrons, ions and neutral and an rf power of 50 W was used to generate the plasma at a pressure of 400 mTorr, with a fixed treatment time of 1 min. Ti/Au and Ni/Au metallization were employed as the Ohmic and Schottky contacts, respectively, on the test diodes. Prior to the Schottky metal deposition, the samples were treated to ozone plasma/dilute HCl solution/ozone treatments to remove surface contamination. A Schottky barrier height of 1.1 eV was obtained for the reference sample without plasma treatment, and an ideality factor of 1.001 was achieved. From the forward I-V characteristic, as illustrated in Figure 1, the diodes exposed to CF4 showed a 0.25V shift from the I-V of the reference sample due to a Schottky barrier height lowering around 13.9%, as show in Table 1. The diodes exposed with N2 and O2 plasmas showed a decrease of Schottky barrier height of 2.5 and 6.5 % with O2 or N2 treatments, respectively, as shown in Figure 2. The effect of plasma exposure on the ideality factor of diodes treated with these plasmas was minimal; 0.2% for O2 and N2, 0.3% for CF4, respectively, as shown in Figure 2. The reverse leakage currents were 0.0012, 0.0022 and 0.0048 mA/cm2 for the diodes treated with O2, and CF4, and N2 respectively, as shown in Figure 3. The effect of downstream plasma treatment on diode on-resistance and on-off ratio were also very minimal as shown in Figure 4 and 5.[1] Jiancheng Yang, Zachary Sparks, Fan Ren, Stephen J. Pearton, and Marko Tadjer, J. Vac. Sci. & Technol. B36, 061201-1-9 (2018). Figure 1

Applicability of Atmospheric Pressure Plasma Jet (APPJ) Discharge for the Reduction in Graphene Oxide Films and Synthesis of Carbon Nanomaterials

Atmospheric pressure plasma jets (APPJ) are widely used in industry for surface cleaning and chemical modification. In the recent past, they have gained more scientific attention especially in the processing of carbon nanomaterials. In this work, a novel power generation technique was applied to realize the stable discharge in N2 (10 vol.% H2) forming gas in ambient conditions. This APPJ was used to reduce solution-processed graphene oxide (GO) thin films and the result was compared with an established and optimized reduction process in a low–pressure capacitively coupled (CCP) radiofrequency (RF) hydrogen (H2) plasma. The reduced GO (rGO) films were investigated by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Effective deoxygenation of GO was observed after a quick 2 s treatment by AAPJ. Further deoxygenation at longer exposure times was found to proceed with the expense of GO–structure integrity. By adding acetylene gas into the same APPJ, carbon nanomaterials on various substrates were synthesized. The carbon materials were characterized by Raman spectroscopy, scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX) analyses. Fullerene-like particles and graphitic carbon with short carbon nanotubes were detected on Si and Ag surfaces, respectively. We demonstrate that the APPJ tool has obvious potential for the versatile processing of carbon nanomaterials.