Measurements of plasma parameters in a hollow electrode AC glow discharge in helium

Measurements of the plasma parameters of coaxial gridded hollow electrode alternating current(AC)discharge helium plasma were carried out using an improved probe diagnostic technology.The measurements were performed under well-defined discharge conditions(chamber geometry,input power,AC power frequency,and external electrical characteristics).The problems encountered in describing the characteristics of AC discharge in many probe diagnostic methods were addressed by using an improved probe diagnostics design.This design can also be applied to the measurement of plasma parameters in many kinds of plasma sources in which the probe potential fluctuates with the discharge current.Several parameters of the hollow electrode AC helium discharge plasma were measured,including the plasma density,electron temperature,plasma density profiles,and changes in plasma density at different input power values and helium pressures.The characteristics of the coaxial gridded hollow electrode plasma determined by the experiments are suitable for comparison with plasma simulations,and for use in many applications of hollow cathode plasma.     

Theoretical research on the transport and ionization rate coefficients in glow discharge dusty plasma

The electron kinetic model for investigating the transport and ionization rate coefficients of argon glow discharge dusty plasma is developed from the Boltzmann equation.Both of the electron-neutral and electron-dust collisions are considered as collision terms in the kinetic equation.The kinetic equation is simplified by employing the local approximation and nonlocal approach under the same discharge conditions,and the corresponding simplified kinetic equations are known as local and nonlocal kinetic equations respectively.Then the electron energy distribution function(EEDF)is obtained by numerically solving the local and nonlocal kinetic equations and the dust charging equations simultaneously.Based on the obtained EEDFs,the effective electron temperature,electron mobility,electron diffusion coefficient and ionization rate coefficient are calculated for different discharge conditions.It is shown that the EEDFs calculated from the local kinetic model clearly differ from the nonlocal EEDFs and both the local and nonlocal EEDFs are also clearly different with Maxwellian distributions.The appearance of dust particles results in an obvious decrease of high energy electrons and increase of low energy electrons when axial electric field is low.With the increase of axial electric field,the influence of dust particles on the EEDFs becomes smaller.The electron mobility and diffusion coefficients calculated on the basis of local and nonlocal EEDFs do not differ greatly to the dust-free ones.While,when dust density nd=10^6 cm^?3,the electron mobility increases obviously compared with the dust-free results at low axial electric field and decreases with the increasing axial electric field until they are close to the dust-free ones.Meanwhile,electron diffusion coefficients for dusty case become smaller and decrease with the increasing axial electric field.The ionization rate coefficients decrease when dust particles are introduced and they approach the dust-free results gradually with the increasing axial electric field.     

Investigation of working pressure on the surface roughness controlling technology of glow discharge polymer films based on the diagnosed plasma

The effects of working pressure on the component, surface morphology, surface roughness, and deposition rate of glow discharge polymer(GDP) films by a trans-2-butene/hydrogen gas mixture were investigated based on plasma characteristics diagnosis. The composition and ion energy distributions of a multi-carbon(C_4H_8/H_2) plasma mixture at different working pressures were diagnosed by an energy-resolved mass spectrometer(MS) during the GDP film deposition process. The Fourier transform infrared spectroscopy(FT–IR), field emission scanning electron microscope(SEM) and white-light interferometer(WLI) results were obtained to investigate the structure, morphology and roughness characterization of the deposited films, respectively. It was found that the degree of ionization of the C_4H_8/H_2 plasma reduces with an increase in the working pressure. At a low working pressure, the C–H fragments exhibited small-mass and high ion energy in plasma. In this case, the film had a low CH_3/CH_2 ratio, and displayed a smooth surface without any holes, cracks or asperities. While the working pressure increased to 15 Pa,the largest number of large-mass fragments led to the deposition rate reaching a maximum of 2.11 μm h~(-1), and to hole defects on the film surface. However, continuing to increase the working pressure, the film surface became smooth again, and the interface between clusters became inconspicuous without etching pits.     

