Volume added surface barrier discharge plasma excited by bipolar nanosecond pulse power in atmospheric air: optical emission spectra influenced by gap distance

In this paper, volume barrier discharge with different gap distances is added on the discharge border of high-voltage electrode of annular surface barrier discharge for generating volume added surface barrier discharge （V-SBD） excited by bipolar nanosecond high-voltage pulse power in atmospheric air. The excited V-SBDs consist of surface barrier discharge （d = 0 mm） and volume added surface barrier discharges （d = 2 mm and 3 mm）. The optical emission spectra are recorded for calculating emission intensities of N2 （C3 ∏u → B3∏g） and N2＋ （B2 ∑u＋ → X2 ∑g＋）, and simulating rotational and vibrational temperatures. The influences of gap distance of V-SBD on emission intensity and plasma temperature are also investigated and analyzed. The results show that d = 0 mm structure can excite the largest emission intensity of N2 （C3 ∏u → B3 ∏g）, while the existence of volume barrier discharge can delay the occurrence of the peak value of the emission intensity ratio of N2 ＋ （B2 ∑u＋ → X2 ∑＋g）/N2 （C3 ∏u → B3 ∏g） during the rising period of the applied voltage pulse and weaken it during the end period. The increasing factor of emission intensity is effected by the pulse repetition rate. The d = 3 mm structure has the highest threshold voltage while it can maintain more emission intensity of N2 （C3 ∏u→ B3∏g） than that of d = 2 mm structure. The structure of d = 2 mm can maintain more increasing factor than that of the d = 3 mm structure with varying pulse repetition rate. Besides, the rotational temperatures of three V-SBD structures are slightly affected when the gap distance and pulse repetition rate vary. The vibrational temperatures have decaying tendencies of all three structures with the increasing pulse repetition rate.

Electrical characters and optical emission spectra of VBD coupled SBD excited by sine AC voltage in atmospheric air

In this paper,volume coupled surface barrier discharge（V-SBD） with three structures possessing different volumes is excited by sine AC power in atmospheric air.Discharge images,waveforms of applied voltage and discharge current,and optical emission spectra simulating rotational and vibrational temperatures are recorded and analyzed.The effects of applied voltage on emission intensities of N2（C^3Πu→ B^3Πg） and N2^＋（B^2∑u^＋ → X^2Eg^＋）,and rotational and vibrational temperatures are investigated.The results show that as applied voltage rises,emission intensities and rotational temperatures increase while vibrational temperatures decrease.In addition it is found that,as applied voltage varies,the rotational temperature of surface discharge changes faster than that of volume discharge.

Study of Non-Thermal DC Arc Plasma of CH_4/Ar at Atmospheric Pressure Using Optical Emission Spectroscopy and Mass Spectrometry

Non-thermal C/H/Ar plasmas are widely applied to carbonaceous material production and processing.In this work,plasma parameters and gaseous species of the atmospheric non-thermal C/H/Ar plasmas produced by an atmospheric-pressure DC arc discharge generator in CH_4/Ar were investigated.The voltage-current characteristics were measured for different CH_4/Ar ratios.Optical emission spectroscopy was employed to analyze the electron excitation temperature,gas temperature and electron density under various discharge conditions.The hydrocarbon molecules produced in the CH4/Ar plasmas were detected with photoionization mass spectrometry.The optical spectral results demonstrated that the electron excitation temperature was 0.4-1 eV,the gas temperature was 2800-4200 K and the electron density was in the range of（5-20）×10^15 cm^-3.The mass spectrum indicated that a variety of unsaturated hydrocarbons（C2H4,C3H6,C6H6,etc.） and several highly unsaturated hydrocarbons（C4H2,C5H6,etc.） were produced in the non-thermal arc plasmas.

Discharge Modes Suggested by Emission Spectra of Nitrogen Dielectric Barrier Discharge with Wire-Cylinder Electrodes

In this paper,nitrogen dielectric barrier discharge（DBD） plasma was generated in a quartz tube with coaxial wire-cylinder electrodes at atmospheric pressure.By varying the nitrogen gas flow（FN） in the range of 0-1 m3/h,the plasma optical emission spectra（OES） were measured and studied.The vibration（T_（vib）） and rotation temperature（T_（rot）） of nitrogen were obtained,by fitting the rovibronic bands of N_2（C^3∏_u-B^3∏_g,0-1）,and by the Boltzmann plot method for purposes of comparison.T_（vib） increased up to 2481 K with increasing nitrogen flow till0.2 m3/h,and then decreased with further increasing FN,while Trot decreased monotonously and approached to-350 K for FN ≥ 0.6 m^3/h.The intensity of N_2（C^3∏_u-B^3∏_g,0-0,1-0,0-3） and N_2~＋（B^2∑_u~＋-X^2Σ_g~＋,0-0） exhibited similar evolution with increasing FN to those of the T_（vib） and Trot,respectively.The discharge photos revealed that the discharge filaments gradually decreased with increasing FN,and eventually disappeared,which implied that a discharge mode transition emerged with increasing FN.The possible mechanism for the discharge mode transition is studied in detail according to the vibration（T_（vib）） and rotation temperature（T_（rot）） of nitrogen.

