Micro-dielectric Barrier Discharge Research Articles

Simulation study of a pulsed DBD with an electrode containing charge injector parts

By using a multispecies fluid model, the tunability and controllability of plasma parameters such as distributions of electron density, electron energy, ion density, and electric field in a microdielectric barrier discharge (DBD) with a charge injector electrode and driven by negatively polarized nanosecond pulsed voltage superimposed on a positive DC bias voltage are investigated. To this end, the effects of changing features of pulsed voltage like pulse rise time (10–20 ns), pulse peak width (10–15 ns), and pulse fall time (20–30 ns) on characteristics of argon plasma formed inside the reactor are studied. The results show that with the increase in pulse width and pulse rise time, the density of electron and ion increases, while fall time change does not significantly affect the plasma parameters. Generally, the results of this study explicitly prove the possibility of controlling plasma formed inside DBD reactors driven by negative pulse voltage combined with a positive DC voltage, which is very important in waste gas conversion applications.

Ionization wave propagation characteristics under different polarity of pulse waveforms in micro-DBD device driven by bipolar nanosecond pulse waveform

Different spatiotemporal modes of ionization wave propagation at opposite polarity of bipolar pulses in a micro-dielectric barrier discharge structure device are investigated. The device is fabricated on a heavily doped n-type silicon substrate, and a 1 cm × 1 cm square cavity is formed on the 180 μm-thick polyimide film. Different modes of ionization wave propagation determined by the polarity of bipolar pulses are observed, and the details of streamerlike mode and wavelike mode under positive and negative half cycles of pulses are investigated, respectively. The propagation speeds of streamerlike ionization waves and wavelike ionization waves are ∼120 km/s and ∼40 km/s on average and ∼150 km/s and ∼70 km/s in maximum, respectively. Different parameters of bipolar pulses, especially the rising time of pulses, are applied to the proposed device to explore the variation of ionization wave propagation properties. The results show that the modes of the ionization wave propagation are barely changed when the device is driven by different rising time pulses. However, the initial plasma generation time and propagation speed are greatly changed. With a decrease in the rising time from 400 ns to 50 ns, the initial plasma generation time is brought forward over 200 ns, and the ionization wave propagation speed is improved over 30% for both cases. The results imply great significance in the exploration of the dynamics of plasma discharge evolution and regulation of plasma discharge properties through manipulating the pulse parameters.

Preventing the Solid Cancer Progression via Release of Anticancer-Cytokines in Co-Culture with Cold Plasma-Stimulated Macrophages.

Non-thermal atmospheric pressure plasma sources operated in ambient environments are known to generate a variety of reactive oxygen and nitrogen species which could be applied for various biomedical applications. Herein, we fabricate a micro-dielectric barrier discharge plasma device by using screen-printing technology and apply it for studying immuno-stimulatory effects. We demonstrate a tumor-suppressive role for plasma-stimulated macrophages in metastatic solid cancers that directly elicit proliferation and are responsible for tumor relapse mediated by mesenchymal shift. Using microarray analysis, we observed that cold plasma stimulates and differentiates monocyte cells into macrophages as demonstrated by expression of several cytokine/chemokine markers. Moreover, plasma treatment stimulates the differentiation of pro-inflammatory (M1) macrophages to a greater extent. These stimulated macrophages favor anti-tumorigenic immune responses against metastasis acquisition and cancer stem cell maintenance in solid cancers in vitro. Differentiation of monocytes into anticancer macrophages could improve the efficacy of plasma treatment, especially in modifying pro-tumor inflammatory microenvironment through effecting highly resistant immunosuppressive tumor cells associated with tumor relapse.

Changes in the biomechanical properties of a single cell induced by nonthermal atmospheric pressure micro-dielectric barrier discharge plasma.

Mechanical properties of a single cell are closely related to the fate and functions of the cell. Changes in mechanical properties may cause diseases or cell apoptosis. Selective cytotoxic effects of nonthermal atmospheric pressure micro-dielectric barrier discharge (DBD) plasma have been demonstrated on cancer cells. In this work, changes in the mechanical properties of a single cell induced by nonthermal atmospheric pressure micro-DBD plasma were investigated using atomic force microscopy (AFM). Two cervical cancer cell lines (HeLa and SiHa) and normal human fibroblast cells (HFBs) were exposed to micro-DBD plasma for various exposure times. The elasticity of a single cell was determined by force-distance curve measurement using AFM. Young's modulus was decreased by plasma treatment for all cells. The Young's modulus of plasma-treated HeLa cells was decreased by 75% compared to nontreated HeLa cells. In SiHa cells and HFBs, elasticity was decreased slightly. Chemical changes induced by the plasma treatment, which were observed by Raman spectroscopy, were also significant in HeLa cells compared to SiHa cells and HFBs. These results suggested that the molecular changes induced by micro-DBD plasma were related to cell mechanical changes.

