Plasma Chemistry and Plasma Processing

A new 3-phase ac plasma reactor has been developed within the framework of research on hydrocarbon cracking for the production of carbon black and hydrogen. (1,2) One of the main characteristics of the system is related to the 3-phase, 50 Hz ac current plasma generator which induces a very particular arc motion affecting the heat and mass transfer inside the reactor. In a first step, the general flow inside the reactor in the absence of hydrocarbon injection has been studied. A simplified approach to characterize the heat and mass transfer inside the reactor is presented in this paper. The arc zone analysis is carried out simultaneously by a theoretical analysis of the electromagnetic forces and by an ultrahigh-speed cine-camera analysis. The flow in the reactor is modeled with a CFD commercial code. Results are compared with experimental temperature measurements.

Hollow particles have attracted considerable attention owing to their unique properties. In this study, hollow monoclinic zirconia particles were directly synthesized from inkjet droplets of a zirconyl hydroxychloride aqueous solution via atmospheric-pressure plasma processing. Hollow structures with craggy surfaces were obtained in the plasma at gas temperatures above 1000 K. The steep solvent evaporation rate induced by the localized high-energy reaction field of the atmospheric-pressure plasma may have induced solute condensation near the droplet surface and contributed to the formation of hollow particles. The average diameter of the synthesized particles was ~ 3 μm, while their size distribution was narrow (coefficient of variation: 0.06–0.10). The high reproducibility of the synthesized particles was attributed to the small variations in inkjet droplet size. The proposed method enables the rapid synthesis of hollow particles of various inorganic materials, while controlling their number and composition.

The axial distribution of the electrostatic potentials ofan electron-cyclotron resonance (ECR) processing plasma confined in a do magnetic mirror geometry was characterized. The potential profiles far argon and helium at 8.0x 10−4 and 4.0 × 10−4 Torr were measured using electron emissive probes. The experimental measurements were then compared with the predictions of a one-dimensional, electrostatic, particle-in-cell computer code which runs on a personal computer. The potential profiles as predicted by the code showed good agreement with the experimental measurements.

The mass transfer and energy efficiency in the “batch” and “Circulating” gliding arc configuration reactors for the direct discharges and degradation of pollutants in the aqueous solution have been investigated. The mass transfer characterization and energy efficiency in this study showed that the “Batch” configuration would be more efficient than the “Circulating” reactor. The difference between these reactors is due to the plasma (gas)–solution (liquid) contact time, therefore the gas–liquid transfer phenomenon. The lowest value of pH (2.5) and high temperature obtained in the “Batch” reactor contributes to better nitrogen oxides (NOx) transfer and solubility consequently the high conversion of the phenol in this reactor configuration (100% after 10 min of treatment) relative to that obtained in the circulating reactor (≈ 50% after 30 min) with pH 4.7.

Non-thermal plasma (NTP) was produced in a dielectric barrier discharge reactor for degradation of acetaldehyde and benzene, respectively. The effect of volatile organic compounds (VOCs) chemical structure on the reaction was investigated. In addition, acetaldehyde was removed in different background gas. The results showed that, no matter in nitrogen, air or oxygen, NTP technology always exhibited high acetaldehyde removal efficiency at ambient temperature. However, it also caused some toxicity by-product such as NOx and ozone. Meanwhile, some intermediates such as acetic acid, amine and nitromethane were formed and resulted in low carbon dioxide selectivity. To solve above problems, Co–OMS-2 catalysts were synthesized and combined with plasma. It was found that, the introduction of catalysts improved VOCs removal efficiency and inhibited by-product formation of plasma significantly. The plasma-catalysis system was operated in a recycling experiment to investigate its stability. The acetaldehyde removal efficiency can be kept at 100 % in the whole process. However, slight deactivation in ozone control was observed at the later stage of the experiment, which may be ascribed to deposition of VOCs on the catalysts surface and reduction of catalysts surface area.