Time-resolved characteristics of a nanosecond pulsed multi-hollow needle plate packed bed dielectric barrier discharge

In this paper,self-designed multi-hollow needle electrodes are used as a high-voltage electrode in a packed bed dielectric barrier discharge reactor to facilitate fast gas flow through the active discharge area and achieve large-volume stable discharge.The dynamic characteristics of the plasma,the generated active species,and the energy transfer mechanisms in both positive discharge(PD)and negative discharge(ND)are investigated by using fast-exposure intensified charge coupled device(ICCD)images and time-resolved optical emission spectra.The experimental results show that the discharge intensity,number of discharge channels,and discharge volume are obviously enhanced when the multi-needle electrode is replaced by a multihollow needle electrode.During a single voltage pulse period,PD mainly develops in a streamer mode,which results in a stronger discharge current,luminous intensity,and E/N compared with the diffuse mode observed in ND.In PD,as the gap between dielectric beads changes from 0 to250μm,the discharge between the dielectric bead gap changes from a partial discharge to a standing filamentary micro-discharge,which allows the plasma to leave the local area and is conducive to the propagation of surface streamers.In ND,the discharge only appears as a diffusionlike mode between the gap of dielectric beads,regardless of whether there is a discharge gap.Moreover,the generation of excited states N_(2)^(+)(B^(2)∑_(u)^(+))and N2(C^(3)Π_(u))is mainly observed in PD,which is attributed to the higher E/N in PD than that in ND.However,the generation of the OH(A^(2)∑^(+))radical in ND is higher than in PD.It is not directly dominated by E/N,but mainly by the resonant energy transfer process between metastable N_(2)(A^(3)∑_(u)^(+))and OH(X^(2)Π).Furthermore,both PD and ND demonstrate obvious energy relaxation processes of electron-to-vibration and vibration-to-vibration,and no vibration-to-rotation energy relaxation process is observed.

Tuning Effect of N_2 on Atmospheric-Pressure Cold Plasma CVD of TiO_2 Photocatalytic Films

To deposit TiO2 films through plasma CVD, the partial pressure ratio of O2 to TIC14 should be greater than the stoichiometric ratio （PO2/PTiCl4 〉 1）. However, this may lead to the formation of powder instead of film on the substrate when using volume dielectric barrier discharge （volume-DBD） at atmospheric pressure. In this study, by adding N2 into the working gas Ar, TiO2 photocatalytic films were successfully fabricated in the presence of excess O2 （PO2/PTiC14 = 2.6） by using a wire-to-plate atmospheric-pressure volume-DBD. The tuning effect of N2 on the deposition of TiO2 film was studied in detail. The results showed that by increasing the N2 content, the deposition rate and particle size of the TiO2 film were reduced, and its photocatalytic activity was enhanced. The tuning mechanism of N2 is further discussed.

Preparation of Copper Nanoparticles Using Dielectric Barrier Discharge at Atmospheric Pressure and its Mechanism

Dielectric barrier discharge （DBD） cold plasma at atmospheric pressure was used for preparation of copper nanoparticles by reduction of copper oxide （CuO）. Power X-ray diffraction （XRD） was used to characterize the structure of the copper oxide samples treated by DBD plasma. Influences of H2 content and the treating time on the reduction of copper oxide by DBD plasma were investigated. The results show that the reduction ratio of copper oxide was increased initially and then decreased with increasing H2 content, and the highest reduction ratio was achieved at 20% H2 content. Moreover, the copper oxide samples were gradually reduced by DBD plasma into copper nanoparticles with the increase in treating time. However, the average reduction rate was decreased as a result of the diffusion of the active hydrogen species. Optical emission spectra （OES） were observed during the reduction of the copper oxide samples by DBD plasma, and the reduction mechanism was explored accordingly. Instead of high-energy electrons, atomic hydrogen （H） radicals, and the heating effect, excited-state hydrogen molecules are suspected to be one kind of important reducing agents. Atmospheric-pressure DBD cold plasma is proved to be an efficient method for preparing copper nanoparticles.

