Direct Current Discharge Plasma Research Articles

Ion temperature profile importance in collisional sheath modelling

A plasma fluid model is being developed for the simulation of a direct current plasma discharge simulation including the sheath regions. The code uses a second order centered finite difference scheme and time integration is done by strong stability preserving third order Runge-Kutta method. The separation of scalar and vectorial quantities in two different grids gives stable results. After validation by comparison with theoretical ion sheath profiles, a one dimensional direct current argon discharge was simulated and compared to 1D3v particle-in-cell simulation results. It is shown that the inclusion of a non constant ion temperature profile is mandatory in fluid models in order to recover correct increase of ion velocity in sheaths and thus to simulate direct current (DC) discharges where collisions are not negligible in the sheaths.

Control of NV, SiV and GeV centers formation in single crystal diamond needles

Single-crystal diamond needles in the form of micrometer-scale sharp pyramids were produced using a combination of chemical vapor deposition and thermal oxidation processes. These diamond needles with atomic-sized fluorescent defects (color centers) attract great interest for practical applications as a platform for creation of optical quantum sensing probes and optical quantum communication elements. Particular color centers localization providing suitable luminescence characteristics is required for these applications. Here we report about formation of nitrogen-vacancy (NV), silicon-vacancy (SiV) and germanium-vacancy (GeV) centers in the single-crystal diamond needles during direct current discharge plasma enhanced chemical vapor deposition (PE CVD). Luminescent characteristics of the needles were observed by confocal photoluminescence mapping. Analysis of experimental results allows us proposing possible mechanisms of impurities introduction into the crystal lattice during needles growth. These mechanisms include gas decomposition and etching of substrate material during CVD. The proposed mechanisms were approved by realization of CVD process protocols resulting in controllable formation of NV, SiV and GeV centers in single-crystal diamond needles including formation of thin NV- and SiV- enriched layers by tuning deposition process parameters. The proposed experimental approaches in combination with revealed mechanisms for introduction of favorable impurities make single-crystal diamond needles promising candidates for scanning quantum sensing, quantum communication and hyperpolarization experiments.

Recent advances towards aqueous hydrogen peroxide formation in a direct current plasma–liquid system

The aqueous phase hydrogen peroxide (H2O2aq) produced from the plasma–liquid interactions can directly or synergistically (with other substances) affect the liquid chemistry, and therefore it is important to unfold the H2O2aq formation mechanism. However, up to now, a consensus on the H2O2aq formation mechanism is not reached. This review aims to survey the recent advances on the understanding of the H2O2aq formation mechanism in the system of a direct current discharge plasma operated over a liquid electrode. Theoretical and experimental analyses indicate that the recombination of dissolved OH radicals (OHaq) is the dominant process for the H2O2aq formation, while the purported plasma-induced photolysis of water and the dissolution of gaseous H2O2 are ruled out.

Two-dimensional simulation of an Ar/H2 direct-current discharge plasma

We created a two-dimensional, mathematical fluid model of a plasma in a spherical direct-current (DC) glow-discharge chamber based on the finite-element method (FEM) using the commercial FEM solver COMSOL Multiphysics. The model is based on the Boltzmann transport equation, and we solved it to simulate the transient physical characteristics of an Ar/H2 glow-discharge plasma, including the distributions of electron density, electrical potential, electron temperature, and other physical characteristics in the reaction chamber. We simulated a 5%H2/95%Ar DC glow discharge at 500 V and 2 Torr, and the results show that the electron density is distributed between the anode and the cathode, with a maximum electron density of 2.76 × 1015 m−3 and a maximum electron temperature of 5.37 eV. We also studied the effects of the discharge voltage and pressure on the electron density. The mathematical model simulates well the variation of the electron density in the chamber, and it shows that the electron density increases with increasing pressure or driving voltage.

