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The surface flashover phenomenon across vacuum-dielectric interface severely limits the service life and operational reliability of high voltage electrical equipment. The surface modification by atmospheric pressure plasma treatment is a promising method to improve the surface insulating performance of polymers. In order to explore the effect mechanism of plasma processing on the vacuum flashover characteristics of polymer materials, the atmospheric pressure plasma deposition was used to treat polytetrafluoroethylene (PTFE) film. The surface parameters under different processing conditions, such as surface chemical composition, surface resistivity, surface charge decay and trap distribution, were tested and analyzed. The space charge distribution of PTFE and the flashover voltage in vacuum were measured. The results show that Si-O-Si and Si-OH groups are introduced on the surface of PTFE, and the characteristic peaks of PTFE are gradually weakened with the increase of processing time. The surface trap density increases and more traps with lower energy level arise with longer processing time. The plasma deposition changes the space charge distribution in PTFE body, and leads to positive charge accumulation inside the sample. The flashover field strength respectively increases by 15% and 70% in direct current (DC) voltage and microsecond pulse voltage afrter plasma deposition. The rapid dissipation of surface charge is the main reason for pulse flashover voltage enhancement, while the increase of surface leakage current due to lower surface resistivity and space charge accumulation in PTFE body make the DC flashover voltage reach the saturation point. Therefore the surface insulating and body performance of polymer materials after plasma modification processing should be considered comprehensively based on different applications.

Electric field calculations based on the Poisson equation have been widely used in high voltage and plasma technology. However, in practical applications, the electric field distribution in space is relatively complex, and the simulation technology based on the traditional method is often a simplification of reality, which leads to a large error between the simulation and the actual measured value. In the actual application process, due to the limitation of measurement methods, it is necessary to infer the electric field data at other locations in space according to the measurement results. Physics informed neural networks (PINNs) are introduced into the electric field calculation. PINNs are considered partial differential equation solvers based on deep neural networks. In this paper, 2D and 3D electric field distributions are discussed and compared with the finite element method. A method of dividing the dielectric distribution based on the sigmoid function is proposed, which can be effectively used to construct the spatial electric field model of the homogeneous dielectric. The combination of the data and physical model based on PINNs establishes a method to solve the inverse problem of the relative permittivity in the electric field. The results show that PINNs can calculate the distribution of the electric field according to the physical equations and different types of constraints and parameters.

With respect to successful applications of pulsed power in gas discharges, the enhanced generation of desired active species and control of plasma parameters as required are always decisive issues. In this study, a bipolar nanosecond high-voltage pulse generator with a maximum pulse repetition rate (PRR) of up to 1 MHz (i.e. a minimum pulse interval of 1 µs) in burst mode is developed, based on the principle of full-bridge converter and pulse transformer. This pulse source is used to generate an atmospheric-pressure plasma jet in Ar + 1%CH4 gas flow, and the influence of pulse intervals (from 1–10 µs) is explored. It is found that the pulse interval can strongly modulate the active species, i.e. a short pulse interval enhances the generation of C2 radial and H atom due to the accumulation effect, when the pulse interval is comparable with their lifetime, while it slightly suppresses the generation of Ar excited states and the energy fraction into electronic excitation. Reduced pulse intervals also prominently increase the energy fraction of vibrational excitation. This study demonstrates how the PRR effectively modulates active species and energy branching and enhances the generation of certain active species in atmospheric-pressure plasma driven by pulsed power.

In this paper, the radiation of argon and hydrogen in a repetitively pulsed diffuse discharge formed in an inhomogeneous electric field at elevated gas pressure have been studied. The emission spectra are measured under a series of short voltage pulses with the durations of 0.7 ns and 160 ns. It is shown that for the diffuse discharge in pure argon, the transition of argon dimers (Ar2*, λ max = 126 nm) have the highest intensity. Small addition of Xe to Ar contributes to the disappearance of Ar2* radiation bands and the appearance of those of ArXe* and Xe2* dimers in the plasma emission spectrum. In hydrogen, emission at the maximum wavelength of 160 nm is dominated in the spectra of the diffuse discharge and the luminescence intensity in the region of 220–280 nm is relatively low. If Ar is added to H2, the diffuse discharge behaves non-uniform. The luminescence band with the peak at 160 nm narrows, strong argon ion line (Ar+, λ max = 191 nm) appears in the spectra, and the luminescence in the spectral region 220–280 nm reappears. The excitation temperature and electron density are diagnosed using emission spectra and images of the discharge plasma. It is shown that both of the excitation temperature and electron density increase as the electric field is enhanced, while the excitation temperature decreases as the gas pressure increases.

