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An experimental investigation of the dissociation of CO2 in a symmetric pin-to-pin dielectric barrier discharge (DBD) is presented. The reactor geometry allows for an accurate control of the number of filaments (microdischarges) and is used to study the impact of one single filament on the CO2 dissociation. We show the number of filaments per half cycle follows a power-law as a function of the injected power and does not depend on pressure, flow or other process parameters. It is shown that for pressures between 200 and 700 mbar approximately 0.5 W per filament is required and the charge transferred per filament remains constant at 0.5 nC. Furthermore, the dependence of CO2 conversion on only specific energy input (SEI) is shown to be valid down to a single filament. Additionally, by using quantum cascade laser absorption spectroscopy the absolute number of CO molecules produced per filament is measured and is found to be in the range from 5.1011 to 2.1012. The conversion degree of CO2 into CO is estimated to be lower than 0.1% within a single filament and increases with SEI. In the presence of a couple of filaments, the maximum energy efficiency obtained is 25%. A comparison of the conversion degrees in pin-to-pin DBD and plane-to-plane DBD configuration shows that these two reactor geometries follow the same power law. This means the geometry is not the most important parameter in CO2 dissociation in DBDs, but the SEI and thus the number of filaments ignited per unit of time. This result means that the dependence of conversion degree on the SEI can be extended to a single filament. This observation leads to the conclusion that the SEI appears to be valid as a universal scaling parameter down to very low values. [ABSTRACT FROM AUTHOR]

The densities of N, NH, N[H.sub.2] radicals in a remote Ar-N[H.sub.3]-Si[H.sub.4] plasma used for high-rate silicon nitride deposition are examined for different gas mixtures and plasma settings by using cavity ringdown absorption spectroscopy and threshold ionization mass spectroscopy. It is concluded that N and N[H.sub.2] radicals are mainly responsible for the N incorporation in the silicon nitride film, while Si atoms are most likely brought to the surface in the form of Si[H.sub.x] radicals.

A roll-to-roll high-current diffuse dielectric barrier discharge at atmospheric pressure was operated in air and Ar/N2/O2 gas mixtures. The exhaust gas from the discharge was studied using a high-resolution Fourier-transform infrared spectrometer in the range from 3000 to 750 cm-1 to unravel the plasma-polymer interactions. The absorption features of HxNyOz, COx, and HCOOH (formic acid) were identified, and the relative densities were deduced by fitting the absorption bands of the detected molecules. Strong interactions between plasma and polymer (Polyethylene-2,6-naphthalate, or PEN) in precursor-free oxygen-containing gas mixtures were observed as evidenced by a high COx production. The presence of HCOOH in the gas effluent, formed through plasmachemical synthesis of COx, turns out to be a sensitive indicator for etching. By adding tetraethylorthosilicate precursor in the plasma, dramatic changes in the COx production were measured, and two distinct deposition regimes were identified. At high precursor flows, a good agreement with the precursor combustion and the COx production was observed, whereas at low precursor flows an etching-deposition regime transpires, and the COx production is dominated by polymer etching. [ABSTRACT FROM AUTHOR]

The aim is to determine the role of carbon atoms in the growth of hydrogenated amorphous carbon (a-C:H) films by means of an argon/acetylane expanding thermal plasma. Only at low acetylane flows, the condition when soft polymer-like films are deposited, carbon atoms are detected close to the substrate and possibly contribute to the film growth.

