Pulsed Dc Discharge Research Articles

Optical emission spectroscopy of 50 Hz pulsed dc nitrogen–hydrogen plasma in the presence of active screen cage

ABSTRACTThe N2-H2 plasma gas mixture, generated in a 50 Hz pulsed dc discharge system with active screen cage, is characterized by optical emission spectroscopy (OES), as a function of gas pressure, the fractions of hydrogen and current density. The N2 dissociation degree and N atomic density was measured with actinometery where argon gas is used as actinometer. It was shown that the increase in hydrogen fraction enhances the dissociation of N2, until the maximum of 40%. The excitation temperature is determined from Ar-I emission line intensities by using the simple Boltzmann plot method. The dissociation fraction and excitation temperature is found to increase with hydrogen mixing in nitrogen plasma.

Evolution of Striation in Pulsed Glow Discharges**supported by National Natural Science Foundation of China (Nos. 10875010 and 11175017)

In this work, striations in pulsed glow discharges are studied by experiments and Particle-In-Cell/Monte Carlo Collision (PIC/MCC) simulation. The spatio-temporal evolution of the potential and the electron energy during the discharge are analyzed. The processes of striation formation in pulsed glow discharges and dielectric barrier discharges (DBD) are compared. The results show that the mechanisms of striation in pulsed DC discharge and DBD are similar to each other. The evolution of electron energy distribution function before and after the striation formation indicates that the striation results from the potential well of the space charge. During a pulsed breakdown, the striations are formed one by one towards the anode in a weak field channel. This indicates that the formation of striations in a pulsed discharge depends on the flow of modulated electrons.

Time-resolved curling-probe measurements of electron density in high frequency pulsed DC discharges

A plasma-induced shift in the resonance frequency of a curling probe measured by using a network analyzer (NWA) yields the electron density. This technique was applied here for measuring time-varying electron density in pulsed DC glow discharges. Using the NWA in an on-sweep synchronization mode with the discharge pulse allows measuring at pulse frequencies below 0.5 kHz. For higher pulse frequencies, an on-point mode was introduced which enabled time-resolved measurements of electron density at pulse frequencies reaching 25 kHz, with the minimal time interval of 2 µs, typically for nitrogen discharge at 10 Pa. In the afterglow regime, the decay time constant of electron density was measured for nitrogen and argon discharges at 40 Pa. In the case of argon, the electron density was observed to decrease in three steps. This characteristic behavior was tentatively attributed to a bi-Maxwellian electron energy distribution and Ramsauer effect, supported by Langmuir probe measurements.

Characterization of thick and soft DLC coatings deposited on plasma nitrided austenitic stainless steel

Thick and soft a-C:H:Si coatings containing more than 45% hydrogen (thickness: 25–27μm, hardness: 6GPa, Young's Modulus 38GPa and low ratio of sp3 bonds) were deposited by PACVD with a DC pulsed discharge on nitrided (duplex sample) and non-nitrided austenitic stainless steel (coated sample). After deposition, the chemical, microstructural and tribological properties were studied. Finally, the adhesion and the atmospheric corrosion resistance of a-C:H:Si coatings were also investigated.In pin-on-disk tests, the friction coefficient using an alumina pin of 6mm in diameter as counterpart, under 0.59GPa Hertzian pressure was 0.05 for the coated samples and 0.076 for the duplex samples. These values were more than one order of magnitude smaller than the friction coefficient of the nitrided sample without coating, which was around 0.65. In the coated samples, the wear loss could not be measured. In ball-on-disk tests under dry sliding conditions, the coatings were tested under different Hertzian pressures (1.29, 1.44 and 1.57GPa) using a steel ball with a diameter of 1.5mm as counterpart. Using a normal load of 9N, the a-C:H:Si coating of the coated samples was broken and detached thus leading to a coefficient of friction of around 0.429. However, in contrast to that, the friction coefficient of the duplex samples remained stable and reached as maximum a value of 0.208.In abrasive tests, mass loss was undetectable in both duplex and coated samples. Furthermore it could be seen that the a-C:H:Si film showed only some smaller grooves and no severe damage or deformation. On the contrary, severe damage was observed in the only nitrided sample. With respect to adhesion, the critical load to break the coating was higher in the duplex sample (27N) than in the only coated sample (16.3N). By chemical analysis using the salt spray fog test, the duplex sample remained clean, but the coated sample failed and presented film delamination as well as general corrosion.