Plasma catalysis is an emerging process electrification technology for industry decarbonization. Plasma-catalytic dry reforming of methane relies on the mutual effects of the plasma and the catalyst leading to the higher chemical conversion efficiency. The effects of catalyst surfaces on the plasma are predicted to play a major role, yet they remain unexplored. Here, a 1D plasma fluid model combined with 0D surface kinetics is developed to reveal how the surface reactions on platinum (Pt) catalyst affect the redistribution of the gas-phase particles. Two contrasting models with and without the surface kinetics as well as the Spearman rank correlation coefficients are used to quantify the effect of the key species (H, CH, CH2) on the CO generation. Advancing the common knowledge that Pt catalyst can influence the plasma chemistry directly by changing the surface loss/production of particles, this study reveals that the catalyst can also affect the spatial distributions of active species, thereby influencing the plasma chemistry in an indirect way. This result goes beyond the existing state-of-the-art which commonly relies on over-simplified 0D models which cannot resolve the spatial distribution. Further analysis indicates that the species spatial redistribution is driven by the dynamic catalyst surface adsorption-desorption processes. This work enables the previously elusive account of active species redistribution and may open new opportunities for plasma-catalytic sustainable chemical processes.

A homogeneous discharge with a large volume is a desirable plasma source for many applications. Nanosecond-pulsed high-voltage (HV) excitation is believed to be a promising strategy for obtaining homogeneous or diffuse discharges at atmospheric pressure. In this paper, using a knife–plate geometry driven by a nanosecond-pulsed generator, a diffuse plasma sheet with a gap distance of 1 cm and a length of 12 cm is generated in atmospheric air, maintaining a low gas temperature of ∼330 K. However, time-resolved images reveal that the discharge, which appears diffuse to the naked eye, actually consists of multiple individual streamers that propagate from knife (HV) to plate (ground). The appearance of two processes, namely primary and secondary streamers, is consistently verified by discharge images, electric field evolution and fluid simulation. This further proves that the entire discharge belongs to an intermediate state between corona and spark. This work aids a deeper understanding of the intrinsic characters of similar diffuse discharges and optimizing parameters in practical applications.

In this work, the reaction mechanism in non-thermal plasma enabled methane conversion driven by different sources, i.e., microwave discharge, nanosecond pulsed dielectric barrier discharge, and spark discharge, is investigated. The behavior of reactive species is monitored using the optical emission spectroscopy (OES), while the final gaseous products are analyzed with gas-chromatograph (GC). A zero-dimensional (0D) reaction kinetics model is established to reveal the underlying reaction processes. By changing pressure, different plasma modes (diffuse and filamentary) are achieved and the relative emission intensity of different species and distribution of gaseous productions are manipulated. It is found that there is a strong correlation between the intensity-ratio of C2 A→X and CH A→X, gas temperature, and the relative concentration of C2 products in different discharge forms, i.e., higher intensity ratio of C2/CH is associated with higher gas temperature and more unsaturated hydrocarbon products. Assisted with the 0D reaction kinetics model, the relationship between the intermediate species and final products is illustrated. Based on the investigation in this work, it is proposed that intensity-ratio of OES can be widely adapted as a non-intrusive and real-time process monitoring in the plasma-enabled methane conversion.

Heteroatom doping is an effective method to improve the capacitive performance of graphene-based materials. In this work, a facile and efficient radio-frequency (RF) plasma treatment strategy has been employed to achieve simultaneous doping and reduction of graphene oxide (GO). As a result, boron-doped and nitrogen-doped reduced graphene oxide (denoted as B-rGO and N-rGO) have been synthesized rapidly under relatively low temperatures compared with conventional thermal methods. The B-rGO and N-rGO present significantly improved specific capacitances as high as 345 F g−1 and 365 F g−1 at 0.2 A g−1, respectively, exhibiting a fourfold increase compared to that of GO before plasma treatment. Interestingly, the N-rGO shows better rate capability than the B-rGO. Furthermore, the mechanism of simultaneous doping and reduction by RF plasma treatment is discussed based on the diagnosis of emission spectroscopy. The high energy electrons and plasma-excited ions and radicals render effective reduction, etching, and doping of GO at the same time. Compared with high-temperature carbonization and wet chemical methods, our plasma treatment method is more energy-saving and eco-friendly. We believe this rapid and straightforward plasma treatment method reported here can be extended to the incorporation of various heteroatoms into graphene lattice for broad applications.