Calculation of Temperature Conditions of a Plasma Sputtering Cell for Decontamination of Nuclear Power Plant Constructions

We are developing a technology for the “dry” deactivation of nuclear power plant (NPP) constructions by ion sputtering of surfaces with microsized radioactive contaminants. Our technology is implemented by plasma discharge in an inert gas medium with mass transfer of sputtered material and its deposition in the diffusion mode on the anode substrate. In our technology, unlike traditional radiochemical methods, radionuclides do not transfer into liquid radioactive waste (LRW), but condense in a compact solid form, which makes it possible to use them. Since our technology can be applied both during decommissioning of NPPs (including deactivation of neutron-irradiated nuclear graphite) and during routine operation and scheduled repairs of nuclear reactors, it is possible to extract the necessary isotope concentrate in the required quantities using an intense neutron flux in the NPP. The design of a plasma sputtering cell to remove radionuclides from the irradiated graphite and the NPP construction surfaces involves ignition of a direct current plasma discharge in an inert gas (for example, argon) at a pressure P ~ 0.1–1 atm and control of the temperature conditions of the sputtering material deposition. In this paper, the temperatures of the anode (tantalum collector) and the cathode (sputtered graphite) have been calculated depending on the input power to the argon plasma discharge. Data on the temperatures of the cathode and anode (collector) surfaces make it possible to control the elemental composition of the sputtered atoms and to form nanoscale layers of radionuclide concentrate on collector for use in radiation medicine and new betavoltaic batteries. Thus, the technology is important not only for the deactivation of NPPs but also for the formation of nanoscale layers of beta-active materials.

Calculation of temperature conditions of a plasma sputtering cell for decontamination of nuclear power plant constructions

We are developing a technology for the “dry” deactivation of nuclear power plant constructions by ion sputtering of surfaces with micro-sized radioactive contaminants, our technology is implemented by plasma discharge in an inert gas medium with mass transfer of sputtered material and its deposition in diffusion mode on the anode substrate. Unlike traditional radiochemical methods, in our technology, radionuclides do not transfer into liquid radioactive waste, but condense in a compact solid form, which makes possible to use them. Весаuse our technology can be applied both during decommissioning nuclear power plants (including deactivation of neutron-irradiated nuclear graphite), and during routine operation and scheduled repairs of nuclear reactors, so it is possible to extract the necessary isotope concentrate in the required quantities, due to intense neutron flux in nuclear power plant. The design of plasma sputtering cell to remove radionuclides from the irradiated graphite and NPP construction surfaces involves ignition of a direct current plasma discharge in an inert gas (for example, argon) at the pressure of P ~ 0.1–1 atm and control of the temperature conditions of the sputtering material deposition. In this paper temperatures of the anode (tantalum collector) and the cathode (sputtered graphite) were calculated depending on the input power to the argon plasma discharge. Data on temperatures of cathode and anode (collector) surfaces make it possible to control the elemental composition of the sputtered atoms and to form nanoscale layers of radionuclide concentrate on collector for using in radiation medicine and new beta-voltaic batteries. Thus, the technology is important not only for the deactivation of nuclear power plants, but also for the formation of nanoscale layers of beta-active materials.

Observation of ring–shaped pulsed DC discharge plasma source using single pole magnet setups for material processing

ABSTRACTA pulsed direct current (DC) discharge ring–shaped plasma source has been proposed using single pole magnet arrangements, including a center magnet, with magnets in the setups (a): one circle, (b): two circles, and (c): three circles. The three-dimensional (3D) magnetic flux lines profiles, gyro–radii and Hall parameters of the electrons and ions, the electrical discharge characteristics, the temporal evolutions of the ion saturation currents were investigated to characterize the proposed plasma source. The calculated electron gyro–radii, re were 0.17, 1.64, 5.82 mm for setups (a), (b), and (c), respectively. The strong ring–shaped plasma discharges was observed for all setups. The typical discharge voltages were 1.0, 0.6, and 0.6 kV for setups (a), (b), and (c), respectively. The ion saturation currents, Iisat were 1.44, 2.88, and 2.2 mA for setups (a), (b), and (c), respectively at r = 45 mm and t = +10 μs. The Iisat of setup (b) is less fluctuating, whereas Iisat of setup (a) and (c) is highly variable in all positions, so that setup (b) has the best profile among the setups.

