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To gain a better understanding of the mechanism governing arc dynamics in 3D gliding arc discharge (GAD) plasma, the spatio-temporal evolution of GAD was investigated in a reverse vortex flow by a novel reactor with ring (powered electrode, PE) and truncated cone (ground electrode, GE) electrodes. A newly underlying mechanism governing arc evolution in 3D GAD was gained with combination of flow field simulation, synchronous electrical characteristics, intensified charge coupled device images and high-speed photos. The spatio-temporal analysis indicates that, being different from the well-known ignition–gliding–extinction mechanism occurring in traditional GAD, the PE arc root glides continuously in the 3D GAD, but the GE arc root features jumps from the end of the gliding path to the beginning of the next one. By means of the jumping, the arc auto-sheds the caudal part of the arc and a new arc tail is simultaneously generated, rather than rebuilding a new arc channel back to the shortest electrode gap. With this special behavior of caudal autotomy and regeneration, the main part of the arc remains for each jump. This new insight improves the understanding of the discharge mechanism governing arc evolution in 3D GAD and provides a reference for optimization design of gliding arc plasma in a vortex flow.

With the aim of understanding the arc behaviors of a gliding arc (GA) in a magnetic field, arc shape, voltage-current characteristics, plasma parameters, rotational frequency and force analysis of a GA in a magnetically-driven 3D GA reactor are investigated. Three modes, straight (S-arc), forward-bent (FB-arc) and reverse-bent (RB-arc) arcs are observed by moving the magnet axially. It is demonstrated that the arc mode depends on the magnetic field, but is independent of the discharge current. The GA is a glow-like discharge with similar plasma parameters in all the modes. The S-arc mode has the fastest rotational frequency and the highest gas-treated fraction, thus is the optimal for gas conversion (ammonia decomposition to hydrogen as a model reaction in this work). A criterion of the S-arc mode, that the azimuthal Lorentz force applied by the magnetic field is proportional to the square of the radial distance, is deduced from the force analysis. In addition, a method to calculate the arc rotational frequency based on the torque balance between the Lorentz and drag forces is set up to explain why the S-arc has the fastest rotational frequency, and the calculated values agree well with the experimental results.

As an intermedia between cold and thermal plasmas, warm plasma generated by gliding arc (GA) discharge possesses high conversions and high energy efficiencies for CO2 conversion. However, there is little understanding for the arc characteristics in CO2-based atmosphere and reaction mechanism of CO2 in GA plasma from experimental approach. For this purpose, a magnetically driven GA plasma with a constant length of arc is designed to study GA characteristics and reaction mechanism by changing H2/CO2 molar ratio (from pure CO2 to pure H2). In the entire range of H2/CO2 ratio, the arc channel shows a glow-like feature. The plasma parameters are arc temperature of 2200–3600 K, electron density of ~1.6 × 1014 cm−3 and electron temperature of ~3 eV. From pure CO2 to pure H2, arc diameter decreases from 1.3 to 0.6 mm, whereas rotational frequency increases from 20 to 64 Hz. Based upon the optical emission spectra of CO, OH, H and O species, chemluminescence spectra of CO and O recombination and CO production rate detected online by gas chromatographs, the reaction mechanism at low and high H2/CO2 molar ratios is disclosed.

The conventional double-probe technique was improved with a combination of self-powering and radio-frequency (RF) choking. RF perturbations in dual-frequency capacitively coupled discharge were effectively eliminated, as judged by the disappearance of self-bias on the probes. The improved technique was tested by spatially resolved measurements of the electron temperature and ion density in both the axial and radial directions of a dual-frequency capacitive plasma. The measured data in the axial direction were compared with simulation results, and they were excellently consistent with each other. The measured radial distributions of the ion density and electron temperature were influenced significantly by the lower frequency (LF) power. It was shown that superposition of the lower frequency to the higher frequency (HF) power shifted the maximum ion density from the radial center to the edge region, while the trend for the electron temperature profile was the opposite. The changing feature of the ion density distribution is qualitatively consistent with that of the optical emission intensity reported.

To deposit TiO2 films through plasma CVD, the partial pressure ratio of O2 to TiCl4 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/pTiCl4 = 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.