The primary aims of this paper are to provide a better understanding of surface dielectric barrier discharge based on annular geometry and to investigate the effect of segmented grounding electrodes on their electrical and optical properties. To this end, four grounding electrode conditions are considered: 10-segment, 20-segment, and 30-segment ones as the experimental conditions, and an unsegmented (termed 0-segment) one as the control. A great number of current pulses with lower amplitudes are observed under the segmented conditions compared to the 0-segment condition. In the former case, the current pulse number and the peak value are observed to be inversely and directly proportional to the number of segments, respectively. However, the average currents corresponding to the various segmentations are observed to be nearly identical, and each of them is lower than that under the 0-segment condition. Secondly, the discharge uniformity under the 30-segment condition is observed to be better than under the 0-segment condition, because even though the discharge is usually concentrated at covered regions, it spreads spanwise to the adjacent uncovered regions as the number of segments is increased. Consequently, the airflow induced by spanwise-spread plasma extends the effective range of plasma action. Moreover, the Lissajous figures corresponding to the four conditions are ascertained to be approximately parallelogram-shaped. However, the slopes of the discharge phases are dependent on the voltage, as the variations of equivalent capacitance in dark and discharge phases are distinct. A higher amount of power is consumed under the 30-segment condition than under the 0-segment condition, although the maximum transported charge is much lower in the former case. Finally, in the quasi-sinusoidal external electric field distribution induced by the segmented grounding electrode, a slightly lower-than-average electric fields avoid the creation of obvious separated channels, while a moderate peak-to-peak difference of electric field improves the electric field distortions caused by existing micro-discharges. This phenomenon serves as a satisfactory explanation of the differences between the discharge channel developments and the plasma distributions under different conditions. Based on the obtained results, we conclude that the performance of discharge plasma can be improved by arranging the electrodes optimally.

Non-thermal plasmas show great potential in low-temperature activation of methane (CH4) owing to the abundant energetic active species. Motivated by the fact that the chemical reactions in plasma-based CH4 conversion are dominated and regulated by the energetic electrons and various radicals, the temporal evolution of the electron energy distribution function (EEDF) and its relation to hydrogen (H) radical generation in an atmospheric-pressure CH4 needle–plane discharge plasma have been investigated numerically. The simulations are carried out using one-dimensional particle-in-cell Monte-Carlo collision and fluid dynamic models. It can be shown that during the formation and development of the streamer, a characteristic time exists, before and after which the evolution characteristic of the EEDF is reversed. This is mainly attributed to the competition between the energies continuously obtained from the electric field and the increasingly strong inelastic collisions and fast-growing low-energy electron population. When the amplitude of the applied voltage is increased, the fraction of electrons with high enough energy to participate in dissociation or ionization reactions of CH4 increases, leading to an increased H density. Besides, the characteristic time decreases exponentially, and the energy efficiency of the activation of CH4 molecules is decreased. An appropriate electron energy distribution and H radical density should be chosen to ensure acceptable product selectivity and conversion rate without excessive energy consumption; this will depend on the required products. The results presented in this work provide a partial theoretical basis for effectively optimizing the content of high-energy electrons and H radicals.

A double layer DBD plasma jet driven by a pulsed generator is used for SiO x thin film deposition. The effects of dielectric and conductive targets on discharge behavior and film properties are analyzed. Different fraction of N2 is added to the working gas (argon) to modulate film growth rate. The electrical property and optical emission of the plasma jet at different positions are characterized. Film surface morphology, composition and thickness are measured for different targets. The excitation temperature (T exc ), vibrational temperature (T vib ) and rotational temperature (T rot ) are calculated. The relationship between the discharge behavior and the film properties is studied. Results show that the relative optical emission intensities of Si, O, Ar and N2, as well as T exc , T vib and T rot are higher for the PMMA target between the high voltage electrode and the grounded electrode (referred to as the H–G region). However, T vib and T rot appear to be higher on the surface of the copper (conductive) target. The morphology of SiO x film is different: nanoparticles are formed on the PMMA (dielectric) target while dense amorphous film is formed on the copper target. With the same depositing parameters, the thickness of the SiO x film is higher on the copper target. The presence of PMMA at the downstream of the plasma can be considered as a small capacitor with a large resistance, which inhibits the current into the primary circuit passing through the H–G region. Although the presence of a dielectric target enhances the discharge in the H–G region, the charge accumulation on the PMMA surface limits the film growth. These results may contribute to the development of next-generation plasma surface modification technologies for diverse applications in electronics, material synthesis and other related areas.

Lift-enhancement–drag-reduction technology is strongly required by near-space vehicles with low Reynolds number. It is known that a flow control method by a surface dielectric barrier discharge (SDBD) plasma can play an important role in this field. In order to obtain the discharge characteristics and evaluate the flow control effect of a SDBD actuator, the nanosecond-pulse discharge and induced flow field by the SDBD plasma are simulated at various altitudes using discharge-aerodynamics models. The results show that the ignition voltage decreases with altitude and it is very easy to discharge in near space. Compared with a SDBD at ground level, the plasma is produced on both sides of the exposed electrode and distributes more uniformly in near space. Although the body force generated by the SDBD actuator is less effective in inducing a jet with nanosecond-pulse excitation than that with alternating voltage excitation, the induced jet by body force is with longer extent, thicker profile and higher velocity in near space than at ground level. The plasma bulk heating should be taken into account for nanosecond-pulse excitation. The Joule heating of electrons is the main source of plasma bulk heating which acts as a micro-explosion and mainly induces pressure perturbation. The discharge at ground level is like a ‘point explosion’, but a ‘region explosion’ in near space, which indicates a diffuse distribution of energy, should be responsible for the fact that the effect of bulk heating is reduced in near space.