The effect of changing the magnetic field strength on HiPIMS deposition rates

The marked difference in behaviour between HiPIMS and conventional dc or pulsed-dc magnetron sputtering discharges with changing magnetic field strengths is demonstrated through measurements of deposition rate. To provide a comparison between techniques the same circular magnetron was operated in the three excitation modes at a fixed average power of 680 W and a pressure of 0.54 Pa in the non-reactive sputtering of titanium. The total magnetic field strength B at the cathode surface in the middle of the racetrack was varied from 195 to 380 G. DC and pulsed-dc discharges show the expected behaviour that deposition rates fall with decreasing B (here by ~25–40%), however the opposite trend is observed in HiPIMS with deposition rates rising by a factor of 2 over the same decrease in B.These observations are understood from the stand point of the different composition and transport processes of the depositing metal flux between the techniques. In HiPIMS, this flux is largely ionic and slow post-ionized sputtered particles are subject to strong back attraction to the target by a retarding plasma potential structure ahead of them. The height of this potential barrier is known to increase with increasing B.From a simple phenomenological model of the sputtered particles fluxes, and using the measured deposition rates from the different techniques as inputs, the combined probabilities of ionization, α, and back attraction, β, of the metal species in HiPIMS has been calculated. There is a clear fall in αβ (from ~0.9 to ~0.7) with decreasing B-field strengths, we argue primarily due to a weakening of electrostatic ion back attraction, so leading to higher deposition rates. The results indicate that careful design of magnetron field strengths should be considered to optimise HiPIMS deposition rates.

Surface morphology and mechanical strength of AISI M2 tool steel treated in abnormal glow region of plasma

Plasma ion nitriding is a flexible and multi-functional casehardening technique used in the given study for surface modifications of AISI M2 tool steel (0.9% C, 4.2% Cr, 5.0% Mo, 6.0% W, and 2.0% V). By varying the plasma treatment time from 1 to 4 hour and the filling gas pressure from 1 to 4 mbar, the surface nitriding was carried out in the abnormal glow region of a pulsed DC discharge. Pre-plasma treatment heating of the samples was performed by a heating unit; the substrate temperature was raised to 500°C, with the ramping rate of 15°C. The resultant microscopic changes in the surface properties of the plasma treated tool steel were studied with different surface characterization techniques such as X-ray Diffraction, Scanning Electron Microscopy and Vickers’s micro-hardness testing. These investigations confirmed the formation of a compound layer on the plasma exposed surface. It was observed that the layer thickness initially increases and then decreases with nitriding time. Similar results were obtained for increasing filling gas pressure. The X-ray Diffraction results showed a down-shift in the original diffraction peaks, which confirms the nitrogen diffusion into the exposed surface and a compound layer formation. A significant improvement in the surface hardness was also vivid, which might be due to the nitrogen diffusion and the formation of a compound layer on the target surface.

Friction and Wear Behavior Evaluation of DLC Films Grown in Multilayer of Carbon and Silicon

Excellent tribological properties of hard materials surface are desirable in several sectors of industry. Diamond-like carbon (DLC) coatings are well known for their low friction, excellent wear resistance, and high hardness. In this work, DLC films were deposited on AISI M2 steel using a modified PECVD pulsed-DC discharge. Multilayer of carbon and silicon were grown, alternately. Samples were produced with different layer thickness for carbon and silicon, and the same parameters for each material layer, in order to investigate friction coefficient in each layer, evaluate rate deposition variation and the gradient behavior of different layers. Raman spectroscopy was used to verify the structural arrangement of carbon atoms. The films were also characterized by scanning electron microscopy and EDX. Tribological tests were performed to observe adhesion between layers and substrate, friction, and wear. The results showed the variation of friction coefficient and that deposition rate declines when increasing number of layers.

Adherence Analysis of DLC Films Grown on AISI M2 Steel Substrates as a Function of Silicon Interlayer Thickness

Diamond-like carbon (DLC) films are widely known for their attractive properties. High adhesion between coating and substrate is necessary to ensure these properties. The bombardment by energetic species during growth tends to generate high intrinsic compressive stresses levels, which have several consequences in coating performance. However, this problem can be solved with the deposition of a thin interlayer with intermediary properties. In this work, films were grown on M2 steel using a modified plasma enhanced chemical vapor deposition PECVD pulsed-DC discharge. In order to improve the coating adherence on the substrate, a silicon interlayer was deposited varying the growth time, which generated different interlayer thickness. Tribological tests were performed to study adhesion and friction gradient. Raman spectroscopy was used to verify the structural arrangement of carbon atoms. The results showed that thickness variation in silicon interlayer leads to significative changes in adhesion between coating and substrate.