Influence of dust particles on spatial distributions of particles and fluxes in positive column of glow discharge

The research herein examined the results of numerical simulations of the positive column of a glow discharge in argon dusty plasma using COMSOL Multiphysics software under conditions similar to the project known as PK-4. Various scenarios dealing with formations of spatial distributions of densities and fluxes for charged particles were studied, and evaluations of the influence of dust particles on the discharge were obtained in a wide range of dust densities. Two extreme cases were distinguished: weak dust influence when the densities, fluxes and electric field profiles are not perturbed, and strong dust influence when all three density profiles(electrons, ions and charged dust) in the dust cloud are similar(parallel) to each other, resulting in all created charges in the dust cloud being lost inside the cloud. In such a case, the ambipolar field and the transport of charged particles are decreased in the dust cloud, and any ambipolar flux is almost absent within the cloud.     

Study on water treatment effect of dispersion discharge plasma based on flowing water film electrode

To improve the utilization rate of plasma active species,in this study,a closed non-uniform air gap is formed by a flowing water film electrode and a sawtooth insulating dielectric layer to realize the diffuse glow discharge in the atmosphere.Firstly,the electric field distribution characteristics of non-uniform air gap in the sawtooth dielectric layer are studied,and the influence of aspect ratio on the characteristics of diffuse discharge plasma is discussed.Subsequently,the effects of wire mesh,the inclination angle of the dielectric plate,and liquid inlet velocity on the flow characteristics of the water film electrode are analyzed.The results show that the non-uniform electric field distribution formed in the sawtooth groove can effectively inhibit the filamentous discharge,and the 1 mm flowing water film is directly used as the electrode,and high-active plasma is formed directly on the lower surface of the water film.In addition,a plasma flowing water treatment device is built to treat the methyl orange solution and observe its decolorization effect.The experimental results show that after 50 min of treatment,the decolorization rate of the methyl orange solution reaches 96％,which provides a new idea for industrial applications of wastewater treatment.     

A comparative study on the activity of TiO2 in pulsed plasma under different discharge conditions

In the present study,a combination of pulsed discharge plasma and TiO_2(plasma/TiO_2)has been developed in order to study the activity of TiO_2by varying the discharge conditions of pulsed voltage,discharge mode,air flow rate and solution conductivity.Phenol was used as the chemical probe to characterize the activity of TiO_2in a pulsed discharge system.The experimental results showed that the phenol removal efficiency could be improved by about 10%by increasing the applied voltage.The phenol removal efficiency for three discharge modes in the plasma-discharge-alone system was found to be highest in the spark mode,followed by the spark–streamer mode and finally the streamer mode.In the plasma/TiO_2system,the highest catalytic effect of TiO_2was observed in the spark–streamer discharge mode,which may be attributed to the favorable chemical and physical effects from the spark–streamer discharge mode,such as ultraviolet light,O_3,H_2O_2,pyrolysis,shockwaves and high-energy electrons.Meanwhile,the optimal flow rate and conductivity were 0.05 m~3l~(-1)and 10μS cm~(-1),respectively.The main phenolic intermediates were hydroquinone,catechol,and p-benzoquinone during the discharge treatment process.A different phenol degradation pathway was observed in the plasma/TiO_2system as compared to plasma alone.Analysis of the reaction intermediates demonstrated that p-benzoquinone reduction was selectively catalyzed on the TiO_2surface.The effective decomposition of phenol constant(D_e)increased from 74.11%to 79.16%when TiO_2was added,indicating that higher phenol mineralization was achieved in the plasma/TiO_2system.     