Propagation characteristics of dust–acoustic waves in presence of a floating cylindrical object in the DC discharge plasma

The experimental observation of the self–excited dust acoustic waves (DAWs) and its propagation characteristics in the absence and presence of a floating cylindrical object is investigated. The experiments are carried out in a direct current (DC) glow discharge dusty plasma in a background of argon gas. Dust particles are found levitated at the interface of plasma and cathode sheath region. The DAWs are spontaneously excited in the dust medium and found to propagate in the direction of ion drift (along the gravity) above a threshold discharge current at low pressure. Excitation of such a low frequency wave is a result of the ion–dust streaming instability in the dust cloud. Characteristics of the propagating dust acoustic wave get modified in the presence of a floating cylindrical object of radius larger than that of the dust Debye length. Instead of propagation in the vertical direction, the DAWs are found to propagate obliquely in the presence of the floating object (kept either vertically or horizontally). In addition, a horizontally aligned floating object forms a wave structure in the cone shaped dust cloud in the sheath region. Such changes in the propagation characteristics of DAWs are explained on the basis of modified potential (or electric field) distribution, which is a consequence of coupling of sheaths formed around the cylindrical object and the cathode.

Sheath structure transition controlled by secondary electron emission

In particle-in-cell Monte Carlo collision (PIC MCC) simulations and in an experiment we study sheath formation over an emissive floating Al2O3 plate in a direct current discharge plasma at argon gas pressure 10−4 Torr. The discharge glow is maintained by the beam electrons emitted from a negatively biased hot cathode. We observe three types of sheaths near the floating emissive plate and the transition between them is driven by changing the negative bias. The Debye sheath appears at lower voltages, when secondary electron emission is negligible. With increasing applied voltage, secondary electron emission switches on and a first transition to a new sheath type, beam electron emission (BEE), takes place. For the first time we find this specific regime of sheath operation near the floating emissive surface. In this regime, the potential drop over the plate sheath is about four times larger than the temperature of plasma electrons. The virtual cathode appears near the emissive plate and its modification helps to maintain the BEE regime within some voltage range. Further increase of the applied voltage U initiates the second smooth transition to the plasma electron emission sheath regime and the ratio Δφs/Te tends to unity with increasing U. The oscillatory behavior of the emissive sheath is analyzed in PIC MCC simulations. A plasmoid of slow electrons is formed near the plate and transported to the bulk plasma periodically with a frequency of about 25 kHz.

Etching of carbon nanowalls during synthesis in the plasma of direct current discharge

Anisotropic etching of carbon nanowalls by hydrogen during synthesis in plasma discharge of direct current is considered. This effect brings about generation of defects in the bottom part of the side surface of the nanowalls during their vertical growth. Based on the theoretical model of the discharge, it is shown that a decrease in the intensity of such etching is accompanied by an increase in the concentrations of such hydrocarbon radicals as C, CH, CH2, C2H, C3, and C3H, which indicates their possible role in the so-called process of healing of vacancies in the structure of nanowalls. In addition, it has been shown that an increase in synthesis temperature also can contribute to a decrease in the etching intensity.

Plasma-based ambient sampling/ionization/transmission integrated source for mass spectrometry.

Better sensitivity and interface of ambient sampling/ionization mass spectrometry remain a challenge. Herein, a novel, plasma-based, ambient sampling/ionization/transmission (PASIT) integrated source in a pin-to-funnel configuration was developed for the sensitive analysis of complex samples. With the funnel sleeve directly affected by direct-current discharge plasma, PASIT combines the ability to sample/ionize analyte molecules and then efficiently collect/transport charged mass species under atmospheric pressure and consequently shows an improved sensitivity. The integrated source enhances the signal intensity by more than 2 orders of magnitude compared with the previous pin-to-plate plasma source without significant background addition. A surface limit of detection (LOD) of 130 fmol mm-(2) (S/N = 3) has been achieved for clenbuterol on filter paper with an argon carrier gas. Demonstrated applications include the direct determination of active ingredients from drugs and symbolic compounds from natural plants and cholesterol from mouse brain tissue sections.