The dissociation of CO2 in a geometrically symmetric dielectric barrier discharge has been analysed by means of numerical modelling. A time- and space-dependent fluid model has been used, taking into account the spatial variation of the plasma between the plane-parallel dielectrics covering the electrodes. The main features of the model, including an extensive reaction kinetics for the vibrational states of CO2, are given. The modelling studies have been performed for different applied voltages, discharge frequencies, pressures, gas temperatures, and relative permittivities of the dielectrics. The model calculations show that the discharges in the positive and negative half-cycles are different for the considered standard condition, leading to a spatially asymmetric distribution of the stable neutrals like CO molecules and O atoms. The generation of CO mainly takes place during the discharge pulses, and it is dominated by electron impact dissociation. The specific energy input obtained for the broad range of parameters considered and determined for residence times reported in the literature agrees well with the corresponding experimental values. In accordance with these experiments, the calculated degree of CO2 conversion has been found to increase almost linearly with the specific energy input. Remaining discrepancies between the measured and calculated energy efficiencies are discussed. [ABSTRACT FROM AUTHOR]

We present a time-resolved imaging and electrical study of an atmospheric pressure spark discharge. The conditions of the present study are those used for nanoparticle generation in spark discharge generator setups. The oscillatory bipolar spark discharge was generated between two identical Cu electrodes in different configurations (cylindrical flat-end or tipped-end geometries, electrode gap from 0.5 to 4 mm), in a controlled co-axial N2 flow, and was supplied by a high voltage capacitor. Imaging data with nanosecond time resolution were collected using an intensified CCD camera. This data were used to study the time evolution of plasma morphology, total light emission intensity, and the rate of plasma expansion. High voltage and high current probes were employed to collect electrical data about the discharge. The electrical data recorded allowed, among others, the calculation of the equivalent resistance and inductance of the circuit, estimations for the energy dissipated in the spark gap. By combining imaging and electrical data, observations could be made about the correlation of the evolution of total emitted light and the dissipated power. It was also observed that the distribution of light emission of the plasma in the spark gap is uneven, as it exhibits a "hot spot" with an oscillating position in the axial direction, in correlation with the high voltage waveform. The initial expansion rate of the cylindrical plasma front was found to be supersonic; thus, the discharge releases a strong shockwave. Finally, the results on equivalent resistance and channel expansion are comparable to those of unipolar arcs. This shows the spark discharge has a similar behavior to the arc regime during the conductive phase and until the current oscillations stop. [ABSTRACT FROM AUTHOR]

The dissociation of CO2 and the formation of CO, O3, and O2 were studied in a dielectric barrier discharge (DBD) at atmospheric pressure by means of ex-situ infrared absorption spectroscopy. CO mixing ratios of 0.1%-4.4% were determined for specific injected energies between 0.1 and 20 eV per molecule (0.3-70 kJ/l). A lower limit of the gas temperature of 320-480 K was estimated from the wall temperature of the quartz reactor as measured with an infrared camera. The formation of CO in the DBD could be described as function of the total number of transferred charges during the residence time of the gas in the active plasma zone. An almost stoichiometric CO:O2 ratio of 2:1 was observed along with a strongly temperature dependent O3 production up to 0.075%. Although the ideal range for an efficient CO2 dissociation in plasmas of 1 eV per molecule for the specific injected energy was covered, the energy efficiency remained below 5% for all conditions. The present results indicate a reaction mechanism which is initiated by electron impact processes followed by charge transfer reactions and non-negligible surface enhanced O and CO recombination. While electron-driven CO2 dissociation is relatively energy inefficient by itself, fast O recombination and the low gas temperatures inhibit the synergistic reuse of atomic oxygen in a secondary CO2 + O dissociation step. [ABSTRACT FROM AUTHOR]