Growth of mirror-like ultra-nanocrystalline diamond (UNCD) films by a facile hybrid CVD approach

In this study, growth of mirror-like ultra-nanocrystalline diamond(UNCD) films by a facile hybrid CVD approach was presented. The nucleation and deposition of UNCD films were conducted in microwave plasma CVD(MPCVD) and direct current glow discharge CVD(DC GD CVD) on silicon substrates, respectively. A very high nucleation density(about 1?×?10~(11) nuclei cm~(-2)) was obtained after plasma pretreatment. Furthermore, large area mirrorlike UNCD films of Φ 50 mm were synthesized by DC GD CVD. The thickness and grain size of the UNCD films are 24 μm and 7.1 nm, respectively. In addition, the deposition mechanism of the UNCD films was discussed.     

Effect of megapore particles packing on dielectric barrier discharge,O3 generation and benzene degradation

Recently, packed-bed discharge plasma technologies have been widely studied for treatment of volatile organic compounds (VOCs), due to the good performance in improving the degradation and mineralization of VOCs. In this paper, a coaxial cylindrical dielectric barrier discharge reactor packed with porous material of micron-sized pores was used for degradation of benzene, and the discharge characteristics and ozone generation characteristics were studied. When the discharge length was 12 cm and the filling length was 5 cm, the packed particles in the discharge area significantly increased the number of micro-discharges, and the current amplitude and density increased with the pore size of packed particles, but the discharge power and ozone concentration showed a trend of first increasing and then decreasing. The discharge power and ozone production reached the maximum when the size of pore former was 75 μm, correspondingly, the degradation efficiency of benzene was the highest.     

Influence of Ti3C2Tx(MXene)on the generation of dielectric barrier discharge in air

The formation of homogeneous dielectric barrier discharge(DBD)in air is a key scientific problem and core technical problem to be solved for the application of plasmas.Here,we report the effect of two-dimensional(2D)nanomaterial Ti3C2Tx(Tx=-F,-O and/or-OH)on regulating the electrical discharge characteristics.The field emission and weak bound state property of Ti3C2Tx can effectively increase the seed electrons and contribute to the generation of atmospheric pressure homogeneous air DBD.The electron avalanche development for the uneven electrode structure is calculated,and the discharge mode transition is modeled.The comparative analyses of discharge phenomena validate the regulation of Ti3C2Tx on the discharge characteristics of DBD.The light emission capture and the voltage and current waveforms verify that the transition of Townsend discharge to streamer discharge is effectively inhibited.The optical emission spectra are used to characterize the plasma and confirm that it is in a non-equilibrium state and the gas temperature is at room temperature.This is the first exploration of Ti3C2Tx on the regulation of electrical discharge characteristics as far as we know.This work proves the feasibility of Ti3C2Tx as a source of seed electrons to form homogeneous DBD,establishing a preliminary foundation for promoting the application of atmospheric pressure non-equilibrium plasma.     

Synthesis of (B-C-N) Nanomaterials by Arc Discharge Using Heterogeneous Anodes

In spite of the current prevalence of the CVD-based processes,the electric arc remains an interesting process for the synthesis of carbon nanoforms,thanks to its versatility,robustness and easiness.It also allows performing in-situ substitution of carbon atoms by hetero-elements in the graphene lattice.Our work aims to establish a correlation between the plasma properties,type and chemical composition (and the substitution rate) of the obtained single-wall carbon nanotubes.The plasma was characterized by optical emission spectroscopy and the products were analyzed by high resolution transmission electron microscopy and core level Electron Energy-Loss Spectroscopy (EELS).Results show that a high boron content leads to a plasma temperature decrease and hinders the formation of nanotubes.This effect can be compensated by increasing the arc current and/or yttrium content.The optimal conditions for the synthesis of boron-and/or nitrogen-substituted nanotubes correspond to a high axial plasma temperature associated to a strong radial gradient.EELS analysis confirmed that the boron incorporates into the graphenic lattice.     