The millimeter-wave spectrum of CCN [formula omitted]: A combined Fourier transform and direct absorption study

New pure rotational measurements have been conducted for the CCN radical (X∼2Πr) in the Ω=1/2 spin–orbit ladder using a combination of Fourier transform millimeter-wave (FTmmW) and millimeter direct absorption techniques. For the FTmmW work, the species was created in a supersonic jet in a pulsed DC discharge from a dilute mixture of CH4 and (CN)2 in argon, while an AC discharge of pure (CN)2 in argon was used for the direct absorption study. Six hyperfine components arising from the nitrogen nuclear spin in the lambda-doublets of the J=5/2→3/2 transition of CCN near 59GHz were recorded, partly as a test of the new FTmmW system. Four transitions were also measured in the 224–296GHz range using direct absorption, as well; at the higher frequencies, lambda-doubling was resolved but the hyperfine structure was effectively collapsed. All measurements required significant signal-averaging. The spectra were analyzed in a combined fit with the previous J=3/2→1/2, FTMW data, using a case (a) Hamiltonian. Rotational and lambda-doubling constants for CCN were significantly improved over previous analyses. Based on the hyperfine and quadrupole constants, the most dominant resonance structure for this radical is the CCN arrangement. Unlike CN and C3N, however, there is still substantial electron density on the nitrogen nucleus such that the CCN structure is also important, as found for CCP and CCAs.

Electrical and optical characteristics of Ar plasma generated by low-frequency (60Hz) power source

The optical and electrical properties of Ar plasma, ignited using low-frequency (LF, 60 Hz) power source, were investigated. The plasma resistance, electron temperature, and density were found to be ∼102 Ω, ∼3 eV and ∼108 cm−3, respectively. The plasma parameters are strongly dependent on Ar pressure and discharge current, and the results of optical emission analysis showed similar tendency to the probe measurements. The properties of Ar plasma are similar to DC-pulsed discharge, and the polarity of AC power can be possible of depositions and modifications on both conductive and insulating materials, unlike DC discharge. The a-C films, which are deposited using Ar-diluted CH4 plasma, showed a smooth surface, high transmittance (>80%), high sp3 concentration (∼40%) and wide optical band-gap (3.55 eV). Consequently, the LF power source was found to be a very simple, convenient and inexpensive tool to generate plasmas for various applications.

Pulsed dc discharge in the presence of active screen for nitriding of high carbon steel

A discharge of nitrogen - hydrogen mixture by 50 Hz pulsed dc in the presence of active screen cage is investigated by optical emission spectroscopy (OES). The aim is to identify the parameters (mixture ratio, filling pressure and current density) that may lead to high concentration of active species (N2, N2+ and N). The maximum concentration in this experiment is found with 40% H2 - 60% N2, at filling pressure of 3 mbar with current density of 5 mAcm-2. High carbon steel samples are nitrided for 1 - 4 hours at 500ºC and the treated samples are characterized by X - ray diffraction (XRD), Raman spectroscopy and Vicker's micro - hardness testing. The XRD spectra show different phases of nitrides (Fe2 - 3N, FeN, CrN) along with iron oxides. The Raman spectra confirm the formation of α - β (C3N4). The surface hardness is found to increase up to six times for 2 hour treatment.

Investigation of 50 Hz Pulsed DC Nitrogen Plasma with Active Screen Cage by Trace Rare Gas Optical Emission Spectroscopy

Optical emission spectroscopy is used to investigate the nitrogen-hydrogen with trace rare gas (4% Ar) plasma generated by 50 Hz pulsed DC discharges. The filling pressure varies from 1 mbar to 5 mbar and the current density ranges from 1 mA·cm−2 to 4 mA·cm−2. The hydrogen concentration in the mixture plasma varies from 0% to 80%, with the objective of identifying the optimum pressure, current density and hydrogen concentration for active species ([N] and [N2]) generation. It is observed that in an N2-H2 gas mixture, the concentration of N atom density decreases with filling pressure and increases with current density, with other parameters of the discharge kept unchanged. The maximum concentrations of active species were found for 40% H2 in the mixture at 3 mbar pressure and current density of 4 mA·cm−2.