Dusty or complex plasmas are rich in reactive species and micro-/nano-particles, and they are present in different systems such as interstellar clouds, Tokamak, and plasma-based microfabrication, among others. Depending on the experimental conditions, the particles may affect the plasma properties, which in turn affects the particle characteristics. So far, the interdependent plasma–particle system remains ambiguous, particularly in the case of non-thermal plasmas at atmospheric pressure. Herein, we study the properties of a pulsed nanosecond discharge in ambient air in contact with fluorapatite microparticles of variable size distributions (d1: 20–38 μm, d2: 38–106 μm, and d3: 106–150 μm), at different gap distances. The obtained results demonstrate that the discharge electrical and optical properties are strongly dependent on the size distribution of particles, but not on the gap distance. For instance, homogeneous discharge emission is observed in the presence of d1 particles; however, the emission obtained with d2 and d3 particles is filamentary. Based on emission spectroscopy and other analyses, Ca, Ca+, and F elements are present in the plasma (band intensities vary depending on the conditions), which indicates that the plasma–particle interactions are strong. Furthermore, the morphology of the processed particles shows signatures of heating and melting in the case of d1, while in the case of d3, discharge processing induces matter ejection from the particles by electro-erosion-like phenomenon. These changes in particle morphology are consistent with the emission data, as d1 particles are suspended in the plasma, whereas d3 particles remain at the electrode surface. Overall, the findings reported here contribute to the understanding of plasma–particle interactions, as well as to the development of applications related to particles processing.

The oxidation of 12 aromatic compounds using either high voltage or radio frequency glow discharges has been studied. The reactions have been carried out by making the oxygen plasma reach the low vapor pressure substrate. This (1.5-3 ml) was placed into a double-walled glass vessel that was cooled down to temperatures close to its freeing point. Oxygen pressure was of the order of 0.05-0.30 torr, the ratio p(Oa) / (vapor pressure of the liquid) being in the range 10–120. The results obtained in the plasma-liquid interactions of this work considerably differ from those in the homogeneous gas phase. Neither fragmentation products in the traps nor polymers on the reactor walls have been detected. Product formation has proved to be more selective as well. For anisole and five monoalkylben:enes o-, m-, and p-alkvlphenols amounted to 65–86% of the total yield, ortho derivatives being the most important products. The oxidation of the alkyl side chain was observed at a lower level than the aromatic hydroxylation. No ipso substitution was detected. Diand trimethylbenzenes were also studied, di- and trirnethylphenols being the major products. For all substrates dihydroxy derivatives were the most important byproducts. Total conversion, i.e., mass transformed against initial mass of substrate, has been studied as a Junction of temperature of the liquid and oxygen fog, rate in the reactor, this ranging from 10 to 20 mMol/h. The optimum conversions were 7 to 40%. A correlation between these results and the behavior of the O(3P) population in the discharge has been /bund.

Analytical results of the thermophoretic force on an evaporating spherical particle immersed in a rarefied plasma with a large temperature gradient are presented for the extreme case of free-molecule regime and thin plasma sheath. It has been shown that the existence of a temperature gradient in the plasma causes a nonuniform distribution of the local heat flux density on the sphere surface with its maximum value at the fore-stagnation point of the sphere, although the total heal flux to the whole particle is independent of the temperature gradient existing in the plasma. This nonuniform-distribution of the local heat flux density causes a nonuniform distribution of the. local evaporated-mass flux and related reaction force around the surface of an evaporating particle, and thus causes an additional force on the particle. Calculated results show that the thermophoretic force on an evaporating particle may substantially exceed that on a nonevaporating one, especially for the case of a metallic particle (with infinite electric conductivity). The effect of evaporation on the thermophoretic force is more pronounced as the evaporation latent heat of the particle material is comparatively low and as high plasma temperatures are involved.

In this paper computational results are presented which reveal the effects of the Knudsen number on heat transfer and drag of small particles in a flowing thermal argon plasma. The Knudsen number is restricted to moderate values so that “temperature jump” and “velocity slip” conditions may be employed, and for the governing equations the continuum approach remains valid. It is shown that the ratio of the heat fluxes with and without the Knudsen effect is almost identical to the ratio obtained by the authors for the case of pure heat conduction. This fact is very important for modeling of the behavior of particles injected into an actual plasma reactor when the Knudsen effect has to be taken into account.

Superparamagnetic iron oxide nanoparticles were synthesized by injecting ferrocene vapor and oxygen into an argon/helium DC thermal plasma. Size distributions of particles in the reactor exhaust were measured online using an aerosol extraction probe interfaced to a scanning mobility particle sizer, and particles were collected on transmission electron microscopy (TEM) grids and glass fiber filters for off-line characterization. The morphology, chemical and phase composition of the nanoparticles were characterized using TEM and X-ray diffraction, and the magnetic properties of the particles were analyzed with a vibrating sample magnetometer and a magnetic property measurement system. Aerosol at the reactor exhaust consisted of both single nanocrystals and small agglomerates, with a modal mobility diameter of 8–9 nm. Powder synthesized with optimum oxygen flow rate consisted primarily of magnetite (Fe3O4), and had a room-temperature saturation magnetization of 40.15 emu/g, with a coercivity and remanence of 26 Oe and 1.5 emu/g, respectively.