Pulsed PECVD for Low‐temperature Growth of Vertically Aligned Carbon Nanotubes

Pulsed plasma‐enhanced (PE)CVD is used for the growth of vertically aligned multiwall carbon nanotubes (CNTs) at a low temperature range of 350–490 °C. A pulsed plasma is generated by the application of a rectangular negative pulse to the substrate electrode with an on time of 4.5 μs, off time of 5.5 μs, duty cycle of 45%, and pulse repetition frequency of 100 kHz. CNTs are synthesized from Ni catalyst film of 20–40 nm thickness deposited on a silicon substrate under pressures of 0.01 and 0.5 Torr by magnetron sputtering. The effect of Ni catalyst film morphology on low temperature growth of CNTs by pulsed PECVD is studied. It is found that CNTs grown from Ni catalyst films depend on the process pressure employed to prepare the film by magnetron sputtering. A comparison with the direct current (DC) discharge‐produced CNTs reveals that the growth rate of pulsed plasma‐produced CNTs is two times higher. CH species density is studied using optical emission spectroscopy (OES) by an actinometrical approach, which shows that DC discharge plasma has a higher CH concentration, but still has a lower growth rate. Further, it is observed that using pulsed plasma, growth of CNTs is possible at temperatures down to 350 °C, whereas in the case of DC discharge plasma, CNTs growth is possible only at temperatures down to 450 °C in the present experimental set‐up. Possible reasons for the better performance of pulsed plasma in respect of growth rate and low temperature growth are discussed.

Initial Performance Evaluation of a Gridded Radio Frequency Ion Thruster

Helicon plasma sources are devices that are capable of efficiently producing high-density plasmas. There is growing interest in using a helicon plasma source in space propulsion as a replacement to the direct current plasma discharge in ion engines. A radio frequency ion engine is developed that combines a helicon plasma source with electrostatic grids and a magnetically shielded anode. Thruster performance evaluation includes estimation of the discharge plasma ion density and electron temperature and measurement of the plume current density, grid currents, and thrust. The radio frequency ion engine is tested across the following operating parameter ranges: 343–600 W radio frequency power, 50–250 G magnetic field strength, argon flow rate, and 100–600 V discharge voltage range at an operating pressure of , discharge ion density and electron temperature range and 5.4–9.9 eV, respectively. Maximum beam current extracted is 120 mA at a 600 V discharge and is primarily limited by ion impingement on the grids due to insufficient grid potentials. The maximum measured thrust is 2.77 mN with an average uncertainty of . Further optimization of the thruster operating conditions and grid assembly can improve thruster performance.

High-speed camera observation of solution plasma during nanoparticles formation

The direct-current discharge plasma during nanoparticles formation was observed using a high-speed camera. Metallic plates of Au, Ni, Ti, and Zn were used as a cathode, and a Pt wire was used as an anode. Both electrodes were immersed in a 0.1M NaOH solution. The solution plasma with light emission was generated via the vapor layer surrounding the cathode by applying 190 V. The current concentration occurred at a certain point of the electrode surface, in which the electrode surface was partially melted to produce nanoparticles. According to the high-speed observation, many light-emitting points appeared on the metallic plate and immediately disappeared when a certain point was strongly heated to produce nanoparticles. Additionally, light emission points moved in a chain reaction; after the first emission point was generated, the next emission point tended to be generated in the space surrounding the first emission point. During electrolysis, holes were generated on the cathode. The current concentration strongly heated certain spots on the electrode, and the electrode momentarily melted or vaporized, resulting in the formation of nanoparticles.

Growth of a Carbon Nanotube Forest on Silicon using Remote Plasma CVD

CVD enhanced by remote plasma is applied to produce carbon film material on silicon substrates. The layers of aligned multiwall carbon nanotubes (MWCNT) are obtained without a metal catalyst by deposition from a methane/hydrogen gas mixture activated by a direct current (DC) discharge. The individual MWCNTs in the films consist of 4–7 concentric carbon shells with a spacing of 0.34 nm. An absence of metallic impurities in the MWCNT film material is confirmed using energy dispersive X‐ray (EDX) and electron energy loss spectroscopy (EELS) analyses. A proposed model explains the formation of the cylindrical seeds of the MWCNTs via condensation of carbon atoms from the activated gaseous environment on the inner surfaces of pores, created in the substrate material as a result of silicon etching by ionized hydrogen, generated in the DC discharge plasma. Further growth of these seeds during the CVD process leads to the formation of MWCNT forests.