Antioxidant Potential and Antibacterial Efficiency of Caffeic Acid-Functionalized ZnO Nanoparticles.

We report a novel zinc oxide (ZnO) nanoparticle with antioxidant properties, prepared by immobilizing the antioxidant 3-(3,4-dihydroxyphenyl)-2-propenoic acid (caffeic acid, CA) on the surfaces of micro-dielectric barrier discharge (DBD) plasma-treated ZnO nanoparticles. The microstructure and physical properties of ZnO@CA nanoparticles were characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), infrared spectroscopy, and steady state spectroscopic methods. The antioxidant activity of ZnO@CA nanoparticles was evaluated using an ABTS (3-ethyl-benzothiazoline-6-sulfonic acid) radical cation decolorization assay. ZnO@CA nanoparticles exhibited robust antioxidant activity. Moreover, ZnO@CA nanoparticles showed strong antibacterial activity against Gram-positive bacteria (Staphylococcus aureus) including resistant bacteria such as methicillin-resistant S. aureus and against Gram-negative bacteria (Escherichia coli). Although Gram-negative bacteria appeared to be more resistant to ZnO@CA nanoparticles than Gram-positive bacteria, the antibacterial activity of ZnO@CA nanoparticles was dependent on particle concentration. The antioxidant and antibacterial activity of ZnO@CA may be useful for various biomedical and nanoindustrial applications.

Fabrication of SiNx Thin Film of Micro Dielectric Barrier Discharge Reactor for Maskless Nanoscale Etching.

The prevention of glow-to-arc transition exhibited by micro dielectric barrier discharge (MDBD), as well as its long lifetime, has generated much excitement across a variety of applications. Silicon nitride (SiNx) is often used as a dielectric barrier layer in DBD due to its excellent chemical inertness and high electrical permittivity. However, during fabrication of the MDBD devices with multilayer films for maskless nano etching, the residual stress-induced deformation may bring cracks or wrinkles of the devices after depositing SiNx by plasma enhanced chemical vapor deposition (PECVD). Considering that the residual stress of SiNx can be tailored from compressive stress to tensile stress under different PECVD deposition parameters, in order to minimize the stress-induced deformation and avoid cracks or wrinkles of the MDBD device, we experimentally measured stress in each thin film of a MDBD device, then used numerical simulation to analyze and obtain the minimum deformation of multilayer films when the intrinsic stress of SiNx is −200 MPa compressive stress. The stress of SiNx can be tailored to the desired value by tuning the deposition parameters of the SiNx film, such as the silane (SiH4)–ammonia (NH3) flow ratio, radio frequency (RF) power, chamber pressure, and deposition temperature. Finally, we used the optimum PECVD process parameters to successfully fabricate a MDBD device with good quality.

Selective cytotoxic effect of non-thermal micro-DBD plasma

Non-thermal plasma has been extensively researched as a new cancer treatment technology. We investigated the selective cytotoxic effects of non-thermal micro-dielectric barrier discharge (micro-DBD) plasma in cervical cancer cells. Two human cervical cancer cell lines (HeLa and SiHa) and one human fibroblast (HFB) cell line were treated with micro-DBD plasma. All cells underwent apoptotic death induced by plasma in a dose-dependent manner. The plasma showed selective inhibition of cell proliferation in cervical cancer cells compared to HFBs. The selective effects of the plasma were also observed between the different cervical cancer cell lines. Plasma treatment significantly inhibited the proliferation of SiHa cells in comparison to HeLa cells. The changes in gene expression were significant in the cervical cancer cells in comparison to HFBs. Among the cancer cells, apoptosis-related genes were significantly enriched in SiHa cells. These changes were consistent with the differential cytotoxic effects observed in different cell lines.

Contribution of electrons, Ar(3P0,2), H2O+, and H3O+ to production of OH(A2Σ+) in a micro-dielectric barrier discharge of Ar/H2O

The numerical simulation of a micro-dielectric barrier discharge of an Ar/H2O gas mixture has been performed to understand the behavior of OH(A) in three-dimensionally integrated micro-solution plasma. During the discharge ignition sequence in the period of 0–200 ns, the major OH(A) production reactions are the dissociation of H2O by high-electron-temperature electrons and metastable excited Ar(3P). After the main discharge, OH(A) production is dominated by the dissociative recombination of H3O+ and low-electron-temperature electrons.