In situ quantum cascade laser (QCL) absorption spectroscopy is used to investigate the effect of admixed water in a pulsed CO2 glow discharge on the vibrational excitation of CO2 and CO and the conversion of CO2. Time-resolved transmittance spectra of the non-equilibrium CO2 plasma are measured with a 100 μs time resolution. A custom fitting routine is used to extract the time evolution of the gas temperature, rotational temperature and vibrational temperatures of CO2 and CO, while the CO2 conversion is determined from measured CO2 and CO number densities. Rotational Raman scattering is additionally performed in the centre of the reactor to verify measured rotational and vibrational temperatures from line-of-sight absorption spectroscopy. The plasma is operated at 6.7 mbar, with up to 10% water admixed, and is pulsed with a 5–10 ms on-off cycle, with a current of 50 mA supplied during the plasma on-time. Vibrational temperatures and CO2 conversion are not significantly affected by water admixtures below 0.5%. However, the asymmetric stretch temperature of CO2 (T3) shows considerable quenching upon admixing 10% water vapour, with the maximum elevation above the rotational temperature (Trot) decreasing from 580 ± 86 K to 230 ± 63 K. For the vibrational temperature of CO (TCO), a similar trend is measured. However, the slopes of T3 and TCO within the first few hundred μs after the start of the plasma remain unchanged, even when admixing 10% water vapour, suggesting equal excitation of the vibrational modes through e–V and V–V interactions. The conversion decreases by almost a factor of 4 when admixing 10% water. We argue that vibrational quenching of CO2 by water can explain part of the decrease. Changes in electron density and temperature and reactions between CO and OH can also play a role. [ABSTRACT FROM AUTHOR]

State resolved densities of high rovibrationally excited hydrogen isotopologues H2, HD, and D2 in the electronic ground state have been measured in a supersonically expanding plasma jet. The obtained state distributions differ substantially from thermal equilibrium. Moreover, the distributions are not the same for H2, HD, and D2 indicating different formation and relaxation rates for each isotopologue. Mechanisms for this deviation from a Boltzmann distribution are given and compared to hydrogen reactions in other environments. The difference between the measured highest occupied rovibrational states in H2, HD, and D2 is ascribed to an isotope effect in the dissociation process. [ABSTRACT FROM AUTHOR]

The remote plasma deposition of hydrogenated amorphous carbon (a-C:H) thin films is investigated by in situ spectroscopic ellipsometry (SE). The dielectric function of the a-C:H film is in this paper parametrized by means of B-splines. In contrast with the commonly used Tauc–Lorentz oscillator, B-splines are a purely mathematical description of the dielectric function. We will show that the B-spline parametrization, which requires no prior knowledge about the film or its interaction with light, is a fast and simple-to-apply method that accurately determines thickness, surface roughness, and the dielectric constants of hydrogenated amorphous carbon thin films. Analysis of the deposition process provides us with information about the high deposition rate, the nucleation stage, and the homogeneity in depth of the deposited film. Finally, we show that the B-spline parametrization can serve as a stepping stone to physics-based models, such as the Tauc–Lorentz oscillator. [ABSTRACT FROM AUTHOR]

The direct simulation Monte Carlo (DSMC) method was used to investigate the efficiency of differential pumping in linear plasma generators operating at high gas flows. Skimmers are used to separate the neutrals from the plasma beam, which is guided from the source to the target by a strong axial magnetic field. In this way, the neutrals are prevented to reach the target region. The neutral flux to the target must be lower than the plasma flux to enable ITER relevant plasma-surface interaction (PSI) studies. It is therefore essential to control the neutral gas dynamics. The DSMC method was used to model the expansion of a hot gas in a low pressure vessel where a small discrepancy in shock position was found between the simulations and a well-established empirical formula. Two stage differential pumping was modeled and applied in the linear plasma devices Pilot-PSI and PLEXIS. In Pilot-PSI a factor of 4.5 pressure reduction for H2 has been demonstrated. Both simulations and experiments showed that the optimum skimmer position depends on the position of the shock and therefore shifts for different gas parameters. The shape of the skimmer has to be designed such that it has a minimum impact on the shock structure. A too large angle between the skimmer and the forward direction of the gas flow leads to an influence on the expansion structure. A pressure increase in front of the skimmer is formed and the flow of the plasma beam becomes obstructed. It has been shown that a skimmer with an angle around 53° gives the best performance. The use of skimmers is implemented in the design of the large linear plasma generator Magnum-PSI. Here, a three stage differentially pumped vacuum system is used to reach low enough neutral pressures near the target, opening a door to PSI research in the ITER relevant regime. [ABSTRACT FROM AUTHOR]