Laser-Induced Fluorescence Detection of OH Radicals Generated by Atmospheric-Pressure Nonequilibrium DC Pulse Discharge Plasma Jets

It has been proven that many types of radicals released from atmospheric-pressure plasma can provide effective surface treatment and modification of materials. However, the method for measuring radicals generated with atmospheric-pressure plasma and their reaction mechanisms have not become clear in material surface processing. The OH radical distribution was measured successfully in nonequilibrium atmospheric-pressure dc pulse discharge plasma jet by use of the laser-induced fluorescence system. The OH transition [A2Σ+(v' = 1) ← X2Π(v = 0)] at 282 nm was used to monitor the ground-state OH radicals.

Study of gas heating mechanisms in millisecond pulsed discharges and afterglows in air at low pressures

A self-consistent model is developed to study the temporal variation of the gas temperature in millisecond single dc pulsed discharges and their afterglows in air-like mixtures (N2–20%O2) at low pressures. The model is based on the solutions to the time-dependent gas thermal balance equation, under the assumption of a parabolic gas temperature profile across the discharge tube, coupled to the electron, vibrational and chemical kinetics. Modelling results provide a satisfactory explanation for recently published time-resolved experimental data for the gas temperature in a 5 ms pulsed air plasma with a current of 150 mA and the corresponding afterglow at a pressure of 133 Pa (1 Torr). It is shown that the main heating mechanisms during the first millisecond of the pulse come predominantly from O2 dissociation by electron impact through the pre-dissociative excited state and the quenching of nitrogen electronically excited states , B 3∏g, , a 1∏g, w 1Δu) by O2, agreeing with other studies on fast gas heating in air plasmas. As the pulse duration increases, other gas heating sources become important, namely V–T N2–O energy exchanges, recombination of oxygen atoms at the wall, N2(A) quenching by O(3P) and reaction N(4S) + NO(X) → N2(X, v ∼ 3) + O, contributing altogether to an additional smooth increase in the gas temperature until the end of the pulse. In the first instants of the early afterglow, the gas temperature decreases very rapidly as a consequence of the minor role played by electronic collisions and due to a fast decay of N2 electronic states. For afterglow times up to 10 ms, the gas temperature continues to decrease, following the time-dependent kinetics of [N2(X, v)], [N(4S)], [O(3P)] and [NO(X)]. Sensitivity of the model to different input parameters such as thermal accommodation coefficient and probabilities for atomic recombination at the wall are reported.

Plasma Polymerization Inside Tubes in Hexamethyldisiloxanes and Ethyne Glow Discharges: Effects of Deposition Atmosphere on Wetting and Ageing in Solvents

The coating deposition inside tubes becomes increasingly important for fluidic applications, in which inner surfaces are chemically and mechanically strained by the flowing liquid and by scratching of particles. The developed process for tube coating, presented in this work, is based on the discharge in the precursor gas atmosphere between two mesh electrodes at the ends of the tube. The gas mixture is introduced on one end and pumped through the electrode on the other end. Igniting plasma inside the tube, the tube walls are the barrier to the atmosphere. Especially pulsed DC discharges for plasma polymerization in this alignment lead to good coating results, which is shown in this work focusing on deposition in pure and mixed hexamethyldisiloxane, ethyne, and oxygen atmospheres. Chemical binding, wetting, and ageing are strongly influenced by the choice of the gas mixtures. Sufficient oxygen partial pressure in the deposition atmosphere leads to hydrophilic behavior of the SiO2-like polymer-like carbon coatings, all other applied atmospheres to generally hydrophobic behavior of pure and Si-doped plasma polymers, respectively.

Surface deactivation of vibrationally excited N2 studied using infrared titration combined with quantum cascade laser absorption spectroscopy

The wall de-excitation probability of vibrationally excited nitrogen molecules was determined using infrared (IR) titration with CO, CO2 and N2O. Gas mixtures of N2 with 0.05–0.5% of CO (CO2 or N2O) were excited by a pulsed dc discharge at p = 133 Pa in a cylindrical discharge tube. During the afterglow, the vibrational relaxation of titrating molecules was monitored in situ with quantum cascade laser absorption spectroscopy. The value of was deduced from measured vibrational relaxation times using a model of vibrational kinetics in N2. It was found that adsorption of IR tracers on the surface may increase the value of by a factor up to two, depending on the molecule and the surface material. It was demonstrated that N2O is the most inert and reliable tracer and it was used for the determination of on silica, Pyrex, TiO2, Al2O3 and anodized aluminum. Pretreatment of the silica surface by low-pressure plasma was found to have a strong effect on the vibrational de-excitation. Values of measured after O2, Ar and N2 plasma pretreatment of the same silica discharge tube were 5.7 × 10−4, 8.2 × 10−4 and 11 × 10−4, respectively. This study clearly demonstrates that the presence of adsorbed atoms and molecules on the surface may significantly alter the value of .