The effect of power balance on the heating mode transition in micro-dielectric barrier helium glow discharges

A particle-in-cell simulation has been adapted to analyze the helium discharge characteristics of a planar micro-dielectric barrier discharge with a gap distance less than 100 µm for variation of driving frequency from 10 to 500 MHz. Two scaling laws for the breakdown voltage were obtained with the change in heating mechanism from the γ- to the α-mode with increasing frequency. The optimal condition for the efficient generation of high-density plasmas with minimized power is obtained when the ratio of ion transit time to the RF period is about 1/4, so that the secondary electrons emitted by ion bombardment on the dielectric-gap boundary are accelerated in phase with the sheath potential. Under this condition, the plasma density as well as the ion current is maximum, the displacement current and the electron current are almost the same, and the ratio of the power delivered to the plasma is 50% of the total power. At high frequencies, the collisional electron heating increases by the induced bulk electric field, enhancing excitation and elastic collisions, and thus ionization efficiency decreases while excitation efficiency increases.

Methane conversion into higher hydrocarbons with dielectric barrier discharge micro-plasma reactor

We reported a coaxial, micro-dielectric barrier discharge (micro-DBD) reactor and a conventional DBD reactor for the direct conversion of methane into higher hydrocarbons at atmospheric pressure. The effects of input power, residence time, discharge gap and external electrode length were investigated for methane conversion and product selectivity. We found the conversion of methane in a micro-DBD reactor was higher than that in a conventional DBD reactor. And at an input power of 25.0 W, the conversion of methane and the total C2+C3 selectivity reached 25.10% and 80.27%, respectively, with a micro-DBD reactor of 0.4 mm discharge gap. Finally, a nonlinear multiple regression model was used to study the correlations between both methane conversion and product selectivity and various system variables. The calculated data were obtained using SPSS 12.0 software. The regression analysis illustrated the correlations between system variables and both methane conversion and product selectivity.

Influence of gas pressure and applied voltage on Xe excimer radiation from a micro dielectric barrier discharge for plasma display panel

We present the influence of gas pressure and applied voltage on Xe excimer radiation from a microdielectric barrier discharge (micro-DBD) in Ne/Xe gas mixture for plasma display panel. Measurements show that the excimer radiation with the 172 nm band lines is strongly observed in the afterglow, and drastically increases with an increase in gas pressure and applied voltage. It is also found that for high gas pressure and low voltage, excimer molecule (Xe2∗) is efficiently produced because of less infrared emission from Xe excited atom. The reaction processes of Xe metastable atom (Xe1s5∗), which is a precursor for Xe2∗, are theoretically analyzed using a one-dimensional fluid model. Increasing gas pressure results in large excimer radiation due to the enhancement of the following three processes, i.e., the conversion process from Xe1s5∗ to Xe2∗, the direct electron impact excitation from ground state to Xe1s5∗, and the collisional de-excitation process from upper level (Xe∗∗) to Xe1s5∗. The simulation analytical result shows that for lower voltage, Xe1s5∗ is efficiently produced due to the increase in the ratio of direct excitation to Xe1s5∗ from ground state and the suppression of the stepwise ionization from Xe1s5∗ by electron collisions. While for high voltage operation of micro-DBD, the recombination process with Xe molecular ion (Xe2+) and electron contributes to the total excimer radiation, which can be responsible for the large excimer radiation observed in experiment.

Optimum conditions for ozone formation in a micro dielectric barrier discharge

Micro-dielectric barrier discharge has attracted attention as a practical solution to realize an atmospheric pressure glow discharge process suitable for the surface treatments including ozone generation. In order to investigate the optimum conditions for the effective ozone formation in a micro-dielectric barrier discharge, measurements of ozone concentration were carried out for various conditions such as the gap length, the dielectric material and the operating gas. It was found that the optimum discharge conditions extremely differed in the kinds of the operating gases and the dielectric materials. In dry air, the dielectric material with the low dielectric constant and thermal conductivity, which might contribute the restriction of the gas temperature rise in the discharge region, was effective to obtain the high ozone yield and concentration. The optimum gap length was considered to be in the range of 0.6–0.8 mm. In oxygen, using a quartz glass disk as the dielectric material, the required condition to obtain the high ozone yield and concentration was expanded.