Formation of honeycomb-Kagome hexagonal superlattice pattern with dark discharges in dielectric barrier discharge

A honeycomb-Kagome hexagonal superlattice pattern with dark discharges is observed in a dielectric barrier discharge system for the first time.The spatiotemporal structure of the honeycomb-Kagome hexagonal superlattice pattern with dark discharges is investigated by an intensified charge-coupled device and the photomultipliers show that it is an interleaving of three different sub-lattices,which are bright-spot,invisible honeycomb lattice,and Kagome lattice with invisible frameworks and dim-spots,respectively.The invisible honeycomb lattices and Kagome lattices are actually composed of dark discharges.By using the optical emission spectra method,it is found that the plasma parameters of the three different sub-lattices are different.The influence of the dark discharges on pattern formation is discussed.The results may have significance for the investigation of the dark discharges and will accelerate the development of self-organized pattern dynamics.

Influence of oxygen addition on the discharge characteristics of an argon plasma jet at atmospheric pressure

Plasma jet is an important low-temperature plasma source in extensive application fields.To promote the production of active oxygen species,oxygen is often introduced into the inert working gas.However,the influence of oxygen content on the discharge characteristics of an argon plasma jet is not clear.Aim to this status,an argon plasma jet in a singleelectrode geometry is employed to investigate the influence of oxygen concentration(CO)on discharge aspects.Results indicate that with increasing CO(≤0.6%),the plume transits from a diffuse morphology to a hollow structure.Electrical and optical measurements reveal that both discharge number per voltage cycle and pulse intensity alter with varying CO.Moreover,discharge morphologies of negative and positive discharges obtained by fast photograph also shift with varying CO.Besides,optical emission spectra are collected to investigate atomic CO,electron density,and electron temperature.The results mentioned above are explained qualitatively,which are believed to be of great significance for the applications of atmospheric pressure plasma jet.

Plasma Treatment of Industrial Landfill Leachate by Atmospheric Pressure Dielectric Barrier Discharges

An dielectric barrier discharge （DBD） system in atmospheric pressure utilized for the treatment of industrial landfill leachate is reported. The discharge parameters, such as the operating frequency, gas flow rate, and treating duration, were found to affect significantly the removal of ammonia nitrogen （AN） in industrial landfill leachate. An increase in treating duration leads to an obvious increase in the removal efficiency of AN （up to 83%） and the leachate color changed from deep grey-black to transparent. Thus the dielectric barrier discharges in atmospheric pressure could degrade the landfill leachate effectively. Typical waveforms of both applied voltage and discharge current were also presented for analyzing the discharge processes under different discharge parameters. Optical emission spectra measurements indicate that oxidation species generated in oxygen DBD plasma play a crucial role in removing AN, oxidizing organic and inorganic substances and decolorizing the landfill leachate.

Atmospheric air dielectric barrier discharge excited by nanosecond pulse and AC used for improving the hydrophilicity of aramid fibers

In this paper, a long line-shape dielectric barrier discharge excited by a nanosecond pulse and AC is generated in atmospheric air for the purpose of discussing the uniformity, stability and ability of aramid fiber treatment. The discharge images, waveforms of current and voltage,optical emission spectra, and gas temperatures of both discharges are compared. It is found that nanosecond pulsed discharge has a more uniform discharge morphology, higher energy efficiency and lower gas temperature, which indicates that nanosecond pulsed discharge is more suitable for surface modification. To reduce the water contact angle from 96° to about 60°, the energy cost is only about 1/7 compared with AC discharge. Scanning electron microscopy,Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy are employed to understand the mechanisms of hydrophilicity improvement.

Discharge characteristics and reactive species production of unipolar and bipolar nanosecond pulsed gas–liquid discharge generated in atmospheric N2

In this paper,unipolar pulse(including positive pulse and negative pulse)and bipolar pulse voltage are employed to generate diffuse gas–liquid discharge in atmospheric N2with a rumpetshaped quartz tube.The current–voltage waveforms,optical emission spectra of excited state active species,FTIR spectra of exhaust gas components,plasma gas temperature,and aqueous H2O2,NO2-,andNO3-production are compared in three pulse modes,meanwhile,the effects of pulse peak voltage and gas flow rate on the production of reactive species are studied.The results show that two obvious discharges occur in each voltage pulse in unipolar pulse driven discharge,differently,in bipolar pulse driven discharge,only one main discharge appears in a single voltage pulse time.The intensities of active species(OH(A),and O(3p))in all three pulsed discharge increase with the rise of pulse peak voltage and have the highest value at 200 ml min-1of gas flow rate.The absorbance intensities of NO2and N2O increase with the increase of pulse peak voltage and decrease with the increase of gas flow rate.Under the same discharge conditions,the bipolar pulse driven discharge shows lower breakdown voltage,and higher intensities of excited species(N2(C),OH(A),and O(3p)),nitrogen oxides(NO2,NO,and N2O),and higher production of aqueous H2O2,NO2-,andNO3-compared with both unipolar positive and negative discharges.