Application of dielectric barrier discharge (DBD) plasma packed with glass and ceramic pellets for SO2 removal at ambient temperature: optimization and modeling using response surface methodology

Air pollution is a major health problem in developing countries and has adverse effects on human health and the environment. Non-thermal plasma is an effective air pollution treatment technology. In this research, the performance of a dielectric barrier discharge (DBD) plasma reactor packed with glass and ceramic pellets was evaluated in the removal of SO₂ as a major air pollutant from air in ambient temperature. The response surface methodology was used to evaluate the effect of three key parameters (concentration of gas, gas flow rate, and voltage) as well as their simultaneous effects and interactions on the SO₂ removal process. Reduced cubic models were derived to predict the SO₂ removal efficiency (RE) and energy yield (EY). Analysis of variance results showed that the packed-bed reactors (PBRs) studied were more energy efficient and had a high SO₂ RE which was at least four times more than that of the non-packed reactor. Moreover, the results showed that the performance of ceramic pellets was better than that of glass pellets in PBRs. This may be due to the porous surface of ceramic pellets which allows the formation of microdischarges in the fine cavities of a porous surface when placed in a plasma discharge zone. The maximum SO₂ RE and EY were obtained at 94% and 0.81 g kWh⁻¹, respectively under the optimal conditions of a concentration of gas of 750 ppm, a gas flow rate of 2 l min⁻¹, and a voltage of 18 kV, which were achieved by the DBD plasma packed with ceramic pellets. Finally, the results of the model's predictions and the experiments showed good agreement.     

Formation of amorphous carbon on the surface of poly(ethylene terephthalate) by helium plasma based ion implantation

The surface modification of poly(ethylene terephthalate) (PET) by helium plasma based ion implantation (He PBII) was studied. The effect of the main process parameters (acceleration voltage, fluence and fluence rate) on the alterations of the surface chemical composition and structure were investigated by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy.According to SRIM calculations, at ion energies above 2 keV the stopping power of PET for He⁺ ions is dominated by the electronic component and the contribution of the nuclear component is relatively small. Degradation of the ester group and carbonisation were observed by XPS due to elimination of O-rich fragments. The total C-content of the modified layer increased with the increase of fluence rate and acceleration voltage of particles, enabling the purposeful alteration of the surface composition. A strong broadening was detected in the Raman spectrum between 1000 and 1700 cm⁻¹, testifying to the intense formation of amorphous carbon. The area ratio of the D (∼1410 cm⁻¹) to G (∼1570 cm⁻¹) band increased with the increase of particle fluence and the increase of acceleration voltage, offering the possibility of tailoring the chemical structure of the amorphous carbon layer created by the He PBII treatment.     

Density functional theory study of H,C and O chemisorption on UN(001) and(111) surfaces

We performed density functional theory calculations of H, C, and O chemisorption on the UN(001) and(111) surfaces using the generalized gradient approximation(GGA) and the Hubbard U parameter and revised Perdew-Burke-Ernzerhof(RPBE) exchange-correlation functional at non-spin polarized level with the periodic slab model. Chemisorption energies vs. distance of molecules from UN(001) and UN(111) surfaces have been optimized for four symmetrical chemisorption sites, respectively. The results show that the Hollow, N-top, and Hollow adsorption sites are the most stable sites for H, C, and O atoms with chemisorption energies of 13.06,25.50 and 27.34 kJ/mol for UN(001) surface, respectively. From the point of adsorbent(UN(001) and UN(111)surfaces in this paper), interaction of O with the chemisorbed surface is of the maximum magnitude, then C and H, which are in agreement with electronegativities of individual atoms. For the UN(001) surface, U-N bond lengths change relatively little( 9%) as a result of H chemisorption, however C and O chemisorptions result in remarkable changes for U-N bond lengths in interlayer( 10%). Electronic structure calculations indicate that Bridge position is equivalent with Hollow position, and the most stable chemisorption position for H, C,and O atoms are all Bridge(or Hollow) position for the UN(111) surface. Calculated electronic density of states(DOSs) demonstrate electronic charge transfer between s, p orbitals in chemisorbed atoms and U 6d, 5f orbitals.