Discharge and optical characteristics of long arc plasma of direct current discharge

Arc discharge with argon and nitrogen was generated across the long electrode gap distance of 400mm to produce a stable thermal plasma and large volume with low power under atmospheric pressure. In the case of nitrogen as plasma forming gas, an increase of the gas flow rate increases the arc voltage, thus the center temperature of the arc column reached higher. For the argon arc, the arc voltage decreases with increasing gas flow rates. Due to the arc constriction in both cases of nitrogen and argon, the center temperature of the arc column increases under the constant current. The result of emission intensity distribution across nitrogen arc column reveals that the arc column diameter increases with increasing gas flow rate because of the increase of the input power. On the other hand, in the argon arc, the diameter decreases by strong thermal pinch in large flow rate of argon. The measured excitation temperature is uniform along with axial direction of the arc column which is an important feature for waste materials processing due to the long direct current arc plasma that provides long residence time for injected materials.

Ionic liquids as novel catalysts for methane conversion under a DC discharge plasma

Nine imidazolium-based ionic liquids were investigated as novel catalysts for methane conversion in a direct current (DC) discharge plasma reactor. The conversion of methane increased from 21.2% to 43.6% in the plasma system when the C6MIMHSO4 IL was used as a catalyst, C6MIMBF4 successfully achieved 91.0% selectivity for C2 hydrocarbons. The results of the optical emission spectroscopy indicate that the intensity of the C2, CH, H, C+, and C active species from methane decomposition increased when C6MIMHSO4, C6MIMCF3COO, and C6MIMBF4 were introduced into the plasma system. FTIR analysis indicates that C6MIMBF4 is steady in the DC discharge plasma.

Ionization mechanism in a direct current discharge plasma in an argon-oxygen mixture

We present the results of calculating the charged particle formation and decay rates in a dc discharge positive column in argon-oxygen mixture. The calculations are based on a combined solution to the Boltzmann equation; vibration kinetics equations for the ground state of the O2 molecule; and chemical kinetics equations for Ar and O atoms, O2 molecule, and their excited states under the experimentally determined reduced electric field strength, E/N, for various gas phase compositions, currents, and pressures. The role is clarified of particular components in the ionization processes. Electron and positive ion decay is shown to take place predominantly at the reactor walls on which they fall due to joint diffusion. The negative oxygen ions affect the diffusion rate.

The surface structure and chemical characters of activated carbon fibers modified by plasma

ABSTRACTViscose‐based activated carbon fibers (VACFs) were treated by a direct‐current (DC) discharge plasma at different conditions. The surface functional groups of VACFs were modified to improve the adsorption and catalysis capacity for SO2 and NOx. The surface properties of the untreated and treated VACFs which were modified in different feed gases (air, O2, N2 and NH3) were performed by scanning electron microscopy, BET, X‐ray photoelectron spectroscopy (XPS) and Fourier transform infrared attenuated total reflection spectroscopy (FTIR‐ATR). The results show that after being treated with plasma, more particles appeared on the external surface of the VACFs and the surface became rougher, the average pore size decreased but the pore volume and the surface area decreased first and then increased. XPS revealed that DC discharge plasma can remarkably change the distribution of functional groups on the surface of VACFs. FTIR‐ATR also indicated that more nitrogen atoms were introduced to the VACFs surface when the feed gas contained nitrogen atoms and N and O atoms were introduced to the surface of VACFs at the same time when the feed gas was air, but it did not happen in the feed gas of N2, or O2, or NH3. Discharge voltage and treatment time were important factors and for our experiment in the feed gas of nitrogen, the optimum modification conditions were 8 kV and 5 min, respectively. © 2011 Curtin University of Technology and John Wiley & Sons, Ltd.

Raman scattering analysis of silicon dioxide single crystal treated by direct current plasma discharge

Low-k materials such as silicon dioxide (SiO2) play an important role in the semiconductor industry. Plasma has become indispensable for advanced materials processing. In this work a treatment of SiO2 single crystal by direct current plasma discharge is studied in detail. Offline metrology is conducted for silicon dioxide wafers by Raman scattering, energy-dispersive x-ray spectroscopy, and ellipsometry. Broad Raman peak at around 2800 cm−1 is observed for the treated SiO2 wafers. Effects of plasma treatment on position of this peak are reported in the paper. An analysis of this correlation could be a framework for creating virtual etch rate sensors, which might be of importance in managing plasma etching processes.