Generation of atmospheric pressure air diffuse discharge plasma in oxygen enriched working gas with floating electrode

In this work,a floating electrode is employed to generate a stable large-area diffuse discharge plasma under an open oxygen-rich environment.The discharge image and the optical emission spectra of the N2(C-B),N2+(B-X),N2(B-A),and O(3p–3s,777 nm)are measured to analyze the morphological and optical characteristics of the discharge.The effects of applied voltage,gas flow rate,and electrode gap on the reactive species,vibrational temperature and rotational temperature are investigated,and the discharge mode is discussed by simulating the electrostatic field before the breakdown.It is found that the changes of applied voltage and electrode gap causes the transition of the discharge modes among corona mode,diffuse discharge mode and spark mode.It is shown that the floating electrode can inhibit the transition from discharge to spark mode to a certain extent,which is conducive to maintaining the stability of discharge.As is vividly illustrated in this study,the increase of applied voltage or the decrease of electrode gap contributes to the generation of more active particles,such as N2(C)andN+2(B).Furthermore,the Joule heating effect becomes more evident with the increased applied voltage when the electrode gap is 15 and 20 mm.Moreover,as the applied voltage increases,the vibrational temperature increases at the electrode gap of 25 mm.

Argon plume transition from a hollow swell to a diffuse swell with increasing amplitude of a trapezoidal voltage

Atmospheric pressure plasma jets can generate a remote plasma plume,which usually presents a conical or cylindrical morphology.Despite a few morphologies being observed,efforts should be made to obtain more plume structures because streamer dynamics may be revealed from them.For this purpose,an argon plasma plume excited by a trapezoidal voltage is investigated,which presents two kinds of swells(a hollow swell and a diffuse swell)with increasing voltage amplitude(V).The results indicate that there are two positive discharges(Dand D)and one negative discharge(D)per voltage cycle for both of the swells.With increasing V,the inception voltage and discharge intensity increase for every positive discharge,while they decrease for the negative discharge.Fast photography reveals that the positive streamer(D)leaves different tracks in the two swells,which are curved in the hollow swell and randomly branched in the diffuse swell.The different tracks of Dare explained with the consideration of applied field strength and residual positive ions of D.The existence of residual positive ions is finally verified from optical emission spectra.

Discharge Characteristics of Series Surface/Packed-Bed Discharge Reactor Diven by Bipolar Pulsed Power

The discharge characteristics of the series surface/packed-bed discharge （SSPBD） reactor driven by bipolar pulse power were systemically investigated in this study. In order to evaluate the advantages of the SSPBD reactor, it was compared with traditional surface discharge （SD） reactor and packed-bed discharge （PBD） reactor in terms of the discharge voltage, discharge current, and ozone formation. The SSPBD reactor exhibited a faster rising time and lower tail voltage than the SD and PBD reactors. The distribution of the active species generated in differ- ent discharge regions of the SSPBD reactor was analyzed by optical emission spectra and ozone analysis. It was found that the packed-bed discharge region （3.5 mg/L）, rather than the surface discharge region （1.3 mg/L） in the SSPBD reactor played a more important role in ozone gener- ation. The optical emission spectroscopy analysis indicated that more intense peaks of the active species （e.g. N2 and OI） in the optical emission spectra were observed in the packed-bed region.

Spectroscopic study of bipolar nanosecond pulse gas-liquid discharge in atmospheric argon

Atmospheric gas-liquid discharge with argon as a working gas is presented by employed nanosecond pulse power. The discharge is presented in a glow-like mode. The discharge powers are determined to be less than 1 W, and remains almost constant when the discharge duration time increases. Bountiful active species are determined by capturing optical emission spectra, and their main generation processes are also discussed. The plasma gas temperature is calculated as 350 K by comparing the experimental spectra and the simulated ones of N2（C3Ⅱg→B3Ⅱg, △v=-2）. The time resolved vibrational and rotational temperature is researched to present the stability of discharge when pulse voltage and discharge duration vary. The electron density is determined to be 1016 cm-3 according to the Stark broadening effect of the Ha line.