Achieving the high sensitivity necessary for trace gas detection in the midinfrared molecular fingerprint region generally requires long absorption path lengths. In addition, for wider application, especially for field measurements, compact and cryogen free spectrometers are definitely preferable. An alternative approach to conventional linear absorption spectroscopy employing multiple pass cells for achieving high sensitivity is to combine a high finesse cavity with thermoelectrically (TE) cooled quantum cascade lasers (QCLs) and detectors. We have investigated the sensitivity limits of an entirely TE cooled system equipped with an ∼0.5 m long cavity having a small sample volume of 0.3 l. With this spectrometer cavity enhanced absorption spectroscopy employing a continuous wave QCL emitting at 7.66 μm yielded path lengths of 1080 m and a noise equivalent absorption of 2×10-7 cm-1 Hz-1/2. The molecular concentration detection limit with a 20 s integration time was found to be 6×108 molecules/cm3 for N2O and 2×109 molecules/cm3 for CH4, which is good enough for the selective measurement of trace atmospheric constituents at 2.2 mbar. The main limiting factor for achieving even higher sensitivity, such as that found for larger volume multi pass cell spectrometers, is the residual mode noise of the cavity. On the other hand the application of TE cooled pulsed QCLs for integrated cavity output spectroscopy and cavity ring-down spectroscopy (CRDS) was found to be limited by the intrinsic frequency chirp of the laser. Consequently the accuracy and advantage of an absolute internal absorption calibration, in theory inherent for CRDS experiments, are not achievable. [ABSTRACT FROM AUTHOR]

We investigated the efficiency and formation mechanism of ammonia generation in recombining plasmas generated from mixtures of N2 and H2 under various plasma conditions. In contrast to the Haber-Bosch process, in which the molecules are dissociated on a catalytic surface, under these plasma conditions the precursor molecules, N2 and H2, are already dissociated in the gas phase. Surfaces are thus exposed to large fluxes of atomic N and H radicals. The ammonia production turns out to be strongly dependent on the fluxes of atomic N and H radicals to the surface. By optimizing the atomic N and H fluxes to the surface using an atomic nitrogen and hydrogen source ammonia can be formed efficiently, i.e., more than 10% of the total background pressure is measured to be ammonia. The results obtained show a strong similarity with results reported in literature, which were explained by the production of ammonia at the surface by stepwise addition reactions between adsorbed nitrogen and hydrogen containing radicals at the surface and incoming N and H containing radicals. Furthermore, our results indicate that the ammonia production is independent of wall material. The high fluxes of N and H radicals in our experiments result in a passivated surface, and the actual chemistry, leading to the formation of ammonia, takes place in an additional layer on top of this passivated surface. [ABSTRACT FROM AUTHOR]

The densities of N, NH, and NH2 radicals in a remote Ar–NH3–SiH4 plasma used for high-rate silicon nitride deposition were investigated for different gas mixtures and plasma settings using cavity ringdown absorption spectroscopy and threshold ionization mass spectrometry. For typical deposition conditions, the N, NH, and NH2 radical densities are on the order of 1012 cm-3 and the trends with NH3 flow, SiH4 flow, and plasma source current are reported. We present a feasible reaction pathway for the production and loss of the NHx radicals that is consistent with the experimental results. Furthermore, mass spectrometry revealed that the consumption of NH3 was typically 40%, while it was over 80% for SiH4. On the basis of the measured N densities we deduced the recombination and sticking coefficient for N radicals on a silicon nitride film. Using this sticking coefficient and reported surface reaction probabilities of NH and NH2 radicals, we conclude that N and NH2 radicals are mainly responsible for the N incorporation in the silicon nitride film, while Si atoms are most likely brought to the surface in the form of SiHx radicals. [ABSTRACT FROM AUTHOR]