Evaluation of the mechanical behaviour of a DLC film on plasma nitrided AISI 420 with different surface finishing

DLC films have a high hardness, extremely low friction coefficient, and they are chemically inert. Plasma nitriding can serve as a pre-treatment to increase adhesion and improve tribological performance of the DLC coating.In this work, the mechanical and tribological behaviour of a DLC film deposited on martensitic stainless steel is studied. Two different surface finishing techniques were accomplished before the nitriding process: paper grinding and polishing with diamond powder. Plasma nitriding was carried out in a DC pulsed discharge, and the DLC film was deposited by Plasma Assisted CVD. The nitrided layer and film were observed with an optical microscope and SEM. The tribological behaviour was analysed in sliding wear tests and adhesion was tested. The coating was 3μm thick, and the nitrided layers were about 11μm in the polished samples and only 8μm in the grounded ones. The coated samples yielded a better wear resistance than the pre-nitrided samples but only on the polished nitrided samples did the film present an acceptable adhesion.

Resonance-Enhanced Photon Excitation Spectroscopy of the Even-Parity 3p5(2P1/2)nl′ [K′]J (l′=1, 3) Autoionizing Rydberg States of Ar

Metastable 40Ar* atoms are produced in the two metastable states 3p54s [3/2]2 and 3p54s′ [1/2]0 in a pulsed DC discharge in a beam, and are subsequently excited to the even-parity autoionizing resonance series 3p5np′ [3/2]1,2, 3p5np′ [1/2]1, and 3p5nf′ [5/2]3 using single photon excitation with a pulsed dye laser. The excitation spectra of the even-parity autoionizing resonance series from the metastable 40Ar* are obtained by recording the autoionized Ar+ ions with time-of-flight ion detection in the photon energy range of 32500–35600 cm−1 with an experimental bandwidth of <0.1 cm−1. A wealth of autoionizing resonances are newly observed, from which more precise and systematic spectroscopic data of the level energies and quantum defects are derived.

Structure modification of titanium oxide thin films by rf-plasma assistance in Ti–O2 reactive dc and pulsed dc sputtering

Titanium oxide films have been deposited by Ti–O2 reactive direct current (dc) and pulsed dc sputtering under the presence of the secondary radio-frequency (rf)-plasma to the primary cathode discharge at various discharge pressures in order to reveal effects of rf-plasma assistance on film structure and properties. At low discharge pressures, film structure changes from anatase to rutile by rf-plasma assistance to dc and pulsed dc discharge. At high discharge pressures, films changes from the mixtures of weakly ordered anatase to amorphous by rf-assistance. Refractive index of films deposited increases by rf-plasma assistance in either of dc and pulsed dc sputtering, showing a highest index of 2.57 at a discharge pressure of 3.0Pa. The enhancement of refractive index becomes remarkable at high discharge pressures. Furthermore, the surface of films becomes smoother by the presence of the secondary rf plasma. Being similar to the tendency of the refractive index change, the smoothening of film surface by the presence of the secondary rf plasma is remarkable at high discharge pressures. It is concluded that film structure changes from rutile to amorphous and is simultaneously densified by rf-plasma assistance to dc and pulsed dc discharge at a high discharge pressure, yielding films with a high refractive index and smooth surface.

Pulsed DC discharge for synthesis of conjugated plasma polymerized aniline thin film

The polymerization of aniline in pulsed dc plasma is studied and the effects of variation of pressure, power, frequency and duty cycle on the chemical structure of the obtained film are examined. During the film deposition optical emission spectroscopy is used to investigate the molecular dissociation of aniline. The chemical structure of the films is characterized using Fourier transform infra-red spectroscopy. The surface morphology is studied using atomic force microscopy. Results show the retention of polyaniline like structure having conjugated nature at some particular discharge conditions. Moreover, it is observed that a strong dependence of film chemistry is obvious on the discharge power, reactor pressure, pulse repetition frequency and duty cycle. The advantages of the pulsed dc for deposition of conjugated plasma polymerizes thin film have been highlighted.