Wang Wenchun

The Study of Active Atoms in High-Voltage Pulsed Coronal Discharge by Optical Diagnostics

In this study, the emission spectra of active atoms O (3p5P → 3s5S20 777.4 nm), Hα (3P → 2S 656.3 nm) and N (3p4P → 3s4S0 742.3 nm, 744.2 nm, 746.8 nm) produced by the positive high-voltage pulsed corona discharge (HVPCD) of N2 and H2O mixture in a needle-plate reactor have successfully been recorded against a severe electromagnetic interference coming from the HVPCD at one atmosphere. The effects of the peak voltage, the repetition rate of pulsed discharge and the flow rate of oxygen on the production of those active atoms are investigated. It is found that when the peak voltage and the repetition rate of the pulsed discharge are increased, the emission intensities of those active atoms rise correspondingly. And the emission intensities of O (3p5P → 3s5S20 777.4 nm), Hα (3P → 2S 656.3 nm) and N (3p4P → 3s4S0 742.3 nm, 744.2 nm, 746.8 nm) increase with the flow rate of oxygen (from 0 to 25 ml/min) and achieve a maximum value at a flow rate of 25 ml/min. When the flow rate of oxygen is increased further, the emission intensities of those atoms visibly decrease correspondingly. The main physicochemical processes of interaction involved between electrons, neutrals and ions are also discussed.

Synthesis of Nano-Size AlN Powders by Carbothermal Reduction from Plasma-Assisted Ball Milling Precursor

Nano-size aluminum nitride (AlN) powders have been successfully synthesized with a high efficiency method through annealing from milling assisted by discharge plasma (p-milling) alumina (Al2O3) precursors. The characterization of the p-milling Al2O3 powders and the synthesized AlN are investigated. Compared to conventional ball milling (c-milling), it can be found that the precursors by p-milling have a finer grain size with a higher specific surface area, which lead to a faster reaction efficiency and higher conversion to AlN at lower temperatures. The activation energy of p-milling Al2O3 is found to be 371.5 kJ/mol, a value that is much less than the reported value of the unmilled and the conventional milled Al2O3. Meanwhile, the synthesized AlN powders have unique features, such as an irregular lamp-like morphology with uniform particle distribution and fine average particle size. The results are attributed to the unique synergistic effect of p-milling, which is the effect of deformation, fracture, and cold welding of Al2O3 powders resulting from ball milling, that will be enhanced due to the introduction of discharge plasma.

Effect of OH Radicals on Formaldehyde Removal in Dielectric Barrier Discharge

The removal of formaldehyde by dielectric barrier discharge in a coaxial cylindrical reactor has been studied at atmospheric pressure. The emission spectra of OH (A 2 Σ→X 2 Π, 0-0) emitted from the dielectric barrier discharge have been successfully recorded. The relationship between the removal efficiency of formaldehyde and the emission intensity of OH (A 2 Σ→X 2 Π, 0-0) has been investigated at different applied voltages, driving frequencies, concentrations of argon and oxygen. We find that the removal efficiency of HCHO increases when the emission intensity of OH (A 2 Σ→X 2 Π, 0-0) increases with rising applied voltage, driving frequency, and concentration of argon. However, the removal efficiency of HCHO decreases when the emission intensity of OH (A 2 Σ→X 2 Π, 0-0) decreases with an increase in the concentration of oxygen. The removal efficiency of HCHO is 93.8% in N2+HCHO mixed gas at 11.5 kV applied voltage and 9 kHz driving frequency.

Dust Charging in the Sheath of an Electronegative Plasma

We theoretically investigate the dust charging in the sheath of an electronegative plasma, by using a single dust grain model based on a previous sheath structure [Chin. Phys. Lett 20 (2003) 1537] in which cold positive ions and hot negative ions have been assumed. It is found that dust grains are first charged negatively at the sheath edge and then begin to be charged positively in the sheath. Moreover, both the temperature ratio of electrons to negative ions and the density ratio of negative ions to positive ions have effects on the neutral point of the dust charge.

Sheath Structures of Strongly Electronegative Plasmas

The sheath structures of strongly electronegative plasmas are investigated on basis of the accurate Bohm criterion obtained by Sagdeev potential. It is found that the presheath transition between the bulk plasma and the sheath almost does not exist there, and that distributions of electrons, negative and positive ions in the sheath form a pure positive ion sheath near the boundary of the electrode. Furthermore, the density distribution of space net charge has a peak near the sheath edge, the spatial potential within the sheath falls faster, and the sheath thickness becomes thinner.

Characteristics of Collision, Capacitive Radio Frequency Sheath

A simple collisional radio frequency (rf) sheath fluid model, which is not restricted by the ratio of rf frequency to ion plasma frequency (β = ωrf/ωpi), was established and solved numerically. In the ion balance equation, the effect of the collision on the ion and the ion velocity is assumed to be a direct ratio to ion velocity. The ion energy distributions (IEDs) calculated in the model in comparison with the experimental data [M. A. Sobolewski, J. K. Olthoff, and Y. C. Wang, J. Appl. Phys. 85, 3966 (1999)], proved the validity of the model. And the effect of the collision on the sheath characteristic was obtained and discussed. This paper demonstrates that the collision frequency is another crucial parameter as well as the ratio β to determine the rf sheath characteristics and the shape of IEDs.

Surface Potential of Dust Grains at the Sheath Edge of Electronegative Dusty Plasmas

In this paper we investigate the dust surface potential at the sheath edge of electronegative dusty plasmas theoretically, using the standard fluid model for the sheath and treating electrons and negative ions as Boltzmann particles but positive ions and dust grains as cold fluids. The dust charging model is self-consistently coupled with the sheath formation criterion by the dust surface potential and the ion Mach number, moreover the dust density variation is taken into account. The numerical results reveal that the dust number density and negative ion number density as well as its temperature can significantly affect the dust surface potential at the sheath edge.