The densities of NH and NH2 radicals in an Ar–NH3 plasma jet created by the expanding thermal plasma source were investigated for various source-operating conditions such as plasma current and NH3 flow. The radicals were measured by cavity ringdown absorption spectroscopy using the (0,0) band of the A 3Π←X 3Σ- transition for NH and the (0,9,0)-(0,0,0) band of the à 2A1←X 2B1 transition for NH2. For NH, a kinetic gas temperature and rotational temperature of 1750±100 and 1920±100 K were found, respectively. The measurements revealed typical densities of 2.5×1012 cm-3 for the NH radical and 3.5×1012 cm-3 for the NH2 radical. From the combination of the data with ion density and NH3 consumption measurements in the plasma as well as from a simple one-dimensional plug down model, the key production reactions for NH and NH2 are discussed. [ABSTRACT FROM AUTHOR]

A combination of experiment [optical emission and cavity ring-down spectroscopy (CRDS) of electronically excited H atoms] and two-dimensional (2D) modeling has enabled a uniquely detailed characterization of the key properties of the Ar/H2 plasma within a <=10-kW, twin-nozzle dc arc jet reactor. The modeling provides a detailed description of the initial conditions in the primary torch head and of the subsequent expansion of the plasma into the lower pressure reactor chamber, where it forms a cylindrical plume of activated gas comprising mainly of Ar, Ar+, H, ArH+, and free electrons. Subsequent reactions lead to the formation of H2 and electronically excited atoms, including H(n=2) and H(n=3) that radiate photons, giving the plume its characteristic intense emission. The modeling successfully reproduces the measured spatial distributions of H(n>1) atoms, and their variation with H2 flow rate, FH20. Computed H(n=2) number densities show near-quantitative agreement with CRDS measurements of H(n=2) absorption via the Balmer-β transition, successfully capturing the observed decrease in H(n=2) density with increased FH20. Stark broadening of the Balmer-β transition depends upon the local electron density in close proximity to the H(n=2) atoms. The modeling reveals that, at low FH20, the maxima in the electron and H(n=2) atom distributions occur in different spatial regions of the plume; direct analysis of the Stark broadening of the Balmer-β line would thus lead to an underestimate of the peak electron density. The present study highlights the necessity of careful intercomparisons between quantitative experimental data and model predictions in the development of a numerical treatment of the arc jet plasma. The kinetic scheme used here succeeds in describing many disparate observations—e.g., electron and H(n=2) number densities, spatial distributions of optical emission from the plume, the variation of these quantities with added flow of H2 and, when CH4 is added, absolute number densities and temperatures of radicals such as C2 and CH. The remaining limitations of the model are discussed. [ABSTRACT FROM AUTHOR]

The evolution of the rotational and vibrational distributions of molecular hydrogen in a hydrogen plasma expansion is measured using laser induced fluorescence in the vacuum-UV range. The evolution of the distributions along the expansion axis shows the relaxation of the molecular hydrogen from the high temperature in the upstream region to the low ambient temperature in the downstream region. During the relaxation, the vibrational distribution, which has been recorded up to v=6, is almost frozen in the expansion and resembles a Boltzmann distribution at T≈2200 K. However, the rotational distributions, which have been recorded up to J=17 in v=2 and up to J=11 in v=3, cannot be described with a single Boltzmann distribution. In the course of the expansion, the lower rotational levels (J<5) adapt quickly to the ambient temperature (≈500 K), while the distribution of the higher rotational levels (J>7) is measured to be frozen in the expansion at a temperature between 2000 and 2500 K. A model based on rotation-translation energy transfer is used to describe the evolution of the rotational distribution of vibrational level v=2 in the plasma expansion. The behavior of the low rotational levels (J<5) is described satisfactory. However, the densities of the higher rotational levels decay faster than predicted. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]