Non-thermal plasma at atmospheric pressure was explored for the preparation of polysilicon from SiCl4. The power supply sources of positive pulse and alternating current (8 kHz and 100 kHz) were compared for polysilicon preparation. The samples prepared by using the 100 kHz power source were crystalline silicon. The effects of H2 and SiCl4 volume fractions were investigated. The optical emission spectra showed that silicon species played an important role in polysilicon deposition

A green process for the oxygen-free conversion of methane to high-concentration ethylene and hydrogen in a plasma-followed-by-catalyst (PFC) reactor is presented. Without any catalysts and with pure methane used as the feed gas, a stable kilohertz spark discharge leads to an acetylene yield of 64.1%, ethylene yield of 2.5% and hydrogen yield of 59.0% with 80.0% of methane conversion at a methane flow rate of 50 cm3/min and a specific input energy of 38.4 kJ/L. In the effluent gas from a stable kilohertz spark discharge reactor, the concentrations of acetylene, ethylene and hydrogen were 18.1%, 0.7% and 66.9%, respectively. When catalysts Pd-Ag/SiO2 were employed in the second stage with discharge conditions same as in the case of plasma alone, the PFC reactor provides an ethylene yield of 52.1% and hydrogen yield of 43.4%. The concentrations of ethylene and hydrogen in the effluent gas from the PFC reactor were found to be as high as 17.1% and 62.6%, respectively. Moreover, no acetylene was detected in the effluent gas. This means that a high concentration of ethylene and oxygen-free hydrogen can be co-produced directly from methane in the PFC reactor.

Vibrational and rotational temperatures were simultaneously determined in a kilohertz alternating current (ac) gliding arc discharge by using overlapped emission spectra of N2(C 3Πu–B 3Πg) with OH(A 2Σ+–X 2Πi) and with . The simulated emission spectra of OH(A 2Σ+–X 2Πi) and were largely overlapped by radiation transition bands of N2(C 3Πu–B 3Πg) when the rotational temperature was elevated from 500 K to 2500 K. The temporally resolved vibrational and rotational temperatures in a discharge voltage period suggested that the rotational temperature from OH(A 2Σ+–X 2Πi) was remarkably larger than that from N2(C 3Πu–B 3Πg) and . The ratio of number densities of excited electronic states was also determined based on the overlapped emission spectra.

Achieving complete oxidation, good humidity tolerance and low energy cost is the key issue that needs to be addressed in plasma catalytic volatile organic compounds removal from air. For this purpose, Ag/HZSM-5 catalyst-packed dielectric barrier discharge using a cycled system composed of a storage stage and a discharge stage was studied. For dilute benzene removal from simulated air, Ag/HZSM-5 catalysts exhibit not only preferential adsorption of benzene in humid air at the storage stage but also almost complete oxidation of adsorbed benzene at the discharge stage. Five 'storage?discharge' cycles were examined, which suggests that Ag/HZSM-5 catalysts are very stable during the cycled 'storage?discharge' (CSD) plasma catalytic process. High oxidation rate of absorbed benzene as well as low energy cost can be achieved at a moderate discharge power. In an example of the CSD plasma catalytic remedy of simulated air containing 4.7?ppm benzene with 50% RH and 600?ml?min?1 flow rate, the energy cost was as low as 3.7 ? 10?3?kWh?m?3?air. This extremely low energy cost to remove low-concentration pollutants from air undoubtedly makes the environmental applications of the plasma catalytic technique practical.

Surface dielectric barrier discharge (DBD) was used for atmospheric-pressure plasma CVD of TiO2 films from TiCl4 and O2 for the first time. Under this experiment, the deposition rate was estimated at 22 nm min −1 by scanning electron microscope observation and the as-deposited TiO2 films were amorphous as evidenced by Raman analysis. The photocatalytic application of TiO2 films in removing HCHO from simulated air was examined in a continuous flow reactor. The TiO2 films after calcination at 350 or 450 ◦ C were notably photocatalytically active for complete oxidation of formaldehyde to an innocuous product (CO2), which was consistent with the results of Raman analysis. Using the TiO2 films, an extremely harmful by-product, CO, was not detected from photocatalytic oxidation of HCHO in a simulated air stream. (Some figures in this article are in colour only in the electronic version)

Stable kilohertz spark discharges with a unique waveform of discharge current were studied for high-efficiency conversion of methane to hydrogen and acetylene at atmospheric pressure for the first time. Discharge stability was confirmed by the concentration variation in the product streams with 220 min discharge time using pure methane as the feed gas. Utilizing the stable kilohertz spark discharges, the hydrogen concentration in the product stream could be as high as 72.1 vol% at an energy cost of 6.7 eV per H2 molecule produced and 81.5% of methane conversion. The acetylene concentration could reach 18.4 vol% at an energy cost of 21.2 eV per C2H2 molecule produced and 70.3% of methane conversion. For methane conversion ranging from 51.9% to 81.5%, the energy costs were 6.6‐10.7 eV per CH4 molecule converted, 4.4‐6.7 eV per H2 molecule produced and 16.9‐27.6 eV per C2H2 molecule produced. This study showed the high-concentration production of COx-free hydrogen and acetylene from methane at a low energy cost by stable kilohertz spark discharges. (Some figures in this article are in colour